EarthScanner Bearing Accelerated LifeTest BrianJ.Dietz',StevenG.VanDyk", Roamer E.Predmore"" Abstract TheModerate Resolution Imaging Spectrometer (MODIS) opticalinstrument forNASAGoddard willmeasure biological andphysical processes ontheEarth'ssurface andintheloweratmosphere. Akeycomponent of theinstrument isanextremely accurate scanmirrormotor/encoder assembly. Ofprimeconcern inthe performance andreliability ofthescanmotor/encoder isbearing selection andlubrication. Thispaper describes lifetestingofthebearings andlubrication selected fortheprogram. Introduction Fivescanmotor/encoders havebeenassembled andtested. Theseincludeanengineering model, qualification modelandthreeflightmodels. Thequalification scanmotor/encoder hascompleted qualification testingandiscurrently undergoing lifetesting. Thefirstflightmechanism waslaunched December 18,1999andiscurrently undergoing initialoperating procedures. Thesecond mechanism has beenintegrated onthePMspacecraft andisundergoing environmental testing. Aswithmostmechanisms, bearing lifewasdetermined tobeacriticalfactorintheoperation ofthescan motor/encoder. Bearings andlubricant werechosen astheresultofaprevious (phase1)screening program. Eachscanmotor/encoder contained twoduplexbearing pairs.Asthesynthetic hydrocarbon lubricant (Pennzane SHFX-2000) wasrelatively newtospaceflightapplications, accelerated and operational speedbearing lifetestingwasrequired inparalleltoverifythelifeofthePennzane earlyinthe program. Thequalification lifetestmechanism wasrequired tosurviveagroundtestandfive-year mission lifeof57millionrevolutions. Thispaperpresents theresultsforoneoffourbearing lifeteststations. The otherstations, including thequalification model,continue tooperate. Thisteststationwasanaccelerated bearing lifetestrunning at50RPM(theoperational speedofthe scanner was20.3RPM).Inordertocompensate forthehigherspeed,heaters wereaddedinorderto decrease theviscosity ofthelubricant. Theoriginal planspecified alifetestduration of57million revolutions. Afterthebearing lifetestsuccessfully achieved therequired 57millionrevolutions, thetest torquedatawasreviewed anditwasconcluded thattherewasnoindication offailure,sothetestwas allowed tocontinue. Steel(440C), DFduplexpairbearings andPennzane SHFX-2000 with2.5%leadnapthanate and0.6% antioxidant additives wereselected fortheMODIS scanner assembly. Eachbearing paircontained approximately 280mgoftotallubricant (approximately 75mgofwhichwassaturated intotheretainers). Thedecision todisassemble theteststationwasfinallymadeaftertheteststationhadexperienced equipment failure(heatercontrol) andthelifeduration hadbeengreatlyexceeded. Upondisassembly ofthe teststation, twoofthethreebearing pairsweredryandvisibledamage totheballs,racesandretainers was observed. Oneofthethreebearing pairsremained wetwithlubricant andcomponents ofthebearing appeared tobeingoodcondition. Bearing torqueforallpairsremained wellbelowtheoperational requirements ofthesystem [0.13N-m(18oz-in)dragtorque]throughout thelifetest. •Moog,Inc.,Schaeffer Magnetics Division, Chatsworth, CA "Raytheon Systems Company, SantaBarbara Remote Sensing, SantaBarbara, CA "*"NASAGoddard SpaceFlightCenter, Greenbelt, MD Proceedings ofthe34_Aerospace Mechanisms Symposium, Goddard SpaceFlightCenter,May10-12,2000 303