Gears & Gear Drives - Regal Power Transmission · PDF fileGEAR TYPES G ears are compact,...

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GEAR TYPES G ears are compact, positive-engagement, power transmission elements that determine the speed, torque, and di- rection of rotation of driven machine elements. Gear types may be grouped into five main categories: Spur, Helical, Bevel, Hypoid, and Worm. Typically, shaft orientation, ef- ficiency, and speed determine which of these types should be used for a partic- ular application. Table 1 compares 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 parallel to the rotational axis. The tooth form is based on the involute curve, Figure 1. Practice has shown that this design accommodates mostly rolling, rather than sliding, contact of the tooth surfaces. The involute curve is generated during gear machining processes us- ing gear cutters with straight sides. Near the root of the tooth, however, 99%. Some sliding does oc- cur, however. And because contact is simultaneous across the entire width of the meshing teeth, a contin- uous series of shocks is pro- duced by the gear. These rapid shocks result in some objectionable operating noise and vi- bration. Moreover, tooth wear results from shock loads at high speeds. Noise and wear can be minimized with proper lubrication, which reduces tooth surface contact and engagement shock loads. Spur gears are the least expensive to manufacture and the most commonly used, especially for drives with parallel shafts. The three main classes of spur gears are: external tooth, internal tooth, and rack-and-pinion. External-tooth gears — The most common type of spur gear, 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 the tool traces a trochoidal path, Fig- ure 2, providing a heavier, and stronger, root section. Because of this geometry, contact between the teeth occurs mostly as rolling rather than sliding. Since less heat is produced by this rolling action, mechanical effi- ciency of spur gears is high, often up to GEARS AND GEAR DRIVES 2001 MSD Motion System Design A145 Figure I — Involute generated by unwrapping a cord from a circle. Figure 2 — Root fillet trochoid generated by straight tooth cutting tool. GEAR TYPES .....................................................A145 GEAR BASICS ...................................................A153 SPEED REDUCERS ............................................A158 SELECTING GEAR DRIVES ................................A162 ANALYZING GEAR FAILURES ...........................A168

Transcript of Gears & Gear Drives - Regal Power Transmission · PDF fileGEAR TYPES G ears are compact,...

Page 1: Gears & Gear Drives - Regal Power Transmission · PDF fileGEAR TYPES G ears are compact, positive-engagement, power transmission elements that determine ... GEARS AND GEAR DRIVES 2001

GEAR TYPES

G ears are compact,positive-engagement,power transmission

elements that determinethe speed, torque, and di-rection of rotation of drivenmachine elements. Gear types may begrouped into five main categories:Spur, Helical, Bevel, Hypoid, andWorm. Typically, shaft orientation, ef-ficiency, and speed determine which ofthese types should be used for a partic-ular application. Table 1 comparesthese factors and relates them to thespecific gear selections. This sectionon gearing and gear drives describesthe major gear types; evaluates howthe various gear types are combinedinto gear drives; and considers theprinciple factors that affect gear driveselection.

Spur gearsSpur gears have straight teeth cut

parallel to the rotational axis. Thetooth form is based on the involutecurve, Figure 1. Practice has shownthat this design accommodatesmostly rolling, rather than sliding,contact of the tooth surfaces.

The involute curve is generatedduring gear machining processes us-ing gear cutters with straight sides.Near the root of the tooth, however,

99%. Some sliding does oc-cur, however. And becausecontact is simultaneousacross the entire width ofthe meshing teeth, a contin-uous series of shocks is pro-duced by the gear. Theserapid shocks result in some

objectionable operating noise and vi-bration. Moreover, tooth wear resultsfrom shock loads at high speeds. Noiseand wear can be minimized withproper lubrication, which reducestooth surface contact and engagementshock loads.

Spur gears are the least expensive tomanufacture and the most commonlyused, especially for drives with parallelshafts. The three main classes of spur

gears are: external tooth, internaltooth, and rack-and-pinion.

External-tooth gears — Themost common type of spur gear,Figure 3, has teeth cut on the out-side perimeter of mating cylindri-cal wheels, with the larger wheelcalled the gear and the smallerwheel the pinion.

The simplest arrangement ofspur gears is a single pair of gearscalled a single reduction stage,where output rotation is in a direc-tion opposite that of the input. Inother words, one is clockwise whilethe other is counter-clockwise.

Higher net reduction is pro-duced with multiple stages in

the tool traces a trochoidal path, Fig-ure 2, providing a heavier, andstronger, root section. Because of thisgeometry, contact between the teethoccurs mostly as rolling rather thansliding. Since less heat is produced bythis rolling action, mechanical effi-ciency of spur gears is high, often up to

GEARS AND GEAR DRIVES

2001 MSD ● Motion System Design ● A145

Figure I — Involute generated byunwrapping a cord from a circle.

Figure 2 — Root fillet trochoid generatedby straight tooth cutting tool.

GEAR TYPES .....................................................A145GEAR BASICS ...................................................A153SPEED REDUCERS ............................................A158SELECTING GEAR DRIVES................................A162ANALYZING GEAR FAILURES ...........................A168

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which the driven gear is rigidly con-nected to a third gear. This third gearthen drives a mating fourth gear thatserves as output for the second stage.In this manner, several output speedson different shafts can be producedfrom a single input rotation.

Internal (ring) gears — Ringgears produce an output rotation thatis in the same direction as the input,Figure 4. As the name implies, teethare cut on the inside surface of a cylin-drical ring, inside of which aremounted a single external-tooth spurgear or set of external-tooth spurgears, typically consisting of three orfour larger spur gears (planets) usu-ally surrounding a smaller centralpinion (sun).

Normally, the ring gear is station-ary, causing the planets to orbit thesun in the same rotational directionas that of the sun. For this reason,this class of gear is often referred to asa planetary system. The orbiting mo-tion of the planets is transmitted tothe output shaft by a planet carrier.

In an alternative planetary ar-

closure in these applications, butsome type of cover may be provided tokeep dirt and other contaminantsfrom accumulating on the workingsurfaces.

Helical gearsHelical gearing differs from spur in

that helical teeth are cut across thegear face at an angle rather than

rangement, the plan-ets may be restrainedfrom orbiting the sunand the ring left freeto move. This causesthe ring gear to rotatein a direction oppositethat of the sun. By al-lowing both theplanet carrier and thering gear to rotate, adifferential gear drive

is produced, the output speed of oneshaft being dependent on the other.

Rack-and-piniongears — A straightbar with teeth cutstraight across it, Fig-ure 5, is called a rack.Basically, this rack isconsidered to be aspur gear unrolledand laid out fiat.Thus, the rack-and-pinion is a specialcase of spur gearing.

The rack-and-pin-ion is useful in con-verting rotary motionto linear and viceversa. Rotation of thepinion produces lin-ear travel of the rack.Conversely, move-ment of the rack causes the pinion torotate.

The rack-and-pinion is used exten-sively in machine tools, lift trucks,power shovels, and other heavy ma-chinery where rotary motion of thepinion drives the straight-line actionof a reciprocating part. Generally, therack is operated without a sealed en-

A146 ● Motion System Design ● MSD 2001

Figure 3 — Spur gears have straight teethcut parallel to the rotational axis.

Figure 4 — Internal (ring) gears produce acomplex form of output with a planetaryconfiguration of sun, planets, and ring.

Figure 5 — Rack-and-pinion gearingproduces linear travel from rotationalinput. Shown here is spur gearing. Helicalgearing is also available, but is not ascommon because the helical teeth createthrust, which produces a force actingacross the face of the rack. Worm rack isalso available, the axis of the worm(pinion) being parallel to, rather thanperpendicular to, the rack.

Circularthickness

Chordalthickness

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straight, Figure 6. Thus, the contactline of the meshing teeth progressesacross the face from the tip at one endto the root of the other, reducing thenoise and vibration characteristic ofspur gears. Also, several teeth are incontact at any one time, producing amore gradual loading of the teeth thatreduces wear substantially.

The increased amount of sliding ac-tion between helical gear teeth, how-ever, places greater demands on thelubricant to prevent metal-to-metalcontact and resulting premature gearfailure. Also, since the teeth mesh atan angle, a side thrust load is pro-duced along each gear shaft. Thus,thrust bearings must be used to ab-sorb this load so that the gears areheld in proper alignment.

The three other principle classes ofhelical gears are: double-helical, her-ringbone, and cross-helical.

Double-helical gears — Thrustloading is eliminated by using twopairs of gears with tooth angles op-posed to each other, Figure 7. In thisway, the side thrust from one gearcancels the thrust from the othergear. These opposed gears are usuallymanufactured with a space betweenthe opposing sets of teeth.

Herringbone gears — Teeth inthese gears resemble the geometry ofa herring spine, with ribs extendingfrom opposite sides in rows of paral-lel, slanting lines, Figure 8. Herring-bone gears have opposed teeth toeliminate side thrust loads the sameas double helicals, but the opposedteeth are joined in the middle of thegear circumference. This arrange-ment makes herringbone gears more

shafts. This overhung load (OHL)may deflect the shaft, misaligninggears, which causes poor tooth con-tact and accelerates wear. Shaft de-flection may be overcome with strad-dle mounting in which a bearing isplaced on each side of the gear wherespace permits.

There are two basic classes ofbevels: straight-tooth and spirals.

Straight-tooth bevels — Thesegears, also known as plain bevels,have teeth cut straight across the faceof the gear, Figure 9. They are subjectto much of the same operating condi-tions as spur gears in that straight-

tooth bevels are effi-cient but somewhatnoisy. They producethrust loads in a di-rection that tends toseparate the gears.

Spiral-bevels —Curved teeth providean action somewhatlike that of a helicalgear, Figure 10. Thisproduces smoother,quieter operationthan straight-toothbevels. Thrust load-ing depends on the di-rection of rotation andwhether the spiralangle at which theteeth are cut is posi-tive or negative.

Hypoid gears

Hypoid gears resemble spiral-bevels, but the shaft axes of the pinionand driven gear do not intersect, Fig-ure 11. This configuration allows bothshafts to be supported at both ends. Inhypoid gears, the meshing point of thepinion with the driven gear is aboutmidway between the central position

compact than double-heli-cals. However, the gear cen-ters must be preciselyaligned to avoid interfer-ence between the matinghelixes.

Cross-helical gears —This type of gear is recom-mended only for a narrowrange of applications whereloads are relatively light.Because contact betweenteeth is a point instead of aline, the resulting high slid-ing loads between the teethrequires extensive lubrica-

tion. Thus, very littlepower can be trans-mitted with cross-helical gears.

Bevel gearsUnlike spur and

helical gears withteeth cut from a cylin-drical blank, bevelgears have teeth cuton an angular or conical surface.Bevel gears are used when input andoutput shaft centerlines intersect.Teeth are usually cut at an angle sothat the shaft axes intersect at 90 deg,but any other angle may be used. Aspecial class of bevels called mitergears have gears of the same size withtheir shafts at right angles.

Often there is no room to supportbevel gears at both ends because theshafts intersect. Thus, one or bothgears overhang their supporting

2001 MSD ● Motion System Design ● A147

Figure 6 — Helical gears have teeth cutacross the face at an angle for gradualloading.

Figure 7 — Double helicalgearing uses two pairs ofopposed gears to eliminatethrust.

Figure 8 — Herringbonegears have opposed teethjoined in the middle.

Figure 9 — Straight-tooth bevel gears areefficient but somewhat noisy.

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of a pinion in a spiral-bevel and the ex-treme top or bottom position of aworm. This geometry allows the driv-ing and driven shafts to continue pasteach other so that end-support bear-ings can be mounted. These bearingsprovide greater rigidity than the sup-port provided by the cantilever mount-ing used in some bevel gearing. Alsoadding to the high strength and rigid-ity of the hypoid gear is the fact thatthe hypoid pinion has a larger diame-ter and longer base than a bevel or spiral-bevel gear pinion of equal ratio.

Although hypoid gears are strongerand more rigid than most other types,they are one of the most difficult to lu-bricate because of high tooth-contactpressures. Moreover, the high levelsof sliding between tooth surfaces re-

ends. This configuration allows theworm to engage more teeth on thewheel, thereby increasing load capacity.

In worm-gear sets, the worm ismost often the driving member. How-ever, a reversible worm-gear has theworm and wheel pitches so propor-tioned that movement of the wheel ro-tates the worm.

In most worm gears, the wheel hasteeth similar to those of a helical gear,but the tops of the teeth curve inwardto envelop the worm. As a result, theworm slides rather than rolls as itdrives the wheel. Because of this highlevel of rubbing between the wormand wheel teeth, the efficiency ofworm gearing is lower than other ma-jor gear types.

One major advantage of the wormgear is low wear, due mostly to thefull-fluid lubricant film that tends tobe formed between tooth surfaces bythe worm sliding action. A continuousfilm that separates the tooth surfacesand prevents direct metal-to-metalcontact is typically provided by a rela-tively heavy oil, which is often com-pounded with fatty or fixed oils suchas acidless tallow oil. This adds filmstrength to the lubricant and furtherreduces friction by increasing the oili-ness of the fluid.

duces efficiency. Infact, the hypoid com-bines the sliding ac-tion of the worm gearwith the rolling move-ment and high toothpressure associatedwith the spiral bevel.In addition, both thedriven and drivinggears are made ofsteel, which furtherincreases the de-mands on the lubri-cant. As a result, spe-cial extreme pressurelubricants with bothoiliness and anti-weld

properties are required to withstandthe high contact pressures and rub-

bing speeds in hy-poids.

Despite these de-mands for special lu-brication, hypoidgears are used exten-sively in rear axles ofautomobiles withrear-wheel drives.Moreover, they arebeing used increas-ingly in industrialmachinery.

Worm gearing

Worm gear sets,Figure 12, consist of ascrew-like worm(comparable to a pin-ion) that meshes witha larger gear, usuallycalled a wheel. Theworm acts as a screw,several revolutions ofwhich pull the wheelthrough a single revo-lution. In this way, awide range of speedratios up to 60:1 andhigher can be ob-tained from a singlereduction.

Most worms are cylindrical in shapewith a uniform pitch diameter. How-ever, a double-enveloping worm has avariable pitch diameter that is narrow-est in the middle and greatest at the

A148 ● Motion System Design ● MSD 2001

Figure 10 — Spiral bevel-gears havecurved teeth for smoother operation.

Figure 11 — Hypoid gears resemble spiralbevels, but the shaft axes do not intersect.Therefore, both shafts can be supported atboth ends.

Figure 12 — Worm gearing hasperpendicular, nonintersecting shafts inwhich the worm acts as a screw. Severalrevolutions of the worm pull the wheelthrough one revolution.

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Worm-gear friction is further re-duced through the use of metals withinherently low coefficients of friction.For example, the wheel is typicallymade of bronze and the worm of ahighly finished, hardened steel. Theselow-friction materials can be used inworm gears because pressures are moreuniformly distributed over the toothsurface than most other gear types.

Worm-gear shafts are perpendicu-lar, non-intersecting, and may be po-sitioned in a variety of orientations.

Noncircular gears Though often overlooked, noncircular

gears can provide several types of un-usual motion or speed characteristics.

Cams and linkages can providethese special motion requirements aswell, but noncircular gears often rep-resent a simpler, more compact, ormore accurate solution. Servo sys-tems may also be able to do the job,but they are usually more expensiveand require more expertise to solvemotion problems.

Common requirements handled bynoncircular gears include converting aconstant input speed into a variableoutput speed, and providing severaldifferent constant-speed segments dur-ing an operating cycle. Other applica-tions require combined translation androtation, or stop-and-dwell motion.

Variable speed. Several types ofnoncircular gears, particularly ellipti-cal gears, generate variable outputspeeds. Other, less commonly usedtypes are triangular and square gears.

Elliptical. A set of like ellipticalgears can run at a constant center dis-tance, but deliver an output speedthat changes as they rotate. Ellipticalgears come in two basic types:unilobe, Figure 13, which rotatesabout one of two fixed points on itslong axis, and bilobe, Figure 14,which rotates about its center. Thespeed-reduction ratio of these gears

than elliptical gears. Constant-speed segments.

Where an application requires severalconstant-speed periods within a cycle,multispeed gears, Figure 15, may bethe answer. These gears make thetransition between speeds by usingspecial function segments on the gearperimeter between the constant-speed sections.

Translation and rotation. Forapplications requiring both transla-tional and rotational motion, certaingears serve as cam substitutes. Oftenused in labeling machines, the camgear duplicates the shape of a part tobe labeled and a cam-following rackcarries the labeling device at a con-stant surface speed.

Stop-and-dwell motion. Somemachines must provide either stop-and-dwell motion or reverse motion.This is achieved by combining noncir-cular gears with round gears and adifferential (epicyclic gear train).

Stop-and-dwell motion is common inindexing mechanisms. Reverse motionis required where a transfer devicemust operate between two locations.

GEAR BASICS

Types of gearsSpur gears. Gears with teeth straightand parallel to the axis of rotation.Helical gears. Gears with teeth that

varies from 1/K to K during each cycleof rotation, where practical values ofK range up to 3.

As the gears in Figure 14 rotate,the radii of the driving and drivengears change, so that speed first de-creases for 1/4 revolution, then in-creases for 1/4 revolution, etc. Theseperiods of increasing or decreasingspeed occur four times per revolution.

Elliptical gears are commonly used inpackaging and conveyor applications.

Triangular. A pair of triangulargears has three lobes, or high pointson the perimeter, rather than the twolobes in elliptical bilobe gears. So tri-angular gears deliver six periods ofspeed increase or decrease per revolu-tion, rather than four.

Square. Gears that are square havefour lobes, so they produce eight peri-ods of speed increase or decrease perrevolution.

Both triangular and square gearshave a smaller range of speed ratios

2001 MSD ● Motion System Design ● A149

Driving gear Driven gear

Spe

ed r

educ

tion

ratio

1/K

K

Rotation of driving gear, deg0 360

Figure 13 — Elliptical unilobe gears provide variable output speed.

0

Driving gear

Driven gear

Spe

ed r

educ

tion

ratio

1/K

K

Rotation of driving gear, deg360

Figure 14 — Elliptical gears with two lobes (bilobes) provide twice as many periods ofvariable output speed.

0Driving gear

Driven gear

Spe

ed

Rotation of driving gear, deg360

Figure 15 — Multispeed gears give one constant speed for part of a cycle and a differentconstant speed for a second part of the cycle.

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spiral around the body of the gear. External gears. Gears with teeth onthe outside of a cylinder.

be an external gear.) Bevel gears. Gears with teeth on theoutside of a conical-shaped body (nor-

Internal gears. Gears with teeth onthe inside of a hollow cylinder. (Themating gear for an internal gear must

A150 ● Motion System Design ● MSD 2001

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mally used on 90-deg axes). Worm gears. Gearsets in which onemember of the pair has teeth wrapped

worm-gear axis.) Face gears. Gears with teeth on theend of the cylinder.

around a cylindrical body like screwthreads. (Normally this gear, calledthe worm, has its axis at 90 deg to the

2001 MSD ● Motion System Design ● A151

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Hypoid gears. Similar in generalform to bevel gears, but operate onnon-intersecting axes.

Elements of gear teethTooth surface. Forms the side of agear tooth. Tooth profile. One side of a tooth ina cross section between the outsidecircle and the root circle. Flank. The working, or contacting,side of the gear tooth. The flank of aspur gear usually has an involute pro-file in a transverse section. Top land. The top surface of a geartooth. Bottom land. The surface at the bot-tom of the space between adjacentteeth.Crown. A modification that results inthe flank of each gear tooth having aslight outward bulge in its centerarea. A crowned tooth becomes gradu-ally thinner toward each end. A fullycrowned tooth has a little extra mate-rial removed at the tip and root areasalso. The purpose of crowning is to en-sure that the center of the flank car-ries its full share of the load even ifthe gears are slightly misaligned ordeflect under load. Root circle. Tangent to the bottom ofthe tooth spaces in a cross section. Pitch circle. Concentric to base cir-cle and including pitch point. Pitchcircles are tangent in mating gears. Gear center. The center of the pitchcircle.Line of centers. Connects the cen-ters of the pitch circles of two engag-ing gears; it is also the common per-pendicular of the axes in crossedhelical gears and worm gears.Pitch point. The point of a gear-toothprofile which lies on the pitch circle ofthat gear. At the moment that thepitch point of one gear contacts itsmating gear, the contact occurs at thepitch point of the mating gear, andthis common pitch point lies on a lineconnecting the two gear centers.

by which a tooth space exceeds thethickness of the engaging tooth on theoperating pitch circles.

Angular dimensionsHelix angle. The inclination of thetooth in a lengthwise direction. If thehelix angle is 0 deg, the tooth is paral-lel to the axis of the gear and is reallya spur-gear tooth. Lead angle. The inclination of athread at the pitch line from a line 90-deg to the shaft axis.Shaft angle. The angle between theaxes of two non-parallel gear shafts. Pitch angle. In bevel gears, the an-gle between an element of a pitch coneand its axis. Angular pitch. The angle subtendedby the circular pitch, usually ex-pressed in radians.

RatiosGear-tooth ratio. The ratio of thelarger to the smaller number of teethin a pair of gears.Contact ratio. To assure smooth,continuous tooth action, as one pair ofteeth passes out of action, a succeed-ing pair of teeth must have alreadystarted action. It is desired to have asmuch overlap as possible. A measureof this overlapping action is the con-tact ratio.Hunting ratio. A ratio of numbers ofgear and pinion teeth which ensuresthat each tooth in the pinion will con-tact every tooth in the gear before itcontacts any gear tooth a second time.(13 to 48 is a hunting ratio; 12 to 48 isnot a hunting ratio.)

General Terms Runout. A measure of eccentricityrelative to the axis of rotation. Runoutis measured in a radial direction andthe amount is the difference betweenthe highest and lowest reading in 360deg, or one turn. For gear teeth,runout is usually checked by eitherputting pins between the teeth or us-ing a master gear. Cylindrical sur-faces are checked for runout by a mea-suring probe that reads in a radialdirection as the part is turned on itsspecified axis.Undercut. When part of the involuteprofile of a gear tooth is cut away nearits base, the tooth is said to be undercut.Undercutting becomes a problem when

Line of action. The path of action forinvolute gears. It is the straight linepassing through the pitch point andtangent to the base circle.Line of contact. The line or curvealong which two tooth surfaces aretangent to each other. Point of contact. Any point at whichtwo tooth profiles touch each other.

Linear and circularmeasurements

Center distance. The distance be-tween the parallel axes of spur gears orof parallel helical gears, or the crossedaxes of crossed helical gears or ofworms and worm gears. Also, it is thedistance between the centers of thepitch circles.Offset. The perpendicular distancebetween the axes of hypoid gears oroffset face gears. Pitch. The distance between similar,equally spaced tooth surfaces along agiven line or curve.Diametral pitch. A measure of toothsize in the English system. In units, itis the number of teeth per inch ofpitch diameter. As the tooth size in-creases, the diametral pitch de-creases. Diametral pitches usuallyrange from 25 to 1. Axial pitch. Linear pitch in an axialplane and in a pitch surface. In helicalgears and worms, axial pitch has thesame value at all diameters. In gear-ing of other types, axial pitch may beconfined to the pitch surface and maybe a circular measurement. Base pitch. In an involute gear, thepitch on the base circle or along theline of action. Corresponding sides ofinvolute gear teeth are parallelcurves, and the base pitch is the con-stant and fundamental distance be-tween them along a common normalin a plane of rotation. Axial base pitch. The base pitch ofhelical involute tooth surfaces in anaxial plane.

Lead. Theaxial ad-vance of athread or ahelical spiralin 360 deg(one turnabout theshaft axis).Backlash.The amount

A152 ● Motion System Design ● MSD 2001

Figure 16 — Backlash and tip relief.

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the number of pinion teeth is small.Flash temperature. The tempera-ture at which a gear tooth surface iscalculated to be hot enough to destroythe oil film and allow instantaneouswelding or scoring at the contact point. Full depth teeth. Those in which theworking depth equals 2.000 dividedby normal diametral pitch.Tip relief. A modification of a toothprofile, whereby a small amount ofmaterial is removed near the tip ofthe gear tooth to accommodatesmooth engagement of the teeth.

SPEED REDUCERSA speed reducer is a gearset assem-

bled with appropriate shafting, bear-ings, and lubrication in a sealed hous-ing, generally an oil-tight case. Suchunits are also sometimes referred toas gear boxes, speed increasers, andgear reducers.

Speed reducers are available in abroad range of power capacities andspeed ratios depending on gear sizeand type, with most having a maxi-mum speed limit of 3,600 rpm, andusually driven at a full-load speed of1,750 rpm or less.

Lubrication of speed reducers is ac-complished either by a splash or cir-culating system. Bearings may be lu-bricated automatically with the sameoil as the gearset, or they may haveseparate systems. The specific lubri-cant required for a speed reducer de-pends on a number of factors includ-ing operating speed, ambienttemperature, loads, and method of lu-bricant application. More informationon lubrication is discussed in the PTAccessories Product Department.

Gear types — Most speed reduc-ers use one or more of the commongear types previously discussed. How-ever, the use of helical, worm, andbevel gearsets is particularly preva-lent, especially in small and medium-sized speed reducers. Helical gearsare often used in combination withspiral-bevel or worm gears.

Selection of a particular style ofspeed reducer for an application de-pends primarily on shaft arrange-ment, type of gearing, ratio range,and horsepower range. Three broadcategories of speed reducers, groupedaccording to mounting arrangements,are base-mounted, shaft-mounted,and gearmotor, Figures 17 to 19.

Base-mounted speed reducers

angle. Parallel shaft units typicallyuse helical or spur gears, whereasright-angle units contain bevel, beveland helical, or worm gearing.

A particular type of base-mountedreducer is the free-shaft type, whichusually has one high-speed inputshaft connected to a prime mover anda single or double output (low-speed)shaft connected to the load. In generalthis type is a good choice if the primemover is remotely located or if only asimple change in direction in thepower train is required. Free-shaft re-ducers are also recommended if aproper-sized gearmotor or motorizedreducer is not available, or if severalloads must be driven by a singleprime mover.

Shaft-mounted reducers —Some gear reducers are furnishedwith a hollow output shaft that slipsover the driven shaft, which in turnsupports the reducer. Free rotation ofthe housing is prevented by some typeof reaction member, often a torquearm or flange attached to a stationaryportion of the machine. The gear unitmay or may not support the primemover. Shaft-mounted reducers gen-erally use helical or spur gears for par-allel-shaft arrangements and wormgearing for right-angle arrangements.Though many shaft-mounted reducersare now in use, they are generallyavailable only in a limited range ofsizes and are used most widely in ma-terial handling applications.

Gearmotors — A gearmotor com-bines an enclosed gearset with a mo-tor. The frame of one component sup-ports the other, and the motor shaft istypically common with or coupled di-rectly to the gear input shaft. In oneversion, called an integral gearmotor,the speed reducer and motor share acommon shaft. Another type uses aninput flange, Figure 20, for connec-

— Gear reducer units that have feetbolted to a stationary pad or otherstructural member, account for thelargest number of speed reducer ap-plications. In general, the primemover is mounted on the same struc-ture as the reducer or on the reduceritself, but can be mounted on a sepa-rate structure.

The input and output shafts of thesebase-mounted speed reducers can bearranged horizontally or vertically,and parallel to each other or at a right

Figure 17 — Based-mounted speedreducer with shafts mounted at rightangles.

Figure 18 — Shaft-mounted speed reducerwith torque arm to prevent housingrotation.

Figure 19 — Gearmotor.

Figure 20 — Input flange on a speedreducer accepts a C-face motor.

2001 MSD ● Motion System Design ● A153

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tion to a C-face motor. Motorized reducers resemble gear-

motors and perform much the samefunction. This reducer type commonlyincorporates a scoop, Figure 21, whichsupports the motor. Generally, thedistinction between motor-reducerunits depends on whether the motoris an integral part (as in a gearmotor)or a modular part (as in a reducer-mo-

tor combination). Gearmotors and motorized reduc-

ers often are specified when the re-ducer is the first component followingthe prime mover in a power train. Ofthe two, the motorized reducer is be-ing used more extensively because themotor can be changed quickly andeasily, standard motors are readilyavailable for replacement, and a widerange of options can be provided froma few basic components.

Traction drivesTraction drives, Figure 22, trans-

mit mechanical power from source toload by means of mating metalrollers, which may be consideredgears with infinite numbers of teeth.

These rollers can be cones, cylin-ders, discs, rings, spheres, or toroids.The speed ratio is determined by theradius of rotation of the driver rolleron the driven member, the distancebetween the rollers, or their orienta-tion with respect to each other.

Traction drives are available in twotypes — dry and lubricated. Dry trac-tion drives eliminate the need for lu-bricant and allow nearly 100% effi-ciency in power transmission.Slippage between driving and drivenmembers is prevented by a spring-loaded system.

Other traction drives use syntheticfluids. Previously, conventional lubri-

and higher speed ratios.Constant-ratio and other

roller drive designs use oneor more rows of steppedplanet rollers to provide ahigh speed ratio in a singleplanetary stage. The drivefunctions as either a speedreducer or increaser depend-ing on whether power is in-troduced through the sunroller or the outer roller ring.

Cycloidal drivesCycloidal drives, Figure

23, transmit power equal to that ofgears, but in a smaller and more effi-cient package. In contrast to the cir-cular motion of gears, cycloidal drivesuse noncircular or eccentric compo-nents to convert input rotation into awobbly cycloidal motion. This cy-cloidal motion is then converted backinto smooth, concentric output rota-tion. In the process, speed reductionoccurs.

The term cycloidal is derived fromhypocycloidal, which is defined as thecurve traced by a point on the circum-ference of a circle that is rotating in-side the circumference of a largerfixed circle. A common example of thismotion is the path traced by a tooth ofa planetary pinion rotating inside aring gear.

Whereas worm gearing experiencesa dramatic loss of efficiency in goingfrom low to very high input/outputspeed ratios; and helical gearing losesefficiency at high ratios because twoor more stages of reduction are re-quired; cycloidal drives achieve reduc-tion ratios as high as 200:1 in a singlestage, while still maintaining moder-ately high efficiencies. Moreover, be-cause cycloidal drive components in-teract in a rolling fashion, failure isgenerally not catastrophic. As in abearing, fatigue in the rolling sur-faces of a cycloidal drive causes noiselevels to gradually increase, servingas a warning long before completedrive failure occurs.

Heat generation, attributablemainly to mechanical losses and thepower being transmitted, is readilydissipated through the large surfacearea of other types of gears. But cy-cloidal drives, like worm gears, mustdissipate heat through a smallerhousing surface area. However, be-cause efficiency in the cycloidal drive

cants were used so that tractiondrives had to be designed with veryhigh contact pressures betweenrolling members to carry the loadwithout slip. Today’s traction fluidsreduce this required contact pressureso drives can be designed with longerlives and higher power capacities. Un-der the high pressure, viscosity of thefluid increases dramatically so thefluid behaves more like a plastic ma-terial than a liquid. This plastic-likefilm enables the drive to transmitpower without appreciable metal-to-metal contact.

Another contributor to tractiondrives’ increased power capacity isimproved high-grade bearing steelthat resists fatigue, thereby extend-ing the life of the drives.

Traction drives are compact andcan be used instead of belts, gears, orchain drives where space is limited.Moreover, traction drives are quieterthan most other mechanical drivesbecause there is no engagement shockloading or backlash. Other inherentadvantages include high speed reduc-tion or multiplication, low vibration,excellent rotational accuracy, andhigh efficiency at all speeds.

Many traction drives are capable ofadjusting the speed ratio infinitelythroughout a range. These variable-ratio traction drives are discussed inthe Adjustable-speed Drives ProductDepartment of this handbook. Morerecently, constant-ratio tractiondrives have been developed to accom-modate increased power, longer life,

A154 ● Motion System Design ● MSD 2001

Figure 21 — Scoop-mount motor-reducer.

Figure 22 — Concept drawing shows howtraction drives transfer power from theinput to output shaft with a set of matingrollers. By varying radius, R, outputspeed can be adjusted while input speedremains constant.

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is higher than worm gearing of equalcapacity and ratio, less heat is gener-ated in cycloidal units. Consequently,the auxiliary cooling often requiredfor worm units is usually not neededfor cycloidal drives.

There are various types of cycloidaldrives currently available. Harmonicand planocentric drives are generaltypes of cycloidals in which speed re-duction occurs in converting the inputmotion into cycloidal motion. Double-reduction cycloidals achieve greaterratios because speed is reduced twice:first in converting input rotationalmotion into cycloidal motion, andagain in converting the cycloidal motion back into rotational outputmotion.

Although not as efficient as spur orhelical gearing, cycloidal drives offersubstantially higher efficiency thanworm gearing. The concentric shaftorientation also proves valuable, asdoes the drive’s compact size and highreduction capability.

shafts, and concentricshafts.

Speed ratio — Theratio of input speed tothat of the output is an-other significant factordictating the type ofgearing to be selected.By examining efficiencyand gear type, the usercan determine if a sin-gle high-ratio stage issufficient or if multi-stage gearing is re-quired.

Geared systems canbe driven at constantor varying speeds, de-pending on the require-ments of the applica-tion. When a gearedsystem is selected foran adjustable-speedapplication, operatingspeed should be deter-mined along with oper-ating cycle and powerrequirements. From

this information, a lubrication systemcan be selected, the heat dissipationevaluated, the effect of speed varia-tion on the dynamic characteristicsdetermined, and the need for specialbalancing determined.

Gears can be custom-designed tomeet specific speed requirements, orstandard ratios may be used frommanufacturers’ catalogs. Generally, astandard ratio results in a less expen-sive drive. Standard ratios estab-lished by AGMA are a series of valuesbased on the 1.5 geometric progres-sion, which is a modification of theASA 10 series ranging from 1.225 to1,810.0:1. Tolerances for these ratiosare �3% for single reduction, �4% fordouble and triple reduction, and �5%for quadruple reduction. There are noAGMA standards for worm-gear ra-tios. However, popular ratios for sin-gle reduction systems range from 5 to70, and for double reduction from 75to 5,000.

Design style — The applicationshould be evaluated to determine ifindividual open gears will be suffi-cient or if an enclosed speed reducer isrequired. In general, individual gearsrequire rigid shafting to keep thegears aligned. And shafts should notintroduce external loads to the gear-

SELECTING GEARDRIVES

Gears can be selected, rated, in-stalled, and maintained by most usersthrough common standards and prac-tices developed by the American GearManufacturers Association. However,the services of a gear-engineering spe-cialist are generally required to makemore detailed, in-depth analysis incases where severe duty, extreme reli-ability, unusually long service, orother extraordinary conditions exist.In either case, the major selection fac-tors include: shaft orientation, speedratio, design style, nature of load, ser-vice factor, environment, mountingposition, ratio, lubrication, and instal-lation practices. All these factorsmust be carefully considered in select-ing gears for optimal operation in aparticular application.

Shaft orientation — Probably thefirst consideration in selecting a geartype is to determine the required ori-entation of the input to the outputshaft. According to the type of gear se-lected, various shaft arrangementsare available including: parallelshafts, shafts at right angles with in-tersecting axes, shafts at right angleswith nonintersecting axes, skewed

Figure 23 — Cycloidal drives, such as thisplanocentric unit, convert input rotationinto a wobbly cycloidal motion. Thiscycloidal motion is then converted backinto smooth circular output rotation, withspeed reduction occurring in the process.

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ing. Typically, an enclosure aroundthe gears with oil lubrication is thepreferred design, but grease-lubri-cated open gears can be used in rela-tively clean environments.

Nature of load — Theoretically,gear teeth and bearings understresses below the endurance limitand lubricated properly will last in-definitely, provided the operatingloads are within the design specifica-tions. Thus, determining the natureof the load is important in selectinggears for long life and reliable ser-vice. Gears may be subjected to occa-sional loads (e.g., 15 to 30 minutesper day), intermittent loads (appliedseveral minutes per hour), or contin-uous service loads (10 to 24 hours perday). The life of the geared system isthe period of operating time or cyclesduring which the system can trans-mit a required load. Life of the gearmay end with a fracture or opera-tional failure of a component or withthe development of excessive noise,vibration, or heat.

Determining the nature of a gearload involves the consideration ofmaximum horsepower, drive iner-tia, overhung load, and speed limitof the gear.

Maximum horsepower or maximumtorque of the prime mover is one of themost significant considerations onwhich selection of a geared unit isbased. A gear is rated approximatelyas a constant-torque machine, withthe horsepower rating varying almostdirectly with the input speed. Eitherconstant-torque or constant-horse-power motors are used with gears, de-pending on the application. For exam-ple, constant-torque motors arerequired for applications such as con-veyors, stokers, and reciprocatingcompressors. Constant-horsepowermotors are required for lathes, boringmills, radial drill presses, etc.

Drive inertia also requires torque toovercome more than the drive load ofthe output, because a drive is seldomsubjected to only a single-intensityload. For example, if an electric motoris the prime mover, peak startingtorques as high as 400% of the motorrating can be applied to the drive dur-ing start-up. The geared unit is onlyone member of the system that maycontain members with high inertia,unbalance, or torsional stiffness.

The shafts of geared units are typi-

quired life, operating duty, and dy-namic characteristics of the drivingand driven machines. Typically, thisservice rating is determined by multi-plying the required horsepower by theappropriate service factor based onequipment, duty cycle, and type ofprime mover.

For most speed reducers (spur, he-lical, herringbone, and bevel), AGMArecognizes three load classificationsfor determining service factors: uni-form, moderate shock, and heavyshock. Based on field experience, nu-merical values have been assigned tothese classifications for intermittentservice, for service between 3 and 10hours per day, and for service beyond10 hours per day. The factors are de-pendent upon the prime mover. More-over, the load classification and resul-tant service factor varies with theapplication of different types ofgeared units.

Using these service factors, a max-imum momentary or starting load of200% of rated load can be allowed formost gears, with rated load definedas the unit rating with a service factor of 1.0.

Service factors for worm gears aredetermined somewhat differently,with normal starting or momentarypeak loads up to 300% of rated loadpermissible.

Environment — The type of gearselected must also compensate for aless-than-ideal environment, whichcan adversely affect gear-system per-formance if proper precautionarysteps are not taken. The most com-mon types of hostile environments aredust, heat, wide variation in tempera-ture, moisture, and chemicals. Gener-ally, each has particular adverse ef-fects on lubricant, gears, bearings, orseals. Dusty atmospheres may con-taminate the lubricant, for example.Moreover, heat may accelerate lubri-cant breakdown, affect the perfor-mance of synthetic contact oils, orlower gear capacity by lowering mate-rial properties and distorting thegear. Wide temperature variationmay cause improper lubrication forgears and bearings, thereby shorten-ing the life of the unit. Moisture infil-tration may accelerate lubricantbreakdown, corrode components, andaccelerate the wear of contact seals.

Contact seals should be used on in-put and output shafts when the unit

cally subjected to torque loads, axialloads, and radial loads. These last twotypes are usually called externally ap-plied thrust loads and externally ap-plied overhung loads, and they can becaused by chains, pinions, V-belts, orflat belts.

The type of load introduced by theprime mover depends on the opera-tional characteristics of the primemover. Electric motors and turbines,for example, produce relativelysmooth operation whereas an inter-nal-combustion engine does not affordso smooth a load.

Overhung load (OHL), if producedby the drive, requires the use of out-board bearings or speed reducers thatwill accept this OHL. OHL is imposedon the shaft when a pinion, sprocket,sheave, pulley, or crank is mountedon the input or output shaft.

To calculate the magnitude of theload, multiply the transmitted forcethat is tangent to the pitch circle ofthe mounted member by the OHLfactor. For a single or multiple-chaindrive, this OHL factor is 1.00. Thefactor is 1.25 for a cut pinion runwith gear teeth, 1.50 for a single ormultiple V-belt drive, and 2.50 for aflat-belt drive.

OHL given by manufacturers areusually specified in catalogs at oneshaft diameter from the housing face.Load-location factors are also given sothat, regardless of where the load isacting, it can be converted to the ref-erence position and compared withthe cataloged value.

The magnitude of the OHL that cansafely be applied depends upon sev-eral factors including bearing life orpressure, elastic shaft deflection,shaft strength, and bolt strength.

Speed limit is based on a maximumpitch line velocity of 5,000 fpm andpinion speeds of 3,600 rpm or less. Forworm gearing, speed limits are basedon a maximum sliding velocity of6,000 fpm and worm speeds of 3,600rpm and less. Occasionally, the speedlimitation may be that for a bearing orcontact oil seal, based on the manu-facturer’s limiting speed.

Service factor — Basic ratingstake into account minimum designcriteria for gears, lubricants, shafts,bearings, and other gearset compo-nents. Service factors are then used toadjust the basic rating to a servicerating that is compatible with the re-

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operates in dusty environments orwhere water is splashed around theunit. In atmospheres laden with abra-sive dust or in areas hosed down withwater under pressure, two contactseals may be required on each shaft.

Mounting position — Mountingposition is an important selection cri-teria, because most gear units are de-signed to operate in either a horizon-tal or vertical position only. In someapplications, however, the gearedunit may have to operate in a positioninclined to either axis or inclined toboth axes. In this case special consid-erations must be given in selectingthe gear systems regarding oil level,air-vent position, and location of oil-drain holes.

Gear rating — All components ofa commercial enclosed geared unitare usually rated by established prac-tices, with the rating for the entiresystem determined by the lowest rat-ing for any one part for a given set ofoperating conditions. Generally, thereare two types of ratings for a gearedunit: mechanical and thermal.

The mechanical rating is based onthe strength of the gears, shafts,bolts, etc., or the load compatible withthe required bearing life or pressure,or the resistance of the gears to pit-ting or scoring.

The thermal rating specifies thepower that can be transmitted with-out exceeding a specified rise aboveambient in the operating temperatureof the unit. Typically, enclosed drivesoperate at temperature rises of 70 to100 F above ambient temperature.Generally, a maximum oil-sump tem-perature of 200 F is permissible forgearmotors and shaft-mounted speedreducers, even though thermal rat-ings are not specified by manufactur-ers or defined by AGMA.

When the applied horsepower ex-ceeds the thermal horsepower, auxil-iary cooling methods are used includ-ing: use of an oil-exclusion pan toreduce churning of the oil and the re-sulting higher temperatures, air-cooling the housing, circulating wa-ter or some other cooling mediumaround the unit, using a separate oilsump for greater heat dissipation, orusing a cooler mounted inside thegear housing.

Lubrication — The most impor-tant factor in a lubricating system isreliability, because failure to supply

lubricant to the bearings and gearswill result in their damage. As a re-sult, the type of lubricating systemshould be chosen carefully, becausethis may be the most critical compo-nent of the entire system.

In addition, accepted practicesmust be observed by the user to main-tain proper lubrication during the lifeof the geared unit. One of the mostcommon causes for gear damage, forexample, is the failure to fill the unitto the proper level with the firstchange of lubricant, so data on the lu-brication plate and all other instruc-tions must be followed carefully. Givespecial attention to warning tags.

The user should also know the tem-perature range in which the unit isdesigned to operate. If the unit is op-erating where temperatures varywidely, the oil viscosity should bechanged to suit the conditions. Forlow-temperature operation, the oilshould have a pour-point lower thanthat of the extreme minimum temper-ature encountered. And the oil mayrequire pre-heating under extremelycold starting conditions.

Several lubrication system typesare available. Enclosed gear units arecommonly lubricated by splash sys-tems in which one of the gears dipsinto an oil bath and transfers the lu-bricant to the contacting teeth as itrotates. For low-speed operation,scrapers close to the side of the gearmay be used so that splash from thegears reaches oil-feed troughs to lu-bricate the bearings.

Pressure or forced-spray lubrica-tion reduces oil churning. In this sys-tem, lubricant is pumped under pres-sure to the gear train. After passingthrough the gears, it is returned tothe reservoir to be recirculated. Thismethod uses the gear oil as a coolingmedium. The lubricant may be deliv-ered as a stream running over thegearing or as a spray from jet orspray nozzles. These nozzles may di-rect the lubricant to the entering orthe trailing side of the gear mesh, de-pending on speed.

Many gear units (particularly ver-tical units) have grease-lubricatedbearings. Grease is normally intro-duced at the factory and grease fit-tings are provided for lubrication inthe field. Some units designed to oper-ate in extremely adverse conditionshave double seals mounted in cages.

They allow grease to be purged from achamber between the seals.

Installation practicesSuccessful performance of a gear

unit is vitally dependent upon instal-lation practices. The foundation mustbe rigid and level, the shafts aligned,the accessories properly installed, therequired grade and quality of lubri-cant added, and any special instruc-tions followed carefully.

Foundation — The gear unit mustprovide a level, secure base on whichto operate. The base should bemounted horizontally, unless the unithas been designed for mounting in atilted position. Moreover, the baseshould be precisely leveled, since onlya few degrees of tilt might adverselyaffect lubrication efficiency. If a unitis to be mounted in a position differ-ent from that for which it was de-signed, changes might be necessary toprovide proper lubrication.

Most gear units have a drain plugin the base for oil drainage. The unitshould be mounted above floor level,or a sump hole used for this purpose.Drain plugs may be replaced with avalve or with pipe extensions for con-venient drainage. However, guardingmust be provided to prevent acciden-tal breakage. Drain valves may beprovided with a lock to prevent acci-dental or unauthorized opening.

When mounting a unit on struc-tural steel beams, a base plate shouldbe used between the two beams towhich the unit is attached. The thick-ness of the base should be equal to orgreater than the thickness of the unitfeet, and it should extend under theentire unit. Mountings that are notsufficiently rigid can lead to excessivevibration and deflection.

Procedures — When installingthe geared unit bring it into align-ment by placing broad, flat shims un-der all mounting pads. Start at thelow-speed end and level across thewidth and then along the length of theunit. Most units have leveling sur-faces for reference. Use a feeler gageunder all pads to make certain thatall are carrying equal loads. This pre-vents distortion of the housing whenfoundation bolts are secured.

Units equipped with backstopsshould receive special attention duringassembly. If the motor is connected to

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start in the wrong direction, the suddenshock can immediately break the back-stop or cause premature failure shortlythereafter. Thus, disconnect the motorcoupling and check the rotating direc-tion of the backstop by rotating the gearunit by hand. Then check the rotation ofthe motor by an actual start.

The user should also make sure thatall couplings, shims, and pinions areproperly installed so that no destructiveload is applied to the gear unit or tothese parts because of misalignment.Hubs, pinions, sprockets, pulleys, andother components should be carefullyinstalled (not driven on with a hammer)on shafts. Couplings should be alignedso that the angular and parallel align-ment are within the limits specified bythe coupling manufacturer. The dis-tance between the shaft ends or cou-pling hubs should also be checked to en-sure they agree with the specifications.

Maintenance — Proper mainte-nance is essential in keeping a gearunit running properly. After a week ormore of service, check all external boltsand pipe plugs to make sure they aretight. Also, check chains, belts, sprock-ets, and pinions, to make sure that theload is being distributed evenly andthat there is no unusual condition.

After a few hundred hours of opera-tion, the user should drain all oil,flush the system with an oil of similargrade, and refill the lubricating sys-tem to the proper level. Of course, oillevel should be periodically checkedunder all conditions.

ANALYZING GEARFAILURES

By following a step-by-step proce-dure, engineers can diagnose gearfailures and develop solutions. Here’show to conduct a failure analysis.

Procedures can vary depending onfailure conditions. For example, if thegears are still able to function, youmay continue their operation andmonitor the rate at which damageprogresses via periodic inspection andsound and vibration measurements.If reliability is crucial, examine thegears by magnetic particle inspectionto ensure that they have no cracks.

Failure inspectionBefore starting the inspection, col-

lect background information, includ-

where needed. Mark each componentso it is clearly identified. Mark thebearings so their position in the gear-box can be determined later.

Gear geometry. Obtain the fol-lowing data, which will be neededlater to calculate the load capacity ofthe gearset:

• Number of teeth.• Outside diameter.• Face width.• Gear housing center distance for

each gearset.• Whole depth of teeth.• Tooth thickness (span and top

land).Test specimens. Take broken

parts for laboratory testing. Oil sam-ples can be very helpful. But, an effec-tive oil analysis depends on havingrepresentative samples. For a gear-box drain or reservoir, take a samplefrom the top, middle, and bottom.Check the oil filter and magnetic plugfor wear debris and contaminants.

Determine type offailure

Now examine all of the informationto determine how the gear(s) failed.Several failure modes may be present.Identify the primary mode of failure,and any secondary modes that mayhave contributed to the failure. Thefour most common failure modes arebending fatigue, contact fatigue,wear, and scuffing.

ing part numbers, service history, andlubricant type. Interview those in-volved in the design, installation, op-eration, and maintenance of the gear-box. Encourage them to telleverything they know about the gear-box even if it seems unimportant.

Visual examination. Before dis-assembling the gearbox, inspect itsexterior and record data that wouldotherwise be lost. For example, thecondition of seals and keyways mustbe recorded before disassembly. Oth-erwise, it will be impossible to deter-mine when these parts may have beendamaged. Take gear tooth contactpatterns before completely disassem-bling the gearbox (see next section).

After the external examination,disassemble the gearbox and inspectall components. Examine the gearteeth and bearings and record theircondition. Look for signs of corrosion,contamination, and overheating.

After the initial inspection, washthe parts with solvent and re-examinethem. This is often the most importantphase of the investigation and mayyield valuable clues. A low powermagnifying glass and pocket micro-scope are helpful tools for this phase.

The bearings often provide clues asto the cause of gear failure:

• Bearing wear can cause exces-sive radial clearance or end play thatmisaligns the gears.

• Bearing damage may indicatecorrosion, contamination, electricaldischarge, or lack of lubrication.

• Deformation between rollers andraceways may indicate overloads.

• Gear failure often follows bear-ing failure.

Gear tooth contact patterns.(Complete this step before disassem-bling gearbox components). The wayin which mating gear teeth contact in-dicates how well they are aligned,Figure 24. If the gears are still able tofunction, record tooth contact pat-terns under either loaded or unloadedconditions. For no-load tests, paintthe teeth of one gear with markingcompound. Then, roll the teeththrough mesh so the contact patterntransfers to the unpainted gear.

For loaded tests, use machinist’slayout lacquer to paint the teeth, thenrun the gears to wear off the lacquerand establish the contact patterns.

Document observations. De-scribe your observations in writing,using sketches and photographs

A158 ● Motion System Design ● MSD 2001

(a)

(b)

Figure 24 — Typical gear tooth contactpatterns: (a), aligned, and (b),misaligned.

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Bending fatigue. This type of fail-ure, caused by repeated loading,starts as a crack that grows until thepart fractures. As a fatigue crackpropagates, it leaves “beach marks”that correspond to positions wherethe crack stopped, Figure 25. The ori-gin of the crack is usually surroundedby several of these beach marks.

Most fatigue failures occur in thetooth root fillet, Figure 26.

Contact fatigue. In anothermode, called contact fatigue, repeatedstresses cause cracks and detachmentof metal fragments from the toothcontact surface, Figure 27. The mostcommon types of contact fatigue aremacropitting (visible to the nakedeye) and micropitting.

With macropitting, fatigue cracksgrow until they cause a piece of thesurface to break out, forming a pit.

cantly.Finally, polishing is fine abrasion

that imparts a mirror-like finish togear teeth, Figure 30. It is promotedby fine abrasives in the lubricant.

Severe polishing removes all ma-chining marks. The surface may bewavy or it may have wear steps at theends of the contact area and in the dedendum.

Scuffing. Severe adhesion (scuff-ing) transfers metal from one tooth toanother, Figure 31. It usually occursin bands along the direction of slid-ing. Surfaces have a rough or mattetexture.

Mild scuffing is generally nonpro-gressive. Moderate scuffing occurs inpatches, and it can be progressive.

Severe scuffing occurs on signifi-cant portions of a tooth (for example,

entire addendum or dedendum), andis usually progressive. In some cases,material is deformed and displacedover the tooth tip or into the toothroot.

Tests and calculationsIn many cases, inspection informa-

tion isn’t enough to determine thecause of failure. When this happens,gear design calculations and labora-tory tests are needed.

Design calculations. The gear ge-ometry data collected earlier aids inestimating tooth contact stress, bend-ing stress, lubricant film thickness,and tooth contact temperature. Thesevalues are calculated according toAmerican Gear Manufacturers Asso-ciation standards (ANSI/AGMA 2001-B88 for spur and helical gears). Com-paring these calculated values with

Progressivemacropitting con-sists of pits largerthan 1 mm diam.

Micropittinghas a frosted,matte, or graystained appear-ance. Magnifica-tion shows thesurface to be cov-ered by pits lessthan 20 �m deep.

Wear. Toothwear involves re-moval or displace-ment of material

due to mechanical, chemical, or electri-cal action. The three major types areadhesion, abrasion, and polishing.

Adhesion is the transfer of materialfrom one toothto another dueto weldingand tearing,Figure 28. It iscategorized asmild or mod-erate.

Mild adhe-sion usuallyoccurs duringgearset run-in and sub-sides after itwears imperfections from the surface.Moderate adhesion removes machin-ing marks from the contact surface,and it can become excessive.

Abrasion is caused by contaminantsin the lubricant such as scale, rust,machining chips, grinding dust, weldsplatter, and wear debris. It appears

as smooth,parallelscratches orgouges, Fig-ure 29.

Severeabrasion re-moves allmachiningmarks, and itmay causewear steps atthe ends ofthe contactsurface andin the deden-dum. Tooththicknessmay be re-duced signifi-

Beach mark

Fracture zone

Ratchet marks

Origin

Origin

Fatigue zone

Beach mark

Figure 25 — Bending fatigue fracturesurfaces of gear teeth. Upper tooth hasmultiple origins of failure.

Figure 26 — Fatigue crack in a gear tooth root fillet.

Fatigue crack

Tooth retrieved from oil sump

Figure 27 — Fatigue failure (pitting) in the contact surface of a gear tooth. Beach marks are visible in some of thelarger pits.

Crack growth

Beach marks

Origins

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A160 ● Motion System Design ● MSD 2001

AGMA allowable values helps to de-termine the risk of macropitting,bending fatigue, and scuffing.

Laboratory tests. A microscopicexamination is useful for confirmingthe failure mode or finding the originof a fatigue crack.

If tooth contact patterns indicatemisalignment, check the gear accu-racy on gear inspection machines.Conversely, where contact patternsindicate good alignment, check formetallurgical defects.

Conduct nondestructive tests be-fore any destructive tests. These non-destructive tests, which help detectmaterial or manufacturing defects,include:

• Surface hardness and roughness.

completed, form a hypothesis for theprobable cause of failure, then deter-mine if the evidence supports the hy-pothesis. Evaluate all of the evidencecollected during the investigation.

Finally, after testing the hypothe-sis against the evidence, you reach aconclusion about the most probablecause of failure.

A failure analysis report shoulddescribe the inspections and tests,and give conclusions. It usually con-tains recommendations for repairingthe equipment, or changing its de-sign or operation to prevent futurefailures.

Extracted from an article by Gear-tech in the March 1994 issue of PTD.■

• Magnetic parti-cle inspection.

• Acid etch inspection.• Gear tooth accuracy inspection.Then conduct destructive tests to

evaluate material and heat treat-ment. These tests include:

• Microhardness survey.• Microstructural determination

using various acid etches.• Determination of grain size and

nonmetallic inclusions.• Examination of fracture surfaces

with scanning electron microscope(SEM).

Conclusions and reportWhen all calculations and tests are

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

Figure 28 — Adhesion type wear of gear teeth.Figure 30 — Polishing type wear.