Developments-for-High-Performance-Machine-Tool-Spi

AnnalsoftheCIRPVol.56/1/2007-395-doi:10.1016/j.cirp.2007.05.092DevelopmentsforHighPerformanceMachineToolSpindlesC.Brecher1(2),G.Spachtholz1,F.Paepenmüller11LaboratoryforMachineToolsandProductionEngineering,Aachen,GermanyAbstractOneimportantdemandonspindlesystemsinmodernmachinetoolsistorealisehigherrotationalspeedsinordertoincreasethemachiningefficiency.Additionally,foragivenspeedrangeabetterrobustnessisdemandedsothatthespindlesystemisdesensitisedagainstimproperoperatingconditions.Thepaperpresentsresearchresultsinvariousfieldswhichcontributetotheimprovementofspindle-bearingsystems.Atfirst,newresultsforalternativespindlebearingkinematicswithfourcontactpointsarepresented.Secondly,anewsolutionforfloatingbearingarrangementsisdiscussed.Amodifiedcylindricalrollerbearingispresentedwhichcanbeoperatedathigherspeeds.Finally,thepotentialofcoatedbearingcomponentsisdiscussedinthecontextofimprovedfail-safeproperties.Inthispaperbothanalyticstudiesandexperimentaltestsarepresented.Keywords:Machine,Spindle,Bearing1INTRODUCTIONTheproductivityofmodernmachinetoolsismainlydeterminedbytherotationalspeedlimitsandtheloadcarryingcapacitiesoftheirmainspindleunits.Ontheonehand,themachiningofaluminiumormagnesiumwithmoderncuttingtools,equippedwithcubicboronnitride(CBN)orpolycrystallinediamond(PCD)inserts,allowscuttingratesfrom5,000m/minupto10,000m/min.Inthecaseoftheapplicationofendmillswithdiametersbetween20and30mmtherealisationoftheseveryhighcuttingspeedsrequiresspindlespeedsofmorethan100,000rpm.Accordingtotheactualstateoftheartofrollingbearingtechnology,thisdemandcancurrentlyonlyberealisedbyspindlebearingswithameandiameterof30mm.However,duetotheseextremeoperatingconditions,allfunctionalcomponentsofamainspindleunit–thespindlebearings,therotorofthemotoraswellastherotatingunions–areloadeduptotheirphysicallimits.Ontheotherhand,themainspindlesalsoneedtobesuitableforversatilemachinetoolapplications.Thesearecharacterisedbyvaryingdemands.Theroughmachiningofsteel,forexample,ischaracterisedbyhighcuttingforcesandmomentsandmoderaterotationalspeeds.Inthosecases,biggerspindleandbearingdiametersareessentialtobeartheseloads.Thefollowingbasicapproachesforthedesignofmainspindlescanbederivedfromthedivergingdemandspresentedabove.Tofulfilthosedemands,thecharacteristicspeedcoefficientnxdmhastobeincreasedupto3.5–4.0x106mm/min,ensuringasufficientstiffnessandrobustnessofthespindlebodyandthespindlebearings.Figure1presentsatypicalmotorspindlewithapoweroutputof80kWandamaximumrotationalspeedof30,000rpm.Thestatorofthedriveiswater-cooled.Thespindlebodyhasahollowshafttaperandisrotationallysupportedbyanelasticallypreloadedback-to-backspindlebearingarrangement.Inordertodevelopanimprovedspindleandbearingdesigntheoptimisationofthefixedbearingunit(variedelasticallypreloadedback-to-backarrangementHSKA63/80bearingdiameter:70mmmax.rotationalspeed:30,000rpmsource:WeissGmbHFigure1:Millingmotorspindlewithrollingbearings.innerbearinggeometry),ofthemovablebearingunit(elasticcylindricalrollerbearings)aswellasofthetribologicalproperties(surfacecoatings,lubrication)shallbeanalysed.Thesetopicsarediscussedinthefollowing.2MULTIPOINT(3P,4P)-SPINDLEBEARINGS2.1MotivationfortheOptimisationoftheBearingGeometrySpindlebearingsfortheapplicationinmainspindlesofmodernmachinetoolshavetofulfilhighestdemandsonrunningaccuracyandstiffness.Inthepast,variousmodificationshavebeendevelopedinordertoimprovethebearingperformance.Amongothers,onecanenumeratespeciallubricantsuppliesthroughtheouterring,smallerorceramicballsaswellasoptimisedcages.Nevertheless,highestspeedsextremelyreducethelifetimeofspindlebearings.Theunderlyingmaineffectsduringtheoperationprocesswereinvestigatedbyvariousauthorsandwere,e.g.,summarisedin[1],[2],[3],[4].Especially,thespeeddependentdeviationofthecontactanglesontheinnerandouterringcausesadecreaseofaxialandradialstiffness.Inaddition,thecontactareason-396-theouterringarestronglyloadedbythecentrifugalforcesactingontheballs.Wecketal.presentedin[1]spindlebearingswithvariedinnergeometriesinordertoreducethenegativeeffectsofthecentrifugalforcesactingontheballs.Inaddition,therobustnessofthebearingsshouldbeimproved.Wecketal.providedadditionalcontactpointsontheracewaysoftheinnerandouterringofconventionalspindlebearings.Duetothis,theaxialandradialmovementoftheballsispreventedandconstantcontactanglesandareducedaxialdisplacementoftheinnerringcanbeensuredoverawidespeedrange.Thebearingconceptsintroducedin[1]areshowninFigure2(a),(b).Figure2(c)presentsathird,novelconceptintroducedinthispaper.(a)Multipoint(3P)-bearing(b)Multipoint(4P)-bearing(c)Multipoint(4P)-bearing,internalspringpreloadFigure2:Differentalternativesofmultipoint(3P,4P)-spindlebearings.2.2AnalyticInvestigationofMultipoint(4P)-BearingsThecontentsof[1]focusontheoperatingbehaviourofmultipoint(3P)-bearings,boththeoreticandexperimentalinvestigation.ThetestbearingsweremanufacturedattheLaboratoryforMachineToolsandProductionEngineering(WZL)basedonconventionalspindlebearings.Alsosomecharacteristicsofmultipoint(4P)-Bearingswereanalysedbymeansofn