FLUID POWER DRIVES

HYDRAULIC HYDRAULIC ADJUSTABLE-SPEED DRIVES...... A197 • PV-MF (variable ADJUSTABLE-SPEED ROTARY ACTUATORS ...... A199 pump, fixed motor) DRIVES FLUID MOTORS...... A201 • PF-MV (fixed pump, HYDRAULIC PUMPS...... A203 variable motor) ydraulic, or fluid, • PV-MV (variable adjustable speed pump, variable motor) H drives are manufactured to plications calling for close speed or • PF-MF (fixed pump, fixed motor) deliver 1 to 5,000 hp. These smooth power regulation, because of their with flow control valves. The general operating units can be controlled high overall operating efficiencies. characteristics of these units are sum- manually at the unit or by remote Axial piston pumps and motors com- marized in Table 1. control devices. All of the following monly have wobble or swash plates PV-MF — This is the most common drives are designed for installation that determine effective piston design. Output speed is adjusted by between a prime mover, such as an stroke, and hence displacement. varying the pump displacement. The electric motor, and the driven load. Output speed of a hydraulic motor pump can be stroked “across center” Some units are made with the prime is a direct function of its displacement to reverse the direction of oil flow and mover integral with the fluid drive. and the volume of fluid supplied. the direction of motor shaft rotation. The major types of these fluid Hence, varying the displacement of Since motor displacement is constant, drives include hydrostatic hydroki- either pump or motor varies drive output torque is constant if pressure netic, and hydroviscous. speed. is constant, Figure 1. Horsepower is Output torque-horsepower re- variable and a direct function of out- Hydrostatic lationships — Hydraulic adjustable put shaft speed at a given pressure speed drives are frequently classified setting. Applications for this drive in- Hydrostatic drives, typically rated according to their output characteris- clude machine tools, printing and pro- from 10 to 300 hp, incorporate posi- tics. Analysis of the load characteris- cessing machinery, foundry and con- tive displacement pumps and positive tics will usually determine the best tinuous casting equipment, textile displacement motors. Mechanical en- drive type for the job. machinery, elevators, and hoists and ergy is transmitted from the prime Broadly speaking, varying the vol- winches. mover to the pump. The pump im- ume of fluid delivered to a fixed dis- PF-MV — Pump output is constant parts energy through a fluid to a hy- placement motor will produce varying with a constant input speed. Hy- draulic motor. The prime mover can speed, constant torque, and varying draulic motor displacement is varied be an electric motor, a gasoline or horsepower at the output shaft. A to change output speed. Reducing mo- diesel engine, or a take-off from the fixed rate of fluid supplied to a vary- tor displacement increases output main machine drive. ing displacement motor develops speed. However, if this displacement Hydrostatic drives offer several fea- varying speed, variable torque, and is reduced to near zero, the drive tures that make them particularly constant horsepower. When both speed will attempt to reach an infinite adaptable to many adjustable speed pump and motor are variable dis- value, Figure 2. Therefore, output applications: infinitely adjustable placement types, speed, torque, and speeds are usually kept within design stepless change of speed, torque, and horsepower can be varied. limits by limiting displacement ex- horsepower; no damage even if stalled Hydrostatic adjustable speed tremes with mechanical stops. The at full load; operation in both direc- drives fall into four standard drive displacement range of a variable dis- tions of rotation at controlled speeds; categories based on the types of placement motor is commonly 4:1. set speeds held accurately against pumps and motors (fixed or variable Output torque is inversely propor- driving or braking loads; small size displacement): tional to output speed, maintaining and low weight per horsepower output. Output speed can be adjusted if ei- Table 1 — Performance characteristics of hydrostatic drives ther the pump or motor has variable based on constant pump speed and torque measured at displacement. If both are fixed dis- constant pressure. (C denotes constant; V denotes variable.) placement units, valves control fluid flow to the hydraulic motor. Pump Motor Torque Output speed Power Variable displacement pumps and motors are usually piston or vane de- Variable Fixed C V V signs. (Gear pumps and motors are Fixed Variable V V C generally not adaptable to variable Variable Variable V V V displacement.) Piston units have Fixed Fixed C C C proven to be the most capable in ap-

1998 PT Design A197 relatively constant power. This pattern is particularly useful in rewind drives that require con- stant web speeds. PV-MV — A vari- able-pump, variable- motor de- sign offers the widest operating Figure 1 — Torque and horsepower characteristics of PV-MF drive. range of any hydrostatic drive. Output speed can be varied by adjusting Figure 4 — Typical integrated hydrostatic the displacement of the transmission. pump, the motor, or both. Output torque and through the speed range. power are infinitely ad- PF-MF (with valving) — Pump justable across the com- output volume and motor torque are plete speed range in both constant, and output speed and horse- directions of rotation. power are variable. The flow-control Increasing pump dis- valve limits the amount of fluid avail- placement and main- able to the motor and thus controls taining the hydraulic output speed. Efficiency of this sys- motor at maximum dis- tem is relatively low compared with placement, delivers es- other hydrostatic drives, because of sentially constant torque the throttling action of the valve. Low output, at a constant in cost, this drive is usually applied pressure, while increas- where only small speed changes are ing output speed and required. horsepower, Figure 3. Integrated hydrostatic trans- With pump displace- missions — Frequently termed Figure 2 — Torque and horsepower characteristic of PF-MV drive. ment and system pressure held con- IHSTs, these units consist of a pump stant, the drive delivers essentially and motor contained in the same constant horsepower. housing and share a common valving If motor displacement surface. These compact units offer ex- increases, speed tremely short paths for oil flow, and drops, and torque in- eliminate high-pressure oil leaks creases proportion- either to the reservoir or to the out- ally, and vice versa. side, Figure 4. In some installations, pump and motor dis- Hydrokinetic placement are con- trolled simultane- Usually applied in applications ously to provide rated 1 to 5,000 hp, hydrokinetic ad- constant horsepower justable speed drives vary the output speed and torque by varying the amount of fluid in the vortex that cou- ples the input to the output, Figure 5. The fluid vortex size can be varied during operation by controlling the Figure 3 — Torque and angle of the scoop tube that removes horsepower fluid from the vortex. The tube angle characteristic of PV-MV can be controlled manually or by elec- drive. tric, hydraulic, or pneumatic means.

A204 1997 Power Transmission Design Since the output is only “connected” are splined to the in- to the input by the fluid, and without put shaft. The plain any direct mechanical connection, steel discs are keyed there is a 2 to 4% slip. This slip re- to the output shaft. duces efficiency, but offers good shock Both sets of discs are protection to the driving and driven free to slide axially. A equipment. A heat exchanger re- pump provides cool- moves the heat generated by this slip. ing oil to the inner di- Hydrokinetic drives offer an operat- ameter of the compo- ing speed range of about 8:1. In most sition-faced discs, cases, the minimum speed is specified which have a pattern to allow the oil to Figure 7 — Rack-and-pinion. because operating at too low a speed flow through the disc pack. This oil will cause drive instability. also flows between the discs and trans- In addition to controlling the shaft mits the torque from the input to the Rack-and-pinion — One side of output speed and torque, these units of- output plates. the piston contains a gear rack that fer a smooth or soft-start characteristic. The output shaft contains a cylin- engages a pinion to turn the output der and an actuating shaft. Output torque can be doubled piston that produces an in versions built with two parallel pis- axial force on the disc ton-rack units. High tolerance for side stack. This controls the and end loading makes these actua- oil-film thickness be- tors suitable for heavy-duty applica- tween the rotating tions, Figure 7. plates, and adjusts the Piston-chain — Two pistons, a output torque. For max- chain, and a sprocket convert fluid imum output, the out- pressure into torque. The larger pis- put plates are forced ton pulls the chain in response to pres- firmly against the input surized fluid, while the smaller piston plates with no slip. seals the return side of the chain A heat exchanger dis- against leakage. Torque remains con- sipates the heat gener- stant throughout the stroke. Design Figure 5 — Hydrokinetic adjustable speed ated by the slipping action that occurs constraints include the strength of the drive. during reduced speed operation. chain and sprocket as well as the bulk of the unit itself, Figure 8. Hydroviscous ROTARY ACTUATORS Sometimes termed oil shear drives, these units are adjustable-speed, con- Devices known as stant-torque, variable-slip drives. rotary actuators They are similar to other variable-slip produce limited re- drives in efficiency and application. ciprocating rotary Capable of operating at zero slip, hy- force and motion by droviscous drives are rated from 25 to rotating an output over 300 hp. shaft through a The heart of the drive is the disc fixed arc. Typically, stack — alternating layers of steel they accomplish discs. Alternate discs are faced with a their task at higher composition material, Figure 6, and instantaneous torques and relatively Figure 8 — Piston-chain. lower speeds than fluid motors, which produce lower torques and unlim- Scotch-yoke — Two pistons are ited arcs at higher speeds. connected by a common rod. At the be- ginning and end of the stroke, torque Types output is twice the value produced at the stroke’s midpoint. Applications Since most types of ro- that require a high breaking torque to tary actuators are perma- move the load find this type of actua- nently lubricated and tor appropriate, Figure 9. sealed, they are especially Enclosed piston-crank — Also suitable for harsh and de- termed linear cylinders, these units Figure 6 — Hydroviscous drive. manding environments. The types feature adjustable stroke for variable include: shaft rotation up to 110 deg. In opera-

1997 Power Transmission Design A205 Figure 9 — Scotch-yoke.

Figure 12 — Single-vane.

Figure 10 — Enclosed piston-crank.

Figure 13 — Double-vane.

Figure 11 — Helical-spline terparts. Two vanes and barri- tion, a pin-ended rod connected to a ers provide a balance that crank arm drives the rotating shaft. counteracts the tendencies of Fail-safe versions of this actuator con- unbalanced loads, Figure 13. tain a spring that returns the shaft to Bladder — A pair of rubber a safe position in the event of power bladders are alternately pres- failure or loss of fluid, Figure 10. surized and exhausted to pro- Helical-spline — A short piston duce the driving force. When with a high helical-angle internal pressurized, the bladder thread meshes with a central helical pushes against a cup-shaped drive shaft. Alternating fluid pres- lever arm that rotates the out- sure causes the piston to reciprocate, put shaft. Zero internal leak- reversing shaft rotation. Standard ro- age makes this actuator highly tations vary from 100 to 370 deg, accurate and resistant to con- Figure 11. tamination. Bladder and fluid Single-vane — Differential pres- compatibility is the only re- sure applied across the vane rotates quirement for fluid choice. the drive shaft. Rotation is stopped by Versions that use rack-and-pinion Figure 14 — Bladder type. a stationary barrier. To reverse rota- mechanisms in conjunction with tion, pressure is reversed, Figure 12. precharged bladders provide the Double-vane — These units offer highest torques (to 45,000 lb-in.), twice the torque and less than half while standard versions are typically the rotation of their single-vane coun- rated to 2,750 lb-in, Figure 14.

A206 1997 Power Transmission Design Applications Rotary-abutment motors contain vanes that roll in contact with outer General applications for rotary actu- sealing services, Figure 17. This de- ators include the following operations: sign maintains a virtually frictionless • Mixing seal that is relatively insensitive to • Dumping wear. It also offers low and high- • Intermittent feeding speed capability. • Screw clamping • Positioning Rotary Abutments (2) • Opening and closing • Turning over Figure 15 — Radial-vane motor. • Automated transfer • Providing constant tension • Material handling however, the more popular types are • Indexing discussed here. • Lifting Vane-type motors — Radial-vane • Pushing and pulling motors have vanes that slide radially in slots that are ma- chined into a rotor, Figure 15. The ec- centric rotor is mounted on a shaft Figure 17 — Rotary-abutment motor. Timing gears that keep rotor and within a cylindrical abutments in proper relationship are not bore referred to as a shown. cam ring. The vanes are spring loaded to maintain contact with the cam ring Gear-type motors — Gear-on- during startup and gear motors consist of a pair of at low speeds. At matched gears enclosed in a housing. high speeds, cen- Both gears have the same tooth form trifugal force assists and are either spur or helical types. in maintaining One gear is connected to an output vane-to-cam con- shaft; the other functions as an idler. tact. As fluid enters As the gears rotate within the hous- the motor, pressure ing, fluid is swept from the inlet to the against the vanes outlet, Figure 18. turns the rotor, Gear-within-gear motors consist of sweeping the fluid an inner and outer gearset, Figure 19. from the inlet to the FLUID MOTORS outlet port. Axial-vane motors use vanes that Fluid-powered motors offer the rotate about fixed points on the cir- highest power-per-pound ratio of any cumference of the rotor, Figure 16. industrial motor, thus enabling them Small fixed clearances reduce friction to be used in space-restricted areas. for better start and run-torque effi- Since they are totally enclosed, fluid ciencies in a motor that has low and motors can operate in highly contami- high speed capabilities. nated areas, and do not require ex- pensive protective devices in explo- sive or flammable atmospheres. Figure 18 — Gear-on-gear motor. Generally, hydraulic and pneu- matic motors are designed along simi- lar lines. Though the internal design of each type may be quite different, operating principles are much alike.

Motor types Fluid motors use one of three basic designs to convert fluid pressure into rotary motion: vane, gear, or piston. Several variations of each type exist; Figure 16 — Axial-vane motor. Figure 19 — Gear-within-gear motor.

1997 Power Transmission Design A207 In other designs, just the inner gear rotates. Another design, the roller-vane gerotor, uses rollers between the inner and outer gears to reduce wear, Fig- ure 20. Regardless of the basic design, gear-type motors offer greater power density and higher torque at low speeds than vane-type motors. Piston-type motors — Figure 20 — Roller-vane gerotor. These motors use pistons to con- Figure 21 — Axial-piston motor. vert fluid pressure into rotary motion. When compared to other fluid motors, Often referred to as gerotors, the in- piston motors are very efficient, offer ner gear has one less tooth than the inner gear rotates on a shaft that is high torque at low speed, but gener- outer gear. The shape of the teeth is mounted eccentric to the bore, fluid ally have limited high-speed capabil- such that all teeth of the inner gear travels from inlet to outlet port. ity. Two basic designs include pistons are in contact with some portion of the Several variations of the gear- arranged in either a radial or axial outer gear at all times. Because the within-gear motor exist, with the configuration. inner gear has one less tooth than the most apparent differences occuring in Axial-piston motors, Figure 21, outer gear, a fluid pocket is formed the gerotor design. In most designs, contain several pistons (usually seven within the remaining cavity. As the both the inner and outer gear rotate. to nine) that bear against a swash

Table 3 — Typical fluid motor specifications Hydraulic motors Radial Radial piston piston Radial Axial Rotary Gear-on- Gear-in- Roller- Axial (rotary (fixed vane vane abutement gear gear gerotor piston barrel) cylinder)

Maximum continuous pressure (psi) 2,500 3,000 3,000 3,000 2,000 4,500 2,500-5,000 3,000 5,000 Displacement (in.3/rev) 12 10 12 20 5 57 44 98 2,310 Maximum torque (lb-in.) 4,000 3,200 4,800 6,000 1,500 16,400 17,500 46,000 1,100,000 Starting torque (% of theoretical) 70 97 95 70 70 68 72 88 97 Running torque (% of theoretical) 90 95 95 90 87 85 93 95 97 Continuous speed range (rpm) 80-4,000 0-2,000 5-2,000 100-3,000 100-5,000 0-1,000 50-4,500 1-2,000 0-300 Maximum continuous power (hp) 140 70 65 200 100 32 413 250 300 Leakage (% of max theoretical displ.) 2.5 3 10 8 10 15 2.5 3 3.5 Weight-to-power ratio (lb/hp) 0.3 1.1 1.3 0.2 0.2 1.5 0.7 2 2 Pneumatic motors Maximum continuous pressure (psi) 150 150 150 Rated pressure(psi) 90 90 90 Maximum rated power (hp) 25 3.5 25 Maximum design speed (rpm) 6,500 3,000 2,000

A208 1997 Power Transmission Design plate. Fluid pressure causes the pis- tons to create a rotating force in the swash plate, which causes the output Figure 22 — Radial-piston motor. shaft to rotate. Radial-piston motors use pistons that radiate out from the driveshaft, Figure 22, and are available in sev- • Variable displacement — eral design variations. displacement per cycle can be Piston motors generally cost more varied from zero to maximum than vane or gear-type motors of com- volumetric capacity. parable horsepower. Typically, axial- piston motors have better high-speed Selection capabilities than radial-piston mo- tors, but are limited at low speeds. To determine a pump’s suit- Conversely, radial-piston motors HYDRAULIC PUMPS ability for a given application, three have better low speed capabilities different ratings are typically consid- than axial-piston motors, but are lim- Two general categories of pumps ered: ited at high speeds. Both axial and ra- are used for converting rotating to hy- Pressure ratings are a major design dial-piston motors are available in draulic power: consideration that tell the specifier fixed or variable-displacement mod- • Nonpositive displacement how much of a load the pump can els, which enable their torque-speed • Positive displacement withstand and still maintain flow. curves to be fine tuned to application Nonpositive displscement — Since a pump does not create pressure requirements. Made with a large clearance space be- but rather moves fluid, this rating tween rotating and stationary pump deals with the load on the fluid. The Selecting fluid motors parts, these pumps are generally used load creates a resistance to flow, for low-pressure, high-volume appli- which in turn creates pressure. Table 3 provides typical perfor- cations. Operations such as transfer- Flow ratings tell the pump specifier mance specifications for various hy- ring fluids, supplying coolant, and su- at what rate a specific volume of fluid draulic and pneumatic motors. Many percharging the inlet of other pumps is delivered. Also known as size, ca- sizes are available in each type with are examples of suitable applications. pacity, or delivery, the rating is nor- only the maximum values shown. Positive displacement — On the mally expressed in gallons per minute Therefore, on the basis of torque and other hand, positive displacement (gpm). horsepower, more than one type of pumps have a very close clearance be- Speed ratngs limit the permissible motor may meet application require- tween rotating and stationary parts. speed range to avoid cavitation or ments. In these cases, performance These pumps deliver a specific other damage. curves should be obtained from the amount of fluid to the system for each manufacturer to optimize motor se- revolution. Pumps used for hydraulic Gear pumps lection. power are positive displacement pumps that can be further subdi- Variations abound for internal and Figure 23 — Classification of the most vided, Figure 23, into two categories. external type gear pumps. All vari- common positive-displacement hydraulic Fixed delivery — provides a specific eties share the same principles of op- pumps. • volume displacement per revolution. eration; that is, gear pumps produce

1997 Power Transmission Design A209 within a cylinder block. As pistons re- ciprocate, they con- vert rotary shaft mo- tion into radial motion. One version has cylindrical pis- tons, while another uses ball-shaped pis- tons. Another classifi- cation refers to port- ing: check-valve Figure 24 — Spur-gear pump. Figure 26 — Axial-flow, three-screw radial piston pumps use a rotating pump. cam to reciprocate pistons; pintle- flow by using the teeth of two meshing valve pumps have a rotating cylinder gears to move the fluid. block, and piston heads contact an ec- External gear pumps can be Piston pumps centric stationary reaction ring. equipped with straight spur (the most common type), helical, or herringbone High performance char- gears. In operation, the drive gear and acterizes these pumps in driven gear rotate, creating a partial general, especially the bent- vacuum at the pump inlet (where gear axis variety used in teeth unmesh) that draws fluid into aerospace applications. gear teeth. Gear teeth mesh at the This means piston pumps outlet, forcing fluid out of the pump. can supply high flows at Internal gear pumps contain one in- high rpm. Two basic types ternal and one external gear. They of piston pumps — axial pump fluid in the same manner as ex- and radial piston—are ternal spur gear pumps. In the basic manufactured in both fixed design, the internal gear, which drives and variable displacement the outer gear, has one tooth less than versions. Figure 28 — Balanced-vane pump. the outer gear. As they mesh, the Axial piston pumps contain one or teeth create sliding seal points. An- more pistons that convert rotary shaft other design, the crescent pump, uses motion into axial reciprocating mo- Vane pumps a crescent-shaped seal to separate the tion. An angled cam or wobble plate two gears. The gerotor internal gear rotates, causing pistons to reciprocate Typically, a circular rotor mounted pump supplies a seal by means of slid- eccentrically in a ing contact, Figure 25. circular chamber Screw pumps operate with low noise comprises a vane because fluid does not pulsate; it moves pump. Vanes in- linearly. Single-screw pumps contain a side the rotor ex- spiraled rotor that rotates eccentrically tend and retract in an internal stator. Two-screw pumps due to centrifugal use two rotors that remain in edge con- loading as the ro- tact as they mesh inside a close-toler- tor spins. A high anced housing. The same type of hous- pressure area ing is used for a three-screw pump, tends to develop on where a central drive rotor meshes one side of the ro- with two idler rotors, Figure 26. tor and a low pres- Figure 27 — Axial-piston pump. sure area on the other, hence, the name unbalanced vane pump. To and take fluid in as they move eliminate the high bearing loads that toward the thin part of the result from this tendency, designers plate, Figure 27. Fluid is ex- offered the balanced vane pump, Fig- pelled as pistons approach the ure 28. It uses two diametrically op- thick end. In one version, the posed high pressure areas to equalize bent-axis design, both pistons forces on the pump shaft. Variations and shaft rotate, making a on the standard vane pump include a wobble plate unnecessary. version in which springs hold vanes Bent-axis pumps use drive against the housing, and another in shaft rotation to rotate pistons. which the vanes are moved outward Figure 25 — Crescent pump, a type of Radial piston pumps are character- by pressurized pins. ■ gear pump. ized by a radial piston arrangement

A210 1997 Power Transmission Design