1、09FTM12AGMA Technical PaperThe Anatomy of aMicropitting InducedTooth Fracture Failure- Causation, Initiation,Progression andPreventionby R.J. Drago, R.J. Cunningham,and S. Cymbala, Drive SystemsTechnology, Inc.The Anatomy of a Micropitting Induced Tooth FractureFailure - Causation, Initiation, Progr
2、ession and PreventionRaymond J. Drago, Roy J. Cunningham, and Steve Cymbala, Drive SystemsTechnology, Inc.The statements and opinions contained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractMicropitti
3、ng has become a major concern in certain classes of industrial gear applications, especially windpowerandotherrelativelyhighlyloadedsomewhatslowspeedapplications,wherecarburizedgearsareusedtofacilitatemaximumloadcapacityinacompactpackage.Whilebyitselftheappearanceofmicropittingdoesnotgenerallycausem
4、uchperturbationintheoveralloperationofagearsystem,theultimateconsequencesofa micropitting failure can, and frequently are, much more catastrophic.Micropittingismostoftenassociatedwithparallelaxisgears(spurandhelical)however,theauthorshavealsofound this type of distress when evaluating damage to carb
5、urized, hardened and hard finished spiral bevelgears.This paper presents a discussion of the initiation, propagation and ultimate tooth fracture failure mechanismassociatedwithamicropittingfailure. Thesubjectispresentedbywayofthediscussionofdetaileddestructivemetallurgical evaluations of several exa
6、mple micropitting failures that the authors have analyzed on bothparallel axis and bevel gears.Copyright 2009American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314September 2009ISBN: 978-1-55589-965-33The Anatomy of a Micropitting Induced Tooth Fracture Fa
7、ilure- Causation, Initiation, Progression and PreventionRaymond J. Drago, Roy J. Cunningham, and Steve Cymbala,Drive Systems Technology, Inc.ForewordMicropitting has become a major concern in certainclasses of industrial gear applications, especiallywindpowerandotherrelativelyhighlyloadedsome-what s
8、low speed applications, especially wherecarburized gears are used to facilitate maximumloadcapacityinacompact package. Althoughmostfrequently associated with lower speed gearsystems, micropitting can also be observed in highand very high speed gear systems as well thoughthe failure sequence can be s
9、omewhat different atthehighspeedendofthespectrum. Whileof andbyitself, the appearanceof micropitting, Figure 1doesnot generallycausemuchperturbationintheoveralloperation of a gear system, the ultimate conse-quencesof amicropittingfailurecan, andfrequentlyare, much more catastrophic.Figure 1. Typical
10、 micropitted regionUnfortunately, the micropitting phenomenon,including the underlying causes and analysismethods directed at prevention in the design stage,is not fully understood. Indeed, even the conditionto which the term should be applied is subject tosome discussion and disagreement. We do not
11、propose, herein, to address the greater subject ofclassification, analyticalevaluationandterminologyina“Standard”sense. Ratherthispaperpresentsadiscussionoftheinitiation,propagationandultimatetooth fracture failure mechanism associated with amicropitting failure.Thesubject ispresentedbywayofthediscu
12、ssionofdetailed destructive metallurgical evaluations ofseveral example micropitting failures that theauthors have analyzed as a part of larger investiga-tions of tooth fracture failures. Micropitting is mostoften associated with parallel axis gears (spur andhelical) however, the authors have also f
13、ound thistype of distress when evaluating damage tocarburized, hardened and hardfinished spiralbevelgears. Micropitting observed on both parallel axisand bevel gears will be addressed in thispresentation. Although no specific failure “case” ispresented,informationhasbeenextractedandcon-densed from s
14、everal individual actual tooth fracturefailure investigations conducted by the authors sothat a better understanding of the specific condi-tions that lead to micropitting and the actualprogression from micropitting to fracture can bebetter understood.Before we can discuss the occurrence andpropagati
15、on of micropitting (also known as greystaining), we will have to understand what it is andhow it differs from classic fatigue pitting. Micropit-ting has become a serious problem in high quality,usually ground or otherwise hard finished,carburized industrial gearing, especially in criticalapplication
16、s such as wind turbine, conveyor andsomelowerspeedaerospacegearboxes. Whiletheproblem is more often observed in parallel axisgears, it is also observed in bevel gears where thecontact conditions are “right.”MacropittingUntil fairly recently, surface durability of gears hasbeen defined by macroscopic
17、 pitting (macropitting)in which a crack initiates at a subsurface locationwhere the shear stress exceeds the shear allow-able, Figure 2A. When such a crack propagates to4the tooth surface, a small piece (or, more often,several smallpieces) of material, Figure2C, arelib-erated leaving an inverted cov
18、e shaped defect, asshown in Figure 2B.As this process is repeated, more and more pitsappear and eventually the tooth surface is heavilydamaged, as Figure 3A shows. Eventually, if theloads are high enough, the pitting damaged regionof the tooth acts as a significant stressconcentration and bending fa
19、tigue failure of thetoothmayoccurthroughthepittedregion, asshownin Figure 3B.MicropittingMore recently (over the last 10 to 15 years) amicroscopicpittingphenomenon,generallyreferredto as micropitting, has become a very problematicfailuremodeincertainapplications. Typicallywherehighloadingispresent a
20、t lowerspeedsunder lowormarginal film thickness conditions micropittingbecomes a significant risk. It is important to note,however, that though it usually appears in a some-what different presentation, micropitting is also afactorintheoperationofhigherspeedgearsaswell.In the latter instance, micropi
21、tting is frequentlypresentasa“hardlinethatleadsfairlyquicklytotheformationof largespallsthat mayleadratherrapidlyto tooth fracture failures thus the original micropit-ting“evidence” isoften lost in thefailure. Extremelycarefulmetallurgicalevaluationofthefracturescan,however, often pinpoint the micro
22、pitting “connec-tion” (Figures 14C and 15 show this type ofmicropitting failure).The cause of micropitting is still not fully andcompletely understood. Initially, it was thought thatthe cleanliness of the steel might be playing asignificant role; however, even where very cleansteels are used, microp
23、itting still occurs. Micropit-ting appears to occur at local surface irregularitiesincluding tooling witness lines (Figure 4) andgeneral surface roughness peaks. It has beendemonstrated that micropitting capacity can beimproved through the use of improved finishingtechniques, especially “super finis
24、hing” processeswhich reduce the surface finish down well below 10RMS. The use of some extreme pressure (EP) ad-ditive oils to avoid scoring type failures has alsobeen shown, at least anecdotally, to increase thetendency for micropitting to occur, at least withsome formulations.Figure 2. Classical ma
25、cro pitting fatigueFigure 3. Heavy pitting often leads to local tooth fracture5Figure 4. Micropitting aligned with finishing tool witness linesThe lubricant connectionWhile some testing has been accomplished to in-vestigatetherelationbetweenmicropittingandspe-cific lubricant additive package combina
26、tions, it isstill not well understood. Figure 5 shows, for exam-ple, the results of one set of FZG tests thatcompared the efficacy of a biodegradable, straightmineral with an ester based fluid in the incidence ofmicropitting.Figure 5. Effect of lubricant selection on micropitting performance6The amo
27、unt of micropitting damage that occurredduring these controlled tests shows that thelubricant in use, under identical, controlled testconditions can significantly affect the rate of micro-pitting formation. Equally interesting, however, isthe finding that micropitting occurred with bothlubricants th
28、ough at differing rates. New oils havebeen developed specifically aimed at providingimprovements in the lubricated contact that willforestalltheoccurrenceandreducetheprogressionof micropitting on gear teeth. These new lubricantshave been successfully applied and have proven tobe good solutions in sp
29、ecific applications. Our pur-pose here, however, is to understand the specificnature of the micropitting failure mode so that wecan postulate ways in which the basic design of thegears can be modified to reduce the probability ofmicropitting occurring at all.Designing against micropittingThe hill an
30、d valley effect of the surface topographyproduced by the specific finishing method used is abit easier to understand, at least qualitatively. Nospecific analytical technique has yet beendeveloped to allow the prediction of micropitting ingeneralthusdesigningagainst micropittingremainsa bit of a chal
31、lenge. The best course of actionappears to lie in recognizing the general circum-stances that lead to micropitting and then applyingknown enhancements to reduce the probability ofmicropitting occurring. These enhancementsinclude treatments of the gear itself (finer surfacefinishes and some coatings)
32、; reducing slidingvelocity along the tooth profile, reducing the frictioncoefficient and the use of specially developedanti-micropitting oils.Effect of micropitting on operationThe simple existence of micropitting of and by itselfis, however, not generally an operational problemfor most gear systems
33、. The progression of micro-pitting to a more catastrophic form, however, canand often does cause a cessation of function of theentire gear system. It is this progression that is therealcausedforconcernandthephenomenathatwewill deal with here.Figure 6 shows a helical gear tooth which exhibits afine m
34、icropitting pattern in the flank region. Thisvery fine damage, which is very difficult to seewithout special lighting effects, would appear to beof little consequence. After progression, however,the ultimate result of this micropitting is the toothfractures shown in Figure 7.Figure 6. Helical gear t
35、ooth exhibiting fine micropitting patterns (red arrows) in flankFigure 7. Helical gear tooth after partial tooth fractures7Certainly, the very minordamage shownin Figure6would not, at first review, seem serious enough tocause the very extensive, service ending damageshown in Figure 7. After reviewin
36、g a number ofgearsfromsimilarsystemsat variousstagesofpro-gression, theauthorswereable todevelop ahistorythat clearly shows the relation between this minorinitial damage and the catastrophic result.Figure 8 shows two views of the same generalregion of micropitting on a helical gear tooth. Theregions
37、 shown are magnified views of one of themicropitted regions denoted by the Red arrows inFigure 6. This gear, which initially appeared quitesimilar to the one shown in Figure 6, eventuallyexperienced many service ending tooth fracturefailuressuchasthoseshowninFigure7. Inordertoexamine the mechanism b
38、y which the seeminglymild micropitting distress progressed to a completetooth fracture we will examine this failure in detail.1)The micropitting mechanismVisual examination of each tooth fracture surfaceindicated several zones of bending fatigue initiationand propagation. Fatigue was indicated by th
39、emany semi-circular beach marks which are plainlyvisible. Bending fatigue initiated sub-surface fromthe pitting/spalling and propagated up and over thetip from the drive side to the coast side of the toothresulting in removal of portions of the tooth tip.Fatigue characteristics are revealed in Figur
40、e 8which is an enlarged view of the fatigue and roughsurface topography shown in Figure 7.As the micropitting progressed it began to initiatesmall fan or triangular shaped spalls which are notreadilyapparent throughvisualexamination,Figure9A, but are visible with some additionalmagnification, Figure
41、 9B.As the spalls continue to initiate and grow, theypropagate by a crack mechanism as Figure 10shows. Whilethesecracksaresmall,overtimetheyprogress and result in larger spalls occurring on thetooth surface. Continued propagation of thesecracks into the body of the tooth, Figure 11, led tothe tooth
42、fracture failures which caused the gear tobe removed from service.Figure 8. Magnified view of sub-surface fatigue characteristics_1)The figures used to illustrate this discussion were taken from several related individual failure investigationsconductedby theauthors thuseach figuredoes notnecessaril
43、y showthe samegear. Each figurewas selectedfromthe large number of images accumulated during the course of these investigations so as to best illustrate the pointunder discussion.8Figure 9. Typical micropitted tooth surface appearanceFigure 10. Cracks emanating from small spalls in micropitted regio
44、nFigure 11. Small cracks propagating from spalls shown in figure 99Micropitting also very often occurs when theamount of involute modification provided at the tipsand flanks of a mating pair of gear teeth isinadequate to compensate for the deflections thatoccur under load. In this case, a very fine
45、line ofmicropitting occurs on the tips of the teeth on onemember and on the flanks of the mating memberveryclosetothelowestpointofcontactonthetooth.The appearance of this type of micropitting, shownon the tip of one tooth in Figure 12, is slightly differ-ent from that typically observed on the tooth
46、 flanksin general but the basic failure mechanism is quitesimilar.A similar line of micropitting will be observed on theflankof thematinggear. This latterdamage isoftenreferredtoasa“hardline.”Thepresenceof thehardline in the lower dedendum portion of the toothwhere the bending stresses are quite hig
47、h poses aparticulardanger of tooth fracture. Themechanismby which this occurs is shown in Figure 13.Figure 12. Micropitting at the tip of a gear tooth due to inadequate profile modificationFigure 13. Initiation of a spall in a line of micropitting (the “hard line”)10The spalls progress from the hard
48、 line, Figure 13A,via cracks that initiate at or near the surface in themicropitted region and then extend into the geartooth body until a piece breaks away from the toothsurface. This crack mechanism is shownschematically in Figure 13B and in sequence inFigure 13A.The profile modification connectio
49、nAs noted previously, micropitting is often thought ofas being related solely to lower speed operation.This is not true, but the mechanism of themicropitting induced failure takes on a somewhatdifferent appearance. For higher speed gears, es-pecially those which experience significant toothdeflections, if the profile modifications (i.e., tip and/or flank modifications) are not sufficient to preventhard contact near the extremes of contact (ate thetip and flank of the tooth in the profile direction) themicropitting often first appears in the form of thevery narrow “h
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