1、11FTM15AGMA Technical PaperMicropitting - A SeriousDamage? Testing,Standards andPractical ExperienceBy B. Pinnekamp, T. Weiss, andG. Steinberger, RENK AGMicropitting - A Serious Damage? Testing, Standards andPractical ExperienceDr. Burkhard Pinnekamp, Dr. Toni Weiss, and Dr. Gregor Steinberger, RENK
2、 AGThe 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.AbstractMicropittingisasurfacefatiguephenomenononhighlyloadedcasehardenedgearflanks. Maincontributorsare the tribological
3、 system tooth flank / lubricant and the operating conditions. To determine the lube oilperformance with respect to micropitting, different test methods have been established. Proposals areevaluated for adopting suitable calculation methods for micropitting resistance to the ISO 6336 gear ratingstand
4、ards. Butismicropitting necessarilya seriousdamage inanycase?Practical experienceshowsthatacertainlevelofmicropittingcanactuallybeacceptableandleadtoevenmorefavorableloaddistributionandstable flank condition with the gear performing without problems for the designed service live.Thepaperdescribestes
5、ting,calculationapproachesandpracticalcaseswithrespecttomicropittingonwindturbine and high speed gears.Copyright 2011American Gear Manufacturers Association1001 N. Fairfax Street, 5thFloorAlexandria, Virginia 22314October 2011ISBN: 978-1-61481-014-83 11FTM15Micropitting - A Serious Damage? Testing,
6、Standards and Practical ExperienceDr. Burkhard Pinnekamp, Dr. Toni Weiss, and Dr. Gregor Steinberger, RENK AGIntroductionAdvanced design methods, production technologies and calculation standards have been developed overdecadesto ensurereliable operationof industrialpower transmissions. Forthe mostp
7、art, inhigh powergeartransmissions case carburized and ground gears are used. International standards have been developed toprovide calculation methods for safety factors againstS Macro-pitting (ISO 6336-2)S Tooth breakage (ISO 6336-3)S Scuffing (ISO TR 13989).It is obvious that tooth breakage is a
8、serious damage and in most cases causes loss of drive of the gearsystem. For thruhardenedgears, acertainlevelof macro-pittingis acceptable. For high performancecasecarburizedgears,however,evenminormacro-pittingisnotacceptable,althoughthegearsmightstillbeoper-able. With carburized gears, macro-pittin
9、g reduces the actual safety against tooth breakage and in manycasescausesdeteriorationinnoiseandvibrationbehavior. Scuffingcaninaninstancesignificantlydamageamajor portion of the gear flanks and therefore safely needs to be avoided.Micropitting is a surface phenomenon which mainly occurs with case c
10、arburized gears. Macro-pitting wouldbe the prevailing form of damage for thru hardened gears.Micropittingcausesmultiplemicroscopicparticlestobreakoutoftheloadedflanksurfaceofacarburizedgearand to create gray appearing areas. ISO TR 15144-1 provides a method to determine the micropitting loadcapacity
11、 of cylindrical gears.In the primary stage of micropitting, the surface wear does not create a relevant change in tooth geometry.Likewise, the surface roughness does not necessarily increase and may decrease by smoothing the grayarea. Powertransmissionandnoiseandvibrationbehaviorarenotaffectednegati
12、velybutmayevenimprove.Therefore, not every level and appearance of micropitting can be considered a serious damage. However, ifthe abrasive wear due to micropitting does not come to a stop at an acceptable stage and after a reasonabletime of operation, micropitting becomes serious. Significant chang
13、es in lead and profile cause transmissionerrors, noise and vibrations as well as load concentration in other flank areas. Moreover, significantmicropitting can increase the risk of severe macro-pitting damage and subsequent tooth breakage.This paper intends to give an overview on micropitting and ra
14、ise the sensitivity to distinguish between suchharmless initial (running-in) micropitting and a serious surface damage.Definition of micropittingMicropitting is a gear surface phenomenon which is strongly influenced by the conditions of the tribologicalsystem consisting of the tooth flank surface, t
15、he lubricant and the operating conditions. Micropitting is mostcommonly observed on gear flanks with a high surface hardness.Thephenomenonofmicropittingischaracterizedbyalargenumberofmicroscopicsurfacecracks. Smallpitsoriginating from these cracks result in a removal of surface material and create l
16、ocal flank wear, Figure 1. Incontrast to the development of micropitting, macro-pitting starts from sub-surface crackspropagating tothesurface and leading to major material parts to break off.4 11FTM15Figure 1. Left: Change of involute caused by progressive profile deviations due to micropitting;Rig
17、ht: View on tooth flankTherewereseveralattemptstodefinemicropittingwhenWorkingGroup15ofISOTC60,Sub-committeeSC2, Gear Capacity Calculation, started its work and there were different opinions with respect to the influencefactors and the importance for gear performance. Finally, the following wording
18、was agreed and adopted toISO TR 15144-1:“Micropitting is a phenomenon that occurs in Hertzian type of rolling and sliding contact that operates inelastohydrodynamic or boundary lubrication regimes.Micropitting is influenced by operating conditions such as load, speed, sliding, temperature, surface t
19、opo-graphy,specificfilmthicknessandchemicalcompositionofthelubricant(Figure 2). Micropittingismorecom-monly observed on materials with a high surface hardness.Figure 2. Influence factors on the micropitting load capacity of gears5 11FTM15Micropittingis thegenerationof numeroussurfacecracks. Thecrack
20、s growat ashallowangletothesurfaceforming micropits. The micropits are small relative to the size of the contact zone, typically of the order10 - 20 mmdeepandabout25 - 100mmwide. Themicropitscancoalescetoproduceacontinuousfracturedsurface which appears as a dull, matte surface during unmagnified vis
21、ual inspection. Micropitting is the pre-ferred name for this phenomenon, but it hasalso beenreferred toas greystaining, greyflecking, frostingandpeeling.Micropitting may arrest. However, if micropitting continues to progress, it may result in reduced gear toothaccuracy, increased dynamic loads and n
22、oise. If it does not arrest and continues to propagateit candevelopinto macro-pitting and other modes of gear failure.”Micropitting test methods 2The FVA-FZG micropitting test as defined in 3is wellestablished asa standardtest methodto evaluatethemicropittingloadcapacityofgearlubricants. Thetestprov
23、idesaquantitativeevaluationoftheinfluenceofthelubricant (especially additives) on the occurrence of micropitting, characterizes oils according to their micro-pittingloadcapacityandenablesthechoiceofalubricantwithsufficientmicropittingresistance. Theresultsofthe test are used for a micropitting capac
24、ity rating method for gears.The FZG-FVAmicropitting testconsists ofa loadstage testwith step-by-stepincrease ofthe contactstressfollowedbyanendurancetest. Intheloadstagetest,theabilityofthetribologicalsystemtoresistmicropittingisdeterminedbythefailureloadstage. Thistestmethodprovidespreciseresults,b
25、utcreateshighcostandisquitetimeconsuming. Anefficientandreliablestandardizedscreeningtestmethodtoevaluatethepropertiesof a lubricant with respect to micropitting would be beneficial. Therefore, the existing FVA-FZG micropittingtestwassupplementedbyastandardizedscreeningtestmethodtoclassifycandidate
26、lubricantscorrespond-ing to the standard test method.FZG micropitting test GT according to FVA-Information Sheet 54/7Figure 3schematically showsthe testprocedure ofthe FVA-FZGmicropitting testaccording toFVA-inform-ation sheet 54/7 3.Figure 3. FZG-FVA Micropitting test procedureAfter running-in (1 h
27、our at load stage 3), the load is stepwise increased from load stage 5 thru load stage 10(HertzianstresspC= 795 N/mm21547 N/mm2). Running time at every load stage is 16 hours. Aftereach6 11FTM15load stage, the test gears are inspected and the development of micropitting -if any- is documented bymeas
28、uring the profile deviation and evaluating the micropitting area and the weight loss. If the mean profiledeviation exceeds the limit of 7.5 mm (corresponding to a change of gear accuracy from grade 5 to grade 6according to ISO 1328-1), the failure load stage is considered to have been reached.If the
29、 failure load stage is higher or equal to 8, the load stage test is followed by an endurance test with thesame gear pair for 80 hours at load stage 8 and maximum 5 times 80 hours at load stage 10. After each testsequence,thegearsareinspectedandtheprofiledeviationismeasured. Theendurancetestistermina
30、ted,iftheprofiledeviationexceeds20mm(correspondingtoadeteriorationofgearaccuracytograde9accordingtoISO 1328-1) or macro-pitting occurs. A more detailed description of the test procedure can be found in 3.The test is performed on a standardized FZG back-to-back test rig, Figure 4, with a center dista
31、nce of91.5 mm. Test gears type C-GF with a specified surface roughness, Ra=0.5 0.1 mm, are used.Based on the test results, the lubricants are given a load capacity class according to Table 1, Figure 5illustrates typical test examples of lubricants with different micropitting load capacities.Figure 4
32、. FZG back-to-back test rigTable 1. Classification of lubricantsGF-class (micropitting class) Failure load stage, FLS, at the loadstage testGFT-low (low micropitting load capacity) 5, 6 and 7GFT-medium (medium micropitting load capacity) 8 and 9GFT-high (high micropitting load capacity) 10 or more7
33、11FTM15Figure 5. Typical results in FZG-FVA micropitting testFZG micropitting short test according to DGMK 575 2Within the scope of a research project 4, a standardized screening test method called DGMK-FZGmicropittingshorttest(GFKT)wasdevelopedtocharacterizecandidatelubricantswithrespecttomicropitt
34、ingload capacity. This new short test method can categorize candidate lubricants in classes of micropittingloadcapacitywithagoodcorrelationtothestandardFZGmicropitting test(see Table 1). Tothis end,the testcon-ditions for the short test were chosen as close as possible to the standard FZG micropitti
35、ng test.The GFKT is a load stage test, but with a reduced number of load stages as compared to the standard FZGmicropittingtest. Afterrunning-infor1hour. atloadstage3,the testis continuedfor 16hours atload stage7followed by third run for 16 hours at load stage 9 (see Figure 6).Figure 6. Test procedu
36、re of FZG micropitting short test GFKT according to DGMK 575 48 11FTM15After each load stage, the test gears are dismounted to inspect the tooth flank condition and to measure theprofile deviation of the gear flanks on at least three teeth evenly distributed over the circumference of thepinion.The m
37、icropitting short test GFKT is performed with the same test gears (type C-GF) as the standard FZGmicropitting test on an FZG back-to-back test rig with a center distance of 91.5 mm, Figure 4. Unlike thestandard FZG micropitting test according to FVA 54/7, the micropitting short test GFKT uses dip lu
38、bricationwithavolumeofapproximately1.51inthetestgearbox. Adetaileddescriptionofthetestprocedureisgivenin4.To evaluate the test result, the same failure criterion as in the standard FZGmicropitting testapplies, i.e.,thefailure load stage is reached if the mean profile deviationffmexceeds the limit of
39、 7.5 mm. To increase the reli-ability of the test result, it is recommended to repeat the test procedure on the reverse flank side of the testgears. According to Table 2, the candidate lubricants are finally classified into the micropitting load capacityclasses GFKT-low, GFKT-medium and GFKT-high.Fi
40、gure 7 shows typical test results of the FZG micropitting short test GFKT for three differentlubricants. Themicropitting progression is monitored by means of the measured average profile deviation, ffm, over the loadstages for candidate lubricants of the micropitting load capacity classes GFKT-low,
41、GFKT-medium andGFKT-high.Table 2. Classification of lubricants into GF-classes according to FZG micropitting short testGF-Class (micropitting class) according toGFKTFailure load stage FLSGFKTat theGFKTGFT-low (low micropitting load capacity) FLSGFKT=7ffmafter LS 7 7.5 mmGFT-medium (medium micropitti
42、ng load capacity) FLSGFKT=9ffmafter LS 9 7.5 mmGFT-high (high micropitting load capacity) FLSGFKT9ffmafter LS 9 7.5 mmFigure 7. Typical test results for lubricants with a different micropitting load capacity in the FZGmicropitting short test GFKT according to DGMK 5759 11FTM15Figure 8depictsthetooth
43、flankconditionattheendofanFZGmicropittingshorttestGFKT(afterLS9)fortwolubricantswithdifferentmicropittingloadcapacityclasses. Obviously,thetoothflanksafterthetestsarechar-acterized by a different level of micropitting area. This corresponds to different values of the average profiledeviation and ref
44、erring micropitting load capacity classes with respect to the different test lubricants.Withinthescopeoftheresearch,severallubricants,wheretheclassificationbythestandardFZGmicropittingtest is known, were tested using the micropitting short test GFKT. This investigation covers three referencelubrican
45、ts (R1 . R3) and 9 commercially available gear lubricants (T1 . T9). The tested lubricants aredifferentintypeofbaseoil,type ofadditives and/orviscosity. The correlationbetween therating accordingtothestandardFZGmicropittingtest andtheFZGmicropittingshorttestGFKTisgood. Figure 9 illustratestheaverage
46、 profile deviation for the tested lubricant samples after load stage 9 of the FZGmicropitting shorttestGFKT in combination with the failure load stage of these lubricants in the standard FZG micropitting test.Regardingthegeneraltendency,themeanprofiledeviationattheendofFZGmicropittingshorttestGFKT,a
47、sexpected,decreasedwithincreasingmicropittingloadcapacityofthelubricantaccordingtothestandardFZGmicropitting test.GFKT low GFKT highFigure 8. Examples for the tooth flank condition at the end of an FZG micropitting short testGFKT (after LS 9)Figure 9. Correlation of test results, FZG micropitting sh
48、ort test GFKT (ffm, after LS 9) andstandard FZG micropitting test (FLS) according to 54/710 11FTM15Calculation approachThecalculationmethodsandmodelsandthevariousinfluencefactorshavenotbeenfullyvalidated,andthereisnotyetsufficientexperienceinthisfield. Therefore,thecalculationprocedurehasbeenpublish
49、edasanISOTechnical Report and not yet as a part of ISO 6336.This Technical Report, ISOTR 15144-15, providesthe principlesfor thecalculation ofthe micropittingloadcapacity of cylindrical involute spur and helical gears with external teeth.Thebasisforthecalculationofthemicropittingriskofagearsetisthemodeloftheminimumoperatingspecificfilmthicknessinthecontactzone. Thereare alot moreinfluence parameters,such assurface topology,con-tactstresslevel,flankmodificationandlubricantchemistry. Theseparametersareknown
