Fluid Power, Rate Training Manual

DOCUMENT RESUME ED 070 578 SE 014 125

TITLE Fluid Power, Rate TrainingManual. INSTITUTION Bureau of Naval Personnel,Washington, D. C. REPORT NO NAVPERS-16193-B PUB DATE 70 NOTE 305p. EDRS PRICE MF-$0.65 HC-$13.16 DESCRIPTORS Force; *Hydraulics; Instructional Materials; *Mechanical Equipment; Military Personnel; *Military Science; *Military Training; Physics; *Supplementary Textbooks; Textbooks ABSTRACT Fundamentals of hydraulics and pneumatics are presented in this manual, prepared for regular navy and naval reserve personnel who are seeking advancement to Petty Officer Third Class. The history of applications of compressed fluids is described in connection with physical principles. Selection of types of liquids and gases is discussed with a background of operating temperature ranges, contamination control techniques, lubrication aspects, and safety precautions. Components in closed- and open-center fluid systems are studied in efforts to familiarize circuit diagrams. Detailed descriptions are made for .the functions of fluidlines, connectors, sealing devices, wipers, backup washers, containers, strainers, filters, accumulators, pumps, and compressors. Control and measurements of fluid flow and pressure are analyzed in terms of different types of flowmeters, pressure gages, and values; and methods of directing flow and converting power into mechanical force and motion, in terms of directional control valves, actuating cylinders, fluid motors, air turbines, and turbine governors. Also included are studies of fluidics, trouble shooting, hydraulic power drive, electrohydraulic steering, and missile and aircraft fluid power systems. Illustrations for explanation use and a glossary of general terms are included in the appendix. (CC) IDLE AV

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FLUID POWER

BUREAU OF OF NAVAL PERSONNEL

.1% RATE TRAINING MANUAL NAVPERS 16,1937B PREFACE

Fluid Power is written for personnel of the Navy and Naval Reserve whose duties and responsibilities require them to have a knowledge of the fundamentals of hydraulics and pneumatics. The subject matter covers a broad area of the Qualifications for Advancement, NavPers 18068 ISeries), which is common to several rates and ratings. Combined with the necessary practical experience, this training manual will assist nonrated men in preparing for advancement to Petty Officer Third Class in the, specific ratings requiring a knowledge of fluid power. This training manual should also be valuable as a review and reference source for personnel of more senior rates. This basic Rate Training Manual was prepared by the Navy Training Publications Center, NAS Memphis, Millington, Tennessee, for the Bureau of Naval Personnel. Credit is given to the Naval Air Technical Training Center, located at NAS Memphis, Millington, Tennessee, the Naval Ex7 amining Center, Great Lakes, Illinois,and the Naval Air Systems Command for technical reviews.

First Edition - 1966

Reprinted - 1969 1970 Edition

Stock Ordering No. 0500-393-0110 THE UNITED STATES NAVY

GUARDIAN OF OUR COUNTRY The United States Navy is responsible for maintaining control of the sea and is a ready force on watch at home and overseas, capable of strong action to preserve the peace or of instant offensive action to win in war. It is upon the maintenance of this control that our country's glorious future depends; the United States Navy exists to make it so.

WE SERVE WITH HONOR Tradition, valor, and victory are the Navy's heritage from the past. To these may be added dedication, discipline, and vigilance as the watchwords of the present and the future. At home or on distant stations we serve with pride, confident in the respect of our country, our shipmates, and our families. Our responsibilities sober us; our adversities strengthen us. Service to God and Country is our special privilege. We serve with honor.

THE FUTURE OF THE NAVY The Navy will always employ new weapons, new techniques, and greater power to protect and defend the United States on the sea, under the sea, and in the air. Now and in the future, control of the sea gives the United States her greatest advantage for the maintenance of peace and for victory in war.

. Mobility, surprise, dispersal, and offensive power are the keynotes of the new Navy.The roots of the Navy lie in a strong belief in the future, in continued dedication to our tasks, and in reflection on our heritage from the past. Never have our opportunities and our responsibilities been greater.

4 CONTENTS

Chapter Page

1. History and development 1

2. Physics of fluids 5

3. Hydraulic and pneumatic fluids 37

4. Basic systems and circuit diagrams 49

5. Fluid lines and connectors 67

6. Sealing devices and materials 86

7. Reservoirs, strainers, filters, and accumulators 98

8. Pumps and compressors 120

9. Control and measurement of flow 153

10. Measurement and control of pressure 171

11. Directional control valves 198 12. Actuators 216 1 13. Hydraulic and pneumatic systems 234

14. Fluidics 261 Appendix

I. Glossary 288

Index 295 CHAPTER 1

HISTORY AND DEVELOPMENT

Fluid Power, Nav Pers 16193-B, presents Ships Systems Command, Naval Air Systems many of the fundamental concepts in the fields Command, etc. of hydraulics and pneumatics. This maralal is intended as a basic reference for all personnel 3. Maintenance Requirements Cards of the Navy whose duties and responsibilities (MRC' 8). require them to have a knowledge of the fundam- 4. Military Standards and Specifications. entals of fluid power. Consequently, emphasis is placed primarily on the theory of operation of 5. Instructions and Notices. typical fluid power systems and components that have applications in naval equipments.Many FLUID POWER applications of fluid power are presented in this manual to illustrate the functions and operation Fluid power is a relatively new term which of different systems and components. However, was established to include the generation, con- these are only representative of the many appli- trol, and application of smooth, effective power cations of fluid power in naval equipments. Indi- of pumped or compressed fluids (either liquids vidual rate training manuals should be consulted or gases) when this power is used to provide for information concerning the application of force and motion to mechanisms. This force fluid power to specific equipments for which and motion may be in the form of pushing, pul- each rating is responsible. ling, rotating, regulating, or driving. Fluid power includes hydraulics, which involves liquids, and In addition, the examples of systems and com- pneumatics, which involves gases. Liquids and ponents presented in this manual are used to gases are similar in many respects and there- illustrate the functions and operation of certain fore are described as fluids in most sections of types. Since there are many different manufac- this manual. The differences are pointed out in I turers of fluid power systems and components, the appropriate areas. In these areas they are each type presented may represent several dif- treated separately as liquids and gases, or as ferent models or designs. Therefore, this manual in the following sections, as hydraulics and is not intended for use in the maintenance and pneumatics. repair of specific equipments. Applicable technical publications and technical instructions HYDRAULICS should be used for this purpose. These terms, applicable technical publications and applicable The word hydraulics is a derivative of the technical instructions, are frequently referred Greek words hydro (meaning water) and aulis to in this manual. They include such documents (meaning tube or pipe). Originally, the science as the following. of hydraulics covered the physical behavior of 1. Certain chapters of the Naval Ships water at rest and in motion. This dates back Technical Manual. several thousand years ago when water wheels, dams, and sluice gates were first used to con- 2. Operation, maintenance, repair, over- trol the flow of water for domestic use and haul, and parts manuals, which are provided by irrigation. Use has broadened its meaning to the manufacturer of the component, system, include the physical behavior of all liquids. This and/or equipment. These manuals are usually includes that area of hydraulics in which confined approved by the appropriate command Naval liquids are used under controlled pressure to do 1 FLUID POWER work.This area of hydraulics, sometimes reas the other, and if each is supplied with a piston ferred to as "power hydraulics," is discussed that 'fitsit exactly,' then a man pushing the in this manual. small piston will exert a force that will equi- librate (balance) that of one hundred men pushing . Hydraulics can be defined as that engineering science which pertains to liquid pressure and the large piston and will overcome that of ninety- flow. This science includes, for example, the nine men."(This is the basic principle of hy- manner in which liquids act in tanks and pipes, diarlics and is covered in detail in chapter 2.) dealing with their properties and with ways of For Pascal's law to be made effective for utilizing these properties. It includes the laws of practical applications, it was necessary to have floating bodies and the behavior of liquids on sub- a piston that "fit exactly." It was not until the merged surfaces. It treats the flow of liquids latter mat of the 18th century that methods were under various conditions, and ways of directing fonrol to make these snugly fitted parts required this flow to useful ends, as well as many other in hydraulic systems. This was accomplished by related subjects. the invention of machines which were used to There are several other terms which are cut and shape the necessary closely fitted parts sometimes used to more precisely describe the and, particularly, by the development of gaskets behavior of liquids at rest and in motion. These and packings. Since that time, such components termsaregenerallyconsideredseparate as valves,pumps,actuating cylinders, and branches of science and include: hydrostatics, motors have been developed and refined to make that branch of science pertaining to the energy hydraulics one of the leading methods of trans- of liquids at rest; hydrodynamics, the branch of mitting power. science pertaining to the energy of liquid flow and pressure; and hydrokinetics, whichpertains Application to motions of liquids or the forces which produce or affect such motions. Today, hydraulic power is used to operate many different tools and mechanisms. In a Development garage, a mechanic raises the end of an automobile with a hydraulic jack. Dentists and Although the modern development of hydraul- barbers use hydraulic power to lift and position ics is of comparatively recent date, the ancients their chairs to a convenient working height by a were familiar with many hydraulic principles few strokes of a control lever. Hydraulic door- and their applications. The Egyptians and the stops keep heavy doors from slamming. Hy- ancient peoples of Persia, India, and China con- draulic brakes have been standard equipment on veyed water along channels for irrigation and automobiles for approximately 35 years. Most domestic purposes, using dams and sluicegates automobiles are equipped with automatic trans- to control the flow. The ancient Cretans had an missions that are hydraulically operated. Power elaborate plumbing system. Archimedes studied steering isanother application of hydraulic the laws of floating and submerged bodies. The power. Construction men depend upon hydraulic Romans constructed aqueducts to carry water power for the operation of various components to their cities. of their equipment. For example, the blade of a After the breakup of the ancient world, there bulldozer is normally operated by hydraulic were few new developments for many centuries. power. Then, over a comparatively short period, begin- During the period preceding World War II, ning not more than three or four hundred years the Navy began to apply hydraulics to naval ago, the physical sciences began to flourish, mechanisms extensively. Since then, naval appli- thanks to the discovery ofprinciples basic to all cations have increased to the point where many of them, and to the invention of many new me- ingenious hydraulic devices are used in the solu- chanical devices. Thus, the fundamental law tion of problems of gunnery, aeronautics, and underlying the entire science of hydraulics was navigation. Aboard ship, hydraulic power is util- discovered by Pascalinthe17th century. ized to operate such equipment as anchor wind- Pascal's theorem was as follows:"If a lassc,s, power cranes, steering gear, remote vessel full of water, and closed on all sides, has control devices, power drives for elevating and two openings, the one a hundred times as large traininggunsandrocket launchers. Some 2 Chapter 1 HISTORY AND DEVELOPMENT elevators on aircraft carriers utilize hydraulic the most widely used fluid medium. As discussed power to transfer aircraft from the hanger deck in chapter 3, nitrogen has many of the qualities to the flight deck and vice versa. desiredin thefluid medium for pneumatic Most naval aircraft contain a network of systems and is, therefore, recommended foruse hydraulic lines and components. Hydraulic power in some systems. is used to operate such units as wheel brakes, Probably one of gie greatest uses of pneu- landing flaps, antennas, speed brakes, control matic power is in the operation of the va -ious surfaces, and the retraction and extension of typesof pneumatic tools. Pneumatic arills, the landing gear. Many types of guided missiles screwdrivers, and nut setters are operated by contain hydraulic systems for the operation of air motors. Riveting guns and chipping hammers flight control devices. are usually operated by air pressure. Compres- sion tools such as rivet squeezers and punches PNEUMATICS are operated with pneumatic power. Trains, buses, and large trucks are normally equipped The word pneumatics is a derivative of the with air brakes. Greek word pneuma, which means air, wind, or Most models of aircraft contain pneumatic breath.It can be defined as that branch of engi- systems as an emergency means of operating neering science which pertains to gaseous those units that are normally operated hydraul- pressure and flow.As used in this manual, ically. Guided missiles utilize compressed gas pneumatics is that portion of fluid power in which as a power source for electrical generators and compressed air, or other gas, is used to transmit hydraulic pumps. In some missiles, pneumatic and control power to actuating mechanisms. power is utilized to operate flight control devices. Development Pneumatics and hydraulics are combinedfor some applications. This combination is some- There is no record of man's first use of air timesreferred to as hydropneumatics. An to do work. Probably the first were fans used to example of this combination is the lift used in separate the chaff from grain and the use of sails garages and service stations. Air pressure is to move ships. One of the first pneumatic devices applied to the surface of hydraulic fluid in_ a was the blow gun used by primitive man. In the reservoir. The air pressure forces the hydraulic latter part of the 18th century, heated air was fluid to raise the lift. The accumulator, de- used to carry the first balloon aloft. The heated scribed in chapter 7, is another example of this air was allowed to expand in the balloon, and combination. therefore it became lighter than the surrounding air and caused the balloon to rise. ADVANTAGES OF FLUID POWER Every age of man has witnessed the development of devices to utilize air to do work. The extensive use of hydraulics and pneu- However, it can be seen that man used air for matics to transmit power is due to the fact that properly constructed fluid power systems pos- work long before he understood it. sess a number of favorable characteristics. Many of the principles of hydraulics apply to They eliminate the need for complicated systems pneumatics. For example, Pascal's law applies of gears, cams, and levers. Motion can be trans- to gases as well as liquids. Also, like hydraulics, mitted without the slack inherent in the use of the development of pneumatics depended upon solid machine parts. The fluids used are not closely fitted parts and the development of gas- subject to breakage as are mechanical parts, kets and packings. Since the invention of the air and the mechanisms are not subjected to great compressor, pneumatics has become a very re- wear. liable source for the transmission of power. The different parts of a fluid power system Application can be conveniently located at widely separated points, and the forces generated are rapidly The fluid medium for pneumatic systems is transmitted over considerable distances with usually compressed air or nitrogen. Compressed small loss. These forces can be conveyed up air has been used for many years and is still and down or around corners with small loss in 3 FLUID POWER efficiency and no complicated mechanisms. Very a great amount of pressure and/or extremely large forces can be controlled by much smaller accurate control, a hydraulic system should be ones, and can be transmitted through compara- used. tively small lines and orifices. If the system is well adapted to the work it SPECIAL PROBLEMS is required to perform, and if it isnot misused, The extreme flexibility of fluid power ele- it can provide smooth, flexible, uniform action ments gives rise to a number of problems. without vibration, and is unaffected by variation Since fluids have no shape of their own, they of load. In case of an overload, an automatic must be positively confined throughout the entire release of pressure can be guaranteed, so that system. Special consideration must be given to the system is protected against breakdown or the structural organization and the relation of strain. Fluid power systems can provide widely the parts of a fluid power system. Strong pipes variable motions in both rotary and straight- and containers must be provided. Leaks must be line transmission of power. Need for control by prevented. This problem is acute with the high hand can be minimized. In addition, fluid power pressure obtained in many fluid power instal- systems are economical to operate. lations. The question may arise as to why hydraulics The pressures required in fluid power sys- is used in some applications and pneumatics in tems must becontrolled and likewise the others. Many factors are considered by theuser movement of the fluid within the lines and and/or the manufacturer when deteraitning which components. This movement causes friction, power source to use in a specific appi!cation. within the fluid itself and against the containing There are no hard and fast rules to tallow; surfaces, which if excessive, can lead to serious however, past experience has provided some losses in efficiency. Foreign matter must not sound ideas that are usually considered when be allowed to accumulate in the system, where making such decisions. If the application re- it will clog small passages or score closely quires speed, a medium amount of pressure, and fitted parts. Chemical action may cause cor- only fairly accurate control, a pneumatic system rosion. Above all, it is necessary to know how may be used. If the application requires only a a fluid power system and its components operate medium amount of pressure and a more accurate both in terms of the general principles common control, a combination of hydraulics and pneu- to all physical mechamisms and the peculiar- matics may be used. If the application requires ities of the particular arrangethent at hand.

4 9 CHAPTER 2 PHYSICS OF FLUIDS

The operation of any fluid power system, phasize the particular aspects of interest. regardlessof its complexity. can be satis- The terms pertaining tothe principlesof factorily explained as an application of phys- hydraulics and pneumatics are discussed in ics.Physics is that branch of science which this chapter. This dilcussion covers the phys- deals with matter and energy.It is devoted ical properties and characteristics of fluids, to finding and defining problems, as well as including the similarities and differences in searching for their solutions. It not only teach- the characteristics of liquids and gases. Also es a person to be curious about the physical included are the outside factors which influ- world, but also provides a means of satis- ence the characteristics of fluids at rest and fying that curiosity.The science of physics in motion, and the laws which govern the ac- is divided into five major areasmechanics, tion of fluids under specific and fixed con- heat, acoustics (sound), optics (light), and elec- ditions. tricity.Further divisions may include such areas as magnetism, radiation, atomic structure, and nuclear phenomena.For the most STATES OF MATTER part, fluids are included in the area of mechanics.In fact, many physics textbooks re- The material substance which makes up fer to fluids as the mechanics of liquids and the universe is known as matter.Matter is the mechanics of gases.It should be point- defined as any substance which occupies space ed out, however, that the study of fluids is and has weight.Examples covered by this not limited to the area of mechanics.For definition are iron, water, and air.Each of example, heat (changes in temperature) has a these occupies space and has weight.In con- definite effect on the physical characteristics trast, heat, light, and electricity are not in- of fluids. cluded because they do not take up space and In order to operate, service, and main- cannot be weighed.They are forms of en- tain fluid power systems, an understanding of ergy which are described later in this chap- the basic principles of fluids at rest and in ter.Although the three examples of matter motion is essential.There can be no question iron, water, and airare all forms of matter, that the mechanic or technician who possess- each one has distinguishing characteristics.' es this understanding is better equipped to They represent the three states of matter meet the demands placed upon him in his every- solids, liquids, and gases.Solids have a definite volume and a definite shape; liquids flay tasks. have a definite volume, but take the shape In the study of the principles of hydrau- of the containing vessel;gases have neither lics and pneumatics, it soon becomes obvious a definite volume nor a definite shape. Gas- that specific words and terms have specific es not only take the shape of the containing meanings which must be mastered 'from the vessel, but they expand and fill the vessel, start.Without an understanding of the exact regardless of the volume of the vessel. meaning of the term, there can be no real Matter can change from one state to an- understanding of the principles involved in the other. ,Water is a good example. At high tem- use of the term.Once the term is correct- perature:3itis in the gaseous state known ly understood, however, many principles may as steam.At moderate temperatures it is be discussed briefly to illustrate or to em- a liquid, and at low temperatures it becomes 5 10 FLUID POWER

ice, which is definitely a solid state.In this and gases.In this discussion, the two sub- .example, the temperature is the dominant fac- stances are treated as fluids; however, the dif- tor in determining the state that the substance ferencesin their properties and character- assumes.Pressure is another important fac- istics .are des:..:-ibed in the appropriate areas. tor that will effect changes in the state of Also included are some of the outside fac- matter. At pressures lower than atmos- tors which affect fluids in different situations. pheric, water will boil and thus change into steam at temperatures lower than 212° Fahr- enheit (F). Pressure is also a critical fac- DENSITY AND SPECIFIC GRAVITY torin changing some gases to liquids or solids.Normally, when pressure and chill- The density of a substance isits weight ing are both applied to a gas,it assumes per unit volume.The unit volume selected a liquid state.Liquid air, which is a mix- for use in the Llnglish system of measure- ture of oxygen and nitrogen,is produced in ment is1 cubic foot.In the metric system this manner. itis the cubic centimeter.Therefore, den- In the study of fluid power, we are con- sityisexpressed in pounds per cubic foot cerned primarily with the properties and char- or in grams per cubic centimeter. acteristicsof liquids and gases.However, To find thedensity of a substance, its it should be kept in mind that the properties weight and volume must be. known.Its weight of solids also affect the characteristics of liq- is then divided by its volume to find the weight uids and .gases.The lines and components, per unit volume. which are solids, enclose and control the liquid or gas in the respective systems. EXAMPLE:The liquid which fills a cer- taincontainer weighs 1,497.6 pounds. The container is 4 feet long, 3 feet wide, and 2 feet deep.Its volume is 24 cubic feet (4 ft FLUIDS x 3 ft x 2 ft).If 24 cubic feet of liquid weighs The word fluid is derived from the Latin 1,497.6 pounds, then 1 cubic foot weighs149.6 "r4. word "fluidus" meaning to flow. A fluid is defined as a substance which tends to flow or or 62.4 pounds.Therefore, the density of the to conform to the outline of its container. liquid is 62.4 pounds per cubic foot. Since this definition applies to both liquids and This is the density of water at 4° Celsius gases, they are commonly referred to as flu- (C) and is usually used as the standard' for ids. However, liquids and gases are dis- comparing densities of other substances. (Cel- tinct states of matter and, therefore, differ sius, formerly known as centigrade, and other in some respects. The characteristics of temperature scales are described later in this liquids and gases may be grouped under sim- chapter.)Using the metric system of meas- ilarities and differences. urement, the density .of mater is1 gram per cubic centimeter.The standard temperature Similar characteristics are listed as follows: of 4° C is used when measuring the density 1.Each has no definite shape but conforms of liquids and solids. Changes in temperature to the shape of its container. willnot changethe weight of a substance, 2. Both readily transmit pressures. but will change the volume of the substance Characteristics which differ are listed as by expansion or contraction, thus changing the follows: weight per unit volume. The procedure for finding density applies 1.Gases fill their containers completely, to all substances;however, it is necessary while liquids may not. to take pressure into consideration when com- 2.Gases are lighter than equal volumes of puting the density of gases.Also, tempera- liquids. tureis more critical when measuring the 3. Gases are highly compressible, while density of gases than it is for other sub- liquids are only slightly so. stances. The density of a gas increases The following discussion concerns the phys- in direct proportion to the pressure exerted ical properties and characteristics of liquids on it.Standard conditions for the measurement Chapter 2PHYSICS OF FLUIDS ;.. ofthedensities of gases Era re been estab-. solid has the same numerical value as its lished at 0° C for temperature and 76 cen- densityr-becaise water weighs 1 gram per timeters of mercury.(This is the average cubiccentimeter. Sinceair weighs 1.293 pressure of the atmosphere at sea level, which grams pei liter, tlp specific gravity (NI gases is approximately 14.7 pounds per square inch.) doee not equal the ziletric densities.) The density of all gases is computed under Specific gravity and density are independ- these conditions. ent of the size of the example under consid- It is often necessary to compare the den- erationand depend only upon the substance sity of one substance with that of another. of which it is made.See table 2-1 for typ- For this purpose a standard is needed.Wa- ter is the standard that physicists have chosen ical values ofspecific gravity for various with which to compare the densities of all substances. solids and liquids.Air is most commonly A device called a hydrometer is used for used as the standard for gases; however, measuring specific gravity of,liquids.This hydrogen is used in some instances. In phys- device consists of a tubular shaped glass floz.:t ics, the word specific implies a ratio. Weight which is contained within a larger glass tube. is the measure of the earth's attraction for (See fig. 2-1.)The larger glass tube provides a body.The earth's attraction for a body the container for the liquid.There is a small iscalledgravity. Thus,the ratio of the opening at one end of the container and the weight of a unit volume of some standard sub- other end is fitted with a rubber suction bulb. stance, measured under the standard pressure This provides a means s.; ailing or partially and temperature conditions, is called specific filling the container with the liquid.There gravity. The terms specific weight and six., must be enough liquid in the container to raise cific density are sometimes used to express the float and prevent it from touching the bot- this ratio. tom. The float is weighted and has a ver- The following formulas are used to find ticallygraduatedscale. To determine the the specificgravity(sp gr)of solids and specific gravity, the scale is read at the sur- liquids. face of the liquid in whichthe float is immersed. Weight of the substance An indication of 1000 is read when the float sp gr -Weight of an equal volume of water is immersed in pure wat.'.v. immersed or, in a liquid of greater ,it-t3i.y the float rises, Density of the substance indicating a greater specific gravity. For liq- sp Density of water uids of lesser density than water, the float The same formulas are used to find the sinks, indicating a lower specific gravity. specific gravity of gases by substituting air An example of the use of the hydrometer or hydrogen for water. is to determine the specific gravity of the elec- The specific gravity of water is 1 trolyte (battery liquid) in an automobile battery. (62:41 When the battery is discharged, the calibrated: If a cubic foot of a certain liquid weighs 68.64, float immersed in the electrolyte will indicate then its specific gravity is 1.1(::::4)Thus, 1150.The indication of a fully charged battery the specific gravity of the liquid is the ratio is 1270. of its density to the density of water.If the specific gravity of a liquid or solid is known, the density of the liquid or solid may be ob- BUOYANCY tained by multiplying its specific gravity by the density of water.For example, if a cer- A body submerged in a liquid or a gas tain hydraulic liquid has a specific gravity weighs less than when weighed in free space. of 0.8,1 cubic foot of the liquid weighs 0.8 This is due to the upward force that the fluid times as much as a cubic foot of water- exertson thesubmerged body. An object 62.4 times 0.8 or 49.92 pounds. In the metric willfloatif this upward force of the fluid system, 1cubic centimeter of a substance is greater than the weight of the object. Ob- with a specific gravity of 0.8 weighs 1 times jects denser than the fluid, even though they 0.8 or 0.8 grams. (Note that in tfie metric sink readily, appear to lose a part of their system thesixficgravityof a liquid or weight when submerged.An individual can 7

12 FLUID POWER

Table 2-1.Typical values of specific gravity Gases sp Liquids sp Solids (Air standard at 0°C sp Br (Room temperatures) gr and 76.0 centimeters gr of mercury)

Aluminum 2.7 Alcohol, ethyl 0.789 Air 1.000 Bronze 8.8 Gasoline 0.68- Hydrogen 0.069 0.72 Copper 8.9 011 (paraffin) 0.8 Nitrogen 0.967

Ice 0.917 Water 1.00 Oicygen 1.105

lift a larger weight under water than he can TEMPERATURE possibly lift in the atmosphere.The upward force which anyfluid exerts upon a body As indicated previously, temperature is a placed in it is called buoyant force. dominant factor affecting the physical proper- The following experiment is illustrated in tiesof fluids.It is of particular concern figure2-2. The overflow can is filled up when calculating changes in the state of gases. to the spout with water.The heavy metal The three temperature scales used exten cylinder is first weighed in still air and then sively are the Celsius(C), the Fahrenheit is weighed while completely submerged in the (F),and the absolute or Kelvin (K) scales. water.The difference between the two weights The Celsius scale (the centigrade scale has is thebuoyant force of the water. As the been renamed the Celsius scale in recognition cylinderis lowered into the overflow can, of Anders Celsius, the Swedish astronomer the water is caught in the catch bucket. The who devised the scale) is constructed by using volume of water which overflows equals the the freezing point and boiling points of water, volume of the cylinder.(The volume of ir- under standard conditions, as fixed points of regular shaped objects may be measured by zero and 100,respectively, with 100 equal this method.)If this experiment is performed divisions between. The Fahrenheit scale uses carefully, the weight of the water displaced 32° as the freezing point of water and 212° by the metal cylinder exactly equals the buoy- as the boiling point, and has 180 equal di- ant force of the water. visions between. The absolute or Kelvin scale Experiments similar to this were performed is constructed with its zero point established by Archimedes (287-212 B.C.).As a result as 273° C, or 459.4° F.The relations of the of his experiments he discovered that the buoy- other fixed points of the scale are shown in ant force which a fluid exerts upon a sub- figure 2-3. merged body is equal to the weight of the Absolute zero, one of the fundamental con- fluid the body displaces.This statement is stants of physics, is commonly used in the referred to as Archimedes' principle.This study of gases.It is usually expressed in principle applies to all fluids, gases as well terms of the Celsius scale.If the heat en- as liquids.Just as water exerts a buoyant ergy of a given gas sample could be pro- force on submerged objects, air exerts a buoy- gressively reduced, some temperature should ant force on objects submerged in it. bereachedatwhich themotionofthe 8 3 I

Chapter 2PHYSICS OF FLUIDS

OVERFLOW CAN

CATCH BUCKET

k. 1150 DISCHARGED 1270 CHARGED

FP.2 Figure 2-2.Measurement of buoyant force.

molecules would cease entirely.If accurately determined, this temperature could then be taken as a natural reference, or as a true "absolute zero" value. Experiments with hydrogen indicated that if a gas were cooled to -273.16° C (used as -273°for most calculations), all molecular motion would cease and no additional heat could be extracted from the substance. Since this is the coldest temperature to which an ideal gas can be cooled, it is considered as absolute zero.When temperatures are measured with respect to the absolute zero reference, they are expressed as zero on the absolute or Kelvin scale.Thus, absolute zero may be expressed as 0° K, as -273° C, or as -459.4° F (-460° F for most calculations). Personnel working with temperatures must always make sure which system of measurement is being used and how to convert from one to another.The conversion formulas are FP.1 shown in figure 2-3. For purposes of cal- Figure 2-1.A hydrometer. culations, the Rankine scale illustrated in figure 2-4 is commonly used to convert Fahr- enheit to absolute.For Fahrenheit readings above zero,460°isadded. Thus, 72° F 9 14 FLUID POWER 1

KELVIN pp CELSIUS FAHRENHEIT TEMPERATURE ABSOLUTE CONVERSION 373°-C- 100° KELVIN TO CELSIUS AND FAHRENHEIT °K: C+273 si(F-32)+ 273

° 19 F+255 23

CELSIUS TO KELVIN 273° o° AND FAHRENHEIT

°C= K-273 =1(F-32)

FAHRENHEIT TO KELVIN AND CELSIUS

°F=.4(K-273)+ 32 00 -273° =4K -459.4459.4 _9 Ct -C+325

FP.3 Figure 2-3.Comparison of Fahrenheit, Celsius, and Kelvin temperature. equals 460° plus 72° or 532° absolute.If conversions may be used for the calculations the Fahrenheit reading is below zero, it is of changes in the state of gases. substracted from 460°.Thus, -40° F equals The Kelvin and Celsius scales are used 460° minus 40° or 420° absolute. R phould more extensively in scientific work and, there- be pointed out that the Rankine scale does fore, some technical manuals may use these not indicate absolute temperature readings in scales in giving directions and operating in- accordance with the Kelvin scale, but these structions.The Fahrenheit scale is commonly 10 Chapter 2PHYSICS OF FLUIDS

FAHRENHEIT ABSOLUTE FAHRENHEIT (RANKINE)

672° - 212° 640° - - 180° FP.5 Figure 2-5.Exertion of pressure.

540° 80° Gas exerts pressure on all sides because it completely fills the container. Atmospheric Pressure 460° 0° The atmosphere is the whole mass of air surrounding the earth. While it extends upward 410°------for about 500 miles, the section of primary interest is that portion of the air which rests on theearth'ssurface and extends upward for about71/2 miles.This layer is called the troposphere. The higher one ascends in the troposphere, the lower the pressure. This is because air has weight.If a- column of air 1-inch square extending all the way to the. FP.4 "top"of the atmosphere could be weighed, Figure 2-4.Fahrenheit and absolute this column of air would weigh approximately temperature compared (Rankine scale). 14.7 pounds at sea level.Thus, atmospheric pressure at sea level is approximately 14.7 pounds per square inch (psi).(See fig. 2-6.) used in the United States and most people are familiar with it.Therefore, the Fahren- heit scale is used in most areas of this manual.

PRESSURE 16,400 FT ABOVE SEA LEVEL PRESSURE = 7.7 PSIA The term pressure, as used throughout this manual, is defined as a force per unit volume. It is further defined in relation to other factors later in this chapter. Pressure is usually measured in pounds per square inch (psi). SEA LEVEL Sometimes pressure is measured in inches PRESSURE=14.7 PS111 of mercury, or for very low pressure, inches of water. Pressure m_sit be exerted in one direction, several directions, or in all directions.(See fig. 2-5.)The ice (a solid) exerts pressure : downward only.Water (a liquid) exerts press sure on all surfaces with which it comes in FP.6 contact. Gas exerts pressure in all directions. Figure 2-6.Atmospheric pressure. 11

163 FLUID POWER

As one ascends, the atmospheric pressure decreases approximately 1.0 psi for every 2,343 feet.However, below sea level, in excavations and depressions, atmospheric presZure inc reas- es.Pressures under water differ from those under air only because the weight of the water must be added to the pressure of the Atmospheric pressure can be measured by any of several methods. The common labora- 29.92 INCHES tory method employs the mercury column barometer. A mercury column consists of a glass tube approximately 34 inches in length, sealed at one end, then completely filled with mercury, and inverted in an open container partially filled with mercury. (See fig. 2-7.) The mercury in the tube settles down, leaving an evacuated space in the upper end of the tube.The height of the mercury column serves as an indicator of atmospheric pressure.At sea level and at a temperature of 0° C, the height of the mercury column is ATMOSPHERIC approximately 30 inches or 76 centimeters. PRESSURE ACT This represents a pressure of approximately ING ON SURFACE 14.7 psi. The 30-inch column is used as OF MERCURY a reference standard. At higher levels,, the atmospheric pressure on the surface of the mercury in the open container is less than at sea level;hence, the column of mercury in the tube settles lower. These variations in the height of the mercury column represent changes in the atmospheric pressure which May be calibrated in terms of altitude with reference to sea level. Another device used to measure atmospheric pressure is the aneroid barometer. (See fig. 2-8.)The term "aneroid" means without fluid.The aneroid barometer, then, is a fluid- FP.7 less barometer, utilizing the change in shape Figure 2-7.Measurenient of atmospheric of an evacuated metal cell to measure var- pressuremercury barometer. iations in atmospheric pressure. The aneroid barometer gets its name from the pressure-sensitive element used in the in- rapidly at lower altitudes because of the com- strument.An aneroid is a thin-walled metal pressibility of the air, which causes the air capsule or cell, sometimes called a diaphragm, layers close to the earth's surface to be com- that has been either partially or completely pressed by the air masses above them. This evacuated of air.This thin metal moves in effect, however, is partially counteracted by or out with the variation of pressure on its contraction of the upper layers due to cooling. external surface.This movement is trans- The cooling tends to increase the density of mitted through a system of levers to a pointer, the air. which indicates the pressure. Atmospheric pressures are quite large, but in most instances practically the same pressure The atmospheric pressure does not vary ispresent on all sides of objects so that uniformlywithaltitude. It changes more no single surface is subjected to a great load. 12 Chapter 2PHYSICS OF FLUIDS

INCOMPRESSIBILITY AND POINTER COUNTERWEIGHT EXPANSION OF LIQUIDS

Liquids can be only slightly compressed; that is, the reduction of the volume which they occupy, even under extreme pressure, is very small.If a pressure of 100 psi is applied to ADJUSTING body of water, the volume will decrease only SPRING 3/10,000 of its original volume.It would take a force of 32 tons to reduce its volume 10 percent.When this force is removed, the water immediately returns to its original volume. Since other liquids behave in about the same manner as water, liquids are usually considered incompressible. NOTE:In some applications of hydraulics SECTOR GEAR where extremely close tolerances are required, the compressibility of liquids must be considered in the design of the system. In this manual, however, liquids are considered to be in- ANEROID CELL compressible. Almost all forms of matter expand when heated. this action is normally referred to as thermal expansion. The amount of expansion varies with the substance.In general, liquids FP.8 expand much more than solids. For example, Figure 2-8.Simple diagram of the when a liter (1,000 cubic centimeters) of water aneroid barometer. isheated from 0° to 100° C,it increases approximately 43 cubic centimeters in volume; whereas a block of steel of the same volume would expand only 3 cubic centimeters.All The effects of atmospheric pressure on fluids liquids do not expand the same amount for a is covered later in this chapter. certain increase of temperature. If two flasks are placed in a heated vessel, and if one of these Absolute Pressure flasks is filled with water and the other with alcohol, it will be found that the alcohol expands much more than the water for the same rise in As stated previously, absolute temperature temperature. Most oils expand more than water. is used in the calculation of changes in the Hydraulic systems contain provisions for com- state of gas.It is also necessary to use ab- pensating for this increase of volume in order solute pressure for these and other calculations. to prevent breakage of the equipment. Absolute pressure is measured from absolute zero pressure rather than from normal COMPRESSIBILITY AND or atmospheric pressure (approximately 14.7 EXPANSION OF GASES psi).Gage pressure is used on all ordinary gages, and indicates pressure in excess of As mentioned previously, two of the major atmospheric.Therefore, absolute pressure is differences between liquids and gases are in equal to atmospheric pressure plus gage pres- respect to compressibility and expansion. While sure.For example, 100 psi gage pressure liquids are practically incompressible, gases (psigmeaning pounds per square inch gage) are highly compressible.Gases tend to com- equals 100 psi plus 14.7 psi or 114.7 psi ab- pletely fill any container, while liquids fill a solute pressure(psiameaning pounds per. container only to the extent of their normal square inch absolute). volume. FLUID POWER F. Although both liquids and gases expand when of these varying velocities, corresponds to the heated, gases expand much more than liquids temperature of the gas.In any considerable (approximately nine times as much as water). amount of gas, there are so many molecules t Unlike liquids, all gases expand approximately present that in accordance with the "laws of 1 the same.Because of these characteristics, probability" some average velocity can be there are Several laws concerning the com- found which, if it were possessed by every pressibility and expansion of gases. These molecule in the gas, would prodw: e the same laws are discussed in the following paragraphs. effectat a given temperature as the total of the many varying velocities. Kinetic Theory of Gases Boyle's Law The simple structure of gases make them readily adaptable to mathematical analysis from As previously stated compressibility is an which has evolved a detailed theory of gases. outstanding characteristic of gases. The Eng- This iscalled the kinetic theory of gases. lish scientist Robert Boyle was among the first The theory assumes that a body of gas is to study this characteristic, which he called composed of identical molecules (see glossary) the "springiness of air."By direct meas- which behave like minute elastic spheres, spaced urement, he discovered that when the temper- relatively far apart and continuously in motion. ature of an enclosed sample of gas was kept The degree of molecular motion is dependent constant and the pressure doubled, the volume upon the temperature of the gas.Since the was reduced to half the former value; as the molecules are continuously striking against each applied pressure was decreased, the resulting other and against the walls of the container, volume increased.From these observations, an increase i't temperature with the resulting he concluded that for a constant temperature increase in molecular motion causes a cor- the product of the volume and pressure of an responding increase in the number of collisions enclosed gas remains constant.This became between the molecules. The increased number Boyle's law, which is normally stated:"The of collisions results in an increase in pressure, volume of an enclosed dry gas varies inversely because a greater number of molecules strike with its pressure, provided the temperature against the walls of the container in a given remains constant." unit of time. This law can be demonstrated by confining If the container were an open vessel, the a quantity of gas in a cylinder which has a gas would tend to expand and overflow from tightly fitted piston. A force is then applied the container.However, if the container is to the piston so as to compress the gas. in sealed and possesses elasticity (such as a rub- the cylinder to some specific volume. When ber balloon), the increased pressure causes the force applied to the piston is doubled, the container to expand. the gas is compressed to one-half its original volume, as indicated in figure 2-9. For example, when making a long drive on a hot day, the pressure in the tires of an au- In equation form, this relationship may be tomobile increases and a tire which appeared expressed either to be somewhat "soft" in cool morning tem- VP1 = VP2 perature may appear normal at a higher midday 1 2 temperature. Or V= P Such phenomena as these have been ex- 1 2 plained and set forth in the form of laws pertaining to gases and tend to support the kinetic 2 theory. when Vi and Pi are the original volume and At any given instant, some molecules of a pressure, and V2 and P2 are the revised volume gas are moving in one direction, some are and pressure. moving in another direction; some are traveling Example of Boyle's law:4 cubic feet of fast while some are traveling slowly; some may nitrogen are under a pressure of 100 psi even be in a state of rest. The combined effect (gage). The nitrogen is allowed to expand 14 19 Chapter 2PHYSICS OF FLUIDS

Charles' Law The French scientist Jacques Charles provided much of the foundation for the modern kinetictheory of gases.He found that all gases expand and contract in direct proportion to thechangein the absolute temperature, provided the pressure is held constant.Ex- pressed in equation form this part of the law may be expressed VT2 = VT1 1 2 ' or V1T1 17- 2 2. where Vi and V2 refer to the original and finalvolumes,and Ti and T2 indicate the FP.9 corresponding absolute temperatures. Figure 2-9.Gas compressed to half its Since any change in temperature of a gas original volume by a double force. causes a corresponding change in volume, it is reasonable to expect that if a given sample of a gas were heated while confined within to a volume of 6 cubic feet.What is the new a given volume, the pressure should increase. gage pressure?Remember to convert gage By actual experiment, it was found that for pressure to absolute pressure by adding 14.7. each 1° C increase in temperature the increase in pressure was approximately 1/273 of the pressure at 0° C.Because of this fact, it Formula or equation: is normal practice to state this relationship Vivi = VP2 in terms of absolute temperature. In equation 2 form, this part of the law becomes Substituting: PT2 = PT1 4 x (100 + 14. 7) = 6 x P2 1 2 ' or n 4 x 114. 7 P1T1 1r P2 = 78.47 psi absolute 2 In words, this equation states that with a constant volume, the absolute pressure of a gas Converting absolute pressure to gage pres- varies directly with the absolute temperature. sure: Examples of Charles' law: A cylinder of 76.47 gas under a pressure of 1,800 psig at 70° F -14.7 is left out in the sun in the tropics and heats 11777 psi gage pressureAnswer. up to a temperature of 130° F.What is the new pressure within the cylinder? The pressure and temperature must be converted to absolute Changes in the pressure of a gas also affects pressure and temperature. the density.As the pressure increases, its Formula or equation: volume decreases; however, there is no change in the weight of the gas. Therefore, the weV:ht per Ina volume (density) increases.So it follows that the density of a gas varies directly as the pressure, if the temperature is constant.

15 20,u: FLUID POWER

Using the Rankine system: By combining Boyle's law and Charles' law, a single expression can be derived which in- 70° F = 530° absolute cludes all the information contained in both. 130° F = 590° absolute It is referred to as the GENERAL GAS LAW. It states that the product of the initial pres- Substituting: sure,initial volume, and new temperature (absolute scale) of an enclosed gas is equal 1,800 + 14.7 530 to the product of the new pressure, new vol- 2 ume, and initial temperature.It is a mathematical statement whereby many gas problems can be solved involving the principles of Boyle's Then: P2 (590)(1, 814. 7) law and/or Charles' law.The equation may 530 be expressed as P2 = 2, 020 psia PVT2 = PVT 11 221 or Converting absolute pressure to gage pres- P1V1 PV2 sure: -7= T22 2, 020.0 1 -14.7 NOTE: The capital P and T signify ab- 2, 005.3 psig-Answer. solute pressure and temperature, respectively. Itcan be seen by examination of figure General Gas Law 2-10 that the three equations are special cases of the general equation.Thus, if the temper- The facts concerning gases discussed in ature remains constant, T1 equals T2 and both the preceding paragraphs are summed up and can be eliminated from the general formula, illustrated in figure 2-10.Boyle's law is ex- which then reduces to the form shown in (A). pressed in (A) of the figure, while the effects When the volume remains constant, Vi equals of temperature changes on pressure and vol- V2, thereby reducing the general equation to ume (Charles'law)are illustrated in (B) the form given in (B). Similarly, P1 is equated and (C), respectively. to P2 for constant pressure, and the equation then takes the form given in (C). The general gas law applies with exactness only to "ideal" gases in which the molecules are assumed to be perfectly elastic. However, it describes the behavior of actual gases with sufficient accuracy for most practical purposes. Two examples of the general equation follow: 1. Two cubic feet of a gas at 75 psig and 80° F are compressed to a volume of 1 cubic TEMPERATURE CONSTANT foot and then heated to a temperature of 300° F. What is the new gage pressure? vPi vi . - Formula or equation: P1V1 PV2 2 1 2 (AI Using the Rankine system: FP.10 80° F = 540° absolute Figure 2-10.The general gas law. 300° F = 780° absolute 16 Chapter 2-PHYSICS OF FLUIDS

Substituting: pressure, equal volumes of differentgases contain equal numbers of molecules."This (75 + 14.7)(2) P2 (1) theory was proven by experiment and found 540 760 to agree with the kinetic theory, so it has come to be known as Avogadro's Law. Then: Dalton's Law 179.4P2 = If a mixture of two or more gases which 540 760 do not combine chemically is placed in a con- P2(179. 4)(760) tainer, each gas expands throughout the total 540 space and the absolute pressure of each gas P2 = 252. 5 psia is reduced to a lower value, called a partial pressure. This reduction isin accordance with Boyle's law. The pressure of the mixture Converting absolute pressure to gage pres- of these two gases is equal to the sum of the sure: partial pressures.This fact was discovered by Daltcn,an English physicist, and is set 252.5 forth as Dalton's law.Tb.' law states that -14.7 "a mixture of several gat Is which do not 157-1 psig- Answer. react chemically exerts a-essure equal to the sum of the pressures wi ich the several 2.Four cubic feet of a gas at 75 psig and gases would exert separately if each were 80° F are compressed to 237.8 psig and heated allowed to occupy the entire space alone at to a temperature of 300° F.What is the vol- the given temperature." ume of the gas resulting from these changes? Formula or equation: TRANSMISSION OF FORCES P1V1 V2 THROUGH FLUIDS P2 = 1 2 When the end of a solid bar is struck, the main forceof the blow is carried straight Using the Rankine system: through the bar to the other end.(See fig. 2-11 (A).)This happens because the bar is 80° F = 540° absolute rigid.The direction of the blow almost en- 300° F = 760° absolute tirely determines the direction of the transmitted force.The more rigid the bar the Substituting: less force is lost inside the bar ortransmitted outward at right angles to the direction of the (75 + 14.7)(4) (237.8 + 14.7) V2 blow. 540 760 When a force is applied to the end of a column of confined liquid (fig. 2-11 (B)), it is Then: transmitted straight through to the other end and also equally and undiminished in every lytp_ (252. 5)V2 direction throughout the column-forward, back- 780 ward, and sideways-so that the containing vessel is literally filled with pressure. V2358.8 x 760 540 x 252. 6 If a gas is used instead of a liquid, the force is transmitted in the same manner. V2= - 2 cubic feet-Answer. The one difference is that gas, being highly Avogadro's Law compressible, provides a much less rigid force than the liquid, which is practically incompress- An Italian physicist, Avogadro, conceived the ible.(This is the main difference in the action theory that "at the same temperature and of liquids and gases in fluid power systems.) 17 FLUID POWER

FP.12 Figure 2-12.Distribution of force.

liquid resting on (A) pushes equally downward and outward.The liquid on every square inch (A) (8) of the bottom surface is pushing downward and outward in the same way, so that the pres- FP.11 sures on different areas are in balance. Figure 2-11.Transmission of force: At the edge of the bottom the pressures (A) Solid; (B) fluid. act against the walls of the container, which must be strong enough to resist them with a force exactly equal to the force of the An example of this distribution of force liquid.Every square inch of the bottom of is illustrated in figure 2-12.The flat hose the container must also be strong enough to takes on a circular cross section when it is resist the downward pressure of the liquid filled with water under pressure. The outward resting on it.The same balance of pressures push of the water is equal in every direction. The automobile tire and the toy balloonare examples of this distribution of force through the use of gases.

PASCAL'S LAW As related in chapter 1, the foundations of modern hydraulics, and pneumatics, were established in 1853 when Pascal discovered thatpres- sure set up in a fluid acts equally in all directions. This pressure acts at right angles to the containing surfaces.Thus in figure 2-19, A if the liquid standing on a square inch (A) at the bottom of the container weighs 8 pounds 8PSIG (disregarding the atmospheric pressure acting on the surface of the a pressure of 8 FP.13 prig is exerted in every direction at (A). The Figure 2-13.Pressure acting on a container. 18

23 Chapter 2PHYSICS OF FLUIDS exists at every other level in the container, ple,weighs62.4 pounds per cubic foot or though of lesser pressures as one approaches 0.036 pound per cubic inch, while certain oil the surface.Therefore, the liquid remains might weigh 55 pounds per cubic foot, or at restit does not leak out and the container 0.032 pound per cubic inch.It would take does not collapse.The pressure of the liquid 222 inches of head, using water to produce decreases as one approaches the surface be- a pressure of 8 psi and 252 inches using oil. cause the volume of liquid and, therefore, the (See fig. 2-15.) weight above each square inch decreases. This This fluid head, which is sometimes re- is similar to the decrease in atmospheric pres- ferred to as gravity head or altitude head, sure with increase in altitude, as discussed also applies to gases. As discussed previously, previously. atmospheric pressure at any given altitude is One of the consequences of Pascal's law the result of the weight of the air above that is that the shape of the container in no way altitude.In this case, however, several miles alters pressure relations. Thus in figure 2-14, of vertical height are required to produce ap- if the pressure due to the weight of the liquid proximately 14.7 psi at sea level. Therefore, at one point on the horizontal line (H) is 8 when considering a cubic footof gas, the psi, the pressure is 8 psi everywhere at level gravity head is negligible.For example, a (H) in the system. cubic foot of compressed air (100 psi) at 70° Pressure due to the weight of a fluid de- F produces a gravity head which is less than pends, at any level, upon the vertical height 1percent of that produced by a cubic foot of the fluid from the level to the surface of of water. the fluid.The vertical distance between two horizontal levels in a fluid is known as the head of the fluid.In figure 2-14, the liquid FORCE AND PRESSURE head of all points on the level (H) with respect to the surface is indicated. The terms force and pressure are used Pressure due to fluid head also depends frequently in the preceding paragraphs. In upon the density of the fluid. Water, for exam- order to understand how Pascal's law is applied

FP.14 Figure 2-14.Pressure relationship with shape. 19 FLUID POWER F=PxA Pressure equals force divided by the area. By rearranging the formula, this statement may be condensed into

P =A Since area equals force divided by pressure, the formula is written

A =P WATER OIL Figure 2-16 illustrates a device for re- calling the different variations of this formula. FP.15 Any letter in the triangle may be expressed Figure 2-15.Pressure and density as the product or quotient of the other two, relationship. depending upon its position within the triangle. For example, to find area, consider the letter A as being set off to itself, followed to fluid power, a distinction must be made by an equal sign.Now look at the other two between these terms.Force may be defined letters.The setter F is above the letter P; as a push or pull. It is the push or pull therefore, exerted against the total area of a particular A = surface and is expressed in pounds.As previously stated, pressure is the amount of force In order to find pressure, consider the let- on a unit area of the surface acted upon. ter P as being set off to itself, and look at In hydraulics and pneumatics, this unit area the other two letters. The letter F is above is expressed in pounds per square inch. Thus the letter A; therefore, pressure is the amount of force acting upcn 1 square inch of area. P=1- Likewise, to find force, consider the letter Computing Force, F as being set off to itself. The letters P and Pressure, and Area A areside by side; therefore, F = P x A. A formula, similar to those used in con- NOTE: Sometimes the area may not be junction with the gas laws, is titled in com- expressed in square inches.If it is a rec- puting force, pressure, and area in fluid power tangular surface, the area may be found by systems.Although there appears to be three multiplying the length (in inches) by the width formulas, there is only one formula, which (in inches).The majority of areas to be con- may be written in three variations.In this sidered in these calculations are circular in formula, P refers to pressure, F indicates shape.Either the radius or the diameter may force, and A represents area. be given.The radius in inches must be known to find the area.The radius is one-half the Force equals pressure times area.Thus, diameter.Then, the formula for finding the the formula is written area of a circle is used.This is written

20 2. Chapter 2PHYSICS OF FLUIDS

is .a resistance on the output piston (2) and the input piston is pushed downward, a pressure is created through the fluid, which acts equally at right angles to surfaces in all parts of the container. Referring to figure 2-17,if the force (1) is 110 pounds and the area of the input piston (1)is10 square inches, then the pressure in the fluid is 10 psi(). (NOTE: It must be emphasized that this fluid pressure cannot be created without resistance to flow, FP.16 which, in this case, is provided by the 100 Figure 2-16.Device for determining the pound force acting against the top of the out- arrangement of the force, pressure, put piston (2).)This pressure acts on piston and area formula. (2), so that for each square inch of its area it is pushed upward with a force of 10 pounds. In this case, a fluid column of uniform cross A = wr2, where, A is the area, Tr is 3.1416 section is considered so that the area of the (3.14 or 31/7 for most calculations), and output piston (2)is the same as the input r2 indicates the radius squared. piston (1), or 10 square inches.Therefore, the upward force on the output piston (2) is 100 pounds, the same as was applied to the PRESSURE AND FORCE IN input piston (1). All that has been accomplished FLUID POWER SYSTEMS in this system was to transmit the 100-pound force around a bend.However, this principle In accordance with Pascal's law, any force underlies practically all mechanical applications applied to a confined fluid is transmitted in of fluid power. all directions throughout the fluid regardless At this point it should be noted that since of the shape of the container.Consider the Pascal's law is independent of the shape of effect of this in the system shown in figure the container, it is not necessary that the tube 2-17. This is in reality a modification of connecting the two pistons should be the full figure 2-11 (B) in which the column of fluid area of the pistons. A connection of any size, is curved back upward to its original level, shape, or length will do, so long as an un- with a second piston at this point.If there obstructed passage is provided.Therefore,

FORCE 1 100 LBS. FORCE 2 100 LBS.

INPUT OUTPUT PISTON(1) P1STONL2 10 SQ. IN. 10 SQ. IN.

PRESSURE 10 U15. PER SQ. INCH

FP.17 Figure 2-17.Force transmitted through fluid. 21 26..` FLUID POWER

FORCE 1 - 100 LBS. FORCE 2 100 LBS.

PISTON 1 PISTON 2 10 SQ. IN, 10 SQ. IN

PRESSURE 10 LBS. PER SQ. INCH

FP.18 Figure 2-18.Transmitting force through small pipe. the system shown in figure 2-18, wherein a the output force is equal to the input force. relatively small, bent pipe connects two cyl- Consider the situation in figure 2-19, where inders, will act exactly the same as that shown theinputpiston is',much smaller than the in figure 2-17. output piston.Assume that the area of the input piston(1)is 2 square inches. With Multiplication of Forces a resistant force on piston (2), a downward force of 20 pounds acting on piston (1) creates Infigures2-17 and 2-18 the systems 20) contain pistons ofequalareawherein 10 psi in the fluid.Although this force

II FORCE1 FORCE 2 I I20 LBS. 200 LBS. INPUT OUTPUT "L PISTON (11 PISTON (N.2 ) 2 SQ. IN, 20 SQ. I

s`'

.! 4:/PN ( ,r , PRESSURE --- 10 LBS. RV SQ. INCH

41-

FP.19 Figure 2-19.Multiplication of forces. 22 Chapter 2PHYSICS OF FLUIDS is much smaller than the applied force in fig- the force pushing the piston to the left is its ures 2-17 and 2-18, the pressure is the same. area times the pressure, or 80 pounds (20 x This is because the force is concentrated on 4).Therefore, there is a net unbalanced force a relatively small area. of 40 pounds acting to the right, and the.piston will move in that direction. The net effect This pressure of 10 psi acts on all parts is the same as if the piston and cylinder were of the fluid container, including the bottom of just the size of the piston rod, since all other the output piston (2).The upward force on forces are in balance. the output piston (2) is therefore 10 pounds for each of its 20 square inches of area, or 200 pounds (10 x 20). In this case, the original Volume and Distance Factors force has been multiplied tenfold while using the same pressureinthe fluid as before. In the systems illustrated in figures 2-17 In any system with these dimensions, the ratio and 2-18, the pistons have areas of 10 square of output force to input force is always ten inches.Since the areas of the input and out- to one, regardless of the applied force.For put pistons are equal, a force of 100 pounds example, if the applied force of the input piston on the input piston will support a resistant (1) is 50 pounds, the pressure in the system force of 100 pounds on the output piston.At is increased to 25 psi.This will support this point the pressure of the fluid is 10 psi. a resistant force of 500 pounds on the out- A slight force, in excess of 100 pounds, on put piston (2), the input piston will increase the pressure of the fluid, which will, in turn, overcome the The system works the same in reverse. resistance force.Assume that the input piston Consider piston (2) .as the input and piston is forced downward 1 inch. This displaces (1) as the output.Then the output force will 10 cubic inches of fluid.Since liquid is prac- always be one-tenth the input force. Sometimes tically incompressible, this volume must go such results are desired. some place.In the case of a gas, it will com- Therefore, if two pistons are used in a fluid lress momentarily, but will eventually expand power system, the force acting on each is di- Itoitsoriginal volume, at 10 psi.This is rectly proportional to its area, and the mag- provided, of course, that the 100 pounds of nitude of each force is the product of the pres- force is still acting on the input piston. Thus, sure and its area. this volume of fluid moves the output piston. Since the area of the output piston is likewise 10 square inches,it moves 1 inch upward Differential Areas in order to accommodate the 10 cubic inches offluid. The pistons are of equal areas, Consider thespecial situation shown in and will therefore move equal distances, though figure 2-20.Here, a single piston (1) in a cylinder (2) has a piston rod (3) attached to in opposite directions. one side of the piston. The piston rod extends Applying this reasoning to the system in out of one end of the cylinder.Fluid under figure 2-19, it is obvious that if the input pis- pressure is admitted to both ends of the cyl- ton (1) is pushed down 1 inch, only 2 cubic inder equally through the pipes (4, 5, and 6). inches of. fluid is displaced. In order to accom- The opposed faces of the piston (1) behave modate these 2 cubic inches of fluid the output like two pistons acting against each other. The piston (2) will have to move only one-tenth of area of one face is the full cross-sectional an inch, because its area is 10 times that area of the cylinder, say 6 square inches, of the input piston (1).This leads to the sec- while, the area of the other face is the area ond basic rule for two pistons in the same of the .cylinder minus the area of the piston fluid power system, which is that the distances rod, which is 2 square inches.This leaves moved are inversely proportional to their areas. an effective area of 4 square inches on the right face of the piston. The pressure on both Effects of Atmospheric Pressure faces is the same, in this case, 20 psi. Ap- plying the rule just stated, the force pushing Atmospheric pressure, described previous- the piston to the right is its area times the ly, obeys Pascal's law the same as pressure pressure, or 120 pounds (20 x 6).Likewise, set up in fluids.As illustrated in figure 2-13, FLUID POWER

FP.20 1.- Piston.2. Cylinder.3. Piston rod.4. Pipe.5. Pipe.6. Pipe Figure 2-20.Differential areas ona piston. pressures, due to liquid head, are distributed difference, since for a unit of area,pressures equally in all directions.This is also true are in balance. of atmospheric pressures. The situation is the same if these' pressures acton opposite Vacuum and Partial Vacuum sides of any surfaces, or through fluids.In figure 2-21 (A) the suspended sheet ofpaper When an individual drinks soda througha isnot torn by atmosphericpressurei as it straw, he removes some of the air from the would be by an unbalanced force of 14.7 psi, straw.This disturbs the balance ofpressures because atmospheric pressure acts equally which was prevailing between the liquid in on both sides of the paper. the glass and the liquid in th.:: straw. This In figure 2-21 (B), atmosphericpressure results in unbalanced pressures, and atmos- acting on the surface of the liquid is trans- pheric pressure on the liquid in the glass mitted equally throughout the liquid to the walls pushes the soda up into the straw untila new of the container, but is balanced by thesame balance is reached.The soda can be held pressure acting directly on the outer walls at a certain level in the straw.This level of the container.In view (C) of figure 2-21, will always be where the pressure of the head atmospheric pressure acting on the surface of liquid exactly equals the difference between of one piston is balanced by thesame pressure the pressure in the straw and that on thesur- acting on the surface of the other.The dif- face of the liquid in the glass. (See fig. 2-22.) ferent areas of the two surfaces makeno When the straw is removed from the person's 24 29 Chapter 2PHYSICS OF FLUIDS

( A ) (B) 14.7 LBS. PER SQ. INCH

14.7 LBS. 14.7 LBS.

PER SQ. INC PER SQ. INCH

14.7 (C) rie14.7 LBS. PER SQ. INCH

11111111

FP.21 Figure 2-21.Effects of atmospheric pressure. mouth,the soda in the straw is subject to compressor moves the fluid into the system, the same pressure as that on the surface of a low pressure area (partial vacuum) is de- the liquid in the glass.This causes the liq- veloped at the inlet port.This allows atmos- uid in the straw to return to its original level, pheric pressure to push the fluid into the inlet which is the same as that in the glass. port of the pump or compressor. This action A partial vacuum is produced in the straw is discussed in greater detail in chapters 4 that is, a pressure less than the prevailing and 8. atmospheric pressure.The theoretical limit of this process would be a condition of zero pressurea complete vacuum. In actual prac- INPUT AND OUTPUT RELATIONS tice, however, it is impossible to produce a complete vacuum. As illustrated in figure 2-19, an increase This action takes place in the power supply in output force is accompanied by a decrease forfluid power systems.As the pump or . in the distance traveled in exactly the same 25 FLUID POWER

all interchangeable with each other and with work. Some of the many forms which energy can takeand theirinterchangeability are illustrated by a hydroelectricplant. (See fig.2-23.) Here, a body of water is held back by a dam.In this case the water represents potential energy, because it is not doing work at the moment, but is capable of doing work if it is released.If an opening is provided, water will rush out in a high velocity jet representing energy of motion or kinetic energy. If this jet is directed against the blades of a water wheel it will push them a- round, producing a continuous rotary motion. This is work in its true sense because a force is moving through a distance. The water wheel can, in turn, be connected to an electric generator which converts the work into electricity.This electricity can, in turn, be converted back into work by the FP.22 use of an electric motor;or it can be con- Figure 2-22.Partial vacuum. verted into light by the use of an electric bulb or into heat in an electric iron.By means of a motor and a pump, the energy can be ratio.An increase in force can be obtained transformed back intoits original form of only by a proportional decrease in distance potential energy existing as a body of water traveled.This is also true if the system is at an elevation.Thus, all of these forms operated in the reverse directiona distance of energy are interchangeable with each other. increase can be obtained, but only at the ex- In actual mechanisms, there is always some pense of a force decrease in the same ratio. loss in the form of heat, which is produced This leads to the basic statement: Neglecting by friction, at every exchange.However, the friction, in any fluid power system (or any totalenergy, useful and wasted, will always other mechanical system for that matter), the add up to the original input energy. input force multiplied by the distance through For simplicity, friction was disregarded in which it moves, is always exactly' equal to the preceding discussions. However, it is well theoutput force multiplied by the distance known that there is always some friction in through which it travels. actual machines.It is also known that heat is produced whenever work is accomplished Work and Energy against friction.Therefore, heat is a form of energy because it can be produced from Work is defined as a force moving through work. Likewise, heat in the form of fire some distance, and the amount of work done under a 'boiler can be converted into work is the product of the force multiplied by the through the medium of a steam engine. distance through which it moves.Therefore, when friction is neglected, the work output is Friction represents a loss of efficiency, always equal to the work input.Energy in- but this does not mean an annihilation of en- cludes work and, in addition, all forms into ergy itself.It means only that some of the which work can be converted or which can be energy put into the system has been converted converted into work.Work always involves into another form which is not useful for the actual movement, but energy can be at rest particular problem in hand.The energy is and still exist as energy, as long as it is ca- not usable or available, but it still exists as pable of doing work. dissipated heat. Energy can exist in many different forms, In agreement with this fact, any work or but all have one thing in common; they are energy added to the system must in turn come 26 Chapter 2PHYSICS OF FLUIDS

POTENTIAL ENERGY POTENTIAL ENERGY

WATER WHEEL - WORK

GENERATOR - ELECTRICAL ENERGY

RESERVOIR STORAGE RESERVOIR

SUPPLY RESERVOIR

LIGHT - ENERGY MOTOR - WORK IRON HEAT MOTOR WORK

FP.23 Figure 2-23.Potential and kinetic energy. from somewhere else, for it is not possible the gram-centimeter in the metric system. to create or destroy energy.All that can Power is the rate of doing work.The same be accomplished is to change it from one form amount of work may be accomplished in two into other forms, so as to make it more or instances but less time is used in one case less applicable to the purposes at hand.In than in the other.More power is required the case of a hydraulic jack, since there is. where less time is used. Theunit for measuring always some friction both within the liquid power in the English system is the horsepower, and between adjacent parts, the useful work which is at the rate of 33,000 foot-pounds per output will not exactly equal the work input, minute or 550 foot-pounds per second.The but the difference will always exist somewhere metric system uses the centimeter-gram per in some other form of energy.In this case, second as its unit of measurement. it will appear as heat which must escape from the system somewhere at Sometime.In other words, while the usable work output does not FLUID FLOW equal the input, the total energy output in all forms will always exactly equal the total energy In the operation of fluid power systems, input.This is known as the law of the conser- there must be a flow of fluid.The amount vation of energy. of flow will vary from system to system.In order to understand fluid power systems in Work and Power action,it is necessary to become acquainted with some of the elementary characteristics Work and energy are measured in the same of fluids in motion. Among these are volume units, the foot-pound in the English system and and velocity of flow, steady and unsteady flow, 27 32. FLUID POWER streamline and turbulent flow, and, even more alteredthat is, with volume of input unchanged important, the force and energy changes that the velocity of flow increases as the cross occur in flow and the relations of different section or size of the pipe decreases, and the kinds of energy to each other in fluid power velocity of flow decreases as thecross- systems.These characteristics are discussed sectional area increases. In a stream, velocity in the following paragraphs. Additional infor- of flow is slow at wide parts of the stream mation concerning fluid flow as it applies to and rapid at narrow parts even though the volume fluidics is presented in chapter 14. of water passing each part of the stream is the same. In figure2-24,if thecross- sectional area of the pipe is 16 square inches VOLUME AND VELOCITY OF FLOW at point (A) and 4 square inches at point (B) the velocity of flow at (B) is four times the The quantity of fluid that passes a given velocity at (A). point in a fluid power system in a unit of time is referred to as the volume of flow. Volume of flow can be stated in a number of ways; STEADY AND UNSTEADY FLOW for example, 100 cubic feet per minute, 100 gallons per minute, 100. gallons per hour, etc. A fluid may flow as a single continuous Gallons per minute is the usual method of ex- stream, or the volume of flow may increase, pressing volume of flow in hydraulic systems, decrease, or fluctuate from moment to moment. while cubicfeet per minute is common in Such changes in volume constitute unsteady pneumatic systems.The relative pressure of flow. For example, when a faucet is first the fluid is usually considered when expressing opened, the initial flow is unsteady during the the volume of flow.This is especially impor- short time that the rate of flow of the water tant when considering the volume of flow of is increasing from the initial zero rate to the gases, since the;/ are compressible.For ex- fullrateof flow.The flow then becomes ample, at the same temperature, a cubic foot steady and is maintained if the pressure re- of gas at 100 psi contains twice as many mol- mains constant.If the pressure changes, the ecules as a cubic foot of gas at 50 psi. rate of flow once more becomes unsteady until Velocity of flow means the rate or speed a new balance is reached. at which the fluid moves forward at a particular point in the system.It too can be var- iously stated, but the usual method is in STREAMLINE AND TURBULENT FLOW feet per second. At quite low velocities or in tubes of small Volume and velocity of flow are often con- diameter, flow is streamline, meaning that a sidered together.With other conditions un- given particle of fluid moves straight forward

16 SQ. IN. 4 SQ. IN.

...... B

VEL=4

FP.24 Figure 2-24.Volume and velocity of flow. :28 Chapter 2PHYSICS OF FLUIDS without crossing the paths followed by other par- If the stream narrows, however, and the ticles, and without bumping into them. Stream- volume of flow remains the same, the velocity line flow is often referred to as laminar flow, of flow increases.If the velocity increases which is defined as a flow situation in which sufficiently, the water becomes turbulent. (See fluid moves in parallel lamina or layers. As fig.2-26.) Swirls, eddies, and cross-motions an example of streamline flow, consider fig- areset up in the water.As this happens, ure2-25, which illustrates an open stream the logs are thrown against each other and flowing at a slow, uniform rate with logs float- against the banks of the stream, and the paths ing on its surface.The logs represent par- followed by different logs will cross and recross. ticles of fluid.So long as the stream flows Particles of fluid flowing in pipes act in along at a slow uniform rate, each log floats the same manner.The flow is streamline downstream in its own path, Without crossing if the fluid flows slowly enough, and remains or bumping into the other. streamline at greater velocities if the diameter

FP.25 Figure 2-25.Streamline flow.

FP.26 . /figure 2-26.Turbulent flow. 29 34 ,4. FLUID POWER

of the pipe is small.If the velocity of flow fluid power systems, where velocities of 5 or size of pipe is increased sufficiently, the flow feet per second and above are common.In becomes turbulent. streamline flow, losses due to friction increase One effect of turbulent flow is illustrated directly with velocity, while with turbulent flow in figure 2-27, where the length of the hor- these losses increase much more rapidly. izontal arrows indicates the relative velocities of flow at different places in the pipe, from the center to the edge, when the flowis streamline and when the flow is turbulent.In both FACTORS INVOLVED IN FLOW instances the rate of flow varies from thecen- ter of the pipe to the edge, but the streamline An understanding of the behavior of fluids flow varies more in velocity than turbulent in motion, or solids for that matter, requires flow. For streamline flow, the average velocity an understanding of the term "inertia." Inertia is about one-half the maximum velocity, while isthe, term used by scientists to describe for turbulent flow it is about four-fifths. Veloc- that property possessed by all forms of mat- ity of flow varies both vertically and horizon- ter which makes the matter resist being moved tally, or from the center of the pipe outward. ifitisatrest, and likewise,resist any In both streamline and turbulent flow, the fluid change in its rate of motion if it is moving. next to the wall of the pipe has no velocity. While a high velocity of flow will produce The basic statement covering the action of turbulence in any pipe, other factors contribute inertia is: "A body at rest tends to remain to turbulence. Among these are the roughness at rest, and a body in motion tends to continue of the inside of the pipe, obstructions, and the in motion with the same velocity and in the degree of curvature of bends and the number same direction."This is simply saying what of bends in the pipe.In setting up or main- everyone has learned by experiencethat one taining fluid power systems, care should be must push an object to start it moving and offer taken to eliminate or minimize as many causes an opposition to stop it again. of turbulence as possible, since theenergy A familiar illustration is the effort a pitcher consumed by turbulence is wasted. Limitations must exert to make a fast pitch and the opposi- as to the degree and number of bends of pipe tion the catcher must put forth to stop the ball. are discussed chapter 5. Similarly, considerable work must be performed While designers of fluid power equipment by the engine to make an automobile begin do what they can to minimize turbulence, to to roll; although, after it has attained a certain a very considerable extent it cannot be avoided. velocity, it will roll along the road at uniform For example, in a 4-inch pipe at 68° F, flow speed if just enough effort is expended to over- becomes turbulent at velocities overapprox- come friction, while brakes are necessary to imately 6 inches per second or about 3 inches stop its motion.Inertia also explains the kick per second in a 6-inch pipe. These velocities or recoil of guns and the tremendous striking are far below those commonly encountered in .force of projectiles.

FP.27 Figure 2-27.Streamline versus turbulent flow. 30 35 Chapter2PHYSICS OF FLUIDS

Inertia and Force (which is proportional to its weight), and on the rate at which the velocity of the object In order to overcome the tendency to resist is changed. The rule is that the force in pounds any change in its state of rest or motion, some required to overcome inertia is equal to the force which is not otherwise canceled or un- weight of the object, multiplied by the change balanced must act upon the object.Some un- in velocity measured in feet per second, and balanced force must be applied whenever fluids divided by 32.2 times the time in seconds re- are set in motion or increased in velocity; quired to accomplish the change.Thus, the while conversely, forces are made to do work rate of change in velocity of an object is pro- elsewhere whenever fluids in motion are re- portional to the force applied.The number tarded or stopped. 32.2 appears because it is the conversion fac- Ignoring friction, if the force (A) in figure tor between weight and mass. 2-28 produces a velocity of 10 miles per hour As discussed previously, fluids are always (mph) when it is applied to abody for 5 seconds, acted upon by the force of gravity, or in other it will produce a velocity of 20 mph when it words, by their own weight.Also previously is applied for 10 seconds.The same result explained, is the fact that fluids are acted upon of 20 mph would be obtained if a force (B) by atmospheric pressure, or the weight of air equal to twice (A) were applied to the body over the system, if they are exposed to itif, for 5 seconds.Again ignoring friction, the that is, the system is not enclosed. The action body would be returned to rest from a velocity of specific applied force was also explained of 20 mph if force (C), equal to (A) but acting and, in addition, it was pointed out that when- in the opposite direction, were applied to it ever there is movement there is always sow: for 10 seconds, or if a force (D) equal to twice friction. Inertia, just described, completes (C) were applied to it for 5 seconds. the list of forces which control the action of There is a direct relationship between the fluids in motion. magnitude of the force exerted and the inertia There are five physical factors which can against which it acts.This force is dependent act upon a fluid to affect its behavior.All of on two factorson the mass of the object the physical actions of fluids in all systems

0 SECONDS 5 SECONDS 10 SECONDS

FORCE 1 ; 11=111, 6.100-t -..i=--1 0 MI. 1 . 10 MI.---t 20 MI.1...1110 PER HR.I PERP HR.j PER HR.

I I FORCE I .-_. B 0 N. T 20 MI. ---0 PER HR.1 PER HR.

FORCE C_ 104No 20 MI. 0 MI. PER HR. PER HR.

1=1FORCE' D 20 MI. 0 MI.

PER HR. . PER HR.

FP.28 Figure 2-28.Force and velocity. 31 FLUID POWER are determined by the relationships of these (P) is being acted upon by an applied force five factors to each other. Summarizing, these equivalent to a head (A), by atmospheric pres- five factors are as follows: sure to a head (B), and by gravity head (C) I.Gravity, which acts at all times upon produced by the weight of the fluid standing all bodies, regardless of other forces. overit. The particle possesses sufficient inertia or velocity head to rise to level (P1), 2. Atmospheric pressure, which acts on since head equivalent to (F) was lost in fric- any part of a system exposed to the open air. tion as (P) passed through the system.Since 3. Specific applied forces, which may or atmospheric pressure (B) acts downward on the may not be present, but which, in any event, system on both sides, what was gained on one are entirely independent of the presence or side was lost on the other. absence of motion. If all the pressure acting on (P) to force 4.Inertia, which comes into play whenever it through the nozzle could be recovered in there is a change from rest to motion or the the form of elevation head,it would rise to opposite, or whenever there is a change in level (Y).If account is taken of the balance direction or in rate of motion in atmospheric pressure, in a frictionless system, (P) would rise to level (X), or precisely 5.Friction, which is always present when- as high as the sum of the gravity head and the ever there is motion. head equivalent to the applied force. Figure 2-29 illustrates a possible relationship of these factors with respect to a particle Kinetic Energy of fluid (P) in a system.The different forces are shown in terms of head, or in other words, It was previously pointed out that a force in terms of vertical columns of fluid required must be applied to an object in order to impart to provide the forces.At the particular mo- velocity toit or to increase the velocity it ment under consideration, a particle of water already has.Of necessity the force must act

x ATMOSPHERIC PRESSURE B T -1:- 1 17 FRICTION F 1 API

APPUED FORCE

INERTIA ATMOSPHERIC PRESSURE (VELOCITY HEAD) D s

GRAVITY C

FP.29 Figure 2-29.Physical factors governing fluid flow.

32 Chapter 2PHYSICS OF FLUIDS while the object is moving over some distance. It was also previously stated that a force acting over a distance is work, and that work and all forms into which it can be changed FEET areclassifiedas energy.Obviously, then, energy is required to give an object velocity. The greater the energy used, the greater the velocity will be. KINETIC ENERGY AVAILABLE Likewise, disregarding friction, for an ob- TO DO WORK ject to be brought to rest or its motion slowed down, a force opposed to its motion must be applied to it.This force also acts over some distance.In this way energy is given up by the object and delivered in some form to what- ever opposes its continuous motion. The mov- FP.30 ing object is therefore a means of receiving Figure 2-30.Kinetic energy. energy at one place (where its motion is increased) and deliveringit to another point a projectile against the armor of an enemy (where it is stopped or retarded).While it ship. (See fig. 2-31.)The explosive force is in motion, it is said to contain this energy of the powder in the breach pushes the pro- as energy of motion or kinetic energy. jectile out of the gun, giving it a high velocity. Since energy can never be destroyed,it Because ofits inertia the projectile offers follows that if friction is disregarded the en- opposition to this sudden velocity and a reaction ergy delivered to stop the object will exactly isset up which pushes the gun backward equal the energy which was required to increase (kick or recoil).The force of the explosion its speed.At all times the amount of kinetic acts on the projectile throughout its movement, energy possessed by an object depends upon in the gun.This is force acting through a itsweight and thevelocityat which itis distance producing work.This work appears moving. as kinetic energy in the speeding projectile. Thus, infigure2-30,theforce (F)is The resistance of the air produces friction, applied to the body (A), which is at rest. which uses some of the energy and slows down Disregardingfriction, after it has moved 1 the projectile.Eventually, however, the pro- footit will possess kinetic energy equivalent jectile hits its target and because of the inertia to1. During each succeeding foot of move- tries to continue moving.The target, being ment it will gain an equal increment of kin- relatively stationary, tends to remain stationary etic energy, so long as the force is applied. because of its inertia.The result is that a Ifit meets a resistance after moving 5 feet, tremendous force is set up which either leads kinetic energy equivalent to 5 is available to to the penetration of the armor or the shattering do work. Accelerated motion has been a means of the projectile.The projectile is simply a of receiving energy while force (F) was ap- means of transferring energy, in this instance plied to (A), and of delivering it to do work for destructive purpose, from the gun to the at the point (A) reached at that time. enemy ship.This energy is transmitted in the form of energy of motion or kinetic energy. The mathematical relationship for kinetic Referring to figure 2-31, the projectile is energy is stated in the rule:"Kinetic energy shown in four different positions: at (A) where in foot-pounds is equal to the force in pounds itis at rest in the gun, just before firing; at which created it,multiplied by the distance (B), a short distance beyond the muzzle of the through which it was applied, or to the weight gun, when its kinetic energy is at the max- of the moving object in pounds, multiplied by imum; at (C), midway in its flight, where fric- the square of its velocity in feet per second, tion has used up a portion of its original and divided by 64.4." kinetic energy; and at (D), at the moment of The relationship between inertia forces, impact, where its kinetic energy is suddenly velocity, and kinetic energy can be.illustrated transformed into work by its inertia and the by analyzing what happens when a gun fires opposed inertia offered by the target. Energy 33 FLUID POWER

C =M._.- ---.

ti KINETIC ENERGY (LESS FRICTION)

FP.31 Figure 2-31.Relationship of inertia, velocity, and kineticenergy. imparted to the projectile at (A) has been and pressure can alternately be stated in psi transformed in the form of kinetic energy to or in terms of head, which is the vertical do work at (D). height of the column of fluid whose weight In this diagram no effort has been made would produce that pressure. to exhibit the magnitude of the force of gravity In most of the applications of fluid power acting on the projectile.It enters, of course, in the Navy, applied forces greatly outweigh into the path the projectile takes.This sit- all other forces, and in most systems the fluid uation differs from that shown in figure 2-30 is entirely confined. Under thew; circumstances in which the propelling force was continuously it is customary to think of the forces involved applied throughout the period covered by the in terms of pressures.Since the term head diagram, whereas in figure' 2-31, the force was is encountered frequently in the study of fluid applied while the projectile was moving from power, it is necessary to understand what it (A) to (B). means and how it is related to pressure and A similar action takes place in a fluid force. power system in which the fluid takes the place All five of the factors which control the of the projectile.For example, the pump in actions of fluids can, of course, IN expressed a hydraulic system imparts energy to the fluid either as force, or in terms alternately of which overcomes the inertia of the fluid at equivalent pressures or head. In each situation, rest and causes it to flow throuith the lines. however, the different factors are commonly The fluid flows against some type of actuator referred to in the same terms, since on this which is at rest.The fluid tends to continue common basis they can be added and sub-. flowing, overcomes the inertia of the actuator, tracted to study their relationship to each other. and moves the actuator to do work.Friction uses up a portion of the energy as the fluid At this point some terms in general use flows through the lines and components. should be reviewed.Gravity head, when it is of sufficent importance to be considered, is sometimes referred to as head.The effect of atmospheric pressure is referred to simply RELATIONSHIP OF FORCE, as atmospheric pressure. (Atmospheric pres- PRESSURE, AND HEAD sure is frequently and improperly referred to as suction.) Inertia effect, because it is always In dealing with fluids, forces are usually directly related to velocity, is usually called considered in relation to the areas over which velocity head, and friction, because it represents they are applied.As previously discussed, a loss of pressure or head, is usually referred a force acting over a unit area is a pressure, to as friction head. 34 Chapter 2PHYSICS OF FLUIDS

STATIC AND DYNAMIC FACTORS of its original static head is used to impart thisvelocity, which then exists as velocity The firstthreefactorsgravity, applied head. force, and atmospheric pressureapply equally to fluids at rest or in motion, while the latter twoinertia and frictionapply only to fluids BERNOULLI'S PRINCIPLE inmotion. The first three are the static factors and the latter two the dynamic factors. 3sider the system illustrated in figure The mathematical sum ofthe first three 2-32.Chamber (A) is under pressure and is gravity, applied force, and atmospheric pres- connected by a tube to chamber (B), which is sureis the static pressure obtained at any also under pressure. The pressure in chamber one point in a fluid at any given time. Static (A) is static pressure of 100 psi. The pressure pressure existsin addition to any dynamic at some point (X) along the connecting tube factors which may also be present at the same consists of a velocity pressure of 10 psi exerted point and time. in a direction parallel to the line of flow, plus Remember, Pascal's law states that a pres- the unused static pressure of 90 psi, which sure .set up in a fluid acts equally in all di- still obeys Pascal's law and operates equally rections and at right angles to the containing in all directions.As the fluid enters chamber surfaces.This covers the situation only for (B) itis slowed down, and, in so doing, its fluids at rest, or practically at rest.It is velocity head is changed back to pressure head. true onlyfor the factors making up static The force required to absorb its inertia equals head. Obviously, when velocity becomes a fac- the force required to start the fluid moving tor it must have a direction, and, as previously originally, so that the static pressure in chamber explained,the force related to the velocity (B)isagain equal to that in chamber (A), must also have a direction, so that Pascal's although it was lower at an intermediate point. law alone does not apply to the dynamic factors of fluid power. This situation (fig. 2-32) disregards friction, The dynamic factors of inertia and friction and would therefore, not be encountered in are related to the static factors. Velocity head actual practice. Force or head is also required and friction head are obtained at the expense to overcome friction, but, unlike inertia effect, of static head.However, a portion of the ve- this force cannut be recovered again, although locity head can always be reconverted to static the energy represented still exists somewhere head.Force, which can be produced by pres- as heat.Therefore, in an actual system the sure or head when dealing with fluids, is pressure in chamber (B) would be less than in necessary to start a body moving if it is at chamber (A) by the amount of pressure used rest, and is present in some form when the in overcoming friction along the way. motion of the body is arrested.Therefore, At all points in a system, therefore, the whenever, a fluid is given velocity, some part static pressure is always the original static

100 UM PER SQ. IN. 90

FP.32 Figure 2-32.Relation of static and dynamic factorsBernoulli's principle. 35 FLUID POWER pressure less any velocity head at the point in MINIMIZING FRICTION question, and less the friction head consumed in reaching that point.Since both velocity As mentioned previously, fluid power equip- head and friction represent energy which came ment is designed to reduce friction to the lowest from the original static head, and since energy, possible level. Volume and velocity of flow are cannot be destroyed, the sum of the static head made the subject of careful study. The proper velocity head, and friction at any point in the fluid for the system is chosen. Clean, smooth system must add up to the original static head. pipe of the best dimensions for the particular This is known as Bernoulli's principle, which conditions is used, and it is installed along states: "For the horizontal flow of fluid as direct a route as possible.Sharp bends through a tube, the sum of the pressure and and sudden changes in cross-sectional areas the kinetic energy per unit volume of the fluid are avoided.Valves, gages, and other com- is constant." This principle governs the ponents are designed so as to interrupt flow relationsof the static and dynamic factors as little as possible.Careful thought is given to the 'size and shape of the openings.The concerning fluids, while Pascal's law states-the systems are designed so they can be kept clean manner in which the static factors behave when inside and variations from normal operation taken by themselves. can easily be detected and remedied. CHAPTER 3

HYDRAULIC AND PNEUMATIC FLUIDS

During the design of equipment that re- for use with their equipment.Their recom- quires fluidpower, many factors must be mendations are based on the working conditions, considered in the selection of the type of sys- the service required, temperatures expected tem to be usedhydraulic, pneumatic, or a com- both inside and outside the system, pressures bination of the two.Some o f the factors that the liquid must withstand, the possibilities of must be considered are as follows: Required corrosion, etc. In addition to the manufacturer's speed and accuracy of operation, surrounding recommendations, the proper specifications for atmospheric conditions, economic conditions, liquids used in Navy hydraulic systems are de- availability of replacement fluid, required pres- termined by the various systems commands on sure level, operating temperature range, con- the basis of experiments, tests, and trials. For tamination possibilities, cost of transmission example, many experiments and tests were made lines, limitations of the equipment, lubricity, in the search for hydraulic liquids adapted to both safety to the operators, and expected service the subzero Arctic climate and high temperat- life of he equipment. tures of the Tropics. After the type of system has been selected, many of these same factors must be considered in selecting the fluid for the system. The first PROPERTIES part of this chapter is devoted to hydraulic liquids.Included in this part are sections on the If fluidity (the physical property of a sub- properties and characteristics desired of hy- stance that enables it to flow) and incompress- draulic liquids, the basic types of hydraulic liq- ibility were the only qualities required, any liquids, and the types and control of contamination. uid not too thick might be used in a hydraulic The last part of the chapter covers similar infor- system.However, a satisfactory liquid for a mation concerning the gases used in pneumatic particular installation must possess a number of systems. other properties.Some of the properties and characteristics that must be considered when selecting a satisfactory liquid for a particular sys- HYDRAULIC LIQUIDS tem are discussed in the following paragraphs. Liquids are used in hydraulic systemelri- marily to transmit and distribute forces to the Viscosity various units to be actuated. As pointed out in chapter 2, liquids are able to do this because One of the most important properties of a they are almost incompressible. Pascal's law liquid to be used in hydraulic system is its vis- states that a force applied on any area of an en- cosity.Viscosity is the internal resistance of closed liquid is transmitted equally and undim- a fluid which tends to prevent it from flowing. inished to all equal areas throughout the enclos- A liquid, such as gasoline, which flows easily ure.Thus, if a number of passages exist in a has a low viscosity; and a liquid, such as tar, system, pressure can be distributed through all which flows slowly has a high viscosity. The of them by means of a liquid. viscosity of a liquid is affected by changes in temperatures.As the temperature of a liquid Commercial manufactures of hydraulic de- increases, its viscosity (resistance to flow) devices usually specify the type of liquid best suited creases. That is, a liquid flows more easily when 37 FLUID POWER hot than when cold. Also, the viscosity of a liquid will increase as the pressure increases. HEATING UNIT THERMOMETER A satisfactory liquid for a given hydraulic MID BATH system must have enough body to give a good seal at pumps, motors, valves, etc. These components depend upon close fits for creating and maintaining pressure. Internal leakage through these clearances results in loss of pressure, instantaneous control, and pump efficiency. These leakage losses are greater with lighter liquids (low viscosity). A liquid that is too thin will also lead to rapid wearing of moving parts, or of parts having heavy loads. Onthe other hand, if the viscosity of the liquid is too high the internal friction of the liquid will increase which in turn will increase. the flow resistance through clearances of closely fitted parts, lines and passages. This CONTAINER results in pressure drops throughout the system, sluggish operation of the equipment, and an in- 60 C.C. crease in power consumption. MEASUREMENT OF VISCOSITY.The viscosity of a liquid is measured with an instrument FP.33 called a viscosimeter or viscometer. There are Figure 3-1.SayboltViscosimeter. several types, but the instrument most commonly used by Areorioan engineers is the Saybolt Uni- versal 1, iscoshneter. (See fig. 3-1.) This instru- do not have this characteristic. As the tempera- ment measures the number of seconds it takes for ture increases the oil becomes thinnerthe vis- a fixed quantity of liquid (60 cubic centimeters) to cosity decreases; as the temperature decreases, flow through a small orifice of standard length the oil thickensthe viscosity increases. The and diameter at a specific temperature. The time variation is greater for some liquids than for of flow is taken in seconds, and the viscosity others. Pennsylvania crude oils (paraffinic) vary reading is expressed as Second, Saybolt Univer- comparatively little in viscosity with changes in sal (SSU). For example, a certain liquid might temperature; while with Gulf Coast crude (nap- have a viscosity of 80 SSU at 130° F. hthenic or asphaltic), the variation is consider- The Saybolt Viscosimeter consists of a con- ably greater. tainer for the liquid surrounded by a bath heat- In order to obtain a numerical indication of the ed by heating coils to bring the liquid to the tem- degree to which viscosity changes with change in perature at which the viscosity is to be measur- temperature, these two oils (paraffinic and naph- ed.There is a standard viscosimeter orifice thenic) are taken as the basis for a scale. The located in the bottom of the container. Passage change in viscosity of a specific paraffinic oil at through the orifice is blocked with a cork. The temperatures between 100° and 210° F is as- container is filled to a marked level withthe liq- signed a viscosity index (V.I.) value of 100, uid to be tested and a small container marked at whilethe change in viscosity of a specific the 60-cubic centimeter (cc) level is placed unde r naphthenic over the same temperature range the orifice. When the liquid is at the desired is assigned a value of 0. Other liquids are then temperature, the cork is removed. The number assigned a viscosity index in terms of the degree of leconds required for the liquid to reach the to which their viscosity changes over this tem- 6(' -cc level gives the SSU reading. perature range, as compared to the standard oils. VISCOSITY INDEX.One of the properties of The greater the variation in viscosity with an ideal hydraulic liquid would be that of retain- changes in temperature, the lower the V.I. The ing the same viscosity under all temperattire and V.I. figures may range above 100 or below zero, pressure conditions to which it is subjected. if the liquids being measured vary less or great- Many liquids, particularly petroleum baie oils, er in viscosity than the standard oils.For 38

. '43 Chapter 3HYDRAULIC AND PNEUMATIC FLUIDS

example, a liquid with a viscosity index of -10 Excessive temperature, especially extreme- would indicate a variation in viscosity over the ly high temperatures, have a great effect on the standard temperature range to a greaterdegree life of a liquid.It should be noted that the tem- than naphthenic oils; while an oil with a viscosity perature of the liquid in the reservoir of an op- index of 120 would show less change in viscosity erating hydraulic system does not always repre- with changes in temperature than paraffinic oils. sent a true state of the operating conditions Since naval hydraulic systems must operate throughout the system. Localized hot spots occur satisfactorily under wide temperature extremes, on bearings, gear teeth, or at other points where from the Arctic regions to the Tropics and from the liquid under pressure is forced through small below sea level to many miles into space, the orifices. Continuous passage of the liquid liquids used should have as high a viscosity index through these points may produce local temper- as possible, consistant with the other properties atures high enough to carbonize or sludge the liq- the liquid must possess. The viscosity index of a uid, yet the liquid in the reservoir may not indic- liquid is often increased through the use of chem- ate an excessively high temperature. ical additives. Liquids with a high viscosity have a greater resistance to heat than light or low viscosity liquids which have been derived from the same Lubricating Power source. The average hydraulic liquid has a relatively low viscosity.Fortunately, there is a If motion takes place between surfaces in con- wide choice of liquids available for use in the vis- tact, friction tends to oppose the motion. When cosityrange required of hydraulic liquids. pressure forces the liquid of a hydraulic system Liquids may break down if exposed to air, between the surfaces of moving parts, the liquid water, salt, or other impurities, especially if spreads out in a thin film which enables the parts they are in constant motion or subjected to heat. to move more freely. Different liquids, includ- Some metals, such as zinc, lead, brass, and ing oils vary greatly not only intheirlubricating copper, have an undersirable chemical reaction ability, but also in film strength which is the cap- on certain liquids. ability of a liquid to resist being wiped or These chemical processes result in the for- squeezed out from between the surfaces when mation of sludge, gums, and carbon or otherde- spread out in an extremely thin layer. A liquid posits which clog openings, cause valves and will no longer lubricate if the film breaks down, pistons to stick or leak, and give poor lubrica- since the motion of part against part wipes the tion to moving parts. As soon as a small amount metal clean of liquid. of sludge or other deposits are formed, the rate Lubricating power varies with temperature of formation generally increases more rapidly. changes; therefore, the climatic and working As they are formed, certain changes in the phy- conditions must enter into the determination of sical and chemical properties of the liquid take the lubricating qualities of a liquid. Unlike vis- place. The liquid usually becomes darker, higher cosity, which is a physical property, the lubricat- in viscosity, and acids are formed. ing and film strength of a liquid is directly re- The extent to which changes occur in differ- lated to its chemical nature. Lubricating qual- ent liquids depends on the type of liquid, type ities and film strength can be improved by the of refining, and whether it has been treated to addition of certain chemical agents. provide further resistance to oxidation.The stability of liquids can be improved by the addition of oxidation inhibitors. Laboratory Chemical Stability test must be conducted to select the type and quantity of the inhibitor most effective against Chemical stability is another property which oxidation of a particular liquid.However, in- is exceedingly important in the selection of a hibitors selected to improve the stability of a hydraulic liquid.It is defined as the liquid's liquid must also be compatible with the other re- ability to resist oxidation and deterioration for quired properties of the liquid. long periods. Aliliquids tend to undergo unfavor- able changes under severe operatingconditions. Freedom from Acidity This is the case, for example, whena system operates for a considerable period of time at high An ideal hydraulic liquid should be free from temperatures. acids which cause corrosion of the metals inthe 39

. ,44 FLUID POWER system. Most liquids cannot be expected to re- chemicals and, as a result, most hydraulic main completely noncorrosive under severe op- liquids are free of harmful chemicals. Some erating conditions.The degree of acidity of a fire-resistant liquids are toxic, and suitable pro- liquid, when new, may be satisfactory; but after tection and care in handling must be provided. use, the liquid may tend to develop corrosive Containers for toxic liquids must be properly tendencies as it begins to deteriorate. The liq- labeled. quid must be carefully processed with the spec- NOTE: MIL-STD-755A establishes a uniform ific aim of inhibiting harmful acid formation design for symbols to warn users of potential which would attack metal surfaces in the system. hazards involved with the use of materials in con- Many systems are idle for long periods after tainers.The toxic symbol consists of a brown operating at high temperatures. This permits edged square inside of which is abrovm circular moisture to be condensed in the system, resulting patch on a white background. A skull and the in rust formation. following lettering appears in white on the brown patch: DANGER, TOXIC, CONTAINS (name of Certain corrosion-and rust-preventive addi- substance) AVOID INHALING, SWALLOWING, tives are added to 'hydraulic liquids. Some of OR CONTACT WITH THE SKIN. Additional in- these additives are effective only for a limited formation concerning the identification of com- period. Therefore, the best procedure is touse pressed-gas cylinders and pipelines containing the liquid specified for the system, and to pro- hazardous substance is contained in MIL-STD- tect the liquid and the system as much as poss- 101A and MIL-STD-1247B. ible from contamination by foreign matter, from abnormal temperatures, and misuse. TYPES OF HYDRAULIC LIQUIDS Flashpoint Many different liquids have been tested for use in hydraulic systems. The liquids that are Flashpoint is the temperature at which a liq- presently in use include mineral oil, vegetable uid gives off vapor in sufficient quantity to ignite oil,water, phosphate esters, ethylene glycol momentarily o r flash when a flame is applied. compounds, and oil in water. Hydraulic liquids A high flashpoint is desirable for hydraulic liq- are usually classified according to their type of uids because it provides good resistance to com- base. The three znost common types of hyarau- bustion and a low degree of evaporation at normal lic liquids are water base, petroleum base, and temperatures. synthetic base.

Fire Point Water. Base Liquids Fire point is the temperature at which a sub- Water was used as a fluid medium in the stance gives off vapor in sufficient quantity to first hydraulic systems. It is still suitable for ignite and continue to burn when exposed to a certain large commercial hydraulic installations spark or flame.Like flashpoint, a high fire that require high pressure and low operating point is required of desirable hydraulic liquids. speeds, but it does not meet all the requirements for general hydraulic equipment use. Asa hydraulic liquid, water presents many problems. Minimum Toxicity It is limited to temperatures above freezing and below boiling points. It promotes corrosion and Toxicity is defined as the quality, state, or rusting of metal parts and provides no lubrica- degree of being toxic or poisonous. Some liq- tion of moving parts. In addition, the hazard of quids contain chemicals that are a serious toxic foreign matter in the water itself can cause an hazard. These toxic or poisonous chemicals may abrasive action on the smooth surfaces of system enter the body through inhalation, by absorption components.All of these act as factors det- through the skin, through the eyes, or through the rimental to the operating efficiency and long ser- mouth. The result is sickness and in many cases vice life of the equipment. death. Manufactures of hydraulic liquids strive One of the major advantages of water is its to produce suitable liquids that contain no toxic fire resistant qualities. When water is used as 40 45 Chapter 3HYDRAULIC AND PNEUMATIC FLUIDS a hydraulic liquid, it is usually combined with today.This liquid was improved through the certain oils,ethylene glycol, and other sub- use of a number.of additives, such as oxidation stances. When combined with oil, the combin- and corrosion inhibitors, viscosity index im- ation is relatively fire resistant. However, high provers, pour depressants, etc.These addi- temperatures may cause the water to evaporate tives not only permit liquid operation at greater and then the oil might burn. Other combinations temperature extremes, but add to their lubric- can eliminate ignition problems but may have ating qualities and life characteristics. mechanical or economic limitations. During the Petroleum base liquids are the most widely early 1950's, the Navy used Hydrolube, a mix- used media for hydraulic systems.Certain ture of water and polyethylene glycol (antifreeze petroleum base oils are used in a number of compound) in approximately 80 percent of their different applications.For example, MIL-H- aircraft. This liquid contained 35 to 40 percent 5606B is the specification number of the pe- water mixed with polyethylene glycol, rust in- troleum base oil presently used in most Navy hibitors,thickeners, and lubricating agents. Another example After a few years of use, problems were encoun- aircraft hydraulic systems. tered with thin film corrosion on the hydraulic is MIL-F-1'7111 which isthespecification system components, resulting in the Navy dis- number of the petroleum base oil generally continuing its use in aircraft hydraulic systems. approved for ordnance equipment, except guided Similar water base liquids are used by the Navy missiles. Many special petroleum base oils in the catapult systems on some aircraft carri- are required for special applications. ers.This was approved following explosions attributed to petroleum base liquids. Synthetic Base Liquids Petroleum base oils contain most of the de- Petroleum Base Liquids sired properties required of a hydraulic liquid. However, they are flammable unde r normal con- One of the first oils used as a hydraulic liq- ditions and can become explosively dangerous uid was a petroleum base automotive brake when subjected to high pressures and a source of fluid. At that time natural rubber was used in the flame or high temperatures.Water base liq- construction of packings and gaskets. Since nat- uids are relatively fire resistant, but do not have ural rubber is not compatible with petroleum the high lubricity of petroleum base oils. Some base liquids, the use of this type liquid as a hy- water base liquids cause corrosion of compo- draulic medium was limited. As a result, a vegetable base oil containing 50 percent castor oil nents in the hydraulic system. and 50 percent alcohol was used in some appli- In recent years, nonflammable synthetic liq- cations. This solved the problem with packings uids have been developed for use in hydraulic and gaskets, as natural rubber is compatible systems where fire hazards exist. A synthetic with vegatable base oils.However, this liquid material is a complex chemical compound that was unsatisfactory due to oxidation of the castor has been artifically formed by the combining of oil and, since liquid is an excellent conductor of two or more simpler compounds or elements. electricity, resulted in a high degree of elect- Some of the synthetic liquids currently used as rolysis (the chemical disintegration of a sub- a hydraulic medium are chemically described stance accomplished by an electric currentpas- as phosphate esters, chlorinated biphenyls, or sing through it). In addition, vegetable oils tend blends of each.Certain synthetic liquids have to break down under extreme temperature been found to chemically attack packings used changes. in hydraulic systems; therefore, special pack- During the middle 1930's, a light petroleum ings are normally required when these fluids base oil was developed and, when used with as- are used. bestos seals, proved quite successful. By the late 1930's, advancement in packing materials, such as synthetic rubber seals, permitted exten- CONTAMINATION sive use of petroleum base liquids in hydraulic. systems. As a result, a pertoleum base oil was Experience has shown that trouble in a hy- developed about 1940 that proved very satisfac- draulic system is inevitable whenever the liquid tory and, with certain refinements, is in use is allowed to become contaminated. In fact, most

41 FLUID POWER manufacturers and users agree that a large 1. Particles originally contained in the percentage of the malfunctions in hydraulic sys- system. These particles originate during the tems may be traced to some type of contaminant fabrication and storage of system components. in the hydraulic fluid. The nature of the trouble Weld spatter and slag may remain in welded whether a simple malfunction or the complete system components, especially in reservoirs and destruction of a componentdepends to some pipe assemblies. The presence is minimized by extent on the type of contaminant. proper design. For example, seam-welded overlapping joints are preferred, and arc welding of open sections is usually avoided.Hidden Classes of Contamination passages in valve bodies, inaccessible to sand blasting or other methods of cleaning, are the There are many different types of contam- main source of introduction of core sand. Even inants which are harmful to hydraulic liquids. the most carefully designed and cleaned casting These contaminants are divided into two gen- will alMost invariably free some sand particles eral classes and can be distinguished as follows: under the action of hydraulic pressure. Rubber 1. Abrasives, including such particles as hose assemblies always contain some loose core sand, weld spatter, machining chips, and particles.Most of these particles can be re- rust. moved by flushing the hose before installation; 2.Nonabrasives, including those resulting however, some particles withstand cleaning and from liquid oxidation, and soft particles worn or are freed later by the action of hydraulic pres- shredded from seals and other organic compo- sure. nents. The mechanics of the destructive action by Particles of lint from cleaning rags can abrasive contaminants is clear. When the size cause abrasive damage in hydraulic systems, of the particles circulating in the hydraulic sys- especially to closely fitted moving parts.In tem is greater than the clearance between moving addition,' lint in a hydraulic system packs easily parts, the clearance openings act as filters and into clearances between packings and contacting retain such particles. Hydraulic pressure then surfaces, leading to component leakage and de- embeds these particles into the softer materials, creased efficiency. Lint also helps clog filters and the reciprocating or rotating motion of com- prematurely. Rust or corrosion initially present ponent parts develop scratches on finely fin- in a hydraulic system can usually be traced to ished surfaces. Such scratches result in internal improper storage of materials and component component leakage and decreased efficiency. parts.Particles can range in size from large Liquid-oxidation products, usually called flakes to abrasives of microscopic dimensions. sludge, have no abrasive properties. Neverthe- Proper preservation of stored parts is helpful less, sludge may prevent proper functioning of a in eliminating corrosion. hydraulic system by clogging valves, orifices, 2. Particles introduced fromoutside and filters. Frequent changingof hydraulic sys- sources. Particles can be introduced into hy- tem liquid is not a satisfactory solution to the draulic systems at points where either the liquid contamination problem. Abrasive particles con- or certain working parts of the system (e.g., pis- tained in the system are not usually flushed out, ton rods) are at least in temporary contact with and new particles are continually created as the atmosphere. The most common danger areas friction products. Furthermore, even a small are at the refill and breather openings, at cyl- amount of sludge acts as an effective catalyst inder rod packings, and at open lines where to speed up oxidation of the fresh liquid.(A components are removed for repair or replace- catalyst is a substance which, when added to ment. Contamination arising from carelessness another substance. speeds up or slows down during servicing operations is minimized by the chemical reaction, but is itself unchanged at the use of filters in the system fill lines and finger end of the reaction.) strainers in the filler adapter of hydraulic reservoirs. Hydraulic cylinder piston rods incorpo- Origin of Contaminants rate wiper rings and dust seals to prevent the duif that settles on the piston rod during its The origin 01 contaminants in.hydraulic sys- outward stroke from entering the system when tems can be traced to four major areas as the piston rod retracts. Caps and plug are avail- follows: able and should be used to seal off the open lines 42

. 47 Chapter 3HYDRAULIC AND PNEUMATIC FLUIDS during the time a component is removed for systems by condensation of atmospheric mois- repair or replacement. ture, normally settles to the bottom of the reservoir.Oil movement in the reservoir -disperses 3. Particles created within the system during the water into fine droplets, and agitation of the operation. Contaminants created during system liquid in the pump and in high speed passages operation are of two general typesmechanical forms an oil-water-air emulsion. Such emulsion and chemical. Particles of a mechanical nature normally separates out during the rest period are formed by wearing of parts in frictional in the system reservoir; but when fine dust and contact, such as pumps, cylinders, and packing corrosion particles are present, the emulsion gland components.These wear particles can is chemically chanted by high pressures into vary from large chunks of packings down to sludge. The damaging action of sludge explains steel shavings of microscopic dimensions which the need for effective filtration, as well as the are beyond the retention potential of system need for water separation qualities in hydraulic filters. liquids. The major source of chemical contaminants in hydraulic liquid is oxidation.These contaminants are formed under high pressure and Contamination Control temperatures, and are promoted by the chemical action of water and air and of metals like copper Filters (discussed in chapter 7) provides and iron oxides. Liquid-oxidation products adequate control of the contamination problem appear initially as organic acids, asphaltines, during all normal hydraulic system operations. gums, and varnishessometimes combined with Control of the size and amount of contamination dust particles as sludge.Liquid soluble oxi- entering the system from any other source must dation products tend to increase liquid viscosity, be the responsibility of the personnel who service while insoluble types separate and form sedi- and maintain the equipment.Therefore, pre- ments, especially on colder elements such as caution must be taken to insure that contami- heat exchanger coils. nation is held to a minimum during service and Liquid containing antioxidants have little maintenance.Should the system become ex- tendency to form gums and sludge under normal cessively contaminated, the filter element should operating conditions. However, as the tempe ra- be removed and cleaned or replaced. ture increases, resistance to oxidation dimin- As an aid to exercising contamination control, ishes. Hydraulic liquids which have been sub- the following maintenance and servicing pro- jected to excessively high temperatures (above ceduresshould be adhered to at all times: 250° F for most liquids) will break down in sub- 1.Maintain all tools and the work area stance, leaving minute particles of asphaltines (work-benches and test equipment) in a clean, suspended in the liquids. The liquid changes to dirt-free condition. brown in color and is referred to as decomposed 2. A suitable container should always be pro- liquid. This explains the importance of keeping vided to receive the hydraulic liquid which is the hydraulic liquid temperature below specific spilled during component removal or disassem- levels. bly procedures. The second contaminant producing chemical NOTE:The reuse of drained hydraulic liq- action in hydraulic liquids is one which permits uid is prohibited in some hydraulic systems; for these liquids to establish a tendendy to react example, aircraft hydraulic systems. In some with certain types of rubber.This reaction large capacity systems the reuse of fluid is causes structural changes in the rubber, turning permitted. When liquid is drained from the latter it brittle, and finally causing its complete dis- systems, it must be stored in a clean and suitable integration.For this reason, the compatibility container.This liquid must be strained and/or of system liquid with seals and hose material filtered as it is returned to the system reser- is a very important factor. voir. 3.Before disconnecting hydraulic lines or 4. Particles introduced by foreign liquids. fittings, clean the affected area with an approved One of the most common foreign-fluid contami- drycleaning solvent. nants is water, especially in hydraulic systems 4.All hydraulic lines and fittings should be which require petroleum base liquids. Water:, capped or plugged immediately after discon- which enters even the most carefully designed necting. 43 FLUID POWER

5. Before assembly of any hydraulic com- a sample.In either case, while the liquid is ponents, wash all parts with an approved dry- being taken, a small amount of pressure should cleaning solvent. be applied to the system.This insures that 6.After cleaning parts in drycleaning sol- the liquid will flow out of the sampling point vent, dry the parts thoroughly and lubricate with and thus prevent dirt and other foreign matter the recommended preservative or hydraulic liq- from entering the hydraulic system. Hypoder- uid before assembly. mic syringes are provided with some contami- NOTE: Use only clean, lint-free cloths to nation test kits for the purpose of taking sam- wipe or dry component parts. ples. 7.All packings and gaskets should be re- CONTAMINATION TESTING.Various pro- placed during the assembly procedures. cedures are recommended to determine the contaminant level in hydraulic liquids. The 8.All parts should be connected with care filter patch test provides a reasonable idea of to avoid stripping metal slivers from threaded the condition of the fluid.This test consists areas. All fittings and lines should be installed basically of filtration of a sample of hydraulic and torqued in accordance with applicable techni- system liquid through a special filter paper. cal instructions. This filter paper darkens in degree in relation 9. All hydraulic servicing equipment should to the amount of contamination present in the be kept clean and in good operating condition. sample, and is compared to a series of stand- ardized filter discs which, by degree of darken- Contamination Checks ing, indicates the various contamination levels. The equipment provided with one type of contami- Whenever it is suspected that a hydraulic nation testkit is illustrated in figure 3-2. system has become excessively contaminated, When using this liquid contamination test kit, or the system has been operated at temperatures the liquid samples should be poured through the in excess of the specified maximum, a check of filter disc (shown in figure 3-2), and the test the system should be made. The filters in most filter patches should be compared with the test hydraulic systems are designed to remove most patches supplied with the test kit. A microscope foreign particles that are visible to the naked eye. is provided with the more expensive test kits However, hydraulic liquid which appears clean to for the purpose of making this comparison. the naked eye may be contaminated to the point Figure 3-3 shows test patches similar to those that it is unfit for use. supplied with the testing kit. Thus, visual inspection of the hydraulic liquid To check liquid for decomposition, pour new does. not determine the total amount of contami- hydraulic liquid into a sample bottle of the same nation in the system.Large particles of im- size and color as the bottle containing the liquid purities in the hydraulic system are indications to be checked.Visually compare the color of that one or more components in the system are the two liquids.Liquid which is decomposed being subjected to excessive wear.Isolating will be darker in color. the defective component requires a systematic At the same time the contamination check is process of. elimination.Liquid returned to the made, it may be necessary to make a chemical reservoir may contain impurities from any part analysis of the liquid.This analysis consists of the system.In order to determine which of a viscosity check, a moisture check, and a flashpoint check. However, since special equip- component is defective, liquid samples should be ment is required for these checks, the liquid taken from the reservoir and various other loca- samples must be sent to a laboratory, where a tions in the system. technician will perform the test. FLUID SAMPLING.Liquid samples should be taken in accordance with the instructions provided in applicable technical publications for the System Flushing particular system and the contamination test kit.Some hydraulic systems are provided with Whenever a contamination check indicates permanently installed bleed valves for taking impurities in the system or indicates decompo- liquid samples; while on other systems, lines sition of the hydraulic liquid, the hydraulic sys- must be disconnected to provide a place to take tem must be flushed. 44 Chapter 3HYDRAULIC AND PNEUMATIC FLUIDS

FILTER PAPER DISKS (Whatmon No. 40)

GOOCH CRUCIBLE

RUBBER CRUCIBLE HOLDER

FILTER FLASK TO VACUUM PUMP POLYETHYLENE WASHING BOTTLE

7:".;3,U.; 'tr.

FLUID SAMPLE BOTTLE REFERENCE FILTER PATCH

FP.34 Figure 3-2.Liquid contamination test kit.

NOTE:The presence of foreign particles Drain out as much of the contaminated liquid in the hydraulic system indicates a possible as possible. Drain valves are_provided in some component malfunction, which should be cor- systems for this purpose; while on other sys- rected prior to flushing the system. tems, lines and fittings must be disconnected at the low points of the system to remove any A hydraulic system in which the liquid is trapped fluidin thelines and components. contaminated should be flushed in accordance Close all the connections and fill the system with current applicable technical instructions. with the applicable flushing medium. Any of Flushing procedures are normally recommended the hydraulic liquids approved for use in power- by the manufacturer and approved by the Navy. transmission systems may be used for flushing The procedure varies with different hydraulic purposes. In the interest of economy, however, systems. The following procedures forflushing either used or reclaimed liquids should be used hydraulic power-transmission systems used for flushing, provided they are clean and free of with naval ordance are covered in Nav Ord OD water and insoluble contaminants and do not con- 3000, Lubrication of Ordance Equipment. tain acids resulting from oxidation of the liquids. 45

50 FLUID POWER

CAUTION: The system should not be operated while or after draining the liquid. Power-transmissionsystemsandtheir FILTER BOWL SAMPLE interconnected hydraulic controls whose inner surfaces have been inactivated and treated with a corrosion prevention or preservation compound must be flushed to remove the compound. The latest current instructions for flushing and other operations required to reactivate a particular system must be strictly followed to prevent damage. 1. Discoloration as dark as or darker than refer Some hydra u 1 i c systems are flushed by ence disk. forcing new liquid into the system under pres- 2. More than 2 metal chips larger than 0.01 inch in diameter. (About size of sharp pencil dot.) sure, forcing out the contaminated or decomposed liquid. 3. More than 25 very fine but visible metal particles. Hydraulic liquid which has been contaminated by continuous use in hydraulic equipment or has been expended as a flushing medium must not be used again, but should be discarded in accordance with prevailing instructions. DOWNSTREAM SAMPLE CAUTION: Never permit high-pressure air to be in direct contact with petroleum base liquids in a closed system, because of the danger of ignition. If gas pressure is needed in a closed system, nitrogen or some other inert gas should be used. PNEUMATIC GASES 1. Discoloration as dark as or darker than refer ence disk.

2.More than 1 metal chip larger than 0.01 inch biasesserve the same purpose in pneu- in diameter. (About size of sharp pencil dot.) matic systems as liquids, serve in hydraulic 3. More than 10 very fine but visible metal particles. systems. Therefore, many of the same qualities that are considered when selecting a liquid FP.35 for a hydraulic system must be considered when Figure 3-3.Hydraulic liquid con- selecting a gas for a pneumatic system. tamination test patches. QUALITIES

CAUTION: Diesel fuel oil must not be used Theideal fluid medium for a pneumatic for flushing hydraulic systems in active ser- system must be a readily available gas that is vice, because of its poor lubricating qualities nonpoisonous, chemically stable, free from any and its contaminating effect on the subsequent acids that cause corrosion of system compon- fill of hydraulic liquid. ents, and nonflammable. It should be a gas that While being flushed with an approved hy- will not support combustion of other elements. draulic liquid, power-transmission systems can be operated at full load to raise the temperature The viscosity of gases is not a critical qual- of the liquid. Immediately following the warming ity to consider in the selection of a medium for operation, the system should be drained by pneumatic systems. Howeve', it should be noted opening all drain outlets and disconnecting the that, unlike liquids, the viscosity of gases inhydraulic lines to remove as much of the flush- creases as the temperature increases and de- ing medium as possible. All filter elements, creases as the temperature decreases. screens, and chambers should be cleaned with Gases that have these desired qualities do new fluid prior to filling the system with the not have the required lubricating power. There- required service liquid. fore, lubricationofthecomponentsofa 46 Chapter 3HYDRAULIC AND PNEUMATIC FLUIDS pneumatic system must be arranged by other must be charged (filled to the required pres- means. For example, air compressors are pro- sure) at a centrally located air compressor vided with a lubricating system, and compo- aixt then connected to the system. nents are lubricated upon installation or, in some cases, lubrication is introduced into the Nitrogen air supply line. For all practical purposes, nitrogen is con- TYPICAL GASES USED sidered to be an inert gas. (Inert is defined as chemically inactive; not combining with other The two most common gases used in pneu- chemicals.) It is not completely inert like he- matic systems are compressed air and nitro- lium or argon, for there are many nitrogen gen. compounds, such as nitrate used in fertilizers NOTE: Compressed air is a mixture of all and explosives. However, nitrogen is very slow other gases contained in the atmosphere. How- to combine chemically with other elements under ever, in this manual it is referred to as one of normal conditions. Nitrogen, as a gas, supports the gases used as a fluid medium in pneumatic no fires, no living things, and causes no rust systems. or decay of most of the things with which it comes in contact. Due to these qualities, its Compressed Air use is preferred over compressed air in many pneumatic systems, especially aircraftand The unlimited supply of air and the ease of missile systems. compression make compressed air the most widely used fluid for pneumatic systems. Al- Nitrogen is obtained by the fractional dis- though moisture and solid particles must be tillation of air. In many cases, nitrogen is removed from the air, it does not require the obtained as a byproduct of oxygen-producing extensive distillation or separation process re- plants. Such plants are located at many of the quired in the production of other gases. Navy's installations ashore. In addition, some ships, particularly aircraft carriers, are equip- Compressed air has most of the desired ped with oxygen/nitrogen plants. properties and characteristics of a gas for pneumatic systems. It is nonpoisonous and non- A combination of nitrogen and compressed flammable but does contain gases,such as air is used in some pneumatic systems. Com- oxygen, which support combustion. One of the pressed nitrogen is supplied from cylinders, most undesirable qualities of compressed air while a compressor provides compressed air to as a fluid medium for pneumatic systems is replenish any expended nitrogen and maintains moisture content.The atmosphere contains the pneumatic system at the required pressure. varying amounts of moisture in vapor form, the amount depending upon geographic locations POTENTIAL HAZARDS and weather conditions. Changes in temperature of compressed air will .cause condensation of All compressed gases are hazardous. Com- moisture in the pneumatic system. This con- pressed air and nitrogen are neither poisonous densed moisture is very harmful to,the system.nor flammable, but should not be handled care- as it increases the formation of rust and cor- lessly. Some pneumatic systems operateat rosion, dilutes lubricants, and may 'freeze in pressures exceeding 3,000 psi. Lines and fit- lines and components during cold weather. Most tings have exploded injuringpersonnel and pneumatic systems employ devices for the re- property. Literally ,thousands of careless men moval of moisture. These components are de- have blown dust or harmful particles into their scribed in chapter 7. eyes by the careless handling of compressed air outlets. The supply of compressed air at the required Nitrogen gas will not support life, and when volume and pressure is provided by an aircom- released in a confined space will cause asphyxia pressor. In some systems the compressor is (the loss of consciousness as a result of too lit- part of the system with distribution lines leading tle oxygen and too much carbon dioxide in the from the compressor (receiver) to the devices blood). Because compressed air and nitrogen to be operated. Other systems receive their seem so safe in comparison with other gases, do supply from cylinderb. However, the cylinders not let overconfidence lead to personal injury. FLUID POWER

When handling gas cylinders, always abide terns are equipped with one or more devices to by the color code on the cylinders. For example, remove this contamination. These include fil- a cylinder must not be charged with a gas other ters, water separators, and chemical driers, than that so indicated by the color code. The which are discussed in chapter 7. In addition, color codes for compressed air and nitrogen most systems contain drain valves at critical cylinders are as follows. Cylinders for com- low points in the system. These valves are pressed air are painted black. Cylinders con- opened periodically allowing the escaping gas taining oil pumped air have two green stripes to purge a large percentage of the contamin- painted around the top of the cylinder, while ants, both solids and moisture, from the system. cylinders containing water pumped air have one In some systems these valves are opened and green stripe. Nitrogen cylinders are painted closed automatically, while in others they must gray. One black stripe identifies those cylin- be operated manually. ders for oil pumped nitrogen, and two black Complete purging is accomplished by re- stripesidentify those cylindersforwater moving lines from various components through- pumped nitrogen. In addition to these color out the system and then attempting to pressurize codes, the exact identification of the contents the system. Removal of the lines will cause a is printed in two locations diametrically op- high rate of airflow through the system. The posite and parallel to the longitudinal axis to airflow will cause the foreign matter to be ex- the cylinder. For compressed air and nitrogen hausted froth the system. cylinders, the lettering is in white. NOTE: If an excessive amount of foreign NOTE: Oil pumped indicates that the air or matter, particularly oil, is exhausted from any nitrogen is compressed by an oil lubricated com- one system, the lines and components should be pressor. Air or nitrogen compressed by a water removed and cleaned or replaced. lubricated(or nonlubricated) compressor is Upon the completion of pneumatic system referred to as water pumped. Oil pumped nitro- purging and after reconnecting all the system gen can be very dangerous in certain situations. components, the system drain valves should be For example, nitrogen is commonly used to opened to exhaust any moisture or impurities purge oxygen systems. Oxygen will not burn, which may have accumulated. After all drain but it supports and accelerates combustion and valves are closed, the system should be serv- will cause oil to burn easily and with great iced with the approved gas, usually nitrogen or intensity. Therefore, oil pumped nitrogen must compressed air. The system should then be never be used to purge oxygen systems. When given a thorough operational check and an in- the small amount of oil remaining in the nitro- spection for leaks and security. gen cbmes in contact with the oxygen, an ex- History has indicated that the development of plosion may result. In all situations, use only fluid power and its mechanical equipment goes that gas specified by the manufacturer and/or hand in hand with the development of specific recommended by the Navy. fluids for each application. Past breakthrough in high-temperature fluids have shown that, CONTAMINATION CONTROL when future equipment requires fluid power, satisfactory systems and fluids will be avail- Like hydraulic systems, fluid contamination able. For example, hot gas fluid power systems is also a leading cause of malfunctions in pneu- have been developed and are currently used in matic systems. In addition to the solid parti- missile and space vehicles. The hot gases are cles of foreign matter which find a way to obtained by bleeding off the main rocket engine enter the system, there is also the problem of or using a solid or liquid propellent gas gen- moisture, as mentioned previously. Most sys- erator. CHAPTER 4 BASIC SYSTEMS AND CIRCUIT DIAGRAMS

Inorder to transmit and control power COMPONENTS OF A BASIC SYSTEM through pressurized fluids, an arrangement of interconnected components is required. Such The basic components of a fluid power sys- an arrangement is commonly referred to as a tem are essentially the same, regardless of system. The number and arrangement of the whether the system employs a hydraulic or a components vary from system to system, de- pneumatic medium. There are five basic com- pending upon the particular application. In many ponents used in a system. These basic compo- applications, one main system supplies power nents are as follows: to several subsystems, which are sometimes referred to as circuits. The complete system 1. Reservoir or receiver. may be a small compact unit; more often, how- 2. Pump or compressor. ever,the components are located at widely 3. Lines (pipe, tubing, or flexible hose). separated points for convenient control and 4. Directional control valve. operation of the system. 5. Actuating device. Regardless of the arrangement of the com- The term RESERVOIR is associated with ponents, it is difficult,if not impossible, to hydraulic systems and the term RECEIVER understand the operation and interrelationship with pneumatic systems. The primary purpose of the components by simply observing the op- of a reservoir or receiver is to provide a stor- eration of the system.Thisknowledgeis age space for fluid used in the respective required to effectively troubleshoot and main- systems. The two components, however, differ tain a fluid power system. As an aid for per- in some respects. In the case of the receiver, sonnel VOT) must maintain fluid power equip- a volume of fluid (gas) is stored under pressure ment, a:, least one system or circuit diagram and is supplied to the pneumatic system as is LIFIRMT provided in the applicable technical needed to operate the system. The pressure of publicat:on. By utilizing the applicable diagram, the gas provides the flow and force required to the path of fluid may be traced through the operate the system. After the gas is used for operation of the system. Thus, these diagrams an operation, it is exhausted to the atmosphere are valuable assets in diagnosing the cause of and a new supply of fluid is compressed in the malfunctions in fluid power systems. receiver. In converse, the fluid in most hydraulic reservoirs is not pressurized. Although The first part of the chapter describes the some hydraulic reservoirs are pressurized, functions of the components of a basic fluid this pressure does not supply the force to op- power system. Included in this section are des- erate the system but is limited to the amount criptions and illustrations denoting the differ- necessary to insure a supply of fluid to the ences between closed-center and open-center hydraulic pump at all times. In addition, the fluid power systems. The next part of the fluid of a hydraulic system is used over and chapter covers information concerning the dif- over again, flowing from the reservoir to the ferent types of diagrams used to illustrate fluid other components, back to the reservoir, to the power circuits, including some of the symbols, components, etc. Therefore, a well designed used to depict fluid power components on dia- reservoir also serves as a heat exchanger to con- grams. The last part of the chapter describes trol the temperature of the fluid and as a place and illustrates some of the applications of basic to filter and clean the fluid. Reservoirs and re- fluid power systems. ceivers are discussed in detail in chapter 7. 49 54 FLUID POWER

A fluid power system requires a power unit side of the piston is forced out of the cylinder, to convert mechanical energy into fluid energy. through the control valve, and back to reser- In pneumatic systems, this unit is referred to voir through the return line. as a COMPRESSOR, while in hydraulic systems, this unit is called a PUMP. Like the reservoirs and receivers discussed in the preceding paragraph, the pump and compressor differ in RESERVOIR some respects. These differences and detailed information concerning the types and operation of pumps and compressors are covered in chapter 8 of this training manual. Basically, the compressor, which is driven by some outside power source, compresses the fluid (gas) PRESSURE L;NE in the receiver. The hydraulic pump, also driven by an outside power source, provides a flow of fluid to a hydraulic system. The LINES are the medium for transmitting the fluid from one component to another. This is normally accomplished by using pipe, tubing, RETURN LINE CONTROL VALVE IN or flexible hose for interconnecting reservoirs "DOWN" POSITION to pumps, pumps to valves, valves to cylinders, CONTROL VALVE IN etc. The different types of fluid lines and con- "UP" POSITION nectors are covered in chapter 5. The DIRECTIONAL CONTROL VALVE (also WORKING LINES referred to as a selector valve) is a device which directs a flow of fluid to and from the actuating device. Some of the most common types of directional control valves and their ap- ACTUATING UNIT plications are describedandillustrated in FP.36 chapter 11. Figure 4-1.Basic hydraulic system, The ACTUATING DEVICE of a fluid power hand pump operated. system is that component which converts the fluid pressure into useful work. Actuators perform the opposite function of hydraulic pumps When the control valve is moved to the and pneumatic compressors in that they con- DOWN position, as shownin theinsert of vertfluidenergy into mechanical energy. figure 4-1, the fluid flows from the pump Common types of actuating devices are the cyl- through the control valve to the left side of the inder,whichprovideslinearmotion,and actuating cylinder, thus reversing the process. motors, which provide rotary motion. The types Movement Of the piston can be stopped at any and operation of actuating devices are covered time simply by moving the control valve to in chapter 12. neutral. In this position,all four ports are These five basic components, when combined closed and pressurized fluid is trapped in both into one unit, are the basis for a fluid power working lines. system. An example of a basic hydraulic sys- The example shown in figure 4-1 could very tem is illustrated in figure 4-1. well represent a basic pneumatic system by replacing the reservoir with a receiver and the The flow of fluid can be traced readily from pump with a compressor and reversing their the reservoir through the pump to the direc- positions in the system. Thus the fluid would tional control valve. With the control valve in flow from the compressor into the receiver and the UP position, as shown, the flow of fluid from the receiver to the control valve. The re- provided by the pump flows through the valve turn fluid from the actuating cylinder is ex- to the right-hand end of the actuating cylinder. hausted from the control valve; therefore, the Fluid pressure then forces the piston to the return line, as shown in figure 4-1, is not re- left and, at the same time, the fluid on the left quired in a pneumatic system. 50 Chapter 4BASIC SYSTEMS AND CIRCUIT DIAGRAMS

This basic system is one from which any system can be derived. The hand pump may be replaced with a power pump and the actuating device may be a motor. Additions may be added for the purposes of providing additional sources of power, operating additional cylinders, making operation more automatic, or increasing reliability; but these additions are all made on the framework of the basic system. With the addition of a few components, a basic system can be improved into a more workable system. Figure 4-2 shows a basic system with the addition of a power-driven pump and the following essential components: Filter, pressure regulator, accumulator, pressure gage, relief valve, and two check valves. The functions of each of these components aro briefly explained in the following paragraphs. Detailed information concerning the types, construction, and operating features of these components are described later in this manual. The FILTER removes foreign particles from the fluid, preventing dust, grit, and other undesirable matter from entering the system. The purpose of the PRESSURE REGULATOR is to unload or relieve the power-driven pump when the desired pressure in the system is reached. It is therefore often referred to as an unloading valve. When the pressure in the system builds up to the desired point, a valve in the pressure regulator opens and fluid from the pump is bypassed back to the reservoir. The bypass line is shown if figure 4-2 leading from the regulator to the reservoir. At the same time, the pressure in the remainder of the system is maintained at the desired pressure. When this pressure drops, due to the operation of an actuating unit or internal or external leakage, I!. Reservoir. .7. Hand pump. the valve in the regulator closes, allowing the 2. Power pump. 8. Pressure gage. pressure in the system to Wild up to the de- 3. Filter. 9. Relief valve. sired amount. 4. Pressure regulator.10. Cbntrol valve Many fluid power systems do not use apres- 5. Accumulator. 11. Actuating device. sure regulator, but have other means of un- 6. Check valves. loading the pump andmaintaining system FP.37 pressure. Such methods are described later in Figure 4-2.Basic system with addition of this manual. power pump and other components. The ACCUMULATOR serves several purposes in a hydraulic system. It serves as a cushion or shock absorber by absorbing pressure chamber separated from the hydraulic fluid by surges in the system, and it stores enough a flexible diaphragm, synthetic rubber bladder, fluid under pressure to provide for emergency or movable piston. operation of certain actuating unite. It is de- The PRESSURE GAGE indicates the amount signed with a compressed -air (or nitrogen) of pressure in the system. 51 56 FLUID POWER

The RELIEF VALVE is a safety valve installed in the system so that fluid is bypassed through the valve back to the reservoir in case RESERVOIR excessive pressure is built up in the system. CHECK VALVES allow the flow of fluid in one directions only. There are numerous check valves installed at various points in most fluid power systems. A careful study of figure 4-2 will. reveal why the two check valves are necessary in the system.(The arrow on each check valve points toward'the direction of free flow.)One check valve prevents power-pump pressure from entering the hand pump line; the other prevents hand-pump pressure from (A) being directed to the accumulator.In a system of this type, the power pump is used in the normal operation of the system. The hand pump serves as an emergency means of operating the system in case of power pump failure. The hand pump may also be used as a means for checking and troubleshooting many of the components of the system. In the system described in the preceding paragraphs, the fluid in the system from the pump (or regulator) to the directional control valve is under pressure when the pump is operating. Any number of subsystems may be incorporated in such a system with a separate SUCTION PRESSURE RETURN directional control valve for each subsystem. (B) The directional control valves are arranged in parallel whereby system pressure acts equally FP.38 on all control valves. This type system is re- Figure 4-3.Basic open-center ferred to as a closed-center system. hydraulic system. Another type of system which is sometimes used in hydraulically operated equipment is the When one of the directional control valves open-center system. An open-center system is is positioned to operate an actuating device, as one having fluid flow, but no pressure in the shown in view (B) of figure 4-3, fluid is di- system when the actuating mechanisms are idle. rected from the pump through one of the work- The pump circulates the fluid from the reser- ing lines to the actuator. With the control valve voir, through the directional control valves, and in this position, the flow of fluid through the back to the reservoir. (See fig. 4-3 (A).) Like valve to the reservoir is blocked. Thus, the the closed-center system, the open-center sys- pressure of the fluid builds up in the system to tem may employ any number of subsystems overcome the resistance and moves the piston with a directional control valve for each sub- of the actuating cylinder. The fluid from the system. Unlike the closed-center system, the other end of the actuator returns to the control directional control valves of an open-center valve through the opposite working line and system are always connected in series with flows back to the reservoir. each other, an arrangement whereby the system pressure lb e goes through each directional con- Several different types of directional control trol valve. Fluid is always allowed free pas- valves are used in conjunction with the open- sage through each control valve and back to the center system. One type is the manually en- reservoir until one of the control valies is gaged and manually disengaged. After this type positioned to operate a mechanism. valve ismanually movedtoan operating 52 Chapter 4BASIC SYSTEMS AND CIRCUIT DIAGRAMS position and the actuating mechanism reaches 2. Military Standard, Mechanical Symbols the end of its operating cycle, pump output con- for Aeronautical, Aerospacecraft, and Space- tinues until the system relief valve setting is craft Use, Part 2, MIL-STD-17B-2. reached. The relief ,valve then unseats and allows the fluid to flow back to the reservoir. Some of the more pertinent symbols used The system pressure remains at the pressure in fluid power systems have been selected from setting of the relief valve until the directional these two standards and are depicted in tables control valve is manually returned to the neu- 4-1 and 4-2. Symbols from MIL-STD-17B-2 tral position. This action reopens the open- are presented in table 4-1 and symbols from center flow and allows the system pressure to MIL-STD-17B-1 in table 4-2. drop to line resistance pressure. NOTE: The equipment symbols illustrated in table 4-1 show only the basic outline of each Another type of open-center directional con- component. It is stated in MIL-STD-17B-2 that trol valve is the manually engaged and pressure schematic diagrams should show a cutaway sec- disengaged. This type valve is similar to the tion of each component at least in schematic valve discussed in the preceding paragraph; form. however, when the actuating mechanism reaches While the symbols illustrated in this chapter the end of its cycle and the pressure continues are not all encompassing, they do provide a to rise to a predetermined pressure, the valve basis for the man working with fluid power sys- automatically returns to the neutral position tems to build upon. For more detailed informa- and, consequently, to open-center flow. tion concerning the symbols used in fluid power One of the advantages of the open-center diagrams, the above mentioned Military Stand- system is that the continuous pressurization of ards should be consulted. Additional information the system is eliminated. Since the pressure is concerning symbols and the reading of diagrams gradually built up after the directidnal control is contained in Blueprint Reading and Sketch- valve is moved to an operating.position, there ing, NavPers 10077 (Series). isverylittle shock from pressure surges. This provides a smooth operation of the actu- USE OF DIAGRAMS ating mechanisms; however, the operation is slower than the closed-center system in which As emphasized earlier in this chapter, in the pressure is available the moment the direc- order to troubleshoot fluid power systems intel- tional control valve is positioned. Since most ligently, the mechanic or technician must be applications require instantaneous operation, familiar with the system at hand. He must know closed-center systems are the most widely the function of each component in the system used. and have a mental picture of its location in relation to other components in the system. CIRCUIT DIAGRAMS These can beat be achieved by studying the diagrams of the system. As mentioned previously, the ability to read diagrams is a basic requirement for under- There are many types of diagrams; however, standing the operation of fluid power systems. those which are the most pertinent to fluid To understand the diagrams of a system, first power systems may be divided into two classes. requires a knowledge of the symbols used in These diagrams are usually referred to as pictorial and schematic diagrams and are usually 1 the schematic diagrams. provided by the manufacturer to aid the maintenance man in understanding and troubleshoot- SYMBOLS ing the system and equipment. There may be The Navy uses two Military Standards which instances where a schematic diagram will be list mechanical symbols that shall be used in shown as a pictorial diagram, or the diagram the preparation of drawings where symbolic may consist of a combination of the two. representation is desired. These two Military A diagram, whether itis a pictorial or Standards are as follows: schematic diagram, may be defined as a graphic 1. Military Standard, Mechanical Symbols representation of an assembly or system, in- (Other than Aeronautical, Aerospacecraft, and dicating the various parts and expressing the Spacecraft Use), Part 1, MIL-STD-17B-1. methods or principles of operation. As a general 53 58 FLUID POWER

Table 4-I.Aeronautical mechanical symbols.

H, I

BRAKE BUNGEE , A IR -0I 1111 1 11111 A DOWN (OR CLOSE)0000000A COUPLING, SELF-SEALING EMERGENCY PRESSURE dJ D HOSE CONNECTION (RIGID TUBING) CYLINDER, ACTUATING

ROSE, FLEXIBLE T 7. :4 :4'. :4 : :"*". D

RETURN

DEBOOSTER, BRAKE

SUPPLY FLUID (PUMP SUCTION) FAVAVAVAVAVAVAVAVAI

SUCTION GRAVITY 074 FILTER OR STRAINER SUPPLY PRESSURE U

UP (OR OPEN)

FITTING, SWIVEL VENT

I ACCUMULATOR

GAGE, PRESSURE

AIR BOTTLE, EMERGENCY GAGE AND SNUBBER, PRESSURE A

BRAKE CONTROL

IBC I

54 Chapter 4BASIC SYSTEMS AND CIRCUIT DIAGRAMS

Table 4-1.Aeronautical mechanical symbolsContinued.

VALVE, PRESSURE REGULATING PUMP, HAND (UNLOADING) AUTOMATIC 'lb V

PC ]R PUMP, POWER DRIVEN VALVE, PRESSURE REGULATING (UNLOADING) MANUAL T

R RESERVOIR

VALVE, RELIEF

VALVE, CHECK, AUTOMATIC VALVE, RE STR I CTOR, BOTH WAYS LEJ

VALVE, CHECK, VALVE, RE STR I CTOR , PARTIAL MANUAL ONE WAY

VALVE, SHUTTLE

VALVE, BRAKE CONTROL

R P B VALVE OR SELECTOR, R DIRECTIONAL CONTROL P U VALVE, GUN CHARGER CONTROL

55 60 FLUID POWER

Table 4-I.Aeronautical mechanical Table 4-2.--Mechanical symbols other symbols--Continued. than aeronautical--Continued. LINES TO RESERVOIR A- AIR HP - HANDPUMP BELOW FLUID LEVEL

B - BRAKE P - PRESSURE

BC- BRAKE CONTROL R - RETURN ABOVE FLUID D-DOWN(OR CLOSE) S - SUCTION (OR SUPPLY) LEVEL

F-FLUID (LIQUID) U - UP (OR OPEN)

Table 4-2.--Mechanical symbols other than aeronautical. II, II ',! PLUG OR PLUGGED CONNECTION LINES, WORKING

LINES, PILOT TESTING STATION .1 ,11,

LINES, LIQUID DRAIN OR AIR EXHAUST FLUID POWER TAKE-OFF STATION XT LINES, CROSSING 0

RESTRICTION, FIXED X

QUICK. DISCONNECT WITHOUT CHECKS > <

LINES, JOINING 1111 ,01, WITH CHECKS 1 DISCONNECTED

WITH ONE CHECK 0 1 < LINES, FLEXIBLE

WITH TWO CHECKS

FLOW, DIRECTION OF 11111111111° BASIC SYMBOL ENVELOPE

56 Chapter 4BASIC SYSTEMS AND CIRCUIT DIAGRAMS

Table 4-2.Mechanical symbols other than aeronauticalContinued.

PORTS FLUID MOTORS, ROTARY

LINES OUTSIDE ENVELOPE ARE NOT PART OF SYMBOL, BUT REPRESENT FLOW LINES APPROPRIATE SYMBOLS SHALL BE ADDED CONNECTED THERETO. TO INDICATE SHAFTS, CONNECTING LINES, AND METHOD OF CONTROL. *TYPE OF MOTOR SHALL BE INDICATED WITHIN BASIC SYMBOL BY APPROPRIATE SHAFTS, ROTATING LETTERS LISTED BELOW. MF FIXED DISPLACEMENT MOOSCILLATING MV VARIABLE DISPLACEMENT

ARROW INDICATES DIRECTION OF ROTA- TION BY ASSUMING IT IS ON NEAR SIDE OF SHAFT. SINGLE ACTING

PUMPS, HYDRAULIC I I

DOUBLE ACTING SINGLE END ROD

APPROPRIATE SYMBOLS SHALL BE ADnED TO INDICATE SHAFTS, CONNECTING LINES, AND METHOD OF CONTROL. * TYPE OF PUMP SHALL BE INDICATED WITHIN BASIC SYMBOL BY APPROPRIATE DOUBLE END ROD LETTERS LISTED BELOW. PF FIXED DISPLACEMENT PK KINETIC - CENTRIFUGAL PV VARIABLE DISPLACEMENT

COMPRESSORS, AIR FOR LIQUID,VENTED

APPROPRIATE SYMBOLS SHALL BE ADDED TO INDICATE SHAFTS, CONNECTING LINES, PRESSURIZED, AND METHOD OF CONTROL. RECEIVER FOR AIR OR OTHER OASES * TYPE OF COMPRESSOR SHALL BE INDI- CATED WITHIN BASIC SYMBOL BY APPRO- PRIATE'LETTERB LISTED BELOW. CF FIXED DISPLACEMENT CK KINETIC

57 FLUID POWER

MAIN GEM EMERGENCY UPLOCK RELEASE HANDLE (HYDRAUUC SERVICE CENTER FLOOR ACCESS)

DM A. NOSE GEAR UPLOCK AND CABLE RELEASE

NOSE GEAR EMERGENCY A UPLOCK RELEASE HANDLE (FLIGHT STATION PEDESTAL)

CONTROL LEVER SWITCH

SITAR B LANDING GEAR SELECTOR AND DETAILC BRAKE SHUTOFF VALVE LANDING GEAR EXTEND AND MAIN GEAR UPLOCK RETRACT CONTROL ASSEMBLY AND CABLE RELEASE

FP.39 Figure 4-4.Example of a pictorial diagram. rule, the various components are indicated by of a Navy aircraft.It shows the location of symbols (tables 4-1 and 4-2) in schematic each of the components within the aircraft on diagrams, while drawings of the actual com- the principal view. Each letter (A, B, C, etc.) ponents are used in pictorial diagrams. on the principal view refers to a detail view located elsewhere on the diagram. Each detail Pictorial Diagranui view is an enlarged drawing of a portion of the system identifying each of the principal com- Pictorial diagrams show general location, ponents. function, and appearance of parts and assemblies. This type diagram is sometimes referred Diagrams of this type are invaluable to main- to as an installation diagram. An example of a tenance personnel in identifying and locating pictorial diagram is shown in figure 4-4. This components, and understanding the principle of is.a diagram of the landing gear control system operation of the system. 58 Chapter 4BASIC SYSTEMS AND CIRCUIT DIAGRAMS

PRESSURE LINE I 1 IRETURN LINE

ABRAKE OPEN LINE

470BRAKE CLOSE LINE

-11111D- CHECK VALVE

SPEED BRAKES CONTROL VALVE

SPEEO BRAKE CONTROL SWITCH

ACTUATING CY LINOER CLOSED

FP.40 Figure 4-5.Example of schematic diagram.

Schematic Diagrams Figure 4-5 is an example of a schematic diagram of a speed brake system used in one The primary purpose of a schematic diagram model of Navy aircraft. Notice that this dia- is to enable the maintenance man to trace the gram does not indicate the physical location of flow of fluid, component by component. This the individual components within the aircraft, type of diagram does not necessarily indicate but does locate components with respect to the physical location of the various components, each other in the system. For example, the but does show the relation of each component to check valve in figure 4-5 is not necessarily the other components within the system. located immediately above the speed brake 59 FLUID POWER control valve. The diagram does indicate, how- SIMPLE HYDRAULIC AND PNEUMATIC ever, that the check valve is located in the SYSTEMS pressure line and that the pressure line leads into the control valve. There are severalapplicationsof fluid Schematic diagrams of this type are used power which require only a simple system; mainly in troubleshooting. Note that each line that is, a system which employs only a few com- (pressure,return,brakes open, and brakes ponents in addition to the five basic components. closed) is coded for easy reading and tracing A few of these applications are presented in the the flow. An explanation of the codes is given following paragraphs. The operation of these in the legend at the top righthand corner of the systems is briefly explained at this time so that illustration. On many diagrams of this type, the reader knows the purpose of the different different, colors are used to represent the var- components and can better understand the func- ious lines. Each component isidentified by tions and operation of these components as they name, and its location within the system can be are presented in the following chapters. Ex- ascertained by noting which lines lead into and amples of the more complex fluid power systems out of the component. are described in chapter 13.

The system illustratedin figure 4-5 is HYDRAULIC JACK actually one of several subsystems,ope rated by one power system. Although it is a rather com- The hydraulic jack is perhaps one of the plex system, it is shown at this point in the simplest forms of a fluid power system. By manual to emphasize the importance of using moving the handle of a small device, an indi- diagrams in maintaining fluid power systems. vidual can lift a load weighing several tons. A A brief description of the system including the small initial force exerted on the handle is general function of the major components follow. transmitted by means of, a fluidto a much larger area. This becomes readily understood by study- In tracing The flow of fluid through, the sys- ing figure 4-6. The small input piston has an tem, it can be seen that the first component in area of 5 square inches and is directly con- the pressure line is a check valve. The arrow nected to a large cylinder with an output piston on the valve indicates thedirection of flow having an area of 250 scjuare inches. The top of through the valve. This valve is located in the this piston forms a lift platform. pressure line for the purpose of preventing speed brake pressure from dropping when the main system pressure drops momentarily, as when another actuating system is operated. The next unit in the line is the speed brake control valve. This valve is a four-port valve which is connected to pressure, return, and to 41250 LBS. each actuating line. OUTPUT When the control valve is placed in brakes PISTON 250 50.IN. open position, fluid under pressure flows through 25 LBS. the brakes open line to the actuating cylinders, opening the speed brakes. Also in the brakes II& open line is a blow-back relief valve. This valve 5 SO. IN. is also connected to the return line. The purpose of this valve is to protect the speed brakes against excessive wind blasts. For example, if the pilot opened the speed brakes at an airspeed greater than that for which the speed brakes are designed, the blow-back relief valve would 5 LBS. PER open, allowing the excess fluid pressure to re- WINCH turn to the reservoir. Thus, the speed brakes FP.41 would be blown back to the closed position. Figure 4-6.Hydraulic jack. 60 Chapter 4BASIC SYSTEMS AND CIRCUIT DIAGRAMS

If a force of 25 pounds is applied to the input piston, it produces a pressure of 5 pounds per square inch in the fluid, that is, of course, if a sufficient amount of resistant force is acting OUTPUT against the top of the outputpiston. Disregarding PISTON friction loss, this pressure acting on the 250 4-25050.1N. . squareinch area of the output piston will support a resistance force of 1,250 pounds. In CYLINDER I other words, this pressure could overcome a force of slightly under 1,250 pounds. An input force of 25 pounds has been transformed into a working force of more than half a ton. How- ever, for thisto,be true requires that the distance traveled by the input piston be 50 times (A) as great as that traveled by the output piston. Thus, for every inch that the input piston moves, the output piston will move only one-fiftieth of an inch. .1250 LBS.

OUTPUT INPUT This would be idealif the output piston PISTON needed to move only a short distance. However, 4-250 SQ.IN. t)110?1"N. bk", in most instances, the output piston would have CYLINDER 2 25 LBS. to be capable of moving a greater distance to CYUND I 4, serve a practical application. The device shown RESERVOIR in figure 4-6 is not capable of moving the out- ,/VALVE 3 VALVE I put piston farther than that shown. Therefore, 5 LBS. some other means must be employed to raise PER SOANCH V_ V the output piston to a greater height. (B) The output piston can be raised higher and maintained at this height if valves are installed FP.42 as shown in figure 4-7. In this illustration the Figure 4-7.Hydraulic jack; (A) Up jack is designed so that it may be raised, low- stroke; (B) downstroke. ered, or held at a constant height. These results are attained by introducing a number of valves, the back pressure from the output piston, it and also a reserve supply of fluid to be used in opens valve (1), forcing fluid into the pipeline. the system. The pressure from cylinder (1) is thus transmitted into cylinder (2), where it acts to raise Notice that this system contains the five the output piston with its attached lift platform. basic componentsthe reservoir, cylinder (1) When the input piston is again raised, the pres- which serves as a pump, valve (3) which serves sure in cylinder (1) drops below that in cylinder as a directional control valve, cylinder (2) (2) causing valve-(1) to close. This prevents the which serves as the actuating device, and lines return of fluid and holds the output piston with to transmit the fluid to and from the different its attached lift platform at its new level. Dur- components. In addition, this system contains ing this stroke, valve (2) opens again allowing two valves (1) and (2) whose functions are ex- a new supply of fluid into cylinder (1) for the plained in the following discussion. next power (downward) stroke of input piston. As the input piston is raised (fig. 4-7 (A)), Thus, by repeated strokes of the input piston, valve (1) is closed_ by the back pressure from the lift platform can be progressively raised. the weight of the output piston. At the same To lower the lift platform, valve (3) is opened, time, valve (2) is opened by the head of the fluid and the fluid from cylinder (2) is returned to in the reservoir. This forces fluid into cylinder the reservoir. (1). When the input piston is lowered (fig. 4-7' (B)), a pressure is developed in cylinder (1). HYDRAULIC LIFT When this pressure exceeds the head in the res- The hydraulic lift functions very similar ervoir, it closes valve (2) and when it exceeds tothe hydraulicjack. Thebasic 61 FLUID POWER differehce between the two is the media used multiple piston system allows forces to be trans- to transmit the force. mitted to two or more pistons in the manner In the hydraulic lift, air pressure applied indicated in fic,ure 4-9. Note that the pressure to the surface of oil in a reservoir is transmit- set up by the force applied to the input piston ted by the ',1l to the output piston on the hydrau- (1) is transmitted undiminished to both output lic lift. (See fig. 4-8.) Using the compressed air pistons (2) and (3), and that the resultant force as the 1.77xit piston, the oil as a means of trans- on each piston is proportional to its area. The mitting'irse force, and the Lift attached to the multiplication of forces from the input piston output p.ster. a means is prxvided for lifting to each output piston is the same as that ex- a heavy vehicle. When the release valve is plained earlier. opened, the compressed air pr e s s u r e decreases, the oil returns to the reservoir, and The four-wheel brake system commonly used the lift is lowered. on automobiles is a practical application of the multirle piston system. (See fig. 4-1.8.) HYDitatiLIC BRAKES NOTE: Many of the modern automobiles are equipped with power (vacuu:..4 assist for the op- Thki hydraulic brake system used in the eration of the master cylinder. In the last few automobil. .ts a multiple piston system. A years, dual roaster tinders are incorporated

RELEASE INLET VALVE VALVE

FLOOR LEVEL

EARTH

COloPPI:ZSEP

FP.43 Figure 4-8.Hydraulic lift. 62 67 Chapter 4BASIC SYSTEMS AND CIRCUIT DIAGRAMS

automobiles operates similar to the system il- too LBS. lustrated in figure 4-10 and described in the following paragraphs. 1500 LBS. 3000 LBS. When the brake pedal is depressed, the pres- 2 PISTON I ..1.11CPISTON sure on the brake pedal moves the piston within 150 SO. IN. 10 SO. IN. the master cylinder, forcing the brake fluid from the master cylinder through the tubing and flexible hose to the wheel cylinders. The wheel PISTON 3 300 SO. IN. cylinders contain two opposed output pistons, each of which is attached to a brakeshoe fitted inside the brake drum. Each output piston pushes the attached brakeshoe against the wall 10 LR BS. 10 LBS. PER PE SO. INCH of the brakedrum, thus retarding the rotation of SO. IN. the wheel. When pressure on the pedal is released, the springs on the brakeshoes return the wheel cylinder pistons to their released positions. This action forces the displaced brake fluid back through the flexible hose and tubing to the master cylinder. The force applied to the brake pedal produces a proportional forceon each of the output pistons, which in turn apply the brake- F P.44 shoes frictionally to the turning wheels to re- Figure 4-9.Multiple piston system. tard rotation. If all eight output pistons were as a standard safety feature. In addition, some the same size, the force exerted on eachbrake- automobiles are equipped with disc brakes. shoe would be exactly thesame, provided However, the hydraulic system from the master frictional losses from the master cylinder to cylinder (s) to the wheel cylinders on most each wheel were equal. Likewise, if the brake-

WHEEL CYLINDER

MASTER CYLINDER

Figure 4-10.An automobile brake system. 63 68 FLUID POWER

shoes were identical,each wheel would be with the pedals. Airbrakes are a form of power equally retarded. brakes. Being more powerful than the hydraulic In actual brake designs, it is customary to brakes previously described, they must be use a greater piston area for the front wheels larger so that the individual brake will have a than for the rear. This is to compensate for greater braking area. the transfer of weight to the front of the auto- In an airbrake system (fig. 4-11), an engine- mobile when the brakes are applied. Due to driven air pump or compressor is used to com- inertia, the automobile tends to continue to move press air and force it into a reservoir, where when the brakes are applied. This increases it is stored under pressure and made available the concentration of weight at the front end and for operating the brakes. The compressed air decreases the concentration of weight at the is released from the reservoir to the brake rear. As a result, the froht brakes are required lines by an air valve operated by the brake to do more work than the rear and, therefore, pedal. This released air goes to the brake require a greater piston area. chambers located close to the wheel brakes. Each brake chamber contains a flexible dia- DOORSTOP phragm and plate. The force of the compressed airadmitted tothechambercausesthe A doorstop is a mechanism for reducing the diaphragm to move the plate, and operates the speed of a closing door. Doorstops vary in size, brake shoes through mechanical linkage. Con- shape, design, media, and the principle used to siderable force is available for braking since Control the door's movement. Hydraulic door- the operating pressure may be as high as 100 stops are normally used in controlling heavy pounds per square inch. All brakes on a vehicle, doors, while pneumatic doorstops are used for and on a trailer when one is used, are operated lighter type doors. The most commonly used by special regulating devices. doorstop consists of a cylinder and piston especially designed to cushion or buffer the quick AIR-OVER-HYDRAULIC movements of the closing door. BRAKE SYSTEM In the pneumatic doorstop, the cushion or buffer effect is attained by the transmission of The air-over-hydraulic brake system uses the motion to the piston rod, which thereupon the principle of the hydraulic brake to operate forces the piston head against the volume of the wheel brake cylinders and provide braking air within the cylinder. The air is compressed action. However, the hydraulic pressure for the and absorbs most of the motion, while graduated wheel brake cylinders is hot supplied from the holes in the cylinder perthit the air to escape master cylinder. Instead, there are two circuits. slowly and the piston and door are brought grad- The first leads from the air-hydraulic cylinder ually to rest. and admits air pressure which actuates this In the hydraulic type, a liquid is used to cylinder by moving an air piston that is con- cushion the motion of the door. Two cylinders nected to the hydraulic piston. The hydraulic are generally connected by an orifice through piston then applies the hydraulic pressure that which the liquid is forced as the door is opened produces the braking action. The air is admitted or closed. The door can thus close only as ra- by the action of valves controlled by the hy- pidly as the liquid can be moved through the draulicpressure from the master cylinder. orifice. An air-over-hydraulic brake system is shown in figure 4-12. Air pressure is supplied by a AIRBRAKES compressor and stored in reservoirs. The master cylinder is similar to the master cylinder Airbrakes were developed to enable the used inhydraulic brake systems. Also, the driver to apply sufficient braking action to the wheel brake cylinders and wheel brake wheels of high-speed, heavily loaded vehicles. construction are very similar to that used in With modern airbrake systems, the brake pedal conjunction with hydraulic brake systems. The is merely a controlling device for the braking essential difference between the straight hy- force, which is compressed air. On a vehicle draulic brake system and the air-over-hydraulic that tows trailers provided with airbrakes, an brake system lies in the air-hydraulic cylinder. additional hand controller is provided, which This cylinder consists of three essentials: a may be operated separately or in conjunction large diameter air piston; a small diameter 64 69. Chapter 4BASIC SYSTEMS AND CIRCUIT DIAGRAMS

BRAKE ICHAMBER

SLACK ADJUSTER el*

0.1..NOI RESERVOIR

RUBBER HOSE

COMPRESSOR

GOVERNOR QUICK BRAKE RELEASE PEDAL RELAY VALVE VALVE BRAKE VALVE

,aRUBBER HOSE

Figure 4-11.Typical airbrake system. FP.46 hydraulic piston in tandem with the air piston pressure inthe brake linesis always (both on the same rod); and a set of valves con- ina direct ratio to foot pressure on the trolled by hydraulic pressure from the master brake pedal. cylinder for admitting air into the air cylinder section of the air-hydraulic cylinder. Valve action varies with the amount of brake The air-hydraulic cylinder embodies an air pedal pressure. When heavy brake pedal pres- cylinder and a hydraulic cylinder in tandem, sure is applied by the operator for hard brak- each fitted with a piston with a common piston ing,the hydraulic pressure in the master rod between. The air piston is of greater di- cylinder (which operates the valves) causes ameter than the hydraulic piston. This differ- greater valve movement, and therefore the ence inareas of the two pistonsgives a valves admit more air pressure into the air- resultant hydraulic pressure much greater than hydraulic cylinder.This higher air pressure the air pressure admitted to the air cylinder: causes a stronger braking action. With only a Automatic valves, operated by fluid pressure light brake pedal pressure, the valves admit less from the master cylinder, control the air ad- air pressure into the air-hydraulic cylinder and mitted to the air cylinder. Thus, the fluid the braking action is lighter. 65 70t PTO WINDSHIELD WIPERS 411 DUMMY OMPRESSOR a-TO AIR HORN AIR UNES CUT-OUT COCK COUPLING ',GOVERNOR HYDRAUUC UNES TRAILER EMERGENCYCONNECTION UNE TO COUPLING HOSE TO FRONT WHEEL Ina BRAKES BRAKE PEDALMASTER CYLINDER SAFETY VALVE BRAKESWHEELREAR TO CUT-OUT COCK O5 LOW PRESSURE AIR-HYDRAUUC CYLINDER COUPUNG HOSE INDICATOR DUMMY AIR SUPPLY VALVE Figure 4-12.Mr-hydraulic brake system. SERVICE UNE TO TRAILER CONNECTION COUPLING FP. 47 CHAPTER 5

FLUID LINES AND CONNECTORS

The control and application of fluid power 7. The interior surface of the fluid lines would be impossible without a suitable means must be clean upon installation. After instal- of conveying the fluid from the power source to lation, lines must be kept clean by flushing or the point of application. Fluid lines used for purging the system regularly. Any source of this purpose must be designed and installed with contaminant must be eliminated. the same care applicable to the other components of the system. An improperly piped sys- To obtain these required results, attention tem can lead to serious power loss and/or must be given to the various types, materials, harmful fluid contamination.Therefore, the and sizes of lines available for fluid power sys- lines and connectors of fluid power systems are tems. The different types of lines and their designed with several basic requirements in application to fluid power systems are described mind. The following is a list of some of the in the first part of this chapter. The last part most important requirements which must be of the chapter is devoted to the various con- considered: nectors applicable to the different types of fluid lines. 1. The lines must be of sufficient strength to contain the fluid at the required pressure TYPES OF FLUID LINES and, in addition, must be strong enough to withstand the surges of pressure that may develop The three most common types of lines used in the system during any portion of the oper- in fluid power systems are pipe, tubing, and ating cycle. flexible hose. They are sometimes referred to as rigid, semirigid, and flexible. A number of 2. The lines must be of sufficient strength factors are considered when selecting the type to support components which may be mounted of lines for a particular fluid power system. in or on them. These factors include the type of fluid medium, 3. Terminal fittings (flanges, unions, etc.)' required pressure of the system, and the loca- must be provided at all junctions with parts or tion of the system. For example, heavy pipe components that require removal or replace- might be used for a large stationary fluid power ment. system, but comparatively lightweight tubing must be used in aircraft and missile systems 4. Line supports must be capable of damp- because weight and space are critical factors. ing shock waves caused by surges of pressure Flexible hose is required in some installations and changes in direction of flow. where units must be free to move relative to each other. 5. The lines should have a smooth interior surface to reduce turbulent flow of fluid. PIPE AND TUBING 6. The lines must be of the correct size to insure the required volume and velocity of In commercial usage, there is no clear dis- flow with the least amount of turbulence during tinction between pipe and tubing, since the cor- all demands of the system. Lines which provide. rect designation for each tubular product is return flow in hydraulic systems must be large established by the manufacturer. If the man- enough so as not to build up excessive back ufacturer calls a product pipe, it is pipe; if pressure. he calls it tubing, it is tubing. 67 FLUID POWER

In the Navy, however, a distinction is made The wall thickness of pipe increases as the between pipe and tubing. This distinction is schedule numbers increase. For example, a based on the method the tubular product is iden- reference to schedule 40 for a pipe with a nom- tified as to size. inal pipe size of 3 i ches indicates that the wall 3.500 - 3.068) Size thickness is 0.216 . (NOTE: The difference between the outside diameter and the There are three important dimensions of any inside diameter includes two wall thicknesses; tubular productoutside diameter (OD), inside therefore, this difference must be divided by 2 diameter (ID), and wall thickness. A tubular to obtain the wall thickness.) A reference to product is called tubing if its size is identified schedule 80 for a pipe of the same nominal size by actual measured outside diameter and by (3 inches) indicates that the wall thickness is actual wall thickness. A tubular product is cal- 0.300 inch. led pipe if its size is identified by a nominal Tubing differs from pipe in its size classifi- dimension and wall thickness. cation. Tubing is designated by its actual outside In the past, wall thickness of pipe was clas- diameter. (See table 5-2.) Thus, 5/8-inchtubing sified as standard (Std), extra strong (XS), and has an outside diameter of 5/8 inch. As indi- double extra strong (XXS). These designations cated in the table, tubingis available in a are still used to some extent. However, pipe variety of wall thicknesses. The diameter of is manufactured in a number of different wall tubing is often measured and indicated in 16ths. thicknesses and does not always fit into the Thus, No. 6 tubing is 6/16 or 3/8 inch, No. 8 standard, extra strong, and double extra strong tubing is 8/16 or 1/2, etc. classifications. In recent years, a trend has de- The foregoing is a brief description of the veloped toward the use of scheduled numbers standard ways to identify the size and wall to classify the wall thickness of pipe and pipe thickness of pipe and tubing. It should be noted, fittings. These scheduled numbers, established however, that pipe and tubing are sometimes by the American Standards Association, range identified in other ways. For example, some from 10 to 160 and cover 10 distinct sets of tubing is identified by ID rather than by OD and wall thickness. (See table 5-1.) Schedules 40 some pipe is identified by nominal pipe size, and 80 are comparable in wall thickness for by OD, by ID, and by actual measured wall most nominal pipe sizes to the standard and thickness. extra strong class, respectively. Schedule 160 covers pipe with the greatest wall thicknesses Materials in this classification, but are slightly thinner than the double extra strong class. The table The pipe and tubing used in fluid power sys- includes only pipe sizes up through 2-inch tems are commonly made from steel, copper, nominal size, although larger pipe sizes are brass, aluminum, and stainless steel. Each of available. Schedule 10 is for nominal pipe sizes these has their own distinct advantages or dis- larger than 12 inches. advantages in certain applications. A nominal dimension is close tobut not Steel pipe and tubing are relatively inexpen- necessarily identical withan actual measured sive, and are used in many hydrauliC and pneu- dimension. As indicated in table 5-1, a pipe with matic systems. Steel is used because of its a nominal size of 3 inches has an actual meas- strength, its suitability for bending and flanging, ured outside diameter of 3.500 inches. A pipe and its adaptability to high pressures and tem- with a nominal size of 2 inches has an actual peratures. Its chief disadvantage is its com- measured outside diameter of 2.375 inches. In paratively low resistance to corrosion. the larger size (above 12 inches), the nominal Copper pipe and tubing are sometimes used pipe size and the actual measured outside di- for fluid power lines. Copper has high resis- ameter are the same. For example, a pipe with tance to corrosion and is easily drawn or bent. a nominal pipe size of 14 inches has an actual It is unsatisfactory for high temperatures and measured outside diameter of 14 inches. Nom- has a tendency to harden and break due to stress inal dimensions are used in order to simplify and vibration. the standardization of pipe fittings, pipe taps, Aluminum has many of the characteristics and threading dies. and qualities required for fluid power lines. 68

. 73 Chapter 5-FLUID LINES AND CONNECTORS 4

Table 5-1. -Wall thickness schedule designations for pipe.

NominalPipe Inside diameter size OD Sched.Sched.Sched.Sched.Sched.Sched.Sched.Sched.'Sched.Sched. 10 20 30 40 60 80 100 120 140 160

1/8 0.405 0. 269 0.215 1/4 . 540 . 364 . 302 3/8 . 675 . 493 . 423 1/2 . 840 . 622 . 546 0. 466 3/4 1.050 . 824 . 742 . 614

1 1.315 1. 049 .957 . 815 1 1/4 1.660 1. 380 1.278 1. 160 1 1/2 1.900 1. 610 1.500 1. 338 2 2. 375 2. 067 1.939 1.689 2 1/2 2.875 2. 469 2.323 2. 125 3 3. 500 3.068 2.900 2.624 3 1/2 4.000 3. 548 3.364 4 4. 500 4. 026 3.826 3.624 3.438 5 5.563 5.047 4.813 4. 563 4.313 6 6.625 6.065 5.761 5. 501 5. 189 8 8.625 8. 125 8.071 7.9817.813 7.625 7.439 7. 189 7.0016.813 10 10. 750 10. 25010. 13610. 0209.750 9.564 9.314 9.064 8. 7508. 500 12 12. 750 12. 25012. 09011.93411.626 11. 37611.06410. 75010. 50010. 126

It has high resistance to corrosion and is easily Application drawn or bent. In addition, the outstanding characteristic of aluminum is its light weight. Since The material, the inside diameter, and the weight elimination is a vital factor in the de- wall thickness are the three primary consider- sign of aircraft, aluminum alloy tubing is used ations in the selection of lines for a particular in the majority of aircraft fluid power systems. fluid power system. Most of the advantages and Two aluminum alloys are in common use-alloy disadvantages of the metals used for the con- 5052 may be used for lines carrying pressures struction of fluid power lines were covered in up to 1,500 psi, and alloy 6061 for pressures up to 3,000 psi. Stainless steel tubing is used the preceding paragraphs. in certain areas of many aircraft fluid power The inside diameter of a line is important, systems. As a general rule, exposed lines-and since it determines the rate of flow that can be lines subject to abrasion or intense heat are passed through the line without loss of power made of stainless steel. due to excessive friction and heat. Velocity of 69 FLUID. POWER

Table 5-2. -Tubing size designation.

Tube Wall Tube Tube Wall Tube Tube Wall Tube OD thickness ID OD thickness ID OD thickness ID

0.028 0.069 0.035 0. 555 0.049 1. 152 1/8 .032 .061 .042 .541 .058 1. 134 . 035 .055 .049. . 527 .065 1.120 5/8 . 058 .509 1 1/4 . 072 1. 106 0.032 0. 1235 .065 . 495 .083 1. 084 3/16 .035 .1175 .072 .481 .095 1.060 . 083 .459 .109 1.032 . 095 .435 . 120 1.010 0.035 0. 180 .042 .166 0.049 0.652 0.065 1. 370 1/4 .049 . 152 .058 .634 .072 1.356 . 058 .134 .065 .620 .083 1.334 . 065 . 120 3/4 .072 .606 1 1/2 .095 1. 310 . 083 . 584 . 109 1. 282 0.035 0. 2425 . 095 . . 560 . 120 1.260 . 042 .2285 . 109 .532 . 134 1.232 5/16 .049 .2145 .058 . 1965 0. 049 0. 777 0.065 1.620 . 065 . 1825 .058 .759 .072 1.606 . 065 .745 .083 1.584 0.035 0. 305 7/8 . 072 . 731 1 3/4 .095 1. 560 .042 .291 .083 .709 . 109 1.532 3/8 .049 .277 .095 .685 . 120 1.510 . 058 .259 .109 .657 . 134 1.482 . 065 .245 0.049 0.902 0.065 1.870 0.035 0.430 .058 . 884 .072 1. 856 .042 .416 .065 .870 .083 1.834 .049 .402 .072 .856 2 .095 1.810 1/2 .058 .384 1 .083 . 834 . 109 1. 782 .065 .370 .095 . 810 .120 1. 760 .072 .356 .109 .782 .134 1.732 .083 .334 .120 .760 .095 .310 a given flow is less through a large opening the stronger the metal, the higher the bursting than through a small opening. If the inside di- pressure. However, the greater the inside diameter of the line is too small for the amount ameter for a given wall thickness, the lower of flow, excessive turbulence and friction heat the bursting pressure, because force is the cause unnecessary power loss and overheated product of area and pressure. Industrial activ- fluid. ities recommend that rigid lines should have a bursting pressure which provides a safety The wall thickness, the material used, and factor of at least eight; that is, the rated burst- the inside diameter determine the burstingpres- ing pressure should be at least eight times sure of a line or fitting. The greater the wall greater than the maximum working pressure in thickness in relation to the inside diameter and the system. 70 Chapter 5FLUID LINES AND CONNECTORS

The manufacturers of pipe and tubing usually supply charts, graphs, or tables which aid in the selection of the proper lines for, fluid power systems. These tables and charts use different methods of deriving the correct sizes of pipe and tubing. Regardless of the method, some provision must be made forcorrelating the strength of the line in terms of bursting pressure with the inside diameter in terms of flow capacity at recommended velocity. Fluid power systems are designed as com- pactly as practicable, in order to keep the connecting lines short. Every section of line should be anchored securely in one or more places so that neither the weight of the line northe effects FP.49 of vibration are carried on the joints. The aim Figure 5-2.Ideal bend radius. should be to minimize stress throughout. Lines should normally be kept as short and While friction head increases markedly for free of bends as possible.However, tubing sharper curves than this, it also tends to in- should not be assembled in a straight line, crease up to a certain point for gentler curves. because a bend tends to eliminate strain by ab- The increases in friction in a bend with a radius sorbing vibration and also compensates for ther- of more than about 3 pipe diameters result mal expansion and contraction.Bends are fromincreased turbulence near the outside preferred to elbows, because bends cause less edges of the flow. particles of fluid must travel of a power loss. A few of the correct and incor- a longer distance in making the change in direct methods of installing tubing are illustrated rection. When the radius of the bend is less in figure 5-1. than about 2 1/2 pipe diameters, the increased pressure loss is due to the abrupt change in the Bends are described in terms of the ratio direction of flow, especially for particles near of the radius of the bend to the inside diameter the inside edge of the flow. of the tubing or pipe. The ideal bend radius is 2 1/2 to 3 times the inside diameter, as shown Cutting and bending of pipe and tubing are in figure 5-2. For example,if theinside covered in Basic Handtools, NavPers 10085 diameter of a line is 2 inches, the radius of the (Series), in other applicable Rate Training Man- bend should be between 5 and 6 inches. uals, and in applicable technical publications.

1=11RIGHT RIGHT RIGHT

Icto _12.3 WRONG WRONG WRONG FP.48 Figure 5-1.Correct and incorrect methods of installing tubing. 71 FLUID POWER

FLEXIBLE HOSE RUBBER.Flexible rubber hose consists of a seamless synthetic rubber tube covered with Hose is used in fluid power systems where layers of cotton braid and wire braid, and an there is a necessity for flexibility, such as con- outer layer of rubber-impregnated cotton braid. nections to actuating units that move while in The inner tube is designed to withstand the operation, or to units attached to a hinged por- attack of the material which passes through it. tion of theequipment.Itisalso used in The braid, which may consist of several layers, locations that are subjected to severe vibrations. is the determining factor in the strength of the For example, flexible hose is often used for hose. The cover is designed to withstand ex- connections to and from the pump. The vibra- ternal abuse. tion that is set up by an operating pump would ultimately cause rigid and semirigid lines to TEFLON.Teflon hose is a flexible hose fail. designed to meet the requirements of higher operatingpressuresandtemperaturesin Sizes present fluid power systems. This type hose consists of a chemical resin, which is processed The size of flexible hose is identified by a and extruded into tube shape to a desired size. number which refers to the equivalent tubing It is covered with stainless steel wire which is size; for example, No. 8 flexible hose is equiv- braided over the tube for strength and protection. alent to No. 8 tubing. The No. 8 tubing hasan Teflon hose is unaffected by all fluids pres- outside diameter of 1/2 inch (8/16). The inside ently used in fluid power systems. It is inert diameter of No. 8 hose will not be 1/2 inch; it to acids, both concentrated and diluted. Certain will be slightly smaller to allow for wall thick- Teflon hose may be used in systems where op- ness. The actual inside diameter of both the erating temperatures range from -100° F to hose and tubing is the same. As long as the + 500°F. Teflon is nonflammable; however, number of the hose corresponds to the number where the possibility of open flame exists, a of the tubing, the proper size is being used. special asbestos fire sleeve should be used. The size, along with other information, is Teflon hose will not absorb moisture. This, usually stenciled on the outside of the hose. together with its chemical inertness and anti- This information includes the Military Specifi- adhesive characteristics, makes it ideal for cation of the hose followed by a dash number, missile fluid power systems where noncontam- which is thesize.Inaddition, the year of ination and cleanliness are so essential. manufacture and the manufacturer's symbolare also incluvied. 11' the hose is assembled with end Application fittings, the length of the hose is also included. This information appears at intervals of not Flexible hose is available in four pressure more than 9 inches and is connected by a series rangesextra-high-pressure,high-pressure, of dots or dashes. The continuous line of sten- medium-pressure,and low-pressure. Extra- ciled information and dots or dashes also indi- high-pressure hose is used in fluid power sys- cates the natural lay of the hose. Onsome of tems with normal operating pressures in excess the newer types of hose, such as Teflon (discus- of 3,000 psi. High-pressure hose is used in sed later), this information is placed on a metal systems with normal operating pressures up to tag and attached to the hose. and including 3,000 psi. Medium-pressure hose is used with a wide range of pressures (approx- Materials imately 300 psi to 3,000 psi); however, it must be emphasized that the maximum operating In regards to material, there are two types pressure for a particular medium-pressure of flexible hose used in fluid power systems. hose depends upon its size. For example, No. 12 These two types of material are rubber and hose is limited to pressures up to 1,500 psi, Teflon. Although flexible hose made of rubber No. 8 hose to 2,000 psi, and No. 4 hose to 3,000 is the type most commonly used, Teflon has psi. The maximum operating pressure for low- many of the desired characteristics for-certain pressure hose (up to approximately 600 psi) is applications. These two types of materialsare also determined by the size of the hose. The described in the following paragraphs. use of low-pressure hose in fluid power systems 72 Chapter 5FLUID LINES AND CONNECTORS is limited.Itis used in some low-pressure its length. This stripe should not tend to spiral pneumatic systems and as exhaust lines and around the hose. (See fig. 5-3.) drain lines in some high-pressure fluid power Flexible hoseshould be protected from systems. chafing by wrapping lightly with tape, but only Extra-high-pressure hose and some high- where necessary. pressure hose are available only in complete The minimum bend radius for flexible hose assemblies with factory installed end fittings. varies according to size and construction of Some high-pressure hose is available in bulk ihe hose and the pressure under which the sys- form and can be fabricated with end fittings by certain designated activities which have the re- tem operates. Current applicable technical pub- quired special tools and equipment. Medium- licationscontaintables and graphs showing and low-pressure hose are available in bulk minimum bend radii for the different types of and are usually fabricated locally. The fabri- installations. Bends which are too sharp will cation of hose assemblies is covered in ap- reduce the bursting pressure of flexible hose plicable Rate Training Manuals and technical considerably below its rated value. publications. Flexible hose should be installed so that it will be subjected to a minimum of flexing during Flexible hose must not be twisted on instal- operation. Support clamps are not necessary lation, since this reduces the life of the hose with short installations; but with hose of con- considerably and may cause the fittings to loosen siderable length (48 inches for example), clamps as well. Twisting of the hose can be determined should be placed not more than 24 inches apart. from theidentification stripe running along Closer supports are desirable and in some cases required. A flexible hose must never be stretched 10E1=ftwo0=1:M3(11:=C3D3 tight between two fittings. About 5 to 8 percent WRONG of the total length must be allowed as slack to RIGHT provide freedom of movement under pressure. When under pressure, flexible hose contracts in length and expands in diameter. Examples of correct and incorrect installations of flexible hose are illustrated in figure 5-3. Teflon hose should be handled carefully during removal and installation. Some Teflon hose is preformed during fabrication. This type hose tends to form itself to the installed position in the system. To insure its satisfactory function and reduce the likelihood of failure, the following rules should be observed when working with Teflon hose: 1. Do notexceed recommended bend limits. 2. Do not exceed twisting limits. 3. Do not straighten a bent hose that has taken a permanent set. 4. Do not hang, lift, or support objects from Teflon hose. TYPES OF CONNECTORS Some type of connector must be provided to attach the lines to the components of the system Figure 5-3.Correct and incorrect FP.50 and to connect sections of line to each other. installation of flexible hose. There are many different types of connectors 73 78 FLUID POWER provided for this purpose. The type of connector Threaded connectors are made with standard required for a specific system depends on female threading cut on the inside surface. The several factors.One determining factor, of end of the pipe is threaded with outside (male) course, is the type of fluid line (pipe, tubing, threads for connecting. Standard pipe threads or flexible hose) used in the system. Other de- are tapered slightly to insure tight connections. termining factors are the type of fluid medium The amount of taper is approximately three- and the- maximum operating pressure of the fourths of an inch'in diameter per foot of thread. system. Some of the most common types of (See fig. 5-5.) connectors are described in the following paragraphs.

THREADED CONNECTORS There are several different types of threaded connectors, some of which are described later. In the type discussed in this section, both the connector and the end of the fluid line (pipe) are threaded. This type connector is used in some low-pressure fluid power systems. In Navy systems, they are usually made of steel, copper, or brass, and in a variety of designs, some of which are illustrated in figure 5-4.

TAIL PIECE FP.52 Figure 5-5.Standard taper for pipe threads. CAP CCU LINO Metal is removed when a pipe is threaded, thinning the pipe and exposing new and rough surfaces for chemical action. Corrosion agents PLUG NIPPLE work more quickly at such points than else- OROUND SURFACE JOINT where. If pipes are assembled with no protec- TAIL PIE tive compound on the threads, corrosion sets

LONG NIPPLE in at once and the two sections stick together UNION so that the threads seize when disassembly is attempted. The result is damaged threads and REDUCING COUPUNS pipes. To prevent seizing, a suitable pipe thread compound is sometimes applied to the threads

REDUCING as illustrated in figure 5-6. The two end threads BUSHING

TEE

ELIDOW RETURN SEND 451,ELIOT! FP.53 FP.51 Figure 5-6.Application of protective Figure 5-4.Threaded pipe connectors. compound to pipe threads. 74 Chapter 5FLUID LINES AND CONNECTORS are kept free of compound so that it will not manufactured for flange joints, such as the contaminate the fluid. Pipe compound, when threaded connectors illustrated in figure 5-4. improperly applied, may get inside the lines Suitable gasket material must be used between and components and damagepumps,and control the flanges. equipment. This has been such a problem that many manufacturers forbid the use of any com- WELDED CONNECTORS pound when fabricating the piping for fluid power systems. The subassemblies of some fluid power sys- Another type of material used on pipe tems are connected by welded joints, especially threads is sealant tape. This tape, which is in high-pressure systems which utilize pipe for made of Teflon, provides an effective means of fluid lines. The welding is accomplished ac- sealing pipe connections and eliminates the cording to standard specifications which define necessity of torquing connections to excessively the materials and techniques. There are three high values in order to prevent pressure leaks. generalclassesof welded jointsbutt-weld, It also provides for ease of maintenance when- fillet-weld, and socket-weld. (See fig.5-8.) ever it is necessary to disconnect pipe joints. The tape is applied over the male threads, leaving the first thread exposed. After the tape is pressed firmly'against the threads, the joint is connected.

FLANGE CONNECTORS Bolted flange connectors (fig. 5-7) are suitable for most pressures now in use. The flanges' are attached to the piping by welding, brazing, tapered threads (for some low-pressure sys- BUTT-WELD JOINT BACKING RING FILLET-WELD JOINT tems), or rolling and bending 'into recesses. Those illustrated are the most common types of flange joints used. The same types of standard fitting shapes (tee, cross, elbow, etc.) are

WELDING SLEEVE JOINT

45 DEGREE BUTT - WELDING GASKET ELBOW

BUTT-WELDING TEE BACKING RING

FITTING TUBE WELD METAL

LOW-PRESSURE TYPE HIGH-PRESSURE TYPE °ErWITH RING GASKET VAN STONE OR LAPPED

WELD METAL

WELDING NECK FLANGE SLIP-ON FLANGE TUBE BEING INSERTED COMPLETED JOINT BUTT - WELDED TO TUBE WELDED FRONT AND BACK SOCKET-WELDING FITTINGS FP.54 FP.55 Figure 5-7.Four types of bolted Figure 5-8.Various types of flange connectors. welded joints. 75 80 FLUID POWER

AREA OF HEATING CLEARANCE GAP

BAND SILVER BRAZING ALLOY INSERT SCRIBE LIN':, AREA OF SHOULDER STOP CLEARANCE DEPTH OF SOCKET HEATING GAP AL PLUS 1 INCH 1`7:17i'

SILVER BRAZING ALLOY INSERT

SILVER BRAZING ALLOY FILLET DEPTH OF SOCKET FLANGE

TORCH FLAME POINTING AT AN ANGLE TOWARDS TUBE

FP.56 Figure 5-9.Silver-brazed connectors.

BRAZED CONNECTORS The connector consists of a fitting, a sleeve, and a nut, as illustrated in figure 5-10. Silver-brazed connectors (fig. 5-9) are commonly used forjoining nonferrous (copper, brass, etc.) piping in the pressure and temperature range where their use is practical. These practical factors limit the use of this type connector to lines not exceeding 425° F; for cold £AAAAAA lines, these fittings may be used for pressures up to 3,000 psi. The alloy is melted by heating the joint with an oxyacetylene torch. Thiscauses the alloy insert to melt and fill the few thousandths of an inch annular space between the pipe and the fitting. A fitting of this type which has been removed from a piping system can be rebrazed intoa system, as in most cases sufficient alloy re- TUBING mains in the insert groove for a second joint. New alloy inserts may be obtained for fittings FP.57 which do not have sufficient alloy remaining in Figure 5-10.Flared-tube connector. the insert for making a new joint. The fittings are made of steel, aluminum FLARED CONNECTORS alloy, or bronze. The fittings should be of the same material as that of the sleeve, nut, and Flared connectors are commonly used in tubing. For example, use steel connectors with fluid power systems containing lines made of steeltubing and aluminum alloy connectors tubing. These connectors provide safe, strong, with aluminum alloy tubing. Fittings are made dependable connections without the necessity inunions, 45-degree and 90-degree elbows, of threading, welding, or soldering the tubing. tees, and various other shapes. Figure 5-11 78 Chapter 5FLUID LINES AND CONNECTORS Mao

ELBOW ELBOW ELBOW

,.t.._. :-.--11 ,-

giekift /kW; Mr(g*:

FLARED TUBE AND FLARED TUBE AND FLARED TUBE 90° PIPE THREAD 90° PIPE THREAD 45°

TEE TEE TEE

f_----g-

Ofr,t; (04; 1..!,,

FLARED TUBE 3 FLARED TUBE PIPE THREAD ON SIDE PIPE THREAD ON RUN

CROSS NIPPLE loot, 47(0.4-

S'-'

FLARED TUBE FLARED TUBE FLARED TUBE AND PIPE THREAD

UNION ELBOW TEE *ese\ ., li-, 7! trO (-,-,-, ;-%-"---- ,.fre(fk; 0,-

FLARED TUBE FLARED TUBE FLARED TUBE BULKHEAD AND UNIVERSALBULKHEAD UNIVERSAL 90°BULKHEAD AND UNIVERSAL

FP.58, Figure 5-11.Flared-tube fittings. 77 FLUID POWER illustrates some of the most common fittings crosses also are availablein several used with flared connectors. different combinations. Tees, crosses, and elbows are self-explan- atory. Universal and bulkhead fittings can be Tubing used with this type connector must mounted solidly with one outlet of the fitting be flared prior to assembly. The nut fits over extending through a bulkhead and the other out- the sleeve and when tightened, draws the sleeve let (s) positioned at any angle. Universal denotes and tubing flare tightly against the male fitting the fact that the fitting can assume the angle to form a seal. required for the specific installation. Bulkhead The male fitting has a cone-shaped surface denotes that the fitting is long enough to pass with the same angle as the inside of the flare. through a bulkhead and is designed in such a The sleeve supports the tube so that vibration manner that it can be secured solidly to the does not concentrate at the edge of the flare, bulkhead. Figure 5-12 illustrates a universal and distributes the shearing action over a wider bulkhead-mounted fitting. area for added strength. Tube flaring is covered in Basic Handtools, NavPers 10085 (Series), and other applicable Rate Training Manuals. Correct and incorrect methods of installing flared-tube connectors are illustrated in figure 5-13. Tubing nuts should be tightened with a torque wrench to the value specified in applicable technical publications.

1 1/2

DO NOT DEFLECT INTO KAM REPLACE TUILI ASSIMIVI

**CORRECT WILL DAMAGE FLARE OR THRIADS,OR CAUSE SLEEVE TO CRAcK UNDER VIBRATION IF TIGHTENED

HOLD NUT WITH WRENCH WHILE TURNING FITTING INCORRECT MAT MU ON OR DISTORT FLARE If TIGHTENED

COORKTITNTTW AND HOMINID

OGG CLEARANCE BETWEEN FLARE AND SHOULDER FP.59 WON TIGHTENING Figure 5-12.Universal bulkhead- mounted fitting. FP.60 Figure 5-13.Correct and incorrect methods For connecting to tubing, the ends of the of tightening flared connectors. fittingsare, threaded with straight machine threads to correspond with the female threads of the nut. In some cases, however, one end of If an aluminum alloy flared connector leaks the fitting may be threaded with tapered pipe after tightening to the required torque, it must threads _to fitthreaded,ports in pumps, valves, not be tightened further. Overtightening may and other coraponents: Several of these thread severely damage or completely cut off the tub- combinations are shown in figure 5-11. For ing flare or may result in damage to the sleeve example, unions have straight machine threads or nut. The leaking connection must be disassembled and the fault corrected. on both ends, whileelbows . have- straight machine .threads on one end, but may have If a steel tube connection leaks, it may be either tapered pipe threads or straight machine tightened 1/6 turn beyond the specified torque threads on the otherend...Tees and in an attempt to stop the leakage; then if 78 Charter 5FLUID LINES AND CONNECTORS unsuccessful, it must be disassembled and flared-tube fittings. (See fig. 5-11.) An example repaired. of a flareless-tubefitting is illustrated in figure5-15.The fitting has a counterbore Some of the causes of leaking flared con- shoulder for the end of the tubing to rest nectors are as follows: against. The angle, of the counterbore causes 1. Flare distorted into nut threads. the cutting edge of the sleeve or ferrule to cut 2. Sleeve cracked. into the outside surface of the tube when the 3. Flare cracked or split. two are assembled together. 4. Flare out of round. 5. Flare eccentric to tube OD. 6. Insideofflare rough or scratched. 7. Fitting cone rough or scratched. 8. Threads of the fitting or nut dirty, damaged, or broken. ELBOW Undertightening of connections may be serious, as this can allow the tubing to leak at the connector because of insufficient grip on the flare by the sleeve. The use of a torque wrench will prevent undertightening. CAUTION: A nut should never be tightened when there is pressure in the line as this will tend to damage the connection without adding any appreciable torque to the connection.

BITE TYPE CONNECTORS FLARELESS TUBE 90° Bite type connectors are commonly refer- FP.62 red to as flareless-tube connectors. This type Figure 5-15.Flareless-tube fitting. connector eliminates all tube flaring, yet provides a safe, strong, and dependable tube connection. This connector consists of a fitting, The nut presses on the bevel of the sleeve a sleeve or ferrule, and a nut. (See fig. 5-14.) and causes it to clamp tightly to the tube. Re- sistance to vibration is concentrated at this point rather than at the sleeve cut. When fully tightened, the sleeve or ferrule is bowed slightly at the midsection and acts as a spring. This SLEEVE PILOT TUBE spring action of the sleeve or ferrule maintains SLEEVE a constant tension between the body and the nut and thus prevents thenut from loosening. Prior to the installation of a new flareless- tube connector, the end of the tubing must be square,concentric, and free of burrs. Forthe connection to be effective, the cutting edge of the sleeve or ferrule must bite into the per- SLEEVE. CUTTING iphery of the tube. This is accomplished by EDGE presetting the sleeve or ferrule on the tube CONNECTOR NUT using a presetting tool which has the same FP.61 dimensions as the fitting body, and which can Figure 5-14.--Flareless-tube connector. be obtained from the fitting manufacturer. ,If a presetting tool is not available, a suitable male-thread fitting may be used. If a fitting Flareless-tube fittings are available in many must be used, a steel fitting is preferred for of the same shapes and thread combinations as this operation. If an aluminum fitting is used 79 FLUID POWER

as a preset tool,it should not be reused in this is the maximum the fitting may be tight- the system. ened without thepossibility of permanently Applicable Rate Training Manuals, technical damaging the sleeve or the tube. publications, or specifications may be consulted for the proper procedures to be followed for CONNECTORS FOR FLEXIBLE HOSE. presetting flareless-tube connectors. After presetting, the connector is disas- As previously stated, extra-high-pressure sembled for inspection. If the sleeve or ferrule and some high-pressure flexible hose are avail- issatisfactorily installed, the connectoris able only in complete assemblies with factory ready for final assembly in the system. When installed end fittings. The procedures involved making the final assembly in the system, the in the fabrication of low-, medium-, and high- following installation procedures should be fol- pressure hose are contained in applicable Rate lowed: Training Manuals and in applicable technical publications. 1. Lubricate all threads with a liquid that The end fittings most commonly used on is compatible with the fluid that is to be used flexible hose used in fluid power systems are in the system. either for the flared or flareless type con- 2. Place the tube assembly in position and nectors. Hose is also available with fittings check for alignment. adaptable to flange type connectors. Examples 3. Tighten thenut by hand until an of end fittings for flexible hose are illustrated increase in resistance to turning is encountered. in figure 5-16. This indicates that the sleeve or ferrule pilot has contacted the fitting. QUICK-DISCONNECT COUPLINGS 4. If possible, use a torque wrench to tighten flareless tubing nuts. Torque values for Quick-disconnectcouplingsof the self- specific installations are usually listed in the sealing type are used at various points in many applicable technical publications. If it is not fluid power systems.. These couplings are in- possible to use a torque wrench, use the fol- stalled at locations where frequent uncoupling lowing procedures for tightening nuts. of the lines is required for inspection, test, and maintenance. Quick-disconnect couplings After the nut is handtight, turn the nut 1/6 are also commonly used in pneumatic 'systems turn (one flat on a hex nut) with a wrench. Use to connect sections of air hose together and to a wrench on the connector to, prevent it from connect tools to the air pressure lines. This turning while tightening the nut. After the tube provides a convenient method of attaching and assembly is installed, the system, should be detaching tools and sections of lines without pressure tested. Should a connection leak, it losing pressure. is permissible to tighten the nut an additionr.1 1/6 turn (making a total of 1/3 turn). If, after Quick-disconnect couplings provide a means tightening the nut a total of 1/3 turn, leakage of quickly disconnecting a line without the loss still existu, the assembly should be removed of fluid from the system or entrance of foreign and the components of the assembly inspected matter into the system. Several types of quick- for scores, tracks, presence of foreign mate- disconnect couplings have been designed for use rial, or damage from overtightening. in fluid power systems. Figure 5-17 illustrates a coupling that is used with portable pneumatic NOTE: Overtightening a flareless-tube nut tools. The male section is connected to the drives the cutting edge of the sleeve or ferrule tool or to the line leading from the tool. The deeply into the tube, causing the tube to be female section, which contains the shutoff valve, weakened to the point where normal vibration is installed in the pneumatic line leading from could cause tile tube to shear. After inspection the power source. These connectors can be (if no discrepancies are found), reassemble the separated or connected by very little effort on connection and repeat the pressure test pro- the part of the operator. cedures. The most common quick-disconnect coupling CAUTION: Do not in any case tighten the- for hydraulic systems consists of two parts, nut beyond 1/3 tura (two flats on the hex nut); held together by a union nut. Each part contains 80

7.4 4.R.:434i$ Chapter 5FLUID LINES AND CONNECTORS

SWIVEL NUT FITTING SWIVEL NUT FITTING NIPPLE

IL=1JILJEL

, "42..revo .2

SWIVEL NUT FITTING 45° NV' SWIVEL NUT FITTING NIPPLE 45°

FLANGE FITTING FLANGE FITTING NIPPLE

mily...171tIroPtiPmr1t

FLANGE FITTING 45° FLANGE FITTING NIPPLE 45°

FP.63 Figure 5- 16. Flexible hose end fittings. a valve which is held open when the coupling ling is disconnected, a spring in each part is connected, allowing fluid to flow in either closes the valve, preventing the loss of fluid direction through the coupling. When the coup- and entrance of foreign matter. 81 FLUID POWER

coupling by turning the nut. The amount the nut must be turned varies with different styles of couplings. For one style, a quarter turn of the union nut locks or unlocks the coupling. For another style, a full turn is required. Some couplings require wrench tightening; others may be connected and disconnected by hand. Some installations require that the coupling be safe- tied with safety wire, others do not require any FP.64 form, of safetying. Because of these individual Figure 5-17.Quick6disconnect coupling differences, all quick-disconnects should be infor air lines. stalled in accordance with the instructions in the applicable technical publications. The union nut has a quick-lead thread One type of quick-disconnect coupling fox which permits connecting or disconnecting the hydraulic systems is illustrated in figure 5-18

( A )

2 34 5 6 7 8

Immar SI HALF - B)

1.Tubular valve. 6.Protruding nose. 2.Valve spring. 7.Poppet valve. 3.0-ring packing.8.Lock spring. 4.Sleeve. '".4 9.Mounting flange. 5.Union nut teeth. FP.65 Figure 5-18.Typical quick-disconnect coupling. 82 Chapter 5FLUID LINES AND CONNECTORS

HEAD OF PROTRUDING NOSE TUBULAR VALVE 7 LOCK TEETH

MOUNTING FLANGE UNION NUT ASSEMBLY LOCKSPRING ASSEMBLY

LOCKSPRING ASSEMBLY UNLOCKED (PARTS ARE NOT FLUSH) i -INCH MINIMUM GAP 16 LOCK TEETH SPRING PLATE (HIDDEN)

SPRING RETAINER COUPLINGS IMPROPERLY TIGHTENED FINGERS (TEETH DISENGAGED) COUPLINGS PROPERLY TIGHTENED

FP.66 Figure 5-19.Quick-disconnect coupling properly and improperly tightened.

Each coupling consists of two halves, referred The locking action may be followed by re- to as the S1 half and the S4 half. When discon- ferring tofigure5-18.Positive locking is 3 nected, the union nut remains with the S1 half. assured by the locking spring (8) with teeth The 34 half has a mounting flange (9) for at- which engage the ratchet teeth on the union nut tachingto a bulkhead or other structural (5) when the coupling is fully connected. The member. lock spring automatically disengages when the union nut is unscrewed. An 0-ring packing (3) All parts referred to in the following dis- seals against leakage as the coupling halves are cussion are identified in figure 5-18. The two joined. Positive opening of the valves occurs as halves of the coupling may be connected by the halves are connected. placing the tubular valve (1) within the protruding nose (6) of the mating half and rotating When the coupling halves are joined, the the union nut in a clockwise direction. The protruding nose (6) of the S4 half contacts the union nut must be rotated until the teeth (5) sleeve (4) of the Si half. Simultaneously, the fully engage the lock spring (8). A properly head of the tubular valve (1) contacts the face tightened coupling will have compressed the lock of the poppet valve (7), thus preventing foreign spring until a 1/16-inch minimum gap exists matter from entering the system. between the inside lip of the spring retainer' fingers and the spring plate. Figure 5-19 shovis Tightening' the union pulls the coupling halves the coupling both properly tightened and im- together. This causes the nose of the 34 half to properly tightened. push' the sleeve into the Si half, .uncovering 83 FLUID POWER the ports of the tubular valve. At the same the fluid combines into one outlet line to the time, the head of the tubular valve depresses reservoir. Some manifolds are equipped with the poppet valve. the check valves,relief valves, filters, etc., When the coupling halves are fully connected, required for the system. In some cases, the the sleeve and poppet valve have reached the control valves are mounted on the manifold in positions shown in the lower left-hand view of such a manner that the ports of the valves are figure 5-19. The nose of the 54 half has engaged connected directly to the manifold. the 0-ring packing of the Si half, providing a Manifolds are usually one of three types positive seal. sandwich, cast, or drilled. The sandwich type is constructed of three or more flat plates. The Dust caps are usually provided with quick- center plate (or plates) is machined for pas- disconnect couplings to cover the ends of the sages, and the required inlet and outlet ports coupling when it is disconnected. are drilled into the outer plates. The plates are then bonded together to provide a leakproof MANIFOLDS assembly. The cast type of manifold is designed Some fluid power systems are equipped with with cast passages and drilled ports. The cast- manifolds in the pressure supply and/or return ing may be iron, steel, bronze, or aluminum, lines. A manifold is a fluid conductor which depending upon the type of system and fluid provides multiple connection ports. Manifolds medium. In the drilled type manifold, all ports serve to eliminate piping, to reduce joints which and passages are drilled in a block of metal. are often a source of leakage, and to conserve A simple manifold is illustrated in figure space. For example, manifolds may be used in 5-20. This manifold contains one pressure inlet systems that contain several subsystems. One port and several pressure outlet ports. Since common line connects the pump to the mani- any number of the outlet ports can be blocked fold. There are outlet ports in the manifold to off with threaded plugs, this type manifold can provide connections to each subsystem. A sim- beadaptedtosystems containing various ilar manifold may be used in the return system. numbeis of subsystems. A thermal relief valve Lines from the control valves of the subsystem may be incorporated in this manifold. In this connect to the inlet ports of the manifold where case, the port labeled (T) is connected to the

OUTLET PORTS

RELIEF VALVE RETURN PORT

OUTLET PORTS

PRESSURE INLET PORT FP.67 Figure 5-20.Fluid manifold. 84 Chapter 5FLUID LINES AND CONNECTORS return line to provide a passage for the re- sure, the relief valve opens and allows the fluid lieved fluid to flow to the reservoir. to flow from the pressure side of the manifold to the return side. Figure5-21 shows a flow diagram in a Although manifolds are used mostly in hy- manifold which provides both pressure and draulic systems, the demand for them in pneu- return passages. One common line provides matic systems is increasing. pressurized fluid to the manifold, which distributes the fluid to any one of five outlet ports. PRECAUTIONARY MEASURES The return side of the manifold is similar in design. This manifold is provided with a relief The fabrication, installation, and mainte- valve, which is connected to the pressure and nance of specific fluid lines and connectors are return passages. In the event of excessive pres- beyond the scope of this training manual. How- ever, there are some general precautionary measures that apply to the maintenance of all fluid lines. Some of these are discussed in the following paragraphs. PRESSURE OUT - PORT It should be emphasized that regardless of the type of lines or connectors used to make up a fluid power system, make certain that they PRESSPORTURE OUT CHECK are the correct size and strength and perfectly VALVE clean on the inside. All lines must be absolutely RETURN IN clean and free from scale and other foreign PORT matter. Iron or steel pipes, tubing, and fittings PRESS IN RELIEF RETURN OUT PORT can be cleaned with a boiler tube wire brush or VALVE with commercial pipe cleaning apparatus. Rust REPORT and scale can be removed from short, straight PORT pieces by sandblasting, provided there is no PRESSURE OUT RETURN IN danger that sand particles will remain lodged PORT in blind holes or pockets after the piece is RETURN IN flushed. In the case of long pieces or pieces PORT PRESORT OUT bent to complex shapes, rust and scale can be RETURN IN removed by pickling (cleaning metal in a chem- PORT ical bath). Parts must be degreased prior to PRESSURE IN topickling. The manufacturer of the parts PORT should provide complete pickling instructions. (PLUGGED ON -I ASSY) = PRESSURE Open ends of pipes, tubing, hose, and fit- PRESSURE IN IMO RETURN POT tings should be capped or plugged when they are to be stored for any considerable period. PTA Rags or waste must not be used for this purpose, Figure 5-21.Fluid manifold because they deposit harmful lint which can flow diagram. cause severe damage to the fluid power system. CHAPTER 6 SEALING DEVICES AND MATERIALS

As related in chapter 1, Pascal's theorem, The dynamic seal is commonly referred to as from which evolved the fundamental law for the a packing. The packing is used to provide a seal science of hydraulics, was proposed in the 17th between two parts which move in relation to each century.One stipulation that was necessary to other; for example, a piston which moves back make the law effective for practical applications and forth within a cylinder. These two classifi- was a piston that would "fit" the opening in the cations ofsealsgaskets and packingswill vessel "exactly."This was not accomplished apply in most cases; however, deviations maybe until over 100 years later. Itwas late in the 18th found in some technical publications.It should century when an Englishman, Joseph Brahmah, be noted that certain types of seals (for example invented the cup packing which led to the develop- the 0-ring which is discussed later) may be used ment of the hydraulic press. either as a gasket or a packing. The packing was probably the most important Many of the seals in fluid power systems pre- invention in the development of hydraulics as a vent external leakage.These seals provide a leading method of transthitting power. Of course, twofold purposeto seal the fluid in the system the invention and development of machines to cut and to keep foreign matter out of the system. and shape closely fitted parts were also very Other seals simply prevent internal leakage important in the development of hydraulics. within a system. These applications are illus- However, some type of packing is usually re- trated in figure 6-1. Gaskets are installed be- quired to make the piston, and many other parts tween the cylinder wall and the end caps (points of hydraulic components, to "fit exactly." This (A) and (9)) to prevent external leakage. A also applies to the components of pneumatic packing is installed between the piston rod and systems. one end cap (point (D)) which also prevents Through years of research and experiments, external leakage. A packing is also installedon many different materials and designs have been the piston (point (C)) to prevent internal leakage. used in the development of suitable packing NOTE: Although leakage of any kind results devices.The materials must be durable and in a loss of efficiency, some leakage, especially provide effectivesealing. In addition,the internal leakage, is desired in hydraulic systems materials must be compatible with the fluid to provide lubrication of moving parts. This also used in the system. Several different designs are applies to some pneumatic systems in which necessary' to satiny the various requirements of drops of oil are introduced into theflow of air h ,fluid power systems. the system. As a result, a slight amount of in- These packing materials are commonly re- ternal leakage within the system provides lubri- ,,:,'Ierred to as seals or sealing devices. In turn, cation of moving parts. the seals used in fluid power systems and com- The first part of this chapterdeals primarily ponents are divided into two general classes with the characteristics of the different types of static seals and-dynamic seals. The static seal, materials used in the construction of seals. The usually referred to as a gasket, is used to provide next, section is devoted to the different shapes a seal between two parts where no relative motion and designs of seals and their application as is involved. For example, gaskets are used in the gaskets and/or packings in fluid power systems. assembly of cover plates on reservoirs and end Most emphasis is on the 0-ring, since it is the plates or other nonmoving parts of certain types most common seal used in fluid power systems. of pumps, motors,valves,etc. Also included inthis chapter are sections 86 Chapter 6SEALING DEVICES AND MATERIALS

PISTON

PISTON ROD La .2

4,,;7/ A

END CAP FLUID PORTS

FP.69 Figure 6- 1. Application of seals

concerning the functions of wipers and backup materials used in the construction ofseals washers in fluid power systems andon the for fluid power systems are discussedin the selection,storage, and handling of sealing following paragraphs. devices.

SYNTHETIC RUBBER MATERIALS Although experiments were conducted inthe As mentioned previously, several different search for a synthetic rubber in the 1800'sand types of material are used in the construction of early 1900's, it was late in the 1930's seals. before In the early years of fluid power, seals suitable synthetic compositionswere developed. were made of such materials as rope, sawdust, Among other desirable characteristics,some of rags, etc. These materials were jammed into a these compositions were resistant topetroleum stuffing box by means of a packing gland. The base fluids and, therefore, became theleading use of such materials led to extrusion of the materials for fluid power seals.Since then, material through clearance spaces, rapidwear great advancements have been made in thisfield. and continual leakagein varying amounts. New synthetics have been developedand the Therefore, these seals demanded almostcon- earlier synthetics have been improved. stant attention. Natural rubber has many of the charac- The basic substance used in thecomposition teristics required in an effective seal. However, of many synthetic rubbers is petroleumor as discussed in chapter 3, natural rubber is not alcohol. Different chemicals'are added to the compatible with petroleuni base fluids.Since basic substance to obtain differentsynthetics. this type fluid is commonly used inhydraulic The names of these differentsynthetics are systems and petroleum base oilsare used as usually derived from their chemicalcompo- lubricants in pneumatic systems, natural rubber sition.Butyl, bona N, neoprene, polyacrylate, seals have limited use in fluidpower systems. thiokol, ethylene propylene, andfluorosilicone They are sometimes used in automotivebrake are some of the synthetic compositions available. systems, which utilize vegetable base fluids. Seals made of such syntheticsas ethylene propylene and fluorosilicone Today, seals are made of materials which are required in systems containing the newer syntheticfluids. have been carefully chosen or developedfor Seals made of butyl and buna Nare used in specific applications.These materials include some fluid power systems. However, synthetic rubber, cork, neoprene, leather, and metal. which was one of the first syntheticcompositions, Asbestos seals are sometimes used whereheat is the most widely used materialin making is a problem.Some of the moat common seals for fluid power systems.

87 FLUID POWER

Many factors contribute to make synthetic CORK AND RUBBER rubber ideal for fluid power seals. This material is virtually impermeable (prevents Cork and rubber seals are made by com- passage of fluid) in a compressed state and, bining synthetic rubber and cork.This com- 4 therefore, requires less sealing load than many bination allows a sealing material having the other types of seals. Synthetic rubber is easily properties of both of the two materials. This formed and is available in sheet form and in means that seals can be made with the com- molded shapes for different applications. Sev- pressibility of cork, but with a resistance to eral types of synthetic rubber seals are capable fluid comparable to the synthetic rubber on of functioning in temperature ranges as wide as which they are based.Cork and rubber com- - 65° to 300° F. Some of the newer synthetics can positionissometimes used as gaskets for withstand even greater temperature ranges. applications similar to those described for cork gaskets. There are two general classes of synthetic rubber seals.One class is made entirely of a' certain synthetic rubber.The term homo- METAL geneous, which means having uniform structure or composition throughout, is frequently used to One of the most common metal seals used describe this class of Beal. The other class of in Navy equipment is copper.Flat copper seal is made by impregnating woven cotton duck rings are sometimes used as gaskets under or fine-weave asbestos with synthetic rubber. adjusting screws to provide a fluid seal. Molded This class is sometimes referred to as fabri- copper rings are sometimes used as a packing cated seals.(Natural rubber impregnated seals with speed gears operating under high p ressures. are available for some applications.) Either type is easily bent and requires careful handling. In addition, copper becomes hard when used over long periods or is subjected CORK to compression. Whenever a unit or component is disassembled, the copper sealing rings should be replaced.However, if new rings are not Cork has several of the required properties available and the part must be repaired, the which make itideallysuited as a sealing old ring should be softened by annealing. (An- material in certain applications.The com- nealing is the process of heating a metal, then pressibility of cork composition seals make cooling, so as to make the metal more pliable them well suited for confined applications where and less brittle.) no relief for side flow can be provided.In other words, cork can be compressed enough to provide an effective seal with only a limited Metallic piston rings are used as a packing spread of the material. Much of the compression in some fluid power actuating cylinders. These is absorbed by the material itself. rings are similar in design to the piston rings in automobile engines. In some instances, this Cork can be cut to any desired thickness and automotive type ring is made of Teflon. shape to fit any surface and still provide an excellent seal. Itcan withstand sustained temperatures up to approximately 270° F. TYPES OF SEALS

One of the undersirable characteristics of Fluid power seals are usually typed in cork is its tendency to crumble.Therefore, accordance with their shape or design. These if cork seals were used'is packings or in areas types include 0-rings, Quad-rings, V-rings, where there is a high fluid pressure and/or U-rings, cup seals, and flange seals.Figure high flow velocity, small particles wouldbe cast 6-2, illustrates some of these seals. A section oil into the system.For this reason, cork is'cut out of each seal to show the cross- seals have limited use in fluid power systems. sectional shape. A few of the more common Cork gaskets are sometimes used under the seals used in fluid power syStems are discussed inspectiontilates on hydraulic reservoirs., in the following paragraphs. r 88 1

Chapter 6SEALING DEVICES AND MATERIALS

leakage. The 0-ring forms the seal by distortion of its resilient, elastic compound, thus idling the leakage path. Figure 6-3 shows the proper installation of an 0-ring seal.The clearance for the seal is less than its free outer diameter, and the 0-ring is squeezed diametrically out-of- round even before the application ofpressure. (See view (A), fig. 6-3.) 0 -RING When pressure is applied to the 0-ring; the seal moves away from thepressure into the path of the possible leakage(fig. 6-3 (B)).The 0-ring is designed so that the seal flows up to the passage, thus completely sealing it against leakage.Thegreater the pressure V -RING applied, the tighter the seal becomes. When the pressure is decreased, the resiliency and elasticityof the seal results in the 0-ring returning to its natural shape.

U -RING Identification and Inspections Individuals working with fluidpower systems must be able to positively identify, inspect, and install the correct size and type 0-ring for every application in order to insurethe V -RING best possible service. ADAPTERS,MALE A FEMALE The task of procuring and positively identifying the correct seal can be difficultsince part numbers cannot be put directlyon the F13.70 seals.In addition, there is a continual intro- Figure 6 -2. Fluid power seals. duction of new types of seals and obsolescence of others. Most 0-rings are identjfied witha color 0-RINGS code to denote the specificuse for which they are intended. Colored dots, dashes, and stripes, An 0-ring, as shovininfigure6-2, is circular or combinationsof dots and dashes on the in shape, and its cross section is small in surface of the 0-ring indicate the medium (air, relation to its diameter. The cross section is gas,, or other type of fluid medium) in which truly round and has been molded and trimmed the 0 -ring is usable. Identificationmarks are to extremely close tolerances.The ellipitcal read clockwise around the ring. seal is also included in this discussion. This seal is similar to the 0-ring except for its The first mark on the 0-ring indicatesthe cross-sectional shape.' As its name implies, fluid medium. Ablue dot orblue stripeindicates its cross section is elliptical in shape. a seal that is used with air and/or petroleum Some 0-rings are made of natural rubber; base hydraulic fluid.The next mark following however, most are made of one of the synthetic the first identification mark denotesthe manu- compositions. The 0-ring is usually'' into facturer; however, in some instancesmanu- a rectangular groove machined into the mech- facturer's marks are not required. anism, to be sealed.As stated previotsly, Color coding' of 0-rings: is notalways a 07rings may be used as gaskets or packings, complete and reliable means of identification. and are used to prevent external or internal. There are several limitations to thecolor FLUID POWER

(A) (B)

FP.71 Figure 6-3.Properly installed 0-ring. coding. Some coding is not permanent, while it may be omitted on some 0-rings due to manufacturing difficulties or interference with operation. Furthermore, the color system provides no means to establish the size, age, and other 1 2 important data. For these reasons, 0-rings are K25330433-7491 3 made available in individually sealed envelopes PACKING PREFORMED SYN. RUBBER labeled with all the necessary pertinent data. 4 1 EACH (MS28778-5) It is recommended that they be processed and 5 DISC 38329 stocked in these' envelopes. An example of the 6 A -5/70 information printed on the 0-ring envelope is SR 810-13.90 illustrated in figure 6-4. 7 MFD, DATE 4-70 CURE DATE 2Q70 When selecting an 0-ring for installation, ST ILLMANAVBER CO. (MFGR/CONTR) information on the package should be carefully 9 MIL- G- 5510A observed. If an 0-ring cannot be positively identified,it should be discarded.The part number on the sealed package pkovides the most reliable and complete identification. Although an, 0-ring may appear perfect at FP.72 first glance, slight surface flaws may exist. 1. Stock number. 6. Rubber compoLition These are often capable of preventing satis- number. factory 0-ring performance under the variable 2. Nomenclature. 7. Date of manufacture operating pressures of fluid power systems. 3. Part number. and cure date. Therefore, 0-rings should be rejected for any 4. Contract number.8. Manufacturer. flaws that will affect theirplerformance. 9. Military specification By rolling the ring on an inspection cone 5. Preservation. number. or dowel, the inner diameter surface can be checked for small cracks particles of foreign Figure 6-4.-0-ring package identification. matter, and other irregularities that will cause leakage or shortenthe life of the 0-ring. The slight stretching of the ring when it is Following these inspection practices will prove rolled insideout. will help to reveal some to be a maintenance economy.It is far more defects not otherwise visible. A further check desirable to take special care identifying and of each 0-ring should be made by stretching inspecting 0-rings prior to installation than to it between the fingers, but care must be taken repeatedly 'overhaul components because of not to exceed the elastic limits of the rubber. 'faulty seals. 90 Chapter 6SEALING DEVICES AND MATERIALS

0-Ring Replacement this lubricant should be fluid of .the type that is used in the system.In pneumatic systems, Figure 6-5 shows a typical 0-ring instal- these surfaces should be coated with a lubri- lation.When such an installation shows signs cant which has a high melting point.Barium of internal or external leakage, the component and lithium soap grease is recommended for must be disassembled and the seals replaced. use in low-pressure systems, while a silicone Sometimes components must be resealed be- lubricant is recommended for use in pneumatic cause of the age limitations of the seals. Age systems that have pressures of 1,000 psi or limitation is discussed later in this chapter. more.Since this lubricant must be compatible Some of the precautions that must be observed with the seal material, the correct lubricant when replacing 0-ring seals are discussed in is sometimes listed in the technical publications the following paragraphs. for the specific system. Therefore, the applicable technical instructions should be consulted CYLINDER/O-RING PACKING \ before lubricant is applied to seals and sealing surfaces of pneumatic systems. PISTON ROD Felt washers are swnetimes installedon both sides of the 0-ring.These felt washers will retain the lubricant for a long period of 111.111111W0111111111111141110111111011111111111111101,111 time.Installations and fittings can be provided so that the washers can be lubricated periodically. 0-ring installation often requires spanning PISTON HEAD or inserting the 0-ring through sharp, threaded 0RING PACKING areas, ridges,slots, and edges.Such areas should be covered with an 0-ring entering FP.73 sleeve(soft,thin-wall metallic sleeve). If Figure 6-5.Typical 0-ring installation. the recommended 0-ring entering sleeve is not available, paper sleeves and covers may be fabricated by using the seal package (glossy After disassembly ofthe componentin side out) or lint-free bond paper.Adhesive accordance with the applicable technical instruc- tapes should not be used to cover these danger tions,the first step in replacing an 0-ring areas. Gummy substances left by the adhesives is to identify it both as to size and material. are extremely detrimental to fluid power sys- The part number of the seal required for each tems. application should be listed in the applicable parts manual for the specific equipment. This After the 0-ring is placed in the cavity number should correspond with the part number provided, it should be gently rolled with the on the package of the replacement seal.(See fingers to remove any twist that might have item 3, fig. 6-4.)This number will usually be occurred during installation. an MS (Military Standard) number. The com- When removing or installing 0-rings, the plete number must be checked since the dash use of pointed or sharp-edged tools which might number indicates the size -Of the 0-ring. For cause scratching or marring of component example,inthe number M528778-5, the -5 surfaces or cause damage to the 0-ring should indicates the size of the 0-ring. be avoided.'Special tools may be fabricated After determining that the replacentent seal for this purpose.A few examples of tools is made of the correct material and is of the used in the removal and installation of 0-rings proper size, the seal should be inspected for are illustratedin figure 6-6. These tools cuts, nicks, or flaws following the procedures should be' fabiicated from soft metal such as discussed previously.If any defects appear, brass or aluminum; ,however, tools made from the seal should be discarded. phenolic rod, wood, or plastic may also be used.In fact, plastic tools of this type are Prior to installation,the 0 -ring available in kits through the supply system. and all surfaces over which the 0-ring must The 0-ring seal, when used alone, islimited slide should be lubricated. In hydraulic systems, to systems having maximum operating pressures

91 FLUID POWER

SURFACE MUST BE SMOOTH AND FREE FROM SCRATCHES. CORNERS MUST NOT BE DENTED OR BUMPED. .005 RADIUS DESIRED. FP. 75 Figure 6- 7.Quad-ring.

The elimination of the spiral twist will in many instances extend the life of the seal. Quad-rings are ideally suited for both low pressures and extremely high pressures.An FLATTEN AS SHOWN AND POLISH OFF BURRS AND example of a Quad-ring used as a cover gasket EDGES. is illustrated in figuie 6-8.

FP. 74 Figure 6-6.-0-ring removal and installation tools.

QUAD of1,500 psi or less.This is particularly RING true when the 0-ring is used as a packing. JAI systems with operating pressures above 1,500psi; backup washers are installed in conjunction with the seal. Backup washers are FP.76 discussed later in this chapter. Figure 6-8.Quad-ring used as cover gasket.

V-RINGS QUAD-RINGS Several years ago,the V-ring was the The Quad-ring is very similar to the 0-ring predominant seal used in fluid power systems. discussed previously; the major difference being In recent years it has been replaced by the that the Quad-ring has a modified square type 0-ring in most applications.However, V- of cross section, as shown in figure 6-7. Like rings arestill used in some applications. 0-rings, Quad-rings are molded and trimmed to extremely close tolerances in cross-sectional Unlikethe 0-ring the V-ring seal will area, inside diameter, and outside diameter. provide a seal in only one direction. Therefore, The Quad-ring isrelatwely new and is if a piston is to move in two directions under presently used as a packing for reciprocating pressure, two sets of V-rings must be used. or rotary motion and can also be used as a V-rings are always installed with the open end static seal.The composition and the design ofthe V facing the pressure. Male.and female of Quad-rings are such that they could be used adapters are used in conjunction with V-rings in most applications in place of 0-rings. The for reinforcement. A V-ring and male and relatively square cross section of the Quad- female adapters are illustrated in figure 6-2. ring helps to eliminate the spiral twist that is A V-ring installation is illustrated in figure sometimes encountered withthe 0 -ring. 6-9.

to, Chapter 6SEALING DTICES AND MATERIALS

ADJUSTMENT NUTS SHALL BE USED IN ALL V-RING INSTALLATIONS MALE V-RING ADAPTER

1111,1111111[1 1,r111111, ,1,1111111111illlllll ili11111111rl 1 elH111111111111

. V-RING PACKING FEMALE V-RING ADAPTER

FP. 77 Figure 6-9.V-ring installation.

Installation of V-rings is slightly different from that of 0-ring seals.The rings and adapters are placed in their respective grooves, one at a time. After the rings and adapters are seated properly, the adjusting nut is tightened. (See fig. 6-9.) The 'adjusting nut should be tightened enough to hold the seals securely. If possible, the unit should be operated by hand to week the adjustment.

CUP SEAL The cup seal is sometimes used as a piston seal in fluid power systems.The cup seal is generally made of synthetic rubber or leather. Some cup seals are made of fabricated synthetic material, described earlier in this chapter. As the name implies, the cup seal is made in the shape of a cup. A typical cup seal is illustrated in figure 6-10. FP. 78 Figure 6-10.Typical cup seal. U-RINGS where high pressures will be encountered.As U-rings are used to prevent leakage in one with 0-rings, when U-rings are used inpneu- direction only.Typical uses of the U-ring are matic systems, provisions must be made to in automotive hydraulic brake assemblies and lubricate the seal.A typical 1i-ring seal is brake master cylinders. U-rings are never used shown in figure 6-11. 93 FLUID POWER

WIPERS Wipers, which are sometimes referred to as scrapers, are used to clean and lubricate the exposed portion ofpiston rods. This prevents foreignmatter from entering the system and scoring internal surfaces and damaging seals.Wipers may be of the metallic (usually copper base alloys) or felt types.In some application s, they are used together, the felt wiper being installed behind the metallic wiper. In hydraulic systems, the felt wiper FP.79 is normally lubricated with system hydraulic Figure U-ring seal. fluid from a drilled passage or from an external fitting. In pneumatic systems, the felt wiper .is lubricated with the approved lubricant from FLANGE SEALS an external fitting. Wipers are manufactured for a specific Flange seals are sometimes used as packings component and must be ordered for that appli- in some fluid power systems. This type packing cation. Wipers are normally inspected and is recommended for use only in low-pressure changed, if necessary, while component repair applications. Flange packings are the least is in process. desirable of the previously described types of Metallic wipers are formed in split rings seals.They are normally used only where for ease in installation and are manufactured there is not sufficient space for either a V-ring slightly undersize to insure a tight fit.Oie packing or a U-ring packing. Flange packings side of the metallic wiper has a lip which should are sometimes referred to as "hat packings." face outward upon installation. Metallic wipers A typical flange packing is illustrated in figure should be inspected for foreign matter and con- 6 -12. dition and then installed over the piston shaft in the proper order, as dire( :acl by the applicable technical instructions. The felt .riper may be a continuous felt ring or a length of felt with sufficient material to overlap its ends.The felt wiper should be soft, clean, and well saturated with the appropriate lubricant during installations.

BACKUP WASHERS One of the major problems concerning seals FP.80 is the problem of extrusion. Extrusion may be Figure 6-12.Flange packing. defined as distortion, under pressure, of portions of the seal into the clearances between mating metal parts.The extrusion of 0-rings WIPERS AND BACKUP WASHERS is illustrated in figure 8-13.When pressure is applied, the seals will flow into the respective clearances. When the pressure is released, the Although wipers and backup washers are 0-rings return to their original shape. However, not classified as seals, they definitely serve the extrusion groove will appear as a cut be- a vital role in the effectiveness and the -life neath the surface of the 0-ring. Eventually, the of seals in certain applications. Their functions cuts will become more severe, and sections and applications are discussed in the following will be cut out of the 0-rings. This, of course, paragraphs. will lead to the failure of the 0-ring. Chapter 6SEALING DEVICES AND MATERIALS

GRIND 0RING SEAL PSI BACKUP WASHERS BACKUP mwmf 1=4 WASHER V 0 PRESSURE` 0RING SEAL RING

\ PRESSURE ALWAYS FROM THIS PRESSURE FROM ALTERNATE SIDES, DIRECTION. PLACE NONEXTRUSION OR FOR OTHER REASONS. TWO BACK- DEVICE AWAY FROM PRESSURE. )P WASHERS USED. ONE ON EACH SIDE OF (MING FP.81 ORING Figure 6-13.Extrusion of 0-rings.

To eliminate extrusion, the manufacturer FLESH OR CUT SIDE must use harder seals, reduce clearances, or OF LEATHER use backup washers.Backup washers are GRAIN OR HAIR SIOE commonly used for this purpose. A backup OF LEATHER washer is a device normally used behind a FP.82 seal to allow a higher pressure to be applied Figure 6-14.-0-ring with backup washers. to the seal. A backup washer used behind an 0-ring, for example; will extend the allowable seal pressure from 1,500 psi to pressures in Presently, backup washers are made of thin- excess of 3,000 psi.If the 0-ring is subject split metal, bakelite, chrome tanned leather, to pressures from alternating sides, backup or Teflon.Leather and Teflon are the most washers are required on both sides of the widely used. These two types of backup wash- 0-ring.An installation of 0-rings with backup ers are described in the following paragraphs. washers is illustrated in figure 6-14. When a part or component is disassembled and the packing is being replaced, the backup Leather washers should also be thoroughly inspected. Inspection of backup washers should include The chrome tanned leather backup washer a check that surfaces are free from' irregu- is made of leather with hair on the outer eine larities,that edges are clean-cut, and that of the leather.The outer side of the leather scarf cuts are parallel. Tools similar to those is called the grain side, while the cut or inside used in the removal and installation of 0-rings of the leather is called the flesh side. The should be used for the removal and installation backup washer is always installed in the gland of backup washers. (groove).The flesh side of the backup washer The size of a backup washer is indicated by is always next to the gland.This positions a dash number. The dash number of the backup the grain side to the seal.(In this case, the washer should be the same as the dash number seal is an 0-ring.)If the pressure is exerted ofthe packing with which it is to be used. on the seal in one direction only, the washer Most backup washers are packaged in envelopes is placed away from the pressure.If pressure similar to those described for 0-rings. Infor-. . is to be applied alternately from both directions, mationconcerningthe backup washers is one backup washer must be placed on each printed on the envelope.It is recommended side of the 0-ring. that the backup washer be retained in the NOTE: Leather backup washers should envelope until required. never be cut,as results have shown that 4095 FLUID POWER when pressure isapplied,this will be the specify the type of seals to be used in their section most likely to fail. equipment, andtheir instructions should be followed when replacing these items.If the proper seal is not available, careful consider- Teflon ationshould be given to the selectior, of a suitable substitute. Backup washers made ofTeflon do not deteriorate with age, are unaffected by any As discussed in the section on 0-rings, system fluid or vapor, and tolerate temper- applicable technical instructions should be con- ature extremes in excess of those encountered sulted to select the correct replacement seal in high-pressure fluid power systems. Teflon in a specific system. To simplify the selection backup rings may be stocked in individual of many types of packings and gaskets commonly sealed envelopes similar to those in which used in naval service, the Naval Sutps Systems 0-rings are packed, or several may be in- Command has prepared a packing and gasket stalled on a cardboard mandrel. chart(Mechanical Standard Drawing B-153) Teflon backup washers are usually of the showing the symbol numbers and the recom- spiral design, as illustrated in figure 6-15. mended applications of most types of packing When dual backup washers are installed, the and gasket materials.The symbol number split scarfed ends must be staggered. usedtoidentifyeach typeof packing and gasket consists of a four-digit number.The firstdigit indicatestheclass of seal; the numeral1indicates theseal is a packing and the numeral 2 indicates a gasket.The second digit indicates the principal material from which the seal is composed.The third and fourth digits indicate the different styles or forms of the seal. Inaddition tothe Naval Ships Systems Command chart, most ships have a packing and gasket chart made up specifically for each ship.The shipboard chart shows the symbol numbers and the sizes of packings and. gaskets re qu i red in the ship' s piping FP.83 system and equipment. Figure 6-15.Teflon spiral backup washer.

STORAGE AND HANDLING SELECTION, STORAGE, AND HANDLING It has been found through experience that The selection, storage, and handling of all seal materials, especially natural and synthetic types of seals are very important to the effec- rubbers, will deteriorate with age.For this tiveness and to the life of the seal. The correct reason, the Navy has established an age control seal must be selected for the job and must program for these materials.This program be protected from outside elements during the is known as shelf life.Knowing and under- time it is in storage. Some of the precautionary standing shelf life will save many hours of measures that must be considered in the se- unnecessary toil experienced in repacking a lection, storage, and handling of fluid power unit and having it still leak because the packing seals are discussed in the following paragraphs. was defective due to age. Prior to installation of natural and synthetic rubber seals, a check must be made to deter- SELECTION mine if these parts are acceptable for use. All natural and synthetic rubber packing con- The selection of the correct packings and tainers are marked to facilitate an age control gaskets is an important factor in maintaining program.(See item 7, fig. 6-4.)This infor- an efficient fluid power system. Manufacturers mation is available for all seals used regardless

96 Chapter 6SEALING DEVICES AND MATERIALS

of whether the seal is stocked on shipboard, Proper storage practices must be observed at stock distribution points, or furnished as to provent deformation and deterioration of an integral part of the component.Positive seals. Most synthetic rubbers are not damaged identification indicating the source, "cure date," by storage under ideal conditions.However, and "expiration date" must be made of seals. most synthetic rubbers will deteriorate when The age control of all seals is based upon exposed to heat, light, oil, grease, fuels, sol- the cure date stamped on the manufacturer's vents,thinners,moisture, strong drafts, or package. This cure date is denoted in quarters. ozone (form of oxygen formed from an elec- For example, the cure date 2Q70, illustrated tricaldischarge). Damage by exposure is in figure6-4,indicates thatthe seal was magnified when rubber is under tension, com- manufactured duringthe second quarter of pression, or stress.There are several con- 1970. Seals manufactured during any given ditions to be avoided which include the following: quarter are not considered one quarter old until the end of the succeeding quarter. Most 1. Deformation as a result of improper seal age limitations are determined by this stacking of parts and storage containers. cure date, anticipated service life,and re- 2. Creasing caused by force applied to placement schedule. corners and edges, and by squeezing between The age of the seal is computed from the boxr,s and storage containers. cure date.The tPrm cure date is used in '3. Compression and flattening, as a result conjunction with replacement kits which contain of storage under heavy parts. seals,parts, and hardware for shop repair 4. Punctures caused by staples used to of various components, These cure dates also attach identification. provide bases for seal replacement schedules, 5. Deformation and contamination due to which are determined by the service life of hanging the seals from nails or pegs.Seals the seal.The service life (estimated time of should be keptin their original envelopes, trouble-free service)of seals also depends which provide preservation, protection, identi- upon such conditions as use, exposure to certain fication, and cure date. elements, both natural and imposed, and sub- 6. Contamination by piercing the sealed jection to physical stress.Operational con- envelope to store 0-rings on rods, nails, or ditions imposed on seals in one component wire hanging devices. may necessitate seal replacement more fre- 7.Contamination by fluids leaking from parts quently than replacement of identical seals in stored above and adjacent to the seal surfaces. other components.Therefore, it is necessary to adhereto the recommended replacement 8.Contamination caused by adhesive tapes schedule for each individual component.The applied to seal surfaces.A torn seal package age of seals in a spare part is determined should be secured with a pressure-sensitive from the assembly date recorded on the service moistureproof tape,but the tape must not or identification plate and/or the exterior of contact the seal surfaces. the container.All 0-rings over 24 months 9.Retention of overage parts as a result old should be replaced or, if nearing their age of improper storage arrangement or illegible limit (24months),should not be used for identification. Seals should be arranged so replacement. the older seals are used first.

97 CHAPTER 7

RESERVOIRS, STRAINERS, FILTERS, AND ACCUMULATORS

Fluid power systems must have a sufficient automatic transmission of an automobile, the supply of uncontaminated fluid for the efficient reservoir also serves as the housing for the operation of the system. Although the same fluid complete system. Although its primary function is recirculated in hydraulic systems, a con- is to provide a storage space for the hydraulic tainer must be provided for a supply of fluid fluid required by the system, a well constructed in excess of that contained in the lines and com- reservoir provides several additional functions. ponents. Since the fluid is expended during the Among these functions are dissipation of heat, operation of pneumatic systems, containers are trapping of foreign matter, and the separation also required to supply gas to these systems. of air from the system. As stated in chapter 3 and emphasized throughout this manual, fluid must be kept free of all Reservoirs dissipate heat by radiation from foreign matter for efficient operation of the the external walls. In addition, some reservoirs system. Various types of strainers and filters are equipped with internal and/or external radia- are incorporated in the system to provide this ting devices such as cooling fins or coils. The function. trapping of foreign matter usually requires the use of strainers and/or filters, which are dis- The first part of this chapter covers the cussed later in this chapter. The separation of containe rshydraulic reservoirs and pneumatic air from the system is accomplished by the receiversused in fluid power systems. The design of the reservoir, which includes the in- next section of the chapter describes the dif- corporation of baffles to slow the fluid as it ferent types of strainers and filters used in the returns to the reservoir. The air bubbles have filtration of fluids. The last part of the chapter a greater chance of escaping to the surface of is devoted to accumulators, another fluid supply a liquid if the liquid moves at a slow velocity. source commonly used in hydraulic systems. Most reservoirs are equipped with a means whereby the air is vented to the atmosphere. FLUID SUPPLY Many factors are considered when selecting As previously stated, an adequate supply of the size and configuration of a hydraulic the recommended fluid is a very important re- reservoir for a particular system.The res- quirement for the efficient operation of a fluid ervoir must be large enough to store more power system. The reservoir, which provides than the anticipated volume of fluid that the sys, a storage space for fluid in hydraulid systems, tem will require. The recommended reservoir differs to a great extent from the receivers volume is usually based on the gallon-her-minute used for this purpose in pneumatic systems. flow demanded by the system. A reservoir For this reason, the two components are cov- capacity equal to two or three times the max- ered separately in the following sections. imum rate of flow required by the system is usually sufficient. A higher ratio is desirable HYDRAULIC RESERVOIRS for fixed installation, and a somewhat lower ratio may be required for mobile equipment. The reservoir is a basic component Of any Adequatespacemustbeallowedto hydraulic system.In most systems the accommodate thermalexpansion ofthe reservoir is a separate component of the system, hyd r aulic fluid and changes in fluid level whilein other systems,forexamplethe due to system operation. 98 .403 Chapter 7RESERVOIRS, STRAINERS, FILTERS, AND ACCUMULATORS

PUMP MOUNTING SUPPLY AIR PLATE RETURN DRAIN LINE BREATHER LINE RETURN

BAFFLE FLUID DRAIN PLATE STRAINER LEVEL PLUG GAGE

CLEAN OUT PLATE (80TH ENDS)

FP.84 Figure 7-1.Nonpressurized reservoir (ground or ship installation).

Reservoirs are of two general typesnon- ship installations and some aircraft that are pressurized and pressurized. Nonpressurized limited to low-altitude operations. reservoirs are vented to the atmosphere. This prevents a partial vacuum from being formed A typical reservoir for use with ground as the fluid level in the reservoir is lowered. and ship installations is illustrated in figure The vent also makes it possible for any air 7-1. This type reservoir is made of hot rolled that has entered the system to find a means of steel plates and welded seams. The ends extend escape. below the bottom of the reservoir and serve Aircraft and missiles designed for high- as the support. The bottom of the reservoir is altitude operation require pressurized reser- convex and a drain plug is incorporated at the voirs. Pressurizing assures a positive flow of lowest point. fluid to the pump at altitudes where low atmos- Large removable covers are installed on pheric pressure is encountered. each end of the reservoir to provide easy access for cleaning. One of the covers contains Nonpressurized Reservoirs a fluid level indicator and a filler opening with a cap that is secured to the plate with a chain. Hydraulic systems designed to operate equip- Since the fluid level must be checked frequently, ment at or near sea level are normally equip- the indicator is located in a position where it ped withnonpressurizedreservoirs.This can be easily read.A strainer is installed includes the hydraulic systems of ground and in the filler neck to prevent foreign matter 99

C4 FLUID POWER

from entering the reservoir when fluid system. The system must then be operated for is added. a few minutes to allow the fluid in the system In this type reservoir a baffle plate extends to circulate through the reservoir and displace lengthwise through the center of the tank and any air that may have entered the system. It separates the pump supply port from the system should be noted at this point that some hydraulic return port. The baffle usually extends from systems are connected to the reservoir by a the bottom of the tank to approximately two- single line. This line serves as both supply and thirds the height of the normal fluid level. return line to and from the system. Since this Usually,there are several openings in the prevents complete recirculation of the fluid in baffle near the bottom of the reservoir. There- the system, air bleed valves are provided at fore, the purpose of the baffle is not to com- various points in the system. The automobile pletely divide the reservoir into two separate brake system is a good example of this type compartments, but rather to reduce turbulence system. Bleed valves are incorporated in the created by return flow and to prevent the con- system at each wheel. After the air has been tinuous recirculation of the same fluid. It also displaced in either type system, the fluid level allows foreign matter to settle to the bottom must be rechecked and more fluid added if of the reservoir and air bubbles to escape necessary. It should be emphasized that the through the surface of the fluid before the fluid fluid must be maintained at the level indicated is recirculated. on the gage. Remember there is a space pro- The vent (air breather) line contains a filter vided above the fluid level for thermal expan- to purify the air that enters the reservoir. The sion. A lesser amount of fluid than that indi- vent assembly functions to allow the pressure cated may result in improper performance of in the reservoir to balance with atmospheric the system. pressure and to filter the air which enters the Al mentioned previously, some aircraft reservoir when the fluid level is lowered due hydraulic systems are equipped with nonpres- to system operation. surized reservoirs. A typical example of this The pump supply line enters the reservoir type reservoiris illustrated in figure 7-2. at the top and extends to within a few inches of the reservoir bottom. This helps to prevent foreign matter which settles to the bottom of the reservoir from entering the system. Return VENT FILTER lines must be well below the fluid surface level .1,1\ to prevent foaming. The end of the return line - is usually cut at an angle of approximately 45 CHECK VALVE degrees and positioned so that the flow is EMERGENCY directed toward the walls of the tank and away RESERVOIR VENT from the pump intake line. This provides for maximum heat dissipation. The inside of the reservoir is painted with a sealer to minimize oxidation, which can be caused by condensatibn. The sealer must be of a composition that will not react chemically PUMP SUPPLY with the fluid specified for the system. LINE Because of the design of this type reservoir, a large portion of the harmful contaminants in the system will accummulate in the reservoir, especially at the bottom. Therefore, the reservoir should be drained periodically and then PUMP SUPPLY flushed with a solvent compatible with the fluid LINE that is used in the system. After cleaning, the reservoir should be filled to the proper level FP.85 with clean hydraulic fluid of the type that con- Figure 7-2.Nonpressurized reservoir forms to the specification designated for the (aircraft system). 100 Chapter 7RESERVOIRS, STRAINERS, FILTERS, AND ACCUMULATORS

Because of weight limitations in aircraft con- Pressurized Reservoirs struction, the reservoirs are made of welded aluminum. The filler neck incorporates a re- As stated previously, many of the Navy's movable metal screen assembly, which serves aircraft and missile hydraulic systems are as a strainer. equipped with pressurized reservoirs. This A sight gage window for visually checking assures a positive flow of fluid to the pump at high altitudes where low atmospheric pressures the fluid level is located on one end of the are encountered. There are two common types reservoir. The rim of this window is marked of pressurized reservoirsair-pressurized and with lines indicating the refill level. hydraulic-fluid-pressurized. In addition to a filter, the vent assembly on AIR-PRESSURIZED.One type of air-pres- this type reservoir usually contains a bypass surized reservoir is illustrated in figure 7-3. check valve. The check valve allvs air to be The reservoir is cylindrical in shape and has expelled from the reservoir at a greater rate a piston installed internally to separate the air than normal when large volumes of fluid are and the fluid chambers. The end of the piston returned to the reservoir, and also prevents rod protrudes through the air chamber end of unfiltered air from entering the reservoir. the reservoir and indicates the fluid quantity. The quantity indication may be seen by inspect- There are two pump supply line outlets ing the distance the piston rod protrudes through one supplies the main power pump, while the the end of the cylinder. other outlet supplies the emergency pump. The outlet which supplies the main power pump is located a considerable distance above the outlet which supplies the emergency pump. This pro- PORT A vides a reserve supply of fluid in the reservoir for the emergency system in the event that external leakage occurs during normal operation. The reserve supply of fluid should be of sufficient volume to ope rate the subsystems necessary for a safe landing of the aircraft. This includes such operations as the extension of the landing gear, the lowering of the flaps, and the operation of the brakes. The emergency and main system return lines, not shown in figure 7-2, are connected to ports on the side of the reservoir. Maintenance of this reservoir consists FP.86 mainly of cleaning the filler neck strainer and Figure 7-3.Air-pressurized reservoir. replacement of the vent filter. The filler neck strainer should be cleaned with a cleaning fluid that is compatible with the fluid used in the The air pressure is usually provided by system. When cleaning the strainer, inspect bleed air taken from the compressor section for broken mesh (sometimes the result of of the engine. An air filler valve is usually servicing with a funnel). incorporated in the air line to provide a means for pressurizing the reservoir during ground The vent filter element should be replaced checks of the hydraulic system when the engine in accordance with applicable technical publi- is not running. The air is filtered and the pres- cations. Add itional information concerning sure is regulated to the required psi before it strainers and filters are presented later in enters thereservoir through port (C). The this chapter. required pressure is stipulated by the manu- The cleanliness of the reservoir is main- facturer. tained by periodic flushing similar tothe There are at least two portsa supply port 1 procedures described in chapter 3. and a return portin the fluid end of the 101 FLUID POWER reservoir. Some reservoirs contain additional pressurizing the reservoir. A reservoir of this ports to satisfy the requirements of certain type is shown in figure 7-4. This type reservoir systems. Provisions are incorporated, usually is divided into two compartments by a floating in the return line, to fill the reservoir from a piston. The floating piston is forced downward single point. Filling is accomplished by forcing in the reservoir by a spring and by a pressure hydraulic fluid into the filler line. Applicable probe which fits into the piston. technical instructions must be adhered to when The pressure probe is connected to the pump servicing this type reservoir. pressure line. Therefore, when the system is In operation, the regulated air pressure pressurized,hydraulicfluid under pressure enters the reservoir and acts on the piston, enters the probe and aids the spring to force which in turn, transmits this force to the fluid. the piston downward, pressuvizing the fluid in Thus, the pressurized fluid in the reservoir the lower compartment. This pressurizes the provides a positive flow of fluid through the pump supply line to the same pressure. This supply line to the pump. pressure prevents pump starvation at all altitudes. FLUID-PRESSURIZED. S o m e hydraulic This type reservoir has five portspump systems utilize system hydraulic pressure for supply, return, pump pressure, reservoir drain,

RESERVOIR DRAIN PORT RETURN PONT POPPET VALVE INDICATOR ROD

SUCTION PORT PISTON 11.1 PRESSURE SPRING VERBOARD,DRAIN PORT (SIGHT GAGE) ggRETURN PRESSURE PORT( I NTO HOLLOW MI STATIC FLUID PISTON ROD OF RESERVOIR)

SIGHT GAGE FULL FULL

REFILL REFILL REFILL OVERBOARD DRAIN .2ISTON PISTON BOTTOMED AT FULL OUT PISTON LEVEL (NORMAL PRESSURE LEVEL) POPPET PROBE OPENED AIR IN REFILL RESERVOIR PUMP BEGINS. TO CAVITATE

SYSTEM OPERATING SYSTEM STATIC SYSTEM STATIC RESERVOIR PRESSURIZED RESERVOIR EMPTY RESERVOIR FULL

FP.87 Figure7-4.Fluid-pressurized reservoir. 102

547

1 Chapter 7-17E8L.11VOIRS, STRAINERS, FILTERS, AND ACCUMULATORS and overboard drain. Fluid is supplied to the the receiver where it is stored under pressure. pump through the pump supply port. Fluid re- (See chapter 8 for detailed information con- turns to the reservoir from the system through cerning air compressors.) This receiver may the return port. Pressure from the system provide gas under pressure directly to pneu- enters the pressure probe in the top of the matic systems or may be used to charge (fill) reservoir through the pump pressure port. The other receivers, such as cylinders, bottles, etc., reservoir drain port is for the purpose of drain- which are, in turn, used to furnish gas to the ing the reservoir, when draining is necessary. pneumatic systems. The line leading from the overboard drain port Receiver is the term usually associated with is equipped with a sight gage which is used as ground and shipboard pneumatic systems that an aid in servicing the reservoir. employ a compressor as part of the system. When servicing the reservoir, a container The receiver is usually located near the com- should be placed under the line leading from pressor and is considered as part of the the overboard drain port. Fluid is forced into compressor system. the reservoir through a fill port in the return The receiver acts as a storage tank and also line. The liquid should be forced into the res- a supply tank. It stores a volume of air under ervoir until air-free liquid flows through the pressure which is provided by the compressor sight gage. (Air-free liquid is liquid containing and supplies this compressed air t;2: the pneu- no air bubbles.) matic system or to other receivers. Through An indicator rod that protrudes through the the storage of a volume of gas under pressure, top of the reservoir housing is used in deter- the receiver functions to maintain the system mining when the reservoir needs servicing. at a constant pressure and thereby retards the The word REFILL is stamped on the rod guide. frequency and length of the start- stop -start When the reservoir is full, the rod is extended cycles of the compressor. and the word REFILL is hidden; when the rod The size and construction of the receiver is retracted, the word REFILL is exposed and depend upon the maximum pressure and volume the reservoir needs servicing. of gas required to efficiently operate the complete system. The rers-Aver is Q.Indrical in PNEUMATIC RECEIVERS shape and may be rnmr:o..e.1 ,Iiiher horizontally Like hydraulic systems, pneumatic systems or vertically, the position and location being require an adequate supply of fluid for the dependent on the space available for installation. efficient operation of the system. In all cases, Vertically mounted receivers should have the supply of gas for pneumatic systems must convex shaped bottoms to permit proper drain- be stored under pressure. The amount of pres- ing of accumulations of moisture,oil, and sure, and also the volume, depends upon the foreign matter. Each receiver should be fitted requirements of the system. with the following accessories and connections: Receiver, storage cylinder, air bottle, air 1. Inlet and outlet connections. cylinders and flask are all terms used to de- 2. Drain connections and valve. scribe the component of a pneumatic system 3. Connection for operating line to com- which provides the functions similar to those pressor regulator. provided by the reservoir of a hydraulic system. 4. Pressure gage. As described previously in this chapter, the 5. Relief valve. hydraulic reservoir supplies the fluid to the 6. Manhole or handhole plate, depending pump and provides a place for the return fluid on the size of the receiver. from the system. In pneumatic systems the The inlet connection is located near the top receiver stores a volt:ae of gas under the max- of the receiver. The outlet connection is located imum pressure required by the system and at a point some distance above the bottom of supplies it to the system as needed. After the the receiver. This helps to prevent water, oil, gas is used in the operation of the system, it and other foreign matter that settles to the is exhausted to the atmosphere. -- bottom of the receiver from entering the system. A receiver is.tually part of the compressor The line between the compressor and receiver system. The compressor forces the gas into should be kept as short and as free of bends as 103 FLUID POWER possible in order to eliminate excessive vibra- Cooling of the high-pressure air in the tion due to pulsations of air and to reduce storage cylinder will cause some condensation friction caused by the flow of air through the to collect and settle to the lowest point in the lines. cylinder. To insure positive operation of these The receivers in aircraft pneumatic systems systems, the cylinder must be purged of mois- are referred to as storage cylinders or bottles. ture periodically. This is accomplished by The compressed air or nitrogen is stored in slightly opening the moisture drain valve, which these cylinders until required by the actuating connects to the drain tube. As shown in figure system. The cylinders are initially charged 7-5, the other end of the drain tube is positioned with compressed air or nitrogen from an ex- at the lowest point in the cylinder. With the ternal source. Most systems contain an air drain valve slightly open, any moisture that has compressor which replaces the volume and accumulated in the cylinder will be forced out pressure loss through leakage and system of the cylinder through the drain tube. operation. Cylinders are available in different sizes, The storage cylinders are made of steel and the selection of which depends on the require- may be either spherical or cylindrical in shape. ments of the system. The required size is The cylinders are nonshatterable and each ids stipulated by the manufacturer of the system. equipped with a moisture drain valve. The valve The unit of measurement to indicate the volume is incorporated into the end fitting, which is of cylinders is the cubic inch. Cylinders with sometimes referred to as a manifold. This fit- volumes of 100, 200, and 400 cubic inches are ting also contains the inlet and outlet ports. An common in aircraft pneumatic systems. The example of a storage cylinder used in aircraft pressure normally required in aircraft pneu- pneumatic systems is illustrated in figure 7-5. matic systems is 3,000 psi. FILTRATION AND COOLING OF FLUIDS As pointed out in chapter 3, most malfunctions in fluid power systems may be traced to some type of contaminant in the fluid. For this reason, every effort must be made to prevent contaminants from entering the system and to remove those contaminants which do find their way into the system. Filtration devices are incorporated at key points in fluid power systems to remove those contaminants which, in some way, do enter the system. Filtrationdevices for hydraulic systems differ to some extent from those for pneumatic systems and, therefore, are covered separately in the following paragraphs. Cooling devices for hydraulic fluids are also covered in this section. Cooling devices for pneumatic systems are covered under the compressor section of chapter 8. END FITTING HYDRAULIC FLUIDS The importance of keeping hydraulic fluid DRAIN TUNE clean and free of contaminants cannot be over- emphasized. Foreign matter in the system can FP.88 cause excessive wear, increase power loss, Figure 7-5.Compressed gas cylinder for clog valves and other components, and substan- aircraft pneumatic systems. tially increase maintenance and replacement 104 Chapter 7 RESERVOIRS, STRAINERS, FILTERS, AND ACCUMULATORS costs. Although great careis takenwhile dais are deposited in the fluid as a result of servicing, maintaining, and operating hydraulic the normal wear on valves, pumps, and other systems,itis impossible to prevent some components. foreign matter from entering the system. Some contaminants are built-in; that is, small par- The filtering devices used in hydraulic sys- ticles of core sand, weld spatter, metal chips, tems are most commonly referred to as strain- lint,and abrasive ers and filters. Since they share a common dust, resulting from the function, the terms strainer and filter are often manufacturing process, remain in the compo- used interchangeably. As a general rule, de- nents when they are installed in the system. vices used to remove large particles of foreign Also, tiny particles of metal and sealing mate- matter from hydraulic fluids are referred to as strainers, while those used to remove the smallest particles are referred to as filters.

Strainers Strainers usually consist of a metal frame wrapped with a fine mesh wire screen, or a screening element constructed of varyinithick- nesses of specially processed wire. A typical strainer is illustrated in figure 7-6. Strainers do not provide as fine a screening action as filters, but offer less resistance to flow.. Therefore, strainers-a- Wu-sillily used in pump supply lines where the fluid is in a low- pressure area and restriction of flow would result in starvation of the pump. Strainers are also used in the filler ports of most hydraulic reservoirs. Various arrangements for using strainers in a pump supply line are illustrated in figure 7-7. If one strainer causes restriction of flow to the pump, two or more may be used in FP.89 parallelas shown. Since strainers must be Figure 7-6.Typical hydraulic system removed frequently. for cleaning, they need strainer. only be hand tightened during installation,

PUMP INTAKE DISCONNECT UNION TO REMOVE CONNECTIO STRAINERS FOR CLEANING

PIPE JOINTS SUBMERGED

NOTE' ACCESS OPENING SHOULD BE PROVIDED SO STRAINERS MAY BE REMOVED FOR CLEANING WITHOUT DRAINING TANK

FP.90 Figure 7-7.Various arrangements of hydraulic system strainers.

105 FLUID POWER

provided the fittings will remain submerged during the operation of the system. Pump inlet fittings exposed to the atmosphere must be air- tight. BYPASS RELIEF VALVE (50 PSI) Filters The most common devices incorporated in hydraulic systems to prevent foreign particles OUT and contaminating substances from remaining in the system are referred to as filters. They may be located in the reservoir, in the return line, in the pressure line, or in any other location in the system where the designer of the system decides that they are needed to safe- guard the hydraulic system against impurities. Filters are classified as full flow or propor- BODY tional flow. In the full flow type filter, all the fluid which enters the unit passes through the filtering element, while in a proportional flow FILTER type, only a portion of the fluid passes through ELEMENT the element. FULL FLOW FILTER.A full flow filter is illustrated in figure 7-6. This type filter provides a positive filtering action; however, it offers resistance to flow, particularly when the FILTER element becomes dirty. For this reason, a full BOWL flow filter usually contains a valve, which automatically allows the fluid to bypass the element FP.91 when the element cannot handle all the flow Figure 7-8.Full flow hydraulic through the unit. filter. Hydraulic fluid enters the filter through the inlet port in the body and flows around the filter element inside the filter bowl. Filtering takes Some nonbypassing full flow filters are place as the fluid passes through the filtering equipped with a contamination indicator which element and into the hollow core, leaving the operates under the principle of difference in dirt and impurities on the outside of the filter pressure entering the element and pressure element. The filtered fluid then flows from the after it leaves the element. As contaminating hollow core through the outlet port and into the particlescollecton the filter element, the system. differential pressure across the element in- The bypass pressure relief valve in the body creases. When the increase in pressure reaches allows the fluid to bypass the filter element a specific value, an indicator (usually in the and pass directly through the outlet port in the filter head) pops out, signifying that the filter event that the filter element becomes clogged. element must be cleaned or replaced. A low- In most filters of this type, the relief valve is temperature lockout feature is incorporated in set to open when. the differential in pressure most types of contamination indicators to pre- exceeds 50 psi. For example, if the pressure vent actuation below 20°F, thus eliminating at the filter inlet port is 90 psi, and the pres- the possibility of false indications due to cold sure at the outlet port drops below 40 psi, the weather. relief valve will open and allow the "'fluid to Filter elements used in connection with a bypass the element. Additional informationcon- contamination indicator are not normally re- cerning the operation of relief valves is pre- moved or replaced until the indicator is actu- sented in chapter 10 of this manual. ated. This decreases the possibility of system 106 Chapter 7 RESERVOIRS, STRAINERS, FILTERS, AND ACCUMULATORS contamination from the outside sources elementt continues to collect foreign particles. due to unnecessary handling. The pressure differential between the inlet and outlet ports will continue to increase until the The use of the nonbypassing type filter bypass valve opens and directs fluid throughthe eliminates the possibility of contaminated fluid filter element bypass. This is similar to that of passing the filter element and Contaminating the entire system. This type filter will minimize the bypass valve of the filter illustrated in fig- the necessity for flushing the entire system and ure 7-8. lessen the possibility of failure of pumps and PROPORTIONAL FLOW FILTER.Although other components in the system. the full flow filter is the most common type used A bypass relief valve is incorporated in in hydraulic systems, some systems use the pro- some filters equipped with the contamination in- portional flow filter. A cutaway view of the pro- dicator. If the filter element in this type is not portional flow filter is illustrated in figure 7-9. replaced when the indicator signifies, the filter This type filter operates on the venturi

VENTURI THROAT

BAFFLE

BODY

CARTRIDGE

FP.92 Figure 7-9.Proportional flow hydraulic filter. 107 112 FLUID POWER principle. (See glossary). As the fluid passes cellulose paper (micronic), and metal. Some of through the venturi throat a drop in pressure the different types of filter elements are de- is created at the narrowest point. A portion of scribed in the following paragraphs. the fluid flowing toward and away from the throat of the venturi flows through the passages FULLER'S EARTH.Fuller's earth is clay- into the body of the filter. A fluid passage like material and is used in the purification connects the hollow core of the filter with the of mineral and vegetable base oils. It produces throat of the venturi. Thus, the low pressure a fine degree of filtration; however, its use as area at the throat of the venturi causes the fluid a filtering element in present day hydraulic under pressure in the body of the filter to flow systems is limited. Filters employing fuller's through the filter element, through the hollow earth or activated clay should not be used with core, into the low pressure area, and then re- hydraulic fluids containing additives, because turn to the system. Although only a portion of such filters may remove the additives as well the fluid is filtered during each cycle, constant as the impurities. recirculation through the system will eventually MICRONIC. Micronic, a term derived from cause all the fluid to pass through the filter the word micron, could be used to describe any element. Figure 7-9 shows the direction of flow filter element. Through usage, this term has from right to left; however, this type filter become associated with a specific filter with a provides filtering for either direction of flow. filtering element made of a specially treated cellulose paper. The paper is formed in ver- Filter Elements tical convolutions (wrinkles) and is made in a cylindrical pattern. (See fig. 7-10.) A spring in The effectiveness of any filter is measured the hollow core of the element holds the element by the degree of filtration it produces. Degree in shape. of filtration means that a specific filter element will, when new and clean, stop a certain percentage of the particles which measure a certain size or larger while the fluid is operatingunder its designed flow conditions. The unit of measurement used in expressing the degree of filtration is the micron. A micron equals one-millionth of a meter, or 0.00004 inch. For comparison value, consider that the normal lower level of visibility to the naked eye is approximately 40 microns. (A grain of table salt is about 100 microns in size; the thickness of a human hair is about 70 microns; and a grain of talcum powder is about 10 microns in size.) Some hydraulic systems require a much finer degree of cleanliness than others. This is due to the closer tolerances required in some systems, such as the precisely mated parts inthe servomechanisms of missiles. Some systems only require that 98 percent of the particles over 100 microns in size are removed, while other systems require that 100 percent of the particles measuring 2 microns are removed. The degree of filtration of sinter FP.93 depends on the material and design of the filter Figure 7-10.Micronic filter element. element. Filter elements are made of various mate- This typeelement is designed to prevent rials, such as clay, plastic, fuller's earth, the passage of 99 percent of solids greater 108 ..itutpLer 'IRESERVOIRS, STRAINERS, FILTERS, AND ACCUMULATORS than 10 microns in size. The element in the the element is accomplished by dipping it in a full flow filter illustrated in figure 7-8 is of cleaning solvent that is compatible with the this design. liquid used in the system and allowing the sol- The element is normally thrown away when vent to flow through the element. The element removed. It is replaced at periodic intervals should then be dried by blowing dry, filtered in accordance with the applicable technical in- air through the element from the inside to the structions. The element is replaced in the fol- outside. lowing manner: STAINLESS STEEL.Stainless steel filter elements are used in the hydraulic systems of 1. Relieve system pressure. many of the Navy's most modern aircraft. This 2. Remove the bowl from the filter body. type element is similar in construction to the (In some systems, such as aircraft hydraulic sintered bronze element described previously. systems, the bowl is safetywired to the body The design is usually a corrugated sintered and the wire must be cut for removal.) stainlesssteel mesh such as the magnified cross section shown in figure 7-11. One man- 3. Remove the filter element from the ufacturer calls this type a "Dutch Twill" pat- filter body with a slight rocking motion. Do not tern. Elements of this type are capable of twist the element. filtering 95 percent of 5- to 10-micron particles 4. Replace the element with a new one and 100 percent of 25-micron particles from the whenever possible. If a new element is not fluid. The curved passages of the filter element, available, the old element may be cleaned, in- through which the fluid passes, limit the length spected, and reinstalled. of the particles that pass through the element. NOTE: A used micronic filter may be cleaned Most filters that use the stainless steel element by masking the element outlet and rinsing it are equipped with the contamination indicator with a cleaning solvent that is compatible with described earlier in this chapter. the fluid used in the system. After cleaning, inspect carefully to insure that there are no holes in the paper walls of the element. 5. Replace all old 0-ring packings and backup washers with new ones. 6. Reinstall the bowl onto the body. Do not tighten the bowl excessively; handtight is usually sufficient. 7. Pressurize the system and check the filter assembly for leaks. Safetywire if required. SINTERED BRONZE.The sintered bronze FP.94 element consists of minute bronze balls joined Figure 7-11.Cross-section of a stainless together as one solid piece, but still remaining steel filter element. porous. The process of joining the balls is known as the sintered process. This element is capable of filtering particles greater than 5 Stainless steel elements are of the reusable microns, or approximately 0.0002 inch in size. type. The specific time limit on the usage of this type filter differs with the type of system The operation of the sintered bronze filter and equipment. Also, special equipment and is very similar to that of the micronic filter. procedures are required in the cleaning of this Most sintered bronze filters are equipped with type element. For these reasons, the applicable either the bypass relief valve or the contam- technical instructions must be consulted when ination indicator described in this chapter. maintenance is performed on this type filter The sintered bronze element may be cleaned, element. tested, and reused a maximum of four times at Some hydraulic systems have magnetic fil- which time it must be replaced. Cleaning of ters installed at strategic points. Filters of 109

114 _ FLUID POWER

this type are designed primarily to trap any extremely cold atmospheric conditions, some magnetic particles that may be in the system. means must be provided to heat the fluid. In some systems, it is impossible to incorporate a reservoir large enough to dissipate enough Temperature Control heat to provide satisfactory cooling. These systems require some means to cool the fluid. Hydraulic systems operate most efficiently when the fluid temperature is held within a The component used to heat or cool liquids specific range. All hydraulic fluids are designed is usually referred to as a heat exchanger. Air, to provide minimum flow resistance with suit- steam, or another liquid is usually used as the able sealing properties when the fluid temper- heat exchange medium. Some systems use an ature is maintained within the correct range. electric heat probe, immersed in the fluid The recommended temperature range varies within the reservoir for heating the fluid. with different types of fluid; for example, the There are several different designs of heat recommended operating temperature of one type exchangers. One type is similar in design to fluid is -65° F to +275° F, while the recom- the automobile radiator in which the fluid flows mended range of another type is -25° F to through small tubes in the core, and air is +140°F. Operating at a temperature below forced through the honeycomb material around thak of the recommended range results in slug- the core to cool the fluid. Another type is the gish movement of the fluid through orifices and shell-and-tube heat exchanger. This type con- other restrictions in the lines and components. sists of a cylindrical metal shell containing a bundle of metal tubes. The hydraulic fluid flows Temperatures higher than the desired level through the tubes while the heating or cooling reduce the lubricating characteristics of the medium flowsthrough the shell around the fluid andlso cause the fluid to break down, tubes. This type can use steam or hot water as forming s' Age and other contaminants. Heat a heating medium. For cooling, water or refrig- will also cause sealing materials to become erated liquid may be used. brittle and close-fitting precision parts of valves and other components to seize. In addition, high Fin tubing is another type of heat exchanger temperatures lower the viscosity of the fluid used in some hydraulic systems. The fins help which, in turn, reduces the efficiency of the to dissipate the heat from the fluid flowing through the line. Air may be forced around the pump. lines to increase the cooling effect. The fin Excessive ly low fluid temperatures are tubing type heat exchanger can also be im- caused primarily by external sources, for ex- mersed in a heating or cooling medium. An ample, hydraulic systems operating in extremely example of this type installation is illustrated cold climates. External sources are also a infigure 7-12. In this installation, several major cause of excessively high fluid temper- loops of the fin tubing are mounted in one of atures. However, in addition to the effects of the fuel cells of an aircraft. Fluid enters the external temperature, the friction,resulting inlet coupling, flows through the fin wall tubing, from fluid flowing through components and long through the outlet coupling, and then returns to lengths of lines, creates heat. the reservoir. The heat of the fluid flowing through the heat exchanger is absorbed by the In most systems, temperature control is not fins and the fuel. a problem. Although slightly cold temperatures Another type of radiator heatex- may cause sluggish action of the fluid when changer is illustrated in figure 7-13. This first operating a system after it has been idle typeis used in several mode Is of air- for a period of time, the friction resulting from craft to cool the hydraulic fluid.Likethe the flow of fluid through the system will usually fin tubing heat exchanger illustrated in fig- increase the temperature of the fluid to the ure 7-12, the radiator type utilizes fuel desired level. As discussed previously in this as a cooling medium.This heat exchanger chapter, most of the heat is dissipated from the is used to cool the fluid for two separate hy- fluid as it circulates through the reservoir. In drnulic systems. In addition, it has a fuel filter some systems, however, it is necessary to incorporated which filters the fuel supply to control the fluid temperature. For example, in the engine. 110 115 Chapter 7RESERVOIRS, STRAINERS, FILTERS, AND ACCUMULATORS

FP.95 Figure 7-12.Fin tubing heat exchanger.

The radiator unit consists of a cylindrical PNEUMATIC GASES housing containing two cooling coils of 1/2- inch aluminum alloy tubing and a replaceable Clean, dry gas is required for the efficient fuelfilter element. The coolingcoil for operation of pneumatic systems.Due to the one hydraulic system is installed in the right- normal conditions of the atmosphere, free air hand end of the housing; the cooling coil for seldom satisfies these requirements adequately. the other hydraulic system and the filter ele- The atmosphere contains dust and impurities ment are installedin the left-hand end as in various amounts, depending on the locality. viewed in figure 7-13.The housing ends con- Also,the atmosphere generally contains a tain fittings for connecting fuel hoses. Threaded substantial amount of moisture in vapor form. fittings, which are secured to the cooling coil Solids such as dust, rust, or pipe scale in ends, serve to connect the hydraulic lines from pneumatic systems may lead to excessive wear each system. and failure of components, and in some cases In operation, hydraulic fluid returning to prevent the pneumatic devices from operating. the reservoir is directed through the applic- Moisture is also very harmful to the system. able system cooling coil where sufficient heat It washes lubrication from moving parts, thereby is transferred to the engine fuel fo maintain aiding corrosion and causing excessive wear of the temperature of the hydraulic fluid at the components.Moisture will also settle in low desired level.Should the cooling coils be- spots in the system and freeze during cold come clogged, each hydraulic system is equipped weather, causing stoppage of the system or with a bypass relief valve which opens and rupture of lines. bypasses fluid around the coil and directly As discussed in chapter 3,the use of to the reservoir. nitrogen decreases these hazards to some 111 -116 FLUID POWER

FM'. FILTER ELEMENT

ENGINE FUEL SYSTEM NI HYDRAULIC COOLING COIL SUPPLY HOSE MIER BYPASS RELIEF VALVE

STRAP

SUPPORT BRACKET

TEN *2 HYDRAULIC OUTLET LINE TO RESERVOIR

SYSTEM al HYDRAULIC INLET LINE

SYSTEM *2 HYDRAULIC COOLING COIL

FUEL RELIEF VALVE SPRING

FUEL HOSE

HYDRAULIC OUTLET LINE TO RESERVOIR

LEGEND SYSTEM 42 HYDRAULIC INLET LINE INDICATES DIRECTION OF FLUID FLOW

Figure 7-13.Radiator type heat exchanger. FP.96 extent. However, it is not economically feasible Removal of Solids to use nitrogen,except in small systems. The removal of solids from the gas of An ideal filter would remove all dirt and pneumatic systems is generally accomplished moisture from a pneumatic system without caus- by screening (filtering), centrifugal force, or ing a pressure drop in the process. Obviously, a combination of the two.In some cases, such a condition can only be approached; it the removal of moisture is accomplished in cannot be attained. conjunction with the removal of solids. 112

.117 Chapter 7RESEM OIRS, STRAINERS, FILTERS, AND ACCUMULATORS

Some types of air filters are similar in design and operation to the hydraulic filters discussed earlier in this chapter. Some materials used in the construction of elements for air filters are woven screen wire, steel wool, fiber OUTLET INLET glass,and felt fabrics.Elements made of these materials are often used in the unit that filters the air as it enters the compressor. They are used in the same manner as carbu- retor filters on automobile engines. These filters must be checked frequently for the accumulation of foreign matter and the element cleaned or replaced as necessary.If the filter element becomes clogged, the compressor is reduced in efficiency or, in extreme cases, becomes inoperative.Filters of this type are usually designed to remove particles as small as 25 microns. Porous metal and ceramic elements are commonly used in filters that are installed in the compressed air supply lines.These filters also use a controlled airpatl to provide some filtration.Internal design causes the air to flow ina centrifugal path within the bowl. (See fig. 7-14.)Heavy particles and water droplets are thrown out from the airstream and drop to the bottom of the bowl. The air FP. 97 then flows through the filter element, which Figure 7-14.Air filter. filters out most of the smaller particles. This type filter is designed with a drain valve at the bottom of the bowl. When the valve is opened with air pressure in the system, the accumulation of solids and water will be blown out of OUT the bowl.Many air filters of this type have transparent bowls so that the accumulation of impurities can be seen.Some units are provided with automatic drainage. An air filter that employs moving mechanical devices as an element is illustrated in figure 7- 15.As the compressed air passes through the filter the force revolves a number of multi- blade rotors at high speed. Moisture and dirt are caught on the blades of the rotors. The whirling blades hurl the impurities by centrifugal force to the outer rims of the rotors and to the inner walls of the filter housing. Here, contaminating matter is out of the airstream and falls to the bottom of the bowl where it must be drained at periodic intervals. Removal of Moisture FP.99 When it enters the compressor, air con- Figure 7-15.Air filter using rotating blades as tains a certain amount of moisture in vapor form. element. 113

I .t18 FLUID POWER

When it enters the compressor, air con- of springs, or the compressibility of gases. In tains a certain amount of moisture in vapor addition to a source of fluid supply, accumu- form. During the compression process, the lators also provide several other useful func- temperature and pressure of the air increase tions. These functions are described in the last and the moisture remains in vapor form. If the part of this section under "Applications." moisture would remain in vapor form it would have very little effect on the system. However, DEAD-WEIGHT ACCUMULATORS as the compressed air cools, the moisture vapor condenses into water. When it is in this The dead-weight or gravity type accumulator condensed state, moisture causes corrosion and represents the earliest form of accumulators rust, dilutes lubricants, and may freeze in the and is generally used as a single unit to operate lines and components. a multiple battery of machines. Since cooling of the compressed air causes This accumulator consists of a vertical the moisture to condense, cooling devices are cylinder with a smoothly machined bore into installed immediately after the compressor be- which is fitted a piston with suitable packing. fore the air enters the receiver and flows on into (See fig. 7-17.) A platform is mounted on the the system.There is a water separator fitted top of the piston on which is placed a high on the discharge side of the cooler. As the air density material, such as cone rete blocks, metal, is cooled, the moisture condenses and is re- or stone. The force of gravity provides energy moved from the system as the air passes to force the liquid from the cylinder into the through the water separator.The separators hydraulic system.The main advantage of this are of a variety of designs.The removal of type accumulator is that the pressure is con- moisture is accomplished by centrifugal force, stant regardless of the amount of liquid in the impact, or sudden changes in velocity of the cylinder, since the weight of the material on airstream. Many water separators are similar top of the piston remains constant. The main in design and operation to the filters illustrated disadvantage of the dead-weight accumulator is in figures 7-14 and 7-15. its cumbersome size and weight. Chemical driers are incorporated it some pneumatic systems. Their purpose is te absorb SPRING-OPERATED ACCUMULATOR any moisture that may collect in the Urn 3 after the water separator. Air drier, dehydrator, air The spring-operated accumulator is similar purifier, and desiccator are all terms used by in principle and design to the dead-weight type, different manufactures to identify these compon- except that spring tension supplies the force. ents. Figure 7-18 depicts two examples of spring- One type of chemical drier is illustrated in operated accumulators. figure 7-18.This unit consists of the housing, The load characteristics of a spring are a cartridge containing a chemical agent, a filter such that the energy storage depends on the (sintered bronze), and a spring. Various types force required to compress the spring.The of absorbent chemicals are used by the differ- uncompressed length of the spring represents ent manufacturers in the construction of the car- zero energy storage. tridges.To insure proper filtering, the air In this type accumulator the compression re- must pass through the drier in the proper di- sulting from the maximum installed length of the rection.The correct direction of flow is in- spring or springs should provide the minimum dicated by an arrow and the word FLOW printed pressure required of the liquid in the cylinder on the side of the cartridge.The cartridge assembly. As liquid is forced into the cylinder, must be replaced at intervals specified by the the piston is forced upward and the spring or applicable technical springs are further compressed, thus providing ACCUMULATOR a reservoir of potential energy for later use. Hydraulic accumulators are incorporated in some hydraulic systems to store a volume of A1R-OPERATED ACCUMULATORS liquid under pressure for subsequent conversion into useful work.This potential energy The air-operated accumulatorisoften may be the result of gravitation, the elasticity referred to as a pneumatic or hydropneumatic 114

.119 Chapter 7RESERVOIRS, STRAINERS, FILTERS, AND ACCUMULATORS

CARTRIDGE OUTLET HOUSING 41b. SPRING ILTER

DRIER CARTRIDGE INLET

CARTRIDGE AND HOUSING-

FILTER DESICCANT LFR.TER A we:74: s-r assolnOW1110:4"j.

CARTRIDGE FP.99 Figure 7 -16. -- Chemical drier. accumulator.This type accumulator utilizes Separator Type compressed gas (usually air or nitrogen) to apply force to the stored liquid. Air-operated In the separator type of air-operated accumu- accumulators are classified as either nonsep- lator, a means is provided to separate the gas arator or separator types. from the liquid. Three types of separators are bladder or bag, diaphragm, and piston (cylinder). Nonseparator Type BLADDER OR BAG.Figure 7-20 illustrates In the nonseparator type accumulator, no one version of an air-operated accumulator of means are provided for separating the gas the bladder type.This accumulator derives from the liquid.It consists of a fully enclosed its name from the shape of the synthetic rubber cylinder, mounted in a vertical position, con- bladder or bag which separates the liquid and taining a liquid port on the bottom and a pneu- gas within the accumulator.The accumulator matic charging port at the top.Figure 7-19 consists of a seamless high-pressure shell, depictsanonseparatortype air-operated cylindrical in shape with domed ends.The accumulator. bladder is fully enclosed in the shell and is To prevent gas from entering the system, molded to an air stem in the upper end of the there must be some liquid trapped in the bottom shell, as it appears in the illustration.The of the cylinder when this type accumulator is air stem contains a high-pressure air valve. placed in service.Nitrogen or compressed The bottom end of the shell is sealed with a air is then charged into the tcp of the cylinder special plug assembly containing the liquid port until the pressure is equivalent to the minimum and usually a safety feature, which makes it system requirements.As the system forces impossible to disassemble the accumulator with liquid into. the bottom of the cylinder, the gas pressure in the syetem. is further compressed and this compressed The bladder is larger in diameter at the gas is then used as the energy medium. Only top (near the air valve) and gradually tapers about 70 percent of the accumulator liquid is to a smaller diameter at the bottom. The used in the system, as the remaining space synthetic rubber is thinner at the top of the must act as a separator to prevent the gas from bladder than at the bottom.The operation entering the hydraulic system. of the accumulator is based on Barlow's formula FLUID POWER

DIAPHRAGM.Although there are several different modifications of the diaphragm accumulator, it is usually spherical in shape. Figure 7-21 illustrates an example of this type. The shell is constructed of two metal hemispheres, which are either screwed or bolted together. The fluid and gas chambers are seperated by WEIGHTS a snythetic rubber diaphragm. An air valve for pressurizing the accumulator is located in the gas chamber end of the sphere, and the liquid port to the hydraulic system is located on the opposite end of the sphere. This accumulator operates very similar to the bladder type. CYLINDER.A cylinder type accumulator is illustrated in figure 7-22. This accumulator contains a free-floating piston, which separates the gas and liquid chambers.The cylindrical accumulator consists of a barrel assembly, a piston assembly, and two end cap assemblies. The barrel assembly houses the piston and incorporates provisions for securing the end caps. The piston contains two packings. A small drilled passage from the liquid side of the piston to a point between the two seals provides for lubrication of the seal on the air chamber side.

Operation TO HYDRAULIC SYSTEM In the operation of pneumatic type accumulators, the compressed air chamber is inflated with air or nitrogen to a predetermined pres- F P. 100 sure that is somewhat lower than system pres- Figure 7-17.Dead-weight accumulate.r. sure. This pressure is stipulated by the system manufacturer.This initial charge of gas is for hoop stress which stases:the stress in referred to as accumulator preload. a circle is directly propor',ional to its diameter and wall thickness.T'Ais means that for a As an example of accumulator operation, certain thickness, a large diameter circle will assume that an accumulator is designed for a stretch faster than a small diameter circle; preload of 600 psi in a 1,500 psi system. or for a certain diameter, a thin wall hoop The hydraulic system pressure should be willstretch faster than a thick wall hoop. zero when the initial charge of 600 psi preload Thus, the bladder will stretch around the top is introduced into the air chamber of the accum- at its largest diameter and thinnest wall thick- ulator.Some accumulators are equiped with ness, and then will gradually stretch down- air pressure gages for checking the preload. ward and push itself outward against the walls When the accumulator is not so equipped, a of the shell. As a result the bladder is capable suitable high-pressure air gage must be in- of squeezing out all the liquid from the accu- stalled at the air preload fitting for this pur- mulator. Consequently, the bladder accumulator pose. As air pressure is applied through the has a very high volumetric efficiency. In other air pressure port,it moves the separator words, this type accumulator is capable of sup- (bladder, diaphragm, or piston) toward the op- plying a large percentage of the stored fluid to posite end. The 600 psi preload moves or do work. stretches the separator to the extent that the 116 Chapter 7RESERVOIRS, STRAINERS,FILTERS, AND ACCUMULATORS

MULTIPLE SPRINGS SINGLE SPRING Ras

SPRING

RAM ASSEMBLY

1116RAM

CYLINDER TO HYDRAULIC SYSTEM ITO HYDRAULIC SYSTEM

FP.101 Figure 7-18.Spring-operated accumulators. volume of gas under pressure completely fills terior with soapy water, whichwill form bubbles the entire shell of the 'accumulator. After the where the air leaks occur. accumulator has been preloaded to 600 psi, The air valve assembly should be loosened the hydraulic system is operated and the pump to examine the accumulator for internal leaks. will force fluid against the separator of the If liquid comes out of the air valve, it indicates accumulator.The system pressure must in- a leak in the separator.This is caused by crease to a pressure greater than 600 psibe- a ruptured bladder or diaphragm, orby faulty fore the hydraulic fluid can move the separator. packing on the piston type.As a result the Thus, at 601 psi the separator will start to accumulator must be removed, repaired, and move within the accumulator,compressing the reinstalled. gas as it moves. At 1,500 psi, the gas is com- Before removing an accumulator from the pressed to the extent that it occupies less than system for repair, all internal pressure must one-half the space that it did at 600 psi. The be relieved.The pumps must be turned off remaining space stores a volume of liquidunder and system pressure relieved by actuating some pressure for subsequent demands of the system. part of the system until all pressure is dissi- When actuation of hydraulic circuits lowers pated. The preload is released by loosening system pressure, it is evident that the com- theswivel nut on the air filler valve until pressed gas will expand against the separator, all air is completely exhausted, then remove forcing liquid from the accumulator, thus sup- the valve. plying an instantaneous supply ofliquid to the hydraulic system. Applications Pneumatic accumulators should be visually examined for indications of external hydraulic Many of the present day hydraulic systems leaks periodically. They should then be exam- are equipped with one or moreaccumulators. ined for external air leaks by brushing the ex- The storage of liquid under oressure serves 117

. 122 FLUID POWER

HIGH-PRESSURE AIR surge when stopped suddenly by the closing LINE FOR PRECHARGING of a valve and thus develop instantaneouspres- sures two and three times the operating pressure of the system.This shock will result in ob- jectional noise and vibration which cancause considerable damage to piping, fittings, andcom- ponents.The incorporation of an accumulator will enable such shock and surges to be absorbed or cushioned by the entrapped gas, thereby reducing their effects.The accumulator will also dampen pressure surges caused by the FLUID LEVEL pulsating delivery from the pump. AID TO PUMP. There are times when most hydraulic systems require large volumes of liquid for short periods of time.This is due to large cylinders or the necessity of operating two or more circuits simultaneously.It is

TO HYDRAULIC SYSTEM AIR .r* 11411.:d ,P,Atfei

FP.102 *.; 11110.) or Figure 7-19.Air-operated accumulator I? (nonseparator type).

several purposes in hydraulic systems,some of which are described in the followingpara- graphs.Some of the hydraulic systems illustrated and described in Chapter 13 of thisman- ual show the applications of accumulators and their relationship to other components in the system. LEAKAGE COMPENSATOR.In someoper- ations it is necessary to maintain the hydraulic system under a controlledpressure for long periods oftime. It is very difficult to maintain a closed system withoutsome leakage, either external or internal. Even a small LIQUID leak can decrease the requiredpressure. By the proper use of an accumulator, this leakage can be compensated for and the pressure maintained within an acceptable range for longpe- riods of time. In the same manner, theaccumulator can compensate for thermal expansion and contraction of the liquid due to variations in temperature. FP.103 SHOCK. ABSORBER.A liquid, flowing ata Figure 7 -20. Air- operated bladder high velocity in a pipe, will createa backward type accumulator. 118 Chapter 7RESERVOIRS, STRAINERS, FILTERS, AND ACCUMULATORS not economical to incorporate a pump of such capacity in the system for only intermittent usage to supply these applications, particularly if there aresufficient intervals during the working cyclefor an accumulator to store up a volume of liquid to aid the pump during these peak demands. EMERGENCY POWER SUPPLY. The energy stored in an accumulator maybe used to actuate a unit in the event of normal hydraulic system failure.For example, in an aircraft hydraulic system, sufficient energy can be stored in theaccumulator for several applications of the wheel brakes.

AIR 04AMEER

AN VALVE CAP

FP.104 Figure 7-21.Diaphragm accumulator.

HYDRAULIC FLUID POR

ENO CAP

END CAP -BARREL ASSEMBLY

ACKING AND BACKUP RING

LUBRICATION PASSAGE

PISTON

AIR PORT

Figure7-22.Cylinder (piston) accumulator. FP.105

119

241.: CHAPTER 8

PUMPS AND COMPRESSORS

To accomplish work, fluid power systems energy. Many different sources are used to require some means to provide a flow of fluid. providethe mechanical power to thepump. Pumps are utilized to provide this requirement Electric motors,air motors, and gasoline in hydraulic systems. Although the volume and engines are all used as a source ofpower to the pressure of a compressed gas provide the operate hydraulic pumps.Some punips are flow in pneumatic systems, some means mustbe manually operated.Many of the pumps in utilized to compress the gas. The air compressor aircraft hydraulic systems are attached to and is commonly used in pneumatic systemsfor this driven by the aircraft engine. purpose and is, therefore, considered the source The purpose of a hydraulic pump is to supply of power in pneumatic systems.It should be a flow of fluid to a hydraulic system. It should noted that some pneumatic systems do not contain be emphasized that a pump does not create a compressor.In these systems, a container, pressure, since pressure can be created only such as a cylinder or bottle, of compressed gas by a resistance to the flow.As it provides (air or nitrogen) is connected to the system to flow, the pump transmits a force to the fluid. provide the power for the system.However, As a result, a pump can produce the flow and some type of compressor must be used to charge force necessary for the development ofpressure; (fill) these containers with the compressedgas. however, since it cannot provide resistance to Although pumps and compressors doserve its own flow, a pump, alone, cannot produce similar functions in their respective systems, pressure.Resistance to flow is the result of the principles of operation and the design differ a restriction or obstruction in the path of the to some extent.Due to these differences, the flow. This restriction is normally the work two components are discussed separately in accomplished by the hydraulic system, butcan this chapter. also be restrictions of lines, fittings, and valves within the system.Thus, the pressure is controlled by the load imposed on the systemor the PUMPS action of a pressure regulating device. In order for a pump to supply a flow of fluid Pumps are used for a number of essential to the pump outlet and into the system, it must services in the Navy. Pumps supply water to the have a continuous supply of fluid available to boilers, draw condensation from the condensers, the inlet port. Atmospheric pressure (discussed supply sea water to the firemain, circulate in chapter 2) plays an important rolein the cooling water for coolers and condensers,pump supply of fluid to the inlet port.If the pump out bilges, transfer fuel, supply water to the is located at a level lower than that of the distilling plants, and serve many otherpurposes. reservoir,the force of gravity supplements Although the pumps discussed in this chapterare atmospheric pressure. However, in mostsys- those used in hydraulic systems, the principles tems the pump is located above the reservoir. of 'operation of the different types apply to the Aircraft and missiles that operate at high pumps used in other systems. altitudes are equipped with pressurized hydraulic reservoirs (see chapter 7) to compensate PURPOSE for low atmospheric pressure encountered at high altitudes. A hydraulic pump is a device which converts As the pump forces fluid through the outlet Mechanical force and motion into hydraulic port, a partial vacuum or low-pressurearea 120 125 Chapter 8PUMPS AND COMPRESSORS

(often referred to as suction) is created at the Basically, pumps which discharge liquid in a inlet port.This low-pressure area contains a continuous flow are referred to as nonpositive pressure lower than the surrounding atmos- displacement, and those which discharge vol- pheric pressure (approximately 14.7 psi at sea umes separated by a period of no discharge level).When the pressure at the inlet port are referred to as positive displacement. of the pump is lower than the local atmospheric Although the nonpositive displacement pump pressure, the atmospheric pressure acting on normally produces a continuous flow, it does the fluid in the reservoir forces the fluid into not provide a positive seal against slippage the pump. and, therefore, the output of the pump varies as system pressure varies.In other words, the volume of fluid delivered for each cycle is PUMP PERFORMANCE dependent upon the resistance offered to the Pumps are normally rated in terms of flow.This type pump produces a force on the volumetric output and pressure. The volumetric fluid that is constant for each particular speed output is the amount of fluid the pump can of the pump.Resistance in the discharge line deliver to its outlet port in a certain period of produces a force in the opposite direction. time at a given speed.Volumetric output is When these forces are equal, the fluid is in a usually expressed in gallons per minute (gpm). state of equilibrium and does not flow. Since changes in pump speed affect volumetric If the outlet of a nonpositive displacement output,some pumps are rated according to pump is completely closed, the discharge pres- displacement. Pump displacement is the amount sure will increase to the maximum for that of fluid the pump can deliver per cycle. Since particular pump at a specific speed.Nothing most pumps use a rotary drive, the displacement more will happen except that the pump will is usually expressed in terms of cubic inches churn the fluid and produce heat. per revolution. As stated previously, a pump does not create In contrast to the nonpositive displacement pressure. However, the pressure developed by pump, the positive displacement pump provides the restriction in the system is a factor that a positive internal seal against slippage. There- affects the volumetric output of the pump. As fore, this type pump delivers a definite volume the system pressure increases, the volumetric offluid for each cycle of pump operation, output decreases.This drop in volumetric regardless of the resistance offered, provided output is the result of an increase in the amount the capacity of the power unit driving the pump of internal leakage from the outlet side to the is not exceeded.If the outlet of a positive inlet side of the pump. This leakage is referred displacement pump is completely closed, the to as pump slippage and is a factorthat must pressure would instantaneously increase to the be considered in all pumps.Therefore, most point at which the unit driving the pump would pumps are rated in terms of volumetric output stall or something would break. at a given pressure. Some pumps have greater Positive displacement pumps are further internal slippage than others.This indicates classified as fixed displacement or variable the efficiency of a pump and is usually expressed displacement.The fixed displacement pump in percent. deliversthe same amount of fluid on each cycle.The output volume can be changed only by changing the speed of the pump.When a CLASSIFICATION OF PUMPS pump of this type is used in a hydraulic system, a pressure regulator (unloading valve) must be Many different methods are used to classify incorporated in the system. A pressure rag- pumps. Terms such as nonpositive displace- aul tor or unloading valve is used in a hydraulic ment, positive displacement, fixed displacement, system to control the amount of pressure in the variable displacement, fixed delivery, variable system and to unload or relieve the pump when delivery, constant volume, and others are used the desired pressure is reached.This action to describe pumps. The first two of these terms of a pressure regulator keeps the pump from describe the fundamental division of pumps; working against a load when the hydraulic that is, all pumps are either of the nonpositive system is at maximum pressure and not func- displacement or positive displacement type. tioning. During this time the pressure regulator 121 FLUID POWER bypasses the fluid from the pump back to the water against the bottom of the bucketand reservoir.(See chapter 10 for more detailed will not spill, even when the bucket isupside information concerning pressure regulators.) down.By whirling the bucket very rapidly, The pump continues to deliver a fixed volume a force many times the force of gravitycan of fluid during each cycle. Such terms as fixed be developed. delivery, constant delivery, and constantvolume To understand the application of this example are all used to identify the fixed displacement to a centrifugal pump, assume thata number pump. of bottomless buckets rotating abouta center The variable displacement pump is con- are placed as shown in figure 8-1 (A).The structed in such a manner that the displacement bottom ofthe buckets is sealed by contact per cycle can be varied.The displacement with what might be called a continuousbottom is varied through the use of an integralcon- inthe shapeofthe bounding wall against trolling device which controls the working re- which theyrotate. As the buckets rotate, lationship of the internal operating mechanisms centrifugal force pushes theliquid against of the pump. Some of these controlling devices this continuous bottom. are described later in this chapter. If the buckets are shaped like pieces of Pumps may also be classified according to pie,and they completely fill the circle,as the specific design used to create the flow of shown in figure 8-1 (B), then the buckets will fluid. Practically all hydraulic pumps fall within act as a paddle wheel revolving ina drum- three classifications of designcentrifugal,ro- shaped container.The result is a continuous tary, and reciprocating. These classifications liquid pressure due to centrifugalforce over are. best illustrated in the following discussion all the circumference of the container.The of principles of operation. pressure is the result of the restriction to the flow of the liquid caused by thewalls of the container. CENTRIFUGAL PUMPS By enlarging the diameter of the container (fig. 8-1 (C)), a space (A) is created for liquid There are several different types of cen- between the ends of the paddles and the drum. trifugal pumps, but they all operateon the same Therotationof the liquid in the buckets, principle.In each case, the operation depends because of centrifugal force, pushes outward upon thecentrifugal force produced by the against the liquid bottom and therebyimparts rotation of an impeller at high speeds.This a pressure to the liquid in space (A). U the force, in turn, imparts a high velocity to the liquid remains stationarywhich wouldnot actu- fluid delivered through the pump. ally be the case in practicethen it wouldbe aubjected to a true static pressure, the magnitude Centrifugal force is based on the principle of which would be dependentupon the centrifugal of inertia.Although inertia is discussed in force of the liquid between the paddles. detail in chapter 2, for convenience, the basic law is repeated here: "A body at rest tends If an opening were provided in thecontaining to remain at rest, and a body in motion tends drum, the centrifugal force wouldcause the to continue in motion with thesame velocity liquid in space (A) tobe discharged. Ifthe liquid and in the same direction." in space (A) were stationary, this discharge could take any direction because, accordingto Many illustrations of centrifugal force could Pascal's law, be cited. static pressure in a fluid is A familiar one is the tendency of an transmitted equally in all directions. Inprac- automobile to skid towards the outside of the tice, however, another factormut be considered curve when rounding a corner at high speed. which determines the direction of discharge. For the same reason the occupants of the This factor is explained shortly. automobile are forced against the outer side. If a means for continuously filling the bucket An example of centrifugal force, helpful for the is provided, such as a central openingin the understanding ofcentrifugal pumps, can be container,and a source of power to rotate found if one imagines a man whirlinga bucket the paddles is provided, thena continuous flow, of water rapidly around in a circleover his as shown in figure 8-1 (D),exists. This head. If he whirls the bucket rapidly enough, demonstrates one of the principlesthat apply the outward or centrifugal force will holdthe to the operation of the centrifugalpump: 122 Chapter 8PUMPS AND COMPRESSORS

( A ) CB)

When a force which is compelling an object times to impart velocity, and then let go one to move in a curved path is removed, the object of the strings. This principle is utilized in the will travel in a straight line with the speed and centrifugal pump in addition to the outward direction it had at the instant the force was action of centrifugal force previously described. removed. This is illustrated in figure 8-2. Referring to figure 8-1 (D), as soon as some 4 An object is attached to a line (R) and swung of the liquid in space (A) is permitted to escape, around in a circular path.Assume that the a continuous suction or low-pressure area will line breaks when it reaches the vertical position be set up in the center opening.(This center shown. The object will then fly off horizontally opening is commonly referred to as the eye.) (at right angles to the line) with the velocity it Atmospheric pressure and,in some cases, had at that moment.This horizontal path will gravity force acting on the liquid in the reser- be tangent to the circular path the object was voir will force liquid into the eye of the con- following up to this moment, so this action is tainer.The blades or paddles will then start commonly called "going off on a tangent," while rotating the liquid.As the liquid from space the velocity is called tangential velocity,. (A) flows out of the container, the liquid be- This is the ;:rinciple of the old fashioned tween the blades will move out to a greater sling shot, in which a man placed a stone in'a and greater radius.In so doing it will acquire leather holder to which two strings were at- an increasing tangential velocity, because the tached, swung it around in a circular path several angular velocity of the blades is constant, and 123 128 FLUID POWER

FP.107 Figure 8-2.-- Tangential velocity.

as the radius of the circular path increases, the liquid must travel 'a greater distanceper FP.108 revolution. Eventually, it will reach the tip of Figure 8-3.Basic elements ofa centrifugal the blades and enter space (A), which is equiva- pump. lent to "going off on a tangent"as described previously. The liquid will also tend to continue moving directly outward from 'the center ofthe The details of the size and shape of thevolute circle because of its inertia. The two actions used are very important to the efficiencyof the centrifugal and tangentialare then combined. pump, as are likewise the exact shape, size, and speed of rotation of the impeller. In general, This liquid that is thrown off the tips of the it is true that impeller blades thatcurve back- blades has a velocity equal to the tangential ward with respect to their direction of rotation velocity it had the instant it left them. The two give better results than straight blades, butthe actionscentrifugal force and tangential veloci- reasons for this and for the other details of tycause the liquid to flow out of the discharge port. construction are matters which concern the designer primarily, and are outside thescope of Since it is part of the meaning of velocity this training manual. that it have direction (see chapter 2), the direction in which the discharge takes place becomes Types important. In order to make the mostuse of the velocity, the discharge is generally inan approximately tangential direction. The two most common types of centrifugal pumps used in hydraulic systems are the volute The liquid is thrown off the paddles,or im- and diffuser types. In both types, flowisprovid- peller blades, as they are called in thepump, ed by the action of centrifugalforce. At thesame all around the circle. Therefore, liquid is added time, the rotation of the impellerproduces the to the space (A) at all points, butescapes from greater part of the velocity thatforces theliquid this space at only one point. In order tocom- out through the discharge. The major differences pensate for this, it is customary to increase the between the two types are described inthe fol- area of space (A) as the outlet is approached. lowing paragraphs. By this means it is also possible toslow down the tangential velocity at a more gradualrate, and thereby obtain more efficient VOLUTE TYPE. Figure 8-4 illustrates the conversion of operation of a volute type centrifugalpump. the velocity in the pump into flow to thesystem. The impeller discharges the liquid ata high Therefore, it is customary to design spade (A) velocity into a progressively expandingcasing. in the shape like that shown in figure 8-3.This This contour of the casing directsthe movement shapecalled a volutecontinuously expands in of the liquid through the outlet port andinto the a definite ratio. system. 124 Chapter 8PUMPS AND COMPRESSORS

As a rule, each impeller acts separately, dis- NOZZLE charging to the inlet (suction) side of the next stage impeller. For the sake of compactness, the several impellers which make up a multi- VOLUTE)Cla stage pump are almost invariably placed on one 0 shaft. Figure 8-6 shows a possible flow diagram -w 1°1IMPELLER for a four-stage pump. %, The illustrations of centrifugal pumps in this a°/ section emphasize horizontal pumps; that is, VOLUTE VOLUTE pumps with their shaft parallel to the horizontal axis. These pumps can operate just as efficiently FP.109 when installed in such a position that the shaft Figure 8-4.Volute type centrifugal pump. is parallel to the vertical axis. However, these pumps are sometimes classified as horizontal or vertical pumps. DIFFUSER TYPE.Figure 8-5 illustrates the diffuser type pump. It is similar in design to the volute pump,v: ith the addition of a sery of stationary blades incorporated around th outside circumference of the impeller blades. These stationary blades are termed the diffuser. The diffuser blades are curved in the opposite direc- INTAKErj tion from the rotating impeller blades. The diffuser reduces the velocity of the liquid and decreases slippage (previously described), which =» increases the efficiency of the pump.

DISCHARGE FP.111 Figure 8-6.Flow diagram of a four-stage centrifugal pump.

The impellers used in centrifugal pumps may be classified as single-suction or double-suction. The single-suction impeller allows liquid to enter the eye from one direction only; the double- suction type allows liquid to enter the eye from two directions. Impellers are also classified as CLOSED or OPEN. Closed impellers have side walls that extend from the eye to the outer edge of the blade tips; open impellers do not have these side walls. FP.110 Manufacturers sometimes classify centrifu- Figure 8-5.Diffuser type centrifugal pump. gal pumps according to the size and shape of the casing, arrangement of the discharge nozzle in relation to the pumping chamber, and the type Classification of material used in construction of the pump. Centrifugal pumps may be driven by any of Centrifugal pumps are classified in several the sources mentioned previously in this chapter. ways. For example, they may be either single- Many of the centrifugal pumps aboard ship are stage or multistage. A single-stage pump his driven by steam turbines. As a rule, centrifugal only one impeller. A multistage pump has two or pumps are direct drivethat is,the impeller more impellers housed together in one casing. rotates at the same rpm as the power source. 125 FLUID POWER

DRIVEN GEAR

DRIVING GEAR

PUMP PRESSURE

ATMOSPHERIC PRESSURE FP.112 Figure 8-7.Gear type rotary pump.

However, some low-pressure centrifugalpumps In some hydraulic systems a centrifugalpump have reduction gears installed between thepower is used with a variable displacementpump. Both source and the impeller. This allows the power pumps are driven by the same electric motor. source to operate at a high speed and the im- The motor is fully enclosed and is cooled bya peller to operate at a lower speed, thus obtaining flow of liquid from the centrifugalpump. The maximum efficiency from both the power source centrifugal pump also provides a supply of liquid and the pump. to the inlet port of the variable displacement pump. This insures an adequate supply of liquid Applications to the main pump (variable displacement) for all demands of the system. Since centrifugal pumpti are nonpositive dis- placeinent, their use in hydraulic systems is normally lithited to systems that requirea large ROTARY PUMPS volume of flow and operate at a relatively low pressure. Some hydraulic systems require vari- All rotary pumps operate bymeans of ations of flow and pressure. A centrifugalpump rotating parts which trap the fluid at the inlet is sometimes used in combination witha positive (suction) port and force it through the discharge displacement pump to supply the demands of such port into the system. Gears, screws, lobes, and systems. The positive displacement pump main- vanes are commonly used as elements in rotary tains the system during periods requiring low pumps. Rotary pumps operate on the positive volume and high pressure. The centrifugalpump displacement principle andare of the fixed aide the positive displacement pump when the displacement type. system requires large volumes and lowpres- Rotary pumps are designed with very small sure. clearances between rotating parts and between PP'

Chapter 8PUMPS AND COMPRESSORS

rotating parts and stationary parts, in order to entirelycomplete. This overlapping and the minimize slippage from the discharge side back relatively larger space at the center of the gears to the suction side. Pumps of this type are de- tend to minimize pulsations and give a steadier signed to operate at relatively moderate speeds flow than is produced by the spur gear pump. in order to maintain these clearances. Operation The helical gear pump is still another modi- at higher speed causes erosion and excessive fication of the simple gear pump. Because of wear, resulting in increased clearances. the helical gear design, the overlapping of successive discharges from spaces between the Classification teeth is even greater than it is in the herringbone gear pump; therefore, the discharge flow Like centrifugal pumps, there are numerous is smoother. Since the discharge flow is smooth types of rotary pumps and various methods of in the helical pump, the gears can be designed classification. They may be classified as to with a small number of large teeththus allow- shaft positioneither vertically or horizontally ing increased capacity without sacrificing mounted; the type of driveelectric motor, smoothness of flow. gasoline engine, etc.; manufacturer's name; or Figure 8-9 shows a helical gear pump. The service application. However, classification of pumping gears of this type pump are driven by rotary pumps is generally made according to a set of timing and driving gears, which also the type of rotating element. A few of the most function to maintain the required close clear- common types of rotary pumps are discussed ances while preventing actual metallic contact in the following paragraphs. between the pumping gears. (Metallic contact GEAR.The simple gear pump (fig.8-7) between the teeth of the pumping gears would consists of two meshed gears which revolve provide a tighter seal against slippage; however, in a housing. The drive gear in the illustration it would cause rapid wear of the teeth, because in turned by a drive shaft which engages the foreign matter in the liquid would be present power source.The clearances between the on the contact surfaces.) gear teeth as they mesh and between the teeth Roller bearings at both ends of the gear and the pump housing are very small. shafts maintain proper alignment and minimize The inlet port is connected to the fluid the friction loss in the transmission of power. supply line, and the outlet port is connected to Suitable packings are used to prevent leakage the pressure line. Referring to figure 8-7, the around the shaft. drive gear is turning in a counterclockwise The spur, herringbone, and helical are all direction, and the driven (idle) gear is turning classified as external gear pumps. This is in a clockwise direction. As the teeth pass the because both sets of teeth project outward from inlet port, liquid is trapped between the teeth the center of the gears. In an internal gear and the housing. This liquid is carried around pump, the teeth of one gear project outward, the housing to the outlet port. As the teeth but the teeth of the other gear project inward mesh again, the liquid between the teeth is toward the center of the pump, as shown in displaced intotheoutlet port.This action figure 8-10 (A). One gear wheelthe external produces a positive flow of liquid into the system. gearstands inside the otherthe internal gear. A shearpin or shear section is incorporated One type of internal gear pump is illustrated inthe driveshaft.This is to protect the in figure 8-10 (B). The external (drive) gear is power source or reduction gears if the pump attached directly tc the drive shaft of the pump fails because of excessive load or jamming of and is placed off center in relation to the internal parts. gear. The two gears mesh on one side of the The gears of the pump illustrated in figure pump, between the inlet (suction) and discharge 8-7 are referred to as spur gears. Herringbone ports. On the opposite side of the chamber, a and helical gears are also used in the construc- crescent-shaped form is located in the space tion of gear pumps. between the two gears in such a way as to The herringbone gear pump (fig. 8-8) is a provide a close tolerance with them. modification ofthe simple gear pump. The The rotation of the center gear by the drive liquid is pumped in the same manner as in the shaft causes the outside gear to rotate, since spur gear pump. However, in the operation of the two are in mesh. Everything in the chamber the herringbone pump, one discharge phase rotates except the crescent. This action causes begins before the previous discharge phase is liquid to be trapped in the gear spaces as they 127 FLUID POWER

FP.113 Figure 8-8.Herringbone gear pump. the crescent. The liquid is carried from the chamber. The inner gear is connected to the inlet port to the discharge port, where it is drive shaft of the source of power. forced out of the pump by the meshing of the The operation of this type internal gear gears. As the liquid is forced from the inlet pump is illustrated in figure 8-12. To simplify side of the pump, a low-pressure area is created the explanation, the teeth of the inner gear and at the inlet port allowing atmospheric pressure the spaces between the teeth of the outer gear to force liquid into the pump from the source of are numbered. Note that the inner gear has one supply. The size of the crescent that separates less tooth than the outer gear. The tooth form the internal and external gears is the determin- of each gear is related to that of the other in ing factor in the volume 'delivery of the pump. such a way that each tooth of the inner gear is A small crescent allows more volume of liquid always in sliding contact with the surface of per revolution than a larger crescent. the outer gear. Each tooth of the inner gear Another design of internal gear pump is meshes with the outer gear at just one point illustrated in figures 8-11 and 8-12. This pump during each revolution. In the illustration, this consists of a pair of 'gear-shaped elements, point is at the top (X). In view A, tooth 1 of the one within theother, located in the pump inner gear is in mesh with space 1 of the outer 128 Chapter 8PUMPS AND COMPRESSORS

11111.1111....-.

HELICAL GEAR

INTAKE

FP.114 Figure 8-9.Helical gear pump. gear. As the gears continue to rotate in a At one side of the point of mesh, pockets of clockwise direction and the teeth approachpoint increasing size are formed as the gears rotate, (X), tooth 6 of the inner gear will mesh with space while on the other side the pockets decrease in 7 of the outer gear, tooth 5 with space 6, etc. size. In figure 8-12, the pockets on the right- During this revolution, tooth 1 will mesh with hand side of the drawings are increasing in size space 2; and the following revolution, tooth 1 will as one moves down the illustration, while those mesh with space 3. As a result, the outer gear on the left-hand side are decreasing in size. will rotate at just six-sevenths the speed of the The intake side of the pump would therefore be inner gear. For example, if the inner gear rotates to the right and the discharge to the left. In at 1,400 rpm, the outer gear will rotate at 1,200 figure 8 -11, since the right-hand side of the rpm. drawing was turned over to show the ports, the intake and discharge appearreversed. Actually, A in one drawing covers Ain the other.

.SUCTION PORT INNER GEAR OUTER GEAR PORT

SUCTION

DISCHARGE

NG POCICETS NG POCKETS (SUCTION/ ( Al (B) (DISCHRGE) FP.115 FP.116 Figure 8-10.Offcentered internal gear pump. Figure 8-11.Centered internal gear puma. 129

.1 34 FLUID POWER

and, therefore, is sometimes classified as a gear type pump.The screw pump has two or more meshing screws which rotate to develop fluid flair. These screws mesh to form a fluid-tight sbal between the screws and between the screws and the housing. Screw type pumps are available in several different designs; however, they all operate in a similar manner. One design of a screw pump (A) is illustrated in figure 8-13.Since this pump contains three screws, it is referred to as a DECREASING INCREASING triple screw pump. The screws revolve within PACK TS POCKETS a close-fittinghousing. The power screw (rotor)in the center is in mesh with (and drives) the two idling rotors (idlers).

FLEXIBLE COUPLING

UPPER THRUST PLATE

IDLER POWER ROTOR

ROTOR HOUSING IDLER

DISCHARGE SUCTION

SPACER RING

POWER ROTOR HOUSING LOCATING CAP IDLER LOCATING LOWER THRUST CAPS PLATE

FP.118 Figure 8-13.Triple screw pump.

The supply line is connected to the pump FP.117 intake which, in turn, opens into the chambers Figure 8-12.Principles of operation at the ends of the screw assembly. As the of the internal gear pump. screws turn,the liquid flows in between the threads at each end of the assembly.At the SCREW.Another type of rotating element end of the first turn, a spiral-shaped quantity used in rotary pumps is the screw. The design of fluid is trapped when the ends of the threads of this elemetit is similar to that of a worm gear become in' mesh again.The threads carry 130 Chapter 8PUMPS AND COMPRESSORS the fluid along within the housing toward the centerofthe pump to the discharge port. ADJUSTMENT MECHANISM In some screw pumps, the intake port is DISCHARGE located near one end of the pump and the INTAKE discharge near the other.In these pumps the screws are designed in such a manner that fluid enters one end of the pump, is forced through the pump by the screws to the opposite end, and is discharged into the system. LOBE.The principle of operation of the lobe pump is exactly the same as for the external gear pump described previously.The lobes are considerably larger than gear teeth, but there are only two or three lobes on each rotor. A three-lobe pump-is illustrated in figure 8-14. The two elements are rotated, one being directly driven by the source of power, and the other SLIDING ROTATING through timing gears. As the elements rotate, VANES ELEMENT liquid is trapped between two lobes of each FP.120 rotor and the walls of the pump chamber, and Figure 8-15.Vane pump carried around from the intake side to the discharge side of the pump. As liquid leaves the intake chamber, the pressure in the intake power source such as an electric motor, gasoline chamber is lowered, and additional liquid is engine, etc. The rotor is slotted and each slot forced into the chamber from the source of is fitted with a rectangular vane. These vanes, supply. to some extent, are free to move outward in their respective slots.The rotor and vanes DISCHARGE are enclosed in a housing, the inner surface of which is offset with the drive axis. As the rotor turns, centrifugal force keeps the vanes snug against the wall of the housing. The vanes divide the area between the rotor and housing into a series of chambers.The chambers varyin size according totheir respective positions around the shaft. The inlet port is located in that part of the pump where INTAKE the chambers are expanding in size sothat FP.119 the partial vacuum formed by this expansion allows liquid to flow into the pump. The liquid Figure 8-14.Lobe pump. is trapped between the vanes and is carried to the outlet side of the pump. The chambers The lobes are so constructed that there is contract in size on the outlet side, and this a continuous seal at the point of juncture at the action foices the liquid through the outlet port center of the pump. The lobes of the pump and into the sybtem. illustrated in figure 8-14 are fitted with small The pump in figure 8-15 is referred to as vanes, at the outer edge to improve the seal of unbalanced because all of the pumping action the pump. Although these vanes are mechanically takes place on one side of the shaft and rotor. held in their slots, they are, to some extent, This causbs a side load on the shaft and rotor. free to move outward. Centrifugal force keeps Some vane pumps are constructed with an the vanes snug against the chamber and the eliptical-shaped housing which forms two sepa- other rotating members. rate pumping areas on opposite sides of the VANE.Figure 8-15 illustrates a vane pump rotor.This cancels out the side loads; thereof the unbalanced design. The rotor is attached fore, such pumps are referred to as balanced to the drive shaft and is rotated by an outside vane. 131

136 FLUID POWER

Application Hand Pumps There are two types of manually operated Although some rotary pumps are capable reciprocating pumpsthe single-action and the of operating in high-pressure systems (above double-action. The single - action pump provides 1,500 psi), their use is usually limited to systems flow during every other stroke, while the double- which operate at pressures of 1,500 psi or action provides flow during each stroke. Single- below. As compared to other types of hydraulic action pumps are frequently used in hydraulic pumps, rotary pumps are less expensive, have jacks.This type pump is illustrated in figure fewer moving parts, and feature simplicity in 4-6 in chapter 4. designs and construction. A double-action hand pump is illustrated in The efficiency of rotary pumps depends on the figure 8-16.This type pump is used in some close tolerances between the rotating elements aircraft hydraulic systems asa source of and, in m.ostpumps, between the rotating element hydraulic power for emergencies, for testing and the housing of the pump. This efficiency is certain subsystems during preventive mainte- quickly reduced by excessive wear. For this nanceinspections,and for determining the reason, dirt picked up by the liquid must be causes of malfunctions in these subsystems. eliminated by suitable filters. This type pump is also used as a secondary Rotary pumps are usually used in hydraulic source of hydraulic power in hydraulic test systems that do not require the continuous, or benches to test hydraulic components. near continuous, operation of the pump.For example, electric-motor-driven gear pumps are OUTLET CHECK INLET PISTON often used to operate the emergency hydraulic PORT D VALVE71 \ PORT C systems in aircraft. This provides an efficient pump to supply fluid to the system in case of . main system failure.It is far less expensive oo . than the type pump generally used in the main system and, due to its limited use, maintains efficiency. CHECK VALVE A DOUBLE-ACTION HAND ilUMP FP.121 RECIPROCATING PUMPS Figure 8-16.,T-Hydraultc hand pump. The term reciprocating is defined as back- and-forth motion.In the reciprocating pump it This pump consists of a cylinder, a piston is this back-and-forth motion of pistons inside containing a built-in check valve (A), a piston of cylinders which provides the flow of fluid. rod, an operating handle, and a check valve (B) Reciprocating pumps, like rotary pumps, operate at the inlet port.When the piston is moved to on the positive principlethat is, each stroke the left,the force of the liquid in the outlet delivers a definite volume of liquid to the system. chamber and spring tension cause valve (A) to The master cylinder of the automobile brake close. This movement causes the piston to system, which is described and illustrated in force the liquid in the outlet chamber through chapter 4, is an example of a simple recipro- the outlet port (D) and into the system. This cating pump. Most manually operated hydraulic same movement of the piston causes a low- pumps are of the reciprocating type. Several pressure area in the inlet chamber. Atmospheric types of power operated hydraulic pumps, such pressure acting on the liquid in the reservoir as the radial piston and axial piston, are also transmits this pressure to the liquid at the classified as reciprocating pumps. These pumps inlet port (C).This differential of pressures are sometimes classified as rotary pumps, acting on the ball of check valve (B) causes the because a rotary motion is imparted to the spring to compress and open the check valve. pumps by the source of power. However, the This allows liquid to enter the inlet chamber. actual pumping is performed by sets of pistons reciprocating inside sets of cylinders. Some of When the piston completes this stroke to the these different types of reciprocating pumps left, the inlet chamber is full of liquid.This are discussed inthe following paragraphs. liquid eliminates the low-pressure area in the Chapter 8PUMPS AND COMPRESSORS inlet chamber, thereby allowing spring tension to close check valve (B). PISTON CYLINDER BLOCK When the piston is moved to the right, the DISCHARGE force of the confined liquid in the inlet chamber acts on theball of check valve (A).This SLIDE BLOCK acticr. compresses the spring and opens check PINTLE valve (A), allowing the liquid to flow from the (STATIONARY) intake chamber to the outlet chamber. Because of the area occupied by the piston rod, the 4 outlet chamber cannot contain all the liquid SLIDE BLOCK INTAKE discharged from the inlet chamber. Since the CONTROL TOR liquid will not compress, the extra liquid is forced out of port (D) into the system. (A)

CYLINDER BLOCK Radial Piston Pumps

SLIDE BLOCK Figure 8-17 illustrates the operation of the radial piston pump.The pump consists of a PINTLE pintle which remains stationary and acts as a (STATIONARY) valve; a cylinder block which revolves around the pintle and contains the cylinders in which ROTOR the pistons operate; a rotor which houses the SUDE OCK reaction ring of hardened steel 'against which CONTROL the piston heal; press; and a slide block which PISTON is used to control the length of the piston (B) strokes.The slide block does not revolve but houses and supportsthe rotor, which does PISTON YLINDER BLOCK revolve due to the friction set up by the sliding DISCHARGE action between the piston heads and the reaction ring.The cylinder block is attached to the ROTOR drive shaft. PINTLE Referring to view (A) of figure 8-17, assume (STATIONARY) that space (X) in one of the cylinders of the SLIDE BLOCK cylinder block contains liquid and that the N./ 4 3 respective piston of this cylinder is at position SUDE BLOCK INTAKE (1). When the cylinder block and piston are CONTROL rotated in a clockwise direction, the piston is forced into its cylinder as it approaches (C) position (2).This action reduces the volumetric CYLINDER BLOCK PISTON size of the cylinder and forces a quantity of INTAKE liquid out of the cylinder and into the outlet port above the pintle.This pumping action is SLIDE BLOCK dueto the fact that the rotor,in the slide PINTLE block, is off centered in relation to the center (STATIONARY) of the cylinder block. TOR In figure 8-17 (B), the piston has reached 4 position (2) and has forced the liquid out of the SUDE BLOCK open end of the cylinder through the outlet above CONTROL DISCHARGE the pintle and into the system. While the piston moves from position(2) to position (3), the (0) open end of the cylinder passes over the solid. FP.122 part of the pintle; therefore, there is no intake Figure 8-17.Principles of operation or discharge of liquid during this time. As the of the radial piston pump.

,133 FLUID POWER

piston and cylinder move from position (3) to A pump of this type with only one piston position (4), centrifugal force causes the piston would not be practical.Therefore, the radial to move outward against the reaction ring of the piston pump is designed with several pistons. rotor. During this time the open end of the Note that the pump illustrated in figure 8-18 cylinder is open to the intake side of the pintle contains an odd number of pistons. All radial and, therefore, fills with liquid. As the piston pumps are designed with an odd number of moves from position (4) to position (1), the open pistons-7,9, 11, etc.This is to insure that end of the cylinder is against the solid side of no more than one cylinder is completely blocked the pintle and no intake or discharge of liquid by the pintle at any one time. If there were an takes place. After the piston has passed the even number of pistons spaced evenly around pintle and starts toward position (2), another the cylinder block (for example, eight), there discharge of liquid takes place. Alternate intake would be occasions when two of the cylinders and discharge continues as the rotor is revolved would be blocked by the pintle, while at other about its axisintake on one side of the pintle times none would be blocked. This would cause and discharge on the other, as the piston slides three cylinders to discharge at one time and in and out. four at one time, causing pulsations in flow. Notice in views (A) and (B) of figure 8-17 With an odd number of pistons spaced evenly that the center point of the rotor is different around the cylinder block, only one cylinder is from the center point of the cylinder block. It is completely blocked by the pintle at any one the difference of these centers that produces the time. This reduces pulsations of flow. pumping action. If the rotor is moved so that its Some of the principal parts of the radial center point is the same as that of the cylinder piston pump are described in more detail in the block, as shown in figure 8-17 (C), there is no following paragraphs. pumping action, since the piston does not move PINTLE.In figure 8-17 the pintle is shown, back and forth in the cylinder as it rotates with for the sake of simplicity, as a flat bar around the cylinder block. which the rotor turns. Actually, the pintle is a The flow in this pump can be reversed by round bar which serves as a stationary shaft moving the slide block, and therefore the rotor, around which the cylinder block turns.The to the right so that the relation of the centers of pintle shaft, as shown in figure 8-19, hasfour 4 the rotor and the cylinder block is reversed from holes bored from one end lengthwise through the position shown in views (A) and (B) of figure part of its length. Two holes serve as the intake 8-17. Figure 8-17 (D) shows this arrangement. and the remaining two as the discharge. Two Liquid enters the cylinder as the piston travels slots are cut in the side of the shaft so that from position (1) to position (2) and is discharged each slot connects two of the lengthwise holes. from the cylinder as the piston travels from The two slots are in line with the cylinders position (3) to (4). when the cylinder block is assembled on the In the illustrations the rotor is shown in pintle. One of these slots provides the path for the center, the extreme right, or the extreme the liquid to pass from the cylinders to the left in relation to the cylinder block. The amount discharge hole bores into the pintle. The other of adjustment in distance between the two centers slot connects the two inlet holes to the cylinders determines the length of the piston stroke, which during the entrance of liquid.The discharge controls the amount of liquid flow in and out of holes are corrected through appropriate fittings the dylinder. Thus, this adjustment determines to a discharge line so that the liquid can be the displacement of the pump; that is, the volume directed into the system.The other pair of of liquid the pump delivers per revolution. This holes are connected to the inlet line. adjustment may be controlled in different ways. CYLINDER BLOCK.One type of cylinder Manual control by means of a handwheel is the block is illustrated in figure 8-20.This is a simplest. The pump illustrated in figure 8-17 is cylindrical-shaped block of metal with a hole controlled in this way. For automatic control of bored through the center to accommodate the delivery to accommodate varying volume re- pintle.The cylinder holes are bored from the quirements during the operating cycle, a hy- outer edge of the block to the center hole and draulically controlled cylinder may be used to are spaced at equal distances around the cir- position the slide block. A gear-motor controlled cumference of the block. Both the cylinder holes by a pushbutton or limit switch is sometimes and the center hole are very accurately machined used for this purpose. so that liquid loss arouna the pistons and the 134 Chapter 8PUMPS AND COMPRESSORS

C3'.

4.4P

INTL

FP.123 Figure 8-18.Nine-piston radial piston pump. pintle is kept to a minimum. There are several curved plates. These curved plates,sometimes different designs of cylinder blocks. Some, like referred to as curved shoes or slippers, bear the one illustrated in figure 8-20, appear to be against and slide around the inside surface of the almost solid, while others have spokelike cyl- rotor.Like the cylinder walls, the sides of the inders radiating out from the center. pistons are accurately machined to fit the PISTONS.Like cylinder blocks, pistons are cylinders, so there is a minimum loss of liquid manufactured in different designs. Some of these between thewalls of the cylinders and the designs are illustrated in figure 8-21. View (A) pistons.No provision is made for the use of shows a piston with small wheels that roll around piston ringsto helpseal against leakage. the inside surface of the rotor. View (B) shows a ROTOR.Here again, the design may be piston in which the conical edge of the top bears different from pump to pump, as showninfigure directly against the reaction ring of the rotor. 8-22. The rotor consists essentially of a circular In this particular design, while thepiston moves ring, machine finished on the inside, against back and forth in the cylinder, it will rotate which the pistons bear. The rotor rotates within about its axis, so that the top surface will wear the slide block which can be shifted from side to uniformly. View (C) shows a piston attached to side to control the length of the stroke of the 195 140, FLUID POWER

(Al CBI (C) FP.126 Figure 8-21.Pistons for radial piston pump. FP.124 Figure 8- 19. Pintle for radial piston pump.

FP.127 Figure 8-22.Rotors for radial piston pump.

However, in the axial piston pump, the block with its pistons rotate on a shaft in such a way that the pistons reciprocate in "their cylinders FP.125 along lines parallel to the axis of the shaft. Figure 8-20.Cylinder block for This is called axial motion. radial piston pump. The pumping action of this pump is made possible by a universal joint or link.Figure pistons. The slide block has two pairs of 8-23 is a series of drawings which illustrates machined surfaces on the exterior so that it can how the universal joint is used in the operation slide in tracks in the pump case. The sliding of the axial piston pump. motion is controlled by any means covered .First, a rocker arm is installed on a hori- earlier in this chapter. zontal shaft.(See fig. 8-23 (A).)The arm is These parts, toge.ther with the drive shaft, joined to the shaft by a pin in such a way that constitute the main working parts of the radial it can be swung back and forth, as indicated in piston pump.The drive shaft is connected to view (13).Next, a ring is placed around the shaft the cylinder block and is driven by an outside and secured to the rocker arm so that the ring force such as an electric motor. can turn from left to right as shown in view (C). This provides two rotary motions in different Axial Piston Pumps planes at the same time, and in varying proportions as may be desired.The rocker arm Axial piston pumps have the same general can swing back and forth in one arc, and the ring characteristics asradialpistonpumps. can simultaneously move from left to right in 136 141 Chapter 8PUMPS AND COMPRESSORS

Figure 8-23 (E) shows the assembly after the shaft, still in a horizontal position, has been rotated a quarter turn. The rocker arm is still in the same position as the tilting plate, and is now perpendicular to the axis of the shaft. The ring has turned on the rocker pins, so that it has changed its position in relation to the rocker arm, but it remains parallel to, and in contact with, the tilting plate. View (F) of figure 8-23 shows the assembly (A) after the shaft has been rotated another quarter turn.The parts are now in the same position (E) as shown in view (D), but with the ends of the rocker arm reversed.The ring still bears against the tilting plate. As the shaft continues to rotate, the rocker arm and the ring turn about their pivots, with (B) each changing its relation to the other, and with RING the ring always bearing on the plate. Figure 8-23 (G) shows a wheel added to the assembly. The wheel is placed upright and fixed to the shaft, so that it rotates with the shaft.In addition, two rods, (A) and (B), are (F) loosely connected to the tilting ring, and extend through two holes standing opposite each other (C) in the fixed wheel. As the shaft is rotated, the RING fixed wheel turns perpendicular to the shaft at all times. The tilting ring rotates with the shaft ROCKER ARM and always remains tilted, since it remains in contact with the tilting plate. Referring to view (G), the distance along rod (A), from the tilting SHAFT ring to the fixed wheel, is greater than the distarAN3 along rod (B).As the assembly is rotated, however, the distance along rod (A) TILTING PLATE decreases as its point of attachment to the (C) tilting ring moves closer to the fixed wheel, (D) while the distance along rod (B) increases. FP.128 These changes continue until after a half revo- Figure 8-23.Relationship of the universal lution, at which time the initial positions of the joint in operation of the axial piston pump. rods have been reversed.After another half revolution, the two rods will again be in their original positions. another arc, in a plane at right angles to the As the assembly rotates, the rods are moved plane in which the rocker arm turns. in and out through the holes in the fixed wheel. This is the way that the axial piston pump Next, a tilting plate is added to the assembly. works.To get a pumping action, it is only The tilting plate is placed at a slant to the necessary to place pistons at the ends of the axis of the shaft, as depicted in figure 8-23 rods, beyond the fixed wheel, and insert them (D). The rocker arm is then slanted at the same in cylinders.The rods must be connected to angle as the tilting plate, so that it lies parallel the pistons and to the wheel by ball and socket to the tilting plate.The ring is also parallel joints.As the assembly rotates, each piston to, and in contact with, the tilting plate. The moves back and forth in its cylinder.Intake position of the ring in relation to the rocker arm and discharge lines can be arranged so that is unchanged from that shown in figure 8-23 (C). liquid enters the cylinders while the spaces 137 142 FLUID POWER between the piston heads and the bases of the As shown in figure 8-23 (G), the distance cylinders are increasing, and leaves the cyl- the pistons move back and forth in their cylinders inders during the other half of each revolution depends on the tilt or angle of the tilting plate. when the pistons are moving in the opposite In the pump illustrated in figures 8-24 and 8-25. direction. this tilt or angle is fixed by the shape of the These operating principles are illustrated housing and therefore is referred to as a fixed in the different views of the axial piston pump displacement or constant displacethrent pump. shown in figures 8-24 and 8-25. This type pump Pump output is determined by the angle and, consists of a circular cylinder with either 7 or since this angle is fixed, can be changed only 9 equally spaced pistons. by varying the pump speed. The main parts of the pump are the drive With no tilt at all, no pumping action would shaft, pistons, cylinder block, and valve plate. occur since the piston would not move back and There are two ports in the valve plate. These forth. The distances (A) and (B) in figure ports connect directly to openings in the face of 8-23 (G) would be equal, and would remain equal the cylinder block.Liquid is forced in one as the assembly rotates.If the angle of tilt port by atmospheric pressure and forced out given to the tilting plate were reversed, making the other port by the reciprocating action of the distance (A) less than distance (B), the pumping action would be reversed.What had been the pistons. discharge would now become the intake and There is a fill port in the top of the cylinder vice versa.By adding a mechanism to control housing. This opening is normallykeptplugged, the angle of the tilting plate, any variation of but it can be opened for testing the pressure in delivery can be obtained, from a maximum flow the housing or case.When installing a new in onedirection through zero (no flow) to pump or a newly repaired one, this plug must maximum flow in the opposite direction, although be removed and the housing filled with the the drive shaft continues to rotate at a constant recommended liquid before the pump is operated. speed.Axial piston pumps designed with such There is a drain port in the mounting flange a controlling device are referred to as variable to drain away any leakage from the drive shaft delivery or variable displacement pumps, de- oil seal. fined previously in this chapter. When the drive shaft is rotated, it rotates A variable displacement pump is shown in the pistons and cylinder block with it.The figure 8-26.This type pump contains either offset position of the cylinder block causes the seven or nine single-acting pistons. The pistons pistons to move back and forth in the cylinder reciprocate within cylinder bores which are block while the shaft, pistons, and cylinder block evenly spaced around a cylinder barrel. (Note rotate together.As the pistons reciprocate that the term barrel, as used in this discussion, in the cylinder block, liquid enters one port actually refers to a cylinder block which contains cylinders.)The piston rods are attached and is forced out the other. to a socket ring by means of ball-and-socket In figure 8-25, piston (A) is shown at the connections.The socket ring rides on a thrust bottom of the stroke.When piston (A) has bearing carried by a castingthe tilting box rotated- to the position held by piston (B), it or plate. will have moved upward in its cylinder, forcing When the tilting plate is at right angles to liquid through the outlet port during the entire the shaft, and the pump is rotating, the pistons distance.During the remainder of the rotation do not reciprocate; therefore, no pumping action back to its original position, the piston travels takes place. When the tilting box is tilted away downward in the cylinder. This action creates from a right angle, the pistons reciprocate and a low-pressure area in the cylinder; therefore, liquid is pumped. atmospheric pressure forces liquid through the Since the displacement of this type pump is inlet port into the cylinder.Since each one of varied by changing the angle of the tilting box, the pistons performs the same operation in some means must be used to control the changes succession, liquid is constantly being taken into of this angle.Various methods are used to the cylinder bores through the inlet port -and control this movementmanual, electric, pneu- discharged from the cylinder bores into the matic, or hydraulic. The operation of a hydrau- system.This action provides a steady, non- lically controlled variable displacement pump is pulsating flow of liquid. 138 143.. Chapter 8PUMPS AND COMPRESSORS

DRAIN PORT

FILL PORT

VALVE PLATE

FP.129 Figure 8-24.Partial cutaway view of axial piston pump.

pump described previously, the Stratopower pump is available in either the fixed displacement or the variable displacement type.The pump shown in figure 8-27 is the fixed displacement PISTON II type. CYLINDER Two major functions are performed by the BLOCK IN internal parts of the fixed displacement Strato- Our pump.These functions are mechanical drive and fluid displacement. UNIVERSAL The mechanical drive mechanism is shown in LINK figure 8-28.Piston motion is caused by the drive cam displacing each piston the full height of the drive cam each revolution of the shaft. By coupling the ring of pistons with a nutating UN (wobble)platesupported by a fixed center RADIAL MARINO pivot, the pistons are held in constant contact THRUST MARINO with the cam face. As the drive cam depresses RADIAL MARINO DRAIN one side of the nutating plate (as pistons are advanced), the other side of the nutating plate SEAL OUPLINO SHAFT is withdrawn an equal amount, moving the FP.130 pistons with it.The two creep plates are pro- Figure 8-25.Schematic diagram of vided to decrease wear on the revolving cam. axial piston pump. A schematic diagram of the displacement of fluid is shown in figure 8-29. Fluid is displaced by axial motion of the pistons. As each piston described in the hydraulic power drive system advances in its respective cylinder block bore, in chapter 13. pressure opens the check and a quantity of fluid Another type of axial piston pump is illus- is forced past.Combined back pressure and tratedin figure8-27. This type pump is check spring tension closes the check when the sometimes referred toas an inline puffy; piston advances to its foremost position. The however, it is most commonly referred to as low-pressure area occurring in the cylinder a Stratopower pump.Like the axial piston during the piston return allows atmospheric 139 -1:44 FLUID POWER

VALVE PLATE CONTROL SHAFT CONNECTING ROD

TILTING BLOCK

SUCTION OR DISCHARGE

SHAFT TRUNNION BLOCK

FORVIARD REVERSE

TILTING BLOCK POSITIONS

FP.131 Figure 8- 26. Variable displacement axial piston pump.

140 145 Chapter 8PUMPS AND COMPRESSORS

CHECK SPRING

.1% ax DISCFIARGE

4,1 DRIVE COUPLING '`k

SEAL

FP.132 Figure 8-27.Cutaway view of Stratopower hydraulicpumpfixed displacement.

pressure to force fluid to flow from the intake loading groove into the cylinder. CRISP PLATS A fluid flow diagram of the fixed.displacement NOTATING PLATE Stratopower pump is illustrated in figure 8-30. Fluid enters the intake port and is discharged through the outlet port by the reciprocating PISTON action of the pistons. Fluid is circulated through the back of the pump for cooling and lubricating purposes by the centrifugal action of the drive FIXED PIVOT STATIONARY SEARING Cain. DRIVE CAM The internal features of the variable displacement Stratopower pump are illustrated in figure 8-31.This pump operates similar to FP.133 the fixed displacement Stratopowerpump; how- Figure 8-28.Mechanical drive ever, this pump provides the additional function Stratopower pump. of automatically varying the volumeoutput 141 146 FLUID POWER

motion,transmitted by a direct mechanical INTAKI BYPASS POUT CHICK linkage, moves sleeves axially on the pistons, thereby varying the time during which the relief holes are covered during each stroke. Fluid flows through the hollow pistons during IMF the forward stroke, and escapes out the relief holes untilthey are covered by the piston sleeves.The effective piston stroke (delivery) iscontrolled by the piston sleeve position. During nonflow requirements, only enough fluid is pumped to maintain pressure against leakage. During normal pump operatton, three conditions may existfull flow, partial flow, and zero or no flow.During full flow operation /// 11/7/ z/z/z///////// ////// ///z (fig. 8-33), fluid enters the intake port and is PISTON MINIM BLOCK DISCHMOI discharged to the system past the pump checks by thereciprocating actionof the pistons. FP.134 Piston sleeves cover the relief holes for the Figure 8-29.Fluid displacement entire discharge stroke. Stratopower pump. During partialflow, system pressure is sufficient (as bled through the orifice) to move the compensator stem against the compensator This function is controlled by the pressure in the spring force. hydraulic system. For example, assume that a If system pressure continues to build up, pump of this type rated at 3,000 psi, is providing as under nonflow conditions, the stem will be flow to a 3,000psi system. When system pres- moved further until the relief holes are un- sure reaches 3,000 psi and there is no demand covered for practically the entire piston stroke. on the system, the pump unloads (delivers no The relief holes will be covered only for that flow to the system). The pressure regulation and portion of the stroke necessary to maintain flow is accomplished by an internal bypass which system pressure against leakage and to produce automatically adjusts delivery of fluid to the adequate bypass flow. demands of the system. The bypass system is provided to supply Flow cutoff actually begins before the fluid self-lubrication, particularly when the pump is reaches system pressure.For example, in a in nonflow operation. The ring of bypass holes 3,000 psi system, flow cutoff begins at approxi- in the pistons are aligned with the bypass passage mately 2,850 psi and reaches zero flow (unloads) each time a piston reaches the very end of its at 3,000 psi. When the pump is operating in the forward travel.This pumps a small quantity unload condition, the bypass system provides of fluid out of the bypass passage back to the circulation of fluid internally for cooling and supply reservoir and provides a constant chang- lubrication of the pump. ing of fluid in the pump. The bypass is designed Four major functions are performed by the to pump against a considerable back pressure internal parts ofthe variable displacement for use with pressurized reservoirs. Stratopower pump.These functions are mechanical drive, fluid displacement, pressure control, and bypass.Two of these functions AIR COMPRESSORS mechanical drive and fluid displacementare identical to those performed by the fixed dis- As previously mentioned, compreLaors are placement Stratopower pump. used in pneumatic systems for requirements A schematic diagram of the pressure control similar to those required of pumps in hydraulic mechanism is shown in figure 8-32. Pressure systems. However,since gases are highly is bled through the control orifice into the compressible, the gas must be compressed in pressure compensator cylinder where it moves advance, stored in containers, and then released the compensator piston against the force of the in sufficient volume and at regulatedpressures, calibrated control (compensator) 8044. This from the container into the pneumatic system. 142

. 11.47 Chapter 8PUMPS AND COMPRESSORS

CHECK SPRING DISCHARGE

INTAKE CYLINDER BLOCK

FP.135 Figure 8-30.Fluid flowStratopower pump (fixed displacement).

There are two major types of compressors the stationary unit and the portable unit. The COMPRESSOR CLASSIFICATION stationaryunit consists of the compressor, receiver, power source, and controls. Thegas Air compressors are classified in several is compressed and then stored in the receiver ways. A compressor may be single-acting where it is piped to the different workareas. or double-acting, single-stage or multistage, Facilitiesarealsoprovided for charging and horizontal, angle, or vertical,as shown in (filling) bottles and cylinders. The bottles and figure 8-34. A compressor may be designed cylinders are then connected to the pneumatic so that only one stage of compression takes system and the compressed gas released into place within one compressing element,or so the system as required.The portable unit is that more than one stage takes placewithin similar to the stationary unit, except that itis one compressing element. In general, compres- smaller in design and is mounted on wheels sors are classified according to the type of so that it can be easily moved to the different compressing element, the source of driving work areas. Tho operation of aircompressors power, the method by which the driving unit is covered in the following paragraphs.Air is connected to the compressor, and thepres- receivers are di3cussed in detail in chapter 7. sure developed. 143 148 FLUE) POWER

CREEP PLATE BEARING PISTON SLEEVE COMPENSATOR BUSHING BYPASS PORPORT PLATE RIVE CAM

SEAL PLATE COMPENSATOR SPRING

DRIVE COUPLING ADJUSTING

:T 111nr COUPLING ! RETAINER .\'\\ COMPENSATOR SEAL RING Ale STEM PISTON MATING RING -.."'CHECK SPRING

SEAL SPRING CHECK NUTATING PLATE' NUTATING PLATE PIVOT INTAKE PISTON CYLINDER BLOCK INTAKE PASSAGE s CYLINDER BORE FP.136 Figure 8-31.Internal features of Stratopower variable displacement pump.

Types of Compressing Elements

CONTROL COMPENSATOR INTAKE ORIFICE SPRING Air compressor elements may be of the centrifugal,rotary-,or reciprocatingtypes. Most of the compressors used in the Navy 7,61, have reciprocating elements.(See fig. 8-35.) mit V)WX/YAV wmov..rz. In this type compressor the air is compressed RELIEF in one or more cylinders very much like the HOLESSPIDER compression which takes place in an internal combustion engine. RIVE CAM w :41i1E4 Sources of Power / ,/ I/Z' PISTON SLEEVE COMPENSATOR DISCHARGE Compressors are driven by electric motors, PISTON internal combustion engines, turbines, recipro- FP.137 cating steam engines, or hydraulic motors. Figure 8-32.Pressure control mechanism Most oftheair compressors in the naval Stratopower variable displacement pump. service are driven by electric motors. 11 "t:

111111i \/ \\N 111 uij ////// TT I , 11 Nem _dm or

/iIN --irgtgt1111-1111111M1i r ewe \ I U

'1111111w u Ini 4111i 1111 -I. ==-Y , "NAM* 111/ \ N A Effl,\ i \ /

II 5 IS

- -

5 I II - 55 1: II. - It II .6 5 - II II 5 II IS -:; IS - - - II II15 5 IS

I FLUID POWER

SUCTION VALVE DISCHARGE VALVE

DISCHARGE VALVE

N.NX\ X\ X\ XNNN\NNNWW%. CYLINDER PISTON SUCTION VALVE (A) (B) SUCTION VALVES

DISCHARGE VALVE DISCHARGE VALVE /DISCHARGEVALVES

SUCTION SUCTION VALVE VALVE %N 4/4, f CYLI DERS Aoe \ / S / \ MEN'0

PIST CYLINDERS

PISTONRODS

IPISTON

CRANK (0 ) (C)

4TH STAGE

1ST STAGE 1ST STAGE 2ND STAGE

2ND STAG 2ND STAGE

(E)

FP.139 Figure 8-34.Types of compressora. (A) Vortical; (B) horizontal; (C) angle; (D) duplex; (E) multistage.

146 151 .1i Chapter 8PUMPS AND COMPRESSORS

INTERCOOLER RELIEF VALVE

INTERCOOLER UNLOADER *0*010000,40p4040iii 0114 4 9 LIALLM49191441,11A-9-1

NitiLPKIls viCa3P" GUARD GUARD.., -WgN\

FEATHER' NikAu.sahlgu.L.LAil VALVE F ATHER VALVE SEAT -SEAT LOW PRESSURE HIGH PRESSURE cwf DISCHARGE SUCTION

'FIRST STAGE SECOND STAGE PISTON PISTON --r

-OIL RESERVOIR

FP.140 Figure 8-35.A simple two -stage reciprocating low-pressure aircompressor.

147 .1.52` FLUID POWER

the compressor and the motor. When using as required for the operation of various units. steam turbine drives, compressors are usually The complete unit normally includes the com- driven through reduction gears. pressor, filter and moisture separator assembly,an adjustable relief valve assembly, a control system, and an air receiver with a Pressure Classifications pressure gage. (See figure 8-37.) Most reciprocating compressors are sim- Compressors are classified as low-pres- ilarin design and operation. The following sure,medium-pressure,orhigh-pressure. discussion relates to the radial, three-stage, Low-pressure compressors are those which pistontype,high-pressure air compressor. provide a discharge pressure of 150 psi or ' less. Medium - pressure compressors are those which provide a discharge pressure of 151 Principles of Operation psi to 1,000 psi. Compressors which provide a discharge pressure above 1,000 psi are clas- This air compressor consists of three pis- sified as high-pressure. ton type air pumps connected in series. There- Most low-pressureair compressors are fore, the process of compressing ambient air of the two-stage type with either a vertical (see glossary) to a relatively high pressure V (fig. 8-35) or a vertical W arrangement of is accomplished in three stages. Each com- cylinders.Two-stage,V-type, low-pressure pressor stage consists of a cylinder, a re- compressors usually have one cylinder that ciprocating piston, an intake valve mechanism, provides the first (low-pressure) stage of com- and an exhaust valve mechanism. The recip- pression, and one cylinder that provides the rocating pistons are connected to, and actu- second (high-pressure)stage. W-type com- ated by, a master rod assembly attached to pressors have two cylinders for the first stage a shaft that is coupled to the power source by of compression and one cylinder for the second a flexible coupling. stage. This arrangement is shown in figure The intake and exhaust valves are usually 8-36 (A). hardened steel discs,carefully ground and Compressors may be classified according lapped to seat snugly against a shoulder at the to a number of other design features or op- end of the cylinder bore (chamber). The valves erating characteristics. are held against the valve seat by a tempered Medium-pressure air compressors are of steel spring. As the piston travels downward, the two-stage, vertical, duplex, single-acting the cylinder bore forward of the piston is at type.Many medium-pressurecompressors reduced pressure and the intake valve lifts have differential pistons. This type of piston from its seat and allows air to enter the cyl- provides more than one stage of compression inder. As the piston travels upward, the in- on each piston. (See fig. 8-36.) take valve closes and the air is compressed Modern aircompressors are generally until the pressure overcomes the resistance motor-driven (direct or geared), liquid or air of the exhaust valve spring. The exhaust valve cooled,four-stage,single-acting units with is then lifted from its seat and the compressed vertical or horizontal cylinder arrangement. air in the cylinder escapes through the outlet Cylinder arrangements for high-pressure air fitting into the interstage tubing. compressors utilized in the naval service are To compensate for the volumetric decrease illustrated in figure 8-36 (B). Small Capacity of the air as it is compressed, the cylinder high-pressure air systems may have three- bores and piston displacement of the three stage compressors. High capacity air systems stages are progressively decreased. The first- maybe equipped with five- or six-stage stage cylinder is the largest and, therefore, compressors. receives a relatively large volume of ambient air and compresses it into a smaller volume for delivery to the medium size second-stage RECIPROCATING AIR COMPRESSORS cylinder. Here, the compression cycle is repeated. The supercharged inlet air is further An air compressor assembly includes all compressed to a higher pressure and applied the associated equipment required for delivering to the third-stage cylinder. Here, the com- filtered, oil and moisture free compressed air pressioncycleis completed as the air is 148 Chapter 8PUMPS AND COMPRESSORS

2-STAGE WITH DIFFERENTIAL 4-STAGE ARRANGEMENTS PISTON

N 1

2 -STAGE VERTICAL WITH 2 DIFFERENTIAL PISTONS 3-STAGE ARRANGEMENTS (A) (B) FP.141 Figure 8-38.Air compressor cylinder arraggement. (A) Low- and medium- pressure cylinders; (B) high-pressure cylinders. compressed to its ultimate delivery pressure. base is used as the oil sump and oil pump (This is an application of Boyle 's law dis- housing. The oil level can be measured by a cussed in chapter 2.) dip stick, or, in some compressors, by an oil NOTE: These cycles compare with the num- level sight gage that is mounted on the out- ber of stages of a particular compressor. For side of the base. example, a six-stage compressor has six such During compressor operation, sump oil en- stages. ters the pump cylinders through ports in the cylinder walls on the up stroke of the piston. On the down stroke, the piston descends past Compressor Lubrication the ports, preventing the escape of oil. The Unlike hydraulic pumps, which are lubricated oil trapped in the cylinder is forced through by the liquid of the system passing around the a spray tube and is directed against the rapidly moving parts, the pneumatic compressor removing master rod. The impact forms a fine quires a lubrication system for its moving mist of oil which fills the interior of the crank- parts. case and provides lubrication of the master rod, bearings, pistons, connecting links, and Most low-pressure and medium - pressure" other internalparts. Internal lubrication of compressors are lubricated by a simple but the 'cylinders and valve mechanisms is aided effective combination of pressure, splash, and by the small quantity of vapor drawn into the mist principles.Normally,the compressor first-stage cylinder from the crankcase and 149 154 FLUID POWER

THIRD STAGE TO AIR RECEIVER

RELIEF VALVE ASSEMBLY

SECOND STAGE FILTER AND SEPARATOR ASSEMBLY

INTAKE FILTER FIRST ASSEMBLY STAGE ift\ DRAIN WLVE

FP.142 Figure 8-37.Air compressor assembly. passed on to the second and third stages through andisrecirculated. Oil from the outboard the interstage connecting tubes. bearings is carried back to the sump by drain Lubrication Of high-pressure air compres- lines. sor cylinders is generally accomplished by The discharge pressure of lubrication oil means of an adjustable mechanical force-feed pumps varies with different pump designs. A lubricator, which is driven from a recipro- reliefvalve,fitted to each pump, functions cating or a rotary part of the compressor. Oil when the discharge pressure exceeds the pres- is fed from the cylinder lubricator, bysep- sure for which the valve is set. When the re- arate feed lines,to each cylinder. A check lief valve lifts, excess oil is returned to the valve is installed at the end of each feed line sump. to keep the compressed air from forcing the oil back into the lubricator. Lubrication begins automatically asthe compressor starts up. Cooling The amount of oil that must be fed to the cylinder depends upon the cylinder diameter, the In some low-pressure air compressors, the cylinder wall temperature, and the viscosity heat, which normally results from rapidcom- of the oil. pression of a gas, is dissipated before the Lubrication of the other internal parts of temperature attains a troublesome level. This most modern high-pressure compressors, and is accomplished by using aluminum cylinders some medium- and low-pressure compressors, with integral cooling fins and a fan which blows is accomplished by an oil pump. Thepump cooling air past the cylinders and interstage (usually of the gear type) is attached to the tubing. This type of cooling is sufficient for compressor and is driven by the compressor most low-pressure air compressors when op- shaft. The pump is supplied with oil from the erated in well-ventilated spaces. compressor reservoir (crankcase) and delivers Most high-pressure and medium-pressure it,through a filter, to an oil cooler. From compressors aboard ship are cooled by sea the cooler, the oil is distributed to the top of water supplied from the ship's fire, flushing, each main bearing, to spray nozzles forre- or water service mains. The cooling water duction gears, and to outboard bearings. The is generally available to each unit throughat crankshaft is drilled so that oil fed to the main least two sources. Compressors located out- bearings is picked up by the main bearing side the large machinery spaces are generally journals and carried to the crank journals. equippedwith an attached circulating water The connecting rods contain passages which pump as a standby source of cooling water. conduct lubricating oil from the crank beam The cooling water is circulated through ings up to the wrist pin bushings. As oil leaks the compressor in much the same manneras out from the various bearings, it drips back an automobile engine.. The path of water in the to the oil sump (in the base of the compressor) cooling water system of a typical four-stage 150

4155 Chapter 8PUMPS AND COMPRESSORS

FOURTH STAGE COOLER

FIRST STAGE COOLER

WATER INLET 1VATEROUTLET 2 OIL OUTLET OIR COOLER OIL INLET FP.143 Figure 8-38.Cooling water system in a typical multistage air compressor. compressor is illustrated in figure 8-38. Not Each cylinder is fitted with a liner of special all systems have identical paths of water flow; cast iron or steel to withstand the wear of the however, in systems equipped with on coolers, piston. Wheneverpracticable,the cylinder it is important that the coldest water is avail- jackets are fitted with handholes and covers able for the cooler. Valves are usually pro- so that water spaces may be inspected and vided so that the water to the cooler can be cleaned. Jumper lines are generally used to controlled independently of the rest of the sys- make water connections between the cylinders tem. Thus, oil temperature may be controlled and heads, since these prevent any possibility without harmfully affecting other parts of the ofleakageinto the compression spaces. In compressor. some compressors, however, the water passes directly through thejoint between the cyl- Next in importance are the intercoolers inder and the head. With this latter type, ex- and after coolers (discussed later), then the treme care must be taken to insure that the cylinder jackets and heads. High-pressure air joint is properly sealed to prevent leakage compressors require from 8 to 25 gallons of which, if allowed to continue, would ruin both cooling water per minute, while medium-pres- the cylinder liner and the piston. sure compressors require from 10 to 20 gal- The intercoolers and aftercoolers remove lons per minute. the heat generated during compression and When sea water is used as the cooling agent, cause the condensation of any vapor that might all parts of the circulating System must be be present. It is important that this conden- of corrosion-resisting materials. The cylin- sation be drained at regular intervals to pre- ders and heads are therefore composed of vent carryover into the next stage, accumu- gun metal or valve bronze composition, with lation at low points, water hammer, freezing water jackets cast integral with the cylinders. orburstingof pipes in exposed locations, 151 LA.. .4156 FLUID POWER faulty operation of pneumatic systems and com- perature rises above a safe limit, thecom- ponents, and possible damage to electrical ap- pressor will stop and will not restart auto- paratus where air is used for cleaning.The matically.Some compressors are fitted with a removal of heat is also required for economical device that will shut down the compressor if compression. During compression the temper- the temperature of the air leaving any stage ature of the air is increased, thus causing the exceeds a preset value. air to expand to a larger volume which, in turn, requires a corresponding increase of work to Compressor Assembly Components compress it.(The effect of changes in temperature on compressed gas is discussed in As previously stated certain other com- chapter 2.)Multistaging and cooling of the air ponents are usually considered as part of the between stages reduce the power requirement compressor assembly. A brief description of for a given capacity. The intercooling reduces these components is given in the followingpar- the maximum temperature in each cylinder and agraphs. They are described in more detail thereby reduces the amount of heat that must in other sections of the manual, as indicated. be removed by the water jacket at the cylinder. A filter and a moisture separator are in- Also, the resulting temperature in the cylinder corporated in the line between the compressor insures good lubrication of the piston and the and the receiver tank.Their purpose is to valves.Both the intercoolers and the after- remove as much dirt and moisture as possible coolers are of the same general construction, before the air enters the system. Filters and except that the aftercoolers are designed to separators are discussed in chapter 7. withstand a higher working pressure that the intercoolers. An air receiver or reservoir is installed in or near each space housing aircompressors. Water-cooled intercoolers may be of the The receiver acts as a supply tank anda stor- straight tube and shell type or, if size permits, age tank for the pneumatic system.Air re- may be of the coil type.In coolers with air ceivers are explained in chapter 7. pressure above 250 psi, the air flows through the tubes.Suitable baffles are provided in An unloading system that removes thecom- tubular coolers to deflect the airor water in its pression load from the compressor while the course through the cooler. In coil type coolers unit is starting and automatically appliesthe the air passes through the coil, with thewater load after the unit reaches operating speed is flowing around the outside. installed in most systems. Unloading valvesare covered in more detail in chapter 10. Air-cooled intercoolers and aftercoolers may be of the radiator type or may consist ofa bank of finned copper tubes located in the path A pressure relief valve is installed in the of blast air provided by the aircompressor. assembly.It exhausts compressor discharge air to the atmosphere when the pressurein the Automatic temperature shutdown devicesare equipment being charged exceeds a predeter- fitted on all recent designs of high-pressure air mined maximum value. Pressure relief valves compressors.Thus, if the cooling water tem- are described in chapter 10.

152 .3.57 CHAPTER 9 CONTROL AND MEASUREMENT OF FLOW

It is all but impossible to design a practical ommended fluid in the system and for keeping fluid power system without some means of con- the fluid clean. Oxidation, rust particles, and trolling the volume and pressure of the fluid and other foreign materials such as dust, sand, lint, directing the flow of fluid to the operatingunits. etc., can cause considerable damage to preci- This is accomplished by the incorporation of sionvalves.This contamination will cause different types of valves at various points in the valves to stick, may plug small orifices, or system. Different types of valves used in fluid cause abrasion of the valving surfaces, resulting power systems are discussed in this chapter in leakage between the valve element and valve and chapters 10 and 11.A brief introduction to seat when the valve is in the closed position. valves, including their classification and appli- Any one of these can result in inefficient opera- cation, is covered in the first part of this chap- tion or complete stoppage of the equipment. ter. This is followed by detailed descriptions and illustrations of those valves which are used Valves may be controlled manually, electri- to control the flow of fluids. cally, pneumatically, mechanically, hydrauli- Some fluid power systems require devices for cally, or by combinations of two or more of these methods. In some systems the entire se- measuring the quantity or rate of flow through quence of operation of the most 'complicated the system. The latter part of this chapter is equipment may be automatic. The method of con- devoted to various types of flowmeters used for trol depends upon many different factors. The measuring the flow of fluids. purpose of the valve, the design and purpose of the system, the location of the valve within the system, and the availability of the source of INTRODUCTION TO VALVES power are some of the factors that determine An often quoted definition of a valve is "an the method of control. engineered obstruction in a pipe." Although this definition is technically correct, a more precise definition is: A valve is any device by which the CLASSIFICATION OF VALVES flow of fluid may be started, stopped, or regulated by a movable part which opens or ob- Valves are sometimes classified according to structs passage. As applied to fluid power sys- their method of operationsimple, compound, tems, valves are used for controlling the flow or directional. A simple valve requires only a of the fluid, the pressure of the fluid, and the single internal motion for its operation. For direction of the fluid flow. example, fluid acting on one side of the valving Valves must be accurate in the control of element opens it against the resistance of grav- fluid flow and pressure and the sequence of ity or spring tension; or the valve is controlled operation. Usually, no packing is used between manually by turning a screw so that the passage the valving element and the valve seat, since for the fluid is opened or closed. A compound fluidleakageisreducedto a negligible valve involves a combination of internal mo- quantity by precision machined surfaces, re- tions for its operation. Directional valves are sultingincarefullycontrolled,clearances. used to control the direction of the flow of fluid (Packing is required around valve stems, be- along two or more paths. tween lands of spool valves, etc.) This is another Probably, the most common method of clas- very important reason for using only the rec- sifying valves is according to their purpose 153 t FLUID POWER flow control, pressure control, and directional in the closed position allowing no flow. It can be control. This method of classification is very fully opened allowing full flow. The rate of flow similar to the method discussed previously. As is varied by turning the faucet handle clock- a general rule, simple valves include thr,se wise or counterclockwise, which changes the which control flow; compound valves include size of the opening of the faucet. Although some those which control pressure; and, of course, of the flow control valves used in fluid power directional valves control the direction of flow. systems are similar to the water faucet, others All the types of valves available for fluid are more complex in design and operation. Some power systems are too numerous to describe of the different types of flow control valves within the scope of this training manual. Most commonly used in fluid power systems are valves, however, are variations of these three described in the following paragraphs. fundamental classesflow control,pressure control, and directional control. Several representative types in each class are described PLUG VALVES and illustrated in this manual. Flow control A plug valve, sometimes referred to asa valves are discussed in this chapter. Pressure cock, consists of a hollow cylindrical shaped control valves are described in chapter 10, body into which is fitted a tapered cylindrical while directional control valves are covered in plug. Figure 9-1 shows an exploded view ofa chapter 11. plug valve, including a cross-sectional view of the body. The top of the plug extends up through the gland, and can be turned with a wrench. APPLICATIONS In most plug valves, the plug terminates in a handle for manual control of the valve. Each type of valve used in fluid power systems has a specific purpose or, in some appli- The body of the valve is secured in the line cations, a combination of different purposes. with holes or ports in the wall of the cylindrical These applications and purposes are discussed body aligned with the flow of fluid through the in more detail as the different valves are de- line. The plug, which also contains holesor scribed in this chapter and in chapters 10 and ports, fits snugly into the valve body. When the 11. In general, however, the applications of plug is open the ports of the plug are in line valves according to their classification, are wiM the ports of the body, allowing fluid to briefly described in the following paragraphs. flow through the valve. (See fig.9-2.) Flow Flow control valves are used in fluid power is stopped by a quarter turn of the plug, which systems to open and close a line to flow or to aligns the solid areas of the plug with the ports control the rate of flow through the lines. Then in the body. The top of the plug or the handle are sometimes used as ON and OFF valves is usually marked by some method to indicate to isolate circuits of the system during cer- whether the valve is open or closed. tain operations. Although the inside surfaces of plug valves The uses of pressure control valves include are machined to give close contact, the meeting the regulation of system pressure, the protection of metal with metal offers the danger of seizing. of the system and components frompressure When plug valves stand normally in an open overload, and the control of the sequence of oper- position, the parts of the plug that provide the ation of certain components in some systems. seal are not directly in contact with the fluid; Among other applications, directional control but. when the valve is normally closed the fluid valves are used to control the paths of the fluid to will act on only one side of the sealingsur- operating components. For example, these face. Under ordinary conditions, however, plug valves are used to control the direction of rota- valves can be easily opened or closed. tion of actuating motors and the direction of movement of actuating cylinders. Plug valves are used as fully ON or fully OFF valves. They are not designed to be used FLOW CONTROL VALVES in a semiopen position; that is, to throttle or vary the volume of flow. This is especially true if grooves in the walls of the bodyare filled A typical example of a valve used to control with packing to separate metal from metal. In flow is the ordinary water faucet. It is normally a partially open position, this packing would 154 Chapter 9CONTROL AND MEASUREMENT OF FLOW

GLAND

TOP RING

OPEN CLOSED FP.145 PLUG Figure 9-2.Operation of plug valve.

FP.144 Figure 9-1.Plug valve. eventually wear away. In any event, unequal wear is encouraged if the valve remains in the partly open position for any length of time. Plug valves have a limited use influidpower systems. Small plug valves are sometimes used in hydraulic systems to free the system of air. The valve is opened so that the air can escape. When the liquid begins to flow continuously the valve is closed. Plug valves are also used in pneumatic systems to drain condensation from the system. FP.146 Figure 9-3.Cross-sectional view of gate valve. GATE VALVES In the gate valve, flow is controlled by means are attached. When the valve is open, the gate of a wedge or gate, the movement of which is stands up inside the bonnet. The bottom sur- usually controlled with a handwheel. By turning face of the gate is then flush with the wall of the handwheel, the wedge or gate can be moved the line. While the valve is closed, the gate up and down across the line of flow to open and blocks flow by standing straight across the line, close the passage. Figure 9-3 illustrates the where it rests firmly against two seats ex- principal elements of the gate valve in cross tending completely across the line. section. Part (A) shows the line connection.and Gate valves permit straight flow and offer the outside structure of the valve body, while little or no resistance to the flow of fluid when (B) shows the wedge or gate inside the valve the valve is completely open. Gate valves are and the atom to which the gate and handwheel intended for use as fully open or fully closed 155 FLUID POWER

the valve is opened or closed. In figure 9-5 (B), the stem rises outside the valveas the valve is opened, but the stem screw operates inside the body of the valve. In figure 9-5 (C), the stem screw operates at the level of the handwheel, so that the stem rises independently of the wheel as the valve is opened. This is called the outside-screw-and-yoke type valve. Valves with rising stems are used when it is important to know by immediate inspection whether the valve is open or closed. The non- rising stem type is least likely to leak, and requires less space. Gate valves should be opened or closed slowly. Difficulty in opening and closing the valve may be caused by high fluid pressure FP.147 acting against the gate. The gate should not be Figure 9-4.Operation of gate valve. forced against the seat.If the valve fails to seat properly, it should be opened slightly and then closed again. If it still fails to seat, the valves. If the valve is partly open, the face of system must be shut down and the valve dis- the valve stands in the flow of fluid. This flow assembled to locate and correct the trouble. will act on the face of the valve causing it to erode. For this reason, gate valves should not be used to restrict or throttle the rate of GLOBE VALVES flow. Two different types of gates are used in the Globe valves derive their name from the construction of gate valves. One type is a solid globular shape of their bodies. It should be noted, or hollow wedge. This type is satisfactory for however, that other types of valves may also small valves in low-pressure systems. How- have globular-shaped bodies; hence, the name ever, wedges are sometimes difficult to tighten may tend to be misleading. It is the internal and will leak when slightly worn. The other structure of the valve, rather than the external type consists of two facing discs. Byusing discs shape, that distinguishes one type of valve from a better closure is provided, since the discs the other. are forced apart, snug against the valve seats, The controlling member of the globe valve, as they are moved into position. One arrange- called the disc, is attached directly to the end ment for accomplishing this is shown in figure of the stem. The valve is closed by turning 9-4. One of the two facing discs, composing the valve stem in until the disc is seated into the valve, has been removed to show how the the valve seat. Since the fluid flows equally valve isconstructed. Two cams with arms on all sides of the center of support when the extending outward stand opposite each other valve is open, there is no unbalanced pressure on slanting surfaces in the space between the on the disc to cause uneven wear. The opera- discs. As the discs move into the closed position of the globe valve is illustrated in figure tion, the arni of each cam engages a lug on 9-6. the body of the valve and is turned on the slant- The moving parts of a globe valve consist ing cam bearing surfaceforcing the discs of the disc, valve stem, and the handwheel. against the valve gates during closure. Figure 9-7 (A) is an exploded view ofa globe valve. The stem, which connects the handwheel Gates valves are available in different types and the disc, is threaded and fits into the threads of stem connections. Figure 9-5 illustrates in the valve bonnet. Discs are available in three different types. In figure 9-5 (A), the various designs. (See fig. 9-7 (B).) stem screws down into the valve gate as the When the valve is closed, the valve disc valve is opened. In this type the stem does not rests against the valve seat, preventing fluid rise or fall outside the body of the valveas from flowing through the valve. The edge of 156 Chapter 9CONTROL AND MEASUREMENT OF FLOW

(A) (B) (C) FP.148 Figure 9-5.Types of gate valves. the disc and the seat are very accurately ma- have been damaged in this manner. Another chined so that they form a tight seal when the reason for not leaving globe valves in the fully valve is in the closed position. When the valve open position is that it is sometimes difficult is open, the fluid flows through the space be- to determine ifthe valve is open or closed. tween the edge of the disc and the seat. The If the valve is jammed in the open position, rate at which the fluid flows through the valve the stem may be damaged or brokenby someone is regulated by the position of the disc in re- who thinks the valve is closed, and attempts lation to the seat. The valve must always be to open it. installed with the pressure against the face of the disc. The globe valve is commonly used as a fully NEEDLE VALVES open or fully closed valve. This valve may also be used as a throttle valve to control the rate Needle valves are similar in design and of flow. However, since the seating surface is operation to the globe valve. Instead of a disc, a relatively large area, this valve is not suit- a. needle valve has a long tapered point at the able as a throttle valve where fine adjustments end of the valve stem. A cross-sectional view are required in controlling the rate of flow. of a needle valve is illustrated in figure 9-8. The globe valve should never be jammed The long taper of the valve element permits in the open position. After a valve has been a much smaller seating surface area as com- fully opened, the handwheel should be turned pared to the globe valve; therefore, the needle toward the closed position approximately one- valve is more suitable as a throttle valve where half turn. Unless this is done, the valve is likely fine adjustments are required in controlling to seize in the open position making it difficult, the rate of flow. Needle valves are used to con- if not impossible, to close the valve. Many valves trol flow into delicate gages, which might be 15'1 FLUID POWER

FP.149 Figure 9-6.Operation of a globe valve. damaged by redden surges of fluid under pres-

1 sor% Needle valves are also used to control cr.: operation ofwork cycle, where it is de- ar sole that a v.:!rk motion be brought slowly a halt; and 4t other points where precise adjustmeWst of ars necessary and where a small rate of flow is der.ireci, Although many ofthrs neettia valves used in fluid power systems are of the menually operated type illustrated in figure 9 -Li, ran:l- ineations of this type valve are often used as variable restrictors, described in the next fac- tion. 4,3

RESTREi TORS Restrivtors, so' Ifnes referred to as ^r- Vices, arcs used in fluid power systems to limit the speed of movement of certain actuating devices. They do so by serving as restrictions in the line, thereby limiting the rate of flow. Figure 9-9 shows an example of a typi, cal re- strfetor. This type is referred to as s fixed restrictor. Some types of restrictors are constructed so that the amount of restriction can be vaTled. One type of variable restrictor is illustrated in figure 9-10. This type is simply a modification of the needle valve, previously described. Instead of a handwheel control, this valve is constructed so that it can be preadjusted to alter the time of operation of a particular subsystem. It can be adjusted to conform to the (9) requirements of a particular system. This per- FP.160 mits the same type valve to be used in dif- Figure 9-I.Globe valve with various ferent systems. types at discs. ORIFICE CHECK VALVES the direttion of flow. Hovaver, they are used in conjunction with soLie types of flow control Check valves are described in more detail valves. The orifice check valve is an example, in chapter 11, since their purpose is to control and is used i4 fluid power systems to allow 158 Chapter 9CONTROL AND MEASUREMENT OF FLOW

FP.153

OPEN Figure 9-10.Variable restrictor. CLOSED FP.151 Figure 9-8.Cross-sectional view of while the cone is off its seat. When fluid pres- a needle valve. sure is applied through port (1), the force of the fluid and spring tension move the cone to the left and on its seat. This action blocks the flow of fluid through the valve, except through the orifice (3) in the center of the cone. Thus, the size of orifice (3) determines the rate of viiimaimmsfriv flow through the valve as the fluid flows from right to left. FP.152 Figure 9-11 (B) shows a ball type orifice Figure 9-9.Fixed restrictor. check valve. Fluid flow through the valve from left to right forces the ball off its seat and normal speed of operation in one direction and allows normal flow. Fluid flow through the valve limited speed of operation in the other. Since in the opposite direction forces the ball on its this type valve allows normal flow in one direc- seat. Thus, the flow is restricted by the size tion and restricted flow in the other, it is often of the orifice (6) located in the housing of the referred to as a one-way restrictor.. Some valve. typical examples of orifice check valves are illustrated in figure 9-11. In some fluid power systems it is necessary Figure 9-11 (A) illustrates a cone-type ori- that the actuating device (for example, an actu- fice check valve. When sufficient fluid pres- ating cylinder) move slower in one direction sure is applied at port (4), it overcomes spring than the other. In some systems an orifice tension and moves the cone (2) off its seat. check valve is used to accomplish this re- The two orifices(5)in the illustratlem rep- quirement. The valve is installed in the alter- resent several openings located around the nating line that carries the fluid from the actu- slanted circumference of the cone. These ori- ating device as it is actuated in the direction fices allow free flow of fluid through the valve in which slower movement is desired. (The 159

144 FLUID POWER

is retracted. This allows for rapid retraction of the landing gear. When the gear is extended, fluid leaving the cylinder returns through the UP line and must pass through the orifice of the valve. Thus, a cushioning effect results and the gear falls slowly, thereby preventing structural damage. If the restriction were placed in the DOWN line, it would limit the quantity of fluid entering the cylinder, but would have (A) no effect on the fluid leaving. This would not satisfy the situation because the heavy gear tends to fall freely, causing a partial vacuum in the cylinder. Thus, the gear would fall too rapidly, resulting in structural damage to the aircraft. Circuits in which the flow is restricted as the fluid leaves the actuating device, such as the landing gear circuit just described, are commonly referred to as meter-out circuits. (B) That is, the fluid is metered out of the actuating device. Some circuits require the flow of fluid FP.154 to be restricted as it enters the actuating de- 1.Outlet port. 4.Inlet port, vice. These circuits are referred to as meter- 2.Cone. 5.Orifices. in circuits. 3.Orifice. 6.Orifice. A meter-in circuit, utilizing anorifice check Figure 9-11.Orifice check valves. valve, is sometimes used to control these- quence of operation in one direction of two or more actuating devices in a subsystem. A sub- line which delivers fluid under pressure to the system of this type is illustrated in figure 9-12. actuating device for one direction of operation In this system the sequence of operation is becomes the line which carries the return flow controlled during the extension of the piston from the actuating device during the opposite rods. Notice that the directional control valve direction of operation; hence, the term alter- is positioned to deliver system pressure to nating line.) This causes the speed of the actu- the right-hand end of each cylinder which will ating device to be retarded, since the fluid extend the piston rods. Since the line to cyl- cannot escape from the actuating device any inder (1) contains the orifice check valve, the faster than the orifice will allow fluid to flow flow is restricted to the cylinder; therefore, to return. When the device is operated in the the fluid takes the path of least resistance and opposite direction, the alternating line con- flows to cylinder (2). After the piston rod of taining the orifice check delivers fluid under cylinder(2) is fully extended, the restricted pressure to the actuating device. Flow in this flow of fluid through the orifice check valve direction is unrestricted through the valve. will eventually extend the piston rod of cyl- NOTE: The direction of free flow through inder (1). the orifice check valve is indicated by an arrow stamped on the housing. During the retraction stroke of the piston rods, the fluid flows in the opposite direction This type installation is sometimes 'used in the alternating lines. Since this allows free in aircraft hydraulic systems. It is used in flow of return fluid through the orifice check the subsystem that retracts and extends the valve, the. rods will retract at approximately landing gear. The orifice check valve is in-' the same time. It should be noted that if the stalled in the UP line (pressure line for the orifice check valve was replaced witha fixed retraction of the landing gear) in such amanner or variable restrictor, the sequence of opera- that it permits free flow when the landing gear tion could be controlled in both directions. 180 Chapter 9CONTROL AND MEASUREMENT OF FLOW

ORIFICE 0I CHECK VALVE CYLINDER ttl

DIRECTIONAL ALTERNATING LINES CONTROL VALVE

CYLINDER #2 SYSTEM PRESSURE

FP.155 Figure 9-12.Meter-in circuit containing orifice check valve.

COMPENSATED VALVES This flow regulator has only one moving part, the piston (5), as illustrated in figure The flowcontrol valves previously dis- 9-13. This valve will regulate flow from left cussed in this chapter are not compensated to right only. Although the flow from right to for changes in fluid temperature or pressure left is restricted to the size of the orifice in and,therefore,are sometimes referred to the head of piston (5), the flow is not regulated as noncompensating valves. The rate of flow in thisdirection.The body of the valve through these valves can vary at a fixed set- is marked with an arrow to indicate the direc- tingifeitherthe pressure or temperature tion of regulated flow. ofthefluidchanges. Changes in viscosity (discussed in chapter 2), which are often the Operation of the flow regulator can be seen result of temperature changes, can also cause in figure 9-13. View (A) shows the fluid flowing flowvariation through a valve. The valves through the valve in the direction of regulated previously described are satisfactory for use flow; however, in this position the valve allows in fluid power systems in which slight varia- free flow (relative to the size of the orifice tions ct flow are not a critical factor to be in the head of piston (5)). Fluid enters port considered. However, some systems require (3), passes through the orifice in the head of extremely accurate control of the actuating piston (5), through the slots (2) in the side of device. Compensated flow control valves are the piston (5), and out port (6). U the flow en- frequently used for this purpose. They auto- tering port (3) increases to a velocity greater matically change the valve adjustment to com- than the capacity of the orifice in the head of pensate for pressure changes encountered in piston (5), the resistance increases. This in- the system, thereby providing a constant flow crease inresistance results in a pressure at a given setting. differential between the fluid entering the ori- Figure 9-13 illustrates an example of a fice and the fluid leaving it.This pressure compensated flow control valve. This type valve differential, caused by a momentary increase meters a constant flow regardless of varia- in flow, overcomes spring tension and moves tions in system pressure. Although it is usu- the piston (5) to the right. As the piston moves ally used to meter fluid into a circuit, it can to the right,the openings at slots (2) in the also be used to meter fluid as it leaves the piston and regulator body decrease in size circuit. Flow control, flow regulator, and con- (fig.9-13(B)),and an additional restriction stant flow valve are all terms used to describe is placed on the fluid which decreases the rate this valve. In this manual it is referred to as of flow. The piston cannot move to the right a flow regulator. far enough to completely block the slots in the 161 FLUID POWER

to move to the right and thereby decreases the size of the slot openings (2). Thisallows less fluid to flow to the actuating device. Thus, the flow regulator maintains a constantrate of flow regardless of variations in systempres- sure and regardless of the resistance to movement of the actuating device. An example of a typical flow regulator installation is shown in figure 9-14. This installation provides for the fluid to be metered into the circuit. System pressure enters port (5). When the directional control valve (3)is (A) positioned,. fluid flows through the flowreg- ulator (4). Thus, regardless of the direction of movement of the actuating cylinder (1),one of the alternating lines (2) will have regulated flow through it,insuring a smooth operation of the actuator. Flow regulators are available in different flow capacities, and are usually rated ingal- lons per minute (gpm). The type of valve discussed in this section is nonadjustable. Ad- justable compensated valvesare required and are available for some fluid power systems. (B)

FLOW EQUALIZER FP.156 1. Body. 4. Retainer. Flow equalizers, sometimes referred toas 2. Slots. 5. Piston. flow dividers, are used in some hydraulicsys- 3. Input port. 6. Output port. tems to synchronize the operation of two actuating units. To accomplish this, the flow equal- Figure 9-13.Flow regulator. izer divides a single stream of fluid from the directional control valve into two equal streams. Thus, each actuating unit receives thesame rate of flow, and both move in unison. During side of the regulator body. Although thepres- operation in the opposite direction, theflow sure on the right side of the piston will build equalizer combines the two streams of fluid up to a value equal to the pressure in port (3) at an equal rate. Therefore, the flow equalizer and against the left side of the piston, spring synchronizes the movement of the actuating tension will overcome this equalization ofpres- units during both directions of operation.Since sures and will prevent a complete blockage of this valve provides synchronizedflow in both flow. directions, it is said to be dual acting. An increase of pressure normally increases One type of flow equalizer is illustrated in fluid flow through an orifice, but this is not figure 9-15. View (A) shows the valvein the the case with the flow regulator. As the pres- splitting (divided flow) position. Fluid under sure increases, the size of the orifices (slots) pressure from the directional control valve decreases, thus maintaining a constant flow. enters port (3). The fluid pressureovercomes If the resistance to movement of the actuating spring tension, forces the plug (4) down, and device decreases, the fluid flow starts to in- uncovers the two orifices in the sleeve (2). crease. This increase of flow to the actuating The fluid then splits and tends toflow equally device causes a decrease in pressureon the through the two side passages (1)and (5). The right side of piston (5). This allows the piston fluid pressure overcomes spring tensionand 162 Chapter 9CONTROL AND MEASUREMENT OF FLOW

The combining position of the flow equalizer is illustrated in figure 9-15 (B). This shows the valve Joining the two streams of fluid as it flows from the actuating units. In this position, the fluid enters ports (9) and (13) of the valve. The fluid cannot return through the split= ting check valves. Therefore, it takes the path of least resistance, which is around the cylindrical shaped metering piston, and enters the combining check valves (6) and (16) as indicated by arrows. The pressure of the fluid overcomes springtension,opens the check valves, and then the fluid flows through passages (1) and (5) to the orifice sleeve (2). The fluid pressure will then force the orifice sleeve FP.157 upward, which opens the orifices and allows 1.Actuating cylinder. 4.Flow regulator. the fluid to flow out port (3). 2.Alternating lines. 5.Pressure line. 3.Directional control Again for purposes of illustration, assume valve. 6. Return line. that the actuating unit attached to a line from port (9) moves with less resistance than the Figure 9-14.Flow regulator installation. unit attached to a _line from port (13). The rate of flow into port (9) tends to increase but as the fluid leaves the combining checi opens the two splitting check valves (7) and valve (6) and flows through passage (5) it is (15). Then the fluid flows through these splitting restricted by the orifice.Therefore,if the check valves, through the metering grooves flow momentarily increases, the restriction (10) and (14), through ports (9) and (13), and of flow through the orifice will cause an in- through connecting lines to the actuating units. crease in pressure in passage (5). This dif- Any difference in the rate of flow between the ferentialinpressure between passages (5) two passages results in a pressure differen- and (1) will force the metering piston (11) to tial between the two passages. Then the free- the right. This results in a restriction between floating metering piston (11) shifts to equalize the metering groove (10) and the piston land the internal pressure, and the flow equalizes. (8).Thus, the stream of fluid which tends to flowata greaterrate is restricted and To illustrate this equalization action, as- equalizes with the other stream. sume that the actuating unit attached to a line from port (9) meets less resistance than the one attached to a line from port (13). As a re- PRIORITY VALVES sult,the rate of flow through port (9) tends to increase, but in so doing, the rate of flow In systems with two or more circuits, it also increases through passage (5). Since the is sometimes necessary to have some means fluid leaving port (13) meets with more re- of supplying all available fluid to one particular sistance, the flow through passage (1) decreases, circuit in case of a pressure drop in the sys- and the pressure becomes greater than the tem. A priority valve is often incorporated pressure in passage (5). This momentary pres- in the system to insure a supply of fluid to sure differential forces the free-floating me- thecriticalcircuit. The components of the tering piston (11) to the left. This causes the system are arranged so that the fluid to operate space between the piston land (8) and the me- each circuit, except the one critical circuit, tering groove (10) to become smaller, which must flow through the priority valve. A pri- restricts the flow of fluid out of port (9). Thus, ority valve may also be used within a subsys- the flow equalizer imposes a restriction on tem containing two or more actuating units to the fluid that has a tendency to increase in insure a supply of fluid to one of the actuating rate of flow, and the two streams equalize. units. Inthiscasethe priority valve is

183 mar.mmii MV A 1.4. mll. p-

4EN

.sosi imanr a- --Imm°Lim. 11 V liijiij' ca4g-- JIB MEL

m Chapter 9CONTROL AND MEASUREMENT OF FLOW

Nomenclature for figure 9-15.

1.Side passage. 9.Port. 2.Sleeve. 10.Metering groove. 3.Port. 11.Free-floating metering piston. 4.Plug. 12.Piston land. 5.Side passage. 13.Port. 6.Combining check valve. 14.Metering groove. 7.Splitting check valve. 15.Splitting check valve. 8.Piston land. 16.Combining check valve.

reached. (The preset pressure depends on the requirements of the system and is set by the manufacturer.) Assume that the critical circuit or actuating unit requires 1,500 psi. When the prefigure in the valve reaches 1,500 psi, the poppet reaches the end of its travel. As the (A) PRIORITY FLOW pressure increases,the piston continues to move to theright, which unseats the poppet and allows flow through the valve, as indicated in figure 9-16 (A). If the pressure drops below PRIORITY FLOW BODY 1,500 psi, the compressed spring forces the ADJUSTING NUT POPPET PISTON SPRING piston to the left, the poppet seats, and flow through the valve stops. Figure 9-16 (B) shows the priority valve in the free flow position. The flow of fluid moves the poppet to the left, the poppet spring compresses, and the poppet unseats. This allows free flow of fluid through the valve.

(RI FREE FLOW FLOWMETERS FP.159 Although flowmeters are normally associated Figure 9-16.Priority valve. with systems in which fluids are consumed, such as oil, gasoline, water, etc., they are sometimes required in fluid power systems.One of the incorporated in the subsystem in such alocation most important uses of flowmeters in fluid that the fluid to each actuating unit, except the power systems is in test stands which are used criticalunit,must flow through the valve. to test and adjust fluid power systems and/or One type of priority valve is illustrated in components. For example, pumps,. which are figure 9-16. View (A) shows the valve in the rated in gallons per minute (chapter 8), can priority flow position; that is, the fluid must be tested for their rated capacity by the use flow through the valve in the direction indicated of a test stand with a flowmeter incorporated. by the arrows to get to the noncritical circuits or actuating units. With no fluid pressure in Measurement of flow may be expressed in thevalve,spring tension forces the piston units of rate, such as gallons per minute, pounds against the stop and the poppet seats against per hour, cubic feet per second, or in terms the hole in the center of the piston. As fluid of total quantity, such as gallons, pounds, or pressure increases, the spring compresses and cubic feet. (This is similar to the speedometer the piston moves to the right. The poppet fol- of an automobile. The needle indicates the speed lows the piston, sealing the hole in the center of the automobile in miles per hour, which ofthe piston, until the preset pressure is compares to the rate-of-flow type flowmeter. 185 FLUID POWER

The other indicator, called the odometer, in- cavity for the ball (4). The spindle (5)passes dicates total miles and compares to the quan- through the ball and is connected to the counting tity type flowmeter.) gears (fig. 9-18). Disc (6) is attached to the A reciprocating pump that displaces a uni- ball.Both the ball and disc are machined to fit form volume of fluid for each stroke of its piston closely in the chamber. There is a slot (8) in could be used as a meter by installing a device the disc at one point, through which passes the for counting the piston strokes. However, the thin partition (7).This partition divides the pump, would have to be designed to minimize chamber into two equal parts. There are open- leakage and to guarantee uniform displacement. ings in the outer wall of the chamberon each Measurementof flow is accomplished by a side of the partition.Opening (9) is the fluid variety of means, depending 'Upon the quantities, inlet, while opening (10) is the outlet. flow rates, and types of fluid involved. Fluid meters are designed to measure fluids of def- When the ball carrying the disc and the spin- inite specific gravities and characteristics, and dle is tilted as far as possible, the bottom Of must be used only for the purpose and the fluid the disc makes a close contact with the bottom for which they were designed. Each meter is conical surface of the chamber, while the top tested and calibrated before it is shipped from of the disc similarly contacts theupper conical the factory and must be tested and calibrated surface at the opposite end of the disc. Since at periodicintervals throughout its service the disc is flat, while the contacting surfaces life. Several types of flowmeters are described are conical, contact takes place along a straight in the following paragraphs. line on each surface. The lines of contactpro- duce seals, which, when taken in connection with the partition (7), divide both theupper NUTATING PISTON and lower compartments into two parts. The DISC FLOWMETER net effect is that the disc and partition produce four separate compartments in the measuring Inthistype flowmeter the fluid passes chamber, two above the disc and two below. through a fixed volume measuring chamber The top and bottom compartments are separated divided into upper and lower compartments by from each other by the disc, while the pairs a disc. During operation, one or the other com- of compartments respectively above and below partment is continually being filled while the the disc are separated by the seal formedat other is being emptied. As it passes through the line of contact between the disc and the thesecompartments,the force of the fluid conical top and bottom surfaces, and also by causes the disc to roll around in the chamber, the partitions. in a manner described later. This movement Spindle (5) extends from ball (4) andpasses of the disc operates a counter, through suitable through wheel (11) at a point near its outside gearing, to indicate the volume of fluid passed edge. The vertical shaft (12) is attached to wheel through the meter. The counter somewhatre- (11) and rotates with it. This shaft is connected sembles the odometer of an automobile, pre- at its upper end to the measuringgears, and viously mentioned, except that this type flowis mounted directly over ball (4). When the meter is usually designed to register gallons. wheel (11) turns on its axis, the position of the shaft keeps the spindle inclined at just the The heart of the meter is the measuring angleto produce a continuous seal between chamber and the disc piston. Views (A) and disc (6) and the upper and lower surfaces of (B) of figure 9-17 show how the disc is located the measuring chambers. in the measuring chamber. View (C) of figure 9-17 is a sectional view illustrating one-half For the purpose of simplicity, consider the of the measuring chamber. The chamber is meter as a pump driven by some outside force. bound at the top and bottom by conical surfaces The action of the meter can be understood by (1) and (2), which are joined at their outer edges imagining wheel (11) to be revolved bymeans by spherical surface (3). A sectional viewof of shaft (12). The spindle (5) would revolve the entire meter is illustrated in figure 9r18. with the wheel and shaft, and would tracea conical path, as shown in figure 9-19. This Referring to figures 9-17 (A) and (B), the movementof the spindle would control the upper and lower surfaces of the chamber con- positioning of the disc. When the spindle is in verge towards the center to form a spherical position (A) (fig. 9-19), for example, the disc 166 Chapter 9CONTROL AND MEASUREMENT OF FLOW

5 12

4 1,

3 7

FP.160 1.Measuring chamber (top surface). 7.Chamber partition. 2.Measuring chamber (bottom surface), 8.Slot. 3.Measuring chamber (outer edge), 9.Inlet opening. 4.Ball. 10.Outlet opening. 5.Spindle. 11.Wheel. 6.Disc. 12.Shaft. Figure 9-17.Nutating piston disc flowireter (sectional views). would also be in position (A), while the posi- bles up and down (nutates), while the seal lines, tions (B) and (C) for the spindle and disc would formed by the disc and the top and bottom walls also correspond. of the measuring chamber, are made to re- The disc cannot rotate, because partition (7) stands in slot (8). The disc, therefore, wob- volve around the chamber. 167

. 1. i72 FLUID POWER

STUFFING BOX REGISTER

CONTROLROLLER . INTERMEDIATE GEAR TRAIN W.7,7,,,,

OUTER HOUSING

FP.161 Figure 9-18.Nutating piston disc flowmeter (indicatingfluid flow).

Again for simplicity, consider only the lower which was formerly behind it, forward. The partition of the chamber. The seal will always line of flow of the fluid is as shown in figure be along the line which has the greatest inclina- 9-18. tion. As the seal line moves in the direction shown by the arrows infigure 9-18, itwill sweep Obviously, if the wheel (11) were contin- the fluid before it and cause the fluid to be dis- uously rotated, the disc (6) would move a volume charged through the opening (10). At the same of fluid equal to the volume of the lower half time, the compartment behind the moving line of the chamber from the inlet to the outlet, of the seal will be increasing in size. Since the for every rotation. During this same revolu- space is open to the inlet (9), it is filled with tion, the top section is doing the same, except fluid. When the line of seal passes the pail- its seal is always directly opposite the lower tion (7), all the fluid formerly in front of the sealinthe measuring chamber. Therefore, seal will have been forced out of the discharge for every revolution, the piston will displace port. The seal line then starts to push the fluid, the volume of the entire chamber just once. 168 113 Chapter 9CONTROL AND MEASUREMENT OF FLOW

4=1

PATH OF SPINDLE 5 FLOW 1÷

FP.163 Figure 9-20.Propeller type flowmeter.

tates at a speed relative to the velocity of the flow through the line. The revolutions of the propeller are registered through the gearing to the counter, the rate of rotation being volume rate of flow. This type meter is calibrated by the manufacturer for a specific fluid. The POSITIONS accuracy of this meter depends, to varying OF DISC .degrees, on the temperature, pressure, and characteristics of the fluid. FP.162 Figure 9-19.Operation of disc and spindle of nutating piston disc TURBINE TYPE FLOWMETER flowmeter. The turbine type flowmeter is similar to the propeller type just described. Figure 9-21 In the previous discussion, the meter is illustrates an installation of a turbine type described as though it were a pump operated flowmeter. The fluid flows through the helical by the rotation of the wheel (11). Actually the (spiral) impeller. This flow causes the impeller operation of the meter is just the reverse. to rotate. The impeller is connected through It is operated by the slightly greater fluidpres- suitable gearing to the counter. The revolu- sure at the inlet as compared with the outlet. tions of the propeller, which change as the This pressure differential causes the seal line velocity of the flow changes, are counted through to advance around the mevsuring chamber, the gearing to the counter. The rate of rotation and in doing so,it revolves the wheel (11). of the impeller indicates the volume flow rate. This in turn revolves the indicating register Like the propeller type flowmeter, the accuracy of the meter by means of shaft (12) and suitable of. the turbine type depends on the temperature, reduction gears. pressure, and characteristics of the fluid. This Standard meters of this type are suitable type ammeter is calibrated by the manufac- for temperatures up to 200°F, and for pres- turer for a specific type fluid. sures to 150 psi, although models are available for higher temperatures and pressures. The meters are accurate to about 1 percent or less. ROTAMETER The accuracy of the meter is not affected by pressure variations. The rotameter is a device for measuring the rate of flow of a fluid. Figure 9 -22 illustrates the operation of a rotameter. PROPELLER TYPE FLOWMETER The rotameter is an upright glass tube through which the fluid flows. A metal casing Figure 9-20illustrates a propeller type with a Plexiglas window protects the glass flowmeter. The propeller is located in the line tube. The tube is tapered, with the =all end of flow and is connected by suitable gearing at the bottom. Inside, a small metal rotor with to the indicator or counter. The propeller ro- a central hole slides up and down on a guide 169 174 FLUID POWER

COUNTER VENT PLUG

OUTLET FLOW el* HELICAL IMPELLER ROD HOUSING

CUSHION

FP.164 Figure 9-21.Turbine type flowmeter. ROTOR rod.Small vanes cut in the sides of the rotor cause it to spin as it slides freely up and down in the tube. Since the tube is tapered, the space between the rotor and the tube wall increases as the rotor rises, permitting more fluid to pass through that space. Therefore, the rotor always rises to a height corresponding to the rate of flow at any particular time. A scale on the tube is calibrated to indicate the rate of flow in the desired measurementgallons per hour, gallons per minute, pounds per hour, etc.

FLOW SCHEMATIC

FP.165 Figure 9-22.Rotameter.

170 CHAPTER 10 MEASUREMENT AND CONTROL OF PRESSURE

For safe and efficient operation, most fluid in scientific calculations, the pressure of a power systems are designed to operate at a system is referred to as absolute pressure specific pressure or, at least within a close (psia).In this manual, however, system pres- range of a specific pressure. Therefore, sure is referred to as gage pressure (psig). most fluid power systems are provided with (Absolute and gage pressures are defined in a means for measuring and indicating the chapter 2.) pressure in the system.Likewise, a means Most pressure gages used in fluid power must be provided for controlling this pressure. systems are of the direct reading type; that Various types of pressure gages are used to is, both the measuring and the indicating mecha- measure and indicate the pressures in fluid nisms are contained in one housing and the power systems and various types of valves, complete unit is connected directly into the pressure regulators, pressure switches, or system or to a line leading from the system. similar mechanisms are used to control these Some fluid power systems are equipped with pressures. electrically operated (synchro) pressure indi- The operation and applications of various cators. In this type, pressure transmitters types of pressure gages used in fluid power are incorporated in the system at required systems are discussed in the first part of locations.The transmitter operates similar this chapter.The latter part of the 6hapter to the measuring mechanism of the direct is devoted to the valves and other mechanisms reading gage; however, movement of the trans- commonly used in controlling pressure. mitter resulting from changes in pressure is relayed by mechanical and electrical means to PRESSURE GAGES a pressure indicator which can be mounted in a convenient location for the operator. Several Pressure gages are used in fluid power transmitters may be used in a system. Each systems to measure and indicate pressure so transmitter may be connected to separate pres- that the operator of fluid power equipment can sure indicators or several transmitters may be maintain pressure at safe and efficient oper- connected through a selector switch to one atinglevels. Any excess or deficiency of indicator.The operator can then select pres- pressure should be immediately investigated sure readings from any one of the transmitters. with a view of locating and removing the cause of the trouble. Pressure is generally measured in pounds BOURDON TUBE GAGES per square inch, as discussed in chapter 2. Gages used in fluid power systems are there- The most common type of pressure gage fore calibratedin pounds per square inch used in fluid power systems is the Bourdon (psi). Gage readings indicate the fluid pressure spring gage. The name of the gage comes set up by the opposition of forces within the from its inventor, a French engineer, Eugene system.Atmospheric pressure also acts on Bourdon. The Bourdon tube, a C -shaped element, the system, but it can be ignored in practical is the heart of the gage. There are variations operation because its action at one place is of the C-shaped tube used in the construction balanced by its equal and opposite action at of pressure gages. The most common of these another place.When it is taken into account are the spiral and the helical shaped tubes. zit!. FLUID POWER

These three types of Bourdon tubegagm C-shaped, spiral, and helicalare describel the following paragraphs. GROOS SECTION OF PRESSURIZED TUBE

Ittimg,"*.'.NORMAL CROSS SECTION OF TUBE C -Shaped Tube SECTION A-A A simple Bourdon tube pressure gage con- POINTER SCALE sists of a Bourdon tube, a gear-and-pinion mechanism, a dial,and a pointer.These essential parts of a Bourdon tube gage are illustrated in figure 10-1.The curved hollow TIP Bourdon tube (C-shaped) is closed at one end (CLND)OSED and is connected to the fluid pressure at the E other end.When pressure is applied the tube tends to straighten out, like a garden hose when the water is first turned on.Pressure acts LINK equally on every square inch of area inside of SECTOR the tube; but, since the surface area on the GEAR AND outside inner surface of the curve is greater PINION than the surface area on the shorter radius, the force acting on the outer surface is greater BOURDON than the force acting on the inner surface. TUBE SOCKET When the pressure is applied the tube straightens out until the force of the fluid pressure is balanced by the elastic resistance of the material PRESSURE composing the tube.Since the open end of the tube is anchored in a fixed position, changes FP.166 in pressure move the tip (closed end). Through Figure 10-1.Essential parts of suitable linkage and a gear-and-pinion mecha- Bourdon tube pressure gage. nism, this tip movement is used to rotate the indicator pointer around a graduated scale. The scale (dial) is properly calibrated so that SIMPLEX.A simplex Bourdon tube pres- the needle points to the number which corre- sure gage is illustrated in figure 10-2.One sponds to the exact pressure.The tube per- pointer (hand) is connected to the gear shalt forms in the same manner as a spring. When which extends through the dial face.This the pressure is removed, it returns to its original pointer indicates the pressure of the system. position, and the pointer indicates zero pressure. The other pointer, normally painted red, pivots The internal working mechanism of the on the gear shaft and is manually positioned. Bourdon tube gage is housed in a gage case. It is set to the normal workingpressure of Gage cases are made of plastic, corrosion- the system to which the gage is connected. resistant metal,or a combination of these On some gages, red lines are painted on the materials. The case assembly serves to protect dial to indicate ihe minimum and the maximum the working parts of the gage from mechanical pressures that should be carried in the system damage, dirt, sand, and, in some designs, from to which the gage is attached.These lines moisture. Secondarily, it may serve as a should be properly labeled. means of mounting the gage on an instrument A simplex BourdOn tube gage may be used panel, wall, or piece of equipment. for measuring the pressure of steam, air, Simplex, vacuum, compound, hydraulic, dif- water, oil, and similar liquids or gases. ferential,duplex,electric alarm, and depth DUPLEX GAGES.A duplex Bourdon tube are all types of Bourdon tube gages containing gage has two separate tube mechanisms within the C-shaped tube. A few of the more pertinent the same case, each acting independently of of these used influid power systems are the other. A pointer is connected to the gear discussed in the following paragraphs. mechanism of each tube,and each pointer 172 Chapter 10MEASUREMENT AND CONTROL OF PRESSURE

RED HAND POINTER

150 125 115 100 7 _../ 225 SO 250. -25. 300

FP.168 Figure 10-3.Duplex Bourdon tube gage. FP.167 Figure 10-2.Simplex Bourdon tube pressure gage. POINTER NO 1 POINTER NO 2

moves over the face of the dial without in- TUBE NO.I terfering with the other pointer.(See fig. 10-3.) TUBE NO 2 One pointer on the face of the gage illustrated in figure 10-3 points to 0, indicating that there is no pressure in the line to which the respective Bourdon tube is attached.The other pointer of the gage, however, indicates that a pressure of 85 psi is being exerted on the tube to which it is attached. Figure 10-4 shows a duplex Bourdon tube gage with the face removed. Note the position of the pointers, and also the separate connections to the different pressure sources at the base of the cane. Duplex Bourdon tube gages are used for such purposes as showing the pressure drop between the inlet and the outlet side of a strainer. The reading of each pointer indicates whetherthe CONNECTION NO.1 CONNECTION NO. 2 strainer is clean or dirty; that is, if the pressure is much greater on the inlet side of the FP.169 strainer, it indicates that foreign matter on the Figure 10-4.Mechanism of duplex straineris very likely responsible for the Bourdon tube gage. 173 FLUID POWER higher pressure. A duplex gage serves many useful purposes in indicating the ope rating condition of certain parts or types of equipment. HYDRAULIC BOURDON TUBE GAGES. Hy- draulic Bourdon tube gages are used to indicate high pressures, as on hydraulic rams (cylinders) used on ship's steering gear and anchor windlasses. Because the pressure on these gages is so high, they areequipped with a slotted connecting link between the segment gear and the link adjustment to the tube. This prevents the pointer from slamming back to 0 when the pressure is suddenly released.Without such a slotted link, the pointer or the gage mechanism could be damaged by a sudden release of pressure in the tube. Note the slotted link, adjustment shown in the gear mechanism of the hydraulic gage illustrated in figure 10-5. Many systems which employ this type of gage are equipped with gage snubbers (discussed later in this chapter) to prevent pressure surges froth damaging the gage.

FP.171 Figure 10-6.Hydraulic Bourdon tube gage.

scale and slightly over 150 on the other scale. This means that during the operation the highest pressure reached was slightly less than 3,900 tons of load on the ram. At one point during the operation, the highest psi registeredby the main pointer was slightly over 150, and while it was registering this pressure it carried the maximum pointer with it.If the main pointer had registered200, for example, the maximum pointer would be pointing to 5,100, the tons of load that had been exerted on the ram. The spindle of the maximum pointer extends through a small hole in the gage face and has a small knob which screws into the spindle. By turning theknob,the operator can set the maximum pointer. Always check, therefore, to FP.170see if the main pointer carries the maximum Figure 10-5.Slotted link in hydraulic pointer along with it. Bourdon tube gage. Hydraulic Bourdon tube pressure gages on some naval aircraft are calibrated to register from 0 to 2,000 psi; on others they register Some of the hydraulic gages used by the Navyfrom 0 to 4,000 psi.On gages designed for a have dials which indicate both the psi pressurerange of 0 to 2,000 psi, the dial is calibrated and the corresponding tons of load, on the ram.with three major markings, the numerals 0, (See fig.10-6.)In the illustration, the main1,000, and 2,000, and four intermediate gradu- pointer of this gage rests on 0, but the maximumations for reading to the nearest 200 psi. A pointer registers between 3,800 and 3,900 on onegage of this type is shown in figure 10-7. Chapter 10MEASUREMENT AND CONTROLOF PRESSURE

between the tubes and the pointer; therefore, the gage indicates the difference inpressure of the two sources. The dial of the gage shown in figure 10-8 1 1 1 (A) has the 0 located at the bottom-left and allows the pointer tomove only in one direction from 0.This design of differential pressure gage should be used in systems where the pressure from one source is always greater than that from the other. When thepressure from either source may be greater, agage with a 0 at the top of the dial should be used. With the 0 in this position, the pointer has freedom of movement to the right or to the left of 0; thus indicating the source of the highestpressure. CAUTION: When a gage with 0 at the bottom FP.172 left is used, turn on the valve to the highpres- Figure 10-7.Hydraulic pressuregage sure source first, and then the valve to the low (0 to 2,000 psi range). pressure source. Upon admittance to the line, pressure from the low pressure source will cause the pointer on the gage to revert toward 0. On gages designed for a range of 0 to 4,000 psi,the dialis calibrated with five major markings with numerals 0, 1, 2, 3, and 4. One Spiral and Helical Tubes major intermediate graduation between each numeral and four minor intermediate markings Two variations of the C-type Bourdon tube between each major intermediate markingper- pressure gage are the spiral and the helical. mit reading to the nearest 100 psi.On these The helical is sometimes referred toas helix. gages the numeral reading must be multiplied Both are made from tubing with a flattenedcross by 1,000 to obtain the actualpressure in psi. section; both were designed to providemore DIFFERENTIAL PRESSURE GAGE.A dif- travel of the tube tip, primarily for moving, ferential pressure Bourdon tubegage is used to the recording pen of pressure recorders. measure the difference in pressure between two pressure lines. SPIRAL BOURDON TUBE.The spiral form (See fig. 10-8 (A).) Like the of the Bourdon tube (fig. 10-9) is madeby duplex gage, the differentialgage contains two winding the ordinary Bourdon tube in the form Bourdon tubes and two separate connectionsfor of a.1 spiral, having, several turns, insteadof different pressure sources.Unlike the duplex the approximately 250-degreearc of the con- gage, the differential pressure gage has only ventional Bourdon tube. This arrangement one pointer and ,indicates the difference in does not change the operating principle ofthe pressure between the two pressuresources. Bourdon tube,but simply has the effect of The Bourdon tubes in agage of this type producing a tip movement equal to thesum of are connected in a definite mannerso as to be theindividual movements that would result able to record the difference inpressure from from each part of the spiral consideredas a the two sources.The small Bourdon tube simple Bourdon tube. A givenpressure, there- (fig- 10-8 (B)) is connected toa stationary base. fore, causes greater tip movement thanthe Through a system of levers, it is connectedto C-shaped Bourdon type. the large Bourdon tube.The base of the large HELICAL BOURDON TUBE.In the helical tube works on a pivot, so thatthe basecan move gage, the Bourdon tube element is wound in either to the right,or to the left atanytime. the form of a helix, as illustrated in figure The movement of the two tubes counteracteach 10-10.This arrangement increases the tip other until the pressure in one tube isgreater travel considerably. A center shaft is usually than the pressure in the other..Only this installed within the helix, and the linkage is difference in pressureaffectsthelinkage arranged in such a manner that the shaft is FLUID POWER

TUBE

LARGE TUBE

LINKAGE

FP.173 Figure 10-8.Differential pressure Bourdon tube gage. Chapter 10MEASUREMENT AND CONTROL OFPRESSURE

4111111111k

SIDE VIEW OF SPIRAL

1111,

FP.174 Figure10-9.Spiral Bourdon tube.

TIP MOVES HERE HELICAL PRESSURE TUBE

POINTER

SHAFT

(DETAIL VIEW OF TYPICAL HEUX

HELIX

. PRESSURE

TIP MOVES HERE

(SCHEMATIC

FP.175 Figure 10-10.Helical Bourdon tube gage. rotated by the tip of the helix.The pointer, DIAPHRAGM GAGES in turn, is driven through additional linkage by the shaft.This design transmits only the circular A diliphragm gage gives sensitive and reliable part of the tip movement to the pointer; this isindications of small differences in pressure. It the movement that is directly related to the .is often wed to measure air pressure. This type change in pressure. of gage is usually designed by the manufacturer 177 t

FLUID POWER

in accordance with specifications for a specific the diaphragm decreases, the diaphragm returns purpose and is calibrated accordingly. This doestoward its neutral position and pulls the metal not mean, of course, that further adjustmentsspring and pointer with it.Thus, the reading on may not be required when the gage is installedthe scale o f the-- diaphragm gage is directly on equipment. proportional to the amount of pressure exerted The indicating mechanism of a diaphragm gage on it by the force being measured. consists of a tough, pliable, leather or neoprene rubber membrane connected to a metal springSPRING-LOADED PRESSURE GAGE which is attached by a simple linkage system to thegage pointer.Stpdy the diaphragm gage In some fluid power systems, the pressure illustrated in figure 10-11. Note the installationfluctuates rapidly. These fluctuations can damage of the diaphragm in the gage frame and the positionpressure gages, especially a delicate instrument of all the parts of the gage.The size of theas the Bourdon tube gage. Gage snubbers (dis- diaphragm affects the sensitivity in registeringcussed in the next section) are used in some pressurethe larger the diaphragm the greatersystems to dampen these fluctuations; however, the sensitivity. the spring-loaded direct-acting gage is sometimes used to measure pressure in such systems. In the spring-loaded gage a piston is directly actuated by the fluid pressure to be measured. The piston moves through a cylinder against the resistance of a spring and carries a bar or indicator with it over a calibrated scale. In this manner all levers, gears, cams, and bearings are eliminated, and a sturdy instrument can be constructed. Figure 10-12 shows the construction of the gage and the manner in which it operates. The parts up through the middle of the gage; from the needle at the bottom on through the packing piston and rod to the button at the top, form a unit that transmits fluid pressure to the sleeve against which the button rests.The sleeve surrounds the inner barrel of the gage.The sleeve is flanged at its base to provide a seat for a spring coiled around the barrel and for a cup to which the indicator PAR OR isattached. Fluid pressure compresses the BUUNEAD spring, and the barrel rises out of the body, ZERO ADJUST IC SCREW carrying the indicator up with it. The indicator FP.176 moves against a pressure scale on the face of Figure 10-11.Diaphragm pressure gage. the gage. (See fig. 10-13.) The spring-loaded gage is calibrated by com- One side o f the diaphragm is exposed to the paring gage readings with known pressures. A pressure being-measured; the other side is ex small error can be corrected by loosening four posed to the atmosphere. When no pressure is ex-screws on the face of the gage and sliding the erted against the diaphragm, both the diaphragmscale up or down under the pointer. For larger and the attached metal spring are in a neutralerrors, the adjustment screw which holds the position. When pressure is applied, the diaphragmspring in place can be tightened if the gage is moves upward forcing the metal spring ahead of reading too high, or loosened if the reading is too it.The spring is connected to the pointer with low.Turning the adjustment screw varies the a length of kinkless bead chain. As the springcompression of the spring.A sealing strip is moves upward, it moves the pointer to a higherprovided to lock the adjustment screw in place reading on the dial.When the pressure againstafter calibration. 178 Chapter 10MEASUREMENT AND CONTROL OF PRESSURE

ADJUSTMENT BUTTON SCREW NUT

SEALING STRIP

SLEEVE BODY SCREW SPRING

SHELL AND SLEEVE

BARREL ROD AND BUTTON

CUP AND INDICATOR INDICATOR DRAIN CONNECTION

SPRING SEAT

SLEEVE

GRADUATION PLATE LARGE LOCK NUT DISKS PISTON

PLUG (FELT)

NEEDLE LOCK NUT

FP.177 Figure 10-12.Spring-loaded gagecutaway view.

While the spring-loaded pressure gage iscontaminated fluids, and sudden high pressure. satisfactory for ordinary fluid pressure measure-A plug valve should be installed between the ments, it is not as accurate as the Bourdon tubegage and the system so that pressure can be gage. It is not a laboratory gage for exactapplied slowly to the gage, and so that it will readings, but a sturdily constructed working unitbe protected against strain when not in actual for practical use.It is available 'in varioususe. pressure ranges and is especially. recommended These gages should not be used in a system for fluctuating loads.0ne advantage of thisin which maximum pressure may exceed the type gage is that it can be rebuilt very easilymaximum designated gage reading. Dropping and economically. a gage may permanently damage the calibrated The gage should be protected against vibration,units.When gages are not in use they should be excessive temperatures; corrosive or otherwisestowed in a dry place. 179 FLUID POWER

NMI

FP.178 Figure 10-13.Spring-loaded gage scale.

GAGE SNUBBERS FITTI PIN AND PLUNGER The irregularity of impluses appliei to the ASSEMBLY fluid power system by some power pumps/compressors causes the gage pointer to oscillate violently.This makes reading of the gage not only difficult but often impossible.Pressure oscillations and other sudden pressure changes existing in fluid power systems will also effect the delicate internal mechanism of gages and cause either complete destruction of the gage or, 0 -RING HOUSING often worse, partial damage, resulting in false readings.A pressure gage snubber is therefore installed in the line to the pressure gage. FP.179 Figure 10-14.Pressure gage snubber. The purpose of the snubber is to dampen the oscillations and thus provide a steady. reading and a protection for the gage.The basic com- PRESSURE CONTROL DEVICES ponents of a snubber are the housing, fitting assembly with a fixed orince diameter, and a Safe and efficient operation of fluid power pin and plunger assembly, as illustrated in figuresystems, system components, and related equip- 10-14. The snubbing action is obtained byment requires a means of controlling pressure. metering fluid through the snubber. The fixingThere are several different types of pressure assembly orifice restricts the amount of fluidcontrol devices used in fluid power systems. that flows to the gage, thereby snubbing theSome of these devices are used to maintain the force of a pressure surge.The pin is pusheddesired pressure in the system, some are designed and pulled through theorifice of the fittingto prevent excessive pressure buildup and damage assembly , by the plunger, keeping it clean andto the system, and some are used to reduce at a uniform size. pressure toone or more subsystems.For 180 Chapter 10MEASUREMENT AND CONTROL OFPRESSURE example, a system with a number of subsystems may require 3,000 psi to operate all subsystems PESSUR400 LBS PER SQ IN except one, which requires only 1,500 psi. A pressure control valve is used to maintain the . .!:!,: 3,000 psi in the main system, and a pressure reducing valve is used to reduce the pressure to 1,500 psi before it enters the one subsystem. These and several other types of pressure control devices are described in the following sections. Most pressure control valves are of the type which contain two systems of moving parts whose joint action is responsible for the operation of the valve.As stated in chapter 9, this type is classified as compound rather than simple. Some of the reasons compound valves are used for the control of pressure are explained in the next section under relief valves. FP.180 The rr:.. e in which the most complicated Figure 10-15.Pressure acting pressure valve operates will always on a slanted surface. conform to the hydraulic principles discussed in chapter 2. The valve will open or close because pressures are different over equal areas, be- "pop-off" valve on a steam boiler and the safety cause pressures are acting over unequal areas, valve on a hot water tank are relief' valves. or a combination of both reasons.In each The pressure cap on an automobile radiator instance, inequality of opposedforces causes the is a relief valve, used to prevent pressure valve to open or close. The forces may be set from building up too high and bursting the

up by the opposition of fluid pressures, or by radiator. . fluidpressureacting against a mechanical Some fluid power systems, even when resistance; for example, the resistance set up operating normally, may temporarily develop by a spring, dangerous excess pressure; for example, when Like all forces, the force exerted in a certain an unusually strong work resistance is en- direction on the face of a valve is always exerted countereb. Relief valves are used to control on an area standing exactly at a right angle this excess pressure. Their purpose is not to that direction.Thus in figure 10-15, if the to maintain flow or pressure at a given amount, I fluid is under a pressure of 400 psi and the but to prevent pressure tram rising above a slanting surface (A) has an area of 10 square definite level when the system is temporarily inches, the force acting in a slanting direction, overloaded. at right angles to that surface, is 4,000 pounds. The main system relief yam must be large The force acting directly. downward on surface enough to allow the full output of the hydraulic (A), however, depends upon the horizontal area pump to be delivered back to the reservoir. (B), directly beneath (A).If (B) has an area of In the case of a pneumatic system, the relief 8 square inches, the downward force acting valve controls excess pressurby discharging on surface(A,)is 3,200 pounds (400 x 8). the excess fluid into the atmosphere. Smaller relief valves, similar in design and operation to the main system relief valve, are RELIEF VALVES often used in isolated parts of the system where a check valve or a directional control valve A relief valve is simply a pressure limiting prevents pressure from being relieved through device. It is commonly used to prevent the the main system relief valve and where .pres- pressure of a confined fluid from building up sures must be relieved at a point lower than to a point at which the container would burst. that provided by the main system relief valve. Since it is used to protect the system from::These small relief valves are also used to excessive pressures, the relief valve is some- relieve pressures caused by thermal expansion times referred to as a safety valve. The (see glossary) of the fluids. Since the amount FLUID POWER of fluid to be relieved is small, the valve can be small and still do its job. All relief valves have an adjustment for increasing or decreasing the pressure at which they willrelieve. Some relief valves are equipped with an adjusting screw forthis purpose. The adjusting screw is unusally covered with a cap, which must be removed before adjustment can be made. Some type of locking device, such as a lock nut, is usually provided to prevent the adjustment from changing through vibration. Other types of relief valves are equipped with a handwheel for making adjustments to the valve.Either the adjusting screw or handwheel is turned clockwise to increase the pressure at which the valve will relieve. A simple two-port relief valve is shown in figure 10-16.Fluid under system pressure enters port (6) and pushes upward against the ball (5).If the pressure increases to a point high enough to overcome the force of spring (3), the ball will be pushed off its seat, and the fluid from the system can flow out port (4) to return, or to the atmosphere in pneumatic systems. When the system pressure decreases to normal, the spring (3) forces the ball (5) on its seat.The adjusting screw (1) increases or decreases the force of the spring, requiring more or less pressure to unseat the ball. Various modifications of the simple relief valve are used and efficiently serve the require- 6 ments of some fluid power systems. However, FP.181 many systems require compound relief valves. 1.Pressure idjusting 4.Return port. For a better understanding of the operation SUM ofreliefvalves, some of the undersirable 2.Adjusting screwcap. 5.Ball. characteristics of the simple relief valve are 3.Compression spring.6.Pressure port. discussed in the following paragraphs. It is because simple relief valves are Figure 10-16.Simple two-port relief valve. unsatisfactory for some applications that compound relief valves were developed. A simple relief valve, such as the one illustrated in figure In addition, the valve will not close until the 10-16, with a suitable spring adjustment can pressure decreases to a point somewhat below be set so that it will open when the system the opening pressure. As indicated in figure pressure increases to 500 psi, for example. 10-17, the surface area of the valve element When it does open, however, the volume of must be larger than the pressure opening if flow to be handled may be greater than the the valve is to seat satisfactorily. The pres- capacity of the valve, so that pressure in the sure in the system acts on the area of the system may increase as much as several valve element open to it.In each case in hundred psi above the set pressure before the figure 10-17, the force exerted directly upward valve spring brings the pressure under control. by system pressure when the valve is closed A simple relief valve would be effective under depends on the horizontal area across the valve these conditions only if it were very large. In at (A).The moment the valve opens, however, that case, however, it would operate stiffly and the upward force exerted depends on the hori- the valve element would chatter back and forth. zontal area of the valve element at (B), which Chapter 10MEASUREMENT AND CONTROL OF PRESSURE

relief valves for one stage of their actionthat of the pilot valvebut provision is made to limit the amount of fluid that the pilot valve must handle, and thereby avoid the weaknesses of simple relief valves.(A pilot valvt. is a small 7., valve used for operating ,anOther valve ) 4 10114.100% .V1,0,1,3*(144 tsf--4,.4.,/o...: The operation of a compound relief valve is fliggilivi .1AM or....V.tJAVOlifeva.sr* illustrated in figure 10-18.In view (A), the .Y.Y.V.M:752 S'ILOWAYANM:071 tA4C/Sisi?.119".Y.0..vi..ALat'fitPd main valve, which consists of a piston, stem, and spring, is closed, blocking flow from the FP.182 high-pressure line to the reservoir. Fluid in Figure 10-17.Pressure acting on the high-pressure line flows around the stem of different areas. the main valves as it flows to the actuating unit.The stem of the main valve is hollow (the stem passage),and contains the main is necessarily greater than the area at (A). This spring which forces the main valve against causes an upward jump in the action of the valve its seat. When the pilot valve is open the stem immediately after it opens, and it also sets passage allows fluid to flow from the pilot up a greater force opposed to the closing of valve, around the main spring, and down to the the valve than was required to open it.As low-pressure or return line. a result, the valve will not close until the There is also a narrow passage through the pressure has decreased to a point somewhat main .valve piston.This passage connects the lower than the pressure required to open it. high-pressurelinetothevalve chamber. The same pressure acting over different areas produces forces proportionaltothe areas. The pilot valve is a small, ball type, spring- For example, assume that a valve of this loaded check valve, which connects the top of type is set to open at 500 psi. (Refer to fig. the passage from the valve chamber with the 10-17.) When the valve is closed, the pressure passage through the main valve stem. The pilot acts on the area (A).If this area is 0.5 square valve spring tension can be adjusted by turning inch, en upward force of 250 pounds (500 x 0.5) the adjusting screw.The pressure at which will be exerted on the, valve at the moment of the valve will relieve depends on the tension opening.With the valve open, however, the of the spring. pressure acts on the area at (B).U the area Fluid at line pressure flows through the at (B) is 1 square inch, the upward force is narrow piston passage to fill the chamber. 500 pounds, or double the force at which the Because the line and the chamber are connected, valve actually opened.For the valve to close, the pressure in both are equal.The top pressure in the system would have to decrease and bottom of the main piston have equal well below the point at which the valve opened. areas; therefore, the hydraulic forces acting The exact pressure at which the valve would upward and downward are equal, and there is close depends upon certain relations of the no tendency for the piston to move in either particular shapes of the valve element. direction.The only other force acting on the In some hydraulic systems, there is a pres- main valve is that of the main spring, which sure in the return line. This "back pressure" holds, it closed. is caused by restrictions in the return line and, The pilot valve is the control unit. Its of course, will vary in relation to the amount of spring is so adjusted that the ball will unseat fluid flowing to return. This pressure acts on when pressure reaches the preset limit.At the top of the valve element and will increase normal operating pressure the ball remains the force necessary to open the valve and relieve seated, system pressure. When the pressure in the high-pressure It follows that simple relief valves have a line increases to the point at which the pilot tendency to open and close rapidly as they valVe is set, the ball unseats, and opens the "hunt" above and below the set pressure, causing valve chamber through the valve stem passage pressure pulsations and undesirable vibrations -tothe low-pressure return line.(See fig. and producing a noisy chatter. Compound relief 10 -18 (B).)Fluid immediately begins to flow valves use the principle of operation of simple out of 'the chamber, much faster than it can FLUID POWER

PILOT VALVE SPRING PILOT VALVE ANTwING STEM PASSAGE BLI.

VALVE CHAMBER

PISTON MAIN VALVE SPRING PASSAGE PISTON MAIN VALVE

STEM

(C) FP.183 Figure 10-18.Operation of compound relief valve. 1s9184 Chapter 10MEASUREMENT AND CONTROL OF PRESSURE flow through the narrow piston passage. As a The valveis normally located in the line result the chamber pressure immediately drops, between the directional control valve and the and the pilot valve begins to close again, outlet of a vertically mounted actuating cylinder restricting the outward flow of fluid. Chamber which supports weight or must be held in posi- pressure therefore increases, the valve opens, tion for a period of time. The counterbalance and the cycle repeats. valve serves as a hydraulic resistance to the So far, the only part of the valve that has actuating cylinder. For example, counterbalance moved appreciably is the pilot, which functions valves are used in some hydraulically operated just like any other simple spring-loaded relief fork lifts. The valve offers a resistance to the valve. Because of the small size of the piston flow from the actuating cylinder when the fork passage, there is a severe limit on the amount is lowered. It also helps to support the fork in of relief it can give the system. All the pilot the UP position. valve can do is limit fluid pressure in the valve Counterbalance valves are also used in some chamber above the main piston to a preset late model air launched weapons loaders. In maximum pressure by allowing excess fluid to this case, the valve is located in the top of the flow through the piston passage, through the lift cylinder. The valve requires 250 psi to valve chamber, and through the stem passage lower the load. If adequate pressure is not into the return line. When pressure in the available,the load cannot be lowered, thus system increases to a value which is above the preventing collapse of the load due to any flow capacity of the pilot valve, the main valve malfunction of the hydraulic system. opens, permitting excess fluid to flow directly to return. This is accomplished in the following One type of counterbalance valve is illus- manner. trated in figure 10-19. The valve element is a balanced spool valve (4). The spool valve con- As system pressure increases, the upward sists of two pistons permanently fixed on either force on the main piston overcomes the down- end of a shaft. The inner surface areas of the ward force, which consists of the tension of pistons are equal; therefore, pressure acts the main piston spring and the pressure of the equally on both areas regardless of the position fluid in the valve chamber. (See fig. 10-18 (C).) of the valve, and has no effect on the movement The piston then rises, unseating the stem, and of the valvehence, the term balanced. The allows the fluid to flow from the system pres- shaft area between the two pistons provides sure line directly into the return line. This the area for the fluid to flow when the valve is causes system pressure to decrease rapidly, open. A small pilot valve is attached to the since the main valve is designed to handle the bottom of the spool valve. complete output of the pump. When the pressure returns to normal, the pilot spring forces the When the valve in in the closed position, ball on the seat. Pressures are then equal the top piston of the spool valve blocks the above and below the main piston, and the main discharge port (8). With the valve in this posi- spring forces the valve to seat. tion,fluid,flowing from the actuating unit, enters the inlet port (5). The fluid cannot flow As can be seen, the compound valve over- through the valve because the discharge port comes the greatest limitation of a simple relief (8, is blocked. However fluid will flow through valve by limiting flow through the pilot valve the pilot passage (6) to the small pilot piston. to such quantities as it can satisfactorily handle. "As the pressure increases, it actson the pilot This limits the pressure above the main valve, piston until it overcomes Ills preset pressure and enables the main line pressure to open the of spring (3). This forces the spool up and main valve. In this way, the system is relieved allows the fluid to flow around the shaft of the when an overload exists. spool valve and out the discharge port (8). Figure 10-19 shows the valve in this position. COUNTERBALANCE VALVES During reverse flow, the fluid enters port (8): The spring (3) forces the spool valve (4) to The purpose of a counterbalance valve is to the closed position. The fluid pressure over- permit free flow of fluid in one direction and comes the spring tension of check valve (7). The to maintain a resistance to flow in the other check lave opens and allows free flow around direction until a certain pressure is reached. the shaft of the spool valve and out port (5). FLUID POWER

SEQUENCE VALVES As described in chapter 9, the orifice check valve is sometimes used to control the sequence of operation in one direction of two or more actuating devices. Some fluid power systems require a more positive means of controlling the sequence of operation. A sequence valve is used in these cases. As the name implies, a sequence valve is used to control a sequence of operations; that is, enable one unit to automatically set another unit into motion. An example of the use of a sequence valve is in an aircraft landing gear actuating system. In a landing gear actuating system the landing gear doors must open before the landing gear starts to extend. Conversely, the landing gear must be completely retracted before the doors close. A sequence valve installed in each landing gear actuating line performs this function. Figure 10-20 shows an installation of two sequence valves which control the sequence of operation of three actuating cylinders. Fluid from the directional control valve is free to flow into cylinder (A). The first sequence valve (1) blocks the passage of fluid until the piston in cylinder (A) moves to the end of its stroke. At 1.Adjustment screw. 6. Pilot passage. this time, sequence valve (1) opens, allowing 2.Internal drain. 7. Check valve. fluid to enter cylinder (B). This action continues 3.Spring. 8. Discharge outlet until all three pistons complete. their strokes. 4.Spool. or reverse free 5.Pressure inlet or flow inlet. reverse free flow MINDER C outlet. FP.184 Figrure 10-19.Counterbalance valve.

The operating pressure of the valve can be adjusted by turning the adjuitment screw (1), which increases or decreases the tension of the spring. This adjustment depends on the weight that the valve must support. It is normal for small amounts of fluid to leak around the top piston of the spool valve and into the area around the spring (3). An accumulation would co arse additional pressure on the top of the spool valve. This would require additional pressure to open the valve. The drain FP.185 (2) provides a passage for this fluid to flow to Figure 10-20.Installation of sequence port (8). valves. Chapter 10MEASUREMENT AND CONTROL OF PRESSURE

There are various types of sequence valves. setting,the force acting upward against this Some are pressure controlled and some are surface area of the piston is less than the down- mechanically controlled. One type of sequence ward force of the spring (2). This holds the valve operates similar to the counterbalance piston down and the valve is in the closed valve just described. In fact, valves are avail- position as shown in figure 10-21 (A). able which can be utilized as either a counterbalance valve or sequence valve. Two types of When the primary actuating unit completes sequence valves are described in the following its op e r a ti o n, pressure in the line to the paragraphs. The first is pressure controlled actuating unit increases and, therefore, the and thesecond is mechanically controlled. pressurein the valve increases. When the pressure increases sufficiently to overcome the Pressure Controlled force of spring (2), piston (1) rises. The valve ,Sequence Valve is then in the open position, as shown in figure 10-21 (B). The fluid entering the valve takes The operation of a typical pressure con- the path of least resistance and flows through trolled sequence valve is illustrated in figure port (5) to the secondary unit. 10-21. The opening pressure is obtained by A drain passage (6) is provided to allow any fluid, which leaks past the piston, to flow from the top of the valve. In the case of hydraulic systems, this drain line is usually connected to the mai n return line. These v al v usually contain a check valve to allow free reverse flow from the actuating units.However, the sequencing action is provided in only one direction of flow. Mechanically Operated Sequence Valves Figure 10-22 illustrates a mechanically (A) operated sequence valve. This valve is operated . by the in and out movement of the plunger which extends through the body of the valve. 1. Piston. 5. Outlet port to 2. Spring. secondary unit. 3. Entrance port. 6. Drain. 4. Outletport to pri- . mary unit. FP.186 Figure 10-21.Operation of pressure controlled sequence valve.

a d j u sting the tension of spring (2), which normally holds the piston (1)in the closed position. Note '.ths,#;:.,the top. part of the piston has a larger 'dituneter than the lower part. Fluid from the directional control valve enters port (3), flows around the lower part of the piston (1) and enters the outlet port (4), where it flows to the first (primary) unit to be operated. Thig fluid pressure acts against the lower FP.18? surface of the larger part. of the piston. When Figure 10-22.Mechanically operated the pressure of the fluid is below the adjusted sequence valve. FLUID POWER

The plunger is held in the extended position by displacement pumps, which con, ain a pressure a spring. The valve is mounted so that the regulating device as described in chapter 8. plunger will be depressed by the primary unit. Althoughmanufacturers are leaning more toward the use of variable displacement pumps, A check valve is incorporated between the there are many closed-center hydraulic systems fluid ports in the body. The check valve, either that utilize constant displacement pumps and, a ball or a poppet, is held against the seat by therefore, require a pressure regulator. (Open- a spring. The check valve is unseated either by and closed-center hydraulic systems are de- the plunger as it is depressed into the valve or by fluid under pressure entering port (B). scribed in chapter 4.)

Port (A) and the actuator,:4\of the primary Pressure regulators are made in a variety unit are connected with a common line from of types by various manufacturers; however, the directional control valve. Port (B) is con- the basic operating principles of all regulators nected with a line to the a c tuator of the are similar to the one illustrated in figure secondary unit. When fluid under pressure flows 10-23. View (A) shows the regulator in the cut- from the directional control valve to the primary in position and view (B) in the cutout position. unit, it also flows through port (A) to the seated check valve in the sequence valve. In order to A regulator is said to be in the cut-in posi- operate the secondary unit, the fluid must flow tion when it is directing fluid under pressure through the sequence valve. The valve is located into the system. In the cutout positica, the fluid in such a position that the primary unit, as it in the system after the regulator is trapped at completes its operation, will depress the the desired pressure, and the fluid from the plunger. The plu4-er unseats the check valve pump is bypassed into the return line and back and allows the fluid to flow through the valve, to the reservoir. To prevent constant cutting in out port (B), and In the secondary unit. and cutting out (chitter), the regulator is designed to cut in at a pressure somewhat lower This type sequence valve permits flow in than the cutout preessure. This difference is the opposite direction. Fluid enters port (B) known as differential or operating range. For and flows to the check valve. Although this is example, assume that a pressure regulator is return flow from the actuating unit, the fluid is set to cut in when the system pressure drops under sufficient pressure to overcome spring below 600 psi, and cut out when the pressure tension and unseat the check valve and flow out rises above 800-- psi. The differential. or through port (A). operating range is 200 psi. PRESSURE REGULATORS Referring to figure 10-23, assume that the piston (5) has an area of 1 square inch, the As the term implies, pressure regulators steel ball of the bypass valve (1) has a cross- are used in fluid power systems to regulate the sectional area of one-fourth square inch, and pressure.In pneumatic systems, the valve the piston spring (6) provides 600 pounds of commonly referred to as a pressure regulator force pushing the piston down. When the pres- serves to reduce pressure. This type valve is sure in the system is less than 600 psi, fluid discussed later in this chapter under pressure from the pump enters the input port (2), flows reducing valves. The pressure switch, also to the top of the regulator, and to the check described later, is often used in pneumatic valve (3). When the pressure of this input fluid systems to regulate pressure. The pressure increases to a value above the pressure of the regulator described in the following paragraphs fluid in the system side of the check valve and is utilized in hydraulic systems. the, force of the check valve spring, the check Pressure regulators, often referred to as valve opens and fluid flows into the system and unloading valves, are used in hydraulic systems to the bottom of the regulator against the piston to unload the pump and to maintain and regulate (5). Until the pressure is great enough to force system pressure between the maximum and the piston upward and unseat the ball, the fluid minimum. All hydraulic systems do not require is directed through the system connection port pressure regulators. The open-center system (4) to the system. The regulator is then in the does not require a pressure regulator. Many cut-in position, as shown in view (A) of figure systems are equipped with variable 10-23. 188 143 Chapter 10MEASUREMENT AND CONTROL OF PRESSURE

1 7

;

(A) (B) 1. Bypass valve. 5. Piston. 2. Pressure input. 6. Spring. 3. Check valve. 7. Line to 4. System connection. reservoir. FP.188 Figure 10-23.Hydraulic pressure regulator. I When the pressure on the piston builds up to lator is in a balanced state as both the upward 600 psi, the force applied on the piston face will and downward forces are equal. Any pressure be 600 pounds. (Force equals pressure time in excess of 800 psi will move the piston up and areaF = P x A.) In this case, the pressure is push the ball off its seat. Since the fluid will 600 and the area of the piston is 1 square inch; always follow the path of least resistance, it thereforethe force is 600 pounds. Since the will pass through the regulator and back to the spring pushes the piston down with a force of reservoir through line (7). 600 pounds, the two forces on either side of the piston are balanced. However, the force holding When the fluid from the pump is suddenly the ball in place must be considered. This force, allowed a free path to return, the pressure on 600 x1 square inch (cross-sectional area of the input side of the check valve (3) drops and 4 the check valve closes. The fluid in the system the steel ball), equals 150 pounds. This force is then trapped under pressure. The regulator allows the fluid to continue to build up pressure is now in the cutout position, as shown in

in the system. figure 10-23 (B).This fluid will remain pre- A When the pressure in the system increases surized until a unit is actuated, or until pres- to 800 psi, there is 800 pounds of force pushing sure is slowly lost thraugh normal internal upward on the piston. The spring force is con- leakage within the system. stant (600 pounds); therefore,theresultant force is 200 pounds (800 poundsminus. The pUmp continues to operate, although it 600 does not have to force. thefluid against pounds) pushing the piston upward. However, pressure.. Therefore,the pump is not con- the force applied to the steel ball will also be stantly under a load and will operate under 200 pounds (800 xt1 ). At this point the regu-trouble-free conditions for a longer period. 189 1.94 FLUID POWER

With the regulator in the cutout position, it maintains the reduced pressure at a constant there is very little pressure acting on the steel level during operation of the subsystem. ball and this pressure acts on the entire sur- Like all other valves, there are various face area of the ball. Therefore, the 600-pound designs and types of pressure reducing valves. force of the spring (6) is the only force pushing Two typesthe diaphragm-controlled and the downward on the piston (5). When the system pilot-controlledare described in the following pressure decreases to a point slightly below paragraphs. 600 psi,the spring(6) forces the piston (5) down and- closes the bypass valve (1). When the Diaphragm-Controlled bypass is closed, the fluid cannot flow directly to return. This causes the pressure to increase A diaphragm-controlled pressure reducing in the line between the pump and the regulator. valve is illustrated in figure 10-24. This type This pressure opens the check valve (3), and valve is commonly used in pneumatic systems. the fluid will then enter the system and build Because it serves to regulate reduced pressure. up the pressure to 800 psi. to a subsystem,it is often referred to as a Therefore, when the system pressure lowers pressure regulator. a certain amount, the pressure regulator will Control of fluid flowing through the valve is cut in, thus sending fluid into the system. When effected by means of a pressure difference on the pressure increases sufficiently, the regu- opposite sides of the diaphragm. The diaphragm lator will cut out allowing the fluid from the is secured to the valve stem. The fluid flowing pump to flow through the regulator and back to out of the reduced pressure side of the valve the reservoir. As stated previously, the dif- also flows through an internal passage to the ference between the regulator cut-in pressure diaphragm chamber below the diaphragm. An and cutout pressure isthe differential or adjusting spring acts on the upper side of the operating range. This prevents the regulator from cutting in or out with small changes in diaphragm. pressure. The pressure regulator serves to The amount of pressure applied by the fluid take theload off the pump and to regulate to the undersurface of the diaphragm varies system pressure. according to the pressure in the outlet tzeduced pressure) port. When the o'it!et pressure is PRESSURE REDUCING VALVES greater than the force of the spring, the diaphragm is forced upward. Since this is an In some fluid power systems it is desirable, upward seating valve, the upward movement of and often necessary, to operate a subsystem at the stem tends to close the valve or at least to a lower pressure than the main system. Pres- decrease the amount of flow through the valve. sure reducing valves are used for this purpose. When the outlet pressure is less than the force For example, assume the operating pressure of of the spring, the diaphragm and the valve stem a system is 1,500 psi. This pressure is re- are forced downward, opening the valve accord- quired for all subsystems, except one which ingly and increasing the amount of flow through must be limited to 800 psi. A relief valve, the valve. This flow increases the pressure of adjusted to 1,500 psi, installed in the main the fluid in the outlet port. When the pressure system would allow system pressure to rise to of the outlet fluid is equal to the spri:_, pres- the required 1,500 psi; however, the relief valve sure,the valve stem remains stationary and would net limit the subsystem pressure to 800 there is no flow of fluid through the valve. psi. A reltlf valve, adjusted to 800 psi, would A pressure gage is usually installed in the protect the subsystem, but would prevent the line leading from the reduced pressure side of main system from developing the required the valve so the operator can adjust the valve 1,500 psi,Therefore, a system such as this to the desired pressure. Since the outlet pres- requires a relief valve, adjusted to a setting of sure depends upon the force provided by the 1,500 psi, installed in the main system and a spring, the pressure can be varied by changing pressure reducing valve, adjusted to 800,psi, the tension of the spring. The spring tension installed in the subsystem, is changed by turning the adjusting screw. In addition to reducing the pressure, a Turningthe screw clockwise increases the reducing valve regulates the pressure. That is, pressure applied by the spring to the top of 190

.195 Chapter 10MEASUREMENT AND CONTROL OF PRESSURE

ADJUSTING SCREW

,z7/411V

ADJUSTING SPRING

DIAPHRAGM

1 /

......

VALVE STEM

INLET PORT OUTLET PORT LINE REDUCED PRESSURE PRESSURE

VALVE FP.189 Figure 10-24.Diaphragm-controlled pressure reducing valve. the diaphragm, thus tending to open the valve. valve consists of an adjustable pilot valve, which Turning the adjusting screw counterclockwise controls the operating pressure of the valve, and decreases the amount of spring tension on the a spool valve, which reacts to the action of the top of the diaphragm, thus tending to decrease pilot valve. the amount of pressure in the outlet port. The pilot valve consists of a poppet (1), spring (2), and adjusting screw (3). The spool Pilot-Controlled valve assembly consists of a spool valve (10) and a spring (4). Figure 10-25 illustrates the operation of a Fluid under main pressure enters the inlet pilot-controlled pressure reducing valve. This port (11) and, under all conditions, is free to 191 FLUID POWER

14 15

NMI

.+1 1 iiimilif101111111111i

6 10 Mit 7

sql11 (A) (8) 8

1.Poppet valve. 8.Fluid passage. 2.Pilot valve spring. 9.Fluid passage. 3.Adjusting screw. 10.Spool valve. 4.Spool valve spring. 11.High-pressure 5.High-pressure outlet inlet port. port. 12.Fluid chamber. 6.Reduced pressure 13.Fluid passage. outlet port. 14.Fluid chamber. 7.Opening. 15.Drain. FP.190 Figure 10-25.Pilot-controlled pressure reducing valve. flow through the valve and out port (5). Either As the pressure increases in outlet port (6), port (5) or port (11) may be utilized as the this pressure is transmitted through passages (8) high-pressure inlet port. and (9) to chamber (12).This preesure also Figure 10-25 (A) shows the valve in the open acts on the pilot valve poppet (1).When this position.In this position, the pressure in the pressure increases above the preset operating reduced-pressure outlet port (6) has not reached pressure of the valve, it overcomes the force of the preset operating pressure of the valve. The the pilot valve spring (2) and unseats the poppet fluid also flows through passage (8), through (1).This allows fluid to flow through the drain the smaller passage (9) in the center of the spool port (15)(usually connected to the reservoir valve, and into chamber (12). The fluidpressure return line) and, because the small size passage at the outlet port (6) is therefore distributed to (9) restricts the flow into chamber (12), reduces both ends of the spools. When these pressures the pressure ofthefluidin the chamber. are equal the spool is hydraulically balanced. Although it allows the pilot valve to close, Spring (4) is a low-tension spring and applies thi ; momentary reduction of pressure in cham- only a slight downward force on the spool.Its ber (12) results in a differential of pressures main purpose is to position the spool and to acting on the bottom and the top of the spool maintain opening(7)at its maximum size. valve(10). Thisdifferential of pressures 192 197 Chapter 10MEASUREMENT AND CONTROL OF PRESSURE overcomes the downward force of spring (4, element. This sensing element can be a and forces the spool upward until the pressures piston, Bourdon tube, or diaphragm. balance. As the spool moves upward, it re- A pressure-switch which utilizes a piston stricts the flow through opening (7) and therefore type element is illustrated in figure 10-26. causes the pressure to decrease in the reduced- The housing of this pressure switch has three pressure outlet port (6).(See fig. 10-25 (B).) external ports.Two of these ports (A and B) If the pressure in the outlet port (6) continues are connected to the main pressure line. Port to increase to a value above the preset pressure, (C) is connected to the return line.Within the pilot valve will open again and the cycle the housing are two pistons (D and E) and a repeats.This allows the spool valve to move hollow poppet valve (F).Piston (D) is held up higherinto chamber(12); thus, further down by a spring, but is placed so that the end reducing the size of opening (7). This places a of its rod will move the ball upward and close furtherrestrictionof flowinto outlet(6). the passage in the hollow poppet when the pres- These cycles repeat until the desired pressure sure becomes great enough to overcome spring is maintained in outlet (6). tension.Piston (E) is attached to an electrical When the pressure in outlet (6) decreases to switch by a piston rod. Pressure on this piston a value below the preset pressure, the spring (4) controlstheswitch. Upward movement of forces the spool downward, allowing more fluid piston (E) turns the switch ON, and downward to flow through opening (7). motion turns the switch OFF. Reverse free flowfrom the reduced-pressure In operation, fluid under pressure enters port (6)through the valve is possible only if ports(A and B). The fluid which enters pressure at port (6)is lower than the valve port (A) acts on the bottom of piston (E). The setting. If the pressure exceeds the valve fluid which enters port (B) flows through the setting,the valve will close, making reverse internal passage and acts on the bottom of flow impossible.To insure reverse free flow, piston (D).As the pressure increases against this type valve is often equipped with a check piston (D), it overcomes the force of the spring valve.The check valve is located between the and moves the piston upward. When piston high-pressure port (5) and the reduced-pressure (D)isinthisposition,the opening in the port (6).Reverse flow will unseat the check hollow poppet is closed by the ball, and the valve and flow directly from port (6) to port (5) poppet valve is held off its seat.When the and to return. poppet valve is off its seat, the upper chamber is connected to system pressure,and the pressure acting on the top of piston (E) is the same as the pressure acting on the bottom. PRESSURE SWITCHES Since the area of the top of piston (E) is greater A pressure switch is used in an installation than the area of the bottom (because of the which requires an adjustable, pressure-actu- area of the piston rod), the piston will move ated, electric switch to make or break an electric downward moving the switch in the OFF position, circuit at a predetermined pressure setting. The as it appears in figure 10-26. The switch will switch is actuated by a rise or drop in pressure. therefore remain in the OFF position until The switch converts this pressure signal into an system pressure is reduced.As the system electric signal by opening or closing the contacts pressure decreases, the large spring forces in the electric circuit. piston (D) down and allows the poppet valve to This electric circuit seat. Any further decrease in pressure will is ther, used to perform some function, such as allow the ball to move away from the opening opening and closing a valve, or stopping and in the hollow poppet.This action will connect starting the motor which drives a pump or the upper chamber to the return line through the compressor.Since fluid pressure is closely hollow poppet, thus reducing the pressure in the relatEd to other system variables, pressure chamber. This reduction in pressure allows pis- az:tit:hes can also be used to detect fluid level, ton (E) to move upward and snap the switch to the flow, and velocity. (In some instances, tempera- ON position. ture changes can be detected.) The piston type sensing element is best The pressure signal, which can be either suited for high pressures.The major dis- a rise or fall of fluid pressure, is converted advantage of this type element is that the piston is into switch actuation by the pressure sensing relatively insensitive to small pressure changes. 193

198 FLUID POWER

UPPER CHAMBER

PISTON E

PORT B TO PORT A TO PRESSURE PRESSURE LINE LINE PORT C TO RETURN LINE

INTERNAL PASSAGE

ADJUSTING SCREW

FP.191 Figure 10-26.Pressure switch.

The Bourdon tube sensing element is similar and operation to the diaphragm pressure gage in operation to the Bourdon tube pressure gage, described earlier in this chapter.It is highly previously described.Instead of moving a accurate where the diameter of the element is pointer, as in the pressure gage, the movable from 2 to 4 inches.The diaphragm sensing end of the tube turns the switch ON and OFF. element offers fast response to fluid pressure The Bourdon tube sensing element is capable changes. of operating over a wide range of pressures. It is an accurate pressure switch and is capable of operating at pressures as high as 12,000 psi. HYDRAULIC FUSES Its major disadvantages are cost and its in- ability to withstand pressure surges. A hydraulic fuseisa protective device The diaphragm sensing element is ideally designed to assure thatall of the fluid in a suited for low pressures (up to 400 psi). The system does not leak outin case some section diaphragm sensing element is similar in design of the system develops aleak. When hydraulic 194

. 1Q9. Chapter 10MEASUREMENT AND CONTROL OF PRESSURE

fuses are used in a hydraulic system, they are Ifmore fluid attempts to flow through the installed at various appropriate locations in valve than is normally required, the piston the system so that the system is divided into will move to the right until it reaches sleeve sections. When a leak occurs in a section, the (5).Then, the -piston blocks the holes drilled hydraulic fuse limits the fluid loss to the amount through from theouter passages and thus

; contained in the small section of the system stops the flow of fluid.In this position, the that is isolated.Fuses are commonly used in valve is fused, as shown in figure 10-29 (B). aircraft hydraulic systems.If a leak develops in a subsystem while the aircraft is flying, or There is a continual flow of fluid through a hydraulic line is shot away during combat, the fuse until the piston moves far enough a fuse prevents excessive loss of fluid, thus to bloci the holes.Thus, the quantity of fluid permitting operation of theremaining sub- allowed to flow through the valve before the system. valve fuses is dependent upon the time it As the name implies, the hydraulic fuse is takes the pistonto move to the right A very similar to the electric fuse. When too large orifice causes the piston to move taster, much current flows,the electric fuse blows allowing less fluid to flow through the outer and opensthe circuit.The hydraulic fuse passages ofthe valve before it fuses. A stops the flow when too much fluid has passed small orifice causes the piston to move slower, through it. The fuse actually measures the thereby allowing a greater volume of fluid quantity of fluid that flows through it, and is to flow before the valve fuses. sometimes referred to as a quantity measuring fuse. The size of the orifice used is dependent upon the quantity of fluid needed to move the Figure 10-27 shows a schematic of a hydraulic actuating unit toits extreme travel.For fuse in the static position. This is the position example, an actuating cylinder that requires the valve assumes whenever the subsystem 40 cubic inches of fluid to extend fully, requires it protects is not in operation, under normal a fuse of slightly larger capacity than that conditions,and there is no flow in either of the actuating cylinder to insure, under normal direction.The valve (fig. 10-27) can function operating conditions, that the valve will not as a fuse only when the fluid flows from left fuse before the piston of the actuating cylinder to right.It cannot measure the quantity of has reached its full limit of travel. Since fluid if the flow is hi the opposite direction. the orifice cannot be removed from a fuse, For this reason, the hydraulic fuse must be con- fuses are available with different sized orifices. nected to the pressure line leading to the di- Each fuse has the flow capacity clearly marked rectional control valve of the subsystem, or a on its body. fuse must be located in each alternating line. Figure 10-28 shows these two methods of The check valve (4) is incorporated in the installation fuse to allow for reverse flow, should the The operation of a hydraulic fuse is illus- fuse be connected in an alternating line. Figure trated in figure 10-29.View (A) depicts the 10-29 (C)shows the reverse flow position fuse in the flow position.The fluid enters with the check valve open. The sleeve (5) the entrance port (1) where it divides and flows and the piston (3) are forced to the left by along the side passages and on to the center spring tension and the forceof the fluid. of the valve through the drilled holes.Here, The flow of fluid overcomes spring tension, the pressure overcomes spring tension and which opens the check valve (4) and allows free forces the sleeve (5) to the right. This allows reverse flow. the fluid to flow out of the other drilled holes, During normal operation the valve nearly around the outside passages, and out the exit fuses each time the fullactuator capacity Port (6). flows through it. When the actuator completes As the fluid flows through the valve, some the full travel, the flow stops, pressure equal- of the fluid which enters the entrance port (1) izes in all parts sf thefuse, and the large spring flows through the orifice (2). This fluid slowly forces the sleeve and piston to the static position. forces thepiston(3) to the right because, (See :fig. 10-27.)The fuse also resets to the due to fluid flow, there is less pressure on static position when the fluid pressure is relieved the right side of the piston than an the left in the line containing the fuse. 195 230 FLUID POWER

FP.192 Figure 10-27.Hydraulic fuse in static position.

HYDRAULIC FUSE

FP.193 Figure 10-28.Hydraulic fuse installations.

If an alternating line should burst, or a further fluid loss from the system. Although seal In an actuating unit ruptures, the fuse the hydraulic fuse allows some of the fluid allows some of its normal flow capacity to to escape from the system, enoughfluid remainc escape. At thattime it fuses, preventing for normal operation of the other subsystems. "

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e - I CHAPTER 11

DIRECTIONAL CONTROL VALVES

Directional control valves are designed for are the type of control, the number of ports in the specific purpose of directing the flow of the valve housing, Dr the specific function of fluid,at the desired time, to the point in a the valve. One common method of classifi- fluid power system where the fluid is applied cation is by the type of valving element used to accomplish work.It may be desired, for in the construction of the valve.The ball, example, to perform a work operation by driv- cone or sleeve, poppet, rotary spool, and sliding a piston or ram back and forth in its ing spool are the most common types of valving cylinder.A directional control valve, which elements.The ball and cone are commonly functions alternately to admit fluid to and from used as the valving element in check valves each end of the cylinder, is used to make this (discussed later).The basic operating princi- operation possible.Various other teems are ples of the poppet,rotary opool, and sliding used to identify directional control valves. For spool type valving elements are described in example, in the fluid power systems of naval the following paragraphs. aircraft, these valves are commonlyreferred to as selector valves.The terms transfer POPPET and control valve are used by some manufacturers and users of these valves.In this It should be noted that the use of the poppet manual, the term directional control valve is as a valving element is not limited to direc- used in most instances to identify these valves. tional cortrol valves.It is also used in some Directional control valves for hydraulic and of the flow control valves and pressure control pneumatic systems are similar in design and valves discussed in the preceding chapters. operation.One major differencethe return port of a hydraulic valve is ported through a Figure 11-1 illustrates the operation of a return line to the reservoir, while the similar simple poppet valve.The valve consists pri- port of a pneumatic valve is usually ported to marily of a movable poppet which closes the atmosphere rad, therefore,is commonly against a valve seat.In the closed position, referred to as an exhaust port.Any other fluid pressure on the inlet side tends to hold differences are pointed out in the discussion the valve tightly closed. A small amount of of the different type valves. movement from a force applied to the top of the poppet stem opens the poppet and allows Directional control valves may be operated fluid to flow through the valve. by differences of pressure acting on opposite sides of the valving element, or they may be The poppet, usually made of steel, fits into positioned manually, mechanically, or electri- the center bore of the seat.The seating sur- cally.Often two or more methods of operating faces of the poppet and the seatee lapped or the same valve will be utilized in different closely machined, so that the cc:.cer bore will phases of its acton. be sealed when the poppet is seated. The action of the poppet is similar to the valves of an automobile engine. An 0-ring seal is CLASSIFICATION usually installed on the stem of the poppet to prevent leakage past this portion of the poppet. Directional control valves may be classified In most valves the poppet is held in the seated in several ways. Some of the different methods position by a spring.The number of poppets 198 Chapter 11DIRECTIONAL CONTROL VALVES on, owe.. within a stationary sleeve.(See fig. 11-2.) As the core is rotated (generally by a hand lever or a knob) within the stationary sleeve, the passages or recesses connect or block the ports in the sleeve.The ports in the sleeve areconnectedtothe appropriate pressure, Working, and return lines of the fluid pow :r OUTLET system.

SLIDING SPOOL The slidingspoolis probably the most common type of valving element used in direc- tionalcontrolvalves. The operation of a simple sliding spool directional control valve is illustrated in figure 11-3. Th* valve is so named because the shape of the valving element CLOSED resembles_ thatof a spool and because the valving element slides back and forth to block and uncover ports in the housing, Some manufacturers refer to this elemem as a piston type.The ha sr piston (lands) areas are equal. Thus fluid under pressure which enters the valve from the inlet ports acts equally on bpth inner piston areas regardless of the position of the spool.Sealing is usually accomplished by a very closely machined fit between the spooland thevalve body or sleeve.For valves with more ports, the spool ir designed with more pistons or lands on a common sneft,

Like .411 -.7?,asse:_of directional control valves, various methods are utilized for positioning the sliding spool valve.Some of the

OPEN FIXED SLEEVE FP.195 Figure 11-1.Operation of a simple poppet valve in a particular valve depends upon the design and purpose of the valve. There are various methods used for shifting the poppets from one position to another. Some of the most common methods are described later under the different type valves.

ROTARY SPOOL PASSAGES The rotary spool type directiong control FP.196 valve has a round core with one or more pas- Figure 11-2.Parts of a rotary spool sarSti or recesses in it.The core is mounted directional control valve. FLUID POWER

valving elements.The poppet, piston, spool, OUTLET PORT BLOCKED or disc are also used as valving elements RY PISTON in some types of check valves.The force of the fluid in motion opens a check valve, SPOOL SHIFTED while it is closed by fluid attempting to flow TO LEFT in the opposite direction aided by the action of a spring or by gravity. ICLOSED TO FLOW Figure 11-4 shows a swing type check valve (flapper valve).In the open position the flow of fluid forces the hinged disc up and allows PORT OPEN free flow through the valve. Flow in the opposite direction with the aid of gravity, forces SPOOL the hinged disc to close the passage and blocks SHIFTED the flow.This type valve is sometimes de- TO RIGHT min signed with a spring to assist closing the valve. OPEN TO FLOW Figure 11-5 shows a vertical check valve in which the closing of the valve is influenced FP.197 considerably by the force of gravity. Figure 11-3.Operation of a sliding spool Examples of spring-loaded check valves are directional control valve illustrated in figure 11-6.Figure 11-6 (A) shows the cone or sleeve type. As fluid pres- most common methods are described under sure is applied in the direction of the arrow, thedifferent types of valves later in this the sleeve (2)is forced off the check valve seat(3). This allows fluid to flow freely chapter. through thedrilled openings of the sleeve. Figure 11-6 (B) shown a ball type check valve. As the pressure is applied, the ball is forced CHECK VALVE off its seat, thus allowing fluid to flow freely Some authorities classify check valves as through the valve. flow control valves. However, since the check The spring is installed in the valve to hold valve permits flow in one direction and pre- the cone or ball on its seat whenever the fluid vents flow in the other direction, most authori- is not flowing.The spring (1) is relatively ties classify it as a one-way directional control weak, requiring approximately 8 psi of fluid valve. pressure to open the valve.ViLon the fluid Regardless oftheir classification, check attempts to flow in the opposite direction, valves are probably the most widely used the spring helps to force the ball or cone on valves in fluid power systems.The check its seat.Since the opening and closing of this valve may be installed independently in a line type valve is not influenced by gravity,its to allow flow in one direction only.This is location in a system is not limited to the ver- indicated in the simple system described and ticalor near vertical position. For this illustrated in chapter 4.Check valves are refison, this type check valve is commonly used also incorporated as an integral part of some in fluid power systems of mobile equipment, other valve, such as the sequence valve, such as aircraft and missile systems. counterbalance valve, and pressure reducing valve described in chapter 10. A modification ofthe check valvethe orifice check valve STARTING VALVES (described in chapter 9)allows free flow in one direction and a limited or restricted flow Starting valves are commonly used in pneu- in the opposite direction. matic systems of missiles. The starting valve provides a positive block in the discharge Check valves are available in various de- (output) side of the air supply (flask).Many signs.As stated previously, the ball and the of these valves are designed to be opened by cone or sleeve are commonly used as the retracting a pin, either manually or electrically. 200 2l5 Chapter 11DIRECTIONAL CONTROL VALVES

CLOSED OPEN

FP.198 Figure 11-4.Swing type check valve.

FP.199 Figure 11-5.Vertical check valve.

Retraction of the pin permits gas to pass SHUTTLE VALVE from the flask, thus starting the operation of the pneumatic system. In certain types of fluid power systems, the supply of fluid to a subsystem must be from The operation of a starting valve is illus- more than one point of origin in order to meet trated in figure 11-7. When the pin is released, system requirements.In some systems an inlet pressure acting on the piston shoulder emergency system is provided as a source of forces the piston to move to the left.This pressure in the event of normal system failure. allows fluid to flow out of the outlet port For example, in aircraft hydraulic systems the into the system. The function of the spring emergency source of pressure may be com- is merely to absorb the shock of the rapid pressed air or nitrogen. The emergency piston movement. system will usually actuate only those units 201 FLUID POWER

under operating pressure to reach the actuating unit and still not enter the other system, a shuttle valve is installed in the working line to the actuating unit.The shuttle valve is small and simple,but a very important component. The main purpose then of a shuttle valve is to isolate the normal system from an alternate or emergency system. limmm=31111riliv(A1 A cutaway view of a typical shuttle valve isillustratedin figure 11-8. The housing contains three portsnormal system inlet port, alternate or emergency system inlet port, and the outlet port.A shuttle valve used to operate more than one actuating unit may contain additional unit outlet ports. Enclosed in the housing is a sliding part called the shuttle.Its purpose is to seal off either one or the other inlet ports.There IBI is a shuttle seat at each inlet port.During FP.200 operation, the shuttle is held against one of the 1. Spring.2. Cone or sleeve.3. Valve seat. seats, sealing off that respective port. These parts are held in the housing by end caps. Figure 11-6.Spring-loaded check valves. External leakage is prevented by an 0-ring gasket at each end cap. essential in getting the aircraft safely on the The shuttle may be one of four typesa ground and stopping it.This includes such sliding plunger, a spring-loaded piston, a subsystems as the extension of the landing spring-loaded ball, or a spring-loaded poppet. gear, the lowering of flaps, and the actuation In shuttle valves employing a shuttle spring, of the wheel brake system. When using the the shuttle is normally held against the alter- emergency system, the pressurized fluid must nate system inlet port by the spring. be directedtothe unit concerned and yet, When a shuttle valveisin the normal fluid from the emergency system must not operation position, fluid has a free flow from enterthe normal system. To allow fluid the normal system inlet port, through the valve,

OUTLET TO REDUCER

FP.201 Figure 11-7.Starting valve. 202 Chapter 11DIRECTIONAL CONTROL VALVES

FP.202 Figure 11-8.Shuttle valve.

and out through the outlet port to the actuating indicated by the arrows against the pistons unit. The shuttle is seated against the alternate of the spool, the same pressure acts on equal system inlet port, and held there by normal areas on their inside surfaces.In the closed system pressure and by theshuttle valve position, one of the pistons of the spool simply spring,if incorporated.The shuttle remains blockstheinlet port,thus preventing flow inthisposition untilthealternate system through the valve. is operated.This action directs fluid under Although the use of two-way directional pressure from thealternate system to the control valves of this type is limited in fluid shuttle valve mid forces the shuttle from the power systems, a number of features common alternate systeminlet port tothe normal to most sliding spool type valves can be noted system inlet port. Fluid from the alternate in figure 11-9.The small ports at either end system then has a free flow to the unit outlet of the valve housing provide a means whereby port, but is prevented from entering the normal any fluid that leaks past the pistons of the system by the shuttle, thus sealing off the spool will flow to the reservoir. This prevents normal system port. pressure from building up against the ends of the pistons, which would hinder the movement of the spool. When spool valves become TWO-WAY VALVES worn, they may lose balance because of greater leakage on one side of the spool than on the The term two-way indicates that the valve other. In that event, the spool would tend to contains andcontrolstwo functional flow stick when it is moved back and forth. Small portsinlet and outlet.The function of two- grooves are therefore machined around the way directional control valves is very similar sliding surface of the piston; and, in the case to the functions of the ON and OFF valves of hydraulic valves, leaking liquid will encircle described in chapter 9.A two-way, sliding the pistons and keep the contacting surfaces spool directional control valve is shown in the lubricated and centered. open and closed positions in figure 11-9.As the spool is moved back and forth, it either allows fluid to flow through the valve or pre- THREE-WAY VALVES vents flow.In the open position, the fluid enters the inlet port, flows around the shaft Three-way valves contain three portsa of thespool, and through the outlet port. pressure port, a cylinder port, and a return The spool cannot move back and forth by or exhaust port.The three-way directional differences of forces set up viitbin the cylinder, control valve is designed to operate an actuating since the forcesthere are balanced. As unit in one direction;it permits either the 203 208 FLUID POWER

OUT 4" p°,

, OPEN 4 RESERVOIR IN RESERVOIR 3

OUT

4 CLOSED 4 RESERVOIR IN RESERVOIR (A) F P.203 Figure 11 -9. Two -way, sliding spool directional control valve. 0 0 load on the actuating unit or a spring to return the unit to its original position. Two I different designs of poppet type, three-way directional control valves are discussed in the following paragraphs.

V CAM-OPERATED Figure 11-10 illustrates the operation of a cam-operated, three-way, poppet type directional control valve.Figure 11-10 (A) shows -1.111c t Irrit fluid under pressure forcing the piston outward against a load. The upper poppet (2) is unseated by the inside cam (5), permitting fluid to flow from line (3) into the cylinder to actuate the piston.The lower poppet (1) in seated, sealing F P.204 off the flow into the return line (4).As the 1. Lower (return or exhaust) poppet. force of the pressurized fluid extends the piston 2. Upper (pressure) poppet. rod,italso compresses the spring in the 3. Pressure line. cylinder. 4. Return or exhaustport. 5. Inside cam. Figure 11-10 (B)shows the valve with the O. Outside cam. control handle turned to the opposite position. In this position, the upper poppet (2) is seated, Figure 11- 10. Three -way, poppet type blocking the flow of fluid from the pressure directional control valve (cam-operated). 204 .209. Chapter 11DIRECTIONAL CONTROL VALVES line(3). The lower poppet (1) is unseated through the holes in the lower diaphragm to by the outside cam (6).This releases the fill the bottom chamber.This pressure holds pressure in the cylinder and allows the spring the lower poppet tightly against its seat and to expand, which forces the piston rod to re- blocks flow from the inlet port through the tract. The fluid from the cylinder flows through valve.At the same time, due to the common the control valve and out the return port (4). stem, the upper poppet is forced off its seat. In hydraulic systems, the return port is con- Fluid from the actuating unit flows through the nected by a line to the reservoir. In pneumatic open passage, around the stem, and through the systems, the return port is usually open to exhaust port to the atmosphere. the atmosphere. When thepilot chamber is pressurized, PILOT-OPERATED the force acting against the diaphragm forces the poppet down.The upper poppet closes against its seat, blocking the flow of fluid from A pilot-operated, poppet type, three-way the cylinder to the exhaust port. The lower pop- directional control valve is illustrated in figure pet opens, and passage from the supply inlet port 11-11. Valves of this design are often used in pneumatic systems.This is a normally to the cylinder port is open so that fluid can closed valve and is forced to the open position flow to the actuating unit. by fluid pressure entering the pilot chamber. The valve contains two poppets connected to The normally open valve is similar in design each other by a common stem.The poppets to the normally closed valve. (See fig. 11-12.) are integral with diaphragms which hold them When no pressure is applied to the pilot chamber, in a centered position. the upper poppet is forced off its seat and the lower poppet is closed.Fluid is free to The movement of the poppet is controlled flow from the inlet port through the cylinder by the presenCe or absence of fluid pressure. in port to the actuating unit. When pilot pressure thepilot port and the chamber above the is applied, the poppets are forced downward, upper diaphragm.When the pilot chamber is closing the upper poppet and opening the lower not pressurized, the lower poppet is seated poppet.Fluid can now flow from the cylinder against the lower valve seat.Fluid can flow through the valve and out the exhaust port to from the supply line through the inlet port and the atmosphere.

PILOT PRESSURIZED

EXHAUST EXNAUST OPEN CLOSED

SUPPLY CYLINDER SUPPLY

CYLINDER CLOSED OPEN TO CYLINDER

CLOSED

FP.205 Figure 11-11.Three-way, poppet type, normally closed directional control valve (pilot-operated). 205 'FLUID POWER:

PILOT PRESSURIZED

SUPPLY OPEN TO CYLINDER SUPPLY CYLINDER CYLINDER CLOSED

EXHAUST EXHAUST CLOSED OPEN

OPEN CLOSED FP.206 Figure 11-12.Three-way, poppet type, normally open directional control valve (pilot-operated).

FOUR-WAY VALVES POPPET TYPE Most actuating devices require system pressure for operation in either direction.The Figure 11-13 illustrates a typical four-way four-way directional control valve, which con- poppet type direcapnal control valve.This is tains four ports,is utilized to control the a manually operated Salve and consists of a operation of such devicer. The four-way valve group of conventional liring-loaded poppets. is also used in some sy stems to control the The poppets are enclolsedn a common housing operation of other valves.With the exception and are interconnected is so as to direct of the check valve, the fair -way valve is the the flow of fluidin the desired direction. most widely used directional control valve in The poppets are actuated by cams on a cam- fluid power systems. shaft, as shown in figure 11-14. The camshaft may be rotated by an attached control lever The typical four-way directional control (handle) to any one of three positions.Thus, valve has four portsa pressure port, a return the valve has three positionsneutral and two or exhaust port, and two cylinder (or working) workingpositions.The poppets are arranged ports. (Although the term cylinder port is sr) that rotation of the camshaft will open the used here to identify these ports, the four-way proper combinations of poppets to direct the valve may also be used to control other types flow of fluid through the desired working line of actuating devices.For this reason, these to an actuating unit.At the same time, fluid ports are also referred to as working polka) will be (II ;voted from the actuating unit through The pressure port is connected to the main the opposite working line, through the valve, and system pressure line and, in hydraulic systems, back to the reservoir (hydraulic) or exhausted to the return port is connected to the reservoir. the atmosphere (pneumatic). In pneumatic systems, the return port is usually vented to the atmosphere and, therefore, is re- This type valve is provided with a stop for ferred to as the exhaust port. The two cylinder the camshaft.This is an integral part of the ports are connected by lines to the actuating shaft and strikes against a stop pin in the body units.Some of the different types of four-way to prevent overrunning. directional control valves are described and The poppets are held on their seats by illustrated in the following paragraphs. springs, and the camshaft is used to unseat 206 -211 Chapter 11DIRECTIONAL CONTROL VALVES

shaft is placed in the neutral position the cam- ming lobes will not contact any of the poppets. When the shaft is rotated, either clockwise or counterclockwise from neutral, the cam lobes unseat the desired poppets and allow a flow of fluid through the valve.There are four poppets in this type valve. Two ot the poppets control the flow of pressurized fluid through the valve, and the other two control the return flow.One cam lobe operates the two pressure poppets, and the other lobe operates the two return/exhaust poppets. To stop rotation of the camshaft at the exact position, a stop pin is secured to the body and extends through a cutout section of the camshaft flange.This stop pin prevents over-travel by insuring that the camshaft stops rotating at the point where the cam lobes have moved the poppets the greatest distance from their seats and where any further rotation would allow the poppets to start returning to their seats. As stated previously, this directional control valve has three positionsneutral and two working positions. In the neutral position, the camshaft lobes are not contacting any of the poppets. This assures that the poppet springs will bold FP.207 all four poppets firmly seated. With all poppets Figure 11-13.Poppet type, four-way seated, there Is no fluid flow through the valve. directional control valve. This also blocks the two cylinder ports; so when the valve is in neutral, the fluid in the actuating unit is trapped. To allow for thermal them and allow a flow of fluid through the valve. expansion buildup, thermal relief valves must The camshaft is controlled by the movement be installed in both working lines. of the handle.The valve may be operated by NOTE: In another version of this type valve, manually moving the handle, or, in some cases the cam lobes are designed in such a =timer this handle may be connected by mechanical that the two return/edraust poppets are open linkage to a control handle which is located when the valve is in the neutral position. This in a convenient place for the operator some dis- compensates for thermal expansion, because tance from the valve.In either case, an in- struction plate is usually secured to an area both working lines are open to return/exhaust near the controlhandle, to indicate to the when the valve is in the neutral position. operator the differed positions of the valve. By moving the control handle either direction from neutral, the camshaft is rotated. The amrshaft has a raised flange an the This rotates the lobes, which unseat one pres- control handle end to prevent it from moving sure poppet and one re!turn/ednest poppet. through the housing.A not screws on the The valve is now in the working position. Fildd opposite end to secure it to the housing. There 'per entering the pressure port, are three 0-ring seals an the camshaft. They flows through the vertical fluid passages 'abaft are spaced at intervals along the lengib at the pressure poppet seats. Since only onepressure abaft to prevent external leakage around the poppet is unseated by the cam lobe, the fluid ends of the shift and intermit leakage from one flows past the open poppet to the inside ct of the valve clambers to another. The carasheft the poppet seat. From there it flows through has two lobes, or raised portions. 'the contour, the diagonal passages, then out ore cylinder or shape, of these lobes is such that when the port and to the actuating nit. 267

Chapter 11DIRECTIONAL CONTROL VALVES

C I

!ELECTOR Milt /11111041

FP.209 Figure 11-15.Worting view of poppet type four-tray directional control valve. rotary spool valve. Views (A) and (C) show in figure 17. The valve body (8) contains the valve in position to deliver fluid to another four field 11por pressure, return/easiest and while view stews the valve in the two cylinder -perports. A large bore has been valve, (B) of neutral position, with all passages through drilled lengthwise through the body the Ole valve blotted. valve, and all foor fluid ports connect into the main bore at its length. valves can be man- Intervals alum Rotary spool operated There is also a drilled passageway in the body ually, electrically, or by fluid pressure. that runs along the main bore. This passageway is used to connect one of the cylinder &MING SPOOL. VALVE ports to the returnieshanst port Tbe sliding spool four-way directional coo- trol valve is similar in operation to the two-way valve previously described in this chapter. ft is simple in principle at operation and is probably Se most datable and tronble-free of all four-waydrectiosal metro& maven in current use., As previously mentioned Sts type valve M sometimes referred to as a piston type. Although Se first valve described in the following paragraph" Is a isnonsay opersded type, Se snare principle is used In away dir ectional control valves. remotely controlled wiw "tile MP mpg PUMP An amleratmallug at Se operation of this valve will therefore aid is walenetand Se more cow- IA) (el tq phcaled soleaoid-opersted valve dimmed later. PPM° of A entanoty view of a typical four-mty slid- Figure 11-113.--Operatton rotary spool, ing spool directional costroi valve is Rinstrated four-way directional control valve.

214

Chapter 11DIRECTIONAL CONTROL VALVES spring legs and allow them to snap back into (A), the valve is in the neutral position, with the the next detent, which would be another position. detent spring clamped in the center detent of Figure 11-18 illustrates the operation of a the sliding spool.The center land is lined up manually operated sliding spool valve. In view with the pressure port, preventing fluid from

CI C2 NEUTRAL POSITION VIEW CO

CI C2 lIONNING POSITION SLIDE RETRACTED VIEW MI

CI Ct NORM POSITION SLOE EXTENDED !NM ICI FP.212 Figure 11-18.Operation of sliding spool, four-way directional control valve. 211 FLUID POWER flowing into the valve through this port.The valves in the system. (Closed-center andopen. return/exhaust post is also blocked, preventing center systems are discussed in chapter 4. flow through that port. With both pressure and return ports blocked, fluid in the actuating lines Figure 11-19 illustrates tioperation of a is trapped.For this reason, a thermal relief representative open-center, siding spool di- valve is usually installed in each actuating line rectional control valve.When this type valve when this type valve is used. In some instances, is in the neutral position, as shown in view (A), the relief valves are incorporated into the dir- fluid flows into the valve through the pressure ectional control valve. port (P), through the hollow spool, and to return. From here it flows to other valves inthe system Figure 11-18 (B) shows the valve in a working and back to the reservoir. position with the end of the sliding spool retracted.The detent spring is in the outboard detent, locking the sliding spool in this position. The lands have shifted inside the sleeve, and the .ports are opened. Fluid under pressure enters the sleeve, passes through it by way of the drilled holes, ^rd leaves through cylinder port (C2). Return fluid, flowing from the actuator, enters port (C1), flows through the sleeve, and is directed out the return port back to the reservoir or exhaust.2..d to the atmosphere. Fluid cannot flow past the spool lands because of the lapped surfaces. Figure 11-18 (C) shows the valve in the opposite working positionwith the sliding spool extended.The detent spring is in the inboard detent.The center land of the sliding spool is nnw on the other side of the pressure port, and the fluid under pressure is directed through the sleeve and out port (C1).Return fluid flowing in the other cylinder port is directed to the drilled passageway in the body. It flows along this passageway to the other end of the sleeve where it is directed out of the return/exhaust port. The directional control valves previously discussed are for use in closed-center fluid power systems.In these valves the spool is solid end all passages through the valve are blocked when the spool is centered (neutral position) in its FP.213 housing.Directional control valves for open- Figure 11-19.Open center, sliding spool, center systems are slightly different in design. directional control valve. These valves must allow a free flow of fluid through the valve, from the pressure port In view (B) the spool is moved to the right through the return port, when the spool is in of the neutral position.In this position, one the neutral position. In some valves of this de- working line (C1) is open to pressure, and the sign; the pistons on the spool are slotted or other working line (C2) is open, through the channeled so that all passages are open to each hollow spool, to return. View(C) shows the flow other when the spool is in neutral position. In of fluid through the valve with the spool moved other open-center valves, passages to the actu- to the left of neutral. ating unit are blocked when the valve is in There are several other variations of the neutral, while fluid from the pump flows through sliding spool directional control valve.For passages in the spool and out the other side of example, some valves of this type have two re- the valve to the reservoir or to other control turn/exhaust ports.In the case of hydraulic 212 ..212_ Chapter 11DIRECTIONAL CONTROL VALVES systems, the two ports are connected to a com- sometimes referred to as a transfer valve. mon return line to the reservoir. In pneumatic This valve is illustrated in the neutral position systems, of course, the two ports are vented to with no flow through the valve.The valve is the atmosphere.In most cases, these valves operated by energizing either of the solenoids. are designed as special units. However, in some This may be accomplished by a manually oper- systems, several directional control valves are ated switch or, for example in missile systems, combined into one control unit. automatically by the response of the solenoids There are many different methods of control- to electric signals generated by the missile ling the sliding spool valve. Figure 11-20 shows computer network. a schematic view of a rotary directional control The object is to move the actuator, which is valve used as a pilot valve to control flow to the mechanically linked to the device to be operated. sliding spool valve.Fluid pressure can be di- Referrring to figure 11-21, if solenoid (1) is rected to either end of the sliding spool valve energized, it will cause the spool of the valve to whicl, positions the spool and directs flow to and move to the left.This will allow fluid under from the actuating unit. pressure to flow to the right-hand side of the The sliding spool valve, like other types of actuator forcing it to move to the left.If sol- directionalcontrol valves, may be operated enoid (2) is energized, the spool will move to the electrically. Thisis usually accomplished right, causing the actuator to move to the right in through the use of one or more solenoids. a smiliar manner. In either position, fluid can A solenoid may be defined as a hollow tubular flow from the opposite side of the actuator shaped electric coil, made up of many turns of through the valve and out the corresponding refine insulated wire, and possessing the same turn/exhaust port. properties as an electromagnet.The hollow Solenoids are often used to control a small coil imparts linear motion to a movable iron pilot valve which directs flow to either end core (or plunger) placed within the hollow coil of a sliding spool valve.The fluid pressure of the solenoid. acting on the end of the sliding spool valve will position the spool for the desired operation. Solenoids are also used in many situations where the control handle must be located a great dis- ROTARY VALVE tance from the control valve.

SERVO VALVES

The four-way directional control valves discussed in the preceding section are positioned by the operator, either with a control handle, or in the case of solenoid-operated valves, with an electrical switch. This usually provides positive movement of the actuating unit; that is, full movement in either direction.If partial movement of the actuating unit is desired, the operator must control this movement by manually controlling the control handle or switch. This method of control satisfies the requirements of many fluid power systems; however, some systems require a more exact and automatic method of control. The servo valve is often used FP.214 to provide such control.. Figure 11-20.Sliding spool valve controlled by A servomechanism is a device in which the rotary valve. output quantity, such as the rotation of a motor or the distance of travel of a piston in a cylinder, Figure 11-21 shows a solenoid-operated di- is monitored and compared with a desired quan- rectional control valve.This type valve is tity. By way of a feedback system, the difference 213 21.8 FLUID POWER

PRESSURE RETURN IN RETURN 11rt.rcIL VALVE SPOOL

L SOLENOID 1 SOLENOID 2

1111.... ME

FP.215 Figure 11-21.Solenoid-operated sliding spool directional controlvalve. between the two quantities (the error) is If the right-hand solenoid is energized, the used to actuate the system and to generate magnetic reed will move to the right, blocking a rate of change of the output.The feedback off the flow of high-pressure fluid through the signal may be provided by fluid pressure, mech- right-hand nozzle. Pressure will buildup in the anical linkage, electrical signals, or a combina- right-hand pressure chamber. This willmove tion of the three.As applied to fluid power the valve to the left. In moving left, the center systems, the feedback signal positions a servo land will open the high-pressure inlet andper- valve which, in turn, corrects any errors in the mit fluid flow directly to the right-hand side of movement of the actuator. the actuator.At the same time, the left-hand One type of hydraulic servo valve is il- land of the spool will open the low-pressure lustrated in figure 11-22. The valve is controlled return line and permit flow through the return by the two solenoids, which receive the electric to the reservoir from the left-hand side of the feedback signals through a followup system from actuator.This process will cause actuator the actuating unit. With neither of the windings movement to the left.By energizing the left- energized (or a balanced current through both), hand solenoid, the magnetic reed willmove to the magnetic reed is centeredas shown in the left, and the entire process will be reversed, figure 11-22.In this condition, high-pressure the actuator then being moved to the right. fluid from the input line cannot pass to the actuator, since the center land of the spool valve Note that the servo valve is basically a blocks the input port.The pressurized fluid sliding spool valve. The servomechanism is just flows through the alternate routes, through the another method of control. This type valve has two restrictors (labeled fixed orifices),passes many applications in fluid power systems. For through the two nozzles, and returns to theres- example, servo valves are used in the guidance ervoir without causing any movement of the systems of missiles and control system of air- actuator. craft.

214 Chapter 11 DIRECTIONAL CONTROL VALVES

LOW PRESSURE H GH PRESSURE RETURN TO RESERVOIR A/INPUT

FIXED ORIFICE FIXED ORIFICE I I 4( --4111. PRESSURE II PRESSURE 1 I -CHAMBER CHAMBER TO To ACTUATOR ACTUATOR CENTERING CENTERING 1 SCREW SPRING\,sii\ tit - . 1111111111111111111111111111111 111111111111111111111111111111111

/HYDRAULIC RETURN TO RESERVOIR SPOOL VALVE NOZZLES

REED PERMANENT MAGNET 2 SOLENOID WINDINGS

FP.216 Figure 11-22.Servo valve. CHAPTER 12

ACTUATORS

One of the outstanding features of fluid power Leakage of fluid out of the ends of the cy- systems is that force, generated by the power linders and around the circumference of the pis- supply,controlled and directed by suitable ton or ram is controlled by suitable designed valving, and transported by lines, can be con- seals. verted with ease to almost any kind of mechanical motion desired at the very place it is needed. Actuating cylinders for pneumatic and hy- Linear (straight line) or rotary motion can be draulic systems are similar in design andoper- obtained by using a suitable actuating device. ation. Some of the variations of ram and piston type actuating cylinders are described in the fol- An actuator is a device which converts fluid lowing paragraphs. power into mechanical force and motion. Cylin- ders, motors, and turbines are the most common RAM TYPE CYLINDERS types of actuating devices used in fluid power systems. Although the terms ram and piston are often The first part of this chapter is devoted to used interchangeably, a ram type cylinderis the various types of actuating cylinders and usually considered one in which thecross-sec- their applications in fluid power systems. The tional area of the piston rod is more thanone- next part of the chapter covers the different half the cross-sectional area of the movable types of fluid motors used in fluid power systems. element.In most actuating cylinders of this The remainder of the chapter covers air turbines type, the rod and movable element have equal and turbine governors. areas.This type of movable element is frequently referred to as a plunger; therefore, this type actuator may also be referred toas a CYLINDERS plunger type. The ram type actuator is used primarily for An actuating cylinder is a device which con- push functions rather than pull.Some appli- verts fluid power to linear or straight line force cations require simply a flat surfaceon the ex- and motion.Since linear motion is a back and ternal part of the ram for pushingor lifting forth motion along a straight line, this type of the unit to be operated. Other applications actuator is sometimes referred to as a recip- require some mechanical means of attachment, rocating or linear motor: The cylinder consists such as a clevis or eyebolt. The design ofram of a ram or piston operating within a cylindrical type cylinders varies in many other respects bore. Actuating cylinders are normally installed to satisfy the requirements of different appli- in such a manner that the cylinder is anchored cations. Some of these various designs are to a stationary structure and theram or piston discussed in the following paragraphs. is attached to the mechanism to be operated. For this reason, the ram or piston is referred Single-Acting Ram to as the movable element of this type actuator. However, in some applications, the piston orram The single-acting ram applies force in only is anchored to the stationary structure, and the one direction.(See fig. 12-1.) Fluid directed cylinder is attached to the mechanism to beop- into the cylinder displaces theram and forces erated and becomes the movable part of the it outward.Since there is no provision for actuator. retracting the ram by the use of fluidpower, the 216 4. Chapter 12ACTUATORS retracting force can be gravity, or some mechan- extend the ram and apply force. To retract the ical means, such as a spring. This type actuating ram and reduce force, fluid is directed to the cylinder is often used in the hydraulic jack. opposite end of the cylinder. The hydraulic lift, described and illustrated in chapter 4, is equipped with a cylinder of this The elevators used to move aircraft to type. PACKED and from the flight deck and hangar deck on air- GLAND RAM craft carriers employ cylinders of this type. In this case the cylinder is installed horizontally GLAND NUT and operates the elevator through a series of cables and sheaves.

FLUID SUPPLY CYLINDER FOR RETRACTION - HOUSING PACKED GLAND PORT B

RAM LIP

PORT A

FLUID SUPPLY - FOR EXTENDING -m

CYLINDER HOUSING FP.218 Figure 12-2.Double-acting ram type actuating cylinder. FLUID SUPPLY A four-way directional control valve is normally utilized to control the double-acting ram. When the valve is positioned to extend the ram, pressurized fluid enters port (A), (fig. 12-2), IA acts on the bottom surface of the ram, and forces the ram outward.Fluid above the ram lip is 1- free to flow out of port (B), through the control FP.217 valve, and to return/exhaust. Figure 12-1.Single-acting ram type When the directional control valve is posi- actuating cylinder. tioned to retract the ram, pressurized fluid enters port (B) and acts on the top surface of In this type actuating cylinder, fluid pressure the ram lip, forcing the ram down. The fluid is used to force the ram outward and lift an from the bottom of the cylinder is free to flow object.When fluid pressure is released, the out of port (A) and through the directional con- weight of the object (gravity) forces the ram into trol valve to return/exhaust. the cylinder, which, in turn, forces the fluid Normally, the pressure of the fluid is the back to the reservoir. same for either stroke of the ram. However, note the difference of the areas upon which the Double-Acting Ram pressure acts.The pressure acts against the large surface area on the bottom of the ram A double-acting ram type cylinder is illus- during the extension stroke, at which time the trated in figure 12-2.In this cylinder, both ram applies force.Since the ram does not strokes of the ram are produced by pressurized apply force during the retraction stroke, pres- fluid. There are two fluid ports, one at or near sure acting on the small area on the top surface each end of the cylinder. Fluid under pressure of the ram lip provides the necessary force to is directed to the closed end of the cylinder to retract the ram. 217 ..222 FLUID POWER

Telescoping Rams stroke, a still lesser force is required. Rain (3) then moves outward to finish raising and Figure 12-3 shows a telescoping ram type dumping the load. actuating cylinder. A series of rams is nested in the telescoping assembly. With the exception Some telescoping ram type cylinders are of of the smallest ram, each ram is hollow and the single-acting type. Like the single-acting serves as the cylinder housing for the next ram discussed previously, the rams of this smaller ram. The ram assembly is contained type cylinder are retracted by gravity or me- in the main c ylind e r assembly which also chanical force. Some hydraulic jacks are provides the fluid ports. Although the assembly equipped with this type. Such jacks are used to requires a small space with all of the rams lift vehicles with low clearances to the required retracted, the telescoping action of the assembly height. provides a relatively long stroke when the rams Other types of telescoping cylinders, like are extended. the one illustrated in figure 12-3, are of the double-acting type. In this type, fluid pressure is utilized for both the extension and retraction strokes. A four-way directional control valve RAM I is commonly used to control the operation of RAM V the dotible-acting type. Note the small passages 2 in the walls of rams (1) and (2). They provide RAM a path for the fluid to flow to and from the PORT A chambers above the lips of rams (2) and (3). During the extension stroke, return fluid flows RAISE through these passages and out of the cylinder through port(B). It then flows through the directionalcontrol valve to return/exhaust. To retract the rams, fluid under pressure is directed into the cylinder through port (B) and acts against the top surface areas of all PORT B three ram lips. This forces the rams to the retracted position. The displaced fluid from the LOWER opposite side of the rams flows out of the FP. 219 cylinder through port (A), through the direc- Figure 12- 3. Telescoping ram type tional control valve to return/exhaust. actuating cylinder. Dual Rams An excellent example of the application of A dual ram assembly is illustrated in figure this type cylinder is in the dump truck. It is 12-4. This assembly consists of a single ram used to lift the forward end of the truck bed and dump the load. During.the lifting operation, the greatest force is required for the initial lifting of the load. As the load is lifted and begins to dump, the required force becomes SNIP'S RUDDER less and less until the load is completely dumped; LINDER CYLINDER FLUID SOPPY RAM FLUID SUPPLY In the raise position, pressurizedfluid enters the PIPE cylinder through port (A) and acts on the bottom PIPE surface of all three rams. (See fig. 12-3.) Ram (1) has the largest surface area, and therefore, provides the greater force for the initial load. As ram (1) reaches the end of its stroke and the required force decreases, ram (2) moves,- FP.220 providing the lesser force needed to continue Figure 12-4.Dual ram actuating, raising the load. When ram (2) completes its cylinder. 218

",,991 Chapter 12ACTUATORS with a cylinder at either end.Fluid can be threaded connection of the rod and mechanical directed to either cylinder, forcing the ram to connector provides for adjustment between the move in the opposite direction. The ram is rod and the unit to be actuated. After correct connected through mechanical linkage to the adjustment is obtained, the locknut is tightened unit to be operated. A four-way directional against the connector to prevent the connector control valve is commonly used to operate the from turning. The other end of the eyebolt or dual ram. When the control valve is positioned clevis is connected, either directly or through to direct fluid under pressure to one of the additional mechanical linkage, to the unit to be cylinders (for example, the left), the ram is actuated. forced to the right. This action displaces the In order to satisfy the many requirements fluid in the opposite cylinder. This displaced of fluid power systems, piston type cylinders fluid flows back through the directional control are available in various designs. Some of the valve to return/exhaust. most common designs are described in the fol- Dual ram actuating cylinders are used in the lowing paragraphs. steering systems of most ships. In some systems, one assembly is utilized to actuate Single-Acting the rudder in either direction; while in other systems, two assemblies are used for the same The single-acting piston type cylinder is purpose. (These steering systems are described similar in design and operation to the single- and illustrated in chapter 13.) acting ram type cylinder, discussed previously. The single-acting piston type cylinder utilizes fluid pressure toapply forcein only one PISTON TYPE CYLINDER direction. In some designs of this type, the An actuating cylinder in which the cross- force of gravity moves the piston in the opposite sectional area of the piston rod is less than direction. However, most cylinders of this type one-half the cross-sectional area of the mov- apply force in both directions. Fluid pressure able element is referred to as a piston type provides the force in one direction, and spring cylinder. This type cylinder is normally used tension provides theforceinthe opposite for applications which require both push and di r e c t i o n. In some single-acting cylinders, pull functions. Thus, the piston type serves compressed air or nitrogen is utilized instead many more requirements than the ram type and, ofa spring for movement in the direction therefore, is the most common type used in oppositethat achieved with fluid pressure. fluid power systems. Figure 12-5 i 1 lu strate s a single-acting, The housing consists of a cylindrical barrel spring-loaded piston type actuating cylinder. In which usually contains either external or inthis cylinder the spring is located on the rod ternal threads on both ends. End caps with side of the piston. In some spring-loaded cyl- mating threads are attached to the eads of the inders the spring is located on the blank side, barrel. These end caps usually contain the fluid and the fluid port is on the rod side of the ports. The end cap on the rod end contains a cylinder. hole for the piston rod to pass through. Suitable packing must be used between the hole and the piston rod to prevent external leakage of fluid and the entrance of dirt and other contaminants. FLUID PORT I- PISTON PISTON The oppositeend cap of most cylinders is ROO RETURN I/ provided with a fitting for securing the actuating SPRING c y 1 inde rto some structure. For obvious reasons, this end cap is referred to as the anchor end cap. VENT The piston rod may extend through either or SEALS both ends of the cylinder. The extended end of FP.221 the rod is normally threaded for the attachment Figure 12-5.Single-acting, spring- of some type of mechanical connector, such as loaded piston type actuating an eyebolt or a clevis, and alocknut. This cylinder.

219 FLUID POWER MINIIIINIM A three-way directional control valve is The arresting hook extends when fluid pres- normally used to control the operation of this sure is r e l e a s e d from the rod side of the type cylinder. To extend the piston rod, fluid cylinder, allowing the spring to expand. This under pressure is directed through the port spring action insures extension of the arresting and into the cylinder. (See fig. 12-5.) This pres- hook in case of hydraulic failure. In addition, sure acts on the surface area of the blank side the spring holds the arresting hook in theex- of the piston and forces the piston to the right. tended position, yet, through the compressibility This action, of course, extends the rod to the of the spring, acts as a shock absorber allowing right, through the end of the cylinder. This slight movement of the arresting hook when it moves the actuated unit in one direction. During strikes the flight deck during landings. This this action, the spring is compressed between prevents damageto the arresting hook and the rod side of the piston and the end of the other structural members of the aircraft. In cylinder.Within limits of the cylinder, the most late model carrier aircraft, this shock length of the stroke depends upon the desired absorbing device utilizes a c o mbin a tion of movement of the actuated unit. spring tension and compressed air/nitrogen. To retract the piston rod, the directional control valve is moved to the opposite working position, which releases the pressure in the Double-Acting cylinder.The spring tension forces the piston to the left, retracting the piston rod and moving Most piston type actuating cylinders are the actuated unit in the opposite direction. The double-acting, which means that fluid under fluid is free to flow from the cylinder through pressure can be applied to either side of the the port, back through the control valve to piston to provide movement and apply force in return/exhaust. the corresponding direction. The end of the cylinder opposite the fluid One design of the double-acting piston type port is vented to the atmosphere. This prevents actuating cylinder is illustrated in figure 12-6. air from being trapped in this area, Any trapped This cylinder contains one piston and piston air would compress during the extension stroke, rod assembly. The stroke of the piston and creating excess pressure on the rod side of the pisto& rod assembly in either direction ispro- piston. This would cause sluggish movement duced by fluid pressure. The two fluid ports, ofthe piston and could eventually cause a one near each end of the cylinder, alternate as complete lock, preventing the fluid pressure inlet and outlet, depending upon the direction of from moving the piston. flow from the directional control valve. Leakage between the cylinder wall and the piston is prevented by adequate se,i1a.The piston in figure 12-5 contains V-ring seals. (Note that the open end of the V's are placed FLUID PORTS toward the fluid pressure.) V-rings are usedin some cylinders; however, 0-rings are used in most piston type cylinders. There is a machined groove around the circumference of the piston, which serves as a seat for the seals. The spring-loaded cylinder is used inar- resting gear systems on some models of carrier 0 -RING SEAL aircraft. In this case, the cylinder is usually FP.222 designed in such a manner that spring force is Figure 12-6.Double-acting piston used to extend the piston rod and fluid pressure type actuating cylinder is used for retraction. To raise (retract) the arresting hook, flui d pressure is directed through the arresting hook control valve to the This is referred to as an unbalanced actu- rod aide of the cylinder. This force moves the ating cylinder; that is, there is a difference in piston, which, through the rod and mechanical the effective working areas on the two sidesof linkage, retracts the arresting hook. This same the piston. Referring to figure 12-6,assume force compresses the spring, which is on the that the cross-sectional area of the piston is 3 blank aide of the piston. square inches and the cross-sectional area of 220

2f2A5 Chapter 12ACTUATORS the rod is 1 square inch. In a 2,000-psi system, pressure acting against the blank side of the CONTROLVALVE piston creates a force of 6,000 pounds (2,000 x 3). PORT/ PONT A When the pressure is applied to the rod side of the piston, the 2,000 psi acts on 2 square inches (the cross-sectional area of the piston less the cross-sectional area of the rod) and creates a force of 4,000 pounds (2,000 x 2). For this reason, this type cylinder is normally installed FP.223 in such a manner that the blank side of the Figure 12 -7. Three -port, double- piston carries the greater load, that is, the acting actuating cylinder. cylinder carries the greater load during the piston rod extension stroke. Fluid under pressure is directed through A four-way directional control valve is nor- port (A) by a four-way directional control valve mally used to control the operation of this type and moves the pistons outward, thus moving the cylinder. The valve can be positioned to direct mechanisms attached to the piston rods. The fluid under pressure to either end of the fluid on the rod side of each piston is forced cylinder and allow the displaced fluid to flow out of the cylinder through ports (B) and (C) from the opposite end of the cylinder through which are connected by a common line to the the control valve to return/exhaust. directional control valve. This displaced fluid then flows through the control valve to return/ The piston of the cylinder illustrated in exhaust. figure 12-6 is equipped with an 0-ring seal to prevent internal leakage of fluid from one side When fluid under pressure is directed into of the piston to the other. Suitable seals are the cylinder through ports (B) and (C), the two also used between the hole in the end cap and pistons move inward, and the mechanisms to the piston rod to prevent external leakage. In which the piston rode are attached are moved addition, some cylinders of this type have a accordingly. Fluid between the two pistons is felt wiper-ring attached to the inside of the end free to flow from the cylinder through port (A) cap and fitted around the piston rod to guard and through the control valve to return/exhaust. against the entrance of dirt and other foreign matter into the cylinder. The actuating cylinder illustrated in figure 12-8 isa double-acting balanced type. The In some localities the atmosphere contains piston rod extends through the piston and out a large amount of dust and abrasive particles. through both ends of the cylinder. One or both When these particles come in contact with the ends of the piston rod may be attached to a piston rod, they will scratch the smooth surface mechanism to be actuated. In either case, the of the rod; and as the rod moves in and out of cylinder provides equal areas on each side of thecylinder,the particles will damage the the piston so that the amount of fluid and force seals, causing external leakage of fluid. Some required to move the piston a certain distance particles may enter the cylinder and cause in one directionis exactly the same as the further damage. To prevent damage from these particles, cylinders used in such localities are often equipped with flexible, synthetic rubber, FLUID PORTS protective coverings around the exposed end of PISTON the piston rod. This covering is referred to SHAFT as, a boot. Ira Figure 12-7 illustrates a three-port, double- acting piston type actuating cylinder. This type GAINS SUL is used in applications where it is necessary to move two mechanisms at the same time. FP.224 This cylinder contains two pistons and piston Figure 12-8.Balanced, double-acting rod assemblies. piston type actuating cylinder.

221 FLUID POWER amount required to move it an equal distance in The flow of fluid to and from the two the opposite direction. The balanced double- chambers of the tandem actuating cylinder is acting cylinder is commonly used in servo- provided from two independent hydraulicsys- m e chant s m s.(See chapter 11 for further tems and is controlled by two sliding spool information concerning servo valves and directional control valves. In some applications mechanisms.) of this type, the control valves and the actuating Like the unbalanced, double-acting cylinders, cylinder are two separate units. In other appli- both the three-port and balanced cylinders re- cations, the valves and the actuator are directly quire suitable seals around the pistons and connected in one compact unit. Although the two suitable packing and wiper-rings in the end caps control valves are hydraulically independent, around the piston rod. These cylinders may they are interconnected mechanically. Insome also be equipped with protective boots around units, the pistons (lands) of the two sliding the exposed area of the piston rods. spools are machined on one common shaft. In other units, the two sliding spools are con- Tandem Cylinders nected through mechanical linkages witha synchronizing rod. In either case, the movement Some fluid power applications require two of the two sliding spools is synchronized, thus or more independent systems. For example, equalizing the flow of fluid to and from the two most models of naval aircraft have power- chambers of the actuating cylinder. operated flight control systems and current specifications require two independent hydraulic The movement of the sliding spools of the systems for their operation. In these systems, directional control valves is controlled, through each movable control surface is operated bya cables and/or other mechanical linkages, by hydraulic actuator incorporated in the control moving the corresponding control devices linkage. A tandem actuating cylinder is rudder pedals, yoke, etc.in the cockpit. The commonly used in this type system. piston rod of the actuator is connected through mechanical linkages to the correspondingcon- A tandem actuating cylinder is illustrated in trols u r f ac e. Therefore, movement of the figure 12-9. Tandem is defined as a group of controls in the cockpit positions the directional two or more arranged one behind the other. control valves, and hydraulic pressure moves The tandem actuating cylinder consists of two the control surfaces. or more cylinders arranged one behind the other but designed as a single unit. For The two working ports of one directional example, the cylinder illustrated in figure 12-9 control valve are connected to ports A and B of is actually two balanced, double-acting piston the actuator (fig. 12-9), and the working porta type actuating cylinders (fig.12-8) arranged of the other control valve are connected to one behind the other with two pistons connected ports C and D of the actuator. When fluidpres- to a common shaft. sure is applied through ports A and C, the piston moves to the right and the displaced fluid flows out ports B and D, to thecorre- sponding c on t r o 1 valves, and returns to the reservoirs of the respective systems. When fluid pressure is applied through ports B and D, the piston moves to the left and the displaced fluid flows out through ports/ A and Cto return. Since the two control valves operate independently of each other as faras hydraulic pressure is concerned, failure of either hy- draulic2'1/s4s:lei does not render the actuator inoperative. Failure of one system does reduce the output force by one half; however, this FP.225 force is sufficient to permit handling ofthe Figure 12- 9. Tandem actuating aircraft at certain airspeeds (always well above cylinder. that required for a safe landing).

222 27 Chapter 12ACTUATORS fi

Cushioned Cylinders overcomes spring tension and unseats the check valve. This allows an unrestricted flow into the In order to slow the action and prevent cylinder and forces the piston to move to the shock at the end of the piston stroke, some left with no cushioning effect. The fluid on the actuating cylinders areconstructed with a opposite side of the piston is free to flow out cushioning device to slow (cushion) the move- of the cylinder through port (A), through the ment of the piston during part of its stroke. control valve to return/exhaust. This cushion is usually a metering device built into the cylinder to restrict the flow at the LIMITED ROTATION CYLINDERS outlet port, thereby slowing down the movement of the piston. There are various designs used Rotary actuation of fluid powered mecha- to provide this cushioning effect. One such nismsis normally provided by fluid power design is shown in the actuating cylinder il- motors.However, certain actuating cylinders lustrated in figure 12-10. are designed to provide limited rotary actuation. The rotation is normally limited to approximately 180 degree s. Like all fluid power components, there are several differentdesigns of limited rotation cylinders. One design of this type cylinder is illustrated in The hydraulic windshield wiper system shown in figure 12-11.

FP.226 PRESSURE Figure 12-10.Cushioned actuating RETURN cylinder. ALTERNATE

In the cylinder illustrated in figure 12-10, the piston is cushioned near the end of the extension stroke. As the fluid under pressure flows into port (A), it acts against the blank side of the piston, forcing the piston and rod to the right. Fluid from the rod side of the piston is free to flow between the center hole of the cushioning element and the piston rod and out through port (B). Some fluid flows through the metering orifice. However, spring tension and fluid pressure cause the check valve to seat, blocking flow through this point. This action continues until the raised portion of the rod reaches the center hole of the cushioning element. The close tolerance between this portion of the rod and the center hole results in near stoppage of flow around the piston rod. The remainder of the fluid must flow through the metering orifice. This reduced volume of flow offers resistance to the moving piston, thus cushioning the end of the piston stroke. To actuate the cylinder in the opposite direc- FZ 227 tion, fluid under pressure is directed into the Figure 12-11.Application of limited' cylinder through port (B). This pressure rotation actuating cylinder. 223 FLUID POWER

This design is often referred to as the rack and little or no modifications. Therefore, a thorough pinion actuator. knowledge of the pumps described in chapter 8 The windshield wiper must move back and will be extremely helpful for understanding the forth in an arc of approximately 180 degrees. operation of fluid power motors. The rack and pinion actuator provides this lim- Motors serve many applications in fluid itedrotation.The wiper has a mechanical power systems.In hydraulic power drives, osculating device which changes the flow of pumps and motors are combined with suitable fluid to opposite ends of the rack and pinion lines and valves to form hydraulic transmis- actuator. The piston rod contains a piston on sions. The pump, commonly referred to as the either end and has teeth (serrations) machined A-leYi, is driven by some outside source, such along one side of the center section. These as ail electric motor. The pump delivers fluid serrations (the rack) mesh with a gear (the to the motor. The motor, referred to as the pinion) which has teeth around only a portion of B-end, is actuated by this flow, and through its circumference. The fluidflow to the actuator, mechanical linkage, conveys rotary motion and alternating from one end to the other, causes force to the work. This type power drive is the piston to move back and forth. Through the used to operate (train and elevate) many of the meshed serrations between the piawn rod and Navy's guns and rocket launchers. Hydraulic the gear, the reciprocating motion of the piston motors are commonly used to operate the wing rod results in a rotary motion of the shaft to flaps, radomes, and radar equipment in air- which the gear is attached. The windshield craft. Mr motors are used to drive pneumatic wiper is attached to the opposite end of the gear drills. Mr motors are also used in missiles to shaft. convert the potential energy of compressed gas into electrical power, or to drive the pump of APPLICATION AND MAINTENANCE the hydraulic system. These are only a few of the applications of fluid power motors. Only a few of the 'many applications of Fluid power motors are generally rated in actuating cylinders are discussed in the pre- terms of displacement and torque. Displacement ceding paragraphs. Figure 12-12illustrates refers to the amount of fluid necessary to force additional types of force and motion applications the motor. through one complete cycle. Dis- obtainable. To meet the various requirements placement is expressed in cubic inches or of fluid power systems, actuating cylinders are gallons per minute. The torque (twisting or available in many different shapes and sizes. rotating force)is expressed in inch-pounds In addition to its versatility, the cylinder at a certain pressure. type actuator is probably the most trouble-free component of fluid power systems. However, it The output speed and the torque of the is very important that the cylinder, mechanical motor depend upon the power input in terms of linkage, and the actuating unit are correctly pressure and rate of flow. The output speed is aligned. Any misalignment will cause excessive proportionalto the input volume. The ratio wear of the piston, piston rod, and seals. Also, between these two factorsoutput speed and proper adjustment between the piston rod and input volumedepends on the displacement of the actuating unit must be maintained. The ex- the motor. The pressure difference between the posed ends of the piston rods must be cleaned i n1 e t and o utl e t ports and the mechanical as required to guard against the entrance of efficiency of the motor determine the output foreign matter into the cylinder. torque. For example, assume that a hydraulic motor MOTORS is rated at a fixed displacement of 7.5 cubic inches per revolution and is capable of speeds A fluid power motor is a device which .upto a maximum of 1,600 revolutions per converts fluid power energy to rotary motion minute (rpm). Since the motor requires 7.5 and force. Basically, the function of a motor is cubic inches of fluid for one revolution, the just the opposite as that of a pump. However, speed depends upon the volume of flow to the the design and operation of fluid power motors inlet port. In addition, assume that this motor are very similar to pumps. In fact, some hy- is rated at 1,200 inch-pounds of torque at 1,000 draulic pumps can be used as motors with psi (the pressure difference between the inlet

224 Chapter 12ACTUATORS

eillI . NI J1111.1114 SI 101 ill e -18 A Ael

CYLINDER CAN BE USED VITA A THIRD CYLINDER CAN BE USED WITH A DOUBLE SPROCKET WHEEL MAKES THE CLASS LEVER. TRAMMEL PLATE. ROTATION MORE NEARLY OMINOUS.

i 1 11

1 It*I I 1/I -1// 1. II 4 iii, _l III 0 I A A

CYLINDER CAN BE LINKED UP DIRECTLY POINT OF APPLICATION OF PONE TOL- NOTION IS TRANSMITTED TO A DISTANT TO THE LOAD. LOWS THE DIRECTION Or THRUST. POINT IN THE PLANE OF NOTION.

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SIMULTANECVS THRUSTS IN TWO DIFFERENT TWO PISTONS WITHPIKED STROKES POST.. NOTION OP MOVABLE RACK IS TWICE DIRECTIONS APE OBTAINED. TION LOAD IIIANY OF FOUR STATIONS. THAT OF PISTON.

111 C=POWPFPr. I IF 1 111"41444100001.111 IMO* ii

11 IL -i -1\1 g. 4. . F5 0 4 A . in A A111 CYLINDER CAN BE USED WITH A FIRST A TOGGLE CAN BE ACTUATED BY THE OEN SECTOR NOTES RACK INTER- CLASS LEVER. . CYLINDER. DICULAR TO STROKE OF PISTON. FP.228 Figure 12-12.Applications of actuating cylinders. 225 230 FLUID POWER ports of the motor). This means, to produce Although most fluid power motors are 1,200 inch-pounds of torque, the fluid flow at capable of providing rotary motion in either the inlet port must be maintained at a pressure direction, some applications require rotation in high enough to allow for the 1,000 pet differ- only one direction. In these applications, one once. In other words, this motor requires the port of the motor is connected to the system use of 1,000 psi to accomplish 1,100 inch- pressure line and the other port to return or pounds of work; therefore, the pressure at the exhaust. The flc Al of fluid to the motor may be inlet port must be maintained at 1,000 psi or controlled by a flow control valve (ON and OFF more. valve), a two-way directional control valve, or In addition to controlling the speed of a by starting and stopping the power supply; for motor, the displacement also has aproportional example, the pump in a hydraulic system. The effect on the torque. With input flow and pres- speed of the motor may be contro 11 e d by sure remaining constant, a decrease in dis- varying the rate of flow. This can be accom- placement increases the Ppeed of the motor but plished by incorporating a flow control valve reducesthetorque proportionally. In other (restrictor of flow regulator) in the pressure words, the decrease in displacement reduces supply line. In many hydraulic systems, the rate the amount of fluid required for each revolution of flow is controlled by varying the output of of the motor element. Thus, with the same rate the variable displacement pump. of flow,the motor element 'completes more In most fluid power systems, the motor is revolutions per volume of 'fluid. However, this required to provide actuation in either direc- reduced volume of fluid decreases the force tion. In these applications the ports are referred acting on the motor element, thus reducing the to as working ports, alternating as inlet and torque. An increase in displacement requires a outlet ports. The flow to the motor is usually larger volume of fluid for each revolution, thus controlled by either a four-way directional. reducing the speed of the motor. The larger control valve or a variable displacement pump. volume of fluid increases the force acting on thus increasing torque. Fluid motorsare usually classified the motor element, according to the type of internal element, which The speed andtorquerequirements of isdirectly actuated by the flow. The most motors vary in different applications. In some common types of eler..ents are the gear, vane, applications the unit must be actuated at a high and piston. All three of these types are adapt- speed, while very little torque is required for able for hydraulic systems, while only the vane the operation.In other applications, low speed type is utilized in pneumatic systems. and high torque are required to operate a particular unit. The requirements of most appli- GEAR TYPE cations vary between these two extremes. To meet these various requirements, fluid power The gears of the gear type motor are of the motors are available in several designs, sizes, external type and may be of the spur, helical, etc. or herringbone design. These designs are the same as those used in gear pumps and, there- Fluid motors may be of the fixed displace- fore, are described in more detail in chapter 8. ment or variable displacement type. The fixed displacement provides const an t torque and The operation of a gear type motor is illus- variable speed. The speed is varied by control- trated in figure 12-13. Both gears are driven ling the amount of input flow. The variable gears; however, only one is connected to the displacement motor is constructed in a manner output shaft. As fluid under pressure enters which permits the working relationship of the chamber (A), it takes the path of least resis- internal parts to be varied so as to change tance and flows around the inside surface of the displacement. This provides variable torque and housing, forcing the gears to rotate as indicated. variable speed., With input flow and operating The flow continues through the outlet port to pressure remaining constant, the ratio between return.This rotary motion of the gears is torque and speed can be varied to meet load conveyed through the attached shaft to the work requirements by varying displacement. The unit. majority of the motors used in fluid power sys- Although the motor illustrated in figure tems are of the fixed displacement type. 12-13 shows operation in only one direction, 226 231 Chapter 12ACTUATORS

a ;drive shaft.Each slot of the rotor is fitted with a freely sliding rectangular vane. The rotor and vane are enclosed in the housing, the inner surface of which is off set with the drive shaft axis.When the rotor is in motion, the vanes tend to slide outward due to centrifugal force. The distance the vanes slide is limited by the shape of the rotor housing. This motor operates on the principle of differentialareas. When compressed air is directed into the inlet port, its pressure is exerted equally in all directions. Since area (A) INLET (fig. 12-14) is greater than area (B), the rotor will turn counterclockwise. Each vane, in turn, FP.229 assumes the No. 1 and No. 2 position and the Figure 12-13.Gear type motor. rotor turns continuously. The potential energy of the compressed air is thus converted into the gear type motor is capable of providing kinetic energy in the form of rotary motion and rotary motion in either direction. The ports force. The air at reduced pressure is exhausted alternate as inlet and outlet. To reverse the to the atmosphere. The shaft of the motor is directionofrotation,the fluidis directed connected to the unit to be actuated. through the port labeled outlet, into chamber Many vane ty p e motors are capable of (B). The flow through the motor rotates the providing rotation in either direction. A motor gears in the opposite direction, thus actuating of this design is illustrated in figure 12-15. the work unit accordingly. The principle of operation is the same as that of the vane type motor previously described. VANE TYPE The two ports may be alternately used as inlet and outlet,thus providing rotation in either A typical Vane type air motor is illustrated direction. Note the springs in the slots of the in figure 12-14. This particular motor provides rotors.Their purpose is to hold the vanes rotation in only one direction. The rotating against the housing during the initial starting element is a slotted rotor which is mounted on of the motor, since no centrifugal force exists

INLET AREA ''B" SHAFT SPRINGS ROTOR AREA "A"

VANE -4.INLET OUTLET

CHAMBER MALL EXHAUST

HOUSING

CASING VANES FP.230 FP.231 Figure 12-14.Vane type air motor. Figure 12-15.Vane type motor. 227 FLUID POWER until the rotor begins to rotate. Springs are not liquid is introduced into the cylinder bore con- required in vane type pumps because the drive taining piston (1), the piston must move outward shaft provides theinitial centrifugal force. since the liquid cannot be compressed and two bodies cannot occupy the same space. To allow PISTON TYPE for this movement, the cylinder block must revolve in a clockwise direction, since the Like piston (reciprocating) type pumps, the piston, in moving outward, will seek the point most common designs of piston type motors are of greatest distance between the cylinder block the radial and axial. These types of motors are and the rotor. As the force acting on piston (1) most commonly used in hydraulic systems. causes the cylinder block to move, piston (2) Although some piston type motors are con- starts to change position and will approach the trolled by directional control valves, they are position of piston (3). often used in combination with variable dis- placemerit pumps. This pump-motor combination (hydraulic transmission) is used to provide a transfer of power between a driving element (for example, an electric motor or gasoline engine) and a driven element. Some of the applications ROTOR CYLINDER BLOCK for which hydraulic transmissions may be used are speed reducer, variable speed drive, con- FLUID PISTON 3 stant speed or constant torque drive, and torque OUTLET converter. Some ad v an t a g e s of hydraulic transmission over mechanical transmission of power are as follows: 1. Quick, easy speed adjustment over a PISTON 2 PINTLE wide range while the power source is operating VALVE at constant (most efficient) speed. Rapid, smooth acceleration or deceleration. 2. Control over maximum torque aridpowir. FLUID' PISTON 1 3. Cushioning effect to reduce shock loads. INLET 4. Smoother reversal of motion. (Whilestudying the description of piston type motors in the following paragraphs, it may FP. 232 be necessary to refer to chapter 8 for a review Figure 12-16.Operation of radial of the operation and particularly the parts of piston motor. radial and axial piston pumps.) Radial It should be noted that the distance between the cylinder block and the reaction ring of the Figure 8-18 in chapter 8 shows a view of rotor gets progressively shorter on the top and the radial piston pump (motor) and identifies right half of the rotor as is shown in figure the parts. In the radial piston pump, as the 12-16. As piston (2) moves, it is forced inward c ylinde r block revolves, the pistons press and in turn forces the liquid out of the cylinder. against the rotor and are forced in and out of Since there is little or no pressure on this side the cylinder, thereby receiving fluid and pushing of the pintle valve, the piston is easily moved in it out into the system. The motor operates in by the contact With the reaction ring of the reversefluid forced into the cylinder drives rotor. The liquid is easily forced out of the the pistons outward. The pistonspushing against cylinder and back to the reservoir or to the inlet side of the pump. As piston (3) moves past the rotor cause the cylinder block to revolve. the midpoint,or past the shortest distance The operation of a radial piston motor is between the cylinder block and rotor, it enters illustrated in figure 12-16. This type motor the pressure side of the pintle valve and liquid usually contains seven or nine pistons; however, is forced into the cylinder. Piston(3) then for simplicity, only three pistons are shown. If becomes the pushing piston and in turn rotates 228 Chapter 12ACTUATORS the cylinder block. This action continues as long position to the other. In the motor, the position as liquid under pressure enters the cylinders. of the rotor is fixed. The direction of rotation The rate at which the liquid flows into the cylin- of the motor is reversed, however, by reversing ders determines the speed at which the cylinder the output of the pump. With the pump in this block turns. position, liquid enters the motor (fig. 12-16) on When the radial motor is used as a hydraulic the top side of the pintle valve, which causes transmission, the two ports of the motor are the cylinder containing piston (3) to become the connected to the two ports of the pump. To cylinder into which liquid enters under pres- control the motor speed, it is necessary to sure. This forces the cylinder block to rotate control the output capacity of the pump. Since in the opposite direction. the pump is usually a variable displacement Variable displacement radial piston motors type, this is accomplished by controlling the are available for applications where variable degree to which the slide block of the pump is torque is required. If constant speed is required offcenter in relation to the center of the cylinder in these applications, a fixed displacement pump block. Since pressure is the result of resistance is used. If control of both speed and torque is to flow,it is practically independent of the required, both the pump and the motor are of volume output of the pump, and thus the speed the variable displacement type. of the motor. The load on the output shaft of the motor is the resistance to the volume of flow; therefore, the pressure varies proportionally to Axial Piston the load. One design of the axial piston type motor is Assume that in a pump-motor combination illustrated in figure 12-17. This motor is iden- of this type, in which both units are the same tical in design to the constant displacement size,he rotor is set offcenter in the pump a axial piston pump, discussed in chapter 8. distance that is just equal to the offset of the rotor in the motor. For each full discharge on one cylinder in the pump, a piston in the motor OC"4 must move an equal distance, as it receives the tN; same amount of liquid as the pump discharges. CYLINDER BLOCK Ot7c% Therefore, at this setting the cylinder block of the motor revolves at the s3ame speed as the OUTPUT SHAFT cylinder block of the pump. AXIS Now assume that the offset distance in the pump is shortened, so that it requires the dis- OUTPUT SHAFT charge of two cylinders in the pump to fill one PISTONS cylinder in the motor. Under this condition the cylinder block of the pump completes two rev- VALVE PLATE olutions for each revolution of the cylinder block in the motor. Thus, the motor rotates at FP.233 just one-half the speed on the pump. Figure 12-17.Axial piston hydraulic motor. Since the operator has a wide control over the pump end of this combination, he has an The operation.'of the axial piston motor is equal control over the motor enda control similar to that of the radial piston motor. Fluid achieved by regulating the amount of flow from from the system flows through one of the ports the pump. If, in the pump, the rotor and cylinder in the valve plate and enters the bores of the block are centered, no pumping action will take cylinder block that are open to the inlet port. place, consequently no liquid will be delivered (For example, in a nine piston motor, four to the motor erid, and therefore the output shaft cylinder bores are receiving fluid while four of the motor will not rotate. are discharging.) The fluid acting on the pistons in these bores forces the pistons to move away In the pump, the direction of flow is reversed from the valve plate. Since the pistons are held by moving the rotor from one side of neutral by connecting rods at a fixed distance from the

229

.234 ". FLUID POW ER output shaft flange, they can move away from very similar to the axial piston motor just the valve plate only by moving in a rotary described. direction.The pistons move in this direction to a point around the shaft axis which is the The hydraulic motor or B-end can be di- greatest distance from the valve plate. There- rectly connected hydraulically to the pump as fore, driving the pistons axially causes them illustrated in figure 12-18, or the motor can to rotate the drive shaft and cylinder block. be installed at a distance from the pump with While some of the pistons are being driven by thetwo mechanisms connected with piping. liquid flow from the system, others are dis- The speed of rotation of this unit is con- charging flow from the outlet port. trolled in the same manner as the radial piston This type motor may be operated in either unit.When the pump is set to allow a full direction of rotation.The direction of rota- stroke of each piston, each piston of the motor tion is controlled by the direction of flow to the must move an equal distaine.In this condi- valve plate.The direction of flow may be tion, the speed of the motor will equal that of the pump. If the tilting plate (box)of the instantly reversed without damage to the motor. pump is changed so that the piston stroke of This design of axial piston motors is some- the pump is only one-half as long as the stroke times used in aircraft hydraulic systems to ofthe motor, it will require the discharge operate such components as landing flaps and from two pump cylinders to fill one motor radar equipment.In these applications the cylinder and move the piston one full stroke. direction of flow, thus the direction of rota- Therefore, in this position of the tilting plate, tion of the motor, is controlled by four-way the motor will revolve just one-half as fast directional control valves. as the pump.If there is no tilt on the tilting plate, the pumping pistons will not move axially, Figure 12-18 shows a hydraulic transmis- and no liquid will be delivered to the motor. sion consisting of an axial piston pump and Therefore, the motor will deliver no power. motor combination.The A-end is a variable If thetilt of the tilting plate is reversed, displacement axial pump.The motor (B-end) the direction of flow is reversed. Liquid enters is identical to the pump except that it is of the motor through the port by which it formerly the fixed displacement type. Although of slightly was discharged.This reverses the direction different design, the operation of the motor is of rotation of the motor.

VALVE PLATE TILTING BOX CONTROL CYLINDER BARREL TILTING BOX

SOCKET RING

El END CYLINDER BARREL DRIVE (HYDRAULIC MOTOR) SOCKET RING SHAFT A END

FP.234 Figure 12-18.Hydraulic transmission.

230

.235 Chapter 12ACTUATORS

Some applications of the axial piston pump- motor combination require a wide range of torque variation. Variable displacement motors are available for these applications. Like the radial piston combination, the variable displacement axial piston motor may be operated with either a fixed or variable displacement pump.If the pump is of the fixed displacement type, the torque and, to some extent, the speed of the motor is controlled by the tilting plate of the motor.The direction of rotation is reversed by reversing thetilt of the tilting plate.If both the pump and motor are variable displacement units, it is possible to obtain almost unlimited control of any conceivable move- F P.235 ment or sequence of operation.In this com- Figure 12-19.Pinwheel turbine. bination the motor rotates in the same direction as the pump when the two tilting plates are tilted in the same direction, and in the opposite volume and velocity of the gas which escapes. direction when the two are tilted oppositely. The shaft of the air turbine is connected to the unit to be operated. TURBINES MULTIPLE-STAGE TURBINE Anotherdevice used for converting fluid power energy into rotary force and motion is A multiple-stage turbine is illustrated in the turbine.Turbines are commonly used in figure 12-20.The turbine wheel is a solid pneumatic systems for this purpose. For ex- piece of steel having = semicircular recesses ample, an air turbine may be used to drive (buckets) cut into the surface of the outside an electric generator, thus converting the po- circumference.Mounted on the casing around tential energy of compressed gas into electrical the wheel are four nozzles spaced 90 degrees power. A turbine may also be used to drive apart.Within the casing are a series of semi- the pump and supply fluid flow in a hydraulic circular reversing chambers. The compressed system. Some of the most common types of gas is led through a gas manifold ring and air turbines are described in the following passes throughthe nozzles.The gas then paragraphs. impinges at high velocity on the buckets. As the gas passes through the buckets, the PINWHEEL TURBINE direction of flow is reversed 180 degrees.. The gas is then caught by a semicircular reversing One typeof air turbine is the pinwheel chamber in the casing where it is again reversed turbine shown in figure 12-19.This turbine 180 degrees and returned to the wheel.The is a mechanical jet, similar in principle to a process is reversed five times through the garden sprinkler.Compressed gas is led into 90-degree arc ofthe turbine housing after the pinwheel hub through the hollow shaft. The which the gas is exhausted. A similar process compressed gas eventually passes through the takes place in each of the other three 90-degree diametrically opposed nozzles. Since the outlet arcs of the turbine at the same time. Revers- in each nozzle is smaller in diameter than the ingthe flowof gas several times gives a passages . in the turbine, the gas increases in multiple-stage effect, thereby using more of velocity as it exhausts from the nozzles.(See the potential energy of the gas. Bernoulli's principle in chapter 2.) The reaction to the exhaust causes rotation of the pin- SINGLE-STAGE TURBINE wheel. The amount of reaction (thrust) is equal to the force of the escaping gas.This Another type of turbine is the single-stage force can be determined by measuring the turbine shown in figure 12-21.In this turbine 231

.236 FLUID POWER

the gas expelled through the nozzles is not reversed in direction, by makes only one pass through the turbine.

TURBINE GOVERNORS As mentioned previously, turbines are sometimes used as the source of power (prime mover) for electric generators.To provide a

.. constant electric output in such applications, a 4 th governor is commonly used to control turbine REVERSAL speed within very close tolerances. Two types 3rd N ofturbine governors are described in the following paragraphs. REVERSAL .. 2nd 01. REVERSAL Moving Shutter Governor 1st REVERSAL The moving shutter governor is used to control the speed of the pinwheel turbine.It is mounted in the hub of the pinwheel, as shown in figures 12-22 (A) and 12-22 (B). Compressed gas entering the turbine through the hollow shaft must pass through the governor before it can pass through the jet nozzles.The governor consists of a shutter secured to a torsion bar. (See fig. 12-22 (C).) FP.236 Figure 12-20.Multiple-stage turbine. As the turbine rotates, centrifugal force causes the shutter to attempt to align itself in the plane of turbine rotation. Infigure 12-22 (B), NOZZLE NO.2 NOZZLE NO. I the shutter tends to turn clockwise andblock the flow of gas to the nozzles. The torsion bar re- strains the shutter from blocking the outlets. As turbine speed approaches a.specified limit, the force on the shutter begins to overcome the resistance of the torsion bar, thus permitting the shutter to turn clockwise and partially block the outlet to maintain speed at a specified limit.If turbine speed decreases, the torsion bar overcomes the centrifugal force, and the shutter moves counterclockwise to uncover the outlets. Thus, the moving shutter governor continuously controls turbine speed by metering the flow of gas to the nozzles. NOZZLE NO.3 NOZZLE NO.4 Flyball Governor A very common type of turbine governor is the zlybali governor illustrated in figure 12-23. FP.237 The coiled springs provide a restraining force Figure 12-21.Single-stage turbine. on a pair of pivoted counterweights mounted on 232 237 Chapter 12ACTUATORS

OUTLET TO SHUTTER PINWHEEL JET TORSION BAR

INLET HOLLOW SHAFT

PORTS OUTLET TO SHUTTER PARTIALLY OPEN PINWHEEL JET

A B C FP.238 Figure 12-22.Moving shutter governor.

TURBINE HOUSING the turbine shaft. When shaft speed reaches design speed, centrifugal force overcomes spring tension.The counterweights pivot, causing the brake shoes to bear on the brake drum. When design speed is exceeded, the centrifugal force and the braking action increase. As the speed decreases, the centrifugal force lessens, and braking action decreases.Thus, a constant turbine speed is maintained.

Another type of flyball governor controls turbine speed by metering the flow of gas to the turbine.Itsspring and counterweights are coupled to a valve on the input side of the 16. turbine.Centrifugal force causes the weights FP.239 to open and close the valve, thereby controlling Figure 12-23.Flyball governor. turbine speed. CHAPTER 13 HYDRAULIC AND PNEUMATIC SYSTEMS

Some of the many applications of fluid power HYDRAULIC POWER DRIVE SYSTEM are mentioned in chapter 1. Also, some of the simple hydraulic and pneumatic systems are described and illustrated in chapter 4. In addi- The hydraulic power drive has been used tion,several specific applications relative to in the Navy for many years. Proof of its effec- particular components are mentioned in other tiveness is the fact that it has been used to chapters. These examples indicate, to some train (provide horizontal movement around the extent, that many different designs of systems verticalaxis)and elevate (provide vertical are necessary to satisfy the requirements of movement) nearly all caliber of guns, from the various fluid power applications. The type, the 40-mm to the 16-inch turret. In addition quantity, and combinations of components vary to gun mounts and turrets, hydraulic power in different fluid power systems. Systems range drives are used to position rocket launchers from the simple hydraulic jack and the pneu- and missile launchers, and to drive and con- matic brake system described in chapter 4 trol such equipment as windlasses, capstans, to extremely complex systems, some of which and winches. combine the use of hydraulics and pneumatics. In its simplest form, the hydraulic power drive consists of the following: A few typical systems are described in this 1. The prime mover, which is the outside chapter to show some of the different applica- source of powergasoline engine, electric motions of fluid power in the Navy. It must be tor, etc.used to drive the hydraulic pump. emphasized that these are representative sys- In this case the prime mover is an electric tems and may not be the specific system used motor. in a particular application. Applicable technical 2. The A-end, which is a hydraulic pump. publications should be consulted for the opera- It is driven by the electric motor, and its fluid tion,servicing, and maintenance of specific output drives a hydraulic motor. systems used in actual applications. However, 3. The B-end, which is the hydraulic motor. a knowledge of the representative systems dis- The output of the B-end is connected directly cussed in the following sections will defin- to the mount by mechanical gearing and shaft- itely serve as an aid in understanding the spe- ing. Although the A-end is driven at a constant cific systems used in actual applications as speed by the electric motor, it is a variable well as other types of fluid power systems. displacement pump and,therefore, delivers The systems are covered in the following fluid at the rate demanded by the B-end. By order: controlling the output of the A-end, the direction and speed of the B-end, and hence the 1. Hydraulic power drive. direction and speed of the motion of the mount, 2. Electrohydraulic steering. can be controlled. 3. Missile' fluid power systems. 4. A means of introducing a signal to the 4. Aircraft fluid power systems. A-end to control the output. Also included in the last part of this chapter 5. The mechanicalshafting and gearing is a section on the fundamentals of trouble- that transmits the output of the B-end to the shootinglocating and diagnosing malfunctions train and elevation drive pinions. in a fluid power system by means of syste- To satisfythe requirements of different matic checking and analysis. equipments, hydraulic power drives differ in 234 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS

INDICATOR RESERVOIR REGULATOR RELIEF VALVESES (2) FOR REPLDISHING PUMP AND HIGH PRESSURE CONTROL PRESSURE PUMP OIL FILTERS

CONTROL PRESSURE PUMP REPLENISHING PUMP

B-END

MAIN SUMP PUMP ELECTRIC A-END CYLINDER AND OSCILLATOR MOTOR HYDRAULIC AUXILIARY MAIN A-END TRANSMISSION REDUCTION REDUCTION VALVE LINES GEARS GEARS PLATE FP.240 Figure 13 -1. -Train power drivecomponents.

some respect, such as to size, method of con- to the training circle of the gun mount. The trol,etc. However, the fundamental operating function of each major component is described principles are similar. The unit used in the in the following paragraphs. following discussion concerning these fundamental operating principles is representative of the hydraulic power drives used to operate ELECTRIC MOTOR the 5"/38 twin mounts. These mounts are driven in train and elevation by separate systems. The prime mover, or electric drive motor, Since the systems differ only in size and minor drives the A-end of the hydraulic power trans- details, only the train power drive is described. mission. Through reduction gears, it also drives The basic components of the train power the replenishing pump, control pressure pump, drive are shown in figure 13-1. The electric and the sump pump and oscillator.A motor drive motor is constructed with drive shafts controller with associated pushbutton stations at both ends. The forward shaft drives the starts and stops the electric motor and gives A-end pump through reduction gears. The after protectionagainst overload and overvoltage. shaft is coupled through reduction gears to the auxiliary pumps (replenishing, control pressure, and sump and oscillation pumps). MAIN REDUCTION GEAR The ports in the A-end valve plate are con- The A-end pump is designed to operate at nected to like ports in the B-end valve plate a speed of approximately 900 revolutions per by large copper pipes. Fluid pumped by the minute. (The pump speed requirements may A-end causes rotation of the B-end. The output differ with various types and applications of of the B-end is connected by shafts and gears hydraulic power drives.)Since this speed is 235

240 FLUID POWER much lower than that of the electric motor, compensate for misalignment of the B-end shaft it is necessary to reduce the speed of rotation and the final drive gearing, they are connected between the A-end and the electric motor. This byflexibleor self-aligning type couplings. speed reduction is accomplished through a gear reduction mechanism. The reduction gears are contained in a housing which is flange-mounted Auxiliary Pump Cluster between the electric motor and the A-end. Also mounted on the motor drive shaft is a heavy The auxiliary reduction gears are flange- flywheel, which tends to prevent any sudden mounted at the rear of the electric motor. Three change in the speed of reduction gear output. importantpumpsare driven by the motor through these gears. (See fig. 13-1.) REPLENISHING PUMP. This pump is a spur HYDRAULIC TRANSMISSION gear pump, driven at a constant speed by the electric motor through the auxiliary reduction The hydraulic transmission system includes gears. Its purpose is to replenish fluid to the the following components: active system of the power drive. 1. The A-end, an electrically driven var- The pump receives a supply of fluid from iable displacement hydraulic pump. the reservoir and discharges it to the B-end 2. The main cylinder, containing the stroke valve plate.This discharge of fluid from the control shaft. (This assembly is used in con- pump is held at a constant pressure (normally trolling the tilt of the A-end.) between 25 and 50 psi) by the action of a pres- 3. The B-end, a fixed displacement hydrau- sure relief valve. Normally, because the ca- lic motor. pacity of the pump exceeds replenishing de- 4. Auxiliary pumps. mands, tin relief valve is continuously allowing The operation of the hydraulic transmission some of the fluid to flow to return. is illustrated in figure 13-2. SUMP PUMP AND OSCILLATOR. This pump is also driven by the electric motor through A-End the auxiliary reduction gears. Its purpose is twofold. It pumps leakage, which collects in The train A-end is an axial piston, var- the sump of the indicator regulator, to the ex- iable displacement pump. The direction and pansion tank. volume cf its output controls the direction and Its other function is to transmit a pulsating speed of rotation of the B-end. It is driven at effect to the fluid in the response pressure a constant speed by the electric motor through system. Oscillations in the hydraulic response the main reduction gears. The output is con- system help eliminate static friction of valves trolled by the tilt of the tilting box. Movement which cause the hydraulic control to respond of the tilting box is directly controlled by the faster. stroke control shaft. CONTROL PRESSURE PUMP.The remaining pump driven through the auxiliary reduction gears is the control pressure pump. Its B-End function is to supply high-pressure fluid for the hydraulic control system, brake pistons, The B-end is similar in construction to the lock piston,andthe hand- controlled clutch A-end except that the tilt of the tilting box is operating piston. fixed; that is, the B- end is a fixed displace- The control pressure pump is an axial pis- ment motor. Since the B-end pistons must each ton pump. It is similar to the A-end pump ex- ,make a full stroke for Gvery revolution of the cept that the angle of tilt is permanently fixed. output shaft, the speed of the B-end, and there- Consequently, the pump produces a constant fore the speed at which the guns are trained volume output. (or elevated), is directly proportional to the To limit the pressure of the pump output angular displacement of the A-end tilting box to the operating pressures, an adjustable re- from its neutral position. lief valve,identical with that of the replen- The output shaft of the B-end motor is con- ishing pump relief valve, is used. Fluid pres- nected to the training pinions through a mech- sure can be measured by inserting a gage in anical system of gearing and shafting. To the plughole located in the relief valve block. 236

.241 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS

CONNECTED TO ERROR SIGNAL THROUGH STROKE PISTON, WHICH POSITIONS TILT BOX

CONSTANT SPEED ELECTRIC DRIVE MOTOR 104t?40

A-END TILT BOX

HIGH PRESSURE

END 611(

FP.241 Figure 13-2.Operation of hydraulic transmission.

RESERVOIR entry port for adding fluid to the hydraulicsys- tem. A removable strainer is located at the The reservoir provides a reserve supply fluid entry point to minimize the possibility of of fluid for the system. It also provides a large particles of foreign matter entering the hy- cooling surface for the fluid. The reservoir draulic system. is a bronze tank of considerable surface area which is mounted on shock absorbing padson top of the reduction gear housing. External CONTROL piping connects the reservoir to the hydraulic transmission, control devices, and the expan- Control,asit pertains to the indicator sion tank. regulator and the handwheel, is beyond thescope of this manual. For the purposes of this manual, control constitutes therelationship between EXPANSION TANK the stroke control shaft and the tilting box. The stroke control shaft is one of the piston The expansion tank (not shown in fig. 13-1), rods of a double-acting piston type actuating as the name implies, provides space to allow cylinder. This actuating cylinder and its direct for expansion of the volume of fluid in the hy- means of control are referred to as the main draulic system due to changes in temperature. cylinder assembly. (See fig. 13-3.) It is the The expansion tank is mounted at the highest link between the hydraulic followup system point in the hydraulic system and hasa gage and the power drive itself. window in its side to indicate the safe fluid In hand control, the tilting box is mechan- level for operation of the equipment. Aremov- ically positioned by gearing from the hand- able cap on the top of the tank servesas a fluid wheel through the A-end control unit. In local

237 242 FLUID POWER

RESERVOIR OR EXHAUST STROKE CONTROL SHAFT PX PRESSURE (0 PSI)

HPC HPC

STROKE CONTROL SHAFT

MAIN MAIN TILTING CYLINDER PISTON BOX

BRAKE VALVE F P.242 u Figure 13-3.Main cylinder assembly. FP.243 Figure 13-4.Brake valves. and automatic control, the tilting box is positioned by the stroke control shaft. As illustrated BRAKE VALVES in figure 13-3, the extended end of the control shaft is connected to the tilting box. Movement An assembly consisting of two brake valves of the shaft will pivot the tilting box one way ( pietons) acts to force the tilting box to its zero or the other which, in turn, controls the output position whenever control pressure is released. of the A-end of the transmission.The other (See fig.13-4.) These spring-loaded pistons end of the shaft is attached to the main piston. are normally held back against the spring ten- A shorter shaft is attached to the opposite sion to allow the tilting box free movement. side of the piston.This shaft is also smaller This is accomplished by fluid pumped at high in diameter.Thus the working area of the left pressure from the control pressure pump. When side of the piston is twice that of the area of fluid pressure drops, the pistons are moved the right side, as it appears in figure 13-3. by their springs to center the tilting box. This Intermediate high pressure (IHP) is trans- would bring the gun mount to rest under emer- mitted to the left side of the piston, while high gency stop conditions. For example, if control pressure (HPC) is transmitted to the right side. pressure failed with the tilting box in a Li"Rd The HPC is held constant at 1,000 psi. Since position, the pistons of the brake valve td the area of the piston upon which HPC acts immediately center the tilting box and prevent Is exactly one-half the area upon which IHP the gun mount fr,:an driving out of control. acts, the main piston is maintained in a fixed position when IHP is one-half HPC. Whenever IHP varies fro..n normal valve of one-half HPC, OPERATION because of action of various valves, the main piston will move, and thus cause movement of Figure 13-5 is a simplified block diagram the tilting box. showing the main elements of the hydraulic 238 243 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS

ELECTRICAL SIGNAL FROM--- - DIRECTOR COMPUTER 4 B-END RESPONSE _INDICATOR 'REGULATOR STROKING PISTON

B-END AUXILIARY ELECTRIC MOTOR GUN MOUNT PUMP MOTOR UNIT

CLOCKWISE MOVEMENT RESERVOIR

ELECTRICAL SIGNAL FROM-- - -%, DIRECTOR COMPUTER 4 B-END RESPONSE ? 0 4INDICATOR ap... REGULATOR STROKING PISTON

IAUXILIARY B-END PUMP MOTOR a UNIT

RESERVOIR

FP.244 Figure 13-5.Operation of the hydraulic power drive. power drive system under automatic control powerdrive,and this in turn controls the for clockwise and counterclockwise rotation. movement of the gun. There are two principal problems in positioning a gun to fire. One is to get an accurate The indicator regulator receives an initial gun-order signal.This problem is solved by electrical gun-order from the director-com- the gun director-computer combination. The puter, compares it to the existing mount posi- other problem is to transmit the director sig- tion, and sends an error signal to the hydraulic nal promptly to the gun, and in such a manner control mechanism in the regulator. The hy- that the position and movements of the gun draulic control mechanism controls the flow will be synchronized with signals from the to the stroke control shaft which positions the director. This problem is complicated by the tilting box in the A-end of the transmission. fact that movements of the gun tend to fall be- Itstilt controls the volume and direction of hind or to overrun director signals, due in fluid pumped to the B-end, and therefore the part to the lag inherent in transmitting the speed and direction of the drive shaft of the B- signals, but mainly to the inertia of the gun. end. Through mechanical linkage, the B-end out- This inertia tends to keep the gun in move- putshaft moves the gun by an amount and ment ifit is moving, and at rest if it is at inthedirection determined by the signal. rest, whereas the gun-order signal depends At the same time, B-end response is trans- primarily on changes in the location of the mitted to the indicator regulator, and contin- target.These signals are always changing, uously combines, with incoming gun-order sig- not only because of changes in the relative nals to give the error between the two. This position_of the target and the ship, but also error is modified hydraulically, according to becauseofthe roll and pitch of the ship. the rate at which the error is changing, bya The problem of transforming gun-order system of mechanical linkages and valves in signals to mount movements is solved by the the regulator. When the gun is lagging behind power drive and its controlthe indicator reg- thesignal,its movement is accelerated by ulator.The indicator regulator controls the this means; and when it begins to catchup, 239 FLUID POWER its movement is slowed down so that it will variable displacement pumps; others use radial not overrun excessively. piston pumps. Systems representative of the double ram and single ram types are described in the following paragraphs.

ELECTROHYDRiJILIC STEERING Double Ram Electrohydraulic transmissions, similar to Figure 13-6 shows a simple diagram of a the one previously described, are used in many double ram type electrohydraulic steering gear. applications in the Navy. The pump-motor com- In this type, the rudder yoke is connected to binations may be either of the radial or axial two hydraulic plungers or rams. Each ram piston type and, as stated previously, are used is equipped with cylinders at both ends. The to operate such equipment as capstans, am- flow of fluid in the system is provided by either munition hoists, aircraft cranes, and winches. one of two piston type, variable displacement Most steering gear Installations on modern pumps. The pumps are driven by electric mo- naval ships are of the electrohydraulic type, tors. The rate of fluid delivery is regulated but use only the A-end of the hydraulic power by the angle of the tilting box in the hydraulic drive described previously. Since the steering pump, which is controlled electrically or hy- gear requires reciprocating motion, the B-end draulically from the steering wheel on deck. is replaced by a ram type actuating cylinder. The control shaft and gearing are indicated The development of the electrohydraulic type in figure 13-6. steering gear was prompted by the large mo- Note that the after cylinder of the port ram mentary electrical power requirements of elec- and the forward cylinder of the starboard ram tromechanical steeringparticularly for ships are connected to one working line from the oflarge displacement and high speed. Also pump and that the after cylinder of the star- the elimination of direct-current electric power board ram and the forward cylinder of the port from ships made switching and speed control ram are connected to the other working line. of electric motors more difficult. A double-acting pressure relief valve serves Advanitages of the electrohydraulic steering to relieve excessive fluid pressures from one gear are its follows: working line to the other. This protects the 1. Littlefriction and inertia of moving system from excessive strain or probable parts, such as in heavy differential screws damage, should unusual resistance to the rud- and gears. der result in abnormal pressures within the 2. Low power consumption. system. 3. Sensitive response, with little lag,to Starting with a neutral position of the tilt- movement of the steering wheel. ing box and no fluid flow, assume that the steer- 4. Small deck space and headroom required. ing wheel is turned to starboard. Turning the 5. Saving in weight. wheel on the bridge causes an electric signal 6. Flexibility in the arrangement of hy- to be transmitted to the synchronous receiver draulic cylinders, pumps, and control mecha- in the steering gear room. (The synchronous nisms. receiver is part of the electrical steering gear 7. Dependability. control and is described later in this chapter.) The synchronous receiver will then turn cor- respondingly (counterclockwiseinfig.13-6 TYPES as viewed from the left). Shaft (A) is turned clockwise which rotates gear (B) in the same There are various types of electrohydrau- direction. The gears (C), meshing with gear lic gear arrangements in use, but their op- (B) and the internal gear teeth on (E), turn erating principles are similar. Some ships counterclockwise. Therefore,(E) turns coun- are equipped with double hydraulic rams and terclockwise and turns the control shaft (F), cylinders (two dual ram assemblies) mounted which operates the tilting boxes on the pumps. fore and aft; others have a double cylinder A quantity of fluid then flows to the forward single ram (one dual rain assembly) mounted port and after starboard cylinders, as indi- athwartship. Some systems use axial piston, cated in figure 13-6. This moves the port ram Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS

IDLE PUMP CYLINDER YOKE YLINDE IDLE MOT R ORT RA . HAFTS RELIEF TO VALVE SIX-WAY PUMP TRANSFER COCKTILTING

RUDDER CYLINDER CYLINDER -quo/ IIU'MO= RUNNING PUMP RACK AND PINION

RUNNING MOTOR CONTROL SHAFT =- ELECTRIC LEADS FROM STEERING STAND (

SUPPLY SELF-SYNCHRONOUS RETURN RECEIVER PLANETARY I DIFFERENTIAL GEAR FP.245 Figure 13-6.Double electrohydraulicate"gear. aft and the starboard ram forward which, in Single Ram turn, moves the rudder to the right. When the steering wheel and the synchro- The single ram type electrohydraulic steer- nous receiver stop moving, the starboard ram, ing gear, illustrated in figure 13-'7,operates in moving forward, operates the rack and pin- on the same principle as the double ram type. ion and turns gear (D) clockwise. Gear (B) There is but one ram, with port and starboard and shaft (A) are held by the now motionless cylinders, mounted athwartship. When the steer- receiver. Gears (C) and casing (E) turn clock- ing wheelis turned to starboard, fluid is wise, thus returning the tilting boxes to the directed to the port cylinder, forcing theram neutral position which, in turn, stops the flow to move to the right. The displaced fluid in the of fluid. The planetary differentialgear, thereby starboard cylinder returns to thepump. This operates as a followup mechanism. When the movement of the ram causes the rudder to steering wheel is turned to port, the actions move to starboard. described are in the opposite direction. In actual installations, two sets of synchron- CONTROL OF STEERING GEARS ous receivers and two sets of electric motors and hydraulic pumps are provided for reli- Control of the steering gear from the steer- ability and flexibility. The six-way plug valve ing wheel on the bridge may be accomplished makes itpossible to transfer quickly from by any of the following remotecontrol the operating pump to the standby pump. systems:

241 FLUID POWER

STEERINGWHL STEERING WHEEL AUTOMATIC BY.PASS ONAFTERDECKHOUSE ON BRIDGE VALVE ELECTRIC CONTROL) SYSTEM TRANSMITTER UNIT j TRANSMITTER UNIT PORT PORT CYLINDER PORT OR STBD. CABLE TRANSFER SWITCH

CRANK FOR HAND OPERATION OF PORT PUMP

PORT t RUNNING PUMP AFTER SIXWAY PUMP Or-3 DECK TRANSFER 1STARBOARD COCK

OFF RUNNING ELK. MOTOR STARBOARD AFT - CABLE WORM WHEEL

CROSSHEAD RECEIVING UNIT TRANSFER SWITCH TRANSMITS CURRENT THROUGH EITHER CABLE OR EITHER TRANSMITTER PLUNGER TO RECEIVER AS SELECTED FOLLOW UP SHAFT TRICK WHEEL FOR HAND CONTROL

UNIVERSAL JOINT

IDLE PUMP WORM WORM CYLINDER IDLE ELECTRIC WHEEL MOTOR STARBOARD SUPPLY SPIRAL GEARS RETURN /INN

AUTOMATIC BY.PASS VALVE FP.246 Figure 13-7.Single ram electrohydraulic steering gear.

1. Electrically, by means of an alternating- receiving and transmitting units which are current synchronous transmission system. similar to small motors. These units are con- 2. Hydraulically, by means of a telemotor nected to the same alternating-current sup- system. ply. When the transmitter motor is turned, the 3. Electrically, by means of a direct-cur- receiver motor turns at the same speed and rent motor and its controller. in the same direction. 4. Mechanically, by means of shafting or The steering gears illustrated in figures wire rope from the steering station. 13-6 and 13-7 are equipped with this type of The first two systems are the most com- remotecontrol.The complete synchronous monly used and therefore are described in transmission is shown in figure 13-7, while the following paragraphs. only the receiver unit is shown in figure 13-6. The transmitters are located in steering Alternating-Current stands at remote control stations and are mec- Synchronous Transmission hanically connected through gearing tothe steering Wheels. A transmitter at each of the The alternating-current synchronous trans- remote stations is electrically connected to mission type of remote control consists of a receiver in the steering room. The receiver

242 241 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS

ELECTRIC HELM ANGLE INDICATOR

TRANSMITTER DN PILOT HOUSE)

REPLENISHING INLAID MOOD TANK CAGE WHEEL

BYPASS VALVE AND AUTO-STEERING INTERLOCK

'HYDRAULICTUBING STEERING CONSOLE ELECTRIC CONTAINING HYDRAULIC PUMP TRANSMITTER, INOICATOR CROSSHEAD (POW CONNECTION RELIEI1AND CHECK VALVES CABLE TO STEERING MUD

PILLING OR CHARGING CONNECTION

RECEIVER ADJUSTAILIII IUNSTEERING GEAR COIYA MMMMMT) STOPS FP.247 Figure 13-8.Hydraulic telemotor control. (A) Telemotor components. is connected to the control shaft of the variable iliary ships. The control system consists of displacement hydraulic pump through a dif- a steering console (hydraulic transmitter) in ferential, as illustrated in figures 13-6 and the pilot house, a hydraulic receiver in the 13-7. Where more than one remote steering steering gear compartment, and connecting station is provided, a switch is provided for hydraulic tubing. In addition, there is an elec- selecting the desired control station. Indicating tric cable which connects the helm angle trans- lights are provided on the steering stands and mitter on the receiver housing with the helm at the selector switch to indicate the selected angle indicator on the steering console. circuit and that power is available. A hydraulic transmitter is located inside the steering console and under the steering Hydraulic Telemotor Control wheel. The hydraulic transmitter components consist of a pump, hydraulic tubing, two re- The hydraulic telemotor type of remote lief valves, two check valves, a replenishing control (fig. 13-8) is found on many Navy aux- tank, and a bypass valve. The remotely located

243 248 FLUB) POWER

PILOT 00010 TO PILOT NOUS, TOP ITIIIIMO MOIL 1T110011 STATION

D IM 44444 CLUTCH MON MILS AIN. INDICATOR IT..0NO MON PILOT N OM TOP MINIM MIMI AND SWITCH

ILICTIIC OILS ANON IMMO,.

-- CLUTCH WINN 44444 MO PROM PILOT NOUS, TOP

ILICTIIC CANT TO NILO tVIEAVGCON$OL{ ANGLO TIMONIUM,. WIN PILOT NMI/

PILLING CAP

SUING TAN LIMO LITIL 00011 (TOMO LOCATION ON OPPOUTI

AIR COCK

IMPASS YALU

IILIIP YALYI AUTO MIRING tri IWITCHNTIRLOCK S

D YPASS VOLVO CHOCK VOLVO

:7:141.1%."ItfATZNer""

HYDRAULIC MINOT*

AOSUSTAILII STOP

MN A.N. TRANUAITTIR RACE AND PINION

°MITA*" iteP InflUIANI{TURIN MIMIco...itsem

PILLMO OR CHARGING

Figure 13-8.Hydraulic telemotor controlContinued. FP.248 (B) Hydraulic circuit diagram; (C) schematic.

244

.249 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS receiver is a hydraulic ram type unit with engine required for thrust.The APS systems two cylindersone on each end of the receiver provide a source of power for the many devices housingin axial alignment. A double-acting required for successful missile flight.Some plunger (ram) operates in the cylinders.On of the APS systems rely on the main com- the middle portion of this plunger a crosshead bustion chamber as the initial source of power. is connected for mechanical linkage to the Others have their own energy sources completely steering gear control mechanism. separate from the main propulsion unit. What- The direction of the hydraulic fluid movement ever the initial source of power, APS systems depends upon the direction of rotation of the may be broken down into two broad categories steering wheel.Rotating the steering wheel static and dynamic.In static systems, energy actuates bevel and spur gears which in turn is used in the same form in which it is stored. operates a fixed displacement axialpiston Inthe dynamic systems, energy is changed hydraulic pump. In this pump, the tilt or from one form to another by a conversion unit. angle is set permanently at a fixed angle so There are several general requirements for that the pistons are always on stroke.When an APS system:First, the system must be the pump shaft and cylinder barrel are rotated able to deliver the necessary power during all by means of the steering wheel, the pistons conditions of missile flight; second, the system draw fluid in from one fluid line and discharge must be able to respond quickly and accurately itto theother fluidline. Reversing the to demands made onit;third, the system rotation of the steering wheel reverses the must be of minimum size and weight consistent direction of fluid flow through the pump. The with the requirements it must meet; and fourth, pump hasexternal check valves and piping the system must be durable enough to withstand for replenishing the hydraulic system from long storage under severe conditions. the reservoir.Relief valves and a bypass valve are also included, as well as vents for purging air from the system. BASIC SYSTEMS When the hydraulic pump shaft is rotated in one direction, the fluid output is discharged As previously mentioned, the static APS from one side of the pump to one of the receiver systems use energy in the same form in which cylinders. In the other cylinder, hydraulic it is stored.For example, compressed air fluid is displaced to the hydraulic pump. Thus may be used directly to operate control system the hydraulic fluid under pressure moves the components. Static systems require no rotating receiver plunger and produces a linear move- machinery for energy conversion. ment of the crosshead.This motion, in turn, In dynamic systems, energy is changed from is transmitted to the connected steering gear one form to another. For example, the potential control mechanism.Travel of the crosshead energy of compressed air may be changed to and plunger is limited by adjustable stops on electrical energy through an air-driven turbine the receiver housing.In figure 13-8 (C), the and electric generator.Air motors or air solid arrows show direction of 'action for turbines may be used to drive hydraulic energy right rudder, while the broken arrows show transfer units. Figure 13-9 shows a basic the flow of hydraulic fluid for right rudder. pneumatic APS system. The compressed air Air valves and filling or charging connect- flows from the air flask through a pressure ions are provided on the receiver cylinders reducer and thento the air motor or air for venting,filling, or purging the hydraulic turbine. system. Specific filling and purging instructions Hydraulic fluidis also used extensively for telemotor systems should be obtained from in guided missiles to transfer energy. An applicable steering gear or telemotor manuals, example is shown in figure 13-10. Compressed as these instructions vary, depending upon the air or hot gas is used to exert pressure againtit type of unit and thespecific installations. a piston, and hydraulic fluid is then used to operate loads such as the control surfaces. The hydraulic fluid contains no energy in MISSILE FLUID POWER SYSTEMS itself, but merely provides a means of transferring energy within a mechanism. Since, All missiles must contain auxiliary power for all practical purposes, hydraulic fluid is supply (APS) systems in addition to the main noncompressible,it can be used to transfer 245 250 FLUID POWER

axles around which the missile turns like a wheel. At the center, where all three axes intersect, each is perpendicular to the other two. The axis which extends lengthwise through the missile from the nose to the tail is called AIR MOTOR AIR OR TURBINE the longitudinal axis.The axis which extends FLASK PRESSURE RED9CER crosswise, from one sideto the other, is called the lateral axis. The axis which passes throughthe center, from top to bottom, is called the vertical axis. Motion about the longitudinal axis resembles the roll of a ship from side to side. In fact, FP.249 the names used in describing the motion about Figure 13-9.Basic pneumatic system. the three axes of a missile were originally nautical terms.They have been adapted to aeronautical terminology because of the simi- larity of motion between an aircraft or missile, and a ship. Thus, the motion about the longitudinal axis is called roll. Likewise, motion about the lateral PISTON (crosswise) axis is called pitch. This is similar to the pitching motion of an ocean vessel as AIR HYDRAULIC PRESSURE it plows through heavy seas. Finally, a missile HOTI FLUID 1111REGULATOR GASE TO moves about its vertical axis in a motion which HYDRAULIC is termed yaw. f FROM COMBUSTION LOAD The missile control system keeps the mis- CHAMBER OR sile in the proper flight attitude by controlling leAIR FLASK the movementroll, pitch and yawaround the corresponding axeslongitudinal, lateral, and FP.250 Figure 13-10.Air or hot gases working vertical. against hydraulic piston. Methods of Control energy with negligible losses.In addition to Some missiles are controlled by control the air or hot gas method previously mentioned, surfaces similar to aircraft control surfaces. axial and radial piston pumps, driven by turbines, The ailerons control roll about the longitudinal are commonly used in guided missiles. axis;the elevator control pitch around the lateral axis; and the rudder controls yaw about the vertical axis.On many of the late model MISSILE CONTROL missiles, this control is accomplished by con- trollingthe directionin which the exhaust A missile guidance system keeps the missile gases are exhausted from the nozzles (thrust on the proper flightpath from launcher to target, vectors) located at the base of each motor. in accordance with the signals received from Movable jets, movable metal rings, and rotatable control points, from the target, or from other nozzles are some of the devices used to control sources of information.The missile control the direction in which these gases are exhausted. system keeps the missile in the proper flight attitude. NOTE: Fluid injection is another method of Like an aircraft, whenever a missile changes controlling the flight of guided missiles by its attitude in flight, it must turn about one or controlling the direction of flow of the exhaust more of three axes. These axes are imaginary gases. Fluid .injection is an application of lines passing through the missile.The axes fluidics which is discussed in chapter 14 of of a missile can be considered as imaginary this manual.

246 251 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS

Types of Control (not shown). The air relay for the respective control surface (rudder or elevator) receives Regardless of the method used to control the air error signal produced by deviation in the attitude of the missile in flight, power must yaw or pitch of the missile. The relay action beavailable to operate the control system. is such that both force and sense direction The power is provided by one of the APS sys- of the error signal are transmitted to the trans- tems described previously. The power is transfer valve. Thus, the initial direction of dis- mitted from the supply source to the movable placement of the air transfer valve is deter- controls by pneumatic, electrical, or mechan- mined by the sense of the error signal. ical means, or by using a hydraulic transfer Assume that a yaw deviation has occurred, system in conjunction with the sources men- and that the error signal from the air relay tioned above. A combination of the different has caused the air transfer valve to move to types of control systems is usually employed the right, thus opening the right port of the in missile systems to achieve the desired de- actuating cylinder. Air from the high-pressure gree of control over the missile's flight. inlet passes through this port and causes the Pneumatic, electropneumatic, and electro- actuator to move toward the left. Simultane- hydraulic systems are discussed in the fol- ously, air is forced from the left-hand section lowing paragraphs. It must be emphasized that of the actuator cylinder through the port lo- these are representative systems to show the cated in that section. This displaced air is use of fluid power for missile control. Ap- exhausted intothe atomosphere through the plicable technical publications must be con- air transfer valve exhaust port located at the sulted for the operation, servicing, and main- left end of the air transfer valve. tenance of specific systems. The motion of the actuator piston is trans- PNEUMATIC. Although control systems op- mitted through the mechanical linkage to the erated entirely by pneumatics are not com- rudder, which applies corrective control in monly used in missiles today, knowledge of the proper direction to bring the missile back this type system will be helpful in understanding tothe desired yaw position. As the piston the operation of other systems. For simplicity moves, it exerts a force on the followup spring. of illustration, the control surface method of The followup spring is a calibrated coil spring controlis used in the following discussion. connected between the piston rod and the air In a pneumatic system, air from a pres- transfer valve. Movement of the piston puts surized source passes through lines, valves, the spring in a state of tension or compres- and pressure regulators to the transfer (direc- sion, depending upon the direction of piston tional control) valve where it is directed to movement. In either state, the followup spring the actuator. The pressurized source generally exerts a force on the air transfer valve which consistaofhigh-pressureair, or nitrogen opposes the force exerted by the air relay. stored in flasks or bottles. In a total pneumatic Thus, the air transfer valve movement is the system, the compressed gas is also used as difference of the two forces and is in the direc- a power source for the rotors of the gyros. tion of the resultant force. (Gyros are the devices used to measure a mis- Movement of the actuator piston continues sile' a movement about its axes.) In fact, the until the force exerted by the followup spring control information is in the form of varying is equal, but opposite in direction, to the force gas pressures. exerted on the air transfer valve by the air The pneumatic system does not reuse its relay. When this condition is established, the transfer medium after it has performed work. air transfer valve spool is centered, and actu- For that reason, the gas must be stored at a ator piston movement stops. With the air trans- pressure much higher than that necessary for fer valve thus balanced, the piston, and the actuating the load in order to maintain ad- rudder to which it is linked, is held in the cor- equate system pressure as the stored gas sup- rective position ordered by the error signal. ply diminishes. The force applied to the rudder causes the Figure 13-11 illustrates two double-acting missile to turn toward the desired flight at- piston type pneumatic actuators used for rud- titude. Since the missile is now moving in a der and elevator control. Each actuating cyl- direction which is opposite to that during off inder is controlled by an air transfer valve course displacement, an opposing air rate sig- which is mechanicilly linked to an air relay nal is applied to %emir relay. This air signal

247 FLUID POWER

high pawn WM *Awe ps0 mind pad *Asp 1* sir teloy dloplwern kilewup epilog

sir tranehr valve

mehtmledWow

FP.251 Figure 13-11.Pneumatic control system. reduces the force that the air relay is exerting deviates in yaw due to air gusts or its own against the air transfer valve. As the missile flight characteristics. approaches normal attitude in yaw, the force The elevator controlactuator, shown in exerted by the followup spring is greater than figure 13-11, operates exactly the same as that exerted by the air relay. Now the air trans- the rudder except for the manner in which the fer valve moves to the left and air from the mechanical linkage is connected to the elevator. high-pressure inlet forces the actuator piston The elevator provides pitch control. A similar toward its neutral position; that is, the direc- arrangement is used to operate ailerons for tion of motion of the piston is now from left roll control. to right. This action results in partial move- ELECTROPNEUMATIC.The p n e um atic ment of the control force applied to the rudder. control system just described can be combined As the amount of control is reduced, the force with other systems to refine the control ac- exertedby the followup spring is reduced. tion. For example, electric signal pickoffs are Therefore, the rate of turn and consequently more accurate and dependable than those trans- the rate signal are also reduced. Thus, all mitted by compressed air. They can provide forces are continually being reduced as the a signal voltage that is proportional to dis- missile approaches normal attitude. When nor- placement. They have a decided advantage over mal attitude is regained, the air signal is zero, pneumatic systems in transporting information the followup spring force is zero, and the actu- over wires instead of through tubing. ator piston and rudder are again centered. It is difficult to design a small electric All movement ceases until the missile again motor with sufficient speed and power to actuate 248 .253 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS

RELIEF VALVE

a NCE AND CONTROLLER SENSOR UNIT

AIR BOOST SERVO CYLINDER

ACTUATOR

SIGNAL PATHS

I. TORQUE TUBES MECHANICAL Immo 00 000 AERODYNAMIC ELECTRICAL

OD 11.111 CONTROL SURFACE

FP.252 Figure 13- 12. Electropneumatic control system. missile flight controls. Figure 13-12 illustrates boost cylinder makes it possible to obtain large a control system in which the best features control surface deflections in either direction. of electric and pneumatic systems are used. The sensors for a electropneumatic sys- Electrical equipment is used in the front end tem are electric pickoffs that detect gyro dis- and actuates pneumatic servos at the actuating placement and produce a voltage proportional end. to the heading deviation angle. This voltage Pneumatic controls are comparatively slow is small and must be amplified before it can because air is compressible, and time is re- operate a solenoid and air servo valve. quired to build up enough pressure in a cyl- The change from electric to pneumatic op- inder to move the piston. Since the piston is eration takes place at the air servo valve. The linked mechanically to the flight control, there air servo motor rotates the torque tubes which is a time lag between the control signal and are connected to the control surfaces and ex- the movement of the control. However, this tend into the center section of the missile. slow response can be speeded up by adding This system may be used for either pitch or a booster cylinder, as shown in figure 13-12. yaw control. The increase in response speed is obtained The servoamplifier receives a followup sig- by allowing air to escape, through ports, into nal from the control surface through mechanical a relief valve after the servo valve has moved linkage and electrical voltage. The feedback a certain distance from midposition. The re- voltage cancels the input voltage when the con- lief valve allows high-pressure air to enter trol surfaces have deflected a certain amount. the boost cylinder. Then, the piston of the boost The deflection of the control surfaces is there- cylinder moves parallel with the piston of the fore proportional to the input signal. actuator cylinder to move the flight control ELECTROHYDRAULIC.The electr ohy- surface. The additional force provided by the draulic control system is used more than any 249 FLUID POWER

troller, a hydraulic transfer or servo valve, regulates the amount and direction of flow to 1 the actuator. DRIVE HYDRAUUC RELIEF ACCUMU- Fignre13-14 shows an electrohydraulic 11111(11111111 MOTOR PUMP VALVE LATOR system. for roll control. Although the control surface (ailerons) method of control is shown, COUPUNG systems very similar to this are used to controlthe movement of movable metal rings, 111 movable jets, or rotatable nozzles, mentioned TRANSFER previously,The system depicted here uses RESERVOIR VALVE proportional control only, which means that the controls react to information that shows TO I /I I the deviation of the missile axis from the de- = HYDRAUUC CONNECTION LOAD sired flightpath. The displacement signal is 11111111111111111 ACTUATORI ION MECHANICAL CONNECTION proportional to the deviation. Should roll develop, the gyro will detect FP.253 it and cause the synchro to produce an error Figure 13-13.Basic hydraulic controller. signal.The correction signal to the servoamplifier is the difference between the followup signal (electrical) and the gyro signal, as indi- other type to actuate the devices used for con- cated by the minus sign in the circle between the trolling missile flight.This combination is synchro block and the servoamplifier triangle. similar to the electropneumatic system, except that the actuators are moved by hydraulic fluid The difference signal is amplified and used fluid pressure instead of air pressure. This to operate the transfer valves and regulates removes some of the disadvantages of a pneu- the flow of fluid to the actuator. The piston matic system, since the fluid is not compres- in this actuator operates the ailerons (or other sible. The most important advantages of this rollcontroldevices)through mechanical type system are the high speed of response linkage. and the large forces available when using hy- The equipment represented by the block draulic actuators. labeled "jitter" provides an a-c voltage with Figure 13-13 illustrates a simplified block a frequency of about 25 hertz (cycles per sec- diagram of an electrohydraulic controller. This ond). This is applied to the transfer valve and system is composed of the following components: other equipment, to keep them in constant vi- 1. A reservoir, which contains the supply bration and prevent the friction that may develop of hydraulic fluid. when the parts are not moving. This was ac- 2. A motor, as a source of power for the complished by the sump pump and the oscil- pump. It may be either an electric motor or lator in the hydraulic power drive system dis- an air motor. cussed in the first part of the chapter. 3. A pump, to move the fluid through the system, 4. A relief valve,to prevent excessive pressure in the system. AIRCRAFT FLUID POWER SYSTEMS 5. An accumulator, which acts as an auxiliary storage space for fluid under pressure, All modern naval aircraft contain hydraulic and as a dampening mechanism which smooths systems for the operation of various mech- out pressure surges within the system. anisms. The number of hydraulically operated 6. A transfer (directional control) valve, units depends upon the model of aircraft. At which controls the flow of fluid to the actuator. the present time, the average operational air- In some systems of this type, a servo valve craft has a dozen or more hydraulically op- is used instead of a transfer 41ve. The servo erated units. Many aircraft are quipped with valve provides finer control. pneumatic systems for the operation of certain Variations in pitch, roll, and yaw are sensed mechanisms. The pneumatic system is also by gyro reference units and transmitted elec- utilized as an emergency system in case of trically to amplifiers and computers. The con- hydraulic system failure. 250 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS

rteIIYMPlif.".7.,:

.,'REFERENCE:'''l ;:::i COMPUTER ..AMPLIFIER ;: 13:4..7. 4..0%1 :. hVIert.";15.:1Vif',i i'tft1.1/41,',VeiktPe. ''' JITTER rillit .11 T ;c. SERVO op (...y, x ....,.... $d.iPiN ....:

GU ling MISsl LE AERODYNAMICS

ELECT RICALp HYDRAULIC '''''' '' FP.254 Figure 13-14.Electrohydraulic system for roll control.

A complete aircraft hydraulic or pneumatic erating the flight controls. All aircraft which system consists of a power system and any utilize hydraulically actuated flight controls number of actuating systems (subsystems). have at least two hydraulic power systems, The number of actuating systems depends upon one which supplies fluid pressure to the flight the requirements of the specific model aircraft controls only and another which supplies fluid concerned. pressure to the utility or normal system in The power system is generally considered addition to the flight controls. The utility sys- to include the fluid supply (reservoir or air tem operates the landing gear, wing fold, wheel bottles), power supply (pump or compressor), brakes, and other such units. Most manufac- and all other components leading up to, but turers refer to the systems supplying pres- not including, the selector (directional control) sure only for the flight controls as the power valves.The selector valves direct the flow control systems. If there are three hydraulic of fluid to the various actuating units, and each power systems, they are generally referred selector valve is considered a part of its re- to as power control system 1 (PC-1), power lated actuating system. control system 2 (PC -2), and utility system. The hydraulic and pneumatic system of a Each system is equipped with its own reser- modern naval aircraft are described in this voir, power pump, and lines. section. Only the power systems are described Power systems are designed to produce in detail. Most of the actuating systems, which and maintain a given pressure. The pressure areindicatedin the illustrations, are con- output of the Navy's high performance air. trolled by solenoid-operated four-way selector craftis 3,000 psi. However, some aircraft valves. The actuators are piston type actuating hydraulic systems operate at only 1,500 psi. cylinders or, in some subsystems, fluid motors. Figure 13-15 illustrates a utility hydraulic system. This particular aircraft has two other independent systems which are similar to the HYDRAULIC SYSTEMS utility system and provide fluid pressure for theoperation of the flight control systems. Current aircraft hydraulic system specifi- All three systems operate 'at 3,000 psi. The cations require two separate systems for op- utilitypowersystemprovides fluid under FLUID POWER

- UTILITY PRESSURE

RELIEF =11: UTILITY RETURN VALVE PUMP SUPPLY FILTER EI:3

IMPRESSURE SWITCH

CHECK VALVE PC-1 ELECTRICAL CONNECTION

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ARRESTING HOOK

FP.255 Figure 13-15.Aircraft hydraulic system. 252 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS pressure for those actuating systems indicated ponents. A check valve is incorporated in the in the illustration. The number of actuating line to the brake system. In normal operating systems vary in different types of aircraft. conditions, the check valve allows free flow A description of the utility power system com- from the system to the wheel brake system. ponents and their functions follow. When utilitysystem pressure drops below 3,000 psi, the check valve closes. This main tains a limited amount of fluid under pressure Reservoir in the accumulator for brake operation in the event of system failure. The reservoir is the source from which the hydraulic pumps receive their supply of fluid, and a storage container for the return Accumulators fluid displaced by the actuating components. The reservoir is fluid-pressurized. This in- There are two accumulators in the system sures a supply of fluid to the pumps at all illustrated in figure 13-15. One is the brake times, even when the aircraft is flying at high accumulator. Its function is described in the preceding paragraph under check valves. The altitudes. other accumulator isinstalled in the main system line.Its function is to dampen pump pulsations and, therefore, maintain smoothness Pumps in system operation. In addition, this accum- The flow of fluid to the system is provided ulator assists the pumps by pro!ridfng the sys- by two variable displacement axial piston pumps, tem with a limited supply of fluid ...*atier pres- one mounted on each engine. Each pump reg- sure during peak demands. ulates volume deliveryin accordance with Accumulator servicing is accomplished via system flow demands. The flow from the pump the connecting air charge valve. The attached is ported to a manifold line from which branch pressure gage is used to check the accumulator lines lead off to the various actuating systems. air charge and to determine the degree of system pressurization.

Check Valves Filters The system contains several check valves. A check valve is installed in the discharge line Filters are installed in both the system from each pump. These discharge lines con- pressure and return lines. The filters are of nect to one common line to supply the fluid the full flow type with a bypass valve incor- to the system. These check valves allow free porated. The bypass valve allows fluid to flow flow from the pumps but prevent any back- through the top of the filter housing instead feed of system pressure against the pump out- of through the filter element should the element put.In case one pump fails or is inoperative become clogged. Filters are also installed in because the engine on which it is mounted is various actuating systems. not running,the corresponding check valve prevents the fluid flow provided by the other Relief Valve pump from attempting to motor the idle pump (rotating it in reverse), and possibly shearing A system relief valve is installed in the or damaging the pump-to-engine drive spline. system to protect it from detrimental pres- These check valves also protect both pumps in suresurgesand to limit system pressure the same manner when fluid pressure is sup- buildup in excess of 3,850 psi, by relieving plied to the system from an external power the fluid to return. source during ground checking of the hydraulic system. Check valves are installed in the system Pressure Switches return lines to direct return flow back to the reservoir and to prevent pressure from acting. There are two pressure switches installed against the return ports of other system com- in the utility system. They are installed in FLUID POWER the pressure lines leading from the pumps The air compressor may be driven by an and are isolated from one another by the check electric motor or a hydraulic motor. The sys- valves. When the pressure from one or the tem described in this section is hydraulically other, pump drops below a specified value, nriven. (See fig. 13-16.) the pressure switch completes an electrical circuit that illuminates a warning light in the cockpit. Power Supply

;lest Exchanger!: The aircraft utility hydraulic system provides power to operate the hydraulic motor The heat exchangers are of the radiator driven compressor. The air compressor hy- type and are located in the fuel tanks. The draulic actuating system consists of a sole- hydraulic fluid returning to the reservoir is noid-operated selector valve,tilow regulator, routed through the heat exchangers. The heat hydraulic motor, and two check valves. The energy is transferred from the hydraulic fluid selector valve is equipp.bd with only one sole- to the cooler fuel. noid. When the solenoid is energized, the selector valve allows the system to be pressurized and rut. nts hydraulic motor; when deenergized, PNEUMATIC SYSTEMS iho valve blocks off the utility system pressure, stopping the motor. The pneumatic power system supplies compressed Iraq for vatious normal and emergency The flow regulator, compensging for the pneumatic actuating systems. The compressed varying hydraulic system flow and pressures, gas is stored in m.arage bottles in the ac- meters the flow to the hydraulic motor to pre- tuating system until requtred by actuation of vent excessive speed variation andifor over- the system.These bt dies and the power sys- 1.1:c.ading of the compressor. One (heck valve tem manifold lines are initially charge with is located in the motor case drain line. Since compressed air or niirwren from an external this line is connected to the utility system source through a sint, ..ir charge valve. In return line,the check valve prevents system flight,the at: compressor replaces the air return line pressure from entering the motor pressura and volume lost through leakage, and stalling it. The other check valve, which thermal contraction,and system operation. is an integral part of the selector valve, performs several Emotions. When the selector The pneumatic system provides power for valve is in the deeuergized position, the check +he normal operation of the cockpit enclosures valve will open to allow excessive fluid pres- and emergency operation ofsuch,components sure in the motor supply line to flow to return as landing gear tztension and the brake sys- and will close to prevent excessive return line tem. The number of normal and emergency pressure from entering the valve. When the systems operated by the pneumatic system selectorvalve is in the energized position, varies in different model aircraft. the check valve prevents :xcessive return line The emergency operation is usually con- pressure from entering -the selector valve. trolled by a three-way spool type &lector Excessive pressure acting on the lands of the valve. In most cases,the valve is manually spool could possibly rep,. sition the valve. operated from the cockpit through mechir,ical linkage. The compressed air flows into the shuttle valve which is in the working line of Air C.omi ressr the hydraulic actuating system concerned. The air pressure forces the shuttle valve to Close The air compressor maintains the aircraft the hydraulic inlet port. The air then 1Rows pneumatic system pressurization during flight. to the actuator. It is a tour-stage radial piston compressor. The air compressor is supplied with super- As previously mentioned,the air is supplied charged air from the engine air system. This from the engine air system. The compressor insures an adequate supply of air to the com- compressesthis air and delivers it to the pressor at all altitudeti. system. 254

.*,9 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS

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MOISTURE SPARATOR CROW DOWN COMMON) FP.256 Figure 13-16.Pneumatic power system. Another filter is located in the compressor inlet line. Its purpose is to prevent particles One filter is located immediately upstream of foreign matter from entering the absolute of the air charge valve. Its purpose is to pre- pressure regulator and causing it to malfunc- vent the entry of foreign particles into the tion. The filter is a full flow type (with an system from the ground charging source. The integral relief valve) housed in a cylindrical filter element is a stainless steel, wire mesh, body. The filter element (rated at 10 microns) reusable unit. isofwoven monel wire construction. The 255 SO FLUID POWER

integral relief valve allows air to bypass the expansion). The relief valve opens when the filter element when the pressure differential system pressure reaches 3,750 psi and re- is 50 psi or more. sets at 3,250 psi. THERMOSTAT AND HEATING ELEMENT. Thethermostaticallycontrolled wraparound Absolute Pressure Regulator blanket type heating element prevents freezing of the moisture within the reservoir, due to The absolute pressure regulator is located low-temperature atomospheric conditions. The in the compressor inlet line and regulates the thermostat closes at 40° F and opens at 60° F. pressure of the air entering the compressor. This stabilizes the pressure of the air for the compressor. Safety Fitting The safety fitting assembly, installed in the Moisture Separator inlet port of the _moisture separator, is composed of a flame arrester screen, rupture The moisture separator is the sensor-reg- disc, and housing. Its purpose is to protect ulator and relief valve for the pneumatic power the moisture separator from the introduction system. The moisture separator is capable of flames or hot particles, which could result of removing up to 95 percent of the moisture in an explosion, and to relieve excessive pres- from the air compressor discharge line. The sure buildups. automatically operated condensation dump valve The flame arrester,, a cylindrical shaped purges theseparator oil-moisture chamber fine mesh steel screen, is placed immediately by means of a blast of air (3,000 psi) each inside of the safety fitting inlet port. Its pur- time the compressor shuts down. The separator pose is to prevent the passage of flame or assembly contains seven basic components, hot carbon particles, emanating from a buildup each of which performs a specific function. of carbon deposits in the air compressor. The components are described in the following The stainless steel rupture disc is designed paragraphs. toburstandrelieveexcessive pressures PRESSURE SWITCH.The pressure switch (4,750psi),to prevent a shrapnel-like ex- controls system pressurization by sensing the plosion of the moisture separator. system pressure between the check valve and relief valve. It electrically energizes the air compressor solenoid-operated selector valve Chemical Drier when the system pressure drops below 2,750 psi and deenergizes the selector valve when A chemical drier further reduces the mois- the system pressure reaches 3,100 psi. ture content of the air emerging from the mois- CONDENSATION DUMP VALVE.The con- ture separator. densation dump valve solenoid is energized and deenergized by the pressure switch. When energized,it prevents theair compressor Air Charge Valve from dumping air to the atmosphere; and when deenergized, it completely purges the separator The air charge valve provides the entire reservoir and the lines up to the compressor. pneumatic system with a single external ser- FILTER.The filter protects the dump valve vicing point. An air pressure gage, located port from becoming clogged and thus insures near the air charge valve, is used in servicing proper sealing of the passage between tt e the system. The gage indicates the system reservoir and the dump port. pressure. CHECK VALVE.The check valve protects the system against pressure loss during the dumping cycle and prevents backflow through the separator to the air compressor during FUNDAMENTALS OF TROUBLESHOOTING the relief condition. RELIEF VALVE.The relief valve protects The maintenance of fluid power systems the system against overpressurization (thermal includes servicing, performingperiodic 256 261 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS

inspections, repairing, and testing after repair. plunger (1) upward, forming a partial vacuum The procedures for performing these tasks (low-pressurearea)inthe pump body (1). are usually contained in the applicable technical This low pressure causes the gravity check valve publications for the specific system, equipment, (1) to open and, with the aid of spring tension, or component.Therefore, these publications causes the spring-loaded check valve(1)to should be consulted when performing any of close. With atmospheric pressure acting through these tasks.Another very important task in the air vent on the fluid in the reservoir, the the maintenance of fluid power systems is fluid flows from the reservoir into the low- troubleshooting. Troubleshooting,which is pressure area of the pump (1).During this perhaps the most difficult task, is the procedures same downward stroke ofthe jack handle, involved in locating and diagnosing malfunctions plunger (2) moves downward.This force on in a fluid power system by means of systematic the fluid closes the gravity check valve (2) checking or analysis. Applicable technical and overcomes the spring tension and opens publications usually contain schematics and the spring-loaded check valve (2).The fluid charts which serve as aids in troubleshooting; then flows out of the pump into the ram cylinder however, there are some fundamental proce- and this force, in turn, raises the ram in the dures which will help in troubleshooting most, cylinder. A similar action takes place during if not all, fluid power systems. the upward stroke of the jack handle, except As previously stated, troubleshooting is a that pump (2) receives fluid from the reservoir difficult task. However, with a thorough under- and pump (1) forces fluid into the ram cylinder. standing of the operation of a specific system, The relief valve is often referred to as a with the proper use of the applicable schematics safety valve or a safety bypass valve. It and troubleshooting charts, and with a little serves as a safety factor when a load in experience,effective troubleshooting can be excess of the maximum allowable load is applied accomplished.The knowledge and experience to the ram cylinder. (The maximum allowable gained from troubleshooting one specific fluid loadis10percent greater than the rated power system will serve as an aid in trouble- load. For example, the maximum allowable shooting other fluid power systems; however, load for a jack rated at 15 tons is 33,000 the applicable technical manuals, schematics, pounds.)The relief valve is preset at a pres- and troubleshooting charts must be utilized in sure that will cause it to open and bypass all cases. fluid back to the reservoir in the event excessive Thejack hydraulic system illustrated in pressure isbuilt up in the ram cylinder. figure 13-17 is used in the following discussion The reliefvalve is spring loaded and will to demonstrate the fundamental procedures automatically reseat when the pressure de- for troubleshooting a fluid power system. Since creases to the preset pressure of the valve. a thorough knowledge of the specific system is To lower the jack, the release valve must important foreffectivetroubleshooting,the be turned counterclockwise.Since this is a operationofthe jack hydraulic system is needle valve,the distance that the valve is described first. turned determines the speed of the lowering The major components of this system are operation. The rams should be allowed to the reservoir, release valve, relief valve, pump, lower at a slow and even rate of speed. checkvalves,and a telescoping ram type During operation of this system, assume actuating cylinder.The pump is actually two that the rams will not extend. The first question single-action pumps which are mechanically to consider when troubleshooting this malfunc- linkedthrough a pivot arm to one handle. tion is:What causes the rams to extend from This acts similar to one double-action pump, thecylinder? The obvious answer is that since there is constant flow to the system when hydraulic fluid is the sole item that forces the the handle is moved either up or down. rams to extend.As the ends of the rams To raise the jack, the release valve (a needle extend farther from the cylinder, more fluid valve) must be closedby turning it in a clockwise is required to displace the area in the cylinder direction. This prevents the fluid discharge left by the extending rams.Since hydraulic from the pump from flowing back to the reser- fluid is the important item, the reservoir should voir. During the raising operation, the downward be checked for sufficient fluid. Many manhours motion of the jack handle moves the reciprocating have been wasted on the removal and testing 257 262 FLUID POWER

PLUNGER (i)

PLUNGER (2)

PUMP BODY (1) PUMP BODY (2)

GRAVITY SPRING-LOADED SPRING-LOADED CHECK VALVE (I) CHECK VALVE Ill CHECK VALVE (2)

GRAVITY R LEASE VALVE CHECK VALVE (2) EXTENSION SCREW

FILTER

RELIEF VALVE FILLER CAP (VENTED)

HYDRAULIC RESERVOIR

RAM CYLINDER

LEGEND

FLUID FROM HYDRAULIC RESERVOIR FLUID TO RAM CYLINDER

FP.257 Figure 13-17.Jack hydraulic system schematic. 258 263 Chapter 13HYDRAULIC AND PNEUMATIC SYSTEMS of hydraulic system components only to find the case, the system should be flushed and that insufficient fluid was the cause of the replenished with new fluid. malfunction. If the fluid level is low, the Ifthe valvingelement or the spring is reservoir should be replenished to the proper defective, the valve must be repaired or re- level with fresh clean hydraulic fluid of the placed. After this has been accomplished, recommended specification. Then the jack must the fluid level in the reservoir shouldbe checked be operated several times toassure that the and replenishedif necessary.Whenever a malfunction has been corrected. The fluid level component is removed from a hydraulic system, should then be rechecked. therewill be some loss of fluid from the system.After such a repair or replacement NOTE: Loss of fluid from a hydraulic iscompleted,thejack should be operated system indicates that there is an external leak several times to assure that the malfunction at some point in the system. Whenever a system has been corrected and that there are no requires an excessive amount of fluid,the external leaks. entire system must be checked for external If improper adjustment is the cause of the leaks.Since most leaks will appear only when malfunction, the relief valve must be adjusted. the fluid is under pressure, the system should Most jacks are equipped with a threaded test be in operation during this inspection. port forthis purpose.Normally, the test If there is sufficient fluid in the reservoir, port is closed with a threaded plug. To check the next question that must be considered in the and adjust the relief valve, the plug is removed troubleshooting procedure is: What can prevent and a hydraulic pressure gage installed in the the fluid from flowing into the ram cylinder? test port.Then pressure is applied to the Obviously, the pump assembly (including the system by the operation of the hand pump. To gravity and spring-loaded check valve) must obtain the required pressure, a test load must operate properly.If the pump is operating be applied to the jack rams or, in some cases, efficiently,there are other components that the pressure may be applied with the rams could prevent the fluid from enteringthe fully extended. The relief valve should then be cylinder.These include the release valve and adjusted to the pressure listed in the applicable the relief valve. technical publication. Becauseof its accessibility,the release The pump should be the last component tobe valve should be eliminated first as a probable considered in this type system.In fact, a cause.If the release valve is not completely defective pump may be so indicated during the closed, the fluid will take the path of least check of other components.For example, if resistance and return to the reservoir. it is impossible to build up system pressure Once the release valve has been eliminated during the adjustment of the relief valve, it as a possible cause, the relief valve should be is a very good indication that the pump is checked. First, check the load on the jack defective. If the pump is defective, it must be rams.If the jack is overloaded, the relief repaired or replaced and the system checked valve will perform its designed function by thoroughly. opening and allowing the fluid from the pump Although the sequence of steps may differ, to flow back to the reservoir.If the jack is the foregoing procedures may be used for not overloaded, there are several other probable troubleshooting other malfunctions of jack hy- troubles that could cause thereliefvalve draulic systems. As previously mentioned, to remain open or to open too soon.Dirt or these same procedures may be adapted to most other foreign matter between the valving element hydraulic systems and similar procedures may and its seat will hold the valve open. A bent be used to troubleshoot pneumatic systems. or otherwise defective valving element or a The components may differ as to quantity, broken spring will also allow fluid to flow type, and complexity.For example, instead from the pump to thereservoir.Improper of a ram type cylinder, the actuator may be adjustment may cause the valve to open too a hydraulic motor or adouble-action cylinder. soon. Instead of a hand pump, the power source If dirt or other foreign matter is the cause, may be an electrically driven or gasoline- the relief valve must be removed and cleaned. engine-driven pump. Instead of a release valve, This is an indication that the entire system is the control valve may be a solenoid-operated contaminated with foreign particles.If this is spool type selector valve. The relief valve may 259 FLUID POWER be more complex and additional components, only refinements to a basic system, experience such as an accumulator, regulator, priority gained in troubleshooting a basic system, like valve, etc., may be incorporated in the system. the one illustrated in figure 13-17, will defi- There may be several subsystems which operate nitely serve as an aid in troubleshooting the from one power source. Since these are more complex systems.

280

.265 CHAPTER 14

FLUIDICS

Although simple applications of fluid power accepted term to identify all aspects of this are centuries old, its emergence as a leading technology is fluidics. method of transmitting power has taken place in The term fluidics is derived from two words the last 30 to 40 years. Each year new compon- fluid and logic. Both liquids and gases are used ents and systems are designed and developed to as a fluid medium for fluidics; however, air is meet the demands of new and more sophisticated used in the majority of applications in present applications. In order to utilize the power devel- use. Logic is the science dealing with the oped by hydraulics or pneumatics to accomplish criteria or formal principles of reasoning and the required amount of work at the desired tim thought. As applied to control systems, logic is some means must be provided to control a means of making decisions concerning what power.The directional control valve, which s operation to perform, when to perform the oper- frequently referenced throughout the prece ng ation and how (of several ways) to perform it. chapters and is described in detail in chapter Fluidics, therefore, is a method of controlling 11, is commonly used for this purpose. However, fluids to provide switching signals, sensing, it should be noted that these directional control logic,and other control functions. In other valves are usually controlled manually, mechan- words, fluidicscan replace electricity and elec- ically, electrically, electronically, or combin- tronics in the operation of computerized control ations of these power sources. In other words, systems. Although fluidics is primarily used to fluid power supplies the "muscles" to do work, control fluid power systems, it can also be used but depends upon other sources of power to provide the "brain" to control this "muscle." to control other methods of transmitting power. Such arrangements are satisfactory for most The first part of this chapter defines some applications and will probably be used for many of the terms associated with fluidics. This is years. followed with a section which describes the basic logic functions. The next part of the chapter is Electricity and electronics have been used devoted to the fundamental operating principles extensively in complex automatic control appli- of the different types of fluidic devices. Some cationsfor many years. However, certain of the possibleapplications of fluidics are applications require the control system to be covered in the last part of the chapter. operated at extreme temperatures and in nuclear radiation environment.Such conditions pose severe problems for electricity and electronics. TERMINOLOGY These problems motivated efforts to develop some other means for automatic control. In the late1950's and early1960's research and Since fluidics serves as a replacement for experiments were conducted in the use of moving electricity and electronics in the operation of fluids as a method of automatic control. Much computerized controls, many of the terrrz used progress has .been made since these early in the study of fluidics were adopted from com- experiments. During the early years of research puter and electronic terminology. It is beyond anddevelopment,such terms asfluerics, the scope of this manual to describe the opera- fluonics, pneumonics, and pure fluid systems tion of computers; however, a knowledge of have been used to identify this technology. At certain computer terms is necessary to under- the time of this writint, the most generally Etandthe operation and functionof fluidic 261 6 FLUID POWER

devices. Some of the common fluidic terms prepared by man who has applied the thought are discussed inthe following paragraphs. process to a problem to a point when logic Digital Computer Basics, NavPers 10088 (Se- decisions can deliver the correct answer. The ries), and Basic Electronics, NavPers 10087 rules for the equations and manipulations em- (Series), should be consulted for detailed in- ployed by a computer differ in many respects formation concerning the operation ofcom- from the familiarrules and procedures of puters. everyday mathematics. A fluidiccircuitis usually made up of People use many logical truths in everyday several elements. In this case, the term ELE- life without realizing it.Most of the simple MENT is defined as an indivisible part ofa logical patterns are distinguished by such words function or circuit.There are two basic types as AND, OR, NOT, if, else, and then. Once a of elementsactive and passive.An ACTIVE verbal reasoning process has been completed element requires a continuous supply to enable and the results put into statements, the basic itto be operated by input signals, whilea laws of logic can be used to evaluate the proc- PASSIVE element requires no supply andcan ess.Although simple logic operations can be be operated by input signals. performed by manipulating verbal statements, One of the important elements of fluidic the structure of more complex relationships circuits is an amplifier. A FLUID AMPLIFIER can be more usefully represented by the use of is an element which enables a flowor pressure symbols.Thus, the operations are expressed to be controlled by one or more input signals by what is known as symbolic logic. .which are of a lower pressure or velocity of The symbolic logic symbols utilized in dig- flow than the fluid being controlled.A fluid ital computers are based on the investigations amplifier in which the movement of parts within of George Boole, and the resulting algebraic the element controls fluids is called a MOVING system is called Boolean algebra. It is similar PART AMPLIFIER.Most fluidic devices con- in some respects to standard algebra; however, tain no moving parts.Thus, a fluid amplifier it follows different laws and rules. which controls fluid with no moving parts is called a PURE FLUID AMPLIFIER. An amplifier which will give eithera full TRU'Ill TABLES output or no output (either ON or OFF),according to the input signals applied, is referred A truth table is a chart used in conjunction to as a DIGITAL AMPLIFIER.An element with logic circuits to illustrate the states of in which the output can be continuously varied the inputs and outputs of a given stage under all by increasing or decreasing the value ofthe possible signal conditions.It provides a ready input signal is called an ANALOG AMPLIFIER. reference for use in analyzing the operating Most elements have several input ports. theory of the circuit, and is useful in developing The fluid can be controlled by applying a signal the overall signal flow diagram. to any one of these ports. The number of inputi3 In devising a truth table, it is necessary to available on a specific element is calledthe know the number of inputs.All possible states FAN-IN RATIO. FAN-OUT RATIO is thenum- of the inputs are listed in column form, with a ber of elements which can be controlled bya separate column for each input. The output of single element of the same type. each combination of possible input states is In some cases, one element will provide the then determined and noted in the output column. required output at the desired time. However, in To illustrate the truth table, consider the most cases, an assembly of logic elements isre- circuits made up of two-way valves illustrated quired to produce the required output whencer- in figure 14-1.These are normally closed tain conditions are satisfied. In other words, the valves, since the spring forces the spool up- required output is obtained if,, and only if, the ward, closing the ports until an input pressure correct input signals are applied. Suchan as- is applied to the top of the spool.Since the sembly is referred to as a LOGIC FUNCTION valves are connected in series, all the valves or GATE. must be open to obtain an output.This is COMPUTER LOGIC accomplished by applying an input signal (fluid under pressure) to each valve in the series Computers never reason why or think out simultaneously.This input signal is indicated an answer; they operate only on instructions by the arrow at the top of each open valve. 262 Chapter 14FLUIDICS

SUPPLY OUTPUT SUPPLY

OUTPUT -till:1-13111±21\ SUPPLY SUPPLY , \ N = .>- OUTPUT

FP.258 Figure 14-1.Two-way valves connected in series.

A truth table for the two-valve circuit illus- puts and outputs.Using the binary symbols to trated in the upper portion of figure 14-1 is construct the truth table of the three-valve shown in table 14-1. In constructing the table, circuit illustrated in the lower portion of figure all possible input combinations are placed in 14-1 would result in table 14-2. column form, and the output for each combina- NOTE:In this type circuit, the number of tion is determined and noted.(NOTE: Only possible input combinations in the truth table two of the possible combinations are shown will be 2n, where n = number of input lines. In in figure 14-1.) table 14-1, two inputs were considered, therefore, the table contains 22 (2 x 2) or 4 combina- Table 14-1.Truth table for two, two-way tions.In table 14-2, three inputs were con- valves connected in series. sidered; therefore, the table contains 23 (2x2 x 2) or 8 combinations. Output signal Valve A input Valve B input present Table 14-2.Truth table symbols for three, two-way valves connected in series. NO NO NO NO YES NO A B C YES NO NO YES YES YES 0 0 0 0 0 0 1 0 0 1 0 0 In computer logic circuit truth tables, the 0 1 1 0 column headings normally contain letter, des- 1 0 0 0 ignations for the input and output, while the 1 0 1 0 "Yes" and "No" are replaced by symbols to 1 1 0 0 denote the state of the inputs and the output. 1 1 1 1 The symbols most commonly used are the binary l's and 0's.(The binary number sys- The meaning ofthesymbolsin table 14-2 tem has two symbols (0 and 1) and has two as is as follows: its base just as the decimal system uses ten A, B, and C = Input of respectivevalves. symbols (0, 1, ...,9) and a base of ten.) Plus F = Output signal. and minus signs and H and L (High or Low) are 1 = Signal present. sometimes used to denote the state of the in- 0 = No signal present.

263

. 268 FLUID POWER

BOOLEAN ALGEBRA Table 14-4.Combinations of logic symbols. Boolean algebra is a method of dealing with Operations Meaning logic problems in a mathematical way.It is used to determine the "truth value" of the (A + B) (C) A OR B, AND C combination of two or more statements.As Boolean algebra is based upon elements having AB + C A AND B, OR C two possible stable states, it becomes very useful in representing switching circuits. The A B NOT A, AND B reason for this is that a switching circuit can be in only one of two possible stable states at A +B A OR NOT B any given time; that is, a signal (either input or output) is either present or not. These two states may be represented as 1 and 0, respectively.As the binary number system consists LOGIC OPERATIONS of only the symbols 0 and 1, these symbols can be used with the Boolean algebra. The main logic functions are AND, OR, NOT, NOR, and NAND.The first threeAND, OR, In the mathematics with which most people and NOTare basic logic operations, while are familiar, there are four basic operations NOR, NAND, and others are combinations of addition, subtraction, multiplication, and divi- the three basic functions.These five logic sion.In Boolean algebra there are three basic functions are illustrated in figure 14-2.In- operationsAND, OR, and NOT. If (these words cluded are theswitching circuit,the truth do not sound mathematical, it is only because table, and. the block diagram for each function. logic began with words, and not until much The switching circuits are illustrated with later was it translated into mathematical terms. three-way control valves. The two positions of The basic operations are represented in logical the valve are shown in cutaway views at the equations by the symbols in table 14.3. top of the illustration. Each view is accompanied with its corresponding symbol, which is used to Table 14-3.Logic symbols. represent the position of the valve in the switching circuits.With no input (fluid pressure) Meaning applied to the signal port, as shown in the top left-hand view, the spring forces the spool up- A AND B ward, opening the lower inlet port to the outlet port. In this position, the top supply port is A OR B closed to the outlet port. When an input signal is applied, as indicated by the arrow in the A NOT or NOT A right-hand view, the spool moves downward, compressing the spring.In this position, the top supply port is open to the outlet port and The OR operation is indicated by the addition the lower supply port is closed to the outlet symbol, while the AND operation is indicated by port. It should be noted that the output also the multiplication symbol. In addition to the dot, depends upon which supply port is used to sup- parentheses and other multiplication signs are ply fluid to the valve. This is indicated by sometimes used for the AND operation.The either an arrow or a connecting line between negation function may also be indicated by an valves in each of the switching circuits. apostrophe following the letter (A') instead of The operation of these functions is described the dash over the letter (A). Examples of com- in the following paragraphs. binations of these symbols are given in table 14-4. AND Function For an extensive coverage of the binary This function requires that all input signals number system and Boolean algebra, including are present before an output is possible. Thus, simplification techniques, refer to Mathematics, A AND B must be applied simultaneously to pro- Vol. 3, NavPers 10073 (Series). vide an output.This may be written A B = F 264 -

/7. ,ivy. ',der,/ 2 /2 42 /2/4:/.. 4/A ...

, 1 . . . ,,,,.,,, .-,

. : AI -

....;--11

// //, /7////.

S FLUID POWER and read A AND B = F.If either input signal function. That is, with no input signals present A or B is not present, there will be no output. in the respective circuit, the OR circuit pro- This is written A + B = F and is read NOT A vides no output, while the NOR circuit provides OR NOT B = NOT F. The same results may be an output.One input signal in either circuit accomplished with additional input valves con- will reverse these results.That is, one input nected in series; however, the possible com- signal applied to an OR circuit provides an binations of inputs to output (truth table columns) output, and one input signal applied to a NOR will increase.For example, the two circuits circuit results in no output. illustrated in figure 14-1 are AND circuits. The NOR function may be written A.B = F The comparison as to the possible combinations and A + B = F and is read NOT A AND NOT of the two-valve circuit and the three-valve B = F and A OR B = NOT F. circuit can be seen in the truth tables illustrated in tables 14-1 and 14-2. NAND Function OR Function In the NAND function all input signals must In the OR function, any one of a number of be applied to stop the output.The removal of input signals will provide an output. Although any one of the possible input signals will pro- there are only two valves shown for the OR vide an output.This is similar to the NOT switching circuit in figure 14-2, any number function in which the input signals are applied of valves (possible input signals) connected in to stop the output.The NAND function is the a like manner would provide the same results. inverse of the AND function in which all input Of course, additional input signals will increase signals must be applied to provide an output. the truth table columns at a rate similar to The NAND function may be written A+ B = F that of the AND function.In this function, and A B = r and is read NOT A OR NOT B = F either signal A OR signal B will provide an and A AND B = NOT F. output at F.This may be written A + B = F and is read A OR B = F.If there is no signal Flip-Flop Circuit at both A and BLthere will be no output at F. This is written A B = F and is read NOT A Another circuit used in logic operations is AND NOT B = NOT F. the flip-flop or memory circuit.An example of a simple flip-flop circuit is illustrated in NOT Function figure 14-3. The valve symbol used in this illustration is the same three-way valve symbol In the NOT function, an input signalproduces used in the switching circuits in figure 14-2. no output, while no input sipal produces an The fluid supply is connected to the upper output.This is written A = Y and A = F, and inlet ports of valves A and B. The output is read NOT A = F and A = NOT F. port from valve A is connected to the input signal port of valve B and the output line F NOR Function from valve C.The output port of valve B is connected to the lower port of valve C, which The NOR function requires that all input is normally open to the output port. signals are removed before an output is possible. That is, neither A NOR B can be present if output F is required.This function is con -. sidered a combination of the AND and OR functions. The NOR circuit is similar to the AND circuit, in that both require manipulation of all input signals to provide an output.However, they differ in that all input signals must be applied to obtain an output from the ANDcircuit, while all input signals must be removed to obtain an output from the NOR circuit.The FP.260 NOR function is just the inverse of the OR Figure 14-3.Flip-flop (memory) circuit. 266 271 Chapter 14FLUIDICS

A brief input signal at A, as shown in figure applications.This type device is discussed 14-3, will allow supply fluid to flow through latter in this chapter. valve A.This output will apply an input signal In addition to the logic operations described at B, allowing supply fluid to flow through valve previously, a fluidic system alsorequires B, through valve C, and provide output F. sensing devices to provide the initial inputs When the input signal at A is removed, the for the logic operations. Thus, fluidic devices output F continues since this output continues may be grouped into two major categories to apply an input signal at B. The lower inlet logic devices and sensing devices. port of valve A is capped so that the output F is not exhausted when input signal A is removed. A brief input signal at C will stop the output F LOGIC DEVICES and remove the input signal B. Thus, a flip-flop is a means of converting Several methods can be used to classify a brief input signal into a continuous signal fluid logic devices as to type.For example, which can be removed by another brief input they are sometimes classified in two general signal.In the circuit presented in figure 14-3, typespure fluid devices and moving part de- a brief input signal at A provides a continuous vices. Another method of classifying these output at F, and a brief input signal at C will devices is by the basic principle of operation. stop the output F. Using this method, there are five basic types wall attachment,jetinteraction,turbulence amplifier, vortex valve or amplifier, and mov- FLUIDIC DEVICES ing part devices.These five types are described in the following paragraphs. The two- and three-way valves used to illustrate the switching circuits in the preceding Wall Attachment sections show that fluid power components can be used to perform the logic operation, pre- An interesting experiment can be conducted viously limited toelectrical and electronic by placing a spoon under a stream of water in circuits. The first so-called fluidic devices the position as shown in figure 14-4.The were of this type; that is, the device contained stream of water will flow down the convex sur- moving parts.However, in many possible ap- face of the spoon, as illustrated. When a person plications of fluidics, space and weight are very places his hand under a similar stream with critical factors; therefore, moving part devices, such asvalves, diaphragms, etc., must be miniaturized.Reducing the size of component parts increases the accuracy requirements. As pointed out throughout this manual, the sliding surfaces of mating parts in fluid power components must be accurately machined. This becomes much more difficult with small parts. Thus, the difficulties encountered in the design and manufacture of small parts and components result in very expensive systems. In addition, moving parts are subject to wear, resulting in excessive clearance between mating parts.This will diversely affect the extreme precision and accuracy demanded by many applications. Therefore, the replacement of components and parts is very expensive. Because of these and other limitations of most moving part devices, most fluidic systems are made up of pure fluid devices; that is, devices with no moving parts.It should be mentioned FP.261 atthispoint,however, that some types of Figure 14-4.Wall attachment of fluid flow. movingpartdevicesare used incertain 267 FLUID POWER the elbow in any position lower than his hand, of the pipe; that is, the fluid in the center of water will flow down his arm before riripping the stream flows at the greatest velocity and off.In a like manner, when painting a t ening, has no velocity along the wall of the pipe. paint that falls on the hand will flow dawn the This is due to the friction uetween the fluid arm before dropping to the floor. all of and the wall.This same principle applies to these instances, the flow of fluid attaches itself wall atta.chment. to a surface.This is called wall attv.chment. Referring to figure 14-5, as the fluid touches Such terms as atmospheric pressure, partial the surface, the fluid nearest the surface is vacuum, and velocity of flow, which are des- slowed down (stopped) by friction.As shown cribed in chapter 2 can be used to explain Elie by the arrows, the velocity of flow is greater phenomenon. Referring to figure 14-4, as the farther the fluid is from the surface. With the water flows from the faucet, it attracts the these different velocities over the surface, a air around it.This causes the air b move in spaceisleft near the surface. The fluid the same direction.There is unlimited space flowing above this space moves downward to around most of the outside surface area of the fill the space.This causes the fluid to flow stream; therefore, there is an adequate supply in a circular motion which forms a whirlpool. of air to replace that extracted by the action This whirlpool is referred to as a vortex bubble. of the stream.The space between the stream (Vortex may be defined as P. mass of fluid hav- and the spoon, however, is limited; thus, the ing a whirling motion which tends to form a supply of replacement air is restricted. This cavity or partial vacuum in the center and to results in a low-pressure area or partial vacuum draw toward this partial vacuum any substance between the spoon and the stream.Since the thatit,subject to its action.)This partial pressure of the air around the stream is =- vacuum area is responsible for the continued balanced,the stream will move toward the attachement of the fluid flow to the surface. spoon. As the stream gets closer tothe str.face This phenomenon was first described by of the spoon, the greater the attraction will be Henri Coanda in 1932.For this reason, wall since the space between the stream and spoon attachment is often referred to as the "Coanda becomes smaller and smaller and, therefore, effect" and the vortex bubble as the "Coanda the supply of replacement air becomes less and bubble." less. By controlling the vortex bubble, the wall Once the stream touches the spoon, it will attachment principle can be used to perform continue to flow along the surface. When a flow logic operatione The bubble can be controlled of fluid flows from an orifice toward a surface, in several ways.One method of control is it will remain attached to the surface instead utilized in the wall attachment fluid logic device of bouncing off.The reason for this is illus- illustrated-in figure 14-6.It should be empha- trated in figure 14-5 and is explained as follows. sized that the shape of the vents, which help As explained in chapter 2 and illustrated in to stabilize flow, and the passages are very figure 2-27, the velocity of flow through a pipe important for the correct function of these varies from the center of the flow to the wall devices.

VORTEX BUBBLE FP.262 Figure 14-5.The principle of wall attachment. 268 .23 Chapter 14FLUIDICS

View (A) of figure 14-6 shows the wall attachment device with no input signals applied. VENT This device is designed in such a manner that, (A) INPUTS VORTEX BUBBLE with no signals applied, the fluid flowing from A(0) B(0) supply forms a vortex bubble along the wall of the output passage Fl.Therefore, the fluid flow attaches itself to the wall and provides an output at Fl with no output at F2. In this type SUPPLY OUTPUTS device, the flow is said to be biased to output Fl. F2(0) If a small input signal is applied at either A or B, as shown in views (B) and (C), respec- VENT POWER tively, the fluid is dislodged from the wall of NOZZLE VENT passage Fl and is forced to attach itself to the wall of the lower passage and flow out through output F2.The input signal required (B) to change this direction of flow is of a much Asp No) smaller value than that of the output. There- fore, the action of this device is that of an Fl.(0) amplifier,which controls larger flows and pressure with smaller input signals. Since the output of this device is much larger than the required inputs, the output can be utilized as F2(1) input signals to control several other devices of the same type.As described previously, the number of input signals that the output can supply is called the fan-out ratio.This is a monostable .device; that is, a continuous input signal at A or B is required to provide an (C) output at F2. Am Nu The wall attachment device shown in figure 14-6 is sometimes referred to as an OR/NOR F1(0) device.F2 is the OR output and Fl is the NOR output. An input signal must be applied at either A OR B to get an output at F2. There- fore, A + B = F2 and A B = F2. To obtain an F2 (i) output at Fl, neither input signal A NOR B can be present. Therefore, A B = Fl and A + B = Fl. FP.263 This device will also perform the NOT func- Figure 14-6.Wall attachment device tion.This is accomplished by using only one monostable. input signal, for example A, and considering Fl as the required output. .With no input signal at A, there will be an Output at Fl. With an input signal applied at A, the flow is diverted NOTE: The presence and absence of in- to output F2, with no output at Fl. Therefore, put and output signals are indicated in figure = Fl and A =Pl. 14-6 by the subscript after the letter.The Regardless of the type of device, all logic subscript consists of either the binary num- functions cannot be performed by a single ele- ber 0 or1 enclosed in parentheses.For ment. However, by using several elements example, A(0) indicates that there is no signal of the same type, different circuits can be present, while A(1)indicatesthat a signal built up, and changed, to suit different require- is present.This method of indicating signals ments by simply altering the connection ar- is used in most of the remaining illustrations rangement.For example, several wall attach- in this chapter. ment OR/NOR devices can be connected in 269 274 FLUID POWER . different arrangements to satisfy different re- OR/NOR elements as shown in figure14-8. quirements.The AND, NAND, and flip-flop The elements are arranged in such a manner operations can be provided in this manner. that one of the outputs of each element is con- Figure 14-7 illustrates one method in which nected to the input of the other.In this ar- three wall attachment OR/NOR devices can be rangement, only one element can provide an arranged to provide the AND and NAND opera- output at any one time. tions..(The devices are shown in schematic A brief input at A will direct the flow to form with the fluid flow indicated by the broken the lower passage of element 1, thus preventing lines.)In this arrangement, only one input in an output at Fl.Since output Fl is connected each of elements 1 and 2 is utilized. Consider to input C of element 2, and there .is no input Fl as the AND output and F2 as the NAND at B, the fluid flows from supply through the output. upper passage of element 2 and provides an With no input signals at A and B, elements output at F2.Output F2 is connected to input 1 and 2 will switch element 3 by supplying D of element 1; therefore, this input at D will signals at C and D.Therefore, the flow from maintain output at F2 after the input at A is supply through element 3 is directed to F2. removed.If input B is applied, output F2 will Both input signals C and D must be removed stop and the flip-flop will change state to pro- from element 3 before fluid can flow through vide outputFl. One point that should be the upper passage Fl.Since the outputs from emphasized is that a flip-flop of this type will elements 1 and 2 supply the input signals C provide an unpredictable output when the supply and D, respectively, the flow through both ele- is first applied. ments 1 and 2 must be directed away from the The circuit illustrated in figure 14-8 shows upper passages.This is accomplished by ap- that the flip-flop operation can be performed plying input signals at both A and B, as shown with wall attachment OR/NOR elements. Such in figure 14-7. Thus, A AND B must be applied arrangements are used in applications to per- to obtain an output at Fl and both must be re- form this operation.However, the most com- moved to obtain an output at F2.Therefore, mon method of achieving a flip-flop operation A B = Fl and A + B = Fl, which is the AND with wall attachment devices is by using the function. Using F2 as the NAND output, A B = single device illustrated in figure 14-9. and A + B = F2. Figure 14-9 shows this device with an input A flip-flop or memory circuit can be ob- signal applied at A and no input signal at B. tainedbyconnectingtwo wall attachment Signal A forces the flow from supply to the wall

SUPPLY 0(o)

SUPPLY -- =`%, 8(1) F2(0)

SUPPLY

FP.264 Figure 14-7.Wall aftachment OR/NOR devices arranged to perform .AND/NAND operations 270 Chapter 14FLUIDICS YU

undesirable characteristic to aerodynamics. The flow of air over an airfoil tends to attach itself to the surface.A thin layer, called the boundary layer, next to the surface has no velocity. As a result, boundary layer con- SUPPLY trol devices, such as porous surfaces, suction slots, or special attachments, are often incorporated in or on the wings and other airfoils X10) of aircraft to eliminate this boundary layer. The term boundary layer is sometimes used F2(0 to identify the wall attachment device. SUPPLY - - - . The wallattachment,devices designed to perform logic functions are usually very small in size.For example, one type of OR/NOR FP. 265 deviceisapproximately 1 1/2 inches long, Figure 14-8.Wall attachment OR/NOR 1 inch wide, and 1/8 inch thick, excluding devices arranged to perform flip-flop fittings.The size of the power nozzle in this operation. device is approximately 0.010 inch wide by 0.040 inch deep.Such devices can operate at pressure from 1 to 20 psi, but the normal working pressure is from 2 to 3 psi. The signal pres- POWER NOZZLE INPUT LOAD SPLIT TER sure varies with the supply but is generally A A(1) VENT / between 5 percent and 15 percent of supply pressure. OUTPUT. Several different materials may be used to F10) manufacture these devices; however, plastic SUPPLY material is the most popular.The passages are formed in one sheet of plastic with the ports OUTPUT opening on the opposite side. Another sheet of F2 113 plastic is placed over the passages and the two sheets are then secured to each other.The INPUT LOAD ) VENT VORTEX BUBBLE area around each external port i8 embossed to provide connection to other devices. Clear FP.266 plastic tubing ("spaghetti") is commonly used Figure 14-9.Wall attachment device to connect the different ports. Plastic caps bistable. are provided tocover any ports not used. In many cases, several elements are manufactured in one block, which is similar to the of the lower passage. Here, the flow forms the printed circuits used in electronics. Suchblocks vortex bubble, as shown, and attaches itself are referred to as integrated circuit blocks or to the wall, giving an output at F2. After the printed circuits. input signal at A is removed, the flow will remain in this passage until a brief input is Jet Interaction applied at B.This input will dislodge the flow from the lower passage and force it to attach When using a garden hose during a windy itself to the upper passage.This changes the day, the stream of water moves around in every output from F2 to Fl. This device is bistable; direction, seldom falling in the desired area. that is, alternate outputs can be obtained by This, of course, is caused by gusts of wind brief input signals.Like that of the OR/NOR acting against the stream of water. By control- device, the design and shape of the splitter, ling a similar action in a confined space, logic vents, and poWer nozzle are very important operations can be performed.This type of for stable operation. . fluidic device is commonly referred to as jet The principle of wall attachment is not interaction. limited toany particular sizeof surface. The operating principles of the jet interac- For example, wall attachment produces an tion device are illustrated in figure 14-10. 271 FLUID POWER

(A) F2 (0) (A)

SUPPLY 10111.11111 F1(1) SUPPLY F1(1)

F2 (0)

I INPUT A(0) (B)

SUPPLY

PP.268 Figure 14-a.Jet interaction OR/NOR I device. I INPUT A(t) I FP.267 passage, thus, changing the output from Fl to Figure 14-10.Jet interactionprinciples F2.View (B) shows the path of flow with an i of operation. input signal applied at A. ti This type of jet interaction device is very similar to the wall attachment device shown View (A) shows the device with no input signal in figure 14-6. The main difference in the two at A.The flow of fluid from supply is in the devices is the method used to control the flow. form of a power jet which drives the fluid The same combinations of inputs provide the across the gap and into the passage to provide same outputs in both devices. This device can an output at Very little of the fluid is lost provide all the logic operations in a manner across the ga . similar to that of the wall attachment device If an oppo signal is applied to the jet of which was described and illustrated previously. fluid as it flows across the gap, the jet is directed away,from the passage. This is shown Another type of jet interaction device is in view (B) of figure 14-10.- The input signal illustrated in figure 14-12.In this device the applied at A diverts the let stream from the input signal ports oppose each other. With lower passage and into the upper passage, giv- no inputs applied, the fluid will flow from sup- ing an output at F2. Here again, the input signal ply and through the gap.This flow will be can control a Jet of a much higher. value. equally divided by the. splitter giving equal Therefore, this device is an amplifier. outputs at Fl and F2. One method of applying the jet interaction With an input at A, the flow is diverted principles to fluidic devices is shown in figure toward output F2. However, the distance 14-11. With no input signals at4 or B, as shown that it is diverted depends upon the value of in viewl (A), the fluid flows across the gap, the input.As the value of the input increases, providing an output at Fl.. With an input signal the value of the output at' F2 increases and applied at either A or B, the signal flow diverts the value of output Fl decreases.An input. the flow from the upper passage .to. the lower ; signal at B will affect the output in the same 272 Chapter 14FLUIDICS

applied at A and B simultaneously, the fluid .will flow out the center dump with no outputs present.In some devices of this type, a third A output is provided instead of the center dump. All of the devices described thus far are Fi active elements; that is, each device requires a constant power supply for operation.Figure 14-14 shows how the jet interaction principle SUPPLY may be applied to provide a passive element. In this type device, the supply is not constant, since it is furnished by the input- signals. To F2 get an output, both input signals, A ANDS, must be applied simultaneously; therefore, this device can be used to obtain the AND function. The collision of the two jets divert each other FP.269 Figure 14-12.--Jet interaction device proportional.

VENT manner, except that as the signal increases, A the output at Fl increases and the output at F2 decreases. Ft If input signals are applied at A or B simultaneously, the values of the inputs determine SUPPLY the values of the output.If the value of A and B are equal, the two outputs are equal. If the value of input B is greater than A, output Fl F2 will be greater than output F2, and if the value 9f input A is greater than B, output F2 will be CENTER DUMP VENT greater than output Fl. Since the input signals can control a jet FP. 270 of a much greater valuethis device is also Figure 14-13.Jet interaction proportional an amplifier. However, in the devices des- device with center, dump. cribed previously, the input and output signals are either present or not present. Therefore, these devicesare called digital amplifiers. In this device(fig. 14-12), the values of the signals vary,with the value of the outputs proportional to that of the inputs.Therefore A(u) this device is an analog amplifier.(Digital and analog amplifiers are defined earlier in this chapter.) Figure .14 -13 shows a variation of the proportional jetinteractiondevicein which a Fu) center dump vent is added. This device is proportional in that an input signal at A will give an output at F2, and the greater the value of the input signal, the greater the value of the output at F2. The same results can be obtained . from input B and output Fl. With no signals applied, the fluid will flow through the gap and out throughthe center dump. FP.271 Thus, no output signals are obtained with no Figure 14-14.Operation of jet interaction input signals applied.If equal signals are passive element. 273 FLUID POWER enough to cause the flow to give an output illustrates the operating principles of the turbu- through the center passage. lence amplifier. The size, operating pressures, and con- The characteristics of turbulent and stream- struction materials of the jet interaction devices line (laminar) flow are described in chapter 2 of are similar to those described for the wall this manual. If a fluid, at the correct pressure attachment devices.Also, like that of the wall and volume, is applied to one end of a pipe of attachment. devices, the design of the jet inter- the correct proportions, it is possible to pro- action devicesis very important.This is vide a laminar flow through the pipe. The especially true of such items as the shape, results of these conditions are shown in the size, and relative location of the vents, the supply lines in both views of figure 14-15. When splitter, and all the passages in the control the fluid emerges from the end of the pipe, it area.Unlike the wall attachment devices, the will continue to flow in this laminar state for jet interaction devices are designed in such a a considerable distance beforeitbecomes manner as to discourage wall attachment. turbulent. With no input at A, as shown in view (A) of figure 14-15, the fluid will flow Turbulence Amplifier across the gap and provide an output at F. Since the velocity of flow is greatest at the In some respects, the operating principles center of the stream, fluid loss across the gap of the turbulence amplifier are similar to that is very small.If a jet of fluid is applied at of jet interaction. In both types of devices, input A, as shown in view (B), it will collide the output is determined by the interaction of with the laminar flow from supply. This action the supply jet with that of the input jet(s) as causes the flow from supply to become tur- the supply jet flows across a gap.However, bulent, thus preventing an output at F. in the turbulence amplifier, the input jet does This type of device is very much an ampli- not divert the supply jet as a stream to control fier becauseitcan be controlled by much output, but changes the flow conditions of the smaller signals than the outputs obtained. That supply, jjet to prevent an output.Figure 14-15 is, the value of the input signals necessary to

OUTPUT SUPPLY F(t)

II INPUT A(0) (A)

,`v OUTPUT] SUPPLY ..flok! F10) 10:44 !

INPUT A(1) (B) FP.272 Figure 14-15.Operating principles of turbulence amplifier. 274

4 Chapter 14FLUIDICS interrupt the laminar flow is extremely small as compared to the value of the output. Another characteristic of this type device is that there is a difference in the time required (response time) to obtain an output and the response time to stop an output. That is, it requires a longer period of time for a turbulent flow to become SUPPLY laminar than it requires for a laminar flow to become turbulent. As a result, it takes VENT longer to obtain an output than it takes to stop (A) an output.For example, in one design of this type device,theresponse time is about 2 em milliseconds (2 thousandths of a second) to stop an output and about 5 milliseconds to obtain an output. Because the input signals have no effect on SUPPLY No) each other, several input ports can be positioned around the area where the stream crosses the gap.Therefore, it is possible to combine several inputs into one device. Although the number varies in a few designs, most turbulence amplifiers in present use contain four input FP.273 ports. Figure 14-16.Turbulence amplifier with The operation of a turbulence amplifier con- four inputs. taining four inputs is illustrated in figure 14-16. View (A) shows the device with no signals applied.In this condition, the laminar streammust be removed to stop the output at F. Thus, flows across the gap and provides an output at F.A signal applied at any number of the four Figure 14-18 shows how three turbulence inputs, for example input B in view (B), inter-amplifiers can be arranged to provide the rupts the laminar flow and stops the output atAND operation. To simplify this example, only F.Neither A NOR B NOR C NOR D can befour inputs are utilized for the three elements. present if an output is required.ThereforeInput A is used in element 1, input B in element A IT r . D = FandA+B+C+D=F,2, and inputs C and D in element 3. The out- which is the NOR function.As a result, thisput from element 1 provides the input at D, deviceis commonly referred to as a NORand the output from element 2 provides the in- device or a NOR gate. put at C.To obtain an output at F, there must be no inputs at C and D. Consequently, there By using only one input signal, the turbulencemust be no outputs from elements 1 and 2. To amplifier can provide the NOT function.Forstop both of these outputs, input signals must example, if input A is used, the results wouldbe applied to A of element 1 AND B of element be the same as those illustrated in figure 2. Thus A B = F and X = f, the AND 14-15.With no input signal at A, there is anfunction. output at F.With a signal applied at A, there Figure 14-19 shows an arrangement in which is no output at F. Therefore, A = F and A = Y.six turbulence amplifiers can be connected to The OR function can be obtained by con-provide the NAND function.This function can necting two turbulence amplifiers.This ar-be provided with a lesser number of elements; rangement is shown in figure 14-17. A signalfor example, elements 1, 2, 5, and 6 could pro- applied at any of the inputs, A OR B OR C OR D, vide the same results.However, by using six will interrupt the flow through element 1. Since elements, all of 'the possible inputs in element there is no output from element 1 to provide an5 are utilized. input signal at E, the laminar stream flows As stated previously in this chapter, the through the gap of element 2 uninterrupted andNAND function is a combination of the NOT provides an output at F. Therefore, A+ B + C '+function and the AND function.In the NAND = F.All of the input signals to element 1 function, illustrated in figure 14-19, each of five 275 FLUID POWER

D ANY INPUT (1)

SUPPLY *a.4444.

A,1

SUPPLY F(i)

FP.274 Figure 14-17.Turbulence amplifiers arranged to provide the OR function.

F(i)

FP.275 Figure 14-18.Turbulence amplifiers arranged to provide the AND function. elements--1,2,3,4,and '6- provides NOT element 6.In this case K is the only input operation. The of five elements - utilized. Therefore, K = F and K = F. The 1,2,3, 4, andand 5provides AND operation. output from element 5 provides the input signal With the addition of two eleinents, this combina- to K.With no input signals applied to element tion is the AND circuit illustrated in figure 5, as shown in figure 14-19, there is an output 14-18. which applies an input at K.Thus there is Referring to figure 14-19, to obtain an out- no output at F.F.,'If a signal is applied to any put at F, there must be no input applied to of the inputs G, H, or J, the output from 276 Chapter 14FLUIDICS

FP.276 Figure 14-19.Turbulence amplifiers arranged to provide the NAND function. element 5 stops.Thus the input to element 6 provides an input at B, etc.Therefore, all is removed, providing an output at F. four doors must be closed before the output at The outputs from, elements 1, 2, 3, and .4 F ceases and allows the brakes to be released. provide the inputs to element 5.Thus all of This is the condition of the circuit shown in these outputs must be removed to provide an figure 14-19./ output from element 5 and prevent an output A flip-flop circuit utilizing two turbulence at F.If any one of the signals at A, B, C, or amplifiers is shown in figure 14-20.This D is removed,_there_ will be an output at F. circuit is very similar to the wall attachment Therefore, A B C D= F and A +B+C +D= flip-flopcircuitillustrated in figure 14-18. 7, which is the NAND function. Referring to figure 14-20, a brief signal at To better understand the NAND circuit input A stops the output Fl from element 1. illustrated in figure 14-19, assume that the This also removes the input signal from D output from F provides a signal to apply the of element 2.In this condition, element 2 pro- brakes on a 4-door automobile and that the vides an output at F2 and also provides an input doors must be closed before the brakes can be at C of element 1. The circuit in figure 14-20 released.Also assume that the closing of each is shown in this condition. door provides an input to 'one of the four ele- With the circuit in this Condition, there ments (1,2, 3, or 4); that is, one closed door are two input signals applied to element 1; provides an input at A, another closed door. therefore, input A can be removed. Input C will FLUID POWER

C11)

F2(i)

FP.277 Figure 14-20.Turbulence amplifiers arranged to provide flip-flop. maintain the circuit in the same condition. and the gasket are approximately 5/16 inch The output at F2 will continue until a brief thick. input is applied at B of element 2. This stops The line from the manifold to the device the output at F2 and the flip-flop will change has a bore of approximately 1/32 inch and, of states to providean output at Fl. course, the input lines are smaller. The output Like thefluidicdevices described pre- line is approximately the same size as the viously, the turbulence amplifier can be made supply line.This type of design operates on in various sizes.Some are made of brass approximately 11/2 psi and utilizes about tubing; others are channeled in plastic. Figure 0.25 cfm of fluid.The volume, of course, will 14-21 illustrates how several devices can be vary, depending uponthe,number of devices in made in one plastic sheet.View (A) shows use at the same time. how each deiice, including input signal lines, supply lines (from a common manifold), output, Vortex Amplifier and vents are channeled into one side ofa plastic sheet. As explained in chapter 8, the velocity of View (B) shows the underside of view (A) a fluid increasesif the fluid is moved by with the addition of several devices. The first centrifugal force from the, center of a cylindri- six elements are connected, representing the cal container to the outer wall.If this process NAND circuit illustrated in figure 14-19. All is reversed, the velocity of the fluid flow will of the Inputs of element 5 and one input of decrease.This can be demonstrated by drain- element 6 are lettered in figure 14-21 to cor- ing water from a sink. When the drain plug respond to elements 5 and 6 in figure 14-19. isfirst removed,. the water flows out at a Only the letters A, B, C, and D' (inputs)appear fast rate; but, as soon as the water begins to on the actual device.In a usable device, the swirl, the rate of flow decreases. This is the unused ports would be covered with plastic basic principle of operationof the vortex caps. Each deviceis approximately 5/16 amplifier. inch 'wide and 2 3/4 inches long including its One application of this vortex effect is il- portIon Of the manifold. Both sheets of plastic lustrated in figure 14-22.In view (A), the

'278 n- Chapter 14FLUIDICS

COVER OA ET INPUT RIVET

TO INPUT SO ES SUPPLY (A) (B) FP.278 Figure 14-21.Turbulence amplifiers constructed in plastic. fluid from supply enters the cylindrical chamber by striking the curved wall.(The fluid is said to flow tangentially to the cylindrical chamber.)This action causes the fluid to flow in a circular or swirling motion. This swirling motion "uses up" considerable velocity and, therefore, slows down the flow of fluid to the output.If the fluid is applied in the opposite direction, as shown by thearrows in view (B) of figure 14-22,. no swirling motion will take place.Thereforethe velocity of flow will not be affected. This type of device is referred to as a diode, which means that the device al- (A) lows a high velocity of flow in one direction (8) and a low velocity flow in the opposite direction. FP:279 In the device illustrated in figure 14-22, Figure 14-22.Vortex device with no the flow throvsh the device cannot be controlled control inputs. except by, chenges in pressure and velocity of the supply fluid.Figure 14-23 shows a device in which an input signal port has been added control passage islocated near the supply to control the flow through the device. - entrance and the path of flowis at right angles In a device of this type, the supply port is to the flow from supply.View (A) shows the located in a straight line (radially) to the out- device with. noinput signal applied... In this put,as (shown in figure 14-23.The input condition, . the fluid flows in a straight line

. 279 N, ..".

FLUID POWER

SUPPLY SUPPLY SUPPLY SUPPLY PORT PORT PORT PORT

OUTPUT VENTS OUTPUT OUTPUT (A) ( B) (A) (B) FP.280 FP.281 Figure 14-23.Vortex device with one Figure 14-24.Vented vortexamplifier. control input. action will take place in thechamber and de- from supply to the output with littleloss in crease the velocity of theoutput.If the input When a control flow is introduced signal pressure is increased to a certainvalue, velocity. the vortex effect will spread thefluid out into through the input passage, as shown in view then flows out the vent, (B),it will impart a swirling motion tothe a cone shape. The fluid flow from supply. As this input control pres- completely missing the output port. These devices provide definiteadvantages sure increases, the velocityof flow through the It is possible output decreases. in analog (proportional) control. This is a negative gain device; that is, the to atain outputs as high as97 percent with pressure of the input controlfluid must be no input signal appliedto zero output with full input pressure applied. Like most fluidic greater than that of the supply fluid:However, be made in the volume of the input flow is muchsmaller devices the vortex amplifier can than that of the flow from supply;'therefore, various sizes and of various materials. this device is also an amplifier. This device can be made withseveral input Moving Part Devices control ports which receive signals from sev- moving eral different sources.Some devices are de- As stated previously in this chapter, signed so that two inputs oppose eachother. parts are. utilized in some types offluidic de- If one input is applied in this type device,the vices. The two-way and three-way spoolvalves used to illustrate the logic functionsin figures velocity of flow at the output is reduced to, say, type 40 percent of the supply flow.If the opposing 14-1, 14-2, and 14-3 are examples of one input signal is applied, the vortex actionslows of moving part device.Diaphragms, springs, down, increasing the velocity of the output to, and steel balls are some of the othertypes of flow. These moving parts used in these devices.Since the say, 80 percent of the supply operating principles of moving part devices are results, of course, depend upon the pressures components, applied at the two input ports. similar to those of fluid power far are only one example is presented here. This The vortex devices described thus device proportional flow devices. Figure 14-24shows examples :-is the Kearfoot moving ball can be reduced to illustrated in figure 14-25. a device in which the output This moving ball device iri a bistable.ele- zero.This is accomplished by incorporating a input vent around the output passage. ment, since a brief signal af input A or With no signal applied at the inputport, B will maintain an output at F2 orFl, respec- as shown in view (A) of figure14-24, the fluid tively.There are two supply linesone sup- little plies the flow to output 7/1 andthe other to flows from supply to the output with very Each supply line contains a re- loss in velocity.If a signal is applied at the output F2. input poit, as shown in view (B),the vortex strictor that is smaller in crows- sectional area Chapter 14FLUIDICS

14-26.With the vent open, as shown in view (A), most of the fluid from supply vents to the SUPPLY atmosphere, since the area of the vent opening

A (01 is larger than the opening in the restrictor. UPPER NOZZLE In this condition, there is very little fluid flowing from the output.If the vent is closed, the RESTRICT ORS back-pressure forces the supply fluid to flow LOWER NOZZLE out of the output port. The vent may be blocked by placing a finger over the port or by some object coming incontact with the port, as SUPPLY F2(0) shown in view (B). The output from the sensor can be used to control the input to some other FP.282 device in the control system. Figure 14-25.Kearfoot moving ball device. There are several variations of this type device, some of which can be controlled with a pushbuttonsimilar to an electric switch. thanthat . of the nozzles in the body of the One such device is illustrated in figure 14-27. device.If an input signal is applied at B, the This type sensor can be used to provide brief ball is forced against the upper nozzle and inputs to a bistable device.With the push- blocks any flow from the upper supply line to button open, as shown in view (A), the supply the body of the device.This provides an out- fluid is vented to the atmosphere. When the put at Fl. pushbutton is pushed inward, the vents close, Because of the difference in areas of the forcing the fluid to flow from supply through restrictor and the nozzle, the fluid from the the output port.As soon as the pushbutton lower supply line enters the lower nozzle with'is released, the vents open, stopping the flow very little output at F2.The fluid acts on the through the output. large surface area of the ball. This force will Figure 14-28 shows a back-pressure sens- hold the ball on the upper nozzle after signal ing switch in which several different outputs can B is removed.The fluid that is holding the be selected.By sliding the selector switch ball on its seat is also flowing around the ball control up and down, the flow from supply can and out through port A. be connected to any one of four outputs.For If port A is momentarily closed or a brief example, view (A) shows the supply passage input signal is applied, the ball is foiced from connected to output Fl, and view (B) shows the its seat.This would expose the entire surface supply port connected to output F4. area of the ball to pressure so that it is forced In each of the switching sensors described to the bottom nozzle.This action changes the thus far, the output is accomplished by direct state of the bistable element, stopping the out- contact of some object against a port of the putatFl and providing an 'output at F2. device.In many control systems, input signals must be provided without this direct contact. Figure 14-29 shows a sensing device in which SENSING DEVICES an output signal is provided before the object makes contact with the device.In this type As mentioned previously, sensors are re- device, the supply nozzle is designed in such quired in control systems to provide the initial a manner that the fluid forms into a cone- inputs to the logic or proportional device. Sen- shaped bubble as it flows from the device. sors are simply a means of measurement; that This bubble of fluid contains a low-pressure is, they measure changes in distance, tem- area that is positioned over the entrance to perature, speed, sound, etc.As a result of the output port. Therefore, the output signal is these changes in measurement, the sensor pro- slightly below atmospheric pressure. (See vides, or removes, input signals to the control view (A), fig. 14-29.) system. There are many types of these devices, If an object is moved into the outer part some of which are described in the following of the cone, some of the fluid flow is directed pa ragraphs. into the bubble and into the outptirvagjagi. Perhaps the simplest fluidic sensing device the object is moved closer to the nozzle, is the back-pressure device illustrated in figure theoutputsignal increases a proportional

.086 FLUID POWER

RESTRICTOR

SUPPLY

OUTPUT OBJECT

(A) (B) FP.283 Figure 14-26.Back-pressure sensing device.

VENT RESTRICTOR _vr

SUPPLY SUPPLY

OUTPUT OUTPUT

VENT

( A ) (B) FP.284 Figure 14-27.Sensor switchpushbutton operated.

amount.This condition is shown in view (B) SELECTOR of figure 14-29. ,CONTROL I The range of this type sensor depends upon. F10) F10)) the size of the bubble.In turn, the size of the F2 (o) bubble depends upon the design of the nozzle. MO) As a result, there are several different designs SUPPLY SUPPLY used to satisfy different range requirements. 0 An interrupted jet sensor is illustrated in F30) F3 0)) figure 14-30.The operating principles of this F4(0) F40) type sensor are similar to that of the turbulence amplifier in which a laminar flow is interrupted to stop the output.View (A) shows the device with a laminar stream of fluidflow- (A) (B) ing from supply across a gap and providing an output signal.If an object moves across FP:985 the gap, the laminar flow is interrupted and Figure 14-28.Sensing switch with four becomes turbulent. This stops the flow through output selections. the output.

282 s- Chapter 14FLUIDICS

SUPPLY SUPPLY

NO OUTPUT OUTPUT SUPPLY SUPPLY

OBJECT OBJECT (A) (B) FP.286 Figure 14-29.Diverging cone sensor.

,NOZZLE

SUPPLYMillrilIMIMPFIXIMENIZIOUTPUT SUPPLY (A)

41110SJECT SUPPLY SUPPLY Wo-".1.tA=MNO OUTPUT

(B) FP.287 Figure 14-30.Interrupted jet sensor NO OUTPUT single j,t. (A) Another type of interrupted jet sensor is illustrated in figure 14-31. This is a combina- SUPPLY tion of a jet nozzle and an interrupted jet sensor.With no object in range, as shown in view (A), a laminar jet _stream flows from the OBJECT jetnozzleacross a gap and interrupts the laminar flow through the interrupted jet sen- In: sor.This prevents an output signal.When SUPPLY an object is moved across the laminar flow from the jet nozzle, as shown in. view (B), the stream becomes turbulent. This allows the flow through the interrupted jet sensor to become laminar and provides an output signal. A position measuring device is illustrated in OUTPUT figure 14-32.This device can be constructed (B) with several output ports. View (A) shows the device withall four outputs open.As the FP. 288 object moves toward the device, the piston Figure 14-31.Interrupted jet sensor two jet streams. 283 FLUID POWER

OUTPUTS I(i) 20) 3W 4(1)

SUPPLY f OBJECT (A)

1(e) 2(0) 3(0) NM 61 61 hi SUPPLY

(B) FP.289 Figure 14-32.Position measuring device. moves inward, covering the output ports in are utilized to interrupt the laminar flow. sequence. View (B) shows the device after the This device can be designed in such a manner object has moved the piston sufficiently to that it is sensitive to sound waves in a narrow cover three ports.Since the supply is con- frequency only.View (A) shows the device stant, the pressure of the outputs increases as without the applicable sound waves present. the piston moves inward.That is, the pres- In this state the laminar flow through the first sure of outputs 1, 2, and 3 increases with the element is not disturbed.The input proviend piston blocks output 4.There is a further by this laminar flow interrupts the laminar increase in pressure at outputs 1 and 2 when flow through the second element, stopping the output 3 is covered. output. This device measures definite positions View (B) shows the sound-sensitive device (four, in this example) of the object.There- with the applicable sound waves present. The fore, it provides digital outputs. By replacing sound waves interrupt tae laminar flow through the four small output ports with one large the first element. Therefore, since the laminar port, analog output can be obtained.As the flow through the second element is not inter- piston moves inward and outward in this type rupted, an output is provided. of device,it partially covers and uncovers This type of device can be designed with one the output.Therefore, the size of the output element.In such a design, there would be an is proportional to the movement of the piston. output if the applicable sound waves were not Since the movement of the piston is dependent present and the output would stop when the upon the movement of the object, the pressure applicable sound waves were present. of the output indicates the relati'ie position This device can be designed to match the of the object. sound waves produced by different methods. An example of a sound-sensitive device is For example, the device can be designed to illustrated in figure 14-33.This device op- match certain frequencies of electrically pro- erates similarto the turbulence amplifier, duced sound waves. In some situations, a fluidic except that sound waves, instead of fluid jets, device is used to produce sound waves. This is Chapter 14FLUIDICS

NO OUTPUT OUTPUT

SUPPLY SUPPLY

) ) ) SOUND WAVES

SUPPLY SUPPLY

(A) (B)

FP.290 Figure 14-33.Sound-sensitive device. accomplished by incorporating a .partial ob- a result, the laminar flow through the sound- struclon, of the correct design, in a fluid line. sensitiveelement is not interrupted and an As the fluid flows by this obstruction, sound output is provided. waves are emitted. The sound-sensitive device There are several types of temperature can then be matched to this device. sensing devices most of which combine fluidics This combination can be used in a manner with some other type of power, suchas elec- similar to that illustrated in figure 14-31. A tricity.One such device employs a fluid oscil- sound-emitter is used instead of the jet nozzle, lator and an acoustic (sound) to electric trans- and a single-element sound-sensitive device is ducer. An oscillator is an amplifier which used instead of the interrupted jet sensor. generates a given frequency determined by the The sound-emitter produces sound waves which value of the components. Positive feedback interrupt the laminar flow through the sound- is used to reinforce the action in the amplifier sensitive device and prevent any output.If and toreplenish the power lost during the an objectis moved intorange, the sound generation of each cycle.The transducer is waves .are interrupted, similar to that of the used to detect the frequency oscillations and laminar flow in the jet interrupted device. As relate them to temperature. 285

. 290 FLUID POWER

One type of fluidic oscillator is illustrated varies with temperature, the frequency of the in figure 14-34.This is a modification of the oscillator varies with the temperature. These bistable wall attachment device illustrated in frequencies can be detected by an electrical figure 14-9. As an oscillator, this device transducer and a measurement of temperature utilizes a feedback system to provide the in- obtained. put signals. In view (A) of figure 14-34, the fluid from supply, formed by the fluid of which the tem- APPLICATIONS perature isrequired,is flowing through the output passage Fl. The fluid is attached to the As stated previously in this chapter, fluidic wall of this passage.As it flows through this control systems have several advantages over passage, part of the fluid enters the feedback electronic control systems in certain applica- passage in input A, as shown in view (B). This tions. First and perhaps most important, fluidic flow of fluid provides an input control signal at control systems are much more reliable in A.(See view (C).)This input signal switches certain severe environmental conditions, such the flow from supply to output passage F2, as nuclear radiation, high temperature, shock, as shown in view (D).Part of this fluid flows vibration, etc.Such conditions can make con- through the feedback passage and provides an ventional electrical, mechanical, and electronic input signal at B.This switches the supply systems inoperative.As a general rule, the flow back to output Fl.This sequence olop- initial cost of a fluidic control system is less eration repeats, resulting in a continuous switch- than that of a similar electronics system. ing of the supply flow from one output to the Fluidic systems are relatively simple and, in other. most cases, contain no moving parts; therefore, The frequency of switching is determined by they are reliable and are easy to maintain. In the time required for the flow from supply to most cases, the life of a flu1lic device is much be transferred from one passage wall to the longer than that of a similar electronic device. other plus the time taken for the control signal This does not mean that fluidics is a threat to travel around throug7 t the feedback passage. to electronics.For example, fluidics cannot The speed of travel is that of sound (pressure) compare with electronics in speed of operation. waves in the fluid.Since the speed of sound The response time of fluidic devices is usually measured in milliseconds (thousands of a sec-

FEED-BACK one),while the response time of electronic )0.0,...,2r.... PASSAGE devices is usually measured in microseconds (millionths of a second).This is similar to Ft Ft comparing the speed of sound to the speed of light. This is the principle disadvantage of SUPPLY fluidic control. HoWever, where this difference B in speed is not an important factor, fluidic F2 control systems can serve as well or better than electronics. In 1969, the national market for fluidic de- (A) (B) vices and control systems was approximately $22 million.A large portion of this amount was financed by government services. By the late 1970's this amount is expected to be over $300 million per year. SUPPLY SUPPLYAt- The field of fluidics was primarly conceived in the military services, and owes its F2 expanding growth to these services. The Navy, for the past few years, has awarded a number of contracts to private industries for the pur- CC) CD) pose of research and development of fluidics for different applications.These applications FP.291 include:missile and aircraft control, torpedo Figure 14-34.Operation of fluidic oscillator. control,speedc o n t r o 1for steam turbine

286 .291 Chapter 14FLUIDICS generators (pneumatic, all analog, all digital, or hybrid analog/digital fluidic control), iluidic amplifiers, boiler control for shipboarduse, aircraft engine control, and a fluidic controlled NOZZLE LONGITUDINAL computer system to provide automatic control THROAT AXIS ofa submarine hover system. At least one model of Navy missile utilizes one - of the principles of fluidics in part of its guidance control system.In this system, die principle of jet interaction is utilized to defle4 the exhaust gases for directional control. This is referred to as the fluid injection method of control.A brief description of this method follows: As mentioned in chapter 13, rotatable nozzles are often used in missile control systems to deflect exhaust gases for directional control. Under conditionsof extreme temperatures, serious problems have been encountered with rotatable nozzles in respect to nozzle throat erosion and bearing plane "freeze up." When the fluid injection method of thrust vect:vcon- trolisused, these undesirable effects are eliminated. FP.292 This method involves the injection of liquid Figure 14-35.Fluid injection nozzle. freon into the nozzle, downstream of the throat, as illustrated in figure 14-35.This injected freon disrupts the normal flow pattern path of The fluid injection method, as compared exhaust gases, creating an oblique shockwave. to therotatable nozzle method, provides a In passing through the shock wave, the flow weight saving because the weight of rotatable of hot gases is deflected, producing a thrust nozzles(including their hydraulic actuators) vector at an angle to the longitudinal axis of exceeds the weight of a comparable fluid in- the missile.The amount of thrust vector con- jectionsystem.Tht: fluid injection method trol available is a function, among other things, also provides a little more thrust. of the amount of fluid injected. Located at intervals around the nozzle throat As can be seen, fluidics is a rapidly ad- are two injection ports, each consisting of three vancing field.For example, the devices pre- injection orifices. (Figure 14-35 shows ports at sented in this chapter to illustrate the prin- the 0-degree and 180-degree location.) By ciples of operation are only representative of injecting fluid through these ports separately, the many devices available. Through research or through any combination of ports at the same and development, new and improved devices time, the thrust vector can be deflected toany become available each year.Therefore, it desired angle to provide the necessary control may well benefit the reader to keep abreast over the missile. of the advances in fluidics.

287 APPENDIX 1

GLOSSARY

ABSOLUTE PRESSURE.Actual pressure (in- measurescontinuouslyrather than dis- cludes atmospheric pressure). cretely. ABSOLUTE TEMPERATURE. The tempera- AND.A basiclogic function (gate or cir- ture measured using absolute zero as a cuit) which provides an output if, and only reference.Absolute zero is -273.16° C or if, all control signals are applied. -459.69° F. ANEROID.Containing no liquid, as an aneroid ACCELERATION.Time rate of change of ve- barometer. locity. ACCUMULATOR. =-A device for storing liquid under pressuF)e,usuallyconsistingof a BACK PRESSURE.A pressure exerted con- chamber separated into a gas compartment trary to the pressure producing the main and a liquidCompartment by, a piston or flow. diaphragm. Ar accumulator also serves to B AROME TER.An instrument which measures smooth out pressure surges in a hydraulic atmospheric pressure. system. BERNOULLI'S PRINCIPLE.If a fluid flowing ACTIVE ELEMENT.A fluidic device which is through a tube reaches a constriction, or directly attached to the power supply and narrowing of the tube, the velocity of fluid requires a continuous power supply to en- flowing through the constriction increases ableitto be operated by input signals. and the pressure decreases. ACTUATING CYLINDER.An actuator which BINARY.A characteristic, property, or con- converts fluid power into linear mechanical dition in which there are but two possible force and motion. alternatives; for example, the binary num- ACTUATOR.A device which converts fluid ber system using two as its base and using power into mechanical force and motion. only the digits zero (0) and one (1). ADDITIVE.A chemical compound added to a BINARY NUMBER SYSTEM.A number sys- fluid to change its properties. tem with two symbols ("0" and "1") that AFTERCOOLER.A device which cools a gas has two as its base just as the decimal after it has been compressed. system uses ten symbols ("0,1,..., 9") AIR BLEEDER.A device used to remove air and a base of ten. from a hydraulic system.It raybe a needle BISTABLE. The capability of assuming either valve, capillary tubing to the reservoir, or of two stable states; for example, a flu- a bleed plug. idic device having two separate and distinct AMBIENT.Surrounding, such as ambient air outputs which can beobtained by brief meaning surrounding air. input signals. BOOLEAN ALGEBRA.A process of reasoning AMPLIFIER, FLUID.A fluidic element that or a deductive system of theorems using enables a flow or pressure to be control- a symbolic logic, and dealing with classes, led by one or more input signals which propositions, or ON-OFF circuit elements. areof a lower pressure or flow value It employs symbols to represent operations1 than the fluid being controlled. such as AND, OR, NOT, etc., topermit math- ANALOG.The general class of fluidic ele- ematical calculations. Named after George ments or circuits having proportional flow Boole, famous English mathematician (1815- characteristics. A type of computer which 1864). 288 ;2913 ti

Appendix IGLOSSARY

BOYLE'S LAW.The volume of any dry gas CONTAMINATION.Harmful foreign matter in a varies inversely with the applied pressure, fluid. provided the temperature remains constant. CONTROL.A mean:. or device used to regulate BUOYANCY.The upward or lifting force ex- the function of a unit. For example, valves erted on a body by a fluid. may be controlled hydraulically, manually, mechanically, electrically, or pneumatically. CALIBRATE.To make adjustments to a me- In turn, the valve controls the function of ter or other instrument so that it will insome unit. dicate correctly with respect to its inputs. CONVERGENT.That which inclines and ap- CATALYST.A substance used to speed up or proaches nearer together, as the inner walls slow down a chemicalreaction,butis of a tube that is constricted. itself unchanged at the end of the reaction. CORROSION. The slow destruction of materials CELSIUS.The temperaturescale using the by chemical agents and eletromechanical freezing point as zero and the boiling point reactions. as 100, with 100 equal divisions between, COUNTERBALANCE VALVE.A valve which called degrees.This scale was formerly permits free flowin one direction, but known as the centigrade scale, but was re- maintains a resistance to flow in the other named the Celsius scale in recognition of direction to prevent a load from falling. Anders Celsius, the Swedish astronomer who CYCLES PER SECOND. (See HERTZ.) devised the scale. CENTIGRADE.(See CELSIUS.) DENISTY.The weight per unit volume of a CENTRIFUGAL FORCE.A force exerted on a substance. rotating object in a direction outward from DIAPHRAGM.A dividing membrane or thin the center of rotation. partition. CHARLES LAW.If the pressure is constant, DIFFUSER.A duct of varying cross section the volume of dry gas varies directly with designed to convert a high-speed gas flow the absolute temperature. into low-speed flow at an increased pressure. CHECK VALVE.A valve which permits fluid DIGITAL.The general class of fluidic devices flow in one dirction, but prevents flow in or circuivs which have ON-OFF charac- the reverse direction. teristics. .k type of computer which measures CHEMICAL CHANGE.A change which alters discretely rather than continuously. the composition of the molecules of a sub- DIODE.A fluidic element which provides a stance. New substances with new properties higher resistance to flow in one direction are produced. compared to that in the opposite direction. CIRCUIT.An arrangement of interconnected DIRECTIONAL CONTROL VAVLE.A valve component parts. which selectively directs or prevents flow to COANDA EFFECT. (S e e WALL ATTACH- or from desired channels. Also referred to MENT.) as selector valve, control valve, or transfer COMPRESSED AIR.Considered as air at any valve. pressure in excess of the local atmospheric DISPLACEMENT.The volume of fluid which pressure. can pass through a puz.-n. motor, or cylinder COMPRESSIBILITY.The property of a sub- in a single revolution or stroke, stance, such as air, by virtue of which its density increases with increase in pressure. DIVERGENT.Moving away from each other, as COMPRESSOR.A mechanical device used for the inner walls of a tube that flares outward. increasing the pressure of a fluid; for example, an air compressor. DOUBLE-ACTING CYLINDER.An actuating COMPUTER.Adevice capable of accepting cylinder in which both strokes are produced information, applying prescribed processes by pressurized fluid. to the information, and supplying the results DYNAMIC PRESSURE.The pressure of a fluid of these prOcesses.It usually consists of resulting from its motion, equal to one-half input and output devices, storage, arithmetic the fluid density times the fluid velocity and logic units, and a control unit. squared.In incompressible flow, dynamic CONDENSATION.The change from a gaseous pressure isthe difference between total (or vapor) state to a liquid state. pressure and static pressure. 289 FLUID POWER

EFFICIENCY.The ratio of output power to FLUID POWER. Power transmitted and con- input power, generally expressed as a per- trolled through the use of fluids, either centage. liquids or gases, under pressure. ELEMENT.An indivisiblepart ofa logic FOOT-POUND.The amount of work accom- function or circuit.Fluidic elements are plished when a force of 1 pound produces a interconnectedto form working circuits. displacement of 1 foot. Also,used inidentifyingthe principal FORCE.The action of one body on another working parts of fluid power components; tending to change the state of motion of the for example, filter element, valving element, body acted upon. Force is usually expressed etc. in pounds. ENERGY.The ability or capacity to do work. FREE FLOW.Flow which encounters negligible EQUILIBRIUM.A state of balance between op- resistance. posing forces or actions. FREQUENCY. The number of complete cycles FAHRENHEIT.The temperature scale using per second (HERTZ) existing in any form of the freezing point as 32 and the boiling point wave motion; for example, the number of as 212, with 180 equal divisions between, cycles per second produced by a fluidic called degrees. oscillator. FAN-IN RATIO.The number of separate inputs FRICTION. The action of one body or substance available on a single fluidic lement. rubbing against another, such as fluid flowing FAN-OUT RATIO.The number of fluidic ele- against the walls or pipe; the resistance ments that can be controlled by a single to motion caused by this rubbing. similar element. FRICTION PRESSURE DROP.The decrease in FEEDBACK.A transfer of energy from the the pressure of a fluid flowing through a output of a device back to its input. passage attributable to the friction between FILTER.A device which removes insoluble the fluid and the passage walls. contaminants from the fluid of a fluid power FUNCTION, LOGIC.A logic element or an system. assembly of logic elements which can produce FIXED DISPLACEMENT.The type of pump a desired effect when certain conditions or motor in which the volume of fluid per are satisifed; that is, if the correct input cycle cannot be varied. signals are applied the required output is FLASH POINT.The temperature at which a obtained. substance, such as a fluid, will give off a vapor that will flash or burn momentarily GAGE PRESSURE.Pressure above atmospheric pressure. . when ignited. FLIP-FLOP.A fluidic device or circuit that GAGE SNUBBER.A device installed in the line is bistable; that is, capable of assuming to the pressure gage used to dampen pressure two stable states each of which is obtained surges and thus provide a steady reading and by brief input signals.This type of device a protection for the gage. is said to have memory, since it is capable GAS.The form of matter which has neither of storing information. a definiteshape nor a definite volume. FLOW CONTROL VALVE.A valve which is GASKET.A class of seal .which provides a used to control the flow rate of fluid in a seal between two stationary parts. fluid power system. GATE.A type of fluidic device which controls FLOWMETE R.An instrument used to measure the passage of fluid.Also, a type of flow quantity or the flow rate of a fluid motion. control valve in which the flow is controlled FLUID.Any liquid, gas, or mixture thereof. by the up and down movement of a wedge FLUID FLOW.The stream or movement of a across the line of flow. fluid, or the rate of its movement. GRAVITY. The force which tends to draw all FLUIDICS. The technology that uses the inter- bodies toward the center of the earth. The action of flowing gases or liquids to per- weight of a body is the resultant of all form sensing, logic, amplification, and con- gravitationalforcesacting onthe body. trol functions.Although primarily associ- HEAD.A measure of fluid pressure as the ated with devices having no moving pails, height of a fluid column necessary to cause the broad field of fluidics includes moving that pressure at its base. The pressure of part :evices. a fluid owing to its elevation. 290 Appendix IGLOSSARY

HEAT EXCHANGER.A device which transfers INTERCOOLv.:R.A device which cools a gas heat through a conducting wall from one between the compression stages of a mul- fluid to another. tiple stage compressor. HERTZ.The measurement of frequencies in INTERFACE.A device that converts or trans- cycles per second, one hertz being equal lates any typeof information from one to one cycle per second. This measurement given medium into signals of another given was formerly known as cycles per second medium; for example, electrical signals to but was renamed hertz in recognition of fluidic signals, fluidic signals to electronic Henrich Hertz (1847-1894), a German phys- signals, etc. icist, who first detected and measured elec- INVERSE PROPORTION.The relation that ex- tromagnets wave. ists between two quantities when an increase HORSEPOWER.A unit for measuring the power in one of them produces a corresponding of motors or engines, equal to a rate of decrease in the other. 33,000 foot-pounds per minute.The force INVERTER.A fluidic device in which appli- required to raise 33,000 pounds at the rate cation of an input signal will remove the of 1 foot per minute. output signal, and the removal of the input HYDRAULICS.That branch of mechanics or signal will provide an output signal. engineering that deals with the action or use JET INTERACTION.A type of fluidic device in of liquids forced through tubes and orifices which fluid flows are arranged so that small under pressure to operate various mechan- opposing jets control the output by deflect- isms. ing a larger jet flow. HYDROMETER.An instrumentfor determining the specific gravities of liquids. KELVIN SCALE.The temperature scale using absolute zero as the zero point and divisions that are the same size as centigrade degrees. IMPACT PRESSURE.That pressure of a mov- KINETIC ENERGY.The energy which a sub- ing fluid brought to rest which is in excess stance has while it is in motion. of the pressure the fluid has when itdoes not KINETIC THEORY.A theory of matter which flow;thatis,total pressure less static assumes that the molecules of matter are pressure.Impact pressure is equal to dy- in constant motion. namic pressure in incompressible flow; but LAMINA.A layer of fluid. in compressible flow, impact pressure in- LAMINAR FLOW.A smooth flbw in which no cludes the pressure change owing to the cross flow of fluid particles occurs. compressibility effect. LINE.A tube, pipe, or hose which isused as a IMPINGEMENT.The striking or dashing upon conductor of fluid. with a clash or sharp collision, as air im- LIQUID.A form of matter which has a definite pinging upon the rotor of a turbine or motor. volume but takes the shape of its container. IMPULSE TURBINE.A turbine driven by a LOAD. The power that is being delivered by fluid at high velocity under relatively low any power-producing device.The equip- pressure. ment that uses the power from the power- INERTIA.The tendency of a body at rest to re- producing device. main at rest, and a body in motion to continue LOGIC.The science dealing with the criteria to move at a constant speed along a straight or formalprinciplesofreasoning and line, unless the body is acted upon in either thought. As applied to fluidic control systems, case by an unbalanced. force. logic is a means of reducing the require- INHIBITOR.Any substance which retards or ments of a problem into clear-cut decisions. prevents such chemical reactions as cor- LOGIC ELEMENTS.The general category of rosion or oxidation. fluidic devices which provide logic functions INPUT SIGNAL.A pressure or flow of fluid such as AND, OR, NOR, etc. These elements which isdirectedinto an input port to can gate or inhibit signal transmissions control an element or logic function. with the application, removal, or other com- INTEGRAL.Essential to completeness, as an binations of input signals. integral part. (The valve stem is an integral MANIFOLD.A type of fluid conductor which part of the valve.) provides multiple connection ports.

291 FLUID POWER

MATTER.Any substance which occupies space PASCAL'S LAW.Whenever an externalpres- and has weight.The three forms of matter sure is applied to any confined fluid at rest, are solids, liquids, and gases. the pressure is increased at every point MECHANICAL ADVANTAGE.The ratio of the in the fluid by the amount of the external resisting weight to the acting force.The pressure. ratio of the distance through which the force PASSIVE ELEMENT.A fluidic device which is exerted divided by the distance the weight requires no power supply but operates on is raised. input power derived from the output of METER-IN.To regulate the amount of fluid another fluidic device. Into a system or an actuator. PERIPHERY. The outside surface, especially METER-OUT.To regulate the flow of fluid that of a rounded object or body. from a system or actuator. PHYSICAL CHANGE.A change which does not MICRON.A millionth of a meter or about alter the composition of the molecules of a 0.00004 inch. substance. MOLECULE.A small natural particle of mat- PILOT VALVE.A valve used to control the ter composed of two or more atoms. operation of another valve. MOTOR.An actuator which converts fluidpow- PIPE.That type of fluid line the dimensions er to rotary mechanical force and motion. of which are designated by nominal 'r proximate) outside diameter (OD),..:1(1 vicui NAND.A fluidic device or circuit having two thickness. (See also TUBING.) or more inputs and a single output, in which PISTON TYPE CYLINDER.An actuating cyl- the output is OFF if, and only if, all inputs inder in which the cross-sectional area of are ON. The NAND function is a combination thepiston rod is less than one-half the of the AND and NOT functions. cross-sectional area of the movable element. NEOPRENE.A synthetic rubber highly resist- PLUNGER.(See RAM TYPE CYLINDER.) ant to oil, light, heat, and oxidation. PNEUMATICS. That branch of physics pertain- N011.A basic logic function (gate or circuit) ingtothe pressure and flow of gases. which has an output when all control inputs PORT.An opening for the intake or exhaust of are OFF. a fluid. NOT.A basic logic function (gate or circuit) POTENTIAL ENERGY.The energy a subiatance which has an output signal when the single has because of its position, its condition, or input signal is OFF has no output when its chemical composition. the input signal is ON. POWER.The rate of doing work or the rate of OR.A basic logic function(gate or circuit) expanding energy. which has an output if any of its multiple PRESSURE.The amount of force distributed controls hate an input. over each unit of area.Pressure is ex- OSCILLATION.A backward and forward mo- pressed in pounds per square inch (psi). tion, a vibration. PRESSURE DIFFERENTIAL.The difference in OSCILLATOR.A fluidic device which incorp- pressure between any two points of a system orates a. feedback system and generates a or a component. . frequency determined by such factors as the PRESSURE SWITCH.An electrical switchop- length of the feedback loop and the tempera- erated by the increase and decrease of fluid ture and/or viscosity of the fluid passing pressure. through the device. This device can be used PRIME MOVER.The source of mechanical as a frequency generator or temperature power used to drive the pump or compressor. and viscosity sensors. PROPORTIONAL.The general class of fluidic OUTPUT SIGNAL.The pressure or flow of fluid elements or circuits having analoging char- leaving the output port of a fluidic device. acteristics; that is, an increase or decrease OXIDATION.That process by which oxygen in control pressure results in a comparable unites with some other substance, causing increase or decrease at the output. rust or corrosion. PUMP.A device which converts mechanical energy into fluid energy. PACKING.A class of seal which is used to provide a seal between two parts of a unit RAM TYPE CYLINDER. An actuating cylinder which move in relation to each other. in which the cross-sectionalarea of the Appendix IGLOSSARY

piston rod is more than one-half the cross- produced by some other force, such as sectional area of the movable element. Also gravity or spring tension. referred to as plunger. SOLID.The form of matter which has a definite RANKINE SCALE.A thermometer scale based shape and a definite volume. on absolute zero of the Fahrenheit scale, in SPECIFIC GRAVITY.The ratio of the weight of which the freezing point of water is approxi- a given volume of a substance to the weight mately 492° R. of an equal volume of some standard sub- RATIO.The value obtained by dividing one stance. number by another, indicating their relative STABILITY.The resistance of fluid to per- proportions. manent change in properties due to chemical RECEIVER.A container in which gas is stored reaction, temperature changes, etc. under pressure as a supply source for pneu- STEADY FLOW. A flow in which the velocity matic power. components at any point in the fluid do not RECIPROCATING.Moving back and forth, as a vary with time. piston reciprocating in a cylinder. STREAMLINE FLOW.A smooth, nonturbulent RELIEF VALVE.A pressure control valve use flow, essentially fixed in pattern. to limit system pressure. STUFFING BOX.A chamber and closure with RESERVOIR.A container which serves pri- manual adjustment for a sealing device. marily as a supply source of the liquid for SUPPLY LINE.A line that conveys fluid from a hydraulic system. the reservoir to the pump. RESPONSE TIME.The time lag between a sig- SURGE.A momentary rise of pressure in a nal input and the resulting change of output. system. RESTRICTION.A reduced cross-sectional SYNCHRONIZE.To make two or more events area in line or passage which reduces the or operations occur at the proper time with rate of flow. respect to each other. RETURN LINE.A line used for returning fluid SYNCHRONOUS.Happening at the same time. backinto the reservoir or atmosphere. SYNTHETIC MATERIAL. A complex chemical SENSOR.A fluidic component that senses var- compound which is artificially formed by iables and produces a signal in a medium the combining of two or more simpler com- . compatible with fluidic elements. Tem- pounds or elements. perature, sound, and position sensors are

' examples. THEORY.A scientific explanation, tested by ;4 SEPARATOR.A trap. for removing oil and observations and experiments. 1 water from compressed gas before it can THERMAL EXPANSION.The increase in vol- collect in the lines or interfere with the ume of a substance due to temperature efficient operation of pneumatic systems. change. SEQUENCE VALVE.An automatic valve in a TORQUE.A force or combination of forces fluid power system that causes one actuation that produces or tends to produce a twisting to follow another in definite order. or rotary motion. SERVO.A device used to convert a small TRANSDUCER.A device that converts signals movement into a greater movement or force. received inone medium infr outputs in SERVOCONTROL.A control actuated by a feed- some other medium; for example, electrical back system which compares the output with inputs to fluidic outputs. a reference signal and makes corrections TRUTH TABLE.A tabular correlation of input to reduce the differences. and output relationships of fluidic elements. SHUTOFF VALVE.A valve which operates TUBING.That type of fluid line the dimensions fully open or fully closed. of which are designated by actual measured SHUTTLE VALVE.A valve which is used to outside (OD) and by actual measured wall direct fluid automatically from either the thickness. normal source or an alternate source to TURBINE.A rotary motor actuated by the the actuator. reaction,impulse, or both, of a flow of SINGLE-ACTING CYLINDER.An actuating pressurized fluid.A turbine usually con- cylinder in which one stroke is produced by sistsof a series of curved vanes on a pressurized fluid, and the other stroke is centrally rotating shaft. 293

. 29& FLUID POW ER

TURBULENCE.A state of flow in which the VISCOSITY. The internal resistance of a fluid fluid particles move in a random manner. which tends top:-event it from flowing. TURBULENCE AMPLIFIER.A fluidic digital VOLUME OF FLOW.The quantity of fluid that device in which an output is provided by passes a certain point in a unit of time. laminar flowof fluid across a gap and The volume of flow is usually expressed in the output is prevented by a small input jet gallons per minute for liquids and cubic of fluid which causes the flow to become feet per minute for gases. turbulent. VORTEX AMPLIFIER. --A fluidic device in which a swirling motion is imparted to a Zlovi of VACUUM.Pressure less than atmospheric fluid by a small jet, thus controlling the pressure. value of the output signal. VARIABLE DISPLACEMENT. The type of pump or motor in which the volume of fluid per cycle can be varied. WALL ATTACHMENT.A type of fluidic de- VELOCITY.The rate of motion in a particular vice which is based on the tendency of a direction. The velocityof fluid flow is fluid stream to adhere to a nearby surface. usually measured in feet per second. This principle was first described by Henri VENTURI.A tube having a narrowing throat or Coanda in 1932 and is often referred to as constriction to increase the velocity of fluid the "Coanda effect." flowing through it. The flow through 'the WORK.The transference of energy from one venturi causes a pressure drop inthe body or system to another.That which is smallest section, the amount being a func- accomplished by a force acting through a tion of the velocity of flow. distance. 3

INDEX

Absolute pressure, 13 AND function, 264-266, 270, 275 Absolute zero, 8 Applied forces, 32 Accumulators: Areas: air-operated, 114-117 differential, 23 bladder, 115-116 relation to forces, 20 cylinder, 116 relation to pressure, 20 diaphragm, 116 Atmospheric pressure: applications, 51, 117-119 description, 11-12 dead-weight, 114 measurement, 12 spring-operated, 114 Auxiliary pumps, hydraulic transmission, 236 Acidity, 39-40 Avogadro's law, 17 Active element, 262 Axial piston motor, 229-231 Actuating unit, 50 Axial piston pumps, 136-142 Actuators: cylinder, 216-224 Backup washers, 94-95 cushioned, 223 Barometem; limited rotation, 223-224 aneroid, 12-13 piston type, 219-224 mercury, 12 ram type, 216-219 Basic fluid power systems, 49 tandem, 222 13-end, 234-235, 236 telescoping, 218 Bernoulli's principle, 35-36 motor, 224-231 Binary number system, 263-264 gear, 226-227 Bite type connectors, 79-80 piston, 228-231 Bladder type accumulator, 115-116 vane, 227 Boolean algebra, 264 turbine, 231-233 Bourdon tube gage, 171-177 governors 232-233 Boyle's law, 14-15 multiple-stage, 231, 232 Brakes: . pinwheel, 231 air-over-hydraulic, 64-66 single stage, 231-232 hydraulic, 62-64 A-end, 234-235, 236 pneumatic, 64, 65 After coolers, 151-152 Brake valve, 238 Air compressor, 50, 142-147 Buoyancy, 7-8 Aircraft fluid power system, 250-256 Air-operated accumulators, 114-117 Cam-operated valve, 204-205 Air-pressurized reservoirs, 101-102 Celsius temperature scale, 8-11 Amplifier: Centrifugal force, 122-123 analog, 262 Centrifugal pumps, 122-126 digital, 262 Charles' law, 15-16 fluid, 262 Check valves, 52, 200, 201 IA interaction, 271-274 Chemical driers, 114 moving part, 262, 280-281 Chemical stability of liquids, 39 pure fluid, 262 Closed-center system, 52 turbulence, 274-278 Compensated valves, 161-162 vortex, 278-280 Compressed air, 47 wall attachment, 267-271 Compressibility of gases, 13-17 Analog amplifier, 262 Computer logic, 262

295 INDEX

Connectors: Energy:Continued: brazed, 76 kinetic, 32-33 flanged, 75 potential, 26 flared, 76-79 relation to work, 26 flareless, 79-80 Expansion of: for flexible hose, 80-81 gases, 14-15 threaded, 74-75 liquids, 13 welded, 75 Expansion tank, 237 Contamination of fluids: gases, 48 liquids, 41-46 Factors affecting flow, 30-33 Control pressure pumps, 236 Fahrenheit temperature scale, 8-11 Control valves, 50 Fan-in ratio, 262 Cork seals, 88 Fan-out ratio, 262 Counterbalance valve, 185-186 Filter, 51, 106-109 C-shaped tube gage, 172 elements, 108-109 Cup seal, 93 fuller's earth, 108 Cushioned actuators, 223 micronic, 108-109 Cylinder block, 134 sintered bronze, 109 Cylinder type actuators, 216-224 stainless steel, 109 full flow, 106 Dalton's law, 17 proportional flow, 107-109 Dead-weight accumulator, 114 Filtration of fluids: Density, 6-7 hydraulic liquids, 104-110 Diagrams: pneumatic gases, 111-114 pictorial, 58 Fire point of liquids, 40 schematic, 59 Flange seals, 94 symbols, 53, 54-57 Flashpoint, 40 uses, 53 Flip-flop circuit, 266-267, 270, 277 Diaphragm accumulator, 116 Flow: Diaphragm gage, 177-178 factors affecting, 30 Differential areas, 23 atmospheric pressure, 32 Differential pressure gage, 175, 176 force, 31 Diffuser type centrifugal pump, 125 friction, 31 Digital amplifier, 262 gravity, 31 Directional control valves, 50, 198-215 inertia, 30-32 Doorstop: steady and unsteady, 28 hydraulic, 64 streamline and turbulent, 28-30 pneumatic, 64 volume and velocity, 28 Double-acting, piston type actuating cylinder, Flow control valve, 154 220-222 Flow equalizer, 162-163, 164 Double-acting ram, 211 Flowmeters: Dual rams, 218-219 nutating piston disc, 166-169 Duplex gage, 172-173 propeller type, 169-170 Dynamic seals, 86 turbine type, 169, 170 . Fluid amplifier, 262 Electric motor, 235 Fluid contamination: Electrohydraulic steering, 240-245 gases, 48 control, 241-245 liquids, 41-46 double ram, 240-241 Fluidic devices, 267-287 single ram, 241 logic, 267-281 Element, fluidic, 262 sensing, 281 -287 Energy: Fluidics, 261-287 conservation of, 27 Fluid injection control, 287 definition of, 26 Fluid lines and connectors, 67:65 296 1001 FLUID POWER

Fluid power: Handpump, hydraulic, 132 advantages, 3-4 Head, relation to force and pressure, 34 definition, 1 Heat exchangers, 110 special problems, 4 Helical gage, 175, 177 Fluid-pressurized reservoirs, 102-103 Helical gear pump, 127, 129 Fluids, 6, 37-48 Herringbone gear pump. 127, 128 Flushing hydraulic systems, 44-46 Hose, flexible: Flyball governor, 232-233 applications, 72-73 Force and pressure, 19-25 materials, 72 differential areas, 23 sizes, 72 effects of atmospheric pressure, 23 Hydraulic fuse, 194-197 formulas, 20-21 Hydraulic liquids: multiplication of forces, 22-23 contamination control, 41-46 volume and distance factors, 23 properties, 37-40 vacuum, 24-25 chemical stability, 39 Force, transmission of, 17.18 fire point, 40 Formulas: flash point, 40 area, 20-21 freedom of acidity, 39-40 Boyle's law, 14-15 lubricating power, 39 Charles' law, 15-16 toxicity, 40 density, 6 viscosity, 37-39 force, 20-21 types, 40-41 general gas law, 16-17 petroleum base, 41 pressure, 20-21 synthetic base, 41 specific gravity, 7 water base, 40 temperature conversion, 9-10 Hydraulic power drive system, 234-239 Four-way valve, 206-213 Hydraulics: Friction, 31 application, 2-3 Full flow filter, 106 definition, 1-2 development, 2 Gages: Hydraulic transmission, 236, 237 Bourdon tube, 171-177 Hydrometer, 7 C-shaped tube, 172 diaphragm, 177-178 Incompressibility of liquids, 13 differential pressure, 175, 176 Inertia, 30-32 duplex, 172-173 Intercoolers, 151-152 helical, 175, 177 Internal gear pump, 127-130 pressure, 50, 171-180 Interrupted jet sensors, 282-283 simple; 172, 173 spiral, 175, 177 Jack, hydraulic, 60-61 spring-loaded pressure, 178-180 Jet interaction devices, 271-274 Gaskets, 86 Gas laws: Kelvin temperature scale, 8-11 Avogadro's, 17 Kinetic energy, 32-33 Boyle's, 14-15 Kinetic theory of gases, 14 Charles', 15-16 Dalton, 17 Leather backup washers, 95 general, 16-17 Lift, hydraulic, 61-62 Gate valve, 155-156, 157 Limited rotation actuators, 223-224 Gear type motor, 226-227 Liquids, 37-46 Gear type pumps, 127-130 Lobe type pump, 131 General gas law, 16-17 Logic devices, 267-281 Globe valve, 156 -157, 158 Jet interaction, 271-274 Glossary of terms, 288-294 moving part, 280-281 Gravity, 31 turbulence amplifier, 274-278 297 INDEX =Ma Logic devicesContinued: Pictorial diagrams, 58 vortex amplifier, 278-280 Pilot-operated valve, 191-193, 205 wall attachment, 267-271 Pintle, 134 Logic function, 262 Pinwheel turbine, 231 Logic operations, 264-267 Piping: Lubricating power of liquids, 39 applications, 69-71 materials, 68 Main reduction gear, 235-236 sizes, 68, 69 Manifolds, 84-85 Piston type actuating cylinder, 219-224 Metal seals, 88 Piston type motor, 228-231 Meter-in circuit, 160 Plug valye, 154 -155 Meter-out circuits, 160 Pneumatic gases, 46-48 Micronic filter, 108-109 compressed air, 47 Missile fluid power system, 245-250 contamination control, 48 electrohydraulic, 249-250 hazards, 47-48 electropneumatic, 248-249 nitrogen, 47 pneumatic, 247-248 qualities, 46 Moisture separator, 113 Pneumatics: Motors: application, 3 electrical, 235 definition, 3 fluid power, 224-231 development, 3 applications of, 224, 231 Poppet valves, 198-199, 206-208 axial piston, 229-231 Porous metal filtering element, 109, 113 function of, 224 Position measuring device, 284 gear type, 226-227 Positive displacement pump, 121 piston type, 228-231 Potential energy, 26 radial piston, 228-229 Pressure control devices, 180-197 vane type, 227 Pressure, definition, 11 Moving ball fluidic device, 281 Pressure gages, 50, 171-180 Moving part amplifier, 262 Pressure reducing valves, 190-193 Moving part devices, 280-281 Pressure regulator, 188-190 Moving shutter governor, 232, 233 Pressure switch, 193-194 Multiple-stage turbine, 231, 232 Priority valve, 163, 165 Multiplication of forces, 22-23 Proportional flow filter, 107-108 Pumps: NAND function, 266, 270, 275 application of, 50, 126, 132 Needle valve, 157, 159 axial piston, 136-142 Neoprene seals, 87-88 centrifugal, 122 -126 Nitrogen, 47 classification of, 121, 125, 127 Nonpositive displacement pump, 121 gear type, 127-130 NOR function, 266, 269, 275 helical gear, 127, 129 NOT function, 266, 269, 275 herringbone gear, 127, 128 internal gear, 127 -130 Open-center system, 52-53 lobe, 131 OR function, 266, 269, 275 nonpositive displacement, 121 Orifice check valve, 158-160 performance, 121 0-ring seals, 89-92 positive displacement, 121 identification and inspections, 89-90 purpose of, 120 replacement, 91-92 radial piston, 133-136 reciprocating, 132-142 Packhigs, 86 rotary; 126-132 Pascal's law, 18 screw, 130 Passive element, 262 spur gear, 127 Petroleum base liquids, 41 Stratopower, 139-142 Physics of fluids, 5-3 3 types of, 124 298

.343 (9.1.M' FLUID POWER Pumps: Continued: Single-acting, piston type actuating cylinder, vane, 131 219-220 Pure fluid amplifier, 262 Single-acting ram, 216-217 Single-stage turbines, 231-232 Quad-ring seals, 92 Sliding spool valve, 199-200, 209-213 Quick-disconnect couplings, 80-84 Snubbers, 180 Sound-sensitive device, 284-285 Radial piston motor, 228-229 Specific gravity, 6-7 Radial piston pump, 133-136 Spiral gage, 175, 177 Ram type actuator, 216-219 Spring-loaded pressure gage, 178-180 Rankine temperature scale, 9-10 Spring-operated accumulator, 114 Receivers, pneumatic system, 49, 103-104 Spur gear pump, 127 Reciprocating pumps, 132-142 Starting valve, 200-201, 202 Reduction gear, 235-236 States of matter, 5-6 Regulator, 188-190 Static and dynamic factors, 35 Relief valves, 50, 181-185 Static seals, 86 Reservoirs, 49, 98-103 Strainers, 105 nonpressurized, 99-101 Steady flow, 28 pressurized, 101-103 Stratopower pump, 139-142 Restrictors, 158, 159 Streamline flow, 28-30 Rotameter, 169-170 Sump pump and oscillator, 236 Rotary pumps, 126-132 Symbols, 53-57 Rotary spool valve, 199, 208-209 Synthetic base liquids, 41 Synthetic rubber seals, 87-88 Saybolt viscosimeter, 38 Tables: Screw pumps, 130 aeronautical mechanical symbols, 54-56 Schematic diagrams, 59 mechanical symbols other than aeronautical, Seals: 56-57 dynamic, 86 truth, 263 gaskets, 86 tubing size designation, 70 handling of, 96-97 typical values of specific gravity, 8 materials, 87-88 wall thickness schedule designation of pipe, cork, 88 89 metal, 88 Tandem actuating cylinder, 222 synthetic rubber, 87-88 Tangential velocity, 123-124 packing, 86 Teflon washers, 96 selection of, 98 Telemotor control, 243-245 shelf life, 96-97 Telescoping actuator, 218 static, 86 Temperature, 8-11 storage of, 98-97 absolute zero, 8 types, 88-94 measurement scales, 9-11 cup, 93 Temperature control, hydraulic fluids, 110-111 flange, 94 Temperature sensing device, 285 0-ring, 89-92 Three-way valve, 203-205 quad-ring, 92 Toxicity of liquids, 40 U-ring, 93, 94 Transfer valve, 213 V-ring, 92, 93 Truth tables, 262 -263 Sensing devices, 281-286 Tubing: Sensor switch, 281-282 applications, 69-71 Sequence valves, 186-188 materials, 68 Servo valve, 213-215 sizes, 68, 70 Shuttle valve, 201-203 Turbine: Simple fluid power system, 80-66 governors, 232-233 Simplex gage, 172 multiple stage, 231, 232 INDEX

Turbine:Continued: Valve:Continued: pinwheel, 231 priority, 163, 165 single stage, 231-232 relief, 181-185 Turbulence amplifier, 274-278 rotary spool, 199, 208-209 Turbulent flow, 28-30 sequence, 186-188 Two-way valve, 203, 204 servo, 213-215 shuttle, 201-203 Unsteady flow, 28 sliding spool, 199-200, 209-213 U-ring seals, 93, 94 starting, 200-201, 202 three-way, 203-205 Vacuum, 24-25 transfer, 213 Valve: two-way, 203-204 application, 154 Vane type motor, 227 brake, 238 Vane type pump, 131 cam-operated, 204-205 Velocity of flow, 28 check, 52, 200, 201 Viscosimeter, 38 classification, 153, 198 Viscosity, 37-39 compensated, 161-162 Viscosity index, 38 counterbalance, 185-186 Volume of flow, 28 directional control, 50, 198-215 Volute type centrifugal pump, 124 flow control, 154 Vortex amplifier, 278-280 flow equalizer, 162-163 V-ring seals, 92, 93 four-way, 206-213 gate, 155-156, 157 Wall attachment devices, 267-271 globe, 156-157, 158 Washers: needle, 157, 159 backup, 94 orifice check, 158-160 installation of, 95 pilot-operated, 191-193 leather, 95 plug, 154-155 Teflon, 96 poppet, 198-199, 206-208 Water base liquids, 40 pressure reducing, 190-193 Wipers, 94 pressure regulator, 188-190 Work, 26

300 U. 1. 1001,11111111101TNever=OPTICZ 1 1510 433-5S7/3