1、04FTM5Investigations on the Micropitting LoadCapacity of Case Carburized Gearsby: Dr.-Ing. B.-R. Hhn, Dr.-Ing. P. Oster, Dr.-Ing. U. Schradeand Dr.-Ing. T. Tobie, Gear Research Centre (FZG)TECHNICAL PAPERAmerican Gear ManufacturersAssociationInvestigations on the Micropitting Load Capacity ofCase Ca
2、rburized GearsDr.-Ing. B.-R. Hhn, Dr.-Ing. P. Oster, Dr.-Ing. U. Schrade and Dr.-Ing. T.Tobie, Gear Research Centre (FZG)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.Abs
3、tractThe load capacity of power transmitting gears can be limited by different failure modes. Standardizedcalculation methods acc. to German (DIN) or International (ISO) standard are available for rating the pittingresistance and bending strength of gear teeth. A further kind of fatigue damage is mi
4、cropitting that is mostfrequently observed on case carburized gears.Micropitting is controlled by the conditions of the tribological system of tooth flank surface and lubricant. Theoil film thickness has been found to be a dominant parameter. Lubricant of base oil and additive, operatingconditions,
5、surface roughness and gear geometry are known as important influence factors on themicropitting load capacity.Incontinuous work over severalresearch projects major influences on themicropitting loadcapacity ofgearswere systematically investigated.ForevaluatingtheinfluenceoflubricantstheFZGmicropitti
6、ngtestwasdeveloped. Resultsontheinfluenceofcertain parameters such as oil temperature, surface roughness or material were determined by variation ofthe test conditions.Within the scope of actual research work some basic influences of gear geometry, gear size and operatingconditionswereinvestigated.F
7、orthispurposeanextensivetestprogramonspurandhelicalgearsofdifferentsizes and different gear geometry has been carried out. Based on the results of the previous and actualinvestigations an enhanced calculation method to determine the micropitting load capacity of practical gearunits was developed.In
8、accordance to the existing standardized calculation methods regarding pitting resistance and bendingstrength the proposed rating formulas can be used to evaluate the risk of micropitting respectively todetermine a safety factor for micropitting on case carburized gears.The calculation method is base
9、d on the result of the micropitting test as a tribological parameter for thelubricant in use but enables the gear designer furthermore to take major influences as operating conditions,geargeometryandgearsizeoftheactualapplicationintoconsiderationifratingthemicropittingloadcapacityof a gear.The paper
10、 summarizes important results of the continuous experimental investigations and introduces theproposed calculation method for rating the micropitting load capacity of case carburized gears.Copyright 2004American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 2231
11、4October, 2004ISBN: 1-55589-828-91Fig. 1: Severe micropitting on theteeth of a test pinionINVESTIGATIONS ON THE MICROPITTING LOAD CAPACITYOF CASE CARBURIZED GEARSB.-R. Hhn, Professor Dr.-Ing.; P. Oster, Dr.-Ing.; U. Schrade, Dr.-Ing.; T. Tobie, Dr.-Ing.Gear Research Center (FZG), Technical Universit
12、y of Munich, Boltzmannstr. 15, 85748 Garching, GermanyNomenclature A start of line of contact - B lowest point of single tooth contact . - C pitch point . - D highest point of single tooth contact - E end of line of contact - Caamount of tip relief mE1,2Youngs modulus of pinion, gear N/mm2E reduced
13、Youngs modulus .N/mm2R radius of flank curvature. mmRamean value of tooth flank roughness mSGFmicropitting safety factor . - SGFminminimum demanded safety factor . - a center distance mmb gear face width mmffmmean profile deviation .mhCfilm thickness at pitch point (isotherm) . . mhminlocal minimum
14、oil film thickness .mmnnormal module mmp Hertzian contact pressure .N/mm2penormal base pitch mmu tangential velocity m/sx1,2addendum modification factor . - z1,2number of teeth - helix angle transverse contact ratio - overlap ratio . - coordinate in the direction of contact line mm coordinate in the
15、 direction of face width mm dynamic viscosity PasMdynamic viscosity at bulk temperature . . PasG4bMbulk temperature . CGFeffective rel. minimum oil film thickness . - GFPpermissible rel. minimum oil film thickness - relative radius of flank curvature mm1 IntroductionThe load capacity of power transm
16、itting gears canbe limited by different failure modes. Standardizedcalculation methods acc. to 1, 2, 12 are availablefor rating the pitting resistance and bendingstrength of gear teeth. A further kind of fatiguedamage is micropitting that is most frequentlyobserved on case carburized gears (Fig. 1).
17、Micropittingfirstly has beennoticed on highpower trans-mitting gearsoperated withlow viscosity lu-bricants at hightemperatures.The increasinguse of lubricants with EP additives has led to adecrease of the scuffing risk and an increase ofthe transmitted power. As a consequencemicropitting has been ob
18、served in recent years ona diversity of different gear applications. It is the state of knowledge that micropitting ongears is controlled by the conditions of thetribological system of tooth flank surface andlubricant. The oil film thickness has been found tobe a dominant parameter. In continuous wo
19、rk over several research projectsmajor influences on the micropitting load capacityof gears were systematically investigated. Forevaluating the influence of lubricant the FZGmicropitting test was developed. Results on the2Graufleckigkeit an der Flanke des RitzelsE5mm5 mDCBAload cyclesFig. 2: Changes
20、 of the involute profile of a gear toothcaused by progressing profile deviations due tomicropitting and correlation to the tooth flankinfluence of several further parameters such as oiltemperature, surface roughness or gear materialwere determined by variation of the test conditions.For a safe and r
21、eliable rating of the micropittingload capacity of gears an experimentally verifiedcalculation method to evaluate the risk ofmicropitting of gears in practice is required.Therefore some more basic influences such asgear geometry, gear size and operating conditionshas to be known. For this purpose an
22、 extensivetest program on spur and helical gears has beencarried out. Gears with different sizes and differentgear geometry were included in the test program inorder to develop an enhanced calculation methodthat is based on the result of the micropitting testas a tribological parameter for the lubri
23、cant in usebut taking also further major influences intoconsideration when rating the micropitting loadcapacity of a gear.2 Characteristics of MicropittingMicropitting occurs most frequently on tooth flankswith a high surface hardness under unfavorablelubrication conditions. Several parametersinflue
24、nce the damage occurrence, developmentand local intensity on a tooth flank. First indicationsof the occurrence of micropitting are determinedoften after few load cycles at very low loads; thatindicates the beginning of damages of wear type. Cracks and material pits in the further damageprogression p
25、oint out the fatigue character of themicropitting failure. Micropitting on the flanksurfaces normally occurs first on the dedendumflank of the driving gear, i.e. in the area of negativespecific sliding. It propagates gradually over theflank and covers in extreme cases the wholeactive flank surface.F
26、orms of micropitting failure and consequentialdamages:G26 Micropitting is a surface damage. It can causeprofile deviations of wear type on the activeflanks (Fig. 2). In this case the dynamicaladditional forces and the gear noise canincrease.G26 The immediately resulted small pits, that givethe flank
27、 its grey appearance, can build largeflat pits.G26 Some of the many small cracks can propagateand ramify. As a result large deep triangularparticles can break out. That is the way ofoccurrence of pitting and spalling, that in manycases can reach the tip edge of the gear.Typical changes in the involu
28、te profile diagram of atooth flank caused by progressing profiledeviations due to micropitting are shown in Fig. 2as a function of the number of load cycles. Theseprofile deviations in Fig. 2 are located in sectionA-C on the line of contact and are correlated to theclosed grey area on the tooth flan
29、k.A more detailed description of the damage patternas well as of some effects of micropitting onoperating characteristics and gear life are to befound in 10.3 Influences on the Micropitting LoadCapacityMicropitting is controlled by the conditions of atribological system consisting of the tooth flank
30、surface and the lubricant (base oil and additives).The relationship of oil film thickness to surfaceroughness of tooth flank (hmin/Ra) is a dominantparameter. The oil film thickness depends on thelubricant viscosity and the operating conditionsand can be calculated according to the rules ofelastohyd
31、rodynamic (EHD) theory. Fig. 3 shows a differentiation of basic influences3MicropittingLoad CapacityBase Oil and AdditivesOil ViscosityLoadOperating TemperatureCircumferential SpeedSurface RoughnessGear GeometryGear SizeGear MaterialSurface TreatmentOperating ConditionsLubricantMicropittingTestTooth
32、 FlankFig. 3: Influences on the micropitting load capacity failure limit,load stagetest 0246810121416m20load stage testrunning time 16h / load stage check every 80hendurance test5795 945 1094 1245 1395 1547 1245 15476 7 8 9 10 8 1010101010load stagepCN/mm2failure limit,endurancetest test terminatedb
33、ecause of pittinglubricants:ISO VG 32FZGmicropitting testsC / 8,3 / 90micropitting load capacity:GFT - lowGFT - mediumGFT - highmeanprofiledeviationffmFig. 4: Typical micropitting test results for lubricantswith different micropitting load capacityon the micropitting load capacity of gears intothree
34、 main categories: lubricant, tooth flank andoperating conditions.Influences of lubricantIt is well known that the chemistry of base oil andespecially additives has a large influence on themicropitting load capacity of gears. As thechemical interaction of base oil and additives withthe material of th
35、e tooth flank is normally notpredictable the influence of lubricant on themicropitting load capacity has to be determined byan experimental test method as the FZG-micropitting test 7, 8. This is also due to differentadditive performance at different operatingtemperatures.The influence of oil viscosi
36、ty is calculable butminor compared to the influence of chemistry ofthe lubricant. A higher viscosity of the lubricantnormally reduces micropitting due to higher oil filmthickness 10.FZG micropitting testThe FZG-micropitting test provides a quantitativeevaluation of the influence of lubricant (especi
37、allyadditives), lubricant temperature and otherparameters on the occurrence of micropitting. Themicropitting test differentiates oils according totheir micropitting load capacity and enables thechoice of a lubricant with a sufficient micropittingresistance. The FZG-micropitting test consists of aloa
38、d stage test with incremental increasing of thecontact stress and an endurance test.In the load stage test the micropitting load capacityof a lubricant is determined for specified testconditions in form of a failure load stage. Thefailure load stage is reached, if the tooth profilehas changed by a m
39、ean profile deviation of 7.5 m(corresponding to a change of gear accuracy fromDIN 5 to DIN 6). The endurance test providesadditional information on the damage progressionwith higher numbers of load cycles. Fig. 4 showstypical examples of lubricants with differentmicropitting load capacity. A detaile
40、d description ofthe test procedure is to be found in 8.In order to allow a transfer of the test result onpractical gear sets an adequate calculation methodto compare the conditions of the tribologicalsystem in the practical application to the testconditions is necessary. For reliable results thetest
41、 conditions should be as close as possible tothe operating conditions of the application. As theeffect of the additives on the micropitting loadcapacity depends on the lubricant temperature it isparticularly recommended to run the micropittingtest in the temperature range of the application.Influenc
42、es of tooth flank and operating conditionsSeveral influences on the micropitting loadcapacity of gears were systematically investigatedin former research projects by variation of the testconditions 5, 6, 14, 15, 16. Some of the basic re-sults acc. to 10 can be summarized as followed:G26 Surface roug
43、hness is one of the majorinfluences on the micropitting load capacity. Anincrease of the surface roughness of the toothflank leads for almost all operating conditions toa decrease of the micropitting load capacity.4G26 Surface treatments as copper plating mayreduce micropitting due to an improvedrun
44、ning-in effect.G26 Higher loads result normally in strongermicropitting.G26 An increase of the operating temperature leadsto a decrease of operating oil viscosity and oilfilm thickness. This generally results in areduction of the micropitting load capacity butthe effect may be compensated by theindi
45、vidual influence of operating temperature onadditive performance.G26 An increase of the circumferential speedimproves the formation of the lubricant film andrises the micropitting load capacity. For highcircumferential speed an adverse affect may beexpected.In the FZG-micropitting test operating con
46、ditionsas circumferential speed and lubricant temperaturemay be suitably adapted for testing lubricants for alarge variety of applications, but gear geometryand gear size are fixed. Within the scope of theactual research work these influences wereexamined.4 Test Program and Test Conditions4.1 Test G
47、earsThe investigations have been carried out onseveral test gear types, different in gear size andgear geometry.Based on a reference test series GF1718 withcenter distance a = 91.5 mm, module mn= 5 mmand number of teeth z1/z2= 17/18, further testseries with the same center distance (GF1717,GF1818, G
48、F1817) were achieved by differentcombinations of pairing pinion and wheel of thereference test series. Additionally test series withprofile modification, test series with helical gearsand test series with smaller module and highernumber of gear teeth were investigated in thestandard test rig with ce
49、nter distance 91.5 mm.Table 1 summarizes main gear data for the testseries with a = 91.5 mm.The influence of gear size was investigated on testseries GF1718, GF1817 and GF1818 at centerdistances a = 68.6 mm, 140 mm and 200 mm.These gear sets have a tooth profile that isgeometrical equivalent to the reference test serieswith a = 91.5 mm (same values of specific sliding,transverse contact ratio and overlap ratio). For thetest gears with a 91.5 mm a higher flankroughness than the reference value of Ra= 0.50m was chosen, assuming that
