r Improving theRobustness toVibration andTemperature Effects foraScanner Duplex Ball Bearing Assembly MerrittWebb*, Thomas Brewer* &Benjamin Joffe Abstract Precision mechanisms suchasscanners require precise bearing preloads coupled withpreload stability overtemperature andvibration. Thebearing retaining system iscriticaltoachieving therequired preload andmaintaining it.Ascanning motorbearing retaining system wasrecently redesigned toachieve bearing preload stability. Anewretaining system design, coupled withmaterial changes, resulted ina precise andtailorable design. Whenthemotorwassubjected tovibration andthermal variation, themodal properties didnotshiftindicating thebearing preload hadnotchanged. Thiswasasignificant improvement overtheprevious designwhichoften-showed modalshiftsofvibration of25Hzormore. Introduction TheNational Oceanic andAtmospheric Administration andtheNational Aeronautics andSpace Administration havedeveloped andoperated aseriesofpolarorbiting meteorological satellites since 1978.Therewereoriginally knownastheTelevision Infrared Observation Satellites andareknowknown asPolarOrbiting Environmental Satellites (POES). ThePOESsatellite operates inan870kmcircular nearpolarorbit.Theorbitissunsynchronous. Theprimary imaging instrument onthePOESisthe Advanced VeryHighResolution Radiometer (AVHRR). TheAVHRR isasixchannel imaging radiometer. Thesixchannels coverthespectrum fromthevisible andnearinfrared to12microns. Thechannels areco-registered toprovide multispectral data.The instrument hasaninstantaneous fieldofviewof1.3milliradians, providing anominal spatialresolution of 1.1kmatnadir. ThefirstAVHRR, a/1series, waslaunched in1978.Improvements wereimplemented andthefirst/2 serieswaslaunched in1981.Improvements tothescanmotorwereimplemented toprovide moretorque andlongerliferesulting inthe/3series.Thefirst/3AVHRR waslaunched in1995. TheAVHRR images areproduced byarotating scanner sweeping acrosstheearth.Thecollected energy isfocused ondiscrete co-registered detectors. Thedetectors aresampled, asagroup,byananalogto digitalconverter atdiscrete timeintervals. Foranimagetobeassembled fromthisdata,averyprecise rotation rateisrequired ofthescanner. TheAVHRR scanner usesan80-pole hysteresis synchronous motor.Themotorisa2-phase motor drivenby240-Hz square wavesinquadrature. Thescanner isshowninFigure1.Thismotorproduces a rotational rateof37.699 radians/second (360RPM).Themotorisinherently synchronized tothedrive wave,producing verylowjitter.Thecardinal requirements forthescanner arelistedinTable1. Thetwosquare wavesinquadrature alternately energize themotorwindings. Thesquare wavedrive signalisderived fromahighprecision clock,inordertoprovide alowjitterdrivesource. Themotorrotor accelerates duringstart-up untilitbecomes synchronous withtherotating field.Itthenfollows thefield withaphasedelay.Thisphasedelayisreferred toasthe"slipangle".Theslipanglewillvaryiftheload varies. Inordertominimize thetorque hashvariations themotorusesMPB$1724ballbearings (1.0625-inch bore)inaduplexpair.Thebearings areClass7T+,produced toanITT-generated sourcecontroldrawing. *ITTDefense, Aerospace/Communications Division, FortWayne, Indiana. Proceedings ofthe35thAerospace Mechanism Symposium, AmesResearch Center,May2001 35