1、04FTM4Influence of Surface Roughness onGear Pitting Behaviorby: T.C. Jao, M.T. Devlin, J.L. Milner and R.N. Iyer, AftonChemical Corporation and M.R. Hoeprich, The Timken CompanyTECHNICAL PAPERAmerican Gear Manufacturers AssociationInfluence of Surface Roughness on Gear PittingBehaviorT.C. Jao, M.T.
2、Devlin, J.L. Milner and R.N. Iyer, Afton Chemical Corporationand M.R. Hoeprich, The Timken CompanyThe 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.AbstractInearlierstudies,s
3、urfaceroughnesshadbeenshowntohaveasignificantinfluenceongearpittinglife. Withinarelativelysmallrangeofsurfaceroughness(Ra=0.1 - 0.3micron),gearpittinglifeasmeasuredbytheFZGpitting test decreases as gear surface roughness increases. This inverse relationship between gear surfaceroughness and pitting
4、life is well understood in the field. To determine whether this inverse relationship isapplicable to a wider range of surface roughness values, we have conducted a pitting study using gearswhosesurfaceroughnessrangesfrom0.1- 0.6micron. Theresultswerenotcompletelyexpected. Thestudyshows thatthemicrop
5、ittingareais radically larger whenthegear surfaceroughness is close tothe upper limitof the range studied. Plasticity index, which approaches a value of around 3.7 for the rougher gear surface,appearstoberesponsiblefortheformationofsuchlargemicropittingarea. Atthesametime,theformationofapitisalsogre
6、atlydelayed.Notonlyisthepittinglifesignificantlylonger,buttheinitiationofpitscanoccurnearthe pitch line. This paper discusses how high surface roughness introduces a wear mechanism that delaysthe formation of pits.Copyright 2004American Gear Manufacturers Association500 Montgomery Street, Suite 350A
7、lexandria, Virginia, 22314October, 2004ISBN: 1-55589-827-01Influence of Surface Roughness on Gear Pitting BehaviorT.C. Jao, M.T. Devlin, J.L. Milner and R.N. Iyer, Afton Chemical Corporationand M.R. Hoeprich, The Timken CompanyINTRODUCTIONExtended gear fatigue pitting life is not only an es-sential
8、performance requirement for todays auto-motiveandindustrialgearoilsbutalsoforautomatictransmission fluids (ATF) or continuously variablespeedtransmission(CVT)fluids1,2. Paststudieshad shown that both gear surface roughness andchemical and physical properties have a significantinfluence on the fluids
9、 pitting performance 3. Thefluids chemical and physical properties affect oilfilm thickness, boundary frictional coefficient andcorrosiveness. The effect of surface roughness onmetalfatiguebehaviorhasbeenstudiedextensivelyandisapparentlyquitewellunderstood4-8. Ithasbeen well established that surface
10、 roughness is amajor factor influencing the formation of micropit-ting 7-9. It also has been shown that micropittingis the most common cause of pitting in modernclean steels since current steel processing technol-ogy essentially eliminates subsurface inclusions10, 11.Although it is generally accepte
11、d that micropittingcanleadtopitting,thespecificmechanismbywhichmicropitting induces pitting is still poorly under-stood. The lack of in depth understanding of thiscause-and-effect relationship between micropit-tingandpittinghinderstheadvancementofgearoil,ATF and CVT fluid technology with respect to
12、im-provement in the fatigue-pitting life. To overcomethisdeficiency,wehavedevotedconsiderableeffortto increase the fundamental understanding of howmicropitting impacts pitting. Our earlier study indi-cated that in addition to the effects of oils physicalproperties, surface roughness has a large effe
13、ct onthegearsfatigue-pittinglife3. Withinasmallvari-ation of gear surface roughness, increasing thegear surface roughness decreases, almost linearly,its fatigue-pitting life. To extend the model devel-oped in the previous work to higher roughness val-ues, we studied the effect of gear surface rough-
14、ness on fatigue-pitting life by doubling the gearssurfaceroughnessbytestinggearsdesignedformi-cropitting study. The results were not completelyexpected. This paper describes how a small in-crease in surface roughness decreases the fa-tigue-pitting life, but a large increase can actuallydelay the for
15、mation of pits and thus significantly in-crease the gears fatigue-pitting life. In essence, anon-linear reversed effect of surface roughness onfatigue-pitting life has been observed.EXPERIMENTALGears TestedBoth the FZG type PT-C pitting gears and the typeGF-C micropitting gears tested were designed
16、andmade by ZF Friedrichshafen AG 12. The supplierindicated that both types of gears were made fromthe same steel material and hardened by the sameprocess. They had the same tooth-profile geome-try, pitch line diameter, addendum and dedendumdepths. However,thepinionandwheelgears ofthesamebatchofsuppl
17、yareusually notmade fromthesame single melt, but they are hardened by thesameprocessandatthesametime. Thiscouldalsobe true for pinion gears that are supplied as thesame batch of gears. The difference in surfaceroughness between the type PT-C and GF-Cgearswasachievedby speciallydressing thegrind-ingw
18、heelsandthecontrol ofthe surfaceroughnessduring grinding. The specification for the gears re-quires the surface hardness to be HRC 62 12; wedid not independentlyverify thevalue. In thisstudy,before each pitting test was carried out, the surfaceroughness of the gear was measured by a one-di-mensional
19、 profilometer. For the matched pinion/wheel set, three teeth of a pinion were chosen tomeasure the surface roughness of the contact sideby profilometry along the center involute of eachtooth profile. This was repeated for the gear. ThearithmeticmeanvalueofthesixmeasurementswastakenastheRavalueofthep
20、air. The procedureforthemeasurementswasdescribedaspartofthetestprocedure13. Thevaluesofsurfaceroughnessofthe pinion/wheel sets are shown in Table 1.OilsTwo different viscosity oils were prepared with thesame additive package, which was developed for2application in automatic transmissions. Eventhough
21、 both oils use polyalphaolefin (PAO) as thebase oil, different combinations of PAOs were nec-essarytopreparethetwooilsattwo100Ckinemat-ic viscosity levels - 7.5cSt and15cSt. Theoils areshowninTableI. Theboundaryfrictionalproperties,filmformationpropertiesandanti-corrosionproper-tiesofthetestoilswere
22、measuredasdescribedpre-viously 3.Pitting Test MatrixThetwovariablesinvestigatedinthisstudywerethegear surface roughness and oil viscosity. Thus, amatrix of four different pitting tests was carried out.Each test was carried out in duplicate.Pitting Test Run ConditionThetestswereconductedusingtheFZGpi
23、ttingtestPT C/9/90 procedure except the oil temperaturewas set at 120_C 13. The Hertzian stress (Pc)forthe load stage 9 used was 1650N/mm2. The pitchline velocity is 8.3m/s. The expected tip deflectionunderthistestconditionwasaround20-30m.Dur-ing the test, inspection of the tested gear for micro-pit
24、ting and pitting was conductedevery eighthours.Theareasofmicropittingandpittingweremeasuredand recorded at each inspection.Determination of Fatigue-Pitting LifeAccording to the FZG pitting test procedure, the fa-tigue-pitting life was determined when any tooth orthesumofteethinonegearaccumulateda to
25、talpit-tingareaof5mm2. Suchmeasurementforfatigue-pitting life for a regular Type PT-C pitting gear wasstraightforwardwithnoambiguity. However,thefa-tigue-pitting life measurements for the runs involv-ing Type GF-C micropitting gears were more com-plicatedbecausebeforeatotalpittingareaof5mm2was reach
26、ed, the pinion dedendum surfacealreadyhadbeencoveredwithmicropits. Thus,fortherunsinvolvedwithTypeGF-Cmicropittinggears,wede-termined the fatigue-pitting life by two procedures.One procedure used the time when the teeth hadaccumulated a total micropitting area of 448 mm2,which is approximately the s
27、um of the micropittingband (about 2 mm deep and 14 mm wide) areasmeasured by unaided eyes from the individualteeth. The second procedure used the time whenany tooth or sum of teeth had actually accumulatedatotalpittingareaof5mm2. Itisnoteworthythatforthe Type GF-C micropitting gears, the total micro
28、-pitting area of 448 mm2is always reached beforethe total pitting area reaches 5 mm2.Surface Analysis of the Tested GearsSEM was used to analyze the wear and pits of thegear surfaces. A Form Taly Surf was used to mea-sure the deviation of the gear tooth profile from theoriginalgeometry.Tofindouthow
29、crackspropagateand ifany subsurfacenon-metallic inclusionscouldhaveinitiatedthecracks, toothsurfaces, wherepitsor spalls had occurred, were sectioned.Table 1. Two-variable Pitting Test Matrix StudyTest Code Fluids 100C KinematicViscosity cStGear Type Surface Roughness(Ra, micron)02-06-02 (LH1)a7.5 F
30、ZG Type C-Mb0.4302-14-10 (LL1)a7.5 FZG Type C-Pc0.2002-12-08 (LH2)a7.5 FZG Type C-Mb0.4102-07-03 (LL2)a7.5 FZG Type C-Pc0.2302-09-05 (HH1)a15.0 FZG Type C-Mb0.4302-08-04 (HL1)a15.0 FZG Type C-Pc0.2002-10-06 (HH2)a15.0 FZG Type C-Mb0.5002-05-01 (HL2)a15.0 FZG Type C-Pc0.2002-11-07 (HL3)a15.0 FZG Type
31、 C-Pc0.23aThelabelgiveninparenthesisistheabbreviatedcodeforthatparticulartestrun;the firstletterandsecond letterstandforthelevelsofviscosityandsurfaceroughness,respectively. LandHmeanlowandhigh,respectively,whilethenumer-ic value indicates the order of the repeat runs.b FZG Type C micropitting gearc
32、FZG Type C pitting gear3RESULTSSEM Analysis of the Tested GearsTheSEMimagesofthetestedpiniongearofeachofthe four matrix runs are shown in Figures 1-4. Fig-ure 1 shows the SEM images of the No. 12 tooth ofthetestedpiniongearoftheHL3 runat twodifferentmagnifications. The band between the upper andlowe
33、r arrows is the micropitting band. As indicatedin our previous paper 3, pitting starts at the upperedge of the micropitting band.Figure 1. SEM images of the No. 12 tooth of the tested pinion gear of the HL3 run using the 15cSt oil with a FZG Type PT-C pitting gear. The area between the two outer arr
34、ows is themicropitting band.Figure 2. SEM images of the No. 5 tooth of tested pinion gear of the LL2 run using the 7.5 cSt oilwith a FZG Type PT-C pitting gear. The area between the two outer arrows is the micropittingband.Figure 3. SEM images of the No. 2 tooth of the tested pinion gear of the HH2
35、run using 15 cSt oilwith a FZG Type GF-C micropitting gear. The area between the two outer arrows is themicropitting band.4Figure 4. SEM images of the No. 10 tooth of the tested pinion gear of the LH2 run using 7.5 cStoil with a FZG Type GF-C micropitting gear. The area between the two outer arrows
36、is the micro-pitting band.Figure2showstheSEMimagesofthetestedpiniongear of the LL2 run. Both HL3 and LL2 runs usedthe same FZG Type PT-C pitting gear but with twodifferentoils. Thesetwooilsareformulatedwiththesame additive chemistry but different viscositygrades of PAOs to achieve two different 100C
37、ki-nematicviscosities - 15cStversus7.5cSt. Themi-cropitting band widths shown in these two figuresarenarrowas istypically seenin thetested gearsofFZG Type PT-C pitting gears. Figures 3 and 4show the tested pinion gears of the two runs HH2and LH2, both of which used FZG Type GF-C mi-cropitting gears.
38、 Again, the same two oils of differ-ent viscosities were used. The noticeable commonfeature between these two figures is the relativelylarger micropitting band width. Overall, the micro-pitting band width appears to depend only on thetype of gear used. The oil viscosity practically hasno effect on t
39、he micropitting band width. However,all four sets of SEM images appear to have a com-mon,polishedwearbandofapproximatelyconstantband width, which is shown between the lower twoarrows. Table 2 summarizes the observations ob-tained from the SEM images shown in these fourfigures.It is particularly inte
40、resting to compare the two setsof SEM images between Figures 1 and 3. The dif-ferencebetweenthetwoSEMfiguresisthatthefor-mer was obtained from a run that used a FZG TypePT-C pitting gear while the latter was taken from arun that used a FZG Type GF-C micropitting gear.Both runs used the same high vis
41、cosity oil; yet theimpact on the micropitting band is quite dramatic.Therunwiththepittinggearhadafatigue-pittinglifeof 42 hours while the one with the micropitting gearhad a fatigue-pitting life of 170 hours. Neverthe-less, the micropitting gear accumulated a total mi-cropittingareaof448mm2injust 26
42、hours whilethepitting gear accumulated a total micropitting area ofonly around 224 mm2by the time the test reached42 hours. Similar results can be found comparingFigures 2 and 4; this time the comparison is be-tween two runs using the same low viscosity oil butdifferent types of gears.Table 2. Pinio
43、n Dedendum Wear BandComparisonFZG Pitting TestRun No.Pinion dedendumwear band (mm)Totala/LowerbHL3 (No. 12 tooth) 1.11 / 0.48LL2 (No. 5 tooth) 0.79 / 0.50HH2 (No. 2 tooth) 3.12 / 0.48LH2 (No. 10 tooth) 3.37 / 0.48aTotal width consisting of micropitting band andthe polished wear band.bOnly the lower
44、polished wear band.Tooth Profile Changes on the Tested GearsTo investigate why the two different types of gearsproduce such a large difference in the micropittingband width, we used a Form Taly Surf to map thecontours of the tooth profiles. Figure 5 shows thecontours of the four tested pinion gear t
45、eeth, eachrepresenting one of the four matrix runs. LL2(5) isthe profile deviation of tooth No. 5 of LL2 piniongear; HH2(2) is the corresponding profile deviationof tooth No. 2 of the HH2 pinion gear; like wise,HL3(9) is for tooth No. 9 of the HL3 pinion gear andLH2(10) is for tooth No. 10 of the LH
46、2 pinion gear.5Figure 5. Wear-induced tooth profile deviation.There are two sources for the profile deviation: oneis the deviation from the ideal involute geometryeven when the gear is new and the other is due towear. For LL2(5) and HL3(9), the profile measure-ments went over the small pits that for
47、med on theshoulder at the upper edge of the approximatelyhalf-mm wide polished wear band. The measure-ments for HH2(2) and LH2(10) show a greater, butmore continuous and smoother wear that de-creases as it approaches the pitch diameter. Inten-tionally,allfourtracesdo notgo throughany spalltoprevent
48、damaging the instrument. The two almostoverlapping vertical bars between 34 and 35 mmgrid marks indicate the locations where the spallsstarttoformontheLL2andHL3gearteethwhilethetwo vertical barsbetween 36and 37mm gridmarksindicate the locations where the spalls start to formon the HH2 and LH2 gear t
49、eeth. LL2(5) and HL3(9)belong to FZG Type PT-C pitting gears whileHH2(2) andLH2(10) belongto FZGType GF-Cmi-cropitting gears. For LL2(5) and HL3(9), it is clearthat the spall forms at the shoulder, which appearsimmediatelyfollowingthe lowerpolished wearbandandcanserveasastressraisertoinitiatetheforma-tion a pit. The contours of HH2(2) and LH2(10)show that a large deviation from the original toothprofile occurs dueto wear,preventing theformationof a clear shoulder. Such a
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