1、06FTM07Improvement of Standardized Test Methodsfor Evaluating the Lubricant Influence onMicropitting and Pitting Resistance of CaseCarburized Gearsby: B.-R. Hhn, P. Oster, T. Radev, G. Steinberger,and T. Tobie, Gear Research Institute (FZG)TECHNICAL PAPERAmerican Gear Manufacturers AssociationImprov
2、ement of Standardized Test Methods forEvaluating the Lubricant Influence on Micropittingand Pitting Resistance of Case Carburized GearsProf. Dr.-Ing. B.-R. Hhn, Dr.-Ing. P. Oster, Dr.-Ing. T. Radev, Dipl.-Ing. G.Steinberger, Dr.-Ing. T. Tobie, Gear Research Centre (FZG)The statements and opinions co
3、ntained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractMicropitting and pitting are typical fatigue failures, which occur on the flanks of highly stressed, casecarburized gears. Both failure modes are
4、strongly influenced by the lubricating conditions and lubricantproperties. Standardized test methods with defined test conditions are available to evaluate the lubricantperformance regarding micropitting and pitting.TheFVA-FZGmicropittingtestiswellestablishedasthestandardtestmethodforevaluatingthemi
5、cropittingload capacity of gear lubricants, but requires relatively high costs and is quite time consuming. Therefore anefficient and consistent short test method was developed to classify candidate lubricants in terms of theirmicropitting load capacity analogous to the FVA-FZG micropitting test and
6、 to supplement the existing test.The results of the standardized short test method correlate well with the ones of the FVA-FZG micropittingtest.Regarding pitting the standard FVA-FZG pitting test is a well known and widely used test method that hasproved itself to be suitable to discriminate the pit
7、ting load capacity of gear lubricants. In order to improve thepractice relevance ofthe testgears and testresults,to reduce the influence of undesired phenomena and toincrease the reliability of the results an advanced pitting test procedure was developed. This new testprocedure is based on the exist
8、ing pitting test but uses superfinished test gears with adequate flankmodifications. The more suitable test gears will improve the reproducibility of the test results and ensure aclosercorrelationoftheresultstopracticalgearapplications.Thetestalsooffersthepossibilitytobeextendedby additional testing
9、 at a 2nd load stage and is seen as an improvement of the existing pitting test method.The paper describes the new developed test procedures and shows basic examples of test results.Furthermore it discusses the correlation and classification to the existing test methods.Copyright 2006American Gear M
10、anufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2006ISBN: 1-55589-889-01IMPROVEMENT OF STANDARDIZED TEST METHODS FOR EVALUATINGTHE LUBRICANT INFLUENCE ON MICROPITTING ANDPITTING RESISTANCE OF CASE CARBURIZED GEARSProfessor Dr.-Ing. B.-R. Hhn, Dr.-Ing. P. O
11、ster, Dr.-Ing. T. Radev,Dipl.-Ing. G. Steinberger, and Dr.-Ing. T. Tobie, Gear Research Centre (FZG)NomenclatureA start of line of contact -B lowest point of single toothcontact-C pitch point -D highest point of singletooth contact-E end of line of contact -Caamount of tip relief mmCbamount of lengt
12、hwisecrowningmmCfamount of root relief mmGF micro-pitted flank area %GT standard micropitting test(FVA 54/7)-GFT micropitting load capacity -GFKT micropitting short test(DGMK 575)-KS load stage (micropittingtest)-LC50pitting lifetime, 50% failureprobability-MS load (torque) stage (pittingtest)-NPinu
13、mber of load cycles onpinion-N4number of load cycles-en-durance limit-Ramean value of tooth flankroughnessmmSKS failure load stage (micro-pitting test)-T1torque on pinion NmZLplubricant performance fac-tor (pitting)-ZLp50Tlubricant performance fac-torfromtest-a centre distance mmffmmean profile devi
14、ation mmpCHertzian contact pressurein CN/mm2HPdauer, 50%permissible contact stress(endurance limit for 50%failure probability)N/mm2HPstat, 50%permissible contact stress(static limit for 50% failureprobability)N/mm2HP50T, 50%permissible contact stressbased on test result ofFZG pitting test for 50mio.
15、 load cyclesN/mm2Hmod. contact stress acc.to 12N/mm21 IntroductionPittingandmicropittingaretypicalfatiguefailuresofhigh power transmitting, case carburized gears.Both failure modes are strongly related to lubricantinfluences as type of base oil, oil viscosity, type ofadditives and additive content.
16、Since the chemicalinteractionofbaseoilandadditiveswiththematerialof the tooth flank is normally not predictable, ade-quate test methods to evaluate the lubricant influ-ence on the different gear failure modes are re-quired.Some important aspects for such test methodsare:- test method as simple as po
17、ssible- high reliability of test results- good correlation to gears in practice- time-saving and cost-effectiveIn continuous work covering several research pro-jects standardized test methods were developed todetermine the actual lubricant performance regard-ingthedifferentgearfailuremodes.Thesestan
18、dardFZG test methods are based on the use of the FZGback-to-backgeartestrigandareperformedunderdefined test conditions using specified test gears.FVA-FZG micropitting test and FVA-FZG pittingtest are well known and widely used standard testmethodsforevaluatingthemicropittingrespectivelypittingresist
19、anceoflubricantsingeardrives.Never-thelesstherewasademandfromthegearandlubri-cant industry for further supplement and improve-ment of these existing test methods.22 Micropitting TestMicropitting on gears is a surface fatigue damagethat is strongly influenced by the conditions of thetribologicalsyste
20、m consistingof thetooth flank sur-faceandthelubricant.Micropittingismostfrequent-ly observed on gear flankswith ahigh surfacehard-ness and normally starts in flank areas with highnegativespecificsliding.Thedamagepatternofmi-cropitting is characterized by a large number of mi-croscopic cracks. Small
21、pits originating from thesecracks can result in a removal of surface materialand create local flank wear (Fig. 1).Consequently progressing micropitting can causeincreasing changes of the involute profile in form ofprofile deviations. In this case the dynamical addi-tional forces and the gear noise c
22、an increase andthe occurrence of pitting may be influenced.E5mmDCBAload cycles5mmFigure 1. Changes of the involute profile of agear tooth caused by progressing profiledeviations due to micropitting and correlation tothe tooth flankTheFVA-FZGmicropittingtestacc.to3iswelles-tablished as a standard tes
23、t method for evaluatingthe micropitting load capacity of gear lubricants.Thetestprovidesaquantitativeevaluationof thein-fluence of lubricant (especially additives) on the oc-currence of micropitting, differentiates oils accord-ing to their micropitting load capacity and enablesthechoiceofalubricantw
24、ithasufficientmicropittingresistance.Theresultsofthetestcanbeintroducedinto a micropitting capacity rating method for gears6, 10, 11.The FZG micropitting test consists of a load stagetest with incremental increasing of the contactstress followed by an endurance test. In the loadstagetest theability
25、of thetribological system tore-sist micropitting is determined in form of a failureload stage. The endurance test provides additionalinformationonthe damageprogression afterhighernumbers of load cycles. This test method providesprecise results, but requires relatively high costsand is quite time-con
26、suming. An efficient and con-sistent short test method to evaluate the lubricantinfluence on micropitting has been missing so far.Therefore the available FVA-FZG micropitting testwas supplemented by a standardized short testmethod, that is able to classify candidate lubricantsanalogous to the existi
27、ng test method.FZG Micropitting Test GT acc. to FVA 54/7Fig. 2 shows schematically the test procedure ofFZG micropitting test acc. to FVA-informationsheet 54/7 3 (for test conditions see also Fig. 4).running timeloadstageloadstageStufentest Dauertestrunning-in,1h3running-in,1hrunning-in,1h5861071089
28、10 10 10 10running time per load stage16 hrunning time per load stage16 hrunning time per sequence80 hrunning time per sequence80 hFigure 2. Test procedure of FZG micropitting testacc. to FVA 54/7Afterrunningin(1hatloadstage3)theloadisstep-wiseincreasedfromloadstage5throughloadstage10 (pC= 795 N/mm2
29、- 1547 N/mm2). Running timeis16hperloadstage.Aftereach loadstage thetestgears are dismounted and the development of thedamage is documented by measuring the profiledeviation and evaluating the micro-pitted area andthemateriallosses. Ifthe meanprofile deviationex-ceedsthelimitingvalueof 7.5mm (corres
30、pondingtoa change of gear accuracy from DIN # 5toDIN6)the failure load stage is reached.In case of failure load stage 8 the load stage testisfollowedbyanendurancetestwiththe samegearpairfor80hinloadstage8andmaximum5times80h in load stage 10. After each test sequence thegears are inspected and the pr
31、ofile deviation ismeasured again. The endurance test is terminatedif the profile error exceeds 20 mm (corresponding to3a change of gear accuracy to DIN 9) or large pittingoccurs. A detailed description of the test procedurecan be found in 3.The test is performed on the FZG back-to-backtestrigwithcen
32、tredistance91.5mmacc.to2(seeFig.3), requires an oil spray device (total oil quanti-ty: 25 l, oil flow rate 2 l/min) and uses standardizedtest gears type C-GF with a specified surfaceroughness Ra=0.5 0.1 mm.torque indicatordrive gearslever arm withweigth piecesload clutchtemperature sensorpiniongear
33、wheelFigure 3. FZG standard gear test rig (a = 91.5mm)BasedonthetestresultstheFVA-FZGmicropittingtest classifies lubricants acc. to Table 1 in load ca-pacity classes. Fig. 4 illustrates typical test exam-ples of lubricants with a different micropitting loadcapacity.Table 1. Classification of lubrica
34、nts acc. to FZGmicropitting test 3GF-CLASS(= micro-pittingclass)Failure loadstage (= SKS) inthe load stagetestDamage behaviourin the endurancetestGFT-low(= low micropittingload capacity)SKS # 7, largemicro-pittedarea (= GF), GFsometimes morethan 50%1 x 80 h runningtime at load stage10, micropitting,
35、 ffmclearly more than20 mmGFT-medium(= medium micropit-ting load capacity)SKS 8-9, me-dium size mi-cro-pitted area,GF about 30%1-2 x 80 h runningtime at load stage10, micropitting and/or pitting, ffm=10-20 mmGFT-high(= high micropittingload capacity)SKS 10, littleor no micropit-ting, GF lessthan 20%
36、1-5 x 80 h runningtime at load stage10, pitting, possiblemicropitting, ffm7.5 mm)GFKT-medium SKSGFKT= load stage (KS) 9(ffmafter KS 9 7.5 mm)GFKT-high SKSGFKT load stage (KS) 9(ffmafter KS 9 N4) is kept constant 9.11.11.21.31.41.51.61.71.81.92load cycles on pinion110018002000220025001200130014001500
37、16001700SN-curve (straight lubricant)acc. to DIN 3990new“ SN curve fortested lubricantMS 9HP50T,50%HPdauer,50%N4HertzianstresspcN/mm21051061071081095107ZLp50T=HP50T,50%HPdauer,50%Figure 12. Modified SN-curve based on the testresult (ZLp50T) of standard FZG pitting test PTTest experience shows that t
38、he test method PT isable to discriminate the pitting load capacity of dif-ferent gear lubricants but that the test results areoften influenced by some additional, undesiredphenomena.On the one side optical inspection proves that -depending on the lubricant performance and thesurface roughness - many
39、 test runs are accompa-nied by more or less strong micropitting. Profiledeviations due to micropitting may lead to achangeof the nominal local pressure distribution on thegear flanks and increased dynamical additionalforces. Consequently the test conditions are notconstant any more. Furthermore micr
40、o-pitting alsoaffectsthelocationwherethepittingdamagestarts.Fig. 13 shows two examples of pitting damages ontest gears type C-PT. The flank shown in Fig. 13ahas only a small amount of micropitting, the pittingdamage has started near the beginning of toothcontact.The gearflank shownin Fig.13b ischara
41、c-terized by noticeable micropitting. In this case thelocation where the pitting damage has started,clearly has shifted to the upper borderof themicro-pitted area.a) low micropitting b) severe micropittingFigure 13. Examples of gear flanks used instandard FZG pitting test with test gears typeC-PTIns
42、omecasesmicropittingduringpittingtestseemstodelaythepittingdamagebutinothercasesstrongmicropitting can also reduce the pitting lifetime.Consequentlyundesiredmicro-pittingmay resultinincreased scattering of the pitting test results andsome uncertainty regarding the reached pitting life-time.On the ot
43、her side often a scrape mark on testgearstype C-PT is observed. Scrape marks are causedby a premature contact of the unmodified gearflanks of the test gearstype C-PT.In mostpracticalapplications scrape marks are avoided by suitablemodifications on the gear flanks. There-fore moresuitable oriented te
44、st gears with adequate profilemodifications and higher micropitting load capacitywere desired.8FZG Practice Relevant Pitting Test PTX acc. toFVA 371/2Withinthescopeofaresearchproject 8a newtestmethod - called “FVA-FZG Practice Relevant Pit-ting Test (PTX)” - was developed. This new testmethod is bas
45、ed on the existing test method FZGpitting test acc. to 4. In theoretical and experimen-tal work the influences of gear modifications, sur-face roughness and circumferential speed were in-vestigated in order to define a new test geargeometry.The PTX test is an advanced short-test procedurefor the eva
46、luation of the lubricant influence on thepitting capacity of case carburized gears. Depend-ing on the application profile the test method is de-finedasasinglestagetest(forlubricantdevelopers)and as an extended application test (for lubricantusers/gear designer).Test procedureS Single stage test - Pi
47、tting Test PTX C/10/90:The single stage test is basically performed attorquestage10and90Coilsumptemperature.S Application test - Pitting Test PTX C/SNC/90:The application test is an extension of thesinglestage test with additional testing of the lubricantat a 2ndload stage. The 2ndload stage is dete
48、r-mined based on the results of the single stagetest. For lubricants with low pitting capacity theadditionaltestingisperformedattorquestage9,lubricants with higher pitting capacity are testedat torque stage 11.A detailed description of the test procedure is to befoundin5.ThetesttemperatureofPTXtestc
49、anbeadjusted due to the temperature range of the actualpractical application. It is recommended to performat least 3 test runs at torque stage 10 andin caseofan extended application test at least 2 additionaltest runs at the 2ndload stage.The result of the single stage test is the pitting life-timeLC50,10obtainedattorquestage10.Compara-ble to the standard pitting test acc. to FVA 2/IV alsothe factor ZLp50Tcan be calculated and introducedinto the calculation metho
