ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
EffectofAlternate Solvent Diluents onPFPEPerformance Christopher J.Dayton andRobertM.Warden* Abstract Alubrication studyhasbeencompleted whichtestedtheeffectofvarious CFC(Freon) substitutes onthe performance ofperfluoroalkylpolyether (PFPE) lubricants applied torollingelement bearings. Three bearing pairsweretestedconcurrently, differing onlyinthediluentusedforthefinalstepintheprocessing ofthebearings. Theobjectives ofthebearing testswereto:1)quantify intermsofstress-cycles the approximate pointatwhichtheonsetoflubricant polymerization occursand2)determine whateffectthe useofdifferent solvents haveontheperiodoftimepriortotheonsetofpolymerization. Thispaper provides adetailed description oftheprocessing oftheballbearings, thetestarticles, thetextfixture, the datalogging philosophy, andtheresultsofthestudy. Introduction Thetong-term reliable performance ofbearings operating inaspaceenvironment requires careful selection oftheappropriate liquidordrylubricant. PFPEisoneclassofliquidlubricant withextensive spaceflighthistory. Relative tohydrocarbon lubricants, PFPE's generally havelowervaporpressures, whichreduces theevaporative lossfromcriticalinterfaces. Theyalsohavehigherviscosity indexvalues, whichtranslates intolowerbearing torques atlowoperating temperatures. Thistypeoflubricant, however, doesnotpossess goodmiscibility fortraditional, space-proven additives thatreadily dissolve in hydrocarbon lubricants. Also,PFPE's haveatendency topolymerize, especially whenemployed in interfaces wheremetal-to-metal contact isprevalent (theboundary lubrication regime). Unfortunately, this isthetypicaloperating regimeformanycomponents withinspace-borne instruments. Overthepastfifteenyears, thepropensity ofPFPE's to undergo anautocatalytic polymerization when subjected tostress inthe presence ofcertain metals hasbeenthesubject of extensive study.1'2Furthermore,several articles have beenwrittendocumenting the varying response ofthisclass oflubricant relative toits propensity topolymerize with respect toboththecleaning methods andthetypeof contacting materials employed. 3'4'5These latter studies werelargelyprompted bytheMontreal Protocol, whichprohibits theproduction TF),thathadbeenwidelyusedFigure1.Bearing testsetupas-runconfiguration ofcertain ozonedepleting chemicals (ODC's), suchasCFC-113 (Freon ascleaning agentsandPFPEdiluents. ThelinearPFPEoilBrayco815Z,andthegreaseanalogue Braycote 601,havebeenusedformanyyears inspaceflightinstruments. Forexample, whenthelastoftheHubble SpaceTelescope (HST)science BallAerospace &Technologies Corporation, Boulder, CO Proceedings ofthe35thAerospace Mechanisms Symposium, AmesResearch Center,May9-11,2001 25
UsingaBallAerospace-developed lubricant filmthickness program thatcorrects forsideleakage fromthe contact ellipse, andthebearing parameters summarized inTableI,thelambda ratio(elastohydrodynamic filmthickness tocomposite ballandraceway surface roughness) wasdetermined tobeapproximately 0.6 whenthebearings wereoperated atroomtemperature and50rpm.Thiscalculated lambda ratiois probably greater thanwhatactually existed sinceflooded conditions wereassumed. Thesurface roughness termwascalculated fromanaverage ofmeasurements ofthebearing innerraceways, performed bythebearing manufacturer andsummarized laterinthepaperinTableI1. TableI.TestBearing Properties Property PartNumber Number ofGrade10Balls,Diameter PitchDiameterValue SBB3HAR007P5A InitialContact Angle OuterandInnerRaceConformities 0.524and0.520 ThrustLoad MeanInnerRaceway Hertzian Compression Stress18,6.35mm(0.25inch) 45mm(1.771inch) 12.80 approximately 62N(14.0Ib) approximately 450,000 kPa(65,000 psi) Thetestwasperformed inavacuum chamber atlessthan1x105torr,atroomtemperature, withone threedayexcursion to+13degCandanother shortexcursion to+30degC.Eachsetoftestbearings wascontinuously monitored fortorque(method defined intheTestApparatus section). Eachofthesetsofangular contact bearings werehoused inseparate stainless steelbearing cartridges withacompliant axialpreload ofabout62N(14Ib).Thisloadresulted inanominal ball-to-inner-raceway Hertzian contact stressofapproximately 450MPa(65ksi).Theobjective wastooperate theprocessed bearings underboundary lubrication conditions. Theintention ofusingalightpreload wastwofold: itwas representative ofseveral bearing pairsoperating intheHST,anditwasthought thatthiswouldallowfora longertestduration priortotheonsetofpolymerization. Labyrinth sealswithalength-togapratioofapproximately 30to1 enclosed bothsidesoftheduplex pair.Thisresulted inreducing the apparent exit area by approximately 88percent, thus reducing evaporative loss.The labyrinths, coupled withthelow vaporpressure ofthevacuumstripped lubricant resulted ina negligible evaporative loss. Lowsurface energy barrierfilm wasapplied tosurfaces adjacent tothebearings tolimitlossdueto creep. Priortocommencement ofthe test,thepreloaded bearings were run-inunderthesamethermal vacuum conditions thatwouldbe present duringthetest.Theruninperiodwas16days.Thiswas followed byremoval oftheVacuum feed_ IJINItl_ forslgnal_ _illld4_l! places  Vacuum feed-thru  formotor Figure2.TestFixtureShowing 6Bearing Pairs 27
bearingsfromthecartridges,andrecleaningtotheparticlelevelsstatedabove,followedbybarrierfilm applicationtothebearingfaces.Carewastakenduringreassembly toduplicate,ascloseaspossible,the orientationofthebearingswithinthecartridgespresentduringrun-in. TestApparatus Thebearing testassembly, depicted intheas-runconfiguration inFigure1,andacross-sectional viewin Figure 2,wasdesigned andbuilttosimultaneously testseveral pairsofbearings underidentical environmental conditions. Thespeedandtemperature canbevariedbutthechanges applytoallthe bearings. Inorderforthebearings tobetestedunderload,twobearings weremounted inacartridge and compliantly preloaded back-to-back (Figure 3).Inthisway,eachbearing pairispreloaded withinthepair, independent ofthetestfixture. Thepreload wassetby compressing thewavewasher apredetermined amount. Eachwavewasher hadbeen characterized through theTang Bearing ,,---,-- Pair I,generation offorceversus displacement curves. Atang extends fromtheouterpartof thecartridge sothatwhenthe innersubassembly isrotated, theforceonthetangkeeps theoutersubassembly from rotating. Bymeasuring the forceonthistangoverlong periods ofbearing rotation, the effectiveness ofthebearing cleaning andlubrication can bemeasured./Inner Clamp  Spacer \ "1 \'___SpringWasher _ OuterClamp _'_-Outer Housing - Setcrew InnerHousing Figure3.Detailsofloadedbearing pairinsidecartridge Acommon shaftdrivesthe innersubassembly ofeachbearing pairatthesamespeed. Thetangontheouterringisrestrained bya cantilevered beam,whichisattached toanoffsetcolumn. Thedistance fromthecenterofrotation tothe pointofcontact withthebeamis66mm(2.6inches) asshowninFigure4.Thesebeamsactassmallleaf springs, whichareinstrumented withstrain gagestoindicate force.Theforcegagesare calibrated beforebeingassembled intothe vacuum chamber. Thezerovaluecanbe verified atanytimebysimplystopping the driveshaftandbacking itoffthetang. Thevacuum chamber wasasimplebelljar overamulti-port access collar.Oneaccess portwasusedfortherotaryfeed-through whichdrovethedriveshaftbymeans ofa rightanglegearhead. Aflexcoupling was usedbetween therotaryfeed-through and theright-angle gearhead toaccommodate alignment errors.Tan% _-_StrainGage 1_-O°_'_'JO_/y B_21r_Partridge_ _ Figure4.Tang,beamandstraingage Oneoftheearlyconcerns wascross-contamination between thebearing cartridges. Although thisthreat wasslight,itwasconsidered sincetheexitareasassociated withthelabyrinth gapswereline-of-sight between adjacent cartridges. Toreducethiseffect,largediskswereplacedbetween thecartridges. To 28
TestResults Atestendpoint basedonachieving apredetermined torquerelative totheinitialsteady-state valuewas notestablished. Instead, thebearings wereallowed torununtilitwasobvious fromin-situretention force measurements thatthelubricant hadfailed,orwasnearing theendofitsusefullife.FortheFreonTF,and toalesserextent,theVertrelXFtreated bearings, theonsetoflubricant polymerization wascharacterized byasudden increase intorque, followed byagradual decline. TheHFE-7100 processed bearing pair, however, alsofollowed thispattern, butwithaverage slopesmoreshallow relative totheotherbearing pairs(Figure 6). Bray815ZSolvent Study:Continuous Rotation, Boundry Lubrication _°61 --t ..... 1_l_Jto 04 . ,t o Ptlnlrll t I[sFreon TFJ •;illT,e,-,T,a,__LVer,re,XFJ,J,__m[j,._,IIIf-tJJ_ ,_/_la_I Cycling ,-- ,- _r 'W_....  FE7100  ++I (:5 e,i _ _ _u_ _6 Revolutions (millions ofcycles) Figure6,Restraining forceplottedagainst bearing revolutions. ThepointatwhichtheFreonTFprocessed pairexceeded two,andessentially, fourtimestheinitial steady-state restraining forceofapproximately 0.35N(0.08Ib)wasat2.73.106 revolutions. Usingthe summation fortheproduct ofthenumber oftimesagivenspotontheinnerraceway iscompressed times themeanHertzian innerraceway contact stressof450MPa(65ksi),andaccounting forthedifferential speeds oftheinnerraceversus thepitchdiameter, thisnumber ofrevolutions corresponded to approximately 1.37-1012 psi-crossings. Interim torquetraces(clockwise andcounterclockwise) weretakenusingastripchartrecorder shortly afterthisevent.Following theresumption ofthetest,thisbearing paircontinued tooperate atgreater than twotimestheinitialrestraining forceuntilessentially alltheoilhadpolymerized, andwaspushed outofthe weartrack.Thiseventisrepresented inFigure6byagradual decline intorqueuntiltesttermination at 3.88-106 revolutions, or1.95.1012 psi-crossings. 30
ThepointatwhichatwofoldincreaseintherestrainingforcefortheVertrelXFprocessedbearingpairwas achievedwasat4.52.106revolutions (2,27.10+2psi-crossings), and5.04.106revolutions (2.54.10+2psicrossings)forafourfoldincrease.TheouterracerestrainingforcefortheHFE-7100processedbearing pairdoubledat5.32.106revolutions(2.68.10+2psi-crossings). Therestrainingforceincreasedonlyslightly betweenthispointandtesttermination at6.63.106,or3.33.10+2psi-crossings. Torquetraces(strip charts)wereperformed at2.80.106 revolutions forthethreepairs(Figure 7),andafterthetestendpoints foreachrespective bearing pair,butpriortodisassembly (Figure 8).Notethatthepeak-to-peak bearing torquenoisefortheFreonTFdilutedoildecreased from8.5mNom(1.2.z-in)inFigure7ato5.0mN.m (0.7.z-in)inFigure8aduetotheformation, thensubsequent displacement ofpolymerized lubricant. In contrast, theamplitude ofthetorquenoisefortheVertrel andHFE-7100 processed bearings roughly doubled fromtheinterimtracestothefinaltraces. Thisindicated alaginthenoticeable effectofpolymer formation relative totheperformance oftheFreonTFprocessed bearings. +iiii.......iii!fi.....!?i......iiii?ili+ii+?ii :':::'_ii':':i_:i_i_.:_:i_2! 1:2_2121;2_ii++222!2:2_!22;i! Figure7aFreonTFprocessed bearing scan ........................... .++......... ....;............. .........,..,..+.+,.,.. .................. ,........................................ +........... o.................... ....+,.:,., ......... ._,.,...... :......... ;....,+.++,,._+.,. / ....... ,...... ,......................... ;........... .....,+ .............................................................. ,...................,.........,........................... _.2--"'--.u-_%J Figure7c.HFE-7100 processed bearing scan........ ,,..:........ +........... •...:,..,: ............ ,,+. _k / .)Jlill'_ 111 ___L"!....i....i...._....i....i...._....i....:....i...._....:, Figure7b.VertrelXFprocessed bearingscan Alltracesat50rpm.Allscansrepresent slightlygreaterthanonebearinginner shaftrevolution. Figure7.Interimtorquetracesforpreloaded bearing pairsafterapproximately 2.8.10srevolutions !+i..........................i........._.........i........._.........::........._........_........_........--.--:.........i...........•..!....!...._....:....:....!....:....!....i....;....!....i* 1"6.4oz'inl ""_...._....i....i........i....!....i"'+!....i....i....." Figure8b.VertrelXFprocessed bearingscan Fig.8arepresents 3.9"106revolutions. Fig.8band8crepresent 6.4"106revolutions. Allscansrepresent slightlygreaterthanone bearinginnershaftrevolution. Figure8.Torquetracesforbearings priortofinaldisassembly 31
Thebearingsweredisassembled asshowninFigure9.Avisualexamination at7to30Xmagnification of thedisassembled bearingcomponents, aswellasSEMimagingofinnerraceways,wasthenperformed• Forallthreebearingpairs,polymerized productwasevidentintheballpockets(ataminimum)ofeach bearingseparator.Alsoinallthreebearingpairs,wearmarksintheracewaysoftheouterraceswere difficulttoseeat30-powermagnification. Nooilhadcreptpastthebarrierfilmtreatedbearingfacesinany ofthebearings. OntheinnerracewaysoftheFreonTFprocessed bearings, therewasessentially nofreeoilapparent• Therewasnooilinevidence, andverylittlepolymer leftinthebearing weartracks(Figure 9a).Theinner raceway weartrackshadafrosted appearance, typicalofinadequate lubrication. Theraceways exhibited agreatdealmorewearrelative totheotherbearing pairs.Therewasalsoafaintbluetinttotheinner raceway weartracks, indicating thepossibility thatsurface damage duetoheathadoccurred. Theballs wereauniform darkgray,similarinappearance toablackoxidecoating. rF Figure9a.Brayco815ZandFreonTF Figure9b.Brayco815ZandVertrelXF #2 Figure9c.Brayco815ZandHFE7100 Figure9.Post-test imagesofrepresentative bearings 32
TheVertrelXFtreatedbearinghadaslightamountofoilmixedinwiththepolymerpresentinthe (predominantly inner)raceways.Whilewearmarkswerepresentontheinnerraceways,thegrinding groovesintroducedduringthebearingmanufacturing processwerestillreadilyapparent.Theballswerea lightershadeofgrayrelativetothoseassociatedwiththeFreonTFprocessedbearings. Asurprisingamountofoilwaspresentandintermixedwiththepolymerlocatedwithintheinnerracewayof theHFE-7100processedbearings.Otherthanthegreatcirclewearmarks,theballslookedalmost new. Theraceswereonlylightlyworn. Axialprofilometer measurements weretakenfortheinnerracesofeachbearing beforeandaftertesting (TableII).Thepost-test roughness valuesareacomposite average ofmeasurements oftheweartrack plusapproximately anequalaxialdistance outside theweartrack.Notethatonlyinoneinstance didthe surface roughness increase, andnotsurprisingly, itisassociated withoneoftheFreonTFprocessed bearings. Itissurprising, however, thattheotherhalfofthepairwasnotasrough.Thisobservation was supported bySEMimaging. TableII.InnerRaceway AxialSurface Roughness Diluent & Bearing SerialNo. FreonTF;_13 FreonTF;_14 VertrelXF;_11 VertrelXF;"12 HFE-7100; "01 HFE-7100; "02Surface Finish Before Test* 3.77 4.83 5.98 6.92 3.99 3.73(rms) AfterTest** 6.14 2.92 2.64 2.27 3.67 2.47 *Performed bySplitBallbearing, Div.MPBCorp. **Performed byFALEX Corp. Discussion andFutureWork Thestress-cycle valuesobtained frombearing testsareofbenefitonlywhenthetestandactualhardware parameters (materials, stresslevels,cleaning techniques, etc.)aresimilar. Forexample, ithasbeen demonstrated thatanexponential correlation existsbetween stressandusefullubricant life;5therefore, linearextrapolations forprojected bearing performance frombearing testsoperated atreduced stress levelsshouldbeapproached withcaution. Thebearing parameters evaluated inthistest,however, were similartotheparameters thatexistincertain HSTbearings (recently processed withVertrelXFdiluted Brayco 815Z),withtheexception thattheHSTbearings contained varying ballcounts. Thepsi-crossing allowables generated inthistestprovided ameans tocalculate "ballpark" performance margins, with respect tolubricant polymerization, forthesebearings. Fromtheresultsofthislimitedstudy,thereisanindication ofpotential benefitobtained fromtheuseofthe HFE-7100 toprocess Brayco 815Zlubricated 440Cbearings relative tothediluents FreonTF,orVertrel XF.Noattempt wasmadetodetermine thequantity orchemical natureofanypossible impurities associated withthesolvents tested.Theconstruction offurtherbearing testswouldconsider theinclusion ofagreater number ofbearing pairstoincrease thereliability ofthetest,andtheuseofdifferent lotsof HFE-7100. TheuseofHFE-7100DL (harddisklubricant gradewithlowionics,andlessthan1ppm nonvolatile residue) maybeincluded tofurtherinvestigate therolethatimpurity quantity mayplay. 33
Conclusion Theuseofnon-CFC solvents anddiluents compare favorably withthetraditional CFC-113 (Freon) solvent anddiluentandinthistestresulted inimproved ballbearing performance. Theseresultssupport previous findings forVertrelXF,andindicate possible furtherbenefits through theuseofahydrofluoroether diluent ratherthanFreonTFintheprocessing bearings. Acknowledgements Theauthors wishtothankMr.KentRollerforhisexpertise inthepost-test inspection ofthebearing surfaces, aswellasMr.BradScottandMr.HarryPoehlmann fortheircomments inreviewing thispaper. Theauthors alsowanttorecognize thecontributions ofallthemachinists andtechnicians whohelpedto putthistesttogether andkeepitrunning. References 1.Mori,S.,Morales, W.,"Reaction ofPerfluoroalkylpolyethers (PFPE) With440CStainless Steelin Vacuum UnderSlidingConditions atRoomTemperature," NASATechnical Paper2883,1989 2.Carre,D,"ThePerformance ofPerfluoropolyalkylether OilsUnderBoundary Lubrication Conditions," Tribology Transactions, Volume 31,4,pp.437-441 3.Loewenthal, S.H.,Jones, Jr.,W.R.,andPredmore, R.E.,"LifeofPennzane and815ZLubricated Instrument Bearings Cleaned WithNon-CFC Solvents," NASATechnical NoteNASA/TM 1999209392, September, 1999 4.Didziulis, S.V.,Childs, J.L.,andCarre,"Effects ofVarying BallBearing Cleaning Processes on Lubricant Life,"Aerospace Report Number ATR-98(2126)-1, TheAerospace Corporation, El Segundo, CA 5.Jones, Jr.,W.R.,Pepper, S.V.,Jansen, M.J.,Nguyen, Q.N.,Kingsbury, E.P.,Loewenthal, S.H., Predmore, R.E.,"ANewApparatus toEvaluate Lubricants forSpaceApplications -TheSpiralOrbit Tribometer (SOT)," NASATechnical ReportNumber NASA/TM 2000-209935, March,2000 6.Phinney, D.D.,Pollard, C.L.,andHinricks, J.T.,"Experience withDuplex Bearings inNarrow Angle Oscillating Applications," 22ndAerospace Mechanisms Symposium, NASALangley, May4-6,1988 34
Themotorisdrivenopenloopwith2squarewavesinquadrature. Thismakesthedriveelectronics, simple,inexpensive andveryreliable.Theactuallife,on-orbitoftheAVHRRscannerhasaveraged5 years.Failureofthespacecrafthasbeenthemostcommoncausefortheendofscannerlife. Figure1.AVHRR Scanner Table1.AVHRR Scanner Cardinal Requirements Requirement FreeBodyTorque, grossoutput torquedelivered bymotor without losingsynchronism Maximum Allowable Power Rotational JitterValue >0.508N-m 7.5W <34microsecondsComment Whenoperated atasupplyvoltage of20,5to 24.5volts Powermeasured at0.565N-m Synchronization Drift <+3microseconds Measured overa24hourperiod OrbitalLifetime 3years Natural Frequency Temperature>100Hz 10to30C Xaxis:11.44GRMS Yaxis:19.40GRMS Zaxis:8.12GRMSVibrationPeak-to-peak variation ofrotational periodfor 98%ofmeasurements overa20minperiod. Requirement isforthecomplete instrument Operating temperature ofInstrument, measured atthebaseplate TitanIIELVlevels 36
Therelativelylowtorqueoutputcombinedwiththeveryseveredriftandjitterrequirements requirevery highprecisionbearings.Therunningtorqueandtorquehascharacteristics ofthesebearingsmustremain consistentovertime,temperature andvibrationforthescannertooperateproperly.Thescannerisan openloopdevice.Thus,individualparameters mustbecarefullycontrolled.Asaresultofthis,agreat effortismadeinthecleaning,lubrication, andsettingofthepreloadandcontactangleofthebearingsand theinstallationofthebearingsintothescanners. Figure2showsablockdiagramofthescanner.Thisisasimplebutremarkably accuratemodel.This modelisusedtopredictscannerperformance, andhasbeenverifiedbytestresults. Km_t...... I Tvy ! J.: 1/Js_ 1/sn ' ----- /_---J 0 i 1 I Figure2.BlockDiagram ofScanner Basedon"hunting" frequency measurements, takenonascanner, theparameters showninFigure2are defined inTable2.Thehunting frequency isthebasicnatural frequency atwhichtherotoroscillates relative totherotating field. Table2.Scanner MotorParameters Parameter Value 0.0067Units Tv N/A,inputparameter e N/Aoutputparameter radians Kb 0.0036 N-m-s/radians Km 6.3251 N-m/radianDescription Scanner rotating inertia Torque variations frombearing torquehash*,loadvariations etc. Slipanglevariation between motorandrotating electric fieldin windings System damping fromfriction magnetic hysteresis, etc. Electromagnetic "stiffness", essentially therestoring constant between therotorandthe rotating electric field Basedonmeasurements takenonA302scanner *Torque hashisdefined asthenon-repetitive torquevariations thatoccurasabearing rotates. Utilizing thevaluesshowninTable2,aplotofthetransfer function forthedisturbance torqueintothe scanner versusfrequency isshowninFigure3.Ascanbeseen,inFigure3thesystem isverylightly damped. Inthisthedamping isapproximately 1%ofcritical.Typical valuesrangefrom0.5%to1.5%.This meansthatdisturbances atlowfrequencies tendtogreatlyaffectthejitterperformance. 37
ScannerDisturbance TorqueTransferFunction 20.000 t-  0.000  0.( ,E g-20.000 e--e-- <--40.000E;L O9 =-60.000 0 I-- -80.000 Hedz Figure3.Scanner Disturbance Torque Transfer Function Problem Statement Despite theuseofhighprecision bearing andcareful assembly techniques withdocumented work instructions, variations inperformance werenoted.Scanner performance, intermsofjitter,wasfoundto varyfrombearing lottobearing lotandevenfromassembly toassembly usingthesamesetsofpartsand thesameassembly technician. Anexploded viewofthescanner isshowninFigure4.Acrosssection of thescanner showing thebearing retaining system isshowninFigure5. Modaltesting, ontheassembled scanners, showed frequency shifts,asaresultofscanner level vibration. Thefundamental frequency wouldshiftfromapproximately 275Hertzpriortovibration toaslow as225Hertzafterthevibration. Thelossofstiffness sometimes resulted inacondition knowas"howling" basedontheaudible soundheardwhenthescanner wasoperated afteravibration. Thischange in modalfrequency wasindicative ofachange oftheloading onthebearing 1.Thechange inbearing loading resulted inachange inbearing running torquecharacteristics thataffected jitterperformance. Giventhestringent requirements forjitter,achange intheslipangleof6.41e-4 radians (0.04degree) will resultin17microseconds ofjitter.UsingthedatafromTable2,thisisequivalent toatorquevariation Of 4.05e-3 N-m(0.57ozrin)overarevolution. Thistorquecalculation assumes nodynamic amplification, thustheactualallowable torquevariation couldbemuchlessdepending uponthefrequency atwhichit occurs. Analysis indicated thatthevibration loadswereresulting intheyielding ofthebearing retaining system. Asaresultofthisanalysis, material changes wereproposed inpartsofthebearing retaining system. The bearing system partandtheoriginal andhigherstrength materials areshowninTable3. 38