1、11FTM21AGMA Technical PaperGearbox BearingService Life - A Matterof Mastering ManyDesign ParametersBy H. Wendeberg, SKFGearbox Bearing Service Life - A Matter of Mastering ManyDesign ParametersHans Wendeberg, SKFThe statements and opinions contained herein are those of the author and should not be c
2、onstrued as anofficial action or opinion of the American Gear Manufacturers Association.AbstractThe service life of a gearbox is determined by many factors. The bearings in the gearbox play a major rolesincetheythemselvesdeliveranimportantfunctionandinadditioninteractwiththeshafts,thecasingandtheoil
3、. Without a doubt, the sizing of the bearings is of great importance for the gearbox reliability. Since morethan 50 years the bearing dynamic carrying capacity has been used to determine a suitable size needed todeliverasufficientfatiguelifebutdespitetheadvancedcalculationmethodsdeveloped,themethods
4、donotfully predictservicelife. Producers ofhigh quality bearing haveintroduced highperformance class bearingsandlackingbetterwaystoexpresstheimprovedperformance,thisisonlyrepresentedbyincreaseddynamiccarrying capacity.The availability of high-strength shaft materials in combination with bearings wit
5、h high carrying capacityallows slimmer shafts to be used. The modulus of elasticity remains the same, so seat design for bearingsand gears must be given close attention.This paper will cover the following:- Sizing of bearings based on dynamic carrying capacity and how this relates to service life- H
6、owthedesignoftheinterfacebetweenbearingandshaftsshouldbeadaptedtomodernshaftmaterials- How the design of the interface between bearing and gearbox casing influences service life of thegearbox- Influence of modern electric motor speed controls in bearing type selectionCopyright 2011American Gear Manu
7、facturers Association1001 N. Fairfax Street, 5thFloorAlexandria, Virginia 22314October 2011ISBN: 978-1-61481-020-93 11FTM21Gearbox Bearing Service Life - A Matter of Mastering Many Design ParametersHans Wendeberg, SKFIntroductionThe service life of a gearbox is determined by many factors. The bearin
8、gs in the gearbox play a major rolesince they deliver the important function of transmitting rotational movement, in addition interacting with theshafts,thecasingandtheoil. Withoutadoubt,thesizingofthebearingsisofgreatimportanceforthegearboxreliability. For more than 50 years, the bearing dynamic ca
9、rrying capacity has been used todetermine asuit-ablesizeneededtodeliverasufficient fatiguelife;butdespitetheadvancedcalculationmethodsdevelopedinfatigue, subsurface fatigue is not the only mechanism involved in the determination of the service life of thebearings. Producersofhighqualitybearingshavei
10、ntroducedhighperformanceclassbearingsbutstilllackingrepresentative ways to express the improved performance regarding fatigue life; currently, this is onlyrepresentedbyincreaseddynamiccarryingcapacityandpossiblyanotherscaleforthelifemodificationfactor.The availability of high-strength shaft material
11、s in combination with bearings with high carrying capacityallowsslimmershaftstobeused. Themodulusofelasticityremainsthesame,soseatdesignforbearingsandgears must be given close attention. The possibilities of the modern electric motor speed controls put newdemands on the design of the interfaces betw
12、een bearings and casings. This also influences what types ofbearings can be used.This paper covers the following:S Sizing of bearings based on dynamic carrying capacity and how this relates to service life.S Howthedesignoftheinterfacebetweenbearingandshaftsshouldbeadaptedtomodernshaftmaterials.S How
13、 the design of the interface between bearing and gearbox casing influences service life of thegearbox.S Influence of modern electric motor speed controls in bearing type selection.Sizing of rolling element bearings based on dynamic carrying capacityFor modern high quality bearings, the classic basic
14、 rating life can deviatesignificantly fromthe actualservicelife ina givenapplication. Generally speaking,service lifein aparticular applicationdepends notonly onloadin relation to bearing size, but also on a variety of influencing factors including lubrication, the degree ofcontamination, misalignme
15、nt, proper installation and environmental conditions.The first method accepted by ISO for determining a suitable bearing size is the classic Lundberg andPalmgren equation L10=(C/P)P, making it possible to determine a suitable dynamic C-value (which in turndefines the bearing size) needed to satisfy
16、a need for a fatigue life L10.Inthesixties,theinfluenceofthematerialpropertiesandlubricantfilmthicknesswasintroduced,representedby the a23-factor. (See Figure 1.)(1)L10= a23CPPa23is a function of and the materialAs an attempt to take some of those factors into account when determining a suitable bea
17、ring size, the DINISO281:1990/AMD2:2000containsamodificationfactoraSLFtothebasicratinglifeL10=(C/P)P. Themethodmakesprovisionforbearingmanufacturersto recommenda calculationmethodology forthis lifemodificationfactor to be applied to a bearing based on operating conditions. Some life modification fac
18、tors applies theconcept of a fatigue load limit Puanalogous to that used when dimensioning other machine components.Furthermore, the life modification factor makes use of the lubrication conditions and a factor cforcontamination level to reflect the applications operating conditions.4 11FTM21Figure
19、1. Life adjustment factor a23for oil film influenceBasic conditionsThe life modification factor considers bearing load level, oil film thickness and the stress-raising influence ofindentations in raceways and rollers from oil contaminants.Theinfluenceoftheoilfilmthicknessisstrong,andisrepresentedbyt
20、hevalue. isthe ratioof theactualoperating viscosity to the rated viscosity for adequate lubrication 1.The influence of indentations from oil contaminants is very important, and complex to model. The c-value(contamination factor) acts as an inverted stress concentration factor defined as a value betw
21、een 0 and 1,where 0 represents “severe contamination” and 1 represents “extreme cleanliness”. The DIN ISO 281Addendum 4:2003 describes a method to obtain an c-factor for a given application. SKF has developed astandardized method to estimate the c-value using load level, oil film thickness, size and
22、 type ofcontamination particles, mounting practice, filter effectiveness, and seal effectiveness into account, 2, 3.(2)L10m= aSKFCPPaSKF , c, Pu, PInthelatestdecade,SKFandotherbearingproducershaveintroducedhigh performanceclass bearingsthatare given higher dynamic carrying capacity figures and in SK
23、Fs case another scale for the life modificationfactor aSKFto adapt the L10mcalculations to the new technological developments in material andmanufacturing. (Figure 2)Therefore,thedevelopmentfromthepurelysub-surfacefatiguetheory“ActaPolytechnica”from LundbergSeparate “high-performance scale”Limitatio
24、ns of the classical and modified sizing modelsThedifferentissuesoftheL10modelsfordeterminingbearingsizearewellgroundedinmathematicalmodelsand practical testing, but limited in the use of only one failure mechanism, i.e., fatigue of the raceways or therolling elements to deliver a suitable service li
25、fe when determining a suitable bearing size. In the aSKFdia-gram, very thin oil film (e.g., equal to 0.1)is associatedwith alife modificationfactor of0.1 independentlyofload level which is to be understood as predominant surface induced fatigue. In the bearing (and in theadjustment factor model), su
26、rface induced fatigue gains importance at low values of and c.Wear from abrasive particles in the oil film is a complex phenomenon (it will for instance surely change thecontactgeometry,whichinturnwillchangethewear progress )andhenceititsprogressisdifficulttopredict.Workonpredictivemodelsisinprogres
27、s,butwillnotbediscussedindetailinthispaper. Recentfindingsinthefield of surface distress in the thin-film regime applicable to bearings in the lower speed part of gearboxes6 11FTM21andongears - arehowever bringingnew insightsthat arefinding practicalapplications inbearing designandmanufacturing 1.Su
28、rface distressIn many industrial applications having lubricated rolling/sliding contacts (e.g., rolling bearings, gears,cam-followers) the power density has steadily increased in time due to the need for higher efficiency,reduction of weight and costs (i.e., downsizing). However, with the increasing
29、 severity of the workingconditions, e.g., by heavier loads in combination with higher temperatures, thinner oil films and/or boundarylubrication conditions, machine components can sometimes suffer from surface distress 5. Thisphenomenon manifestsitself initiallywith achange ofcoloration/dull appeara
30、nceof thesurface, whichgrowsas the damage progresses. Under the microscope, the affected surface areas show the presence of tinymicro-spalls, microcracks or micropits (see Figure 3).Nowadays it is recognized that surface distress is a surface damage phenomenon associated with poorlubricationconditio
31、ns,thushighlocalfrictionandpressuresatasperitylevel. This phenomenonhas beenthesubject of many recent experimental and numerical studies 7-12.Surface frictionRealcontacts,evenwhenrunningunder“nominalpurerolling”conditions,alwayshaveasmallamountofslip,whichresultsinsomeslidingfrictionandconsequentlyi
32、nthepossibilityofsurfacedistressrisk. Intests,nom-inal pure rolling conditions can also exhibit surface distress. Under equal conditions and number of cycles ithas also been found that as the boundary friction coefficient is increased,surface distressis moresevere. Inconclusion, boundary friction is
33、 a very important factor in promoting surface micro cracks when the contactoperates under boundary or mixed-lubrication.Importance of lubrication and roughnessIn rolling bearings lubrication has a major role in the life performance. This is why life models account for theeffect of the lubrication pa
34、rameter 1. The importance of lubrication and roughness in surface damage isverymuchrelatedtotheeffectoflocalfrictionforcesandstressconcentrations(atasperitylevel). Inboundaryor mixed-lubrication, having irregularities (roughness or indentations, as shown in Figure 4) on the surfacewill influence the
35、 way the dry and lubricated spots are distributed within the contact. Furthermore,discontinuities on surface traction and possible stress concentrations (Figure 5) must also be considered.High roughness or high roughness slopes might promote local film collapse, high contact pressures andtractions.
36、This will enhance stress concentrations in the critical areas of traction discontinuities.Figure 3. Illustration of surface distress7 11FTM21Figure 4. Surface distress appearance on asperity summits (left) and in the area around anindent (right)Figure 5. Schematic representation of the traction disc
37、ontinuities and stress concentration areasin boundary or mixed lubrication regime of rough surfacesIndeed, from the test results and theoretical modelling, surface distress appears first in areas of pressurediscontinuities (high pressure gradients) associated with increased roughness. It can easily
38、be seen on theborders of grooves or in the summits of asperities or surface rises from indentations (Figure 4). Ratherunexpectedly, it is in general the smoother of the two mating surfaces that will first start the distress process.Some observations that may help to explain these observations are di
39、scussed in the next section.8 11FTM21The contact of two rough surfacesIn industrial applications, the contact always takes place between two real surfaces having a certainroughness. This is also the case in tests carried out using a Surface Distress Test Rig (SDTR), which has arotatingrodincontactwi
40、ththreediscs(hardenedbearingsteel). Asobserved, whenthe testrod wasrougherthan the load applying discs, surface distress did not appear in a reasonable time even under the harshestconditions(Figure 6a). However,whenthediscswererougherthantherod(Figure 6b);thensurfacedistresseasilyappearedontherodsur
41、face. This isalso acommon observationelsewhere 11. Asdiscussed in13,the likely explanation for this is the load history from the fatigue micro cycles imposed by the roughness.When the conditions in the contact are in general more towards boundary or mixed-lubrication, then thestress history is impos
42、ed by the dominant rougher surface upon the smoother one as long as there is somesliding. In real contacts, both surfaces will be rough and in movement (with some sliding), but if they havedifferent roughness, the rougher surface will prevail over the smoother when it comes toimposing loadmicrocycle
43、s. Therefore, the smoother surface will be more prone to surface distress in the presence of somesliding, provided that the mechanical properties of both surfaces are the same.Improving wear and metallic surface contact resistanceOne way to enhance wear resistance is to increase the hardness of the
44、components in contact, but this hasconsequences. If hardness increases, toughness is reduced as well as a safe failure mode; spalling occursinstead of cracking, leading to catastrophic failure.Development work was performed to find a way to improve the resistance to surface initiated damageswithoutl
45、oosingthefundamentaladvantagesofourcurrentprocesses. Thebainite hardeningused todaycanbe manipulated to retain toughness, compressive residual stresses and at the same time increase wearand debris contaminated condition life without loss of productivity. The process is patent protected,(Figure 7).Th
46、e process provides products with enhanced lives under harsh running conditions but retain all the benefitsof our long term, successfully used, hardening processes.Figure 6. Effect of roughness location: (a) smooth discs on rough rod, and (b) rough discs onsmooth rod9 11FTM21Figure 7. Bainite transfo
47、rmations with the conventional and the new processWear tests conducted using significant amountsof debrisin theform ofchilled castiron particlessignificantlydelays the onset of heavy wear, (Figure 8).Tests run under marginal lubrication conditions confirm the advantages of the new heat treatment met
48、hod about twice the life is attained under very low kappa conditions (Figure 9).Figure 8. Severe wear damage phase under contaminated conditionsFigure 9. Weibull estimates for bearings tested under severe contaminated conditions10 11FTM21Thus,theenhancedheattreatmentthatwillbecomethefuturestandardwi
49、llenhancetheabilityofthebearingcomponents to withstand environmental threats posed by application conditions encompassing debriscontamination and marginal lubrication conditions.How the design of the interface between bearing and shafts should be adapted to modern shaftmaterialsAnother surface failure related mechanism that may limit the performance of a gearbox is fretting. Modern,high-strengthsteelsoffergreat possibilitiesfor transmittinghigh torqueand loadin gearboxshafts andgearsdespite limited dimens
