1、12FTM20AGMA Technical PaperThe Effect of theSurface RoughnessProfile on MicropittingBy M. Bell, G. Sroka andR. Benson, REM SurfaceEngineeringThe Effect of the Surface Roughness Profile on MicropittingMatt Bell, Dr. Gary Sroka and Dr. Ron Benson, REM Surface EngineeringThe statements and opinions con
2、tained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractA wide choice of surface roughness parameters is available to characterize components, such as gears orbearings,withthegoalofpredictingtheperforman
3、ceofsuchmetal-to-metalcontactingparts. CommonlyinindustrytheRoughnessAverage(Ra)ortheMeanPeak-to-ValleyHeight(Rz(DIN)ischosentocalculatetheSpecific Film Thickness Ratio for both superfinished and honed surfaces. However, these two surfaceroughness parameters fail to adequately predict the performanc
4、e properties of surfaces that aresuperfinished or surfaces that are honed. In this paper, a superfinished surface is defined as a planarizedsurface having a 0.25 mmRa. A honed surface is not considered to be planarized, even with a finish of 0.25 mmRa.Thus, one is falsely led to predict that a plana
5、rized surface or a honed surface, having anequivalent Raor Rz,will perform similarly. Experimentally, an isotropic planarized surface delivers superiorperformance. The following discussion utilizes another roughness parameter, 350, to further explain thisphenomenon.Copyright 2012American Gear Manufa
6、cturers Association1001 N. Fairfax Street, Suite 500Alexandria, Virginia 22314October 2012ISBN: 978-1-61481-051-33 12FTM20The Effect of the Surface Roughness Profile on MicropittingMatt Bell, Dr. Gary Sroka and Dr. Ron Benson, REM Surface EngineeringIntroductionTheroughnessprofileofmetal-to-metalcon
7、tactingpartsisoneofthemostimportantcharacteristics inmak-ing mechanical systems more durable and energy efficient. Unquestionably, it is the interaction of the peakasperities from ground or honed surfaces that contributes to the degree of friction, high operating temperat-ures, deteriorated lubricat
8、ion and system failure 1. Lubricant films attempt to keep such interactions to aminimum, but under high loads and low speeds peak asperity contact still occurs. The surface roughnessprofile may be defined as any parameter that is used to characterize the topographical features of a givensurface. How
9、ever, several parameters areusedcommonly. Table 1represents afew of the most prominentsurface roughness parameters in use by industry and academia today in addition to all parameters citedthroughout this discussion.Two parameters most commonly chosen tocharacterize thesurface textureare theroughness
10、 average, Ra,and the mean peak-to-valley height Rz (DIN). Yet, it is evident that surfaces having an identical Racan havecompletely different characteristics as illustrated in Figure 11)2.Table 1. Definition of parametersSymbol Definitions NotesRaComposite arithmetic mean roughness Commonly used as
11、a standardroughness parameter in industryRqRoot mean squared roughness (RMS) The standard deviation in the “Histogramof Heights” in Figure 5 through Figure 7,based on RqRz (DIN)Mean peak-to-valley height Commonly used as a standardroughness parameter in industryS Composite surface roughness of matin
12、gsurfacesSeveral different equations existdepending on industry/historicalpreferencesR1, R2General R profile used in calculation of S May be Ra, Rq,etc.RSKSkewnessRpmAverage maximum profile peak heightISF Isotropic superfinish or planarizedsurface profile having a 0.25 mm Ra Standard deviation350Rou
13、ghnessbasedonpopulationofpeakswithin 30s of the mean roughness Minimum film thickness divided by thecomposite surface roughnessH, hminMinimum film thicknesstpBearing ratioes, ebHeights of the asperities on thecontacting surfaces “s” and “b”1)Surface finish measurement procedures, general terminology
14、, definitions of most parameters and filteringinformation can be found in ASME B46.1 (2009), ISO 4287:2009 and ISO 4288:1998. Units are in micrometers (mm).4 12FTM20Characteristic surface Ra2.4 mm2.4 mm2.4 mmFigure 1. Illustration of component surfaces having approximately equivalent Ravalues butrad
15、ically different surface textures and performance characteristics in operation(Courtesy of Hommelwerke, GmBH)For well over a decade many companies, universities and research organizations carried out concertedinvestigations of gear and bearing micropitting. Recently these investigations have taken o
16、n even moreurgencywiththegrowthofwindturbinesizeandexponential growthinthenumber ofmegawatt windturbinesin service. Due to the extremely high load and low speed operation of the input stage of wind turbines,micropitting has become an epidemic problem and is now recognized as a failure indicator that
17、 can lead topremature gear and bearing failure. Prior to this epidemic problem in the wind turbine industry, micropittingwas often assumed to be self-arresting and not a failure mechanism in gearboxes.Researchers seem to be in universal agreement that micropitting is initiated by peak asperity inter
18、actions ofthemetaltometalcontactingsurfaceswherebyhighsubsurfacestressesarise34. However,theypaylittleor no attention to the method used to generate the surface or the specific topographical features of the finalsurface. For example, ISO/TR 15144:2010 uses the effective composite arithmetic mean rou
19、ghness tocalculate the local specific lubricant film thickness5:Ra= 0.5Ra1+ Ra2 (1)whereRais effective composite arithmetic mean roughness value;Ra1is arithmetic mean roughness value of pinion;Ra2is arithmetic mean roughness value of wheel.5 12FTM20Although the technical report states explicitly, “A
20、t present Rais used, but other aspects such as Rzorskewness have been observed to have significant effects which could be reflected in the finishing processapplied”, in practice the roughness profile of the surface is ignored.The parameter Rais used repeatedly with little consideration of the actual
21、 topographical features or surfacetexture of the polished surface in the 2004 patent Polished Gear Surfaces 6. In this pivotal patent, gearshaving different surface roughnesses were generatedby radically different techniques (hobbed andshaved,ground, honed, fine grit honed, physicochemically polishe
22、d and electrochemically polished). Although thepatentacknowledgesthateachsurfacehasadifferentroughnessprofile,onlytheRaisusedtopredictcontactfatiguelife,wearresistanceandperformance. Whiletheauthorscorrectlyconcludethemacroscopiccorrela-tionofreducedsurfaceroughnessandstressreductionthespecification
23、ignoresthemicroscopicintricaciesofthe nature of the surface roughness and the resulting variations in the magnitude of stress reduction by thesurface roughness generation technique.The planarized surfaceTheplanarizedsurfaceisthatgeneratedbychemicallyacceleratedvibratoryfinishing(henceforwardreferred
24、to as superfinishing). Figure 2 summarizes the process of superfinishing on an “as-ground” or “as-honed”surface. Depicted in scanning electron microscope images and profilometer traces, the superfinishingprocess removes the peaks resulting in a planarized surface. The planarized surface has no sharp
25、 peakasperities and hence, even if two planarized surfaces come in contact the subsurface stresses aresignificantly less than those generated by surfaces having sharp peak asperity interaction. Eventually eventhe valleys disappear as the superfinishing process continues leaving a planarized, micro-t
26、extured surface.In general, on different surfaces with an equivalent Raand Rz, the planarized surface will give superiorperformancetothat ofagroundsurfaceorhonedsurfacesincethelatter haspeak asperities(Figure 3). Theoptimum surface is that surface where the valleys also have been completely removed.
27、Figure 4 illustrates ground or honed surfaces brought into contact (top) versus two planarized surfacesbrought into contact. The film thickness required to separate the planarized surfaces is much less than thatrequiredtoseparatethegroundorhonedrandomsurfaces. Recently,theperformanceofhonedgearswith
28、anaverage0.25mmRawerecomparedtosuperfinishedgearswithanaverage0.04mmRa7. Thesuperfinishedgears lasted 2,000 hours with no indication of micropitting while the honed gears showed micropitting afteronly 150hours. Onecanarguethat agear with asmoother (honed) surface shouldoutperform agear witharougher
29、surface. However, a0.25 mm Raplanarizedsurfacedevoidofstress raisers(peak asperities)shouldoutperform a honed 0.25 mm Rasurface.A better parameter to characterize the planarized surfaceThe specific film thickness ratio, , is a dimensionless parameter commonly used to predict or define thedegree of s
30、eparation between two contacting and lubricated surfaces. The degree of separation may rangefromfullcontactorboundarylubricationtofullseparationorhydrodynamic(fullfluid)lubrication. Specifically,refers to the minimum lubricant film thickness divided by the composite surface roughness of the matingsu
31、rfaces (equation 2). =Minimum film thickness, HComposite surface roughness, S(2)Historically,therewereseveralderivedmethodsofdeterminingthecompositesurfaceroughness. Examplesare shown in equations 3 through 5.S =R2a1+ R2a20.58(3)S = 0.5Ra1+ Ra21(4)S =R2q1+ R2q20.59(5)6 12FTM20PerformanceSurface at 5
32、00XmagnificationRoughnessaverageWorstStarting condition: Ground surface havingpeak and valley asperities along with adistressed layer of metal at the surfaceRa:0.58mmRz:3.5mmGoodStage 1: Partially superfinished surface wherethe peak asperities have been planarized.Ra:0.30mmRz:2.0mmBetterStage 2: Sup
33、erfinished where only a few valleyasperities remain on the surfaceRa:0.13mmRz:1.1mmBestFinal condition: Superfinished surface with allasperities removed while displaying thebeneficial and inherent microtexture.Ra: 0.025 mmRz:0.17mmFigure 2. Roughness profiles and SEM images at 500X of surfaces forme
34、d by chemicallyaccelerated vibratory finishing7 12FTM20Figure 3. Illustration of film thickness measured on two rough surfacesFigure 4. Graphic showing the interaction between surfaces with the same nominal RaOccasionally, Rais substituted in the preceding formulas with Rz5. Consequently, the -ratio
35、, associatedwith the lubrication regime, is dependent on how the composite surface roughness is calculated. Thus, the-ratio may vary widely. See examples in Table 2.Conway-Jones and Eastman suggest that the best definition of the Lambda Ratio, where sliding and rollingcontact occurs (gears, for inst
36、ance) is better defined by the arithmetic summation of the heights of the peakasperities 13.Table 2. The effect of composite roughness calculations on specific film thickness ratio ()Lubrication regime Lambda ratio, Author/source Dudley 8 Frequently used (Tanaka,Hutchings 10 11)Bhushan 12Fully fluid
37、 or hydrodynamic 1.0 3 5Mixed lubrication 0.4 - 1.0 1-3 1-5Boundary lubrication 0.1 - 0.3 1 18 12FTM20This is representedby equation6where esandebaretheheightsof theasperities ontherespectivesurfaces“s” and“b”. Thenumerator, hmin, is theminimum film thickness requiredfor full hydrodynamic lubricatio
38、nun-der the operating conditions. The authors state that this -ratio correlates more closely to actual test data.Alternatively,theequationmaybeexpressedbyequation7. Inaddition,theauthorsproposethecalculationofthe composite surface roughness according to Equation 8 below. =hmines+ eb(6) =hminR1+ R2(7
39、)S = 350(1) + 350(2)(8)In the three following topographical scenarios the authors calculated various R parameters from opticallymeasured histograms of peak heights. The parameter Rqis defined as the standard deviation of the heightdistribution. Therefore, inthe histogram of heights, one canmultiply
40、Rqby threeto obtainthe 3value. Thisvalue is then multiplied by two to approximate two contacting surfaces.For the bearing ratio, tpthe 350value is calculated by only taking into account tpbetween 0.13% and 50%.Thevery highest peaks (tproughly less than0.13%) onagear or bearingsurfacearequicklycleave
41、dorwornaway during the run-in period and, thus; do not affect surface wear significantly. Likewise, a tpgreater than50%, the material has little or no effect on surface wear and fatigue. Again, the 350value is doubled toapproximate two contacting surfaces.Afterward, for illustrativepurposes,themeanl
42、ineofthehistogramof peakheights isalignedwiththelinethatcorresponds to a tpof 50%. The values of 3 (Rq) and 350are calculated and compared beneath theillustrations.In Figure 5, the 3 of the histogram of heights (left) approximates the 350of the micron depth vs. bearingarea. The values are relatively
43、 close since the ground surface approximates a normal distribution of peaksand valleys on the surface.To obtain full film lubrication for mating surfaces having the same roughness, Ra, see Table 3. To obtain fullfilm lubrication for mating surfaces having the same roughness, see Table 4.Inthiscase,R
44、qunderestimatesthethicknessofthefilmrequiredtoachievefull filmlubrication. Thereasonisthatafirstpolishingofthegroundsurfaceintheaboveexampleintroducedraisedareasresultinginapositiveskew, Rsk(Figure 6). Certaindifficult bearingalloys andhighcarbidealloys displaythis characteristicfeatureduring the in
45、itial stages of superfinishing. Areas of raised peaks, plateaus, or nodules may appear on thesurface. To obtain full film lubrication for mating surfaces having the same roughness, see Table 5.Table 3. Condition 1 - Calculation of 350and 3 (based on Rq)Basis Calculation350 hmin= 2 1.502 = 3.00 mmRq
46、hmin= 2 (3 0.56) = 3.36 mmTable 4. Condition 2 - Calculation of 350and 3 (based on Rq)Basis Calculation350 hmin= 2 0.986 = 1.97 mmRq hmin= 2 0.72 = 1.44 mmTable 5. Condition 3 - Calculation of 350and 3 (based on Rq)Basis Calculation350 hmin= 2 0.280 = 0.56 mmRq hmin= 2 0.51 = 1.02 mm9 12FTM20Figure
47、5. The magnitude of 3 approximates 350: Condition 1 - Two ground surfaceswith no skewFigure 6. The magnitude of 3 underestimates 350: Condition 2 - Two lightly lapped or “firstpolished” surfaces with positive skew10 12FTM20Inthiscase,Rqgreatlyoverestimatesthethicknessofthefilmtoobtainfullfilmlubrica
48、tion. TheRskisnegativeas it is for superfinished surfaces where mostly valleys and very few peaks are present (Figure 7).Experiment - An in situ case studyA through-hardened high grade alloy steel specimen2)having an initial ground surface finish of a 0.8 mm Rawas planarized. The Rq, Raand 350were m
49、easured periodically during the planarization process. Thetheoretical separation film thickness was calculated using the parameters Rqand 350.The results, plotted in Figure 8, show that the minimum film thickness required to obtain full film lubricationdecreases muchmorerapidly when calculatedusing 350versus whencalculated usingthe Rq. Thisoccursbecausethesurfacehasbeenplanarizedwiththeremovalofthepeakasperities. Thus,whenagroundpartissuperfinishedtoa0.23mmRa,
