Cool running Hybrid tapered roller ­spindle bearings

Abstract

Spindle heating can be a problem for equipment that dry machines gear teeth. These large­bore hybrid tapered roller spindle bearings operate at temperatures between 90 and 100°F, significantly cooler than all­steel bearings. The hybrid bearings can also last significantly longer than all­steel equivalents under the same operating conditions.

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8/17/2016 Coolrunning
http://machinedesign.com/print/archive/coolrunning 1/4
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Coolrunning
MachineDesign
Staff
Staff
Thu,2006101211:30
Hybridtaperedrollerspindlebearingshelpmachinetoolscutwithoutcuttingfluids.
AnthonyRomero
SeniorApplication
Engineer
MachineTool
TheTimkenCo.
Torrington,Conn.
Spindleheatingcanbeaproblemforequipmentthatdrymachinesgearteeth.Theselargeborehybridtapered
rollerspindlebearingsoperateattemperaturesbetween90and100°F,significantlycoolerthanallsteelbearings.
Thehybridbearingscanalsolastsignificantlylongerthanallsteelequivalentsunderthesameoperating
conditions.

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ThehybridbearingscomeinprecisionClasses00and000,providingarotationalaccuracybelow1μm(TIR)and
nonsynchronousrunoutof<0.5μm.Thelattermetriciskeytoaccurateandrepeatablecuttingprocesses.Timken
creditstighterbearingtoleranceswiththeimprovedrotationalaccuracy.
Machinetoolsareoneofthemostchallengingenvironmentsforbearings.Thisisparticularlytrueforgearcutting
equipment.Suchmachinesputsignificantloadsonspindlebearingswhentheysimultaneouslycutmultiplegear
teethathighmetalremovalrates.Spindlebearingsalsoneedawidespeedrangebecausebevelandhypoidgear
cutterstypicallyrunbetween200to4,500rpm,dependingontheoperationandworkpiecesize.
Thereisalsoapushtoeliminatecuttingfluidsintheinterestofloweringmaintenanceanddisposalcosts.Suchdry
machiningistypifiedbyfastmetalremovalrates,whichnecessitatesanextremelyrigidspindleabletohandleboth
highrpmandhorsepower.Machiningwithoutcuttingfluidscansetuplargethermalgradientsbetweenthespindle
andtooling,makingitdifficulttoholdsizeonmachinedparts.Standardspindlebearingsundertheseconditions
mayreachtemperaturesabove150°F,exacerbatingtheproblem.
Butspecialhybridceramictaperedrollerbearingsmeetalltheabovedesigncriteriaand,underthesame
conditions,runattemperaturesbetween90to100°F.Thebearingsincorporateceramic(siliconnitride)
taperedrollingelementsinaprecisionclassbearing.Ceramichasamodulusofelasticity50%greaterthansteel,
whichboostsbearingrigidity.Thematerialalsohasalowercoefficientoffrictionandanextremelyfinesurface
finishthatlessensfrictionaltorqueandhelpsthebearingsruncooler.

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Withoptimizedraces,ribs,rollingelements,andspecialhighspeedsyntheticgrease,thehybriddesigncan
outperformconventionalbearingsathighermachiningspeeds,whilemaintainingahighstiffnessandloadcapacity
atlowrpm.Asinglebearingtypegenerallydoesn'tworkwellinbothoperatingregimes.Ballbearings,forexample,
tendtoruncoolerthanotherbearingsathigherrpm(above750,000DN).Taperedrollersareabetterchoicewhen
spindlestiffnessandloadcarryingcapacityarekeydesignconsiderations.
Comparedwithotherbearingsinspindleapplications,suchasangularcontactballbearings(pointcontact)or
cylindricalrollerbearings(linecontact),taperedrollerbearingshaveasignificantlyhigherradialstiffnessandare
lesssusceptibletooverload.Hybridtaperedrollerbearingsalsosimplifyspindledesign.Spindlesneedjusttwoof
thebearings—oneoneachend.Forcomparison,spindlesthatusecylindricaldoublerowbearingsneedoneat
bothendsandanaxialangularcontactbearinginthemiddletoachievesimilarrigiditylevels.
Testsshowtaperedrollerbearings(atzeroclearance)havearadialstiffness4to6thatofcomparablysized
angularcontactballbearings,andtwiceasmuchascylindricalrollerbearings.Taperedrollerbearingsuseangled
racewaystocarrybothradialandthrustloads.Asarule,ashallowtaperisusedforheavyradialloads,andasteep
taper,forheavythrustloads.ThehybridbearingisdesignedwithanangularityorKfactorspecifictothegear
cuttingapplication.Ittakesintoaccountpreload,externalcuttingloads,andoperatingspeed.
Anothertaperedrollerbearingqualitycalledtruerollingmotionfurtherimprovesbearingperformance.Standard
classbearingshavecrownedorotherprofilesonracewaysandrollerstominimizecontactstressattherollerends
underheavyloads.Loadsonmachinetools,incontrast,arebettercharacterizedsothespindlebearingsinsteaduse
astraightraceprofile.Suchprecisionclassbearingsexhibittruerollingmotionofrollersonraceways.Truerolling
motionhelpsbearingsruncoolerandboostsspindlestiffnessandaccuracy.Thismotionistheresultoftwodesign
features:rollertaper;andthecontactbetweentheraceribandthesphericalsurfacegroundonthelargeendofthe
rollers.
Rollersaredesignedsuchthatextensionsofthelinesalongtherollerbodiesconvergetowardthecenterlineofthe
bearingandmeetatanapexonthiscenterline.Theresult:norelativeslipbetweentherollersandraces.The
arrangementalsogeneratesaforcethatseatstherollersphericalendagainsttheracerib.Thisseatingforceisa
functionofthedifferentanglesoftheouterandinnerracesandisdesirablebecauseithelpskeeprollersfrom
skewingoffapex.Alackofskewensurespositiverolleralignment.This,inturn,boostsstiffnessandaccuracy,and
extendsbearinglife.
Analyticalmodelsoptimizetheapexanglesobearingsaccommodateradialandaxialloadsgeneratedduringthe
gearcuttingprocess.Thishelpsmaintainproperrolleralignmentatlowrpmandcontrolsskidding.Modifications
totherib/rollerinterfacefurtherhelpmaintaincontrolatlowrpm.
Bearingstiffnessalsodependsonthebearingloadzone,whichisdirectlyrelatedtobearingsettingandapplied
loads.Settinginataperedrollerbearingsystemcanbedefinedastheamountofaxialclearance(endplay)oraxial
interference(preload)withinamountedsetofbearings.Itistypicallymeasuredintheaxialdirectionbecausethisis
themoststraightforwardwaytoestablishanoptimumvalue.
Aconventionaltaperedrollerbearingwithzeroendplayhasaloadzonecloseto180°.Thespecialhybridbearing,
incontrast,hasaminimum200°loadzoneforaddedrigidity.Settingiskeytothebearingfullybenefitingfromthe
extendedloadzone.Settingvariationcausedbythermalexpansionofthespindlebearinghousingsystemdirectly
affectsspindlestaticanddynamicstiffness.
Itisgenerallyagreedthattheoptimumsettingisthemountedendplaythatgivesmaximumbearinglife.However,
machinetoolandotherapplicationsmayemphasizesystemstiffnessandheatgeneration.Inthiscontext,the
optimumsettingisonethatresultsinminimumdeflection,counterbalancedwithmaximumbearinglife.Oftenin
theseapplications,bearingloadcapacityrelativetoafatiguespalllifecriterioniswellinexcessofthemachine
designlife.

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Inpractice,endusersinstallandpreloadthebearingstomanufacturerspecs.Afixtureappliespushpullforces
equalto3suggestedpreloadandmeasurestheresultingshaftdisplacement.Theoperationtypicallytakesplaceina
temperaturecontrolledcleanroom.Thisgivestotalendplayofthebearingsystemonthespindle.Specialsoftware
usesthedeflectiondatatocalculateapushpullconstantandaspacerlengththatgivestherightdimensional
preload.Dimensionalpreloadfortaperedrollerbearingsisdefinedasthedeflectionoftheinnerrace(cone)relative
totheouterrace(cup)ataspecifiedforce.
UserssubtractthePPCandaddthedimensionalpreloadtoanexistingspacertoestablishafinalsetting.The
approachgivesabearingsettingthataccountsforactualsystemstiffnessanddeflections.
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TheTimkenCo.,www.timken.com
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