1、11FTM16AGMA Technical PaperGear Lubrication -Stopping Micropittingby Using the RightLubricantBy M. Hochmann and H. Siebert,Kluber Lubrication Mnchen KGGear Lubrication - Stopping Micropitting by Using the RightLubricantMichael Hochmann and Hermann Siebert, Kluber Lubrication Mnchen KGThe statements
2、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.AbstractDifferent high-performance gear oils were examined on an FZG back-to-back gear test rig to determine ifchangeovertothesehigh-perfor
3、mancegearoilscouldstopmicropittingformationwhichoccurred withotherindustrial gear oils. The test results showed that high-performance gear oils using advanced additivetechnologies can react at the surface of the tooth flanks after an oil change and stop further micropittingformation. The micropittin
4、g area stagnates.Copyright 2011American Gear Manufacturers Association1001 N. Fairfax Street, 5thFloorAlexandria, Virginia 22314October 2011ISBN: 978-1-61481-015-53 11FTM16Gear Lubrication - Stopping Micropitting by Using the Right LubricantMichael Hochmann and Hermann Siebert, Kluber Lubrication Mn
5、chen KGIntroductionMicropittingformationisoftenreportedtooccurinfieldapplicationseventhoughindustrialgearoilswithahighmicropittingload-carryingcapacity areused. Suchoils offer agoodmicropittingprotectiondeterminedintheload stage test of the micropittingtest accordingto FVA 54/7 5, but showa lowendur
6、ance micropittingper-formance. Therefore, thepossibility of stoppingof micropittingformationwhichoccurredwiththesegearoilswas to be investigated by changing over to high-performance gear oils. Due to advanced additivetechnolo-gies,thesehigh-performancegearoilsshowahighmicropittingload-carryingcapaci
7、tyinthemicropittingloadstage test as well as a high endurance micropitting performance.Micropitting gear failureFigure 1showstypicallimits oftheload-carryingcapacity forcasehardenedgears accordingtoNiemann7.Micropittingis atypeof fatiguefailureoccurringonhardenedtoothflanks of highly loaded gears. T
8、his failureconsists of very smallcracks andpores onthesurfaceof toothflanks. Micropittinglooks greyishandcausesmateriallossandachangeintheprofileformofthetoothflanks,whichcanleadtopittingandbreakdownofthegears. A typical micropitting gear failure of an industrial gear box is shown in Figure 2. In th
9、is case,misalignment was the reason for micropitting formation.The formation of micropitting depends on different influences. Besides material, surface roughness, andgeometry of the tooth flanks, it is the lubricant and the operating conditions which have a major influence onmicropitting formation.
10、See also Table 1.Figure 1 . Typical limits of load-carrying capacity for case hardened gears4 11FTM16Figure 2. Micropitting gear failure of an industrial gear boxTable 1. Influences on the micropitting load-carrying capacitylubricant chemistry of the base oilviscosity of the oiltype and amount of ad
11、ditivestooth flank surface roughnesssurface texturesurface hardnessoperating conditions normal and frictional loadcircumferential speedtemperatureInmoderngearboxes,thegearsareoftenhighlyloadedandrununderconditionsofmixedlubrication. Inthiscase, thetoothflanks of thematinggears arenot fully separated
12、by thelubricant film and theadditives of thelubricant have to protect the tooth flanks against micropitting formation.Test procedureMicropitting load-carrying capacityThemicropittingload-carryingcapacityofgearscanbecalculatedaccordingtoISOTR15144-14,wheretheinfluence of lubricant, operating conditio
13、ns, and surface roughness is considered with the specific lubricantfilm thickness. For this purpose, the specific lubricant film thickness of a practical gear is compared with aminimumrequiredspecificlubricantfilm thickness. Thelatter isthespecificfilm thicknesswherenomicropit-tingriskisgivenforalub
14、ricantandcanbedeterminedbyperformingamicropittingtestaccordingtoFVA54/75.Standard micropitting testThemicropittingtestaccordingtoFVA54/75consistsofaloadstagetestandanendurancetest. TestgearstypeC-GFrunatacircumferentialspeedof8.3m/sandalubricanttemperatureof90Cor60C. Theloadandthe test periods are v
15、aried.Intheloadstagetest, theload is increased stepwisefrom loadstage LS 5 toload stageLS 10with arunningtimeof16hperloadstage. Aftertheloadstagetest,anendurancetestwitharunningtimeof80hinloadstageLS 8 and 580 h in load stage LS 10 is performed. The pinion torque and the corresponding Hertzianpressu
16、re of the different load stages are given in Table 2.5 11FTM16Table 2. Load stages of the micropitting testLoad stage Pinion torque, T1,Nm Hertzian pressure, pc,N/mm2LS 5 70.0 795.1LS 6 98.9 945.1LS 7 132.5 1093.9LS 8 171.6 1244.9LS 9 215.6 1395.4LS 10 265.1 1547.3Attheendoftheloadstagetestandtheend
17、urancetestwiththefirsttestgears,theloadstagetestisrepeatedwith new test gears to check repeatability.Aftereachtestperiod,thetestgearsaredisassembledandtheprofileofthetestedflanksismeasuredusinga3D measurement system.Intheloadstagetest,thefailurecriterionhasbeenreachedif themeanprofileform deviationd
18、uetomicropit-ting exceeds the limiting value of 7.5 mm. The load stage in which the failure criterion is reached is calledfailure load stage. An overview regarding the classification of test results obtained in the micropitting test isgiveninTable3.Lubricants withahighmicropittingload-carryingcapaci
19、ty reachthefailurecriterionof aprofileform deviationof 7.5 mm due to micropitting in load stageLS 10 of the load stage test.Examples for the evaluation of the micropitting test are given in Figure 3 and Figure 4.Table 3. Classification of test results of the micropitting testDescription Failure load
20、stageMicropitted area GF-classLow micropitting load-carryingcapacityLS 7 Sometimes morethan 50%GFT-lowMedium micropitting load-carrying capacityLS 8 - LS 9 About 30% GFT-mediumHigh micropitting load-carrying capacityLS 10 Less than 20% GFT-highFigure 3. Pinion type C-GF with measurement of the profi
21、le, nearly no micropitting failure6 11FTM16Figure 4. Pinion type C-GF with measurement of the profile, micropitting failure in the range ofthe failure criterionIntheendurancetest,astagnationofmicropittingformationcomparedwiththemicropittingareaattheendoftheloadstagetest is preferredbut not required.
22、 For ahigh-performancegearoilonbasis ofpolyglycolwithahighmicropittingperformance, atypicaltest result is given inFigure 5 showingthe profileform deviationdueto micropitting. The profile form deviation of the pinion is below the failure criterion for the whole load stagetest. In the endurance test,
23、the profile form deviation stagnates compared with the step test.Modification of the standard micropitting testThe aim of research is the investigation whether a change from an oil with low endurance micropittingperformance toan oilwith highendurance micropittingperformance canstop micropittingforma
24、tion. There-fore, thestandardmicropittingtestaccordingtoFVA 54/75 wasmodified. For anoilwithahighmicropittingload-carrying capacity, but low endurance performance, a load stage test and an endurance test areperformed with the first test gears. After the repeated load stage test with new test gears,
25、an oil change ismade. The subsequent endurance test is conducted to find out whether a product with a high micropittingload-carrying capacity in the load stage test as well as a high endurance micropitting performance can stopmicropitting formation. The oil temperature is set to 60 C. Compared to an
26、 oil temperature of 90 C, thelubricant film thickness in the gear mesh is higher but the formation of a reaction layer on the tooth flanksurfaces is more difficult. The latter is more critical regarding the formation of micropitting.Figure 5. Micropitting test of a high-performance gear oil (polygly
27、col), measurement of theprofile form deviation7 11FTM16Test equipmentBack-to-back gear test rigThe test runs conducted to determine whether micropitting can be stopped by using high-performance gearoils were performed on a FZG back-to-back gear test rig 3. The schematic setup of the FZG back-to-back
28、gear test rig is shown in Figure 6.Figure 6. FZG back-to-back gear test rigTheFZG back-to-back gear test rigutilizes are-circulatingpowerloopprinciple,alsoknownas afour-squareconfiguration,inordertoprovideafixedtorque(load) toapairof testgears. Test gearbox anddrivegearboxareconnectedwithtwotorsions
29、hafts. Oneshaftisdividedintotwopartsandcontainsaloadcouplingusedtoapplythetorque(load)throughtheuseofweightshungontheloadingarm. Aseparateoilaggregatecontainsheating and cooling elements to control the oil temperature as required by the operating test conditions.Before the oil is injected into the g
30、ear mesh of the test gears, it is filtered with a 10 mm filter.Test gearsIn order to investigate if micropitting can be stopped, test gears type C-GF of the standard micropitting testaccording to FVA 54/7 5 are used. The geometrical data and manufacturing details of the test gears typeC-GF are shown
31、 in Table 4 and Table 5.Tested lubricantsA specialty lubricant manufacturers goal is to supply industrial gear oils on the basis of mineral oil,polyalphaolefin,orpolyglycolshowingahighmicropittingload-carryingcapacityoffailureloadstageLS10intheloadstagetest as wellas astagnationof micropittingformat
32、ionintheendurancetestaccordingtoFVA54/7 5 by selection of advanced additive technologies. See also Fig. 5. These industrial gear oils showexcellentmicropittingprotectionnotonlyatanoiltemperatureof90C. Alsoatloweroiltemperatureof60C,the advanced additive technologies can react on the surface of the t
33、ooth flanks and protect them against8 11FTM16micropittingformation. Additionally,itwastobeinvestigatedifmicropittingformationoccurredwithothergearoils can be stopped by changing over to high-performance gear oils. The oil data of the tested gear oils areshown in Table 6 and Table 7. Both the high-pe
34、rformance gear oils and the reference oil are of ISO VG 320and possess a high micropitting load-carrying capacity according to FVA 54/7. All tested gear oils arespecified accordingto DIN51517 2 which includes the minimum requirements for industrial gear oils andissimilar toAGMA 90051. For thereferen
35、ceoil, themicropittingload-carryingcapacity was only tested at anoiltemperatureof 60Cbecauseat this oiltemperature, it was tobeinvestigatedifmicropittingformationcanbe stopped. See also Figure 7, Figure 8 and Figure 9.Table 4. Geometrical data of the test gears type C-GFDimension Symbol UnitNumerica
36、lvalueShaft center distance a mm 91.5Module m mm 4.5Number of teethPinion z1 16Wheel z2 24Effective face width b mm 14Pressure angle deg. 0 deg. 20Profile shift coefficientPinion wdeg. 22.44Wheel x1 0.1817Tip diameterPinion x2 0.1715Wheel da1mm 82.46Transverse contact ratio da2mm 118.36Tooth correct
37、ionPinion 1.44Wheel Without tip and root relief, no longitudinal crowningTable 5. Manufacturing details of the test gears type C-GFDimension Symbol UnitNumericalvalueBasic materialPinion 16MnCr5Wheel 16MnCr5Surface hardnessPinion 750 HV1Wheel 750 HV1Case hardening depth at 550HV1Pinion mm 0.8 - 1.0W
38、heel mm 0.8 - 1.0Core strengthPinion N/mm21000 - 1250Wheel N/mm21000 - 1250Flank roughnessPinion Ra mm 0.500.10Wheel Ra mm 0.500.10Table 6. Oil data of the reference oilProduct ISO VG Base oilDIN 5157,AGMA 9005designationMicropitting load-carrying capacityaccording to FVA 54/7oil=90C oil=60CReferenc
39、e oil 320 Mineral oil CLP, EP oil GFT-high9 11FTM16Table 7. Oil data of the high-performance gear oilsProduct ISO VG Base oilDIN 5157,AGMA 9005designationMicropitting load-carrying capacityaccording to FVA 54/7oil=90C oil=60CKlberoil GEM 1-320 N 320 Mineral oil CLP, EP oil GFT-high GFT-highKlbersynt
40、h GEM 4-320 N 320 Polyalphaolefin CLP HC, EP oil GFT-high GFT-highKlbersynth GH 6-320 320 Polyglycol CLP PG, EP oil GFT-high GFT-highTest resultsFor a reference oil with a high micropitting load-carrying capacity, but low endurance performance, a loadstage test and an endurance test were performed w
41、ith the first test gears according to FVA 54/7 5. Thereferenceoilalways showedthetest result GFT-highat theendof theloadstagetest. But inthesubsequentendurance test, micropitting formation increased. The endurance test was finished due to pitting formation.Thentheloadstagetestwas repeatedfor therefe
42、renceoilusingnewtest gears. Allrepeatedtestruns ofthereference oil showed a good repeatability compared with the first load stage test of this oil. See Figure 7,Figure 8, and Figure 9.After therepeatedloadstagetest for thereferenceoil, anoilchangetohigh-performancegear oilonbasis ofmineral oil with
43、a high micropitting load-carrying capacity as well as a high endurance micropitting perform-ancewas conducted. This oilchangetoanhigh-performancemineralgear oil usingadvanced additivetech-nologies stopped micropitting formation compared with the reference oil. This shows that these advancedadditive
44、technologies can react at the surface of the tooth flanks after an oil change and build up a newimproved reaction layer. Further micropitting formation was stopped and the micropitting area stagnated.SeeFigure7.The oil change to high-performance gear oil on the basis of polyalphaolefin with a high m
45、icropittingload-carrying capacity as well as a high endurance micropitting performance can also stop the micropittingformationcomparedwiththereferenceoil. Thereasonforthestagnationofthemicropittingareaareagaintheadvanced additive technologies in this high-performance polyalphaolefin gear oil. Even a
46、fter an oil change,theseadvancedadditivetechnologiescanreactatthesurfaceofthetoothflanksandbuildupanewimprovedreaction layer. See Figure 8.Finally, also high-performance gear oil on the basis of polyglycol with a high micropitting load-carryingcapacity as well as a high endurance micropitting perfor
47、mance can stop micropitting formation after an oilchange compared with the reference oil due to the advanced additive technologies. See Figure 9.Figure 7. Stopping micropitting by using a high-performance gear oil (mineral oil)10 11FTM16Figure 8. Stopping micropitting by using a high-performance gea
48、r oil (polyalphaolefin)Figure 9. Stopping micropitting by using a high-performance gear oil (polyglycol)All endurance tests for the high-performance gear oils on the basis of mineral oil, polyalphaolefin, andpolyglycol were stopped after the same running time as the reference oil but without any pit
49、ting formation.ConclusionsHigh-performancegear oils onthebasis of mineraloil,polyalphaolefin, orpolyglycolshowahighmicropittingload-carryingcapacity of failureloadstageLS 10inthe loadstage test as wellas astagnation of micropit-tingformationintheendurancetestaccordingtoFVA54/75byselectionofadvancedadditivetechnologies.Thenewtestresultsshowedthathigh-performancegearoilsusingadvancedadditivetechnologies canreactat thesurfaceof thetoothflanks after an oilchange andstop fur
