1、11FTM17AGMA Technical PaperMorphology ofMicropittingBy R.L. Errichello, GEARTECHMorphology of MicropittingRobert L. Errichello, GEARTECHThe 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
2、Association.AbstractMicropitting occurs in gears and rolling-element bearings that operate in the mixed or micro EHL lubricationregime. It manifests in many different ways depending on the loads, speeds, rolling and sliding velocities,macrogeometry, surface topography, edge effects, metallurgy, and
3、lubricant properties. The failure analystmustdiscernwhetherthemicropittingisaprimaryfailuremodeorasecondaryfailurethatoccursbecauseofprior damage. Understanding the morphology of micropitting is the key to determining the primary failuremode and root cause of failure.Several examples of micropitting
4、 in gears and rolling-element bearings are presented to illustrate themorphological variation that can occur in practice.Copyright 2011American Gear Manufacturers Association1001 N. Fairfax Street, 5thFloorAlexandria, Virginia 22314October 2011ISBN: 978-1-61481-016-23 11FTM17Morphology of Micropitti
5、ngRobert L. Errichello, GEARTECHIntroductionThis paper discusses the morphology of micropittingand givesseveral examplesof micropittingin gearsandrolling-element bearings that illustrate the morphological variation that can occur in practice.General morphologyTotheunaidedeye,micropittingappearsdull,
6、etched,orstainedwithpatchesofgray. Micropittingisdifficulttoseeunderdiffusefluorescentlighting,andisbestobservedwithintensedirectionallighting. Aflashlightwithaconcentrated beam held in the proper direction effectively illuminates micropitting. With intense lighting,micropitting might sparkleor appe
7、arspeckled. Figure 1 isa scanningelectron microscopy(SEM) imagethatshowsthefloorofamicropitcraterslopesgentlydownwardfromitsoriginatthetoothsurface. Thefloorhasarough surface typical of that caused by ductile fatigue crack propagation. A featheredge forms at the backofthe crater due to plastic flow
8、of material over the crater rim. The featheredge appears white in SEM when itbecomes charged with electrons. Material surrounding a micropitgenerally appearssmooth andfeaturelessunless abraded.Gear tooth slidingFigure 2 shows the directions of the rolling (R) and sliding (S) velocities on the drivin
9、g and driven gear teeth.Contactonthedrivertoothstartsneartherootofthetooth,rollsupthetooth,andendsatthetoothtip. Slidingisawayfromthe drivinggear pitchline. Contacton thedriven toothstarts atthe toothtip, rollsdown thetooth,and ends near the tooth root. Sliding is towards the driven gear pitchline.
10、Like macropitting, micropittingcracks grow opposite to the direction of sliding at the gear tooth surface. Consequently,the cracksconvergenear the pitchline of the driver and diverge near the pitchline of the driven gear.Figure 1. SEM image of micropitting4 11FTM17Figure 3showsmetallurgicalsectionsc
11、uttransverselythroughmicropitsthatshow cracksstart ator nearthegear tooth surface and grow at a shallow angle (typically 10 - 30, but sometimes as steep as 45)tothesurface.Hydraulic pressure propagationGear teeth dedenda have negative sliding (direction of rolling velocity is opposite sliding veloci
12、ty). NegativeslidingissignificantbecauseitpromotesHertzianfatiguebyallowingoiltoentersurfacecrackswhereitaccel-eratescrackgrowthbythehydraulic-pressure-propagationmechanismfirstproposedbyStewartWay1andverified many times by experiments such as Littmanns 2.Figure 2. Rolling (R) and sliding (S) direct
13、ionsFigure 3. Micropitting cracks on a driven gear(courtesy of Newcastle University)5 11FTM17Figure 4 shows profile inspection charts that demonstrate typical profile damage due to micropitting on thedrive flanks of a wind turbine high speed (HS) pinion.The charts for the coast flanks show the origi
14、nal accuracy of the pinion was high, but the micropitting causedseveredeteriorationofthedriveflanks. Notethattheentireactivedriveflanksweredamaged,butthedamagewas most severe in the dedenda in the area of negative sliding.Figure 5 shows a form-ground wind turbine intermediate (INT) pinion with micro
15、pitting that crosses thepitchline.Figure 4. Typical profile damage due to micropitting on wind turbine HS pinionFigure 5. Pitchline is readily discernible on a driven wind turbine INT pinion6 11FTM17Because slide directions reverse as the pitchline is crossed, micropitting cracks grow in opposite di
16、rectionsaboveandbelowthepitchline. Figure5showsthatwhenmicropittinggrowsacrossthepitchline,itmakesthepitchline readily discernible because opposite inclinations of the floors of micropit craters scatter light inopposite directions above and below the pitchline.Surface topographyFigure 6 is an SEM im
17、age of micropitting on a high asperity of a ground tooth surface.Micropitting begins by attacking high points on gear tooth surfaces such as crests of undulations, peaks ofcutter scallops, ridges of grinding lay, and edges of grinding scratches. Figure 6 shows the surface of theasperity has been sev
18、erely plastically deformed. Tractional stress from sliding has caused material to flowover the micropit craters and form a featheredge at the exit side of the craters. Growth of the micropits isopposite to the slide direction and begins at the entry (first point reached by the roll direction) and en
19、ds at theexit (last point reached by the roll direction).Figure 7showsaskive-hobbed windturbine lowspeed (LS)wheel withmicropitting onpeaks ofthe hobscal-lops. Figure 8 shows a form-ground wind turbine INT wheel with micropitting on peaks of longitudinal grindscratches. Multiple cracks originate at
20、these sites and coalesce to form micropits along lines that follow highpoints of surface topography. If ridges are periodic, micropitting might form in regularly spaced rows. Micro-pitting generally progresses until surface peaks are removed, and might continue until large areas of toothsurface beco
21、me porous and continuously cracked.Figure 6. SEM image of micropitting on asperity peak of ground tooth surface(courtesy of Newcastle University)7 11FTM17Figure 7. Wind turbine LS wheel with micropitting on peaks of hob scallopsFigure 8. Wind turbine INT wheel with micropitting on peaks of grind scr
22、atches8 11FTM17Figure 8 shows a form-ground wind turbine INT wheel with micropitting on peaks of longitudinal grindscratches.Gear teeth dedenda are vulnerable to micropitting, especially along the start of active profile (SAP) and thelowest point of single tooth pair contact (LPSTC). However, microp
23、itting might occur anywhere on activeflanks. Micropitting might occur at edges of teeth, at boundaries of surface defects such as scratches anddebrisdents,adjacenttodamagefromotherfailuremodessuchasmacropittingorscuffing,andwherevertheelastohydrodynamic lubrication (EHL) film is disrupted.Micropitti
24、ng patternsIf the pinion and wheel were initially accurate and had little runout, micropitting damage might be similarfromtooth-to-tooth. Inthesecasesmicropittingpatternscanbeinterpretedinmuchthesamewaycontactpatternsare used to assess gear tooth alignment and load distribution. For example, Figure
25、9 shows a helical wheelthat had some misalignment.When micropitting damage varies from tooth-to-tooth, it usually means there are tooth-to-tooth variations intooth geometry or surface roughness. Gearsets with non-hunting gear ratios might develop micropittingpat-ternsthatrepeatatthefrequencyofacommo
26、nfactorofthetoothcombination. Forexample,agearsetwitha25/55-tooth combination, and a common factor of five, might have similar micropitting on every fifth tooth.There might be micropitting only on the pinion, only on the wheel, or on both. Generally, the gear with theroughestsurfacecausesmicropittin
27、gonthematinggear,especiallyifitisharderthanthematinggear. Micro-pitting is most damaging when the opposing surface is rough, harder, and faster. Micropitting resistanceimproves when the harder surface is made smooth. A worst case example would be a sun pinion that mateswith multiple planet wheels th
28、at are rougher and harder than the sun pinion.Figure 9. Micropitting pattern on a helical wheel that had some misalignment(courtesy of Caterpillar)9 11FTM17Geometric stress concentration (GSC)Micropitting might occur at GSC such as:S Edges of gear teethS Ends of bearing rollersS Boundaries of surfac
29、e defects such as macropitting, scuffing, fretting corrosion, or debris dentsS Tip-to-root interference at the SAPS Corners of tip reliefS Wherever the EHL film is disruptedFigure 10showsaform-groundwindturbineHSpinionwithmicropittingattheedgeofcontactatthedriveendoftheactivefacewidth.Figure 11 show
30、s a form-ground wind turbine sun pinion with micropitting on shoulders of debris dents.Debris dents are local depressions that cause loss of EHL film thickness and lead to GSC at shoulders ofdents. Cyclic contacts at these sites generate pressure spikes, plastic deformation, and tensile residualstre
31、sses that eventually initiate micropits.Debris dents on rolling-element bearing raceways usually cause micropitting that frequently initiatespoint-surface-origin(PSO)macropitting. Therefore,debrisdentsareacommonrootcauseofbearingfailure.Figure 12 shows a shaved automotive planet wheel. In addition,
32、Figure 12 shows micropitting at edges of aPSO macropit. It is a secondary failure mode that occurred because the PSO macropit disrupted the EHLlubricant film. Other teeth show there is also micropitting on peaks of the shaving marks.Figure 13showsaFZGPT-CpinionwithaPSOmacropitthatinitiatedatthecuspa
33、bovetip-to-rootdamageatthe SAP.The root cause of the PSO macropit shown in Figure 13 is GSC caused by tip-to-root interference 3. TheFVA micropitting test 4 requires the test to be terminated when a macropit occurs.Figure 14showsaFZGGF-CpinionwithaPSOmacropitthatinitiatedattheupperedgeofa2mmhighband
34、of micropitting.Figure 10. Wind turbine HS pinion with micropitting at edge of contact10 11FTM17Figure 11. Wind turbine sun pinion with micropitting on shoulders of debris dentsFigure 12. Micropitting at edges of PSO macropit on shaved automotive planet11 11FTM17Figure 13. FZG PT-C pinion with PSO m
35、acropit starting from GSC at SAP(courtesy of Afton Chemical)Figure 14. FZG GF-C pinion with PSO macropit starting from GSC near pitchline(courtesy of Afton Chemical)12 11FTM17The root cause of the PSO macropit shown in Figure 14 is GSC caused by micropitting 5.FZGGF-Cgears4arethesame asFZG PT-Cgears
36、 inall respectsexcept PT-Cgears havea toothsurfaceroughness of Ra = 0.3 mm, whereas GF-C gears have tooth surface roughness of Ra = 0.5 mm. The roughersurfaces of GF-C gears cause more severe micropitting that removes the cusp at the SAP due to tip-to-rootinterference and prevents initiation of PSO
37、macropitting at the SAP. Therefore, the micropitting prolongs themacropitting life until the micropitting spreads to the pitchline where PSO macropits initiate at the top of themicropittingbandbecauseofGSCcausedbythestepinthetooth profileat theupper edgeof themicropittingcrater 6. Consequently, in t
38、he FZG GF-C test, a lubricant with superior micropitting resistance might give ashorter macropitting life than a lubricant with inferior micropitting resistance.Figure 15 shows a contact line with fretting corrosion on a wind turbine INT wheel.Figure 15 shows micropitting at edges of the fretting li
39、ne. The fretting corrosion occurred when the windturbine was parked. The micropitting occurred laterduring operationdue toGSC atedges ofthe frettingline.Therefore, fretting corrosion was the primary failure mode and micropitting was a secondary failure mode.Micropitting in rolling-element bearingsFi
40、gure 16showsmicropittingontheinnerring(IR)ofacylindricalrollerbearing (CRB)from awind turbineHSpinion. The micropitting reduced the diameter of the IR, increased the bearing internal clearance, increasedtherollerloads,andincreasedstresses. Furthermore,themicropittingcausedtheIRtoconformtotherollersa
41、ndnegatethecrownoftherollers. ThiscausedGSCattheendsoftherollersandresultedinmacropittingateach end of the raceway. Therefore, micropitting was the primary failure mode and GSC macropitting was asecondary failure mode.Figure 17 is an SEM image of the central part of Figure 16 showing an enlarged vie
42、w of the micropitting.Figure 17 shows micropitting in rolling-element bearing components has a directional randomness that isdifferentfromthemoredirectionally-orientedmicropittingthatistypicalingearteeth. Thisis probablycausedby differences in sliding directions, which are more random in rolling-ele
43、ment bearings than in gears.Figure 15. Wind turbine INT wheel with micropitting at edges of fretting line13 11FTM17Figure 16. Micropitting on CRB IR of wind turbine HS pinionFigure 17. SEM image of micropitting on pinion(courtesy of Northwest Labs)14 11FTM17Figure 18showsarollerfromaCRBfromawindturb
44、ineINTpinion. Therollerhasscuffingintwocircumferen-tial bands that were caused by skidding between the roller and the outer ring (OR) raceways.The bearing has a disc-shaped cage than is guided by a groove in the two-piece OR. No contact occursbetween the roller and OR raceway in the central portion
45、of the roller because the OR raceway is interruptedby the cage groove. Consequently, no scuffing occurred in the central portion of the roller. Furthermore, theroller has end reliefs that prevented scuffing at the roller ends.Figure 19 shows a CRB IR from a wind turbine INT pinion that mated with th
46、e roller shown in Figure 18.Figure 19 shows micropitting that occurred in two circumferential bands separated by a central band withoutmicropitting. The two bands of micropitting were caused by the roughness on the rollers due to scuffing.Thereisalsoabandwithoutmicropittingateachendoftheactiveracewa
47、y. GSCmacropittingoccurredontheleft band and on the central band.Figure 20 is a plot of the axial profile of the IR shown in Figure 19.Figure 20 shows the micropitting caused two ruts in the raceway that are up to 64mm deep. Consequently,amajor portion of the load bearing area of the IR raceway was
48、lost, leaving only the central partof theracewayand two ends to support load. This lead to the GSC macropitting that is shown in Figure 19.This example demonstrates a complex series of failure modes that started with scuffing between the rollersand OR raceway, followed by micropitting on the IR race
49、way caused by the rough surfaces of the scuffedrollers,andfinallyGSCmacropittingontheIRcausedbyGSCduetomicropitting. Therefore,scuffingwastheprimary failure mode, and micropitting and GSC macropitting were secondary failure modes.Figure 18. Scuffing on CRB roller from wind turbine INT pinion bearing15 11FTM17Figure 19. Micropitting on CRB IR from a wind turbine INT pinion bearingFigure 20. Axial profile of CRB IR from wind turbine INT bearingFigure 21showstheinfluenceofoiltypeonmicropitting. ThetestresultswereobtainedwithAGMAtestgears7. The pinion and wheel have tip
