空气轴承基本原理

1、AirBearingBasicsWhatareAirBearings?FluidFilmRollingElementBearingBearingBasically,airbearingsuseathinfilmofpressurizedairtosupportaload,thesamewaythepuckonanairhockeytablefloatsonair.Thistypeofbearingiscalledafluidfilmbearing.Fluidfilmbearingshavenosolid-to-solidcontactundertypicalrunningconditions;

2、instead,afilmoflubricatingfluid(inourcasepressurizedair)formsalayerbetweenthesolidmachineelementsandservestotransferforcesfromonetotheother.Tocomparethiswithballbearings,inballbearingstheballsareinconstantcontactwithandformasolidbridgebetweenthemachineelements.ApproximatePressureProfileinanAirBearin

3、gThefluidisabletotransferforcesbecauseasthefluidispushedthroughthebearinggapitgeneratesapressureprofileacrossthebearingarea.Theforcethebearingcansupportisthen:AirBearingBenefits:NosolidtosolidcontactNowear,particlesorlubricantsZerostaticfrictionZerorunningfrictionHighStiffnessSelfCenteringErrorAvera

4、gingForce=Avg.PressurexAreaTheactualpressuredistributioninthebearingvariesbasedonthebearingdesignandotherparametersbutagoodruleofthumbistoexpect30%efficiency.Force=0.3xSupplyPressurexAreaForflatpadsandrectangularbearingsthiscalculationisquitestraightforward,butforjournalandsphericalbearingsweuseafew

5、otherapproximationstoestimateloadcapacity.Fluidfilmbearingsofferanumberofadvantagesovermechanicalbearings.First,becausethereisnocontact,airbearingsdonotsufferfromwearorheatgenerationduetofriction.Theyalsoexhibitnostartingorrunningfriction,evenundertheirhighestdesignloading.Inaddition,thefluidfilmact

6、stocenterandaverageoutsmallscaleerrorsinthecomponentsresultinginmotionwhichcanbemoreaccuratethantheindividualbearingcomponents.Airbearingsalsooffermuchhigherstiffnessthanrollingelementbearingsbecausetheairfilmfullysupportsthecomponents,asopposedtoballsorrollerswhichhavepointorlinecontactandaretheref

7、orelimitedduetoHertziancontactstiffness.WhyUseAir?Whilemostpeoplearefamiliarwithoilfluidfilmbearings-forexamplethecrankshaftjournalbearingsincarengines-mostpeoplehavenotbeenexposedtoairbearings.Rememberingourhighschoolphysicsclass,therearetwobasictypesoffluids-liquidsandgasses.Intermsoffluidfilmbear

8、ings,thedifferencebetweenthesetwoisessentiallytheviscosity-liquidshavemuchhigherviscositythangasses.Whenappliedtoafluidfilmbearing,thisdifferencehasanumberofimplications.Whyweuseair(insteadofoil):Airisclean,contaminantfreeZerofriction-noheatgenerationathighspeedsSmallbearinggapmeanshighaccuracycompo

9、nents,highaccuracymotionAirnitrogenofanyothergascanbeusedFirst,lowerviscositymeansthatforthesameworkingpressuregasbearingshavelowerloadcapacity(liquidfluidfilmbearingstypicallysupportfivetimestheloadofgasbearingsforthesamepadarea).Second,becauseoftheextremelylowviscosityofgasses,gasfilmbearingsopera

10、tewithessentiallyzerostaticandrunningfrictionwhereliquidfluidfilmbearingshavemuchhigherfrictionandpumpinglosseswithinthebearings,whichcancauseheatgeneration.Andthird,gasbearingsrequireverytightbearinggapsforproperoperation(10pmforgascomparedtoupto100pmforliquidbearing)whichtranslatesintoextremelyhig

11、haccuracyrequirementsonthecomponents.Whatthismeansforairbearingsisthatalthoughtheyhavealowerloadcapacity,gasbearingshaveessentiallyzerofrictionatallspeedsandbecausethetightbearingclearancesdemandhighaccuracycomponentsthisresultsinextremelyhighaccuracymotion.Anotherbenefitisthecleanlinessofusingairas

12、alubricatingfluidasopposedtooil,water,oranotherfluid.Sincecompressedairisverycommoninindustrialenvironmentsitisprobablythemostoftenusedgas,howeverothergassessuchasnitrogencanbeusedwheretheyareavailable(suchasincleanroomenvironments).Hydrostaticvs.Hydrodynamic?Thefirstquestionishowtogeneratethepressu

13、rizedsupplyoffluidforthebearing.Therearetwowaystodothis-oneisusinganexternalpressurizedsupply(hydrostatic)andtheotheristousetherelativemotionofthemachinecomponentstogeneratethepressureinternally(hydrodynamicorself-generating).Whilehydrodynamicbearingsarecommonforoilfluidfilmbearings,whichgenerateint

14、ernalpressuresquiteeasilyduetotherelativelyhighviscosityofoil,itismuchrarertoseethistechniqueusedforairbearingsbecausethepressuregeneratedisquitelow(althoughNelsonAirhasbuiltbearingsofthistypeforlowload,highspeedrotaryapplicationssuchasopticalscanners).Hydrodynamic:RelativeMotionGeneratesPressureHyd

15、rostatic:ExternalPressureSupplyOurtypicalairbearingsarehydrostatic(orinourcaseaerostatic)anduseacompressedairsupplytocreatethefluidfilm.Thissupplyshouldbecleanandregulatedtoaconstantpressure-simpleofftheshelfairhandlingcomponentsaremorethanadequateformostapplicationstoclean,dehumidify,andregulatethe

16、supply.Typicaloperatingpressuresrunaslowas20psiupto120psidependingonthestiffness,loadcapacity,andairconsumptionrequirements.Howdoestheairgetintothebearing?PorousCarbonFlatPadDrilledNozzleswithPocketedCompensation(Setbacks)JewelNozzlewithPocketedCompensationAfterenteringthebearingandbeingroutedthough

17、internalpassages,thenextstepistofeedthepressurizedairtowhereitisneeded-namelydirectlyintothebearinggap.Therearetwobasicwaystoaccomplishthis,orificesandporousmedia.Fororifices,theairflowsthroughasmallhole(typically.004to.015dia.)intothebearingarea.Porousmediabearingsuseaporousmaterial(typicallycarbon

18、,bronze,orsteel)whichtheairpenetratesthroughintothebearingarea.POROUSMEDIAFEEDINGORIFICEFEEDINGPOROUSMEDIAFEEDINGORIFICEFEEDINGTherelativebenefitsofeachapproacharedebatable.Whileorificestypicallydonotgenerateasuniformapressureprofileascanbeachievedwithporousmedia,therearewaystoimprovethepressureprof

19、ileusingatechniquecalledpocketedcompensation.Porousmediafeedingprovidesmoreinherentdampingthanorifices,howeverpropersizingoftheorificescanadjustdamping.Orificescanbecomepluggedifaverylargeparticlegetsintotheairsupply(thisisextremelyrare),wheretheporousmediaactsasafilterfortheair.However,becauseofthi

20、sfilteringeffect,overtimetheporousmediaitselfcanbecomeclogged(oilvaporintheairisparticularlybad)andacloggedporousbearingismuchmoredifficulttoclearthanacloggedorifice.Orificefedbearingscanbemadefromonepieceofmaterial(particularlywhenusingdrillednozzles)whereporousbearingsarenecessarilymadeusingvaryin

21、gmaterialsandadhesives.AtNelsonAirwehaveusedbothorificeandporousmediafeeding.Mostofourstandardproductsusepocketcompensatedorifices,howeverourLBseriesoflinearbushingsuseporousbronze.Ourtypicalorificebearingsaremadeusingaspecialtechniquewehavedevelopedtodrillthesmallorificesdirectlyintothebearing,inst

22、eadofusingpressedinorgluedinjewelnozzles.Webelievethatthistechniquereducescostandcomplexitywhileincreasinglongtermstabilityandreliability.However,jewelednozzlesareusedwhenthedesignprecludesdrillednozzleseitherbecauseofgeometryorbecauseespeciallysmallorificesizes(.004forexample)arerequired.Geometry,G

23、eometry,Geometry!AirFilmAveragesoutLocalVariationsBecauseoftheverysmallgapsthatarerequiredforairbearings,thesinglemostimportantfactorintheirmanufactureisabsolutelyflawlessgeometry.Anyvariationinpartsize,straightness,squarenessoroutofroundnesswillresultinaclosingofthegapandwillcausethebearingtoloseit

24、sfrictionlessnatureaswellasdegradeaccuracyandloadcapacity.Typicaltolerancesonourrectangularbearingswhichrunairgapsof0.0005arepartdeviationslessthan.0001overall.Thissametoleranceappliesovertheentirelengthofthebearing,whichforsomemodelscanbeover60.Inaddition,thepartgeometryiswhatdeterminestheaccuracyo

25、fthefinalbearingmotion-eventhoughlocaldeviationsareaveragedoutbytheairfilm,theoverallaccuracyisdictatedbyhowwellthepartsaremade.Becauseofthis,NelsonAirhasdevelopedavarietyoftechniquesandequipmentwhichallowustomeetthesestringenttoleranceswithrepeatabilityandrelativelylowcost.OnAccuracy,PrecisionandRe

26、peatabilityAccuracyPrecisionRepeatabilityTypicalAirBearingSpecs:LinearAccuracy:10pin/in,100pin/36inRotaryAccuracy:1pinTIRRoll/Pitch/Yaw:0.25arcsec/inLinearRepeatability:10pinRotaryRepeatability:1pinPositioningResolution:to1encodercountTostartthisdiscussionitisbesttobrieflydefinewhataccuracy,precisio

27、nandrepeatabilitymean.Accuracyisthemeasureofhowcloselythemotionfollowsanidealprofile.Precisionisthemeasureofhowsmallthemotionincrementscanbe.Repeatabilityisthemeasureofhowwellthemotionmovesalongthesamepath.Airbearingsofferthebestofallworldsbecausetheyhaveexcellentaccuracy,precisionandrepeatability.T

28、heyofferhighaccuracybecausethecomponentsaremanufacturedwithextremelytighttolerancesandbecauseoftheairfilmsaveragingeffects.Typicallinearaccuraciesare10pin/inwithmaximumdeviationof100pin/36in.Rotaryrunoutcanbeaslow1pin.Airbearingsalsoofferextremelygoodtiltcharacteristics-0.25arcsec/in.Becauseairbeari

29、ngsexhibitnostictionorrunningfriction,theprecisionachievableisonlylimitedbytheabilitiesofthemotor/controller/encoder.Positioningprecisionof1encodercountcaneasilybeachievedusinglinearmotors.Therepeatabilityofairbearingsfarexceedsthatofmechanicalbearingsforthesimplereasonthattheyhavenocontactorwear.He

30、atgenerationinmechanicalbearingssometimesrequirewarmupperiodsduringwhichthereisthermalmovement.Also,themechanicalpreloadonrollerbearingscanvarywithtemperatureandcausethebearingstooperatedifferently(varyingfrictionforexample)andlackrepeatability.Whenworkingatthehighestaccuracylevels,theseeffectscanca

31、usemechanicalbearingstolosetheiraccuracyfromhourtohour,daytoday,ormonthtomonth.Becausetheaccuracyofourairbearingsisbuiltintoitssolidmetalcomponents(madefromonetypeofmaterialforuniformthermalexpansion),theycanoperateforyearswithoutanydegradationinaccuracy.Thisway,acalibrationdoneonedaywillstick.Theya

32、lsocanberunattopspeedwithoutanyneedforwarm-uptime(excludingmotorheatingeffects)andmaintaintheirbestaccuracy.PreloadingAirBearingsMethod1:WeightusedtopreloadbearingVACUUMVACUUMMethod4:OpposingAirBearingPreloadWhilesomeconfigurationsofairbearingsareself-preloading(e.g.journalbearings)otherssuchasflatp

33、adsandlinearrectangularbearingscanbenefitfrompreload.Airbearingscanbeusedwithoutanypreload(suchasasimpleflatpadridingonagranitesurface),however,inordertomaximizethestiffnessoftheairbearingandhelpmaintainconstantairgapitistypicaltopreloaditusingoneoffourbasicmethods:WeightPreloadVacuumPreloadMagnetic

34、PreloadOpposingAirBearingPreloadThesimplestmethodisweight.Usingaweightmuchheavierthantheexpectedvariationintheloadingofthebearingpreloadstheairbearingsoitridesatasmaller(andstiffer)airgapandmakesitlesspronetoheightvariations.Thismethodhasthedrawbackofaddingmovingmassbutcanworkverywillwithsystemswhic

35、halreadyhavealargeamountofmassandlowforces(suchaspartsinspection).Needlesstosay,thisonlyworksforbearingswhichoperatehorizontally.Thesecondmethodisusingavacuumpreload.Avacuumisusedtoapplyapreloadingforcetotheairbearing(insteadofusingaweight).Thisisaccomplishedbyirprovidinganareaofthebearingsurfaceove

36、rwhichavacuumisapplied.Becausevacuumislimitedtoaroundnegative14psi(atmosphericpressure)andtypicalabearingpressuresare40-80psitheneteffectisstillaliftingofthebearing,evenwhenthebearingandvacuumareasareequal.Vacuumpreloadenhancesthestiffnessandhelpstomaintainconstantairgap,withoutaddingunnecessarymovi

37、ngmass.Themaindrawbackstothismethodaretheneedforalargerbearingareatoaccommodatebothpressureandvacuumandtherequirementofavacuumsource.Thismethodhasbeenusedsuccessfullyinmanyapplications,especiallyforflatpadsandplanarsystemswhichdonotlendthemselvestootherformsofpreload.irThethirdmethodofpreloadingisma

38、gneticpreload.Magneticattractionbetweenamagnetonthemovingpartandamagneticmaterialonthestationarypartofthebearingloadsthebearingandaddsstiffness.Thisconfigurationworkswellforlinearbearingsandcanbeverycosteffectivebecauseitreducesthetolerancesrequiredfortheairbearingcomponents(ascomparedtoairbearingpr

39、eloadedsystems).However,becausemanyairbearingsaremadefromnonmagneticmaterials,itrequiresaddingothermaterialstothebearing(suchasiron).Anotherdrawbackisthatathighspeeds,themagnetgenerateseddycurrentsintheironwhichopposethemotionandaddadragforce(eliminatingthezerofrictionofairbearings).However,inmanyapplicationsmagneticpreloadprovidesaneffectivemethodofpreloadingairbearings.Thefinalmethodisopposin