AND DRIVES

GEAR TYPES ...... A145 GEAR TYPES 99%. Some sliding does oc- GEAR BASICS ...... A153 cur, however. And because ears are compact, SPEED REDUCERS ...... A158 contact is simultaneous positive-engagement, across the entire width of G power SELECTING GEAR DRIVES...... A162 the meshing teeth, a contin- elements that determine ANALYZING GEAR FAILURES...... A168 uous series of shocks is pro- the speed, torque, and di- duced by the gear. These rection of rotation of driven rapid shocks result in some elements. Gear types may be objectionable operating noise and vi- grouped into five main categories: bration. Moreover, tooth wear results Spur, Helical, Bevel, Hypoid, and from shock loads at high speeds. Noise Worm. Typically, shaft orientation, ef- and wear can be minimized with ficiency, and speed determine which of proper lubrication, which reduces these types should be used for a partic- tooth surface contact and engagement ular application. Table 1 compares shock loads. these factors and relates them to the specific gear selections. This section on gearing and gear drives describes the major gear types; evaluates how the various gear types are combined into gear drives; and considers the principle factors that affect gear drive selection.

Spur gears Spur gears have straight teeth cut Figure I — Involute generated by parallel to the rotational axis. The unwrapping a cord from a circle. tooth form is based on the involute curve, Figure 1. Practice has shown the tool traces a trochoidal path, Fig- Figure 2 — Root fillet trochoid generated that this design accommodates ure 2, providing a heavier, and by straight tooth cutting tool. mostly rolling, rather than sliding, stronger, root section. Because of this contact of the tooth surfaces. geometry, contact between the teeth The involute curve is generated occurs mostly as rolling rather than Spur gears are the least expensive to during gear machining processes us- sliding. Since less heat is produced by manufacture and the most commonly ing gear cutters with straight sides. this rolling action, mechanical effi- used, especially for drives with parallel Near the root of the tooth, however, ciency of spur gears is high, often up to shafts. The three main classes of spur gears are: external tooth, internal tooth, and rack-and-. External-tooth gears — The most common type of , Figure 3, has teeth cut on the out- side perimeter of mating cylindri- cal wheels, with the larger wheel called the gear and the smaller wheel the pinion. The simplest arrangement of spur gears is a single pair of gears called a single reduction stage, where output rotation is in a direc- tion opposite that of the input. In other words, one is clockwise while the other is counter-clockwise. Higher net reduction is pro- duced with multiple stages in

2001 MSD ● Motion System Design ● A145 Chordal thickness Circular thickness

Figure 4 — Internal (ring) gears produce a rangement, the plan- complex form of output with a planetary ets may be restrained configuration of sun, planets, and ring. from orbiting the sun and the ring left free closure in these applications, but to move. This causes some type of cover may be provided to the ring gear to rotate keep dirt and other contaminants in a direction opposite from accumulating on the working that of the sun. By al- surfaces. lowing both the planet carrier and the Helical gears ring gear to rotate, a differential gear drive Helical gearing differs from spur in Figure 3 — Spur gears have straight teeth is produced, the output speed of one that helical teeth are cut across the cut parallel to the rotational axis. shaft being dependent on the other. gear face at an angle rather than Rack-and-pinion which the driven gear is rigidly con- gears — A straight nected to a third gear. This third gear bar with teeth cut then drives a mating fourth gear that straight across it, Fig- serves as output for the second stage. ure 5, is called a rack. In this manner, several output speeds Basically, this rack is on different shafts can be produced considered to be a from a single input rotation. spur gear unrolled Internal (ring) gears — Ring and laid out fiat. gears produce an output rotation that Thus, the rack-and- is in the same direction as the input, pinion is a special Figure 4. As the name implies, teeth case of spur gearing. are cut on the inside surface of a cylin- The rack-and-pin- drical ring, inside of which are ion is useful in con- mounted a single external-tooth spur verting rotary motion gear or set of external-tooth spur to linear and vice gears, typically consisting of three or versa. Rotation of the four larger spur gears (planets) usu- pinion produces lin- ally surrounding a smaller central ear travel of the rack. pinion (sun). Conversely, move- Normally, the ring gear is station- ment of the rack causes the pinion to Figure 5 — Rack-and-pinion gearing ary, causing the planets to orbit the rotate. produces linear travel from rotational sun in the same rotational direction The rack-and-pinion is used exten- input. Shown here is spur gearing. Helical gearing is also available, but is not as as that of the sun. For this reason, sively in machine tools, lift trucks, common because the helical teeth create this class of gear is often referred to as power shovels, and other heavy ma- thrust, which produces a force acting a planetary system. The orbiting mo- chinery where rotary motion of the across the face of the rack. Worm rack is tion of the planets is transmitted to pinion drives the straight-line action also available, the axis of the worm the output shaft by a planet carrier. of a reciprocating part. Generally, the (pinion) being parallel to, rather than In an alternative planetary ar- rack is operated without a sealed en- perpendicular to, the rack.

A146 ● Motion System Design ● MSD 2001 compact than double-heli- shafts. This overhung load (OHL) cals. However, the gear cen- may deflect the shaft, misaligning ters must be precisely gears, which causes poor tooth con- aligned to avoid interfer- tact and accelerates wear. Shaft de- ence between the mating flection may be overcome with strad- helixes. dle mounting in which a bearing is Cross-helical gears — placed on each side of the gear where This type of gear is recom- space permits. mended only for a narrow There are two basic classes of range of applications where bevels: straight-tooth and spirals. loads are relatively light. Straight-tooth bevels — These Because contact between gears, also known as plain bevels, teeth is a point instead of a have teeth cut straight across the face line, the resulting high slid- of the gear, Figure 9. They are subject ing loads between the teeth to much of the same operating condi- requires extensive lubrica- tions as spur gears in that straight- Figure 6 — Helical gears have teeth cut tooth bevels are effi- across the face at an angle for gradual cient but somewhat loading. noisy. They produce thrust loads in a di- straight, Figure 6. Thus, the contact rection that tends to line of the meshing teeth progresses separate the gears. across the face from the tip at one end Spiral-bevels — to the root of the other, reducing the Curved teeth provide noise and vibration characteristic of an action somewhat spur gears. Also, several teeth are in like that of a helical contact at any one time, producing a gear, Figure 10. This more gradual loading of the teeth that produces smoother, reduces wear substantially. quieter operation The increased amount of sliding ac- than straight-tooth tion between helical gear teeth, how- bevels. Thrust load- ever, places greater demands on the ing depends on the di- lubricant to prevent metal-to-metal rection of rotation and contact and resulting premature gear Figure 7 — Double helical Figure 8 — Herringbone whether the spiral failure. Also, since the teeth mesh at gearing uses two pairs of gears have opposed teeth angle at which the an angle, a side thrust load is pro- opposed gears to eliminate joined in the middle. teeth are cut is posi- duced along each gear shaft. Thus, thrust. tive or negative. thrust bearings must be used to ab- sorb this load so that the gears are tion. Thus, very little held in proper alignment. power can be trans- The three other principle classes of mitted with cross- helical gears are: double-helical, her- helical gears. ringbone, and cross-helical. Double-helical gears — Thrust Bevel gears loading is eliminated by using two pairs of gears with tooth angles op- Unlike spur and posed to each other, Figure 7. In this helical gears with way, the side thrust from one gear teeth cut from a cylin- cancels the thrust from the other drical blank, . These opposed gears are usually gears have teeth cut Figure 9 — Straight-tooth bevel gears are manufactured with a space between on an angular or conical surface. efficient but somewhat noisy. the opposing sets of teeth. Bevel gears are used when input and Herringbone gears — Teeth in output shaft centerlines intersect. these gears resemble the geometry of Teeth are usually cut at an angle so Hypoid gears a herring spine, with ribs extending that the shaft axes intersect at 90 deg, from opposite sides in rows of paral- but any other angle may be used. A Hypoid gears resemble spiral- lel, slanting lines, Figure 8. Herring- special class of bevels called miter bevels, but the shaft axes of the pinion bone gears have opposed teeth to gears have gears of the same size with and driven gear do not intersect, Fig- eliminate side thrust loads the same their shafts at right angles. ure 11. This configuration allows both as double helicals, but the opposed Often there is no room to support shafts to be supported at both ends. In teeth are joined in the middle of the bevel gears at both ends because the hypoid gears, the meshing point of the gear circumference. This arrange- shafts intersect. Thus, one or both pinion with the driven gear is about ment makes herringbone gears more gears overhang their supporting midway between the central position

2001 MSD ● Motion System Design ● A147 duces efficiency. In ends. This configuration allows the fact, the hypoid com- worm to engage more teeth on the bines the sliding ac- wheel, thereby increasing load capacity. tion of the worm gear In worm-gear sets, the worm is with the rolling move- most often the driving member. How- ment and high tooth ever, a reversible worm-gear has the pressure associated worm and wheel pitches so propor- with the spiral bevel. tioned that movement of the wheel ro- In addition, both the tates the worm. driven and driving In most worm gears, the wheel has gears are made of teeth similar to those of a helical gear, steel, which further but the tops of the teeth curve inward increases the de- to envelop the worm. As a result, the mands on the lubri- worm slides rather than rolls as it cant. As a result, spe- drives the wheel. Because of this high cial extreme pressure level of rubbing between the worm lubricants with both and wheel teeth, the efficiency of oiliness and anti-weld worm gearing is lower than other ma- Figure 10 — Spiral bevel-gears have properties are required to withstand jor gear types. curved teeth for smoother operation. the high contact pressures and rub- One major advantage of the worm bing speeds in hy- gear is low wear, due mostly to the poids. full-fluid lubricant film that tends to Despite these de- be formed between tooth surfaces by mands for special lu- the worm sliding action. A continuous brication, hypoid film that separates the tooth surfaces gears are used exten- and prevents direct metal-to-metal sively in rear axles of contact is typically provided by a rela- automobiles with tively heavy oil, which is often com- rear-wheel drives. pounded with fatty or fixed oils such Moreover, they are as acidless tallow oil. This adds film being used increas- strength to the lubricant and further ingly in industrial reduces friction by increasing the oili- machinery. ness of the fluid.

Figure 11 — Hypoid gears resemble spiral bevels, but the shaft axes do not intersect. Worm gearing Therefore, both shafts can be supported at both ends. Worm gear sets, Figure 12, consist of a of a pinion in a spiral-bevel and the ex- screw-like worm treme top or bottom position of a (comparable to a pin- worm. This geometry allows the driv- ion) that meshes with ing and driven shafts to continue past a larger gear, usually each other so that end-support bear- called a wheel. The ings can be mounted. These bearings worm acts as a screw, provide greater rigidity than the sup- several revolutions of port provided by the cantilever mount- which pull the wheel ing used in some bevel gearing. Also through a single revo- adding to the high strength and rigid- lution. In this way, a ity of the hypoid gear is the fact that wide range of speed the hypoid pinion has a larger diame- ratios up to 60:1 and ter and longer base than a bevel or higher can be ob- spiral-bevel gear pinion of equal ratio. tained from a single Although hypoid gears are stronger reduction. and more rigid than most other types, Most worms are cylindrical in shape Figure 12 — Worm gearing has they are one of the most difficult to lu- with a uniform pitch diameter. How- perpendicular, nonintersecting shafts in bricate because of high tooth-contact ever, a double-enveloping worm has a which the worm acts as a screw. Several pressures. Moreover, the high levels variable pitch diameter that is narrow- revolutions of the worm pull the wheel of sliding between tooth surfaces re- est in the middle and greatest at the through one revolution.

A148 ● Motion System Design ● MSD 2001 Worm-gear friction is further re- duced through the use of metals with K inherently low coefficients of friction. For example, the wheel is typically made of bronze and the worm of a highly finished, hardened steel. These low-friction materials can be used in 1/K worm gears because pressures are more Driving gear Speed reduction ratio uniformly distributed over the tooth 0 360 surface than most other gear types. Driven gear Rotation of driving gear, deg Worm-gear shafts are perpendicu- Figure 14 — Elliptical gears with two lobes (bilobes) provide twice as many periods of lar, non-intersecting, and may be po- variable output speed. sitioned in a variety of orientations.

Noncircular gears Though often overlooked, noncircular gears can provide several types of un- Speed usual motion or speed characteristics. Cams and linkages can provide these special motion requirements as Driven gear 0 360 well, but noncircular gears often rep- Driving gear Rotation of driving gear, deg resent a simpler, more compact, or more accurate solution. Servo sys- Figure 15 — Multispeed gears give one constant speed for part of a cycle and a different tems may also be able to do the job, constant speed for a second part of the cycle. but they are usually more expensive and require more expertise to solve varies from 1/K to K during each cycle than elliptical gears. motion problems. of rotation, where practical values of Constant-speed segments. Common requirements handled by K range up to 3. Where an application requires several noncircular gears include converting a As the gears in Figure 14 rotate, constant-speed periods within a cycle, constant input speed into a variable the radii of the driving and driven multispeed gears, Figure 15, may be output speed, and providing several gears change, so that speed first de- the answer. These gears make the different constant-speed segments dur- creases for 1/4 revolution, then in- transition between speeds by using ing an operating cycle. Other applica- creases for 1/4 revolution, etc. These special function segments on the gear tions require combined translation and periods of increasing or decreasing perimeter between the constant- rotation, or stop-and-dwell motion. speed occur four times per revolution. speed sections. Variable speed. Several types of Elliptical gears are commonly used in Translation and rotation. For noncircular gears, particularly ellipti- packaging and conveyor applications. applications requiring both transla- cal gears, generate variable output Triangular. A pair of triangular tional and rotational motion, certain speeds. Other, less commonly used gears has three lobes, or high points gears serve as cam substitutes. Often types are triangular and square gears. on the perimeter, rather than the two used in labeling , the cam Elliptical. A set of like elliptical lobes in elliptical bilobe gears. So tri- gear duplicates the shape of a part to gears can run at a constant center dis- angular gears deliver six periods of be labeled and a cam-following rack tance, but deliver an output speed speed increase or decrease per revolu- carries the labeling device at a con- that changes as they rotate. Elliptical tion, rather than four. stant surface speed. gears come in two basic types: Square. Gears that are square have Stop-and-dwell motion. Some unilobe, Figure 13, which rotates four lobes, so they produce eight peri- machines must provide either stop- about one of two fixed points on its ods of speed increase or decrease per and-dwell motion or reverse motion. long axis, and bilobe, Figure 14, revolution. This is achieved by combining noncir- which rotates about its center. The Both triangular and square gears cular gears with round gears and a speed-reduction ratio of these gears have a smaller range of speed ratios differential (epicyclic ). Stop-and-dwell motion is common in indexing mechanisms. Reverse motion K is required where a transfer device must operate between two locations.

GEAR BASICS 1/K

Speed reduction ratio Types of gears Driving gear Driven gear 0 360 Rotation of driving gear, deg Spur gears. Gears with teeth straight and parallel to the axis of rotation. Figure 13 — Elliptical unilobe gears provide variable output speed. Helical gears. Gears with teeth that

2001 MSD ● Motion System Design ● A149 spiral around the body of the gear. Internal gears. Gears with teeth on be an external gear.) External gears. Gears with teeth on the inside of a hollow cylinder. (The Bevel gears. Gears with teeth on the the outside of a cylinder. mating gear for an internal gear must outside of a conical-shaped body (nor-

A150 ● Motion System Design ● MSD 2001 mally used on 90-deg axes). around a cylindrical body like screw worm-gear axis.) Worm gears. Gearsets in which one threads. (Normally this gear, called Face gears. Gears with teeth on the member of the pair has teeth wrapped the worm, has its axis at 90 deg to the end of the cylinder.

2001 MSD ● Motion System Design ● A151 Hypoid gears. Similar in general Line of action. The path of action for by which a tooth space exceeds the form to bevel gears, but operate on involute gears. It is the straight line thickness of the engaging tooth on the non-intersecting axes. passing through the pitch point and operating pitch circles. tangent to the base circle. Elements of gear teeth Line of contact. The line or curve Angular dimensions along which two tooth surfaces are Tooth surface. Forms the side of a tangent to each other. Helix angle. The inclination of the gear tooth. Point of contact. Any point at which tooth in a lengthwise direction. If the Tooth profile. One side of a tooth in two tooth profiles touch each other. helix angle is 0 deg, the tooth is paral- a cross section between the outside lel to the axis of the gear and is really circle and the root circle. Linear and circular a spur-gear tooth. Flank. The working, or contacting, Lead angle. The inclination of a side of the gear tooth. The flank of a measurements thread at the pitch line from a line 90- spur gear usually has an involute pro- deg to the shaft axis. file in a transverse section. Center distance. The distance be- Shaft angle. The angle between the Top land. The top surface of a gear tween the parallel axes of spur gears or axes of two non-parallel gear shafts. tooth. of parallel helical gears, or the crossed Pitch angle. In bevel gears, the an- Bottom land. The surface at the bot- axes of crossed helical gears or of gle between an element of a pitch cone tom of the space between adjacent worms and worm gears. Also, it is the and its axis. teeth. distance between the centers of the Angular pitch. The angle subtended Crown. A modification that results in pitch circles. by the circular pitch, usually ex- the flank of each gear tooth having a Offset. The perpendicular distance pressed in radians. slight outward bulge in its center between the axes of hypoid gears or area. A crowned tooth becomes gradu- offset face gears. Ratios ally thinner toward each end. A fully Pitch. The distance between similar, crowned tooth has a little extra mate- equally spaced tooth surfaces along a Gear-tooth ratio. The ratio of the rial removed at the tip and root areas given line or curve. larger to the smaller number of teeth also. The purpose of crowning is to en- Diametral pitch. A measure of tooth in a pair of gears. sure that the center of the flank car- size in the English system. In units, it Contact ratio. To assure smooth, ries its full share of the load even if is the number of teeth per inch of continuous tooth action, as one pair of the gears are slightly misaligned or pitch diameter. As the tooth size in- teeth passes out of action, a succeed- deflect under load. creases, the diametral pitch de- ing pair of teeth must have already Root circle. Tangent to the bottom of creases. Diametral pitches usually started action. It is desired to have as the tooth spaces in a cross section. range from 25 to 1. much overlap as possible. A measure Pitch circle. Concentric to base cir- Axial pitch. Linear pitch in an axial of this overlapping action is the con- cle and including pitch point. Pitch plane and in a pitch surface. In helical tact ratio. circles are tangent in mating gears. gears and worms, axial pitch has the Hunting ratio. A ratio of numbers of Gear center. The center of the pitch same value at all diameters. In gear- gear and pinion teeth which ensures circle. ing of other types, axial pitch may be that each tooth in the pinion will con- Line of centers. Connects the cen- confined to the pitch surface and may tact every tooth in the gear before it ters of the pitch circles of two engag- be a circular measurement. contacts any gear tooth a second time. ing gears; it is also the common per- Base pitch. In an , the (13 to 48 is a hunting ratio; 12 to 48 is pendicular of the axes in crossed pitch on the base circle or along the not a hunting ratio.) helical gears and worm gears. line of action. Corresponding sides of Pitch point. The point of a gear-tooth involute gear teeth are parallel General Terms profile which lies on the pitch circle of curves, and the base pitch is the con- that gear. At the moment that the stant and fundamental distance be- Runout. A measure of eccentricity pitch point of one gear contacts its tween them along a common normal relative to the axis of rotation. Runout mating gear, the contact occurs at the in a plane of rotation. is measured in a radial direction and pitch point of the mating gear, and Axial base pitch. The base pitch of the amount is the difference between this common pitch point lies on a line helical involute tooth surfaces in an the highest and lowest reading in 360 connecting the two gear centers. axial plane. deg, or one turn. For gear teeth, Lead. The runout is usually checked by either axial ad- putting pins between the teeth or us- vance of a ing a master gear. Cylindrical sur- thread or a faces are checked for runout by a mea- helical spiral suring probe that reads in a radial in 360 deg direction as the part is turned on its (one turn specified axis. about the Undercut. When part of the involute shaft axis). profile of a gear tooth is cut away near . its base, the tooth is said to be undercut. Figure 16 — Backlash and tip relief. The amount Undercutting becomes a problem when

A152 ● Motion System Design ● MSD 2001 the number of pinion teeth is small. angle. Parallel shaft units typically Flash temperature. The tempera- use helical or spur gears, whereas ture at which a gear tooth surface is right-angle units contain bevel, bevel calculated to be hot enough to destroy and helical, or worm gearing. the oil film and allow instantaneous A particular type of base-mounted welding or scoring at the contact point. reducer is the free-shaft type, which Full depth teeth. Those in which the usually has one high-speed input working depth equals 2.000 divided shaft connected to a prime mover and by normal diametral pitch. a single or double output (low-speed) Tip relief. A modification of a tooth shaft connected to the load. In general profile, whereby a small amount of this type is a good choice if the prime material is removed near the tip of mover is remotely located or if only a the gear tooth to accommodate simple change in direction in the smooth engagement of the teeth. power train is required. Free-shaft re- Figure 17 — Based-mounted speed ducers are also recommended if a reducer with shafts mounted at right SPEED REDUCERS proper-sized gearmotor or motorized angles. reducer is not available, or if several A speed reducer is a gearset assem- loads must be driven by a single bled with appropriate shafting, bear- prime mover. ings, and lubrication in a sealed hous- Shaft-mounted reducers — ing, generally an oil-tight case. Such Some gear reducers are furnished units are also sometimes referred to with a hollow output shaft that slips as gear boxes, speed increasers, and over the driven shaft, which in turn gear reducers. supports the reducer. Free rotation of Speed reducers are available in a the housing is prevented by some type broad range of power capacities and of reaction member, often a torque speed ratios depending on gear size arm or flange attached to a stationary and type, with most having a maxi- portion of the machine. The gear unit mum speed limit of 3,600 rpm, and may or may not support the prime usually driven at a full-load speed of mover. Shaft-mounted reducers gen- 1,750 rpm or less. erally use helical or spur gears for par- Lubrication of speed reducers is ac- allel-shaft arrangements and worm complished either by a splash or cir- gearing for right-angle arrangements. culating system. Bearings may be lu- Figure 18 — Shaft-mounted speed reducer Though many shaft-mounted reducers bricated automatically with the same with torque arm to prevent housing are now in use, they are generally oil as the gearset, or they may have rotation. available only in a limited range of separate systems. The specific lubri- sizes and are used most widely in ma- cant required for a speed reducer de- terial handling applications. pends on a number of factors includ- Gearmotors — A gearmotor com- ing operating speed, ambient bines an enclosed gearset with a mo- temperature, loads, and method of lu- tor. The frame of one component sup- bricant application. More information ports the other, and the motor shaft is on lubrication is discussed in the PT typically common with or coupled di- Accessories Product Department. rectly to the gear input shaft. In one Gear types — Most speed reduc- version, called an integral gearmotor, ers use one or more of the common the speed reducer and motor share a gear types previously discussed. How- common shaft. Another type uses an ever, the use of helical, worm, and input flange, Figure 20, for connec- bevel gearsets is particularly preva- Figure 19 — Gearmotor. lent, especially in small and medium- sized speed reducers. Helical gears — Gear reducer units that have feet are often used in combination with bolted to a stationary pad or other spiral-bevel or worm gears. structural member, account for the Selection of a particular style of largest number of speed reducer ap- speed reducer for an application de- plications. In general, the prime pends primarily on shaft arrange- mover is mounted on the same struc- ment, type of gearing, ratio range, ture as the reducer or on the reducer and horsepower range. Three broad itself, but can be mounted on a sepa- categories of speed reducers, grouped rate structure. according to mounting arrangements, The input and output shafts of these are base-mounted, shaft-mounted, base-mounted speed reducers can be and gearmotor, Figures 17 to 19. arranged horizontally or vertically, Figure 20 — Input flange on a speed Base-mounted speed reducers and parallel to each other or at a right reducer accepts a C-face motor.

2001 MSD ● Motion System Design ● A153 tion to a C-face motor. and higher speed ratios. Motorized reducers resemble gear- Constant-ratio and other motors and perform much the same roller drive designs use one function. This reducer type commonly or more rows of stepped incorporates a scoop, Figure 21, which planet rollers to provide a supports the motor. Generally, the high speed ratio in a single distinction between motor-reducer planetary stage. The drive units depends on whether the motor functions as either a speed is an integral part (as in a gearmotor) reducer or increaser depend- or a modular part (as in a reducer-mo- ing on whether power is in- troduced through the sun roller or the outer roller ring.

Cycloidal drives Cycloidal drives, Figure Figure 22 — Concept drawing shows how 23, transmit power equal to that of traction drives transfer power from the input to output shaft with a set of mating gears, but in a smaller and more effi- rollers. By varying radius, R, output cient package. In contrast to the cir- speed can be adjusted while input speed cular motion of gears, cycloidal drives remains constant. use noncircular or eccentric compo- nents to convert input rotation into a wobbly cycloidal motion. This cy- Figure 21 — Scoop-mount motor-reducer. cants were used so that traction cloidal motion is then converted back drives had to be designed with very into smooth, concentric output rota- high contact pressures between tion. In the process, speed reduction tor combination). rolling members to carry the load occurs. Gearmotors and motorized reduc- without slip. Today’s traction fluids The term cycloidal is derived from ers often are specified when the re- reduce this required contact pressure hypocycloidal, which is defined as the ducer is the first component following so drives can be designed with longer curve traced by a point on the circum- the prime mover in a power train. Of lives and higher power capacities. Un- ference of a circle that is rotating in- the two, the motorized reducer is be- der the high pressure, viscosity of the side the circumference of a larger ing used more extensively because the fluid increases dramatically so the fixed circle. A common example of this motor can be changed quickly and fluid behaves more like a plastic ma- motion is the path traced by a tooth of easily, standard motors are readily terial than a liquid. This plastic-like a planetary pinion rotating inside a available for replacement, and a wide film enables the drive to transmit ring gear. range of options can be provided from power without appreciable metal-to- Whereas worm gearing experiences a few basic components. metal contact. a dramatic loss of efficiency in going Another contributor to traction from low to very high input/output Traction drives drives’ increased power capacity is speed ratios; and helical gearing loses improved high-grade bearing steel efficiency at high ratios because two Traction drives, Figure 22, trans- that resists fatigue, thereby extend- or more stages of reduction are re- mit mechanical power from source to ing the life of the drives. quired; cycloidal drives achieve reduc- load by means of mating metal Traction drives are compact and tion ratios as high as 200:1 in a single rollers, which may be considered can be used instead of belts, gears, or stage, while still maintaining moder- gears with infinite numbers of teeth. chain drives where space is limited. ately high efficiencies. Moreover, be- These rollers can be cones, cylin- Moreover, traction drives are quieter cause components in- ders, discs, rings, spheres, or toroids. than most other mechanical drives teract in a rolling fashion, failure is The speed ratio is determined by the because there is no engagement shock generally not catastrophic. As in a radius of rotation of the driver roller loading or backlash. Other inherent bearing, fatigue in the rolling sur- on the driven member, the distance advantages include high speed reduc- faces of a cycloidal drive causes noise between the rollers, or their orienta- tion or multiplication, low vibration, levels to gradually increase, serving tion with respect to each other. excellent rotational accuracy, and as a warning long before complete Traction drives are available in two high efficiency at all speeds. drive failure occurs. types — dry and lubricated. Dry trac- Many traction drives are capable of Heat generation, attributable tion drives eliminate the need for lu- adjusting the speed ratio infinitely mainly to mechanical losses and the bricant and allow nearly 100% effi- throughout a range. These variable- power being transmitted, is readily ciency in power transmission. ratio traction drives are discussed in dissipated through the large surface Slippage between driving and driven the Adjustable-speed Drives Product area of other types of gears. But cy- members is prevented by a spring- Department of this handbook. More cloidal drives, like worm gears, must loaded system. recently, constant-ratio traction dissipate heat through a smaller Other traction drives use synthetic drives have been developed to accom- housing surface area. However, be- fluids. Previously, conventional lubri- modate increased power, longer life, cause efficiency in the cycloidal drive

A154 ● Motion System Design ● MSD 2001 shafts, and concentric shafts. Speed ratio — The ratio of input speed to that of the output is an- other significant factor dictating the type of gearing to be selected. By examining efficiency and gear type, the user can determine if a sin- gle high-ratio stage is sufficient or if multi- stage gearing is re- quired. Geared systems can be driven at constant or varying speeds, de- pending on the require- ments of the applica- tion. When a geared system is selected for an adjustable-speed application, operating speed should be deter- mined along with oper- ating cycle and power requirements. From Figure 23 — Cycloidal drives, such as this this information, a lubrication system planocentric unit, convert input rotation SELECTING GEAR DRIVES can be selected, the heat dissipation into a wobbly cycloidal motion. This evaluated, the effect of speed varia- cycloidal motion is then converted back Gears can be selected, rated, in- tion on the dynamic characteristics into smooth circular output rotation, with speed reduction occurring in the process. stalled, and maintained by most users determined, and the need for special through common standards and prac- balancing determined. tices developed by the American Gear Gears can be custom-designed to is higher than worm gearing of equal Manufacturers Association. However, meet specific speed requirements, or capacity and ratio, less heat is gener- the services of a gear-engineering spe- standard ratios may be used from ated in cycloidal units. Consequently, cialist are generally required to make manufacturers’ catalogs. Generally, a the auxiliary cooling often required more detailed, in-depth analysis in standard ratio results in a less expen- for worm units is usually not needed cases where severe duty, extreme reli- sive drive. Standard ratios estab- for cycloidal drives. ability, unusually long service, or lished by AGMA are a series of values There are various types of cycloidal other extraordinary conditions exist. based on the 1.5 geometric progres- drives currently available. Harmonic In either case, the major selection fac- sion, which is a modification of the and planocentric drives are general tors include: shaft orientation, speed ASA 10 series ranging from 1.225 to types of cycloidals in which speed re- ratio, design style, nature of load, ser- 1,810.0:1. Tolerances for these ratios duction occurs in converting the input vice factor, environment, mounting are 3% for single reduction, 4% for motion into cycloidal motion. Double- position, ratio, lubrication, and instal- double and triple reduction, and 5% reduction cycloidals achieve greater lation practices. All these factors for quadruple reduction. There are no ratios because speed is reduced twice: must be carefully considered in select- AGMA standards for worm-gear ra- first in converting input rotational ing gears for optimal operation in a tios. However, popular ratios for sin- motion into cycloidal motion, and particular application. gle reduction systems range from 5 to again in converting the cycloidal Shaft orientation — Probably the 70, and for double reduction from 75 motion back into rotational output first consideration in selecting a gear to 5,000. motion. type is to determine the required ori- Design style — The application Although not as efficient as spur or entation of the input to the output should be evaluated to determine if helical gearing, cycloidal drives offer shaft. According to the type of gear se- individual open gears will be suffi- substantially higher efficiency than lected, various shaft arrangements cient or if an enclosed speed reducer is worm gearing. The concentric shaft are available including: parallel required. In general, individual gears orientation also proves valuable, as shafts, shafts at right angles with in- require rigid shafting to keep the does the drive’s compact size and high tersecting axes, shafts at right angles gears aligned. And shafts should not reduction capability. with nonintersecting axes, skewed introduce external loads to the gear-

2001 MSD ● Motion System Design ● A155 ing. Typically, an enclosure around cally subjected to torque loads, axial quired life, operating duty, and dy- the gears with oil lubrication is the loads, and radial loads. These last two namic characteristics of the driving preferred design, but grease-lubri- types are usually called externally ap- and driven machines. Typically, this cated open gears can be used in rela- plied thrust loads and externally ap- service rating is determined by multi- tively clean environments. plied overhung loads, and they can be plying the required horsepower by the Nature of load — Theoretically, caused by chains, , V-belts, or appropriate service factor based on gear teeth and bearings under flat belts. equipment, duty cycle, and type of stresses below the endurance limit The type of load introduced by the prime mover. and lubricated properly will last in- prime mover depends on the opera- For most speed reducers (spur, he- definitely, provided the operating tional characteristics of the prime lical, herringbone, and bevel), AGMA loads are within the design specifica- mover. Electric motors and turbines, recognizes three load classifications tions. Thus, determining the nature for example, produce relatively for determining service factors: uni- of the load is important in selecting smooth operation whereas an inter- form, moderate shock, and heavy gears for long life and reliable ser- nal-combustion engine does not afford shock. Based on field experience, nu- vice. Gears may be subjected to occa- so smooth a load. merical values have been assigned to sional loads (e.g., 15 to 30 minutes Overhung load (OHL), if produced these classifications for intermittent per day), intermittent loads (applied by the drive, requires the use of out- service, for service between 3 and 10 several minutes per hour), or contin- board bearings or speed reducers that hours per day, and for service beyond uous service loads (10 to 24 hours per will accept this OHL. OHL is imposed 10 hours per day. The factors are de- day). The life of the geared system is on the shaft when a pinion, , pendent upon the prime mover. More- the period of operating time or cycles sheave, pulley, or crank is mounted over, the load classification and resul- during which the system can trans- on the input or output shaft. tant service factor varies with the mit a required load. Life of the gear To calculate the magnitude of the application of different types of may end with a fracture or opera- load, multiply the transmitted force geared units. tional failure of a component or with that is tangent to the pitch circle of Using these service factors, a max- the development of excessive noise, the mounted member by the OHL imum momentary or starting load of vibration, or heat. factor. For a single or multiple-chain 200% of rated load can be allowed for Determining the nature of a gear drive, this OHL factor is 1.00. The most gears, with rated load defined load involves the consideration of factor is 1.25 for a cut pinion run as the unit rating with a service maximum horsepower, drive iner- with gear teeth, 1.50 for a single or factor of 1.0. tia, overhung load, and speed limit multiple V- drive, and 2.50 for a Service factors for worm gears are of the gear. flat-belt drive. determined somewhat differently, Maximum horsepower or maximum OHL given by manufacturers are with normal starting or momentary torque of the prime mover is one of the usually specified in catalogs at one peak loads up to 300% of rated load most significant considerations on shaft diameter from the housing face. permissible. which selection of a geared unit is Load-location factors are also given so Environment — The type of gear based. A gear is rated approximately that, regardless of where the load is selected must also compensate for a as a constant-torque machine, with acting, it can be converted to the ref- less-than-ideal environment, which the horsepower rating varying almost erence position and compared with can adversely affect gear-system per- directly with the input speed. Either the cataloged value. formance if proper precautionary constant-torque or constant-horse- The magnitude of the OHL that can steps are not taken. The most com- power motors are used with gears, de- safely be applied depends upon sev- mon types of hostile environments are pending on the application. For exam- eral factors including bearing life or dust, heat, wide variation in tempera- ple, constant-torque motors are pressure, elastic shaft deflection, ture, moisture, and chemicals. Gener- required for applications such as con- shaft strength, and bolt strength. ally, each has particular adverse ef- veyors, stokers, and reciprocating Speed limit is based on a maximum fects on lubricant, gears, bearings, or compressors. Constant-horsepower pitch line velocity of 5,000 fpm and seals. Dusty atmospheres may con- motors are required for lathes, boring pinion speeds of 3,600 rpm or less. For taminate the lubricant, for example. mills, radial drill presses, etc. worm gearing, speed limits are based Moreover, heat may accelerate lubri- Drive inertia also requires torque to on a maximum sliding velocity of cant breakdown, affect the perfor- overcome more than the drive load of 6,000 fpm and worm speeds of 3,600 mance of synthetic contact oils, or the output, because a drive is seldom rpm and less. Occasionally, the speed lower gear capacity by lowering mate- subjected to only a single-intensity limitation may be that for a bearing or rial properties and distorting the load. For example, if an electric motor contact oil seal, based on the manu- gear. Wide temperature variation is the prime mover, peak starting facturer’s limiting speed. may cause improper lubrication for torques as high as 400% of the motor Service factor — Basic ratings gears and bearings, thereby shorten- rating can be applied to the drive dur- take into account minimum design ing the life of the unit. Moisture infil- ing start-up. The geared unit is only criteria for gears, lubricants, shafts, tration may accelerate lubricant one member of the system that may bearings, and other gearset compo- breakdown, corrode components, and contain members with high inertia, nents. Service factors are then used to accelerate the wear of contact seals. unbalance, or torsional stiffness. adjust the basic rating to a service Contact seals should be used on in- The shafts of geared units are typi- rating that is compatible with the re- put and output shafts when the unit

A156 ● Motion System Design ● MSD 2001 operates in dusty environments or lubricant to the bearings and gears They allow grease to be purged from a where water is splashed around the will result in their damage. As a re- chamber between the seals. unit. In atmospheres laden with abra- sult, the type of lubricating system sive dust or in areas hosed down with should be chosen carefully, because Installation practices water under pressure, two contact this may be the most critical compo- seals may be required on each shaft. nent of the entire system. Successful performance of a gear Mounting position — Mounting In addition, accepted practices unit is vitally dependent upon instal- position is an important selection cri- must be observed by the user to main- lation practices. The foundation must teria, because most gear units are de- tain proper lubrication during the life be rigid and level, the shafts aligned, signed to operate in either a horizon- of the geared unit. One of the most the accessories properly installed, the tal or vertical position only. In some common causes for gear damage, for required grade and quality of lubri- applications, however, the geared example, is the failure to fill the unit cant added, and any special instruc- unit may have to operate in a position to the proper level with the first tions followed carefully. inclined to either axis or inclined to change of lubricant, so data on the lu- Foundation — The gear unit must both axes. In this case special consid- brication plate and all other instruc- provide a level, secure base on which erations must be given in selecting tions must be followed carefully. Give to operate. The base should be the gear systems regarding oil level, special attention to warning tags. mounted horizontally, unless the unit air-vent position, and location of oil- The user should also know the tem- has been designed for mounting in a drain holes. perature range in which the unit is tilted position. Moreover, the base Gear rating — All components of designed to operate. If the unit is op- should be precisely leveled, since only a commercial enclosed geared unit erating where temperatures vary a few degrees of tilt might adversely are usually rated by established prac- widely, the oil viscosity should be affect lubrication efficiency. If a unit tices, with the rating for the entire changed to suit the conditions. For is to be mounted in a position differ- system determined by the lowest rat- low-temperature operation, the oil ent from that for which it was de- ing for any one part for a given set of should have a pour-point lower than signed, changes might be necessary to operating conditions. Generally, there that of the extreme minimum temper- provide proper lubrication. are two types of ratings for a geared ature encountered. And the oil may Most gear units have a drain plug unit: mechanical and thermal. require pre-heating under extremely in the base for oil drainage. The unit The mechanical rating is based on cold starting conditions. should be mounted above floor level, the strength of the gears, shafts, Several lubrication system types or a sump hole used for this purpose. bolts, etc., or the load compatible with are available. Enclosed gear units are Drain plugs may be replaced with a the required bearing life or pressure, commonly lubricated by splash sys- valve or with pipe extensions for con- or the resistance of the gears to pit- tems in which one of the gears dips venient drainage. However, guarding ting or scoring. into an oil bath and transfers the lu- must be provided to prevent acciden- The thermal rating specifies the bricant to the contacting teeth as it tal breakage. Drain valves may be power that can be transmitted with- rotates. For low-speed operation, provided with a lock to prevent acci- out exceeding a specified rise above scrapers close to the side of the gear dental or unauthorized opening. ambient in the operating temperature may be used so that splash from the When mounting a unit on struc- of the unit. Typically, enclosed drives gears reaches oil-feed troughs to lu- tural steel beams, a base plate should operate at temperature rises of 70 to bricate the bearings. be used between the two beams to 100 F above ambient temperature. Pressure or forced-spray lubrica- which the unit is attached. The thick- Generally, a maximum oil-sump tem- tion reduces oil churning. In this sys- ness of the base should be equal to or perature of 200 F is permissible for tem, lubricant is pumped under pres- greater than the thickness of the unit gearmotors and shaft-mounted speed sure to the gear train. After passing feet, and it should extend under the reducers, even though thermal rat- through the gears, it is returned to entire unit. Mountings that are not ings are not specified by manufactur- the reservoir to be recirculated. This sufficiently rigid can lead to excessive ers or defined by AGMA. method uses the gear oil as a cooling vibration and deflection. When the applied horsepower ex- medium. The lubricant may be deliv- Procedures — When installing ceeds the thermal horsepower, auxil- ered as a stream running over the the geared unit bring it into align- iary cooling methods are used includ- gearing or as a spray from jet or ment by placing broad, flat shims un- ing: use of an oil-exclusion pan to spray nozzles. These nozzles may di- der all mounting pads. Start at the reduce churning of the oil and the re- rect the lubricant to the entering or low-speed end and level across the sulting higher temperatures, air- the trailing side of the gear mesh, de- width and then along the length of the cooling the housing, circulating wa- pending on speed. unit. Most units have leveling sur- ter or some other cooling medium Many gear units (particularly ver- faces for reference. Use a feeler gage around the unit, using a separate oil tical units) have grease-lubricated under all pads to make certain that sump for greater heat dissipation, or bearings. Grease is normally intro- all are carrying equal loads. This pre- using a cooler mounted inside the duced at the factory and grease fit- vents distortion of the housing when gear housing. tings are provided for lubrication in foundation bolts are secured. Lubrication — The most impor- the field. Some units designed to oper- Units equipped with backstops tant factor in a lubricating system is ate in extremely adverse conditions should receive special attention during reliability, because failure to supply have double seals mounted in cages. assembly. If the motor is connected to

2001 MSD ● Motion System Design ● A157 start in the wrong direction, the sudden ing part numbers, service history, and shock can immediately break the back- lubricant type. Interview those in- stop or cause premature failure shortly volved in the design, installation, op- thereafter. Thus, disconnect the motor eration, and maintenance of the gear- and check the rotating direc- box. Encourage them to tell tion of the backstop by rotating the gear everything they know about the gear- unit by hand. Then check the rotation of box even if it seems unimportant. the motor by an actual start. Visual examination. Before dis- The user should also make sure that assembling the gearbox, inspect its (a) all , shims, and pinions are exterior and record data that would properly installed so that no destructive otherwise be lost. For example, the load is applied to the gear unit or to condition of seals and keyways must these parts because of misalignment. be recorded before disassembly. Oth- Hubs, pinions, , pulleys, and erwise, it will be impossible to deter- other components should be carefully mine when these parts may have been installed (not driven on with a hammer) damaged. Take gear tooth contact on shafts. Couplings should be aligned patterns before completely disassem- so that the angular and parallel align- bling the gearbox (see next section). ment are within the limits specified by After the external examination, the coupling manufacturer. The dis- disassemble the gearbox and inspect (b) tance between the shaft ends or cou- all components. Examine the gear Figure 24 — Typical gear tooth contact pling hubs should also be checked to en- teeth and bearings and record their patterns: (a), aligned, and (b), sure they agree with the specifications. condition. Look for signs of corrosion, misaligned. Maintenance — Proper mainte- contamination, and overheating. nance is essential in keeping a gear After the initial inspection, wash where needed. Mark each component unit running properly. After a week or the parts with solvent and re-examine so it is clearly identified. Mark the more of service, check all external bolts them. This is often the most important bearings so their position in the gear- and pipe plugs to make sure they are phase of the investigation and may box can be determined later. tight. Also, check chains, belts, sprock- yield valuable clues. A low power Gear geometry. Obtain the fol- ets, and pinions, to make sure that the magnifying glass and pocket micro- lowing data, which will be needed load is being distributed evenly and scope are helpful tools for this phase. later to calculate the load capacity of that there is no unusual condition. The bearings often provide clues as the gearset: After a few hundred hours of opera- to the cause of gear failure: • Number of teeth. tion, the user should drain all oil, • Bearing wear can cause exces- • Outside diameter. flush the system with an oil of similar sive radial clearance or end play that • Face width. grade, and refill the lubricating sys- misaligns the gears. • Gear housing center distance for tem to the proper level. Of course, oil • Bearing damage may indicate each gearset. level should be periodically checked corrosion, contamination, electrical • Whole depth of teeth. under all conditions. discharge, or lack of lubrication. • Tooth thickness (span and top • Deformation between rollers and land). ANALYZING GEAR raceways may indicate overloads. Test specimens. Take broken • Gear failure often follows bear- parts for laboratory testing. Oil sam- FAILURES ing failure. ples can be very helpful. But, an effec- Gear tooth contact patterns. tive oil analysis depends on having By following a step-by-step proce- (Complete this step before disassem- representative samples. For a gear- dure, engineers can diagnose gear bling gearbox components). The way box drain or reservoir, take a sample failures and develop solutions. Here’s in which mating gear teeth contact in- from the top, middle, and bottom. how to conduct a failure analysis. dicates how well they are aligned, Check the oil filter and magnetic plug Procedures can vary depending on Figure 24. If the gears are still able to for wear debris and contaminants. failure conditions. For example, if the function, record tooth contact pat- gears are still able to function, you terns under either loaded or unloaded Determine type of may continue their operation and conditions. For no-load tests, paint monitor the rate at which damage the teeth of one gear with marking failure progresses via periodic inspection and compound. Then, roll the teeth sound and vibration measurements. through mesh so the contact pattern Now examine all of the information If reliability is crucial, examine the transfers to the unpainted gear. to determine how the gear(s) failed. gears by magnetic particle inspection For loaded tests, use machinist’s Several failure modes may be present. to ensure that they have no cracks. layout lacquer to paint the teeth, then Identify the primary mode of failure, run the gears to wear off the lacquer and any secondary modes that may Failure inspection and establish the contact patterns. have contributed to the failure. The Document observations. De- four most common failure modes are Before starting the inspection, col- scribe your observations in writing, bending fatigue, contact fatigue, lect background information, includ- using sketches and photographs wear, and scuffing.

A158 ● Motion System Design ● MSD 2001 Origin Progressive cantly. macropitting con- Finally, polishing is fine abrasion sists of pits larger that imparts a mirror-like finish to than 1 mm diam. gear teeth, Figure 30. It is promoted Ratchet marks Micropitting by fine abrasives in the lubricant. has a frosted, Severe polishing removes all ma- Origin matte, or gray chining marks. The surface may be stained appear- wavy or it may have wear steps at the Fracture zone ance. Magnifica- ends of the contact area and in the tion shows the dedendum. surface to be cov- Scuffing. Severe adhesion (scuff- ered by pits less ing) transfers metal from one tooth to than 20 m deep. another, Figure 31. It usually occurs Beach mark Wear. Tooth in bands along the direction of slid- Beach mark wear involves re- ing. Surfaces have a rough or matte Fatigue zone moval or displace- texture. ment of material Mild scuffing is generally nonpro- Figure 25 — Bending fatigue fracture surfaces of gear teeth. Upper tooth has due to mechanical, chemical, or electri- gressive. Moderate scuffing occurs in multiple origins of failure. cal action. The three major types are patches, and it can be progressive. adhesion, abrasion, and polishing. Severe scuffing occurs on signifi- Adhesion is the transfer of material cant portions of a tooth (for example, Bending fatigue. This type of fail- from one tooth ure, caused by repeated loading, to another due Beach marks starts as a crack that grows until the to welding part fractures. As a fatigue crack and tearing, Crack growth propagates, it leaves “beach marks” Figure 28. It is that correspond to positions where categorized as the crack stopped, Figure 25. The ori- mild or mod- gin of the crack is usually surrounded erate. by several of these beach marks. Mild adhe- Most fatigue failures occur in the sion usually tooth root fillet, Figure 26. occurs during Contact fatigue. In another gearset run- mode, called contact fatigue, repeated in and sub- Origins stresses cause cracks and detachment sides after it of metal fragments from the tooth wears imperfections from the surface. Figure 27 — Fatigue failure (pitting) in contact surface, Figure 27. The most Moderate adhesion removes machin- the contact surface of a gear tooth. common types of contact fatigue are ing marks from the contact surface, Beach marks are visible in some of the macropitting (visible to the naked and it can become excessive. larger pits. eye) and micropitting. Abrasion is caused by contaminants With macropitting, fatigue cracks in the lubricant such as scale, rust, entire addendum or dedendum), and grow until they cause a piece of the machining chips, grinding dust, weld is usually progressive. In some cases, surface to break out, forming a pit. splatter, and wear debris. It appears material is deformed and displaced as smooth, over the tooth tip or into the tooth parallel root. Tooth retrieved from oil sump scratches or gouges, Fig- Tests and calculations ure 29. Severe In many cases, inspection informa- abrasion re- tion isn’t enough to determine the moves all cause of failure. When this happens, machining gear design calculations and labora- marks, and it tory tests are needed. may cause Design calculations. The gear ge- wear steps at ometry data collected earlier aids in the ends of estimating tooth contact stress, bend- the contact ing stress, lubricant film thickness, surface and and tooth contact temperature. These Fatigue crack in the deden- values are calculated according to dum. Tooth American Gear Manufacturers Asso- thickness ciation standards (ANSI/AGMA 2001- may be re- B88 for spur and helical gears). Com- Figure 26 — Fatigue crack in a gear tooth root fillet. duced signifi- paring these calculated values with

2001 MSD ● Motion System Design ● A159 Figure 28 — Adhesion type wear of gear teeth. Figure 30 — Polishing type wear.

Figure 29 — Excessive abrasion type wear. Figure 31 — Scuffing of gear tooth surfaces.

AGMA allowable values helps to de- • Magnetic parti- completed, form a hypothesis for the termine the risk of macropitting, cle inspection. probable cause of failure, then deter- bending fatigue, and scuffing. • Acid etch inspection. mine if the evidence supports the hy- Laboratory tests. A microscopic • Gear tooth accuracy inspection. pothesis. Evaluate all of the evidence examination is useful for confirming Then conduct destructive tests to collected during the investigation. the failure mode or finding the origin evaluate material and heat treat- Finally, after testing the hypothe- of a fatigue crack. ment. These tests include: sis against the evidence, you reach a If tooth contact patterns indicate • Microhardness survey. conclusion about the most probable misalignment, check the gear accu- • Microstructural determination cause of failure. racy on gear inspection machines. using various acid etches. A failure analysis report should Conversely, where contact patterns • Determination of grain size and describe the inspections and tests, indicate good alignment, check for nonmetallic inclusions. and give conclusions. It usually con- metallurgical defects. • Examination of fracture surfaces tains recommendations for repairing Conduct nondestructive tests be- with scanning electron microscope the equipment, or changing its de- fore any destructive tests. These non- (SEM). sign or operation to prevent future destructive tests, which help detect failures. material or manufacturing defects, Conclusions and report include: Extracted from an article by Gear- • Surface hardness and roughness. When all calculations and tests are tech in the March 1994 issue of PTD.

A160 ● Motion System Design ● MSD 2001