机械外文翻译实验研究激光加工表面微观造型平行的推力轴承Word文件下载.docx
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resultsarecomparedwithatheoreticalmodelandgoodcorrelationisfoundovertherelevantoperatingconditions.Acompari-
sonoftheperformanceofunidirectionalandbi-directionalpartial-LSTbearingswiththatofabaseline,untexturedbearingis
presentedshowingthebenefitsofLSTintermsofincreasedclearanceandreducedfriction.
KEYWORDS:
fluidfilmbearings,sliderbearings,surfacetexturing
1.Introduction
Theclassicaltheoryofhydrodynamiclubricationyieldslinear（Couette）velocitydistributionwithzeropressuregradientsbetweensmoothparallelsurfacesundersteady-statesliding.Thisresultsinanunstablehydrodynamicfilmthatwouldcollapseunderanyexternalforceactingnormaltothesurfaces.However,experienceshowsthatstablelubricatingfilmscandevelopbetweenparallelslidingsurfaces,generally
becauseofsomemechanismthatrelaxesoneormoreoftheassumptionsoftheclassicaltheory.
Astablefluidfilmwithsufficientload-carryingcapacityinparallelslidingsurfacescanbeobtained,forexample,withmacroormicrosurfacestructureofdifferenttypes.Theseincludewaviness[1]andprotrudingmicroasperities[2–4].AgoodliteraturereviewonthesubjectcanbefoundinRef.[5].Morerecently,lasersurfacetexturing（LST）[6–8],aswellasinletrougheningbylongitudinalortransversegrooves[9]weresuggestedtoprovideloadcapacityinparallelsliding.TheinletroughnessconceptofTonder[9]isbasedon‘‘effectiveclearance’’reductioninthesliding
directionandinthisrespectitisidenticaltothepar-tial-LSTconceptdescribedinref.[10]forgeneratinghydrostaticeffectinhigh-pressuremechanicalseals.
VeryrecentlyWangetal.[11]demonstratedexperimentallyadoublingoftheload-carryingcapacityforthesurface-texturedesignbyreactiveionetchingofSiCparallel-thrustbearingsslidinginwater.Thesesimpleparallelthrustbearingsareusuallyfoundinseal-lesspumpswherethepumpedfluidisusedasthelubricantforthebearings.Duetotheparallelslidingtheirperformanceispoorerthanmoresophisticatedtaperedorsteppedbearings.Brizmeretal.[12]demon-stratedthepotentialoflasersurfacetexturingintheformofregularmicro-dimplesforprovidingload-carryingcapacitywithparallel-thrustbearings.Amodelofatexturedparallelsliderwasdevelopedandtheeffectofsurfacetexturingonload-carryingcapacity
wasanalyzed.Theoptimumparametersofthedimpleswerefoundinordertoobtainmaximumload-carryingcapacity.Amicro-dimple‘‘collectiveeffect’’wasidenti-
fiedthatiscapableofgeneratingsubstantialload-carryingcapacity,approachingthatofoptimumconventionalthrustbearings.ThepurposeofthepresentpaperistoinvestigateexperimentallythevalidityofthemodeldescribedinRef.[12]bytestingpracticalthrustbearingsandcomparingtheperformanceofLSTbearingswiththatofthetheoreticalpredictionsandwiththeperformanceofstandardnon-textured
bearings
2.Background
AcrosssectionofthebasicmodelthatwasanalyzedinRef.[12]isshowninfigure
1.AsliderhavingawidthBispartiallytexturedoveraportionBp=αBofitswidth.Thetexturedsurfaceconsistsofmultipledimpleswithadiameter
，depth
andareadensitySp.Asaresultofthehydrodynamicpressuregeneratedbythedimplestheslidingsurfaceswillbeseparatedbyaclearance
dependingontheslidingvelocityU,thefluidviscositylandtheexternalload
ItwasfoundinRef.[12]thatanoptimumratioexistsfortheparameter
thatprovidesmaximumdimensionlessload-carryingcapacity
whereLis
thebearinglength,andthisoptimumvalueishp=1.25.ItwasfurtherfoundinRef.[12]thatanoptimumvalueexistsforthetexturedportionadependingonthebearingaspectratioL/B.Thisbehaviorisshowninfigure2forabearingwithL/B=0.75atvariousvaluesoftheareadensitySp.AscanbeseenintherangeofSpvaluesfrom0.18to0.72theoptimumavaluevariesfrom0.7to0.55,respectively.Itcanalsobeseenfromfigure2thatfora<
0.85nooptimumvalueexistsforSpandthemaximumloadWincreaseswithincreasingSp.Hence,thelargestareadensitythatcanbepracticallyobtainedwiththelasertexturingisdesired.Itisalsointerestingtonotefromfigure2theadvantageofpartial-LST（a<
1）overthefullLST（a=1）forbearingapplications.AtSp=0.5,forexample,theloadWata=0.6isaboutthreetimeshigherthanitsvalueata=1.AfullaccountofthisbehaviorisgiveninRef.[12].
3.Experimental
ThetestedbearingsconsistofsinteredSiCdisks10mmthick,having85mmouterdiameterand40mminnerdiameter.Eachbearing（seefigure3）comprisesaflatrotor（a）andasix-padstator（b）.Thebearingswereprovidedwithanoriginalsurfacefinish
bylappingtoaroughnessaverageRa=0.03lm.Eachpadhasanaspectratioof0.75whenitswidthismeasuredalongthemeandiameterofthestator.Thephotographsoftwopartial-LSTstatorsareshowninfigure4wherethetexturedareasappearasbrightermattsurfaces.Thefirststatorindicated（a）isaunidirectionalbearingwiththepartial-LSTadjacenttotheleadingedgeofeachpad,similartothemodelshowninfigure1.Thesecondstator（b）isabi-directionalversionofapartial-LSTbearinghavingtwoequaltexturedportions,a/2,oneachofthepadends.Thelasertexturingparameterswerethefollowing;
dimpledepth
dimplediameter
anddimpleareadensitySp=0.60.03.Thesedimpledimensionswereobtainedwith4pulsesof30nsdurationand4mJeachusinga5kHzpulsatingNd:
YAGlaser.Thetexturedportionoftheunidirectionalbearingwasa=0.73andthatofthebi-directionalbearingwasa=0.63.Ascanbeseenfromfigure2boththeseavaluesshouldproduceload-carryingcapacityvaryclosetothemaximumtheoreticalvalue.Thetestrigisshownschematicallyinfigure5.An
electricalmotorturnsaspindletowhichanupperholderoftherotorisattached.Asecondlowerholderofthestatorisfixedtoahousing,whichrestsonajournalbearingandanaxialloadingmechanismthatcanfreelymoveintheaxialdirection
.Anarmthatpressesagainstaloadcellandtherebypermitsfrictiontorquemeasurementspreventsthefreerotationofthishousing.Axialloadingisprovidedbymeansofdeadweightsonaleverandismeasuredwithasecondloadcell.Aproximityprobethatisattachedtothelowerholderofthestatorallowson-linemeasurementsoftheclearancechangebetweenrotorandstatorasthehydrodynamiceffectscauseaxialmovementofthehousingtowhichthestatorholderisfixed.Tapwaterissuppliedbygravityfromalargetanktothecenterofthebearingandtheleakagefromthebearingiscollectedandre-circulated.Athermocoupleadjacentto
theouterdiameterofthebearingallowsmonitoringofthewatertemperatureasthewaterexitthebearing.APCisusedtocollectandprocessdataon-line.Hence,theinstantaneousclearance,frictioncoefficient,bearingspeedandexitwatertemperaturecanbemonitoredconstantly.
Thetestprotocolincludesidentifyingareference“zero”pointfortheclearancemeasurementsbyfirstloadingandthenunloadingastationarybearingoverthefullloadrange.Thenthelowestaxialloadisapplied,thewatersupplyvalveisopenedandthemotorturnedon.Axialloadingisincreasedbystepsof40Nandeachloadstepismaintainedfor5minfollowingthestabilizationofthefrictioncoefficientat
asteady-statevalue.Thebearingspeedandwatertemperaturearemonitoredthroughoutthetestforanyirregularities.Thetestendswhenamaximumaxialloadof460Nisreachedorifthefrictioncoefficientexceedsavalueof0.35.Attheendofthelastloadstepthemotorandwatersupplyareturnedoffandthereferencefortheclearancemeasurementsisrechecked.Testsareperformedattwospeedsof1500
and3000rpmcorrespondingtoaverageslidingvelocitiesof4.9and9.8m/s,respectivelyandeachtestisrepeatedatleastthreetimes.
4.Resultsanddiscussion
AsafirststepthevalidityofthetheoreticalmodelinRef.[12]wasexaminedbycomparingthetheoreticalandexperimentalresultsofbearingclearanceversusbearingloadforaunidirectionalpartial-LSTbearing.Theresultsareshowninfigure6forthetwospeedsof1500and3000rpmwherethesolidanddashedlinescorrespondtothemodelandexperiment,respectively.Ascanbeseen,theagreementbetweenthemodelandtheexperimentisgood,withdifferencesoflessthan10%,aslongastheloadisabove150N.Atlowerloadsthemeasuredexperimentalclearancesaremuchlargerthanthemodelpredictions,particularlyatthehigherspeedof3000rpmwhereat120Nthemeasuredclearanceis20lm,whichisabout60%higherthanthepredictedvalue.Itturnsoutthatthecombinationofsuchlargeclearancesandrelativelylowviscosityofthewatermayresultinturbulentfluidfilm.Hence,theassumptionoflaminarflowonwhichthesolutionoftheReynoldsequationinRef.[12]isbasedmaybeviolatedmakingthemodelinvalidespeciallyatthehigherspeedandlowestload.InordertobeconsistentwiththemodelofRef.[12]itwasdecidedtolimitfurthercomparisonstoloadsabove150N.
ItshouldbenotedherethatthefirstattemptstotestthebaselineuntexturedbearingwiththeoriginalsurfacefinishofRa=0.03lmonboththestatorandrotorfailedduetoextremelyhighfrictionevenatthelowerloads.Ontheotherhandthepartial-LSTbearingransmoothlythroughouttheloadrange.Itwasfoundthatthepost-LSTlapp
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