TheRoleofBearing andScanMechanism LifeTesting inFlightQualification oftheMODIS Instrument StevenG.VanDyk', BrianJ.Dietz",Kenneth W.Street÷,William R.Jones+,Jr.,MarkJ.Jansen*, Michael Dube÷+,RajeevSharma *÷*,Roamer E.Predmore ÷++ Abstract TheModerate Resolution Imaging Spectroradiometer (MODIS) isaninstrument aboard theTerra(EOS AM-l)satellite andhasbeenoperating successfully sinceDecember 1999.MODIS hasbeenviewing the entireEarth's surface andgathering datatobetterunderstand theglobaldynamics andprocesses occurring onland,intheoceans, andintheloweratmosphere. Allobservations aremadethrough an extremely highresolution, optically andmechanically precise, scanmirrormotor/encoder assembly. The reliable performance ofthisassembly depends ontwoduplex bearing pairslubricated withPennzane, a synthetic hydrocarbon, formulated withleadnapthanate. Thispaperdescribes theresultsofaccelerated andoperational lifetests.Italsodescribes thepost-test analyses ofthedisassembled bearings. Analyses wereperformed usingmicro-Raman, micro-FTIR, X-rayPhotoelectron Spectroscopy (XPS),Scanning Electron Microscopy (SEM), andSizeExclusion Chromatography (SEC). Ingeneral, thethreesetsof bearings ineachoftheteststations wereinverygoodcondition afteraccumulating 68,144,and209 million revolutions, respectively. Someofthebearings exhibited lubricant degradation, indicated by viscous lubricant deposits onthecageandraceways. Introduction AttheonsetoftheMODIS development, Pennzane hadnotbeenflownonGoddard long-life lubricated spacemechanisms. Accelerated andreal-time lifetestswereconducted onMODIS scanbearings to demonstrate thatPennzane wouldsuccessfully lubricate themechanism forthe5-year(53.4million cycles) life.Threesetsofduplex MODIS bearings weretestedat23°C&20.3RPMfor68million revolutions (Station IV);at37°C&50RPMfor144millionrevolutions (Station III);andat45°C&72RPM for209millionrevolutions (Station II).Sixmonths before launch, the37°Caccelerated lifetestwas disassembled andtwoofthethreebearing setswereseverely worn(Ref.1).Theseresultsraiseddoubts aboutflightworthiness oftheMODIS instrument. Fortunately, aflightqualityscanmechanism hadalso beentestedfor4.7yearsatthattime.Measurements oftorquemargins andoperating parameters ofthe engineering modelscanmechanism withonlyafewmonths ofoperating timeandthelifetestmechanism showed notorquevariation oroperating parameter variation. Fromthesemeasurements, itwasconcluded thatnomeasurable bearing wearorlubricant degradation occurred duringthe4.7-year lifetest.After almost 5yearsofdamage freescanmechanism lifetesting, itwasconcluded thattheTERRNMODIS instrument wassafetofly.Thewisdom ofGoddard's requirement foranexpensive scanmechanism life testratherthanlessexpensive bearing lifetestingwasdemonstrated. Thispaperdescribes thebearing wearandlubricant degradation anddistribution aftertheaccelerated bearing lifetestat45°Candtherealtimelifetestat23°C.After68millioncycles(1.3lives)inthe boundary lubrication regimeat23°C,partoftheoilhadcreptintothesupport structure. Theoilcolorwas darkamber, butinmanycasestheviscosity wasunchanged, allballs,retainers andraceswerewetted andcontained oilmeniscuses, andlittlewearwasobserved ontheballsorraces.After209million revolutions (3.9lives)at45°C,moreoilcreeped intothesupport cups,theoilwasblackandviscous, all balls,retainers andraceswerewetted, smallmeniscuses ofoilwereattheballracejunctions, andvery *Raytheon Systems Company, SantaBarbara Remote Sensing, SantaBarbara, CA **Moog,Inc.,Schaeffer Magnetics Division, Chatsworth, CA +NASAGlennResearch Center, Cleveland, OH **NyeLubricants, Inc.,NewBedford, MA +++NASAGoddard SpaceFlightCenter, Greenbelt, MD Proceedings ofthe35thAerospace Mechanisms Symposium, AmesResearch Center,May9-11,2001