1、99FTM4Gear Oil Micropitting Evaluationby: A.B. Cardis and M.N. Webster, Mobil Technology CompanyAmerican Gear Manufacturers AssociationTECHNICAL PAPERGear Oil Micropitting EvaluationA.B. Cardis and M.N. Webster, Mobil Technology CompanyThe statements and opinions contained herein are those of the au
2、thor and should not be construed as an official action oropinion of the American Gear Manufacturers Association.AbstractAs micropitting of gears has become increasingly recognized as a cause of gearbox failures, gear manufacturers havebegun to require documentation of gear oil micropitting performan
3、ce. The German FVA Procedure No. 54, currently theonly industry recognized test, has limited availability and takes several months to complete. A laboratory test has beendeveloped using a roller disk machine to simulate micropitting of gears. This test was instrumental in the development ofnew micro
4、pitting resistant gear oils. A helical gear test was developed using specially designed gears running on a foursquare test rig to further document micropitting performance prior to field testing the new oils.Copyright 1999American Gear Manufacturers Association1500 King Street, Suite 201Alexandria,
5、Virginia, 22314October, 1999ISBN: 1-55589-742-8GEAROILMICROPITTINGEVALUATIONA. B. Cardisand M. N. Webster,MobilTechnologyCompany,Paulsboro,New JerseyIntroductionUnlike macropitting, micropitting is difficult toDuring the last decade industrial gear see, particularly under the conditions of fieldmanu
6、facturers, particularly in Europe, began to inspections. In the laboratory, with a cleanrequire documentation of micropitting gear mounted under a microscope with goodperformance before approving a gear oil for directional lighting, micropitting takes on theuse in their equipment. The development of
7、 appearance of etched glass. In the field, themicropitting resistant lubricants has been tooth surface must be illuminated from variouslimited both by a lack of understanding of the angles to see if the characteristic matte areasmechanism by which certain lubricant can be revealed.chemistry promotes
8、 micropitting and by a lackof readily available testing for evaluation of the Micropittingmay occur almost anywhere on themicropitting resistance of lubricants. This gear tooth. However, research shows thatpaper reports results of two types of testing: micropitting is most likely to occur at local(1
9、) the use of a roller disk machine to conduct areas of high load or areas associated withsmall scale laboratory studies of the effects of higher sliding during the gear tooth contactindividual additives and combinations of cycle. For this reason micropitting is oftenadditives on micropitting and (2)
10、 a helical gear found in the addendum and dedendum of thetest used to study micropitting performance of tooth profile and at the edge of the gear tooth ifformulated gear oils. the gears are misaligned. Also, it has beenobserved that micropitting will often track local- Background high spots in the s
11、urface topography of thev- gears which will be associated with local highMicropitting is an unexpectedly high rolling stresses.contact fatigue wear phenomenon that isobserved in combined rolling and sliding The progressionofmicropitting may eventuallycontacts operating under Elastohydrodynamic resul
12、t in macropitting. If pits form they oftenLubrication (EHL) or mixed EHUBoundary display a characteristic arrowhead or fanLubrication conditions. Besides operating shape, with the pointed end at the edge of theconditions such as temperature, load, speed, micropitted area. There are also reportedslid
13、ing and specific film thickness, the chemical cases where the micropitting progresses up tocomposition of a lubricant has been found to a point and stops, sometimes described as astrongly influence this wear phenomenon, form of running-in or stress relief. Although itTypically the failure may start
14、during the first may appear innocuous, such loss of metal5 610 to 10 stress cycles with the generation of from the gear surface causes loss of gearnumerous surface cracks. The cracks accuracy, increased vibration and noise andpropagate at a shallow angle to the surface other related problems. The me
15、tal particlesforming micropits with characteristic released into the oil may be too small to bedimensions of approximately lOl_m. The picked up by commonly used filters, but largemicropits coalesce to produce a continuous enough to damage tooth and bearingfractured surface with a characteristic dull
16、 surfaceslmatte appearance variously called graystaining, frosting or micropitting when applied Micropitting Teststo gears. Micropittingis the preferred term.The terms peeling or general superficial The factors that influence micropittinghavespallinghave been usedto describethisfailure been reported
17、2 along with suggestions formode when it occurs on rolling element preventingthe problemas tabulatedbelow:bearings. Micropitting is generally, but not-1- necessarilyexclusively, a problem associated Influencing Factor Suggested RemedyGear surface roughness Reduce to 0.3p.m1. with heavily loaded case
18、 hardened gears. Reduceaustenitel vel RetainedausteniteLubricantviscosity Use highestpracticalvisc 2.32 resulted in a moderate reduction in -Coefficient of friction Reducethe coeff of friction micropitting damage versus virtualSpeed Runat highspeed (toproducethickerEHLfilm) eliminationof micropittin
19、g with polishedOiltemperature Reduceoiltemperature surfacesgivinga specificfilm thickness ofLubricantadditivechemistry Use properlyselected 5.62.additives = Micropittingis drastically reduced at lownon-zeroslide to rollratios(e.g. a slide toThe selectionof properlyadditizedlubricantsis rollratioof 0
20、.0095)the most difficult parameter to determine.Ueno, et alz found in their testing that anti- The variable load method as described inscuffing additives (often referred to as EP reference4 has been used to investigatetheadditives) in a GL-5 type lubricant caused effect of lubricantcompositionon mic
21、ropitting.micropittingto increase. Certain specification Figure 1 shows resultsobtainedfrom the teststests, such as the Timken OK Load Test, Four conducted using a series of ISO VG 100BallEP Test andthe FZG ScuffingTest require industrial gear lubricants. The two sulfur-the use of suchanti-scuffinga
22、dditives, phosphorusgear oilscontainthe anti-wearandanti-scuffing additives required to provideThere is no globally accepted test for Timken OK load results greater than 60 Ibs.determiningthe effect of the lubricanton gear The syntheticPAO based circulatingoil wasmicropitting.However,the testreporte
23、dinthe formulated to provide FZG fail stage 11FVA (ForschungsvereinigungAntriebstechnik, scuffingprotectionbutdoes not providea highGerman Research Association for Drive level of Timken OK load protection. DespiteTechnology)InformationSheet No. 54/I-IV has the scatter associated with the mineral gea
24、rgained widespread acceptance among gear oils,the resultsshow that boththe mineralandbuildersand customers. In this test the failure syntheticbased gear oils yieldsimilar results.is determined by the degree to which The resultsfor the synthetic PAO circulatingoilmicropitting causes a deviation from
25、the suggestthat the use of less aggressiveanti-original gear involute profile. If involutewear additive systems provide directionalmeasurement equipment is not available the improvement in micropitting performance. Themicropitting can be tracked using a results from these gear oil tests compare well
26、 _“combination of micropitting area and weight with results obtained with the same lubricantsloss which is compared with tables and using the FVA micropitting gear test andpictures characteristic of reference lubricants suggest a good correlation between the rollerwith different levelsof micropittin
27、gprotection, disk machineand FVA test methods.Experimental Roller Disk Program To furtherinvestigatethe influenceof additiveson micropittinga test wasconductedon the un-The FVA micropittinggear procedurecan be additizedmineral base oil used in the mineralused to screen the performance of various gea
28、r oil test. The resultsare compared withlubricant options. However, disk machines the fully formulated gear oil in Figure 2. Theoffer a more flexible platform on which to onset of micropittingis delayed and the finalconduct tests to evaluate the influence of result correspondsto the lowest of the th
29、reevariousoperationaland lubricationparameters fully formulated gear oil results. This resulton micropitting. Webster and Norbart4 have confirms the significantimpact that lubricantdescribedthe developmentof a roller disktest additivescan haveon micropitting.procedure that successfullyreproducedmany
30、of the aspectsof micropittingobservedin gear Obviouslygear oils must contain additives totesting. Significant findings from this meet various performance and specificationpreliminary workwere: requirements, not the least of which is toprovide protection against the severe form of, Under rolling/slid
31、ing conditions the slower adhesive wear known as scuffing that canmoving surface is more prone to occur in gear tooth contacts. Thus, themicropitting challenge of developing next generation gear, Increasing the specific film thickness (i.e. lubricants is to arrive at a base stock andratio of lubrica
32、nt film thickness to additive composition that balances the various dcombined surface roughness)from 0.92 to qperformance needs against the requirement to effort made use of the FVA test as the primaryobtain good micropitting protection, tool for the evaluation of micropittingperformance. However, a
33、dditional testing wasIn order to gain an understanding of the impact also conducted on larger gears moreof different component technology, micropitting representative of commercial industrial gears.testswere conducted on different combinations A test program was developed using anof additives and ba
34、se stocks. In a first series available four square gear test rig. In a furtherof tests individual components and development an automated machine visioncombinations typically found in conventional system was employed to provide accurate andI sulfur-phosphorus gear oils were tested in ISO repeatable
35、measurement of micropitting areaL VG 150 mineral base stock. The results on test gears. Information about the gears and, shown in Figure 3 indicate that the sulfur based test conditions may be found in Table 1 andantiwear additive 1 does not promote Figure 5. The test oils are listed in Table 2.micr
36、opitting. Comparing against the mineral“ base stock results from Figure 2 we find that it The machine vision system is based on lightI, may even improve upon base stock only scattering by rough surfaces as shown in! performance. Both the nitrogen and Figure 6. Unworn areas appear dark to thephosphor
37、us antiwear additives showed a camera because most of the incident light issignificant tendency to produce micropitting, reflected away from the camera due to the lowThe addition of the sulfur antiwear to either of angle of incidence of the inspection lights. Anythese two resulted in a significant i
38、mprovement micropitted areas on a gear tooth scatter lightin performance. From these results it was in all directions due to the irregular roughnessconcluded that sulfur additive 1 in some way of the surface. Some of the scattered light isacts to reduce micropitting damage. However, captured by the
39、camera, causing the area toit must be kept in mind that results from appear white. In the absence of other surfacemixtures are not necessarily the sum of the features that may scatter light this approachresults gained on individual components so the gives an accurate assessment of the surface- benef
40、it from the use of sulfur additive 1 may affected by micropitting. It thus provides anv not be reproduced when combined with automated inspection system for following theadditional additive technology, progression of micropitting while avoiding theneed for removing the gears from their shafts.In a s
41、econd series of tests the performance ofa range of alternative sulfur and phosphorus After initial runs to determine optimumbased antiwear additives were evaluated and conditions, the first test was run using thethe results are shown in Figure 4. In this case second side of the test development gear
42、 setwe see that there is a variation in the response with a mineral oil, designated Oil A, that hadwithin a general category of additive. For been rated fail load stage 9, medium, in theexample sulfur additive 2 resulted in a greater FVA test. Observations were made at 100-degree of micropitting tha
43、n found for sulfur hour increments. Images were recorded andadditive 1. Similar variations are found for the the amount of micropitting wear was calculatedphosphorus and mixed sulfur/phosphorus at 100 and 300 hours. The micropitting wasadditives tested. The results showthat there is concentrated in
44、the dedendum and at thea large variation in the micropitting edges of the teeth. At 327 hours, the rigperformance of the anti-scuffing additives that automatically shut down due to vibration in thecan be used to formulate gear oils. This slave box. At this time two large pits and avariation is no do
45、ubt a function of the individual crack were found in pinion tooth #1 and aadditive chemistry and it would be dangerous smaller pit in gear tooth #3.to assume, based on our limited testing, thatany one class of additive has an advantage The gear and pinion were analyzed to identifyover another, the t
46、ype of damage and the cause of pitting. Itwas determined that the initiation of the failureHelical Gear Test Rig was due to rollingcontact fatigue, not adhesivewear. There was also evidenceof movementFollowingthe roller disk machineexperiments and/oralignmentproblemswith the gears. The-,- a program
47、was embarked upon to develop a photograph in Figure 7 shows a fan-shaped!- micropitting resistant gear oil. The formulation area starting in the micropitted area andIterminating with macropits at the pitch line. available due to a problem with the dataThis result appears to support field acquisition
48、 system.observations that have linked the onset ofmacropitting to areas that have previously been As expected, there was a very low level ofdamaged by micropitting, micropitting wear. Since we have aquantitative measure of micropitting throughoutThe second test was conducting usingOil B for the test
49、 it is possible to estimate wear rate as awhich a good rating in the FVA micropitting function of test time. In comparing Oil B withgear test had been obtained. This oil ran for Oil C we found that the progression ofthe entire 1000 hours and had a much lower micropitting wear was quite different. In thepinion micropitting rate compared to the case of Oil B, the rate was consistently low atprevious test. Data for these two tests are 0.2%/100 hrs up to 700 hrs at which point thegraphically compared in Figure 8. Note the rate increased sharply. Between 700 and 900repeated pattern in th
