anominalsteppingfrequencyofabout2kHz.However,accounting fortheadditionaltimetostartand stopthemotor,the2rpm(4kHz)steppingrateseemedtoresultinatimeclosertotherequired5 secondsfora30-degreeincrement. Anunexpected obstaclewasencountered followinginitialassemblyandduringthemodule'sfirststages oftesting.Thisobstaclerequiredlengthytroubleshooting toovercome. Themodulewouldoperate properlyatroomtemperature, butnotatcryogenictemperatures eventhoughthedesignhadbeen carefullyreviewed.Throughaprocessofelimination, possibleculpritssuchasthesteppermotor, clearances onthealuminumhousing,andeventhemagnetoresistive sensorwereeliminated. Itwas determined thattheHWPbearingwasseizingduetoeitherwarpageofthebearingracesorslight materialvariationsinthestainlesssteelusedthroughoutthebearing.Thebearingsupplierworkedwithus toincrementally changethebearing'sdiametralclearancebyreducingtheballsizeuntilunrestricted rotationwasobservedwhilesubmerged inabathofliquidnitrogen.Apparently, thebearing'slevelof precisionwastooclosetowithstandthelargechangeintemperature. Oncethisobstaclewasovercome, furthertestingproceededsmoothly. Testinginthebathofliquidnitrogenprovedtobeaquick,lowcostapproachtocheckingcryogenic operationofthemechanism, particularly whentroubleshooting theseizedbearing.Fiberglass(G10)rods wereusedtomanipulate thecomponents inthebath,andthetemperature ofthecomponent couldbe inferredbymonitoringtheboilingrate.Infact,atonepoint,theentiremodule(minusmotorandsensor) wascompletelysubmerged inabathofliquidnitrogenandverifiedtospinfreely. Furthertestingwasperformedoncethemotorwasattachedtothegearingandaluminumhousing. Minimum MotorCurrent Themotorspecifications callforrunning themotoratanominal current of0.6Aperphase. Ateach temperature (room,nitrogen, andhelium) wemeasured theminimum current required toturntheHWP module. Atroomtemperature, thesystem spunfreelyinbothdirections andatboththe1and2rpm speeds withaminimum current of0.2A.Atliquidnitrogen andliquidheliumtemperatures thesystem required 0.2Atorunat1rpm,and0.3Atorunat2rpm.Fundamentally thismakessense:Morepoweris required tocoveranequaldistance inashorter periodoftime.Withmoreaccurate instrumentation, we mayhaveseenthiseffectatroomtemperature aswell. Atroomtemperature, theresistance acrossthemotorcoilswasmeasured tobe-3.Q;whencooledwith liquidnitrogen theresistance was10-4andatliquidheliumitwas0.6O_..Therefore, giventheminimum currents foundpreviously, themotorshouldtheoretically beabletooperate at2rpmdissipating (0.3A)2x 0.6O.=54mWofpowerperphase(108mWtotal).At1rpm,thisvaluewoulddropto48mWtotal.A largefraction ofthispowerwouldbedissipated through heating ofthemotorcoils. Endurance Testinq Successful endurance testswererunatbothliquidnitrogen andliquidheliumtemperatures. Thesetests wereconducted withamotorcurrent of0.3Aat2rpm.Themodule wasrotated through 72030-degree stepsoftheHWPwithfiveseconds ofrestbetween eachstep.Additionally, testswereconducted with themodule continually operating. Theseparticular testswereconducted atliquidheliumtemperature and dissipated asignificant amount ofheatintothereservoir ofliquidhelium. Asaresult,theflowrateoutof theventing heliumgasthatwasliberated increased significantly. Additionally, themotorcoilresistance gradually increased from0.6_to0.8O.,indicating thatcurrent waswarming themotor.Thistypeoftest wasconducted moreasameasure ofthemodule reliability, because suchoperation wouldnotbe normally experienced. DryLubricant Inanattempt tolowertheheatdissipated bythemotoradrylubricant, Dicronite ®DL-5,wasapplied tothe wormandwormgear.Theminimum current required toturntheHWPatliquidheliumtemperature was againfoundtobe0.3Aat2rpmand0.2Aat1rpm.Theamount ofheatreaching theheliumsurface due tothemotorandHWPmodule wasmeasured byexamining theboil-off rateoftheliquidhelium both beforeandafterapplying thedrylubricant. Themotorwasallowed toruncontinuously at2rpmand0.3A NASA/CP--2002-211506 32