32'~~, G~AR T.CHNOLOOY

Classification of Types ofGear Tooth Wear - Part I.

Louis IF,a:ur,eCM'D Transmissiions,

Camlbrai. France'

IntroductionThe phenomena of deterioration of surfaces

are generally very complexend depend!on nu-merous conditions which include the operatingconditions, the type of load applied. the relativespeeds of surfaces in contact, the temperature,Iubrication, surface hardnessand roughness,and the compatibility and nature of materials.

Wear isa general term covering the localphenomena describing the removal of somematerial and occurring when two surfaces slideonto one another, This term also applies to theremoval of material resulting from the pres-ence of impurities in the lubricant. Other typesof gear failures, such as surface fatigue, corro-sion, plastic flow, and breakage are not cov-ered by this article.

Wear may be divided into two distinct clas-sifications: Qualitative, which is based on theaction modes of different wear phenomena ongear teeth and characterized by surface appear-ance; and quantitative, which takes into ac-count. the intensity of the wear phenomena.

The qualitative wear phenomena we willdiscuss include abrasive wear with two bodies,streaks and scoring, polishing, hot and coldscuffing, abrasive wear with three bodies,scratches or grooves, and interference wear. Inthe quantitative analysis we win define normalwear, moderate wear, and excessive wear. Asynthesis of these two classifications has beenmade under a chart found in Part H.

eurrent Wear (c:aued Two Body Abrasion)Current wear is revealed very early in the

life of the gear or gear train and is evidenced bythe removal of micro-particles of metal onthegear teeth surfaces. This phenomenon is causedby contact and metal-to-metal sliding, which

occurs through the oil film. The distinctivemachining marks of the cutting procedureor finish (for example, the facets resultingfrom hobbing, the streaks left by a gearcutter or by ,3 rack cutter. the surface patternfrom certain grinding operations) are di-m:in:ishedor erased by wear.

This wear brings about a progressive reduc-tion of gear tooth thickness, along with a moreor less marked distortion of the profile in theheavy sliding zones, but without noticeabledegradation of foe surface roughness ..

As indicated in Fig, Lthe wear is almost nil

'Wear on Tooth Tipl

_. iPitchCircl:e

Tooth Root Wear

Fig. 1 . Distinctive aspect at distortion of geartooth profile under wear action

in the pitch zone where the liding peed are[ow or nil, and becomes more and more pro-flounced a we move away from thi zone. Thiszone i maximum at the tipcircle and at theactl ve dedendum circle, wherethe sliding speedsare the highest

The wear zone i generally gray colored andlightly dun, with ometimes lu trou areas and

the presence of scoring. (See Fig. 2). Note thepresence of a dull zone below the pitch, wherethe wear i greater, and of a lu trnu area oneach 'ide of the pitch and slightly higher. Alsonote the transver e coring Ughtl.y markednear the tooth tipand the machine streaks tillvisible near the pitch.

The appearance of thi wear, as well as itsdeveloping peed, variesgreatly according tothe pres ure level between the contact surface ,the hardne of the material . the roughnes ofsurface. and the thickness of lubricant film. Forexample, in 'Ihe ca e of lightly loaded gearsoperating with an oil of relatively high vi co -ity at medium peeds, we will have an oil filmsufficiently thick to avoid metal-to-metal con-tact. Thi will not generate wear except duringstarting or stopping. The original machiningtrace will still be intact en the gear teeth afterlung periods of operation ..

In practice it is not always possible to havea continuous lubricant film between the gearteeth according to the load transmitted. Thencontact occurs between the top 'of the asperitiesmade during machining, and there i a tendencyto polish or score the surface in 'contact

In the ca e of urface-hardened gears ..be-cause of the high hardness of flank in con-tact, we eacounter only slight wear which isoften difficult to see.

Scoring - Str,eaksThis type of wear appears in the form of

fine groove or lines in rhe liding direction.The e streak or core are formed progres-ively in tile zone characterized by a highIiding peed. at the tooth tip and near the root

of the gear teeth. (Fi.g. 3)Tiley are etten developed 011 pinion teeth

and wheel teeth that are in contact at the verybeginning of meshing of mating profiles. Thebottom of the striae are smooth compared tothose faund in traces of scuffing.

Duses. The pre ence of these scores orstreaks reveals the existence of relatively

high pressure locally affectingthe gear teethflank . Under high load action. the asperitiecaused by the roughnes of the mating flank •a well a . foreign particles of small dimen-ions that imprint intothe gear teeth flank •

along with the. lippage effect, cause cavitieswhich appear as streak. .

The formation of coring can be con id-ered as a preliminary stage preceding scuff-ing. These treak generally lead to more e-vere local wear of gear teeth in the zonewhere there is a higher pressure. Frequentlywe note in time a. stabilization of these treakwhen the wear level of the cored urface habeen ufficient '10 generate a beuer distribu-tion of the load. which a a con equenee di-mini hes the maximum pre ure on the gearteeth .. In this case only the black. color base ofstreak, or coring will remain. This tabiliza-tionphenomenon can be accelerated by in-creasing the oil vi cosity and by improvingfiltration. This type of wear is oftenencoun-teredin worm gear " (Fig. 4)

Adheslve WearAdhesive wear appears on 'two surfaces

Iiding aero one another when tile pres urebetween the asperities in contact is sufficienttocau e localized pia tic deformations. rni-era-welding, or local, adhesion . When thereis generation or plastic deformation, there isenergy ab orptlon which leads to 0 erhearingdue to friction.

is 111(' Technical Mmlallt'r01 Ml) Transmissions 0/CamlJrlil. France. HI! isall active member of ISOamJAFNOR (!.ssac/aliallFrancois de Normalis-ation {Jlld the Unltm dtNormulisation lit' laM canique. He Jill also/(Jljglll gear Ihf'Uf.l' oud

inten e, and when the gl'C1r failure anoiysis.

NOVEMBER/DECEMII R 1882 33

The mildest form of adhesive wear i theformarion of t'polishing' on the active flanks ofthe gear teeth, When the load condition andfriction become more

temperature at contact level increases. we maywitness the appearance of localized metal ad-hesions 0[1 the teeth. hot cuffing.or cold cuff-ing (for low speeds and heavy loads).

Polishlng. Polishing is a type of very slowprogressive wear in which the a peri tie of themating flanks are distorted and laminated andthen appear on the gear teeth a very moothand polished surfaces, which take on a mirroraspect. Such wear conditions are caused bymetal-to-metal contact during operation. Gen-erally the polishing occurs in applications atlow speeds «20 mlsec) when the lubricant oilfilm ill elastohydrodynarnic yield rate is notsufficiently thick and is near the limit of thelubricant performance ..Generally, this type of

34 GeAR TECHNOLOGY

wear does not cause large variations in theshape and in the dimensioru of gea:r teeth.However, when examined under a microscope,the structure just below the contact surfacereveals the presence of plastic flows in themateria] under slippage. Sometimes very local-ized overheating traces near the surface are alsoencountered. The polishing encourages estab-lishing gear teeth contact pattern in serviceand allows obtaining a good conformity ofsurfaces in contact.

This type of wear is not "damage" and canbe tolerated in service unless the initial mate-rial specifications forbid it. If the load increases,or if the lubrication conditions become insuffi-cient. this type of gear tooth wear can developbecause the temperature between surfaces incontact will increase and render them superfi-ciaIJy less hard and more sensitive to localizedmicro-welding formations ..The appearance ofadhesion or scuffing traces on the gear teethflanks can then be seen. To prevent such traces,we can, after the appearance of polishing. ill-crease slightly the oil viscosity to obtain innormal working conditions a thicker hydrody-namic film. We can also, when possible. reduceslightly the load to be transmitted, but in allcases we must assure that in service there is norisk of producing severe overloads (no matterhow short they may be), which would damagegreatly the gear teeth surfaces.

Generally the polishing appears quickly onloaded gears made with good precision and

most often 0.11 surface-hardened gear . The mo: t

noticeable examples are found in automotivegears, such as, satellite and planet gears, gear-box pinions, and spiral bevel gears. (Figs ..5-6)

Adhesions .. This phenomenon, which ap-pears on some gear mating flanks, i alwaysvery localized and generally is only present ona small area ofa few gear teeth. For each metalpull-off or adherence, we can almost alwaysdistinguish a zone where a brutal welding hasoccurred between the profiles in contact andwhich was immediately sheared. On one of theflanks, a metallic particle has pulled off and isoften found fixed by adhesion onto the flankof the other gear. The irregularities on the twosurfaces after separation ha ve generated streaksor cratches oriented in the eparation directionof the profiles, starting in the zone where theadherence or metal pull-off occurred.

In the welding zone, the profile i generallyaltered in depth. whereas it is more superficialnear the scratches. which generally are less andIe severe as we go further from the initiallydamaged zone. This type of damage is encoun-tered on gear teeth flank lin the high Iippagezone .where the contact surface is preponder-am or localized becaus e of the presence of aJight excess of material caused by a machiningdefect or a profile with exce ive crowning; bya slight alignment defect of gear teeth causedby the machining operation of the gear of bydistortion under load: or bya local distortionalong the flanks caused by a temperature gradi-ent generated by an irregular flow of heat dur-ing meshing. Thi contact urface Iocallzationcau es uch an increase in load that. it nolonger can be up ported bythe lubricant film •.which leads to metal-to-metal contact and theformation of micro-welding by friction.

The development of this type of damageisencouraged by a tate of "poli hing" on theurface of flank in contact and by the presence

of severe undulation or poor teeth surfaceroughne s. The fir I case is common in sur-face-hardened teeth, and the econd in roughmachine-cut teeth of normalized steel,

The formation of localized metal pull-offcan also be caused by the pa sage of a foreignmetallic panicle between the profile .In fact,after it lamination b stween the gear teeth,such a particle will weld it. elf onto the sur-face of one of the profiles under slippage,creating a local ized excrescence.

This risks generating an overload duringme hing wbi.ch may cause a sudden rupture ofthe oil film. thu causingthe formation of ametal pull-off on the mating profile, When thenumbers of gear teeth on the pinion and on thewheel are in an integer ratio. and such all adhe-ion appears on one tooth, it may cause similar

damage on a limited number of other teeth,and thephenomenon has all the chances oftabilizing by itself afterwards, [fthe numbers

of teeth are prime between them elves, wewill have a general sweep of all the teeth by'those from the pinion and wheel which havebeenaffecaed by the initial. meta] pull-off. Thedeterioration will thus progress (0 all theteeth flanks .. and there is a risk of evolutiontowards hot scuffing (at high speeds) or coldcuffing (at low speed ). Qui~e often this

evolution can occur within minute .We are often tempted to say that the pres-

ence of adhesion on a tooth is not 100 eriousbecause its surface is small. [11 reality, this typeof deterioration, in the ca e of surface-hard-ened teeth. can often go along with cracks thatdevelop from the surface down and which willquickly lead to to til breakage. Such crack" areformed by residual sire s of thermal origin andappear at the moment of instant welding by

slippage ofthe two flanks in contact.Before planning to re-use surface-hardened

gears with or without an eventual grindingsurface operation, it is essential to perform adye penetrant or magnetic particle control 'ofthe teeth flank where the adhe ion occurred.

In the ca e of through-hardened gear. thiscracking risk is practically ncn-existent,

If we ascertain, upon opening a reducer or a

gear wheel. the pre ertce of adhesion. on theteeth, we can come to the following concl usions:

• There has been periodic lubricant filmbreakage on the teeth where contact patternarea w,as preponderant .

• The initial preci iOI1 of the gear or reduceris generally not affected, for we should havenoticed this phenomenon under nominal loadduring start-up of reducer.

The causes of this anomaly may be:I) Distortion of the gear support or reducer,

bringing about an evolution of the contact. sur-

NOV,EMBERIDECEUBER r s a a 35

face toward tile tip of teeth (in cases of adher-ence near the tooth tip).

2) Progressive loss of lubricantoil viscositycharacteristics or temporarily insufficientlu-brication (Cf. case of open gears).

3) Unforeseen and brutal overloads.4) Passage of a foreign body (adherent

metallic particle).When we are faced with such degradation,

the remedie are generally simple. After check-ing that the gear teeth precision and their even-tual profile and helix modifications fulfill theirfunction, one mu t:

1) As ure that the lubricant and lubricatiendevices are in working order and capable ofperforming their function.

2) Eliminate the risk of untimely occa-sional uverloadsapplied to [he gear (Tor-sional vibrati.ons, high torque varlauen,pumping effect. etc.),

For added afety the lubricant viscosity canbe Increa ed (if nece aryl. Fig. 7 gives anexample of metal. adhesions encountered onseveral teeth of a spiral bevel pinion.

Hot Scuffing. This wear phenomenon re-sults from oil film breakage under excessiveoverheating during meshing, which causes ametal-to-metal contact of teeth flanks. localwelds and hearing are alternately producedbetween the contact surfaces and contribute tothe quick pull-off of metallic panicles from theteeth flanks. thus progressively modifying thestate of their profile.

36 GEAR TECHNOLOGY

streaks or scratches with rough bottoms andside. often appearing as band ofvari.able depthwidth oriented in the direction of the height ofthe tooth, and affect isolated zones or theirwhole width. The scuffing traces are generallymore clearly marked at. the tooth tip and root ofthe teeth in the high liding zones.

In the case of hot cuffing; it is the combinedaction of high pressure between urfaces, highsliding speeds, and excessive contact. tempera-ture, resulting from pres ure and tiding speedvalues, which cause oil film rupture betweenthe teeth flanks. During start-up or running-inof certain gear, some local scuffing of les erimportance, which is characterized by shallowtraces and very fine roughness, may appear incertain points of the teeth in the zone where thecontact pressure is maximum. In general.afteracertain time of operation at reduced load, theselocalized traces of scuffing diminish by wear,Once thi happens. the gear may be operatedunder its nominal load. In this case, a sligbtincrease in the lubricantviscosity will allow abetter safety in service. o.n ground gears, wecan see the presence of localized scuffing at thetip and root of the teeth, which is the result ofinsufficient tip relief or too great a deviation inthe profile .. We can abo encounter identicalphenomena near tooth ends due to insufficientlonginidinal correction or too great helix de-viation. As long as . cuffing trace caused bythe e phenomena remain slight or shallow. theymay be tolerated in service. for they win end upwearing off and reducing in time ..(Fig. 8)

If they become coarse, the scuffing canevolve either toward periodic adhesions or to-ward more severe scuffing. which may becomedestructive. In general, the appearance of de-tructive cuffing i revealed in ervice by

brutal oil temperatureri e at the me hing exit.If the load and the operating eonditions aremaintained, the scratches will generalize on thewhole surface of the flanksand will becomedeeper. There is also the risk of local metalpull-off, transfer of metallic particles. and pro-gressive deformations of the profile surfaces.

When this deformation becomes seriousenough. the noise level of the gear will in-crea c. The same is true for the temperature ofthe wheel and pinion bodies.

U the operation is maintained under' uchcondition, signs of overheating will quickly

appear 011 the teeth surface (browm h, bluisn,or violet tempering colors).

[11 the case of urface-hardened gears, crackswill often occur under thermal stress ofleeth.which lead quickly to their breaking.

Ia the ca e through-hardened gears. we cana certain some extreme cases of very eriousrise ,intemperature of gears able 1.0 cause a hotnow of the teeth, bringing their destruction.

General scuffing La brutal phenomenonwhich can develop very quickly on the teeth (inthe matter of a few minutes to a few hours) andbring about irremediable gear jai1I~,-,e. h occursbecause of thermal tress, which lead quickly totooth breakage. (Figs. 9-10)

When the failure occurs 11 long time afterstart-up of the gearing, it is caused by anaccidental faulty lubrication or an overloadre ulting from continuous use of the machinein a manner for which it was not designed.However. if failure occurs a short time aftertart-up, one hould check whether the amount

of heat generated by tile gear is compatiblewith the choice of the gear geometry and theennice of lubricant.

Cold Scuffing. This wear phenomenon isthe result of lubricant (oil or grea e) filmbreakage under excessive pressure action dur-ing meshing. causing a metal-to-metal. contactof teeth flank . The generation mechanism ofdegradation i identical to that of hot cuffing(welding and metal. pull-offj. Jn thi case, itconcerns only the joint action of high pre UJ1ebetween surfaces or extremely low slidingvelocity (linear speed not over approximately4rn1s), which cause the breakage of the lubri-cant film between the contact profiles.

In genera] thi type of scuffing begins withone or more adhesions 011 a few teeth flankand spreads gradually nearer and nearer untilthe whole circumference of the gear is in-volved. The spreading speed of damage is afunctlon of the type of lubricant 11.1 ed, thefini hing proce s, and the hardnes of thegear material .

If we con idera gear of normalized steel. orthrough-hardened, rough-cut finished, and.grease lubricated. the destruction process ofthe teeth can be very quick once the lubrica-tion becomes insufficiern for the load tran .mined (from a few minutes to a few hours).

We then see the formation of successive

deep metal pull-offs and adherences on theflanks, which a:repartially rolled during mesh-ing. thu causing an emission of metallicparticles which remain blended in the grease.These particles generally inieime an abrasivewear which in a way re-establishes a. uniformcontact surface on the teeth. but in many casesis not sufficient [0 stop the scuffing.

This degradation mechanism will resuIt inoverheating of the surface in contact, whichgenerally will remain at an acceptable levelbecause the sliding velocities are low. and .inthe appearance of vibrauons and meshingnoises in the installation. When we stop thegear, we find teeth profile which are oftendeeply altered in their geometry and whichwe can consider as being practically destroyed.

Fig. 9 • Destructive scuffing on the teeth of a spiral bevel wheel. Note thepresence of deep streaks and important deformations of the profile in thecontact zones. This gear wheel can be considered as destroyed.

3__,11'NOVE M B E RID E C E 101B E R 1 119·2 I,

NOVEMBERIDECEIoiBE 111&2 a.a

before planning to re-start the gear.Whenever possible, it is preferable to pro-

ceed to a machining of the teeth at least so asto remove the particles embedded on theteeth flanks.

In the cases where we see the presence oftoo many metal pull-offs, we should make amore complete machining on the wheel flanks,which will require the design ofa new pinion ifwe want to preserve an identical backlash to theone initially planned.

When this scuffing happens on large, opengears used for driving ovens, crushers, orwinches, the repair has to be done most of thetime in situ, for it is often impossible to removesuch gears. (Fig. I I )

We can, as a first operation, remove bymanual grinding all welded metallic particlesthat are too thick in relation to the initial profileof the teeth. Then we must run-in the teethbefore thinking of restarting of the gear toobtain a good contact pattern on the flanks.

For this operation, we can use an abrasivecompound of grain size structure adapted to thesize of the teeth or special synthetic graphitegrease. The running-in should not be too harsh,otherwise we risk deforming the profile of theteeth by abrasive wear.

If we have a surface-hardened and groundgear often used at the low speed stage of areducer, the metal pull-off on the flanks is oftenaccompanied by plastic micro-distortion offlanks due to embedding and micro-welding oflarge particles, which may damage the hard-ened surface layer because it has to supportvery heavy local pressure for which it has notbeen designed.

The surface overheating generated by themicro-weldings may be high and can also some-times generate surface cracks, which may evolverapidly in service through the hardened surfacelayer and lead to the rupture of one or more teeth.

For such gears in general, the appearance ofcold scuffing is brutal and often results in theirbreaking during use.

The possible causes of cold scuffing areinsufficient lubrication or a severe overload ofthe gearing over a sufficiently long period.(These two causes can occur simultaneously).

This type of damage is generally destruc-ti ve for the teeth and develops most often ongrease-lubricated teeth. The surface rough-ness of the teeth is of great importance, andwhen it is great, it encourages the formation ofmicro-welding and the development of deepscuffing .•

Part II of this article will cover wear with threebodies, interference wear, and varying intensi-ties of gear tooth wear.Acknowledgements: Published as AGMA document

88 FTM 4 and in Europower Transmision. April,

1991. Reprinted with permission.