Sliding Bearings

Sliding Contact BearingsSlidingContactBearings

VedatTemizAssistant Professor ofMachine Design

Introduction

A bearing is a machine element which support another moving machine Abearingisamachineelementwhichsupportanothermovingmachineelement(knownasjournal).Itpermitsarelativemotionbetweenthecontactsurfacesofthemembers,whilecarryingtheload.l l d ll h h d h l b Alittleconsiderationwillshowthatduetotherelativemotionbetween

thecontactsurfaces,acertainamountofpoweriswastedinovercomingfrictionalresistanceandiftherubbingsurfacesareindirectcontact,therewillberapidwear.

Inordertoreducefrictionalresistanceandwearandinsomecasestocarry away the heat generated a layer of fluid (known as lubricant) may becarryawaytheheatgenerated,alayeroffluid(knownaslubricant)maybeprovided.

Thelubricantusedtoseparatethejournalandbearingisusuallyamineralil fi d f l b bl il ili iloilrefinedfrompetroleum,butvegetableoils,siliconoils,greasesetc.,

maybeused.

Classification ofBearingsg

1 Depending upon the direction of load to be supported The 1.Dependinguponthedirectionofloadtobesupported.Thebearingsunderthisgroupareclassified as:

(a) Radial bearings and (b) Thrust bearings(a)Radialbearings,and(b)Thrustbearings.

Classification ofBearingsg

2 Depending upon the nature of contact The bearings under 2.Dependinguponthenatureofcontact.Thebearingsunderthisgroupareclassifiedas:

(a) Sliding contact bearings and (b) Rolling contact bearings(a)Slidingcontactbearings,and(b)Rollingcontactbearings.

TypesofSlidingContactB iBearings

The sliding contact bearings in which the sliding action is guided in a Theslidingcontactbearingsinwhichtheslidingactionisguidedinastraightlineandcarryingradialloads,asshowninFig.(a),maybecalledslipperorguidebearings.Suchtypeofbearingsareusuallyfoundincrosshead of steam enginesheadofsteamengines.

Theslidingcontactbearingsinwhichtheslidingactionisalongthecircumferenceofacircleoranarcofacircleandcarryingradialloadsareknownasjournalorjsleevebearings.Whentheangleofcontactofthebearingwiththejournalis360 as shown in Fig (a) then the bearing360 asshowninFig.(a),thenthebearingiscalledafulljournalbearing.Thistypeofbearingiscommonlyusedinindustrialmachinery to accommodate bearing loadsmachinerytoaccommodatebearingloadsinanyradialdirection.


Whentheangleofcontactofthebearingwiththejournalis120,asshowninFig.(b),thenthebearingissaidtobepartialjournalbearing.Thistypeofbearinghaslessfrictionthanfulljournalbearing,butit can be used only where the load isitcanbeusedonlywheretheloadisalwaysinonedirection.Themostcommonapplicationofthepartial journal bearings is found in railpartialjournalbearingsisfoundinrailroadcaraxles.Thefullandpartialjournalbearingsmayb ll d l b i bbecalledasclearancebearingsbecausethediameterofthejournalislessthanthatofbearing.


Whenapartialjournalbearinghasnoclearancei.e.thediametersofthejournalandbearingareequal,thenthebearingiscalledafittedbearing,as shown in Fig. (c).asshowninFig.(c).


Theslidingcontactbearings,accordingtothethicknessoflayerofthel b i t b t th b i d th j l l b l ifi dlubricantbetweenthebearingandthejournal,mayalsobeclassifiedasfollows:

1.Thickfilmbearings.Thethickfilmbearingsarethoseinwhichthef g gworkingsurfacesarecompletelyseparatedfromeachotherbythelubricant.Suchtypeofbearingsarealsocalledashydrodynamic lubricatedbearings.bearings.

2.Thinfilmbearings.Thethinfilmbearingsarethoseinwhich,althoughlubricantispresent,theworkingsurfacespartiallycontacteachothertl t t f th ti S h t f b i l ll d b datleast partofthetime.Suchtypeofbearingsarealsocalledboundarylubricatedbearings.

3.Zerofilmbearings.Thezerofilmbearingsarethosewhichoperatewithoutanylubricantpresent.

4.Hydrostaticorexternallypressurizedlubricatedbearings.Thehydrostatic bearings are those which can support steady loads without anyhydrostaticbearingsarethosewhichcansupportsteadyloadswithoutanyrelativemotionbetweenthejournalandthebearing.Thisisachievedbyforcingexternallypressurizedlubricantbetweenthemembers.

Hydrodynamic LubricatedB iBearings

Inhydrodynamic lubricatedbearings,thereisathickfilmoflubricantbetweenthejournalandthebearing.Alittleconsiderationwillshowthatwhen the bearing is supplied with sufficient lubricant, a pressure is buildwhenthebearingissuppliedwithsufficientlubricant,apressureisbuildupintheclearancespacewhenthejournalisrotatingaboutanaxisthatiseccentricwiththebearingaxis.Th l d b t d b thi fl id ith t t l Theloadcanbesupportedbythisfluidpressurewithoutanyactualcontactbetweenthejournalandbearing.

Theloadcarryingabilityofahydrodynamicbearingarisessimplybecauseaviscousfluidresistsbeingpushedaround.

Undertheproperconditions,thisresistancetomotionwilldevelopapressure distribution in the lubricant film that can support a useful loadpressuredistributioninthelubricantfilmthatcansupportausefulload.

Hydrodynamic LubricatedB iBearings

The load supporting pressure in hydrodynamic Theloadsupportingpressureinhydrodynamicbearings arises from either

1.theflowofaviscousfluidinaconvergingchannel(knownaswedgefilmlubrication),or

2.theresistanceofaviscousfluidtobeingsqueezedout from between approaching surfaces (known asoutfrombetweenapproachingsurfaces(knownassqueezefilmlubrication).

ImportantFactorsfortheFormationofThickOil Film in Hydrodynamic Lubricated BearingsOilFilminHydrodynamicLubricated Bearings

According to Reynolds the following factors are essential for AccordingtoReynolds,thefollowingfactorsareessentialfortheformationofathickfilmofoilinhydrodynamiclubricatedbearings:g

1.Acontinuoussupplyofoil. 2.Arelativemotionbetweenthetwosurfacesinadirection

approximatelytangentialtothesurfaces. 3.Theabilityofoneofthesurfacestotakeupasmallinclination

totheothersurfaceinthedirectionoftherelativemotion. 4.Thelineofactionofresultantoilpressuremustcoincidewith

thelineofactionoftheexternalloadbetweenthesurfaces.

WedgeFilmJournalBearingsg g Theloadcarryingabilityofawedgefilmjournalbearingresultswhenthe

journaland/orthe bearingrotatesrelativetotheload.Themostcommoncaseisthatofasteadyload,afixed(nonrotating)bearingandarotatingjournal.Fig.(a)showsajournalatrestwithmetaltometal contactatAonthelineofactionofthesupportedload.

Whenthejournalrotatesslowlyinthe anticlockwisedirection,asshowninFig.(b),thepointofcontactwillmovetoB,sothatthe angleAOBistheangle of sliding friction of the surfaces in contact at B In the absence of aangleofslidingfrictionofthesurfacesincontactatB.Intheabsenceofalubricant, therewillbedrymetaltometalfriction.Ifalubricantispresentintheclearancespaceofthebearing andjournal,thenathinabsorbedfilm of the lubricant may partly separate the surface but a continuousfilmofthelubricantmaypartlyseparatethesurface,butacontinuousfluidfilmcompletelyseparatingthesurfaceswillnotexistbecauseofslowspeed.

WedgeFilmJournalBearingsg g

Whenthespeedofthejournalisincreased,acontinuousfluidfilmisestablishedasinFig.(c).ThecentreofthejournalhasmovedsothattheminimumfilmthicknessisatC.Itmaybenoted thatfromDtoCinthedirectionofmotion,thefilmiscontinuallynarrowingandhenceisa, y gconverging film.Thecurvedconvergingfilmmaybeconsideredasawedgeshapedfilmofaslipperbearing wrappedaroundthejournal.Alittleconsideration will show that from C to D in the direction of rotation asconsiderationwillshowthatfromCtoDinthedirectionof rotation,asshowninFig. (c),thefilmisdivergingandcannotgiverisetoapositivepressureor asupportingaction.

WedgeFilmJournalBearingsg g

Fig.below showsthetwoviewsofthebearingshowninFig.(c),withthevariationof pressureintheconvergingfilm.Actually,becauseofsideleakage,theangleofcontactonwhich pressureactsislessthan180.

SqueezeFilmJournalBearingq g

In a wedge film journal bearing the bearing carries a steady Inawedgefilmjournalbearing,thebearingcarriesa steadyloadandthejournalrotatesrelativetothebearing.

But in certain cases the bearings oscillate or rotate so slowlyButincertaincases,thebearingsoscillate orrotatesoslowlythatthewedgefilmcannotprovideasatisfactoryfilmthickness.

Iftheloadis uniformorvaryinginmagnitudewhileactinginaconstantdirection,thisbecomesathinfilmor possiblyazerofilmproblem.Butiftheloadreversesitsdirection,thesqueezefilmmaydevelop sufficientcapacitytocarrythedynamic loads without contact between the journal and thedynamicloadswithoutcontactbetweenthejournalandthebearing.

Such bearings are known as squeeze film journal bearing Suchbearingsareknownassqueezefilmjournalbearing.

PropertiesofSlidingContactB i M i lBearing Materials

When the journal and the bearings are having proper Whenthejournalandthebearingsarehavingproperlubricationi.e.thereisafilmofclean, noncorrosivelubricantinbetween,separatingthetwosurfacesincontact,theonly, p g , yrequirementof thebearingmaterialisthattheyshouldhavesufficientstrengthandrigidity.

However,theconditions underwhichbearingsmustoperateinservicearegenerallyfarfromidealandthustheother

ti di d h t b id d i l tiproperties asdiscussedhere mustbeconsideredinselectingthebestmaterial.


1 Compressive strength The maximum bearing pressure is 1.Compressivestrength.Themaximumbearingpressureisconsiderablygreaterthantheaverage pressureobtainedbydividingtheloadtotheprojectedarea.Thereforethebearingg p j gmaterialshould havehighcompressivestrengthtowithstandthismaximumpressuresoastopreventextrusionor otherpermanentdeformationofthebearing.

2.Fatiguestrength.Thebearingmaterialshouldhaveffi i t f ti t th th t it ith t d t dsufficientfatiguestrengthsothatitcan withstandrepeated

loadswithoutdevelopingsurfacefatiguecracks.Itisofmajorimportance in aircraft and automotive enginesimportancein aircraftandautomotiveengines


3 Comformability It is the ability of the bearing material to 3.Comformability.Itistheabilityofthebearingmaterialtoaccommodateshaftdeflections andbearinginaccuraciesbyplasticdeformation(orcreep)withoutexcessivewearandp ( p)heating.

4.Embeddability.Itistheabilityofbearingmaterialtoaccommodate(orembed)smallparticles ofdust,gritetc.,withoutscoringthematerialofthejournal.

5.Bondability.Manyhighcapacitybearingsaremadebybondingoneormorethinlayersofa bearingmaterialtoahigh strength steel shell Thus the strength of the bond i ehighstrengthsteelshell.Thus,thestrengthofthebondi.e.bondability isan importantconsiderationinselectingbearingmaterial.


6.Corrosionresistance.Thebearingmaterial shouldnotcorrodegawayundertheactionoflubricatin oil.Thispropertyisofparticularimportanceininternal combustionengineswherethesameoilisusedto lubricatethecylinderwallsandbearings.Inthecylinder, thelubricatingoilcomesintocontactwithhotcylinderwalls and may oxidise and collect carbon deposits from the wallswallsandmayoxidise andcollectcarbondepositsfrom thewalls.

7.Thermalconductivity.Thebearingmaterial shouldbeofhighthermal conductivity so as to permit the rapid removal of the heatthermalconductivitysoastopermit therapidremovaloftheheatgeneratedbyfriction.

8.Thermalexpansion.Thebearingmaterial shouldbeoflowp gcoefficientofthermalexpansion,so thatwhenthebearingoperatesoverawiderangeof temperature,thereisnounduechangeintheclearance.

Properties ofmetallicbearing materialsp g

MaterialsusedforSlidingC t t B iContactBearings

1 Babbit metal The tin base and lead base babbits are widely 1.Babbit metal.Thetinbaseandleadbasebabbits arewidelyusedasabearingmaterial, becausetheysatisfymostrequirementsforgeneralapplications.Thebabbits areq g pprecommendedwhere themaximumbearingpressure(onprojectedarea)isnotover7to14N/mm2.Whenappliedinautomobiles,thebabbit isgenerallyusedasathinlayer,0.05mmto0.15mmthick,bondedtoan insertorsteelshell.Thecomposition of the babbit metals is as follows :compositionofthebabbit metalsisasfollows:

Tinbasebabbits :Tin90%;Copper4.5%;Antimony5%;Lead0 5%0.5%.

Leadbasebabbits :Lead84%;Tin6%;Anitmony 9.5%;Copper0.5%.pp


3.Castiron.Thecastironbearingsareusuallyusedwithsteeljournals.Suchtypeofbearings arefairlysuccessfulwherel b d d h l dlubricationisadequateandthepressureislimitedto3.5N/mm2 andspeed to40metres perminute.

4.Silver.Thesilverandsilverleadbearingsaremostlyusedinaircraft engines where the fatigue strength is the mostaircraftengineswherethe fatiguestrengthisthemostimportantconsideration.


5.Nonmetallicbearings.Thevariousnonmetallicbearingsaremadeofcarbongraphite,rubber,woodandplastics.The

b h b lf l b d llcarbongraphitebearingsareselflubricating,dimensionallystableovera widerangeofoperatingconditions,chemicallyinert and can operate at higher temperatures than otherinertandcanoperateathighertemperaturesthan otherbearings.Suchtypeofbearingsareusedinfoodprocessingandotherequipmentwhere contaminationbyoilorgreaseq p y gmustbeprohibited.Thesebearingsarealsousedinapplicationswhere theshaftspeedistoolowtomaintainah d d i il filhydrodynamicoilfilm.


Thesoftrubberbearingsareusedwithwaterorotherlowviscositylubricants,particularly wheresandorotherlarge

l dd h h h d fparticlesarepresent.Inadditiontothehighdegreeofembeddability and comformability,therubberbearingsareexcellent for absorbing shock loads and vibrations The rubberexcellentforabsorbingshockloadsandvibrations.The rubberbearingsareusedmainlyonmarinepropellershafts,hydraulicturbinesandpumps.p p

Thewoodbearingsareusedinmanyapplicationswherelowcost,cleanliness,inattentionto lubricationandantiseizingareimportant.


ThecommonlyusedplasticmaterialforbearingsisNylonandTeflon.Thesematerialshave manycharacteristicsdesirableinbearingmaterialsandbothcan be used dry i e as a zero film bearing The Nylon is stronger harder andcanbeuseddryi.e.asazerofilmbearing. TheNylonisstronger,harderandmoreresistanttoabrasivewear.Itisusedforapplicationsinwhich thesepropertiesareimportante.g.elevatorbearings,camsintelephonedialsetc.

TheTeflonisrapidly replacingNylonasawearsurfaceorlinearforjournalandotherslidingbearingsbecauseofthe followingproperties:g g g p p

1. Ithaslowercoefficientoffriction,about0.04(dry)ascomparedto0.15forNylon.2 It b d t hi h t t t b t 315C d t 2. Itcanbeusedathighertemperaturesuptoabout315Cascomparedto120CforNylon.

3. Itisdimensionallystablebecauseitdoesnotabsorbmoisture,andy 4. Itispracticallychemicallyinert.

PropertiesofLubricantsp 1.Viscosity.Itisthemeasureofdegreeoffluidityofaliquid.It

isaphysicalpropertybyvirtue ofwhichanoilisabletoform,p y p p y y ,retainandofferresistancetoshearingabufferfilmunderheatand pressure.Thegreatertheheatandpressure,thegreaterviscosityisrequiredofalubricanttoprevent thinningandsqueezingoutofthefilm.Th f d t l i f i it b d t d b Thefundamentalmeaningofviscositymaybeunderstoodbyconsideringaflatplatemoving underaforcePparalleltoastationary plate the two plates being separated by a thin filmstationaryplate,thetwoplatesbeingseparatedbyathinfilmofafluid lubricantofthicknessh,asshowninFig.

PropertiesofLubricantsp Theparticlesofthelubricantadherestronglytothe movingandstationary

plates.Themotionisaccompaniedbyalinearsliporshearbetweentheparticles throughouttheentireheight(h)ofthefilmthickness.IfAistheareaoftheplateincontactwiththe lubricant,thentheunitshearstressisgivenby

AccordingtoNewton'slawofviscousflow,themagnitudeofthisshearstressvariesdirectly with the velocity gradient (dV/dy) It is assumed thatdirectly withthevelocitygradient(dV/dy).Itisassumedthat(a)thelubricantcompletelyfillsthespacebetweenthetwosurfaces,(b)thevelocityofthelubricantateachsurfaceissameasthatofthesurface,and( ) fl f th l b i t di l t th l it f th l t i li ibl(c)anyflowofthelubricantperpendiculartothevelocityoftheplateisnegligible.

whereZisaconstantofproportionality andisknownasp p yabsoluteviscosity(orsimplyviscosity)ofthe lubricant.

PropertiesofLubricantsp Whenthethicknessofthefluidlubricantissmallwhichisthecasefor

bearings,thenthevelocity gradientisverynearlyconstantasshowninFig.

WhenisinN/m2,hisinmetres andVisinm/s,thentheunitofabsoluteviscosityisgivenby

However,thecommonpractice, pistoexpresstheabsoluteviscosityinmassunits,suchthat

PropertiesofLubricantsp Theviscocity ofthelubricantismeasuredbySaybolt universalviscometer.

Itdeterminesthe timerequiredforastandardvolumeofoilatacertaintemperaturetoflowunderacertainhead throughatubeofstandarddiameterandlength.ThetimesodeterminedinsecondsistheSaybolt

universalviscosity.InordertoconvertSaybolt universalviscosityiny y ysecondstoabsoluteviscosity(in kg/ms),thefollowingformulamaybeused:

Z=Absoluteviscosityattemperaturetinkg/ms,andS Saybolt universal viscosity in secondsS=Saybolt universalviscosityinseconds.

PropertiesofLubricantsp

2.Oiliness.Itisajointpropertyofthelubricantandthebearingsurfacesincontact.Itisa measureofthelubricatingqualitiesunderboundaryconditionswherebasemetaltometalisprevented onlybyabsorbedfilm.Thereisnoabsolutemeasureofilioiliness.


3 Density This property has no relation to lubricating value but is3.Density.Thispropertyhasnorelationtolubricatingvaluebutisusefulinchangingthe kinematicviscositytoabsoluteviscosity.Mathematically

Absoluteviscosity= Kinematicviscosity(inm2/s) where=Densityofthelubricatingoil. Thedensityofmostoftheoilsat15.5Cvariesfrom860to950

kg/m3 (theaveragevaluemay betakenas900kg/m3 ).Thedensityatanyothertemperature(t)maybeobtainedfromthefollowing relation,i.e.

t=15.5 0.000657.t where15.5 =Densityofoilat15.5 C.


4 Viscosity index The term viscosity index is used to denote the4.Viscosityindex.Thetermviscosityindexisusedtodenotethedegreeofvariationofviscosity withtemperature.

5.Flashpoint.Itisthelowesttemperatureatwhichanoilgivesoffp p gsufficientvapour tosupport amomentaryflashwithoutactuallysettingfiretotheoilwhenaflameisbroughtwithin6mmatthe

surfaceoftheoil. 6.Firepoint.Itisthetemperatureatwhichanoilgivesoff

sufficientvapour toburnit continuouslywhenignited. 7.Pourpointorfreezingpoint.Itisthetemperatureatwhichan

l ll fl h l doilwillceasetoflowwhen cooled.

TermsusedinHydrodynamicJ l B iJournalBearings

A hydrodynamic journal bearing is shown in Fig in which O is AhydrodynamicjournalbearingisshowninFig.,inwhichOisthecentreofthejournal andOisthecentreofthebearing.

D=Diameterofthebearing, d = Diameter of the journal d=Diameterofthejournal, and l = Length of the bearing l=Lengthofthebearing.


1 Diametral clearance It the difference between the 1.Diametral clearance.Itthedifferencebetweenthediametersofthebearingandthejournal.Mathematically,diametral clearance,,

c=D dNote:Thediametral clearance(c)inao e e d a e a c ea a ce (c) abearingshouldbesmall enoughtoproducethenecessaryvelocitygradient,so that the pressure built up will supportsothatthepressurebuiltupwillsupporttheload.Alsothe smallclearancehastheadvantageofdecreasingsideleakage.However the allowance must be made forHowever,theallowancemustbemadeformanufacturingtolerancesinthejournalandbushing.Acommonlyusedclearance

d l hinindustrialmachinesis0.025mmpercmofjournaldiameter.


2 Radial clearance It is the difference between the radii of 2.Radialclearance.Itisthedifferencebetweentheradiiofthebearingandthejournal. Mathematically,radialclearance,

3 Diametral clearance ratio It is the3.Diametral clearanceratio.Itistheratioofthediametral clearancetothediameterofthe journal.jMathematically,diametral clearanceratio


4 Eccentricity It is the radial distance between the centre (O) 4.Eccentricity.Itistheradialdistancebetweenthecentre(O)ofthebearingandthedisplaced centre(O)ofthebearingunderload.Itisdenotedbye.y

5.Minimumoilfilmthickness.Itistheminimumdistancebetweenthebearingandthejournal, undercompletelubricationcondition.Itisdenotedbyh0 andoccursatthelineofcentres asshownin Fig.

Itsvaluemaybeassumedasc/4.


6.Attitudeoreccentricityratio.Itistheratiooftheyeccentricitytotheradialclearance.

Mathematically,attitudeoreccentricityratio,

7.Shortandlongbearing.Iftheratioofthe length to the diameter of the journalthelengthtothe diameterofthejournal(i.e.l/d)islessthan1,thenthebearingis saidtobeshortbearing.Ontheotheris saidtobeshortbearing.Ontheotherhand,ifl/d isgreaterthan 1,thenthebearingisknownaslongbearing


Notes:1.Whenthelengthofthejournal(l)isequaltotheg j ( ) qdiameterof thejournal(d),thenthebearingiscalledsquarebearing.

2. Becauseofthesideleakageofthelubricantfromthebearing, thepressureinthefilmisatmosphericattheendsofth b i Th ill b hi h f lthebearing.The averagepressurewillbehigherforalongbearingthanforashortor squarebearing.Therefore,fromthe stand point of side leakage a bearing with a large l/d ratiothestandpointofsideleakage,abearing withalargel/d ratioispreferable.However,spacerequirements, manufacturing,tolerancesandshaftdeflectionsarebettermetwithashortbearing.Thevalueofl/d maybetakenas1to2forgeneralindustrial machinery.Incrankshaftbearings,thel/dratioisf tl l th 1frequentlylessthan1.

BearingCharacteristicNumberandBearing Modulus for Journal BearingsBearingModulusforJournalBearings

The coefficient of friction in design of bearings is of great importance Thecoefficientoffrictionindesignofbearingsisofgreatimportance,becauseitaffordsa meansfordeterminingthelossofpowerduetobearingfriction.Ithasbeenshownbyexperiments thatthecoefficientoffriction for a full lubricated journal bearing is a function of three variablesfrictionforafulllubricatedjournalbearingisafunctionofthreevariables,i.e.

Thereforethecoefficientoffrictionmaybe expressed asbeexpressedas

C ffi i t f f i ti p =Bearingpressureontheprojectedbearing =Coefficientoffriction, =Afunctionalrelationship,Z =Absoluteviscosityofthelubricant in kg / m s

p g p p j gareainN/mm2,=Loadonthejournal l dd=Diameterofthejournal,lubricant,inkg/ms,

N =Speedofthejournalinr.p.m., l=Lengthofthebearing,andc =Diametral clearance.

BearingCharacteristicNumberandBearing Modulus for Journal BearingsBearingModulusforJournalBearings

ThefactorZN/pistermedasbearingcharacteristicnumberandisadimensionlessnumber.

weseethattheminimumamountoffrictionoccursatAandatthispointthe valueofpZN/p isknownasbearingmoduluswhichisdenotedbyK.Thebearingshouldnotbeoperatedatthisvalueofbearingmodulus,p gbecause aslightdecreaseinspeedorslightincreasein pressurewillbreaktheoilfilmandmakethejournal tooperatewithmetaltometalcontact.Thiswill resultinhighfriction,wearandheating.Inorder topreventsuchconditions,thebearingshouldbe designedforavalueofZN/patleastthreetimes theminimumvalueofbearingmodulus(K).

CoefficientofFrictionforJ l B iJournalBearings

C ffi i t f f i ti Coefficientoffriction,(whenZisinkg/msandpisinN/mm2)

whereZ,N,p,dandchaveusualmeaningsasdiscussedinpreviousslides,andk=Factortocorrectforendleakage.Itdependsupontheratiooflengthtothediameterofthebearing(i.e.l/d).g ( / )=0.002forl/dratiosof0.75to2.8.TheoperatingvaluesofZN/pshouldbecomparedwithvaluesgiveninTablesin the following slides to ensure safe margin between operating conditionsinthe following slides toensure safemarginbetweenoperatingconditionsandthepointoffilmbreakdown

DesignDesignvaluesforjournalbearingsbearings

CriticalPressure oftheJournalB iBearing

The pressure at which the oil film breaks down so that metal Thepressureatwhichtheoilfilmbreaksdownsothatmetaltometalcontactbegins,isknown ascriticalpressureortheminimumoperatingpressureofthebearing.Itmaybep g p g yobtainedbythe followingempiricalrelation,i.e.

Criticalpressureorminimumoperatingpressure,

(whenZisinkg/ms)

Sommerfeld Number

The Sommerfeld number is also a dimensionless parameter TheSommerfeld numberisalsoadimensionlessparameterusedextensivelyinthedesignof journalbearings.Mathematically,y,

Fordesignpurposes,itsvalueistakenasfollows:

(whenZisinkg/msandpisinN/mm2)

HeatGeneratedinaJournalB iBearing

The heat generated in a bearing is due to the fluid friction and Theheatgeneratedinabearingisduetothefluidfrictionandfrictionofthepartshaving relativemotion.Mathematically,heatgeneratedinabearing,g g,

=Coefficientoffriction,W=LoadonthebearinginN,=PressureonthebearinginN/mm2 Projectedareaofthebearing inmm2 =p(l d),

bb l /V=Rubbingvelocityinm/s disinmetres,

N = Speed of the journal in r p mN=Speedofthejournalinr.p.m.


After the thermal equilibrium has been reached heat will be dissipated at Afterthethermalequilibriumhasbeenreached,heatwillbedissipatedattheoutersurfaceof thebearingatthesamerateatwhichitisgeneratedintheoilfilm.Theamountofheatdissipatedwill dependuponthetemperature difference size and mass of the radiating surface and on thetemperaturedifference,sizeandmassoftheradiatingsurfaceandontheamountof airflowingaroundthebearing.

However,fortheconvenienceinbearingdesign,theactualheatdissipatingareamaybeexpressedintermsoftheprojectedareaofthejournal.

H t di i t d b th b iHeatdissipatedbythebearing,

C=HeatdissipationcoefficientinW/m2/C,A=Projectedareaofthebearinginm2=l d,tb =TemperatureofthebearingsurfaceinC,andta =TemperatureofthesurroundingairinC.


The average values of C (in W/m2/C) for journal bearings may be taken as TheaveragevaluesofC(inW/m2/ C), forjournalbearingsmaybetakenasfollows:

Forunventilatedbearings(Stillair) =140to420W/m2/C Forwellventilatedbearings 490 to 1400 W/m2/C =490to1400W/m2/ C

Ithasbeenshownbyexperimentsthatthetemperatureofthebearing(tb)isapproximately mid way between the temperature of the oil film (t ) and theapproximately midwaybetweenthetemperatureoftheoilfilm(t0)andthetemperatureoftheoutsideair(ta).In otherwords,


Notes : 1 For well designed bearing the temperature of the oil film should not be Notes:1. Forwelldesignedbearing,thetemperatureoftheoilfilmshouldnotbemorethan60C,otherwisethe viscosityoftheoildecreasesrapidlyandtheoperationofthebearingisfoundtosuffer.Thetemperatureofthe oilfilmisoften called as the operating temperature of the bearingoftencalledastheoperatingtemperatureofthebearing.

2.Incasethetemperatureoftheoilfilmishigher,thenthebearingiscooledbycirculatingwaterthrough coilsbuiltinthebearing.

3.Themassoftheoiltoremovetheheatgeneratedatthebearingmaybeobtainedbyequatingtheheat generatedtotheheattakenawaybytheoil.Weknow that the heat taken away by the oilknowthattheheattakenawaybytheoil,

Qt=m.S.t J/sorwatts where m = Mass of the oil in kg / s wherem=Massoftheoilinkg/s, S=Specificheatoftheoil.Itsvaluemaybetakenas1840to2100J/kg/C, t=DifferencebetweenoutletandinlettemperatureoftheoilinC.

DesignProcedure forJournalB iBearing

The following procedure may be adopted in designing journal bearings Thefollowingproceduremaybeadoptedindesigningjournalbearings,whenthebearingload, thediameterandthespeedoftheshaftareknown.

1.Determinethebearinglengthbychoosingaratioof l/dfromrelatedblTable.

2. Checkthebearingpressure,p=W/l.d fromrelated Table forprobablesatisfactoryvalue.y

3. AssumealubricantfromTable26.2anditsoperating temperature(t0).Thistemperatureshouldbebetween 26.5Cand60Cwith82Casamaximum for high temperature installations such as steam turbinesmaximumforhigh temperatureinstallationssuchassteamturbines.

4. DeterminetheoperatingvalueofZN/pforthe assumedbearingtemperatureandcheckthisvaluewith correspondingvaluesinrelated Table,todetermineth possibilityofmaintainingfluidfilmoperation.

5. Assumeaclearanceratioc/dfromrelated Table.

DesignProcedure forJournalB iBearing

6 Determine the coefficient of friction () by using the relation 6. Determinethecoefficientoffriction()byusingthe relationasdiscussedinprevious slides.

7 Determine the heat generated by using the relation as7. Determinetheheatgeneratedbyusingtherelationasdiscussedinprevious slides.

8.Determinetheheatdissipatedbyusingtherelationasp y gdiscussedinprevious slides.

9. Determinethethermalequilibriumtoseethattheheatdissipatedbecomesat leastequalto theheatgenerated.Incasetheheatgeneratedismorethantheheatdissipatedtheni h h b i i d i d i i ifi i ll l d beitherthe bearingisredesignedoritisartificiallycooledbywater.

SolidJournalBearingg

A solid bearing as shown in Fig below is the simplest form of journal Asolidbearing,asshowninFig.below,isthesimplestformofjournalbearing.Itissimplya blockofcastironwithaholeforashaftprovidingrunningfit.Thelowerportionoftheblockis extendedtoformabaseplateor sole with two holes to receive bolts for fastening it to the frame An oilorsolewithtwoholestoreceiveboltsforfasteningittotheframe.An oilholeisdrilledatthetopforlubrication.Themaindisadvantagesofthisbearingare

1.Thereisnoprovisionforadjustmentincaseofwear,and 2.Theshaftmustbepassedintothebearingaxially,i.e.endwise.

Sincethereisnoprovisionforwearadjustment, therefore this type of bearingadjustment,thereforethistypeofbearingisusedwhenthe shaftspeedisnotveryhighandtheshaftcarrieslightloadsonly.

BushedBearingg

A bushed bearing as shown in Fig below is an improved solid bearing in Abushedbearing,asshowninFig.below,isanimprovedsolidbearinginwhichabushofbrass orgunmetalisprovided.Theoutsideofthebushisadrivingfitintheholeofthecastingwhereasthe insideisarunningfitforthe shaft When the bush gets worn out it can be easily replaced In smalltheshaft.Whenthebushgetswornout,itcanbeeasilyreplaced.Insmallbearings,thefrictionalforceitselfholdsthebushinposition,butforshaftstransmittinghighpower, grubscrewsareusedforthepreventionof

d l d f h b hrotationandslidingofthebush.

SplitBearingorPlummerBlockp g

A splitbearing is used for shaftsAsplitbearingisusedforshaftsrunningathighspeedsandcarryingheavyloads.Asplit bearing,ash i Fi i t f t ishowninFig.,consistsofacastironbase(alsocalledblockorpedestal),gunmetal orphosphorbronzebrasses,bushesorstepsmadeintwohalvesandacastironcap.Thetwohalves ofthebrassesareheldtogetherbyacaporcoverbymeansofmildsteelboltsandnuts.

Sometimes thinshimsare introducedbetweenthecapandthebasetoprovidean adjustment forwear.Whenthe bottomwearsout,oneortwoshimsareremovedandthenthecapistightenedbymeansofbolts.

Design considerationsg Thebrassesareprovidedwithcollarsorflangesoneithersideinorderto

preventitsaxial movement.Topreventitsrotationalongwiththeshaft,thefollowingfourmethodsareusuallyusedin practice.

1.Thesungs areprovidedatthesidesasshowninFig.(a). 2 A sung is provided at the top which fits inside the cap as shown in Fig 2.Asungisprovidedatthetop,whichfitsinsidethecapasshowninFig.

(b).Theoil holeisdrilledthroughthesung. 3.Thestepsaremaderectangularontheoutsideandtheyaremadetofit

d d h l h ( )insideacorresponding hole,asshowninFig.(c). 4.Thestepsaremadeoctagonalontheoutsideandtheyaremadetofit

insideacorresponding hole,asshowninFig.(d).p g , g ( )

DesignofBearingCapsandB ltBolts

Whenasplitbearingisused,thebearingcapistightenedonthetop.Thel d i ll i d b h b i d h b iloadisusuallycarried bythebearingandnotthecap,butinsomecasese.g.splitconnectingrodendsindoubleactingsteam engines,aconsiderableloadcomesonthecapofthebearing.Therefore,thecapandtheholding downboltsmustbedesignedforfullload.

Thecapisgenerallyregardedasasimplysupported beam,supportedbyholding down bolts and loaded at the centre as shown in Fig belowholdingdownboltsandloadedatthecentreasshowninFig.below.

Bendingstress

W=Loadsupportedatthecentre,a =Distancebetweencentres ofholdingdownbolts,l=Lengthofthebearing,andt=Thicknessofthecap

Note: Whenanoilholeisprovidedinthecap,thenthediameteroftheholeshouldbesubtractedfromthelength ofthebearing.


Thecapofthebearingshouldalsobeinvestigatedforthestiffness.Wek h f i l d b l d d h hknowthatforasimply supportedbeamloadedatthecentre,thedeflection,

Thedeflectionofthecapshouldbee de ect o o t e cap s ou d belimitedtoabout0.025mm.


Inordertodesigntheholdingdownbolts,theloadoneachboltistaken33% hi h h h l l d h b l I h d l d33%higherthanthe normalloadoneachbolt.Inotherwords,loadoneachboltistaken 4.W/3.n wherenisthenumberof boltsusedforholdingdownthecap.

Letdc=Corediameterofthebolt,andt =Tensilestressforthematerialofthebolt.

Fromthisexpression,thecorediameter(d ) b l l d Af fi di h(dc)maybecalculated.Afterfindingthecorediameter, thesizeoftheboltisfixed.

OilGrooves

The oil grooves are cut into the plain bearing surfaces to assist in the Theoilgroovesarecutintotheplain bearingsurfacestoassistinthedistribution oftheoilbetweentherubbingsurfaces.It preventssqueezingoftheoilfilmfrom heavilyloadedlowspeedjournalsand bearings.h d l l d h h h d Thetendencytosqueezeoutoilis greaterinlowspeedthaninhighspeedbearings,becausetheoilhasgreaterwedging actionathighspeeds.Atlowspeeds,the journalrestsuponagivenareaofoilfilmforalongerperiodoftime,tendingtosqueezeouttheoil overtheareaofgreatestpressure.Thegroovesfunctionasoilreservoirswhichholdsanddistributes theoilespeciallyduringstartingoratverylowspeeds.p y g g y p

Theoilgroovesarecutatrightanglestothe lineoftheload.Thecircumferentialanddiagonalgroovesshouldbeavoided,ifpossible.Theeffectiveness of the oil grooves is greatly enhanced if the edges of grooveseffectivenessoftheoilgroovesisgreatlyenhancediftheedgesofgroovesarechamfered.Theshallow andnarrowgrooveswithchamferededgesdistributestheoilmoreevenly.Achamferededgeshould alwaysbe

id d h i li f h b iprovidedatthepartinglineofthebearing.

ThrustBearingsg

A thrust bearing is used to guide or support the shaft which is subjected to Athrustbearingisusedtoguideorsupporttheshaftwhichissubjectedtoaloadalongtheaxis oftheshaft.Suchtypeofbearingsaremainlyusedinturbinesandpropellershafts.Thethrustbearings areofthefollowingtwotypes :types:

1.Footsteporpivotbearings,and 2.Collar bearings.g Inafootsteporpivotbearing,theloaded shaftisverticalandtheendof

theshaftrestswithin thebearing.Incaseofcollarbearing,theshaftcontinues through the bearingcontinuesthroughthebearing.

Theshaftmaybe verticalorhorizontalwithsinglecollarormany collars.Weshallnowdiscussthedesignaspects ofthesebearingsinthefollowingarticles.

FootsteporPivotBearingsp g

A simple type of footstep bearing suitable for a slow running and lightly Asimpletypeoffootstepbearing,suitable foraslowrunningandlightlyloadedshaft,isshown inFig.Iftheshaftisnotofsteel,itsend mustbefittedwithasteelface.Theshaftisguidedinagunmetalbush,pressedinto the pedestal and prevented from turning by means of a pinintothepedestaland preventedfromturningbymeansofapin.

FootsteporPivotBearingsp g Sincethewearisproportionaltothevelocityoftherubbingsurface,which(i.e.

rubbingvelocity) increaseswiththedistancefromtheaxis(i.e.radius)ofthebearing,thereforethewearwillbedifferent atdifferentradii.Duetothiswear,thedistributionofpressureoverthebearingsurfaceisnot uniform.Itmaybenotedthatthewearismaximumattheouterradiusandzeroatthecentre.Inorder tocompensateforendwear,thefollowingtwomethodsareemployed.

1.Theshaftiscounterboredattheend,asshowninFig.(a). 2 The shaft is supported on a pile of discs It is usual practice to provide alternate 2.Theshaftissupportedonapileofdiscs.Itisusualpracticetoprovidealternate

discsof differentmaterialssuchassteelandbronze,asshowninFig. (b),sothatthenextdisccomes intoplay,ifonediscseizesduetoimproperlubrication.

FootsteporPivotBearingsp g Itmaybenotedthatafootstepbearingisdifficulttolubricateastheoilisbeing

thrownoutwards fromthecentrebycentrifugalforce. Indesigning,itisassumedthatthepressureisuniformlydistributedthroughout

thebearing surface. Let Let


Whenthepressureinuniformlydistributedoverthebearingarea,thene e p essu e u o y d s bu ed o e e bea g a ea, e

andthetotalfrictionaltorque,

Powerlostinfriction,


Notes:1.Whenthecounterboringoftheshaftisconsidered,thenthebearingo es e e cou e bo g o e s a s co s de ed, e e bea gpressure,

andthetotalfrictionaltorque,a d e o a c o a o que,


Notes : 2 The allowable bearing pressure (p) for the footstep bearings may be Notes:2.Theallowablebearingpressure(p)forthefootstepbearingsmaybetakenasfollows:

(a)Forrubbingspeeds(V)from15to60m/min,thebearingpressureshouldbesuchthatp.V.42,when pisinN/mm2 andVinm/min.

(b)Forrubbingspeedsover60m/min.,thepressureshouldnotexceed0.7N/mm2.N/mm .

(c)Forintermittentservice,thebearingpressuremaybetakenas10.5N/mm2. (d)Forveryslowspeeds,thebearingpressuremaybetakenashighas14N/mm2.

3.Thecoefficientoffrictionforthefootstepbearingmaybetakenas0.015.

CollarBearingsg Inacollar bearing,theshaftcontinuesthroughthebearing.The shaftmay

be vertical or horizontal, with single collar or many collars. A simplebeverticalorhorizontal,withsingle collarormanycollars.Asimplemulticollar bearingforhorizontalshaftisshowninFig.below.

Thecollarsareeitherintegralpartsoftheshaftor rigidlyfastenedtoit.Th t di t f th ll i ll t k 1 4 t 1 8 ti thTheouterdiameterofthe collarisusuallytakenas1.4to1.8timestheinner diameterofthecollar(i.e.diameteroftheshaft).

Thethicknessofthecollariskeptasonesixth diameteroftheshaftandclearancebetween collarsasonethirddiameteroftheshaft.In designingcollarbearings,itisassumedthatthe pressureisuniformlydistributedover the bearing surface.overthebearing surface.

CollarBearingsg Whenthepressureisuniformlydistributedoverthebearingsurface,then

bearing pressure,bearingpressure,

Powerlostinfriction,

andthetotalfrictionaltorque,

Notes:1.Thecoefficientoffrictionforthecollarbearingsmaybetakenas0.03to0.05.2. Thebearingpressureforasinglecollarandwatercooledmulticollaredbearingsmaybetakensameas forfootstepbearings.

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Sliding Bearings