Classification of Typesof Gear Tooth Wear -

Part IILouis Faure

CMD Iransmisslans,Cambrai. F,rance

IneroduetionThe first part of this article included abra ive

wear with two bodies, streaks and scoring. 1'01-ishing, and hot and cold scuffing. This part willdeal with three-body wear, scratches or grooves,and interference wear. Normal, moderate, andexcessive wear will be defined, and a descriptivechart will be presented.

Wear With Three BodiesThis type of wear is characterized by the

presence of a third body (in genera] abrasive)between the two surfaces in contact

Scratches. Grooves (Scratching). This typeof wear appears as isolated cavities and more orless deep streaks on the teeth flanks oriented inthe direction of sliding motion. (See Fig. 1.) Theintervals separating these scratches. their length,and their distribution on the teeth are very ir-

34_ GEAR TECHNOLOGY

regular. These scratches are generally caused bythe introduction of foreign bodies of variablesizes in the gearing. particularly large dust par-ticles. metallic particle. pieces of rust orcalamine, molding sand, etc,

This type or wear can oftenbe seen just afterstart-up, after the repair of an installation, orbefore the filter ha bad a chance to removeimpurities from the lubricant.

Open gears lubricated with grease are, be-cause of insufficient protection, very often sus-ceptible to this type of deterioration. Scratcheson the profiles are a sign of the presence offoreign bodies in the teeth. To avoid the develop-meat of abrasive wear, aile should

• assure the structural integrity of the gearhousing to prevent possible exterior pollution,

•. check the slate of the filters,

• filter the lubricant or change it,• assure that no particles originating at the

foundry may detach from the housing,• lightly polish the scratched zones and assure

that no hard particles are still embedded in theteeth surfaces.

This type of degradation, whose cause is tobe looked for in the vicinity of the gear, is oflittle concern to the teeth if discovered in time,and if appropriate necessary mea uresare takento stop it development

We note a fundamental difference with themetal pun-off or adhesion phenomena. whichare much more cause for concern, for they jeop-ardize the gearing conditions.

Abrasive Wear. This wear phenornon iscaused by the presence affine abrasive particles

ill the lubricant.hese particles may have come from the

exteri r environment of the gearing (du t, sand.mi cellaneou .. impurities) and may have beenintroduced in tile lubricant because of in uffi-cient protection of the teeth. They may also havecome from the inside of the gearbox (foundrysand partieles, particles from metal burrs or pithole .. oil impuritre ).

The appearance of abra ive wear depend on[he ize and nature uf the contaminating par-tides; forexample, a fine abra ivedust will forma mixture with the lubricant which will polishthe teeth. giving them a dull, oft surface.

The presence of and in the oil or grease (inthe case of open gears) will generate a urfaceaspect rougher towards the tooth lip with a fewscratches in the sliding direction and n polishedzone be.low the pitch line.

011 case-hardened gear . the presence of car-bide near the surface may induce abra uve wear.e pecial \y it the load on tHe teeth is heavy, and ifllilesliding lis high (a withllypoid gears) ..

When bigger panicles are found in the lubri-cant, we Illay see the formation of'a few scratchesOil the profiles (Cf. preceding paragraph.) Theyare generally present in the middle of the pol-ihed surface'.

In the ca e of abrasive wear. the particles areconstantly swept from the contact surface (ill

general by lubricanl).and we never encounterlocal adhesion or micro-welding .

Abra: ive wear i thus totally different. fromscuffing phenomena because the cratches arid thematerial removal een are not the results of faullylubrication or inadequate service conditions. Thelubricant is often the vehicle which contains theabrasive panicles and transports them tnJt)\Igholit

the 'y uem, cau ing pollution of the seals. bear-ings. bushings. the pump. and all the meshings,

Abrasi e wear i' generally a phenomeno.ning effect i.11 the zone BB.progres ing very quickly and resulting in all In case-hardened gears iHhe abrasive wearincrease of gear backla '11 in service. which maybring about modifications in dynamic behavior.andlor a distortion in the profile form which isthe ource of noises and vibrations.

If it is 1101 detected, the erosion mechanismwill continueartd result in pointed teelh.11 is notrare to discover worn teeth a hown in' i.g. 2.Most of the Iime.when we attain a criticalevolution a represented [here. the tooth ecuonhas reduced greatly, and we often observe shear-

Profile obtained by abrasive

Curve parallel toinitial profileim:litstinll thedistortions createdby wear

- - .~-~, /' Part of the'profile

with minimum wear• :/' (pitch ina]., / '.

B

A.. '-----Fig. 2 . Abrasive wear mechanism.

i!> intense, i.l will result in a rapid reduction of thethickness in the hardened surface on the teeth,This will bring about the formation of anothertype of deterioration (spelling) which will gen-erally be fntal to the teeth. for the Ihickne S of thehardened layer will no longer be sufficient 10

support the load to berransmiued. (See Fig. 3-4.) If tile abra ive wear is detected in lime. beforeits effect ha . call ed a modificalion of the pro-

Louis Faureis III" Tcrhnira! MOIt(Jger01 eMD Transmissions ofCambrni, France. He is(In (J('lil'e member (If [SOand AFNOR (AssociationFrancais de Normalis ..ation and the Union deNormalisation de laMecaniqlJl', lie hen abotough: gear 1III.'0ry and

files and an inc rea e of backla h incompatible Rear [ailur» Gila/pis.

JANUARYJFEBRUARY 1'~3 35

with the gearing function, and if its causes areperfectly established so that efficient remediescan be undertaken, we will observe a durable

stabilization of the stale of the teeth.Interference Wear

This type of wear occurs when contactbetween teeth happens in had geometricconditions at the beginning or the end ofmeshing. This is the case mainly when thecontact occurs outside of the action segment.The two profiles in contact are no longertangent, and their faulty mating during thisperiod of the meshing generates an overloador hock inthe teeth zones concerned ..

We observe during the beginning of meshinga concentration of the load on the low point. of thedriving element. At the end of the reces ion, weencounter the concentration of the load at the tipof the driving element profile.

This interface mechanism is illustrated onthe sketch in Fig. 5. We have represented on eachside of the segment of action T1 T2 two pairs ofteeth meshing in interference position, In gen-eral, an interference wear appears at the lowpoint of the profile in a narrow wear zone (some-times with the presence of pitting or cratches),and at the tooth tip in the form of streaks prone

36, G~AR TECHNOLOGY

to evolving into scratches and, in extreme cases,metal pun-off.

In general, since we will detect shocks atextreme points of the meshing, this type of wearwill result in an increase in gearing noise. In thecase of through-hardened gears we observe 3.

hollow in the lower parts of the profile, as wellas a light flow on the high parts of the profile,which will resuIt in the formation of a burr ontheactive ridge at the tooth tip ..In general, interfer-ence wear is not dangerous, for as the gear wearsoff, we will observe a reduction of shocks andoverloads at the extreme points of the meshing,The degradation which may be observed on theflanks will end up by running in and disappear-ing, and the noise level will reduce.

In general, intesference wear is not influ-enced by the lubrication quality. We encounterthis type of wear:

• When the design of the teeth was bad(primary interferences),

• When the teeth have undergone overloadswhich have brought about exaggerated deflec-tions of theteeth in service,

• When the gear contains, in the criticalzones of the profiles, excess material whichresults from cutting defects.

• When one or more teeth have been dis-torted. cracked, or broken in service. In thiscase, the meshing of the following tooth and!perhaps a few other following teeth is no longerassured, and we observe shocks at the base of theprofile which generally are extremely severeand can cause the formation of deep hollows.Fig. 6 gives an example of interference wearrecorded on the flanks of a gear wheel.

Normal WearThis is a slow, progressive wear which is .110t

prejudicial to keeping the gear in service for thenormal expected life.

In this class of wear we generally placecommon wear abrasion with two bodies in all itsdifferent possible manifestations. hI the case ofnormal wear, we are not in the presence ofdamage, but ofa normal appearance which showsproper operation of the teeth.

In general,this type of wear requires nospecial inspection or any systematic follow-upif we do not see any notable evolution after acertain time of gear operation. This type of wearappears very slowly in surface-hardened teeth.The same is true for lightly loaded gears.

For gears of moderate hardne. s, the wearwill develop OIl each side of the pitch line wheresliding i null by modifying the shape of theprofile a indicated in Fig. 7. As long as the wearremains [ow, this type of profile form evolutionof the teeth is beaeficialat the meshing level, forit allows better ab erption of the teeth impactduring their engagement.

We will thus often observe the disappear-ance in time of the treaks at the tooth tip 01:

at the root of the teeth, which . ometimesappear soon after first ..tart-up ofa gear (inthe general ea e of gear teeth loaded at lowspeed or medium speed).

As wear progresses, the profile devia-tions will increase with such consequences as anincrease of dynamic efforts during meshing. asrevealed by the increase in noi e and vibrationlevels or an increase in backla h.

We can fix a limit [0 normal wear byconsidering the following:

• The maxirnum value of backlashautho-rized in service by the designer ofthe gear. Thistolerated value win be very much cli.fferentwhether the load direction changes or remains.always applied to the same eries of flanksduring the rotauon of one of the gear wheel. .

•.The vibration level or noise level. whichshould not surpass a certain pre 'et value.

.•The dynamieeffecu during meshing due todeviation of profile caused by wear .. In fact.these deviations generate overloads on the teethfor which the gearing has not neces arily beensufficiently dimen ioned when designed. Theyare generally difficult to estimate. but the ap-pearance or the progression of pitting on theteeth can be a useable erirerion to et [he end ofnormal wear.

We will consider the criteria defined aboveeach time that the mechanism in which the gearis placed transmit alternate effects or is subject10 important torsional vibrations. They shouldalso be considered in the case of gears used fordriving alternative machines or machine ofhigh inertia having long drive hafts or with lowrigidity, orin the case of mechanisms with back-lash compensation or gear used to carry outpreci e di placements.

To follow the evolution of backlash in ser-vice, we can periodically record the distancebetween the profiles, either with. thicknessgauges inserted between the teeth or by "rotat-

711 Wear on11_- loath Tip

Pitch". D' amaterc- "" 11'1_ .. ~

_.~. Pitch Diameter

Tooth Root Wear

Fig. 7a· Distinctive aspect of distortion of gear teeth profiles under wearaction. 7b· PrinCiple of direct wear measurement.

Viewaccord~'ngtoF .. "". ".".""-

". " "

. "..""... "" .... " " Thewearis materialized byrhe print of. ".. ". the mating tooth.. . . J

I

IRuie

Fig. 8 . Measure of wear by comparison with initial profile.

ing" the teeth and measuring with a comparatorthe relative movement of one element of the gearin relation to another near the pitch line,

We can at 0 make a direct evaluation of wearon the teeth by using the following method:

.•.Check the value of Wk over Kteeth. whichgives the distance near the pitch line measuredon the K teeth. M d'lis point onrhe teet h. the wearis lowest, (Cf. Fi.g. 7b.)

.•.Check: the value of the dis tance over the K+ n teeth (n ;;;;;;lor 2) in such a manner as totangent toprofiles near the tip circle.

.•.Check: the value of the distance over the K- n (11 "" 1 or 2) in such a manner as to tangent theprofiles near the dedendum in the place wherethe teeth show the greatest wear (Cf. ig. 8).

• Determine the wear value using the foUow-ing formula:

wear on tooth addendum:

U£..K.±..ll.....R~K + fI}

2wear on tooth dedendum:

(W K - '1, Vb - F K - n)2

with Pb = base pitchThe wear values obtained (reduced from tip

relief if need be) win be used for maximum

J A r.I U A R Y I F E.e FI U A R Y , 9 g 3 .31

38 GEAR TECHNOLOGY

backlash calculation of the gear in operation. Incertain cases, the wear on the flank dedendurnmay be obtained from the print left by the matingwheel by direct measurement with gauges (SeeFig. 8). Generally we can classify as normalwear the following surface aspects:

• Streaks or scratches on the teeth urface,providing that they do not evolve while inservice,

• Polishing, providing that its surface doesnot show localized traces of metal pull-off orscuffing.

• Abrasive wear, providing that its progres-sion is slow or has become stabilized, This typeof wear is often beneficial at the beginning of agear life for it encourages the e tablishing ofgood contact surfaces.

• Interference wear when it is slightlymarked on the teeth .. In the case of through-hardened teeth. we can admit that normal wearis often accompanied by the formation of pit-ting in the heavy loaded zones of the gear. (SeeFigs. 9-]0,)

Moderate WearAt this stage of weal' development. the teeth

appearance is such that we can easily notice thatmaterial removal has occurred during operationnear the tip of the teeth and on tooth flankdedendurn.In general the trace on the operatingpitch line becomes apparent and appears as acontinuous line of low thickne s alii the wholewidth of the teeth. On this, line we do not noticeany wear or material removal.

Moderate wear is characterized by loss ofmaterial along the profiles which is more rapidthan that obtained in the case of normal wear.This wear appears most often when the gearsoperate near their limit rate of lubrication. Manygears, because of voluntary oil viscosity limitsrendered mandatory by use conditions (cold start-ing without. preheating, for example), operateunder such condition .

This type of wear appears almost systemati-cally in the case of highly loaded gears made ofthrough-hardened steel and running at lowspeed. In this ca e. we often ee the appearance,of pitting, which is more or less dense on theteeth profiles. (See Figs. 11-12.)

The presence of particles in the lubricant orthe pollution of the lubrication device can alsolead to the generation of this type of wear. Themoderate wear seen on the teeth is not in most

applications an abnormal phenomenon and maybe considered non-destructive.

It can generally be tolerated. but can bereduced by increasing the oil film thickness,mainly by assuring a better cooliagof the ail(reducing gearing operating temperature), or byincreasing the oil viscosity. We can also possi-bly reduce the load ollt-he teeth (for example bylmproving the dynamic conditions).

If the gearing i operating with lubrication byplashing, we will often obtai II good re ults by

mstalting an injection lubricariun: y tern witha filter to retain the metallic particles in 51.1 -

pension in the oil.ln the ca e of gear wheels lubricated with

grease, we obtain a noticeable improvementby replacing thi lubricant by oil wheneverpossible. This implies that the sealing condi-tions ill relation to the environrnenr, the condi-'lion ofexterior pollution by the oil, orthe ______, afelY conditions (iustallarion temperature)will allow such replacement.

A moderate wear can lead to an increa ing thenoi e and vibration oflhe gear which ometimesre ult in a reduction of anticipated gear life.

EX!cessive and Destructive WearThis type of wear is considered an anomaly

which often leads to rejecting the gear or antici-pating it replacement,

The result is an important. change of teethshape and ill most cases in a great reduction oftheir thickne . Thi deterioration. of teeth canresult from eparaie or joint action of one ormore wear phenomena described in the preced-ing paragraphs. We can, for example, find teethwhose general appearance is comparable to thatof moderate wear, but the volume of materialremoval is very large. We note the appearanceofa step near the tooth root, the very low residualtooth thickness, and the destruction of the over-all shape of the profile. This type of deteriorationis geaerally generated by ahra ive wear, whichcan in certain cases. when the residual thicknesof the teeth becomes weak. end lip in the totalbreakage of whole teeth by hearing.

We can also find destructive wear which ithe result of adhesive wear and is due to directcontact between teeth flanks because the loadtransmitted was 100 large for the lubricant u eel.It i. often the case for highly loaded gearingrunning slowly. where we observe the develop-ment of cold scuffing or of deep scratches.

1Pitch Circle

Pite h eire Ie

Fig. 13- Mechanism of destructive wear.

The sketch in Fig. 13 show. how the destruc-tion occurs on teeth profiles.

In the case of through-hardened gearing,undetected hot scuffing can most of the timeresult in de tructive wear which win take thegear out. 'of service.

Destructive weal' call also be caused by thedevelopment of severe pitti rig. wh ich mo Iof thelime is localized below the pitch line. and whichis cau sed by multiplying the falling in of theteeth surface in the region affected, thus destroy-ing the general profile shape. (See Figs. 14-16 ..)

JANU"RYfFEIiR UA RY , 993. 39

Weer with J bodiesAbr~sille wear Wjtl'12 bodies

Adhesive weer

Wear with Scor,"g Poli,ing Adh.,ions HOI Cold Scratches Abrasive Interllee2 bodres Streaks Or metal scufllng 8cuffit!.q GrODVBs. wear wear

pull·,fl

Normal

use.

140 GEAR TECHNOLOGY

Before trying to find a remedy for excessivewear to save the gear, the underlying cause (orcauses) must be determined.

• In the case where the lubricant is responsible,its performance must be increased (viscositychoice), or the load to be transmitted must bereduced, If the profiles are deeply distorted. thisremedy will be efficient only if a run-in or a re-machining of the teeth is performed to re-establisha good conformity of the contact surfaces (in thecase of through-hardened gears) ..

• In the case of excessive wear caused by abra-sion, an improvement of the housing sealant orlubricant filtration will quite often solve the prob-lem. However. we must be certain that the exces-sive wear demonstrated by increase of backlashbetween the teeth is compatible with the full safetyoperation of the driven element. When there is adoubt, a gear replacement must be performed .

• When we see excessive wear caused by themultiplying of pitting on the teeth and oftenaccompanied by an increase of noise and gearvibration, (in the case of through-hardened teeth)we can proceed to a re-machining of the teeth ora grinding of upper parts of flanks which are toothick, to try to re-establish a good conforrnity ofthe geometry of tile teeth. in contact We can alsoincrease the lubricant viscosity, which wouldeffect a reduction in pitting development.

Ifdespite this treatment, such wear continues todeve lop. it means that the load transmitted:is exces-sive for the material performance and the causeshave to be looked for in the unforeseen overloadswhich happen in service. or in the insufficienthardness of the teeth flanks (in the case of through -hardened teeth). The entire design of 'the gear hasto be re-examined.

ConclusionTo complete this article, we propose a classi-

fication of wear phenomena which has been es-tablished as fellows: We have recorded in a doublechart different wear forms on one side, and on theother side, different characteristic wear levels.(See Table I.)We have placed with referencesand arrows arranged in chart boxes each type ofwear at its initial development stage (which wecan usually ascertain), and possible evolutionwhich we may ascertain for each phenomenonpreviously described .•Acknowledgements: Published as A.GMA documents

88 FTM 4 and in Europower Transmission. Apr.il.1992. Reprinted with permission.