Introduction Themechanism designer oftenneedstosolvetheproblem ofballbearing arrangement andpreload to sustain thelaunchloadwithout degradation whileensuring alowfrictiontorquetoachieve thelubricant lifetime undervacuum. Thehigherthepreload is,thehigherthefrictiontorquewillbeandtheshorterthe lifetime willalsobe.Onthecontrary, thelowerthepreload is,thehigherthegapping amplitude is.This creates internal shocks underdynamic loadsoftencalled"hammering" andcouldleadtoanearlybearing lubrication failure. InEurope, peopleoftenconsider thatforMoS2 dry-lubricated mechanisms, thebearing working contact pressure should bebelow1200MPatoensure asignificant lifetimeandthen,thepreload hastobe defined aslowaspossible, whileensuring agapping amplitude notgreater thanabout301Jmat qualification level.Theserulesaremoreorlessderived fromexperience butjustification hasneverbeen provided todemonstrate thevalidity ofthisdesigncriterion. During vibration, forsafetyreasons itwasgenerally assessed thattheHertzian pressure willremain below2000MPa.Several testsmadeatCNESshowthatevenwithadynamic pressure exceeding 3000 MPa,ahighlifetimecanbereached ifatransient increase ofthefriction torque aftervibration is considered acceptable. Morerecently, theEuropean standard (E.C.S.S) stipulated, whatever thelubrication technique is,thata 1.25factorofsafetyismandatory withrespect tothemaximum allowable stress,whichisdefined as4200 MPaintheISO76standard. Therefore, thesurface hardness hastobegreater thanorequalto60Rc (700Hv)toensure apermanent deformation smaller than1/10000 oftheballdiameter for4200MPa.In thiscase,theE.C.S.S. allowable stressincluding thesafetymargincanbetakenat3360MPa. Theneedtoimprove thedesignrulesfordrylubricated MoS2bearings isobvious andmoreover abetter understanding ofthepressure distribution intheball/races contacts asafunction ofthebearing internal geometry isalsoconsidered veryusefulforthedesigners Theaimofthispaperisthentoanswer thefollowing questions: Isthemaximum contact pressure calculation basedontheHertzian theoryalways validtopredicta ballbearing internal markorindentation? Isthereacontact pressure limitation forMoS2drycoatedbearings andwhatistheassociated value? Whatistheeffectofgapping onthesubsequent bearing surface degradation andfriction torque evolution? Toanswer tothesethreequestions, CNESprepared atestplanwith20ballbearing pairsanddesigned a dedicated benchallowing thepreload andalternative dynamic loadtuning. TheTestBenchDescription Thebearing pairishardpreloaded bytwoground strutsandascrewnutdevice. Itismounted onashaft insideasteelcylindrical housing. Aninductive displacement sensor islocated between theendofthe shaftandthehousing. Thissensormeasures therelative displacement between theinternal andexternal ringsofthebearing pair.Ateachextremity ofthebench,theshaftandthehousing canbeputinthejaws ofapullingmachine. Aloadcellisplacedonthepullingmachine interface. TheMTS810pullingmachine applies theloadthrough hydraulic jacks.Thevibration equivalence isbasedonan800Hzresonance modeapplied duringtwominutes. Thepullingmachine isabletoworkverywellat20Hz,whichmeansa 1hourandtwentyminutetesttoreachthe96,000 equivalent cycles. Thiskindoftestbenchallowsadirectaccess tothebearing loadandinternal displacement andthereare noparasitic effects duetomechanical resonance ortransverse loadgenerally induced byashaker. Moreover, thereisnoneedofadouble integration oftheacceleration measurements toevaluate the bearing displacement. Theloadsensoralsoprovides thecapability tochecktheapplied preload value, whichisdependent onthestrutsandbearings thickness differences andonthebearing's radialgap.In fact,fora15°contact angle,theradialgapshouldbeequalto43.81Jm. Theradialgapmeasurements of thevarious bearing pairspointed outavariation from221Jmto461Jm. NASA/C P--2002-211506 320