ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Introduction TheInternational SpaceStationwillbe,whencompleted, thelargeststructure everassembled inspace. TheSpaceStationwillamongotherthingsinclude8solararrays(4portand4starboard) thatwillprovide powertothestation.Tomaximize efficiency oftheindividual solarcells,thearraysmustbeconstantly repositioned relativetothestation.Thisrepositioning isachieved throughtheuseoftworotaryjoints,the AlphaandBetajoints(seeRef.[1]formoreinformation ontheBetajoint).TheAlphajointisthelargerof thetwojointsandisrotatedbymeansoftheDLA.Bothrotaryjoints(portandstarboard) haveaprimary andredundant DLA.TheprimaryfunctionoftheDLAistorotatetheSARJanditssecondary purpose isto accurately lockthejointatspecificlocations toallowastronaut traveltotheouterportionsofthestation. A similartypeofdesigncalledtheThermal Radiator RotaryJoint(TRRJ)DriveLockAssembly (TDLA)isused torotatethesmallerRadiator joint.Amajorityofthepartsarethesame,however, thereisnoGuide Assembly asthematinggearismuchsmallerandafixedpositionisacceptable (4boltsrigidlyfixthe TDLAtotheTRRJwhilethetwoGuideAssemblies allowtheSDLAtomoverelativetotheSARJstructure). ThefocusofthispaperwillbeontheSDLAbutamajorityofthelessonslearnedstemmed frombothunits. DesignFeatures TheDLAconsistsofseveralsignificant subassemblies (Figure2),eachwithaspecificfunction: EDMAssy DriveMotorAssy Assy ! LeftGuideAssy-- Figure2:SARJDriveLockAssembly TheEnaaae/Disenaaae mechani._m (EDM)(Figure3)isasteppermotor-driven four-barlinkagethatrotates themotorbetween its3mainpositions (Figure4):Drive(Pinionteethengaged), Neutral(neitherLockor Pinionengaged andwithinaspecificbanddefinedasneutral), andLocked(Lockrackengaged). Theother twopositions oftheEDMareLockedandOver-center (sameasthelockedpositionexceptthemechanism 62
hastraveled over-center), andIn-Between (somewhere inbetween theotherpositions). Theposition ofthe EDMismonitored bylimitswitches. Boththepinionandlockhavemechanical fusesthatreacttheload duringatooth-to-tooth crashbetween thepinionorlockandthematingbullgear.Thestepper motoris madeupoftwowindings forredundancy. Duringfinalmanufacturing operations, thismechanism isadjusted toensurefivedegrees ofover-center oneachside. --- Figure3:Engage-Disengage MechanismFuse Difficulties encountered duringdesignincluded configuration ofassembly duringlaunchandmethod of securing adjustable fasteners. Duringlaunch,themechanism mustbepreloaded insomefashion toprevent rotation ofthegeartrainand avoiddamaging theoutputgear.TheTDLAhasarigidmountsothepinionwouldbesusceptible todamage iftheEDMshiftedposition duringlaunch. TheSDLAispositioned awayfromtheraceringduringlaunch, removing thepossibility ofinterference between thepinionandthebullgearandallowing themechanism to bepositioned over-center duringlaunch. Theoriginal planforrestraining theTDLAmechanism duringlaunch wastorelyonthestepper motordetenttorque. Itwaslaterrealized thatthedetenttorquewasnotcapable ofrestraining theestimated 1335N(300Ib)delivered tothestepper motorbythemassofthedrivemotor.A tradestudywasinitiated andanExtra-Vehicular Activity (EVA)operated pippindesignwasselected. This designaddedanextraEVAoperation butremoved theneedforanaddedmechanism. TheEDM,duetothesignificant number ofpieceparts,undergoes aseriesofadjustments toensure adequate backlash, sufficient over-center capability, andreliable telemetry. Theseadjustments aremade withstandard boltsandonceadjusted, getstakedinplace.Theoriginal designusedanepoxymaterial but 63
concernsaroseaboutitscapabilityduringthermalextremessothedesignwasrevisedtoincludelocking nutsasamorepositivelockingfeature. -I t_ PinionEngaged Neutral  t.Locked Figure4:EDMPositions TheMotorA_embly (Figure5)consists ofatwo-phase brushless DCmotorpressed intoanaluminum housing withacommutating resolver packaged inboard. Aninternal geartrainproduces ahighmechanical advantage. Therotating shaftsturnaboutpairsofduplexbearings withonesidefixedandtheopposite side allowed tofloataxiallytoaccommodate anythermal distortions. Theoutputpinioniscrowned andhasboth itstipandrootrelieved. 64
Motor  Stator •It'/// ,'3•"•--_,71=1,1"_ Resolver Rotori| i•I Resolver l_ Stator ___ Intermediate'_ F Gear :- "':_,_. '_<IN:;,:_,;.:_,:t_ "_'_ -/L----] _.>:S_,",:_/_. _===_:_,.-_:Ji_.'.'_'_...'."_ "-'- Output Pinion Figure5:MotorAssembly Earlydesignchallenges included selection oftheoptimum bearingpreload, controloflubrication between intermediate gears,andabilitytominimize resolvererrorvoltage• Bearingpreloadwassethighenoughto preventgappingduringlaunchbutlowenoughtoavoidlifeimpacts. Braycote 601wasusedasalubricant between theintermediate gearmeshwithalargeenoughquantitytoallowtheformation ofagreasedamto preventlossoflubrication. Amethodofstaticresolveralignment (withrespecttothemotor)wasdevisedto allowalignment towithin5-degree positional error(99.6%electrical efficiency). LeftandRiahtGui(;leAssemblies (Figure6)eachconsistof3bearings thatseatthemselves ontheSARJ bullgear,allowing thepiniontocloselytracktheSARJbullgear.Duetothelargesizeofthebullgear (pitchdiameter of3.289m),andtheimportance ofmaintaining atightbacklash, arigidmountoftheDLAto theSARJstructure wasbypassed andinsteadtheguideassemblies wereimplemented toallowtheDLAto closelytracktheraceringandkeepthebacklash withintolerance byfloatingtheDLA.Thelaunch configuration oftheDLAisdifferentthantheon-orbitpositionasaresultofloadingoftheDLAduringlaunch. Thebearingassemblies (followers) aretherefore notengaged duringlaunchbutrequireaseriesofEVA operations toallowrepositioning intoanon-orbitposition(Figure7).Preloadofthebearings againstthe raceringisachieved bythecompression ofbelleville washers usingthemaindrawbolt andcontrolled witha hardstop.Calibration forthepreloadsettingisperformed atacomponent levelusingstraingages. Challenges included ensuring properpreloadofthefollowers againsttheraceringanddetermination ofan efficientmethodtoreconfigure fromlaunchtoanon-orbitposition. Preloadisattained bycompressing belleville washers apresetamountwiththeEVAdrawbolt. The tolerances onthedesirable preloadweretightenoughtorequireuseofanextrafinethreadonthedrawbolt. Tumingthedrawbolt produces motionofthefollowerblock,whichrotatesthearm;oncetheblockbottoms outonthehardstop,thebellevilles areloaded.Withastandard thread,repeatability onthepreloadwould notbeattainable. Inordertoavoiddamagetothebearingassemblies duringlaunch,theDLAislaunched withthefollower bearings removed fromtheracering.ThiscreatestheneedforseveralEVAoperations toreconfigure the unittoitson-orbitposition. Thisisachieved throughtherotationofseveralEVAboltsthatrotatethearmin twodifferentaxis. 65
TheFrameAssembly istheprimary structure fortheDLAandisdesigned toreactlaunchandon-orbit loading• --r_:,.Drawbolt_ ,,:_:;:............ ,_< ""........./ i'_,,i#,-,_ "'"....r'.- .<.i_....st #_,_._ l_ o -- - ,--- ._%\_""/,I Figure6:GuideAssembly Crosssection Design Evolution TheDLAwentthrough asignificant number ofchanges asthedesignprogressed intowhatitistoday.The earlydesigndidnotconsider suchissuesasToothCrashes, Gearratcheting, Pinioncrowning, and Microgravity. Atoothcrash(Figure8)isaphenomenon inwhichthematinggearsarriveatatip-to-tip contact that prevents engagement oftheteeth.Thiscanoccurbothwiththepinionandthelockrack•Theoriginal designallowed fornosoftspringorfusetoreacttheloadingandinstead theloading duringthiseventwent directly intothemechanism. Inordertoguardthemechanism fromunnecessary loading andmore importantly, protectthegearteethfromdamage, amechanical fusewasintroduced intothedesignonboth thepinionandlocksides,whichonceadjusted, wouldreacttheloading fromthetoothcrash.Converting thefour-bar designintoaspring-loaded fourbaralsocreated amuchmorereliable design, asitbecame less sensitive tolinklengthvariability brought aboutbymanufacturing tolerances, initialsetup,andthermal distortions. Gearratcheting mightoccuriftheDLAweretostallduringahighspeedcommand orifanexternally applied loadwereencountered thattendedtodrivethepinionoutofengagement. Typically theloadfromthefuse wouldbesufficient toreactthis,however, thisreaction springwasdesigned softenough toallow compression duringatoothcrashsoalargeenough loadwouldcompress thespringandallowthegearto ratchetaboutthematinggear.Ahardstopwasaddedtothedesignthatprevented fulldisengagement of thepinionandwouldsubsequently berotatedoutofthewaytoallowmotorrotation toaneutralorlock position duringanEDMcommand. 66
TheneedforpinioncrowningwasrealizedonceananalysisonthealignmentoftheDLApinionrelative to thebullgearwascompleted. Theexisting designallowed theedgeofapiniontoothtodirectly contact the faceofthematingtooth,producing highcontact stresses andresulting inearlysurface failureofthepinion. Thecrowning oftheteethremoved theportionofmaterial fromthepinionthatcontacted thefaceofthebull gear. Thebaseline resolver didnothavetheaccuracy required tomeetmicrogravity requirements. Anewresolver withbetteraccuracy wasinstalled intotheunit. 1_I_unr.h_nnfinHr_tinn .-""-.% r"  _ "-.[,._,,+ _.....- tP .+jtJ:_.__;I7,___"J"..,_:;   :ot_,-.:_*JL J,*1_'Z ,,#', r't r+"I /I _.'--.-!ik\-..,._.,__J \,..'" F",/'" Figure7:Stepstoreposition DLA ToothCrash Normal Enqa.qement SDLAPinion GearSDLAPinion Gear .. ..• ..". . .;', ,"_, - ,° ./:. -,. : :: -... \- ..... ,: ..; --.,l'.-",... ; ,.- ,.,:" .. '..... .- ;;:"-..: SARJBullGear.-''. :"".. I"__-..., .;-+. : _;";''" :i_:,::-•` '":.:k.::::;..:;:.:"....-+ :-,.- j'.:. _,..,.'\... ",. •",+•,••-."t,"i"" ;' •"-oi.,:: ""'+'+•Backlash SARJBullGear Figure8:ToothCrashversus Normal Engagement 67
Piniontooth--_r/ f iJ  Tipreleif ',, /_ Original profile  Newprofile i / , Rootrelief Figure9:Modified pinionprofile 2)StePper Kickback DuringThermal Vacuum testingononeoftheDLA's,acommand wasgiventodrivetheunitintothelocked position andthenstop.Thecommand wassentandthedestination wasreached. Thepowerwasthencut andimmediately afterthetelemetry indicating position ofthemechanism changed states,implying thatthe mechanism hadmoved. Troubleshooting commenced anditwaslaterdiscovered thatthemechanism had indeedshiftedastherewasnoevidence ofaswitchfailure(thiswastheprimary suspect sinceonlyoneof thetwoswitches changed states). Thiswasofparticular concern asthemechanism couldbebackdriven justenough tobeneargoingbackover-center butnotenough totriggerthetelemetry. Thestepper motorusedintheEDMisaredundantly wound2-phase brushless dcmotor.Itconsists of12 poles,hasa77:1gearreducer, runsatanominal rateof8Hz,andhasatorquecapability of4882N-m(300 in-lb,whichincludes a70percent reduction duetoelectrical efficiency loss,whichisconservative). Ittakesaparticular number ofstepstodrivethemechanism fromaneutralposition toalockedposition. In ordertoguarantee thatthemotorreaches itsdestination, themechanism is"overdriven" byapprox. 10 percent ofthetotalrequired stepstoreachthehardstop (astepcommand of110wouldbegivenfora destination 100stepsaway). Oncethemechanism reaches thehardstop,theremaining stepsinthe command continue topulsethestepper againstthestop.Oncethestepcommand ends,thestepper holds itsposition astheunitisstillpowered, andthesoftware thencutsthepower,allowing themechanism to relax.Ifasignificant enough forcehadbeenbuiltupduringthelastpulseintothehardstop,thisrelaxation wouldforcethemechanism intheopposite direction, awayfromthehardstop. This"relaxation" ofthe mechanism wasneverdiscussed priortothiseventasitwasbelieved thatsincethemechanism hadgone 69
(Figure11)whileduringsystem-level testing,theunitispositionedmorehorizontally. Bothsetupsrequire off-loading,andtheaffectsoftheoff-loadingwillinfluencethetest.Theoffloadsystemsinthiscasewere completelydifferent.Thisvarianceinoff-loadingmethodscoupledwiththechangeinthegravityvector allowedanissuetogoundetected untilsystem-level testing. .__..-,.--,'<2_.,..",,%-._-, /7;,,,.'../,.'_",:.. Gapping -- WideFollower Bearing  ._lJ.I._ {J_,_,.._<_',_,_..l__'_'_._ _BullGearSegmen, OutboardFollower Bear;_;_ I[[_"''_'_ ._ '_ f_"4._lJl_nboard Fotlower Bearing Figure 10:Follower Bearing Gapping Figure11: DLAoncomponent STEwithoff-loading system 72
Aftermoretestingandanalysis,itwasdiscoveredthatcontrollingtheamountofgappingcouldbeachieved byalteringthebearingpreloadagainsttheracering.Anincreaseinpreloadmeantadecreaseingapping. Unfortunately, anincreaseinpreloadalsomeantlesslifeoutofthebearing.Intheend,anoptimumpreload wasreachedthatpreventedthisfalsetelemetryfromoccurringandalsoallowedthebearingtostaywithin therequiredlifeconstraints. Lesson: -Ensureimpacts ofcomponent-level issuesatasystem levelarewellunderstood. -Testthecomponent initsflightorientation. -Ifpossible, testthecomponent underallknownsystem-level configurations. OtherI_ss0ns learned thr0uqhout theproqram -Theimportance ofprecise off-loading. Astouched oninLesson 5anddiscovered duringadditional system-level testing, ahighpriorityshouldbegiventoensuring anaccurate off-loading systemduring groundtesting. Afteroneparticular runduringsystem-level testing, agroovewasdiscovered ontheSARJ raceringthatranamajority ofthewayaroundtheunit.Afterathorough reviewofthehardware, itwas determined thatthefixedDLAfollower bearing wasnotflushwithitsmatingsurface duringoperation and causedtheformation ofthegroovebyapplying apointloadontheracering.Theoffloadsetupwasreviewed andwasfoundtobebiasingtheunitinonedirection andwasnottrulyoff-loading themechanism. The method ofoff-loading wassubsequently modified andthefixedfollower bearing tracked flushwithitsmating surface. -Picturetakingduringmanufacturing. Apersoncannevertaketoomany. -Consideration ofrepairability duringdesign. Noneofusliketothinkourhardware willfail,butithappens. Conclusion TheDLAisauniqueandinteresting assembly whosedesignhasbeenrefinedthrough arigorous test program. Theproblems encountered duringthistestprogram revealtheimportance oftestandalsothe impacts seemingly minorproblems canhaveatthesystem level.Amajority oftheunitshavealready been integrated intothenextassembly hardware andhavegonethrough systemtesting. Thefirstoftheseunits arecurrently scheduled tobelaunched intheFallof2000. References 1.O'Brien, David.International SpaceStationAlpha's Bearing, Motor,andRollRingModule Developmental Testing andResults, Proceedings ofthe28_Aerospace Mechanisms Symposium, 1994, pp51-61. 2.Alchorn, Brendon. SARJPinionLifeTestReportandWearAnalysis, LMSSC EM-02-ME-510, dated 5/6/98 3.McCully, Sean.TDLAEngage Disengage Mechanism Analysis, LMSSC EM-02-ME-422, dated 11/28195 4.McCully, Sean.SDLAPinionAlternate Material Review, LMSSC EMDLA-1121, dated1/20/97 73
aspectsofbasicmachiningrelativetofeeds,speedsanddimensional stabilityduringheattreatment neededtobedetermined. Hard-on-Hard Manual Override WormDrive Inaddition totheelectromechanical actuation requirements, theBMAwasalsorequired tohaveamanual override inputcapability. Duetothemulti-position finalassembly requirement, themanual override interface needed tobeaccessible fromtwoopposing sides(aspecrequirement). Theoriginal input speedandtorquerequirements totheMO,toachieve sameoutputspeedandtorqueasthepowered drive,werealsoverylimitedinselection duetotheuseofEVAstandard interface toolingavailable atthe startoftheoriginal contract. Thesefactors, combined withthepackage sizeconstraints, forcedtheuse ofaHOHwormdrivetocarry113N-m(1000in°lb)output(atthewormgear)witha50.8-mm (2.0in) pitchdiameter. Thepackage didnothaveenough roomtofacilitate asealedcavityintheareaofthe worm,preventing theuseofanoilbath.Identifying aspace-rated, lowoutgassing, -54°C(-65°F)to71°C (160°F) operational range,high-pressure wormdrivelubrication provedtobeamajorchallenge forthe program. Brushl_ss DCMotorandController Again,duetopackage sizeandpowerconsumption constraints, packaging aDCtoDCpowersupply, a brushless 120voltmotorcontroller, andabrushless motorwasachallenge. Themajorobstacle turned outtobelackofroomtofitaresolver intothepackage. ThisforcedthedesigntoimbedanHED commutation boarddirectly intothe50.8-mm (2.0in)ODstatorwindings. Theproduction methods used toaccomplish thisfeatareinteresting, butarenotwithinthescopeofthispaper. No-Back-drive Device Inordertointerface theMOdrivewiththemaindrive,thewormgeardiscussed abovewasintegrated at theODoftheplanetary lastpassringgear.Thisrequired thatduringmaindrive operation, thewormdrive bepositively lockedtosupplymechanical ground totheringgear.Anymotionallowed bythewormdrive wouldlimittheavailable outputtorqueatthemaindrive.TheFQDCprogram haddeveloped aunique devicethatactedasapositive lock,singleinput,singleoutput, bi-directional torquediode.FortheBMA program, thisunitwasscaleddown25%insize,converted intoatwoinput,singleoutputdevice, withthe potential ofseeing25%moretorque. Tohandlethehighertorqueload,Pyrowear ®675wasalsoused extensively forthisdevice. Program Start Preliminary DesignReview Detailed Design Review QualTestStart QualTestEnd DeltaQualStart DeltaQualEnd FirstProduction UnitShipped LastProduction UnitShippedProgram 12/01/95 02/20/96 10/22/96 08/28/97 02/17/98 08/17/98 09/08/98 08/04/98 04/26/99Chronology andIllustrations: 77
Lessons Learned Pyrowear ®675,General Geararranqement Allgearing intheBMAwasmadefromPyrowear _675.Themaindrive wasa4passplanetary system with48diametrical Pitch(DP)gearsandan833:1overallratio(Figures 5&6).Stages 2,3and4were thesamewiththesuncontaining 16teeth,planets 32teethandtheringgear80teeth,obtaining a6:1 ratioforeach3planetsystem. TheMOgearswere24DPwithtwospurpasses beforethewormshaft andhada12.17:1 overallratio.Thewormsetwasasinglehelix,33tooth,16DP,ClassCset.Because thecenteraxisoftheBMArelative totheunitoutside dimensions wasfixedinthespecification, itwas necessary toproduce thelastpassringgear(attheID)integral tothewormgear(attheOD).When running theMO,thelastpassplanetary setranwiththesungearfixed,theringgearasinputandthe carrier(oroutputspline)asoutput(alsocalledasolarsystem). Thislastpassaccounted fora1:1.2ratio. Thus,113N.m(1000in.lb)inputtothewormgearcreated 135.6N,m(1200in,lb)attheoutput. Also, thehighest stressed member wasthelastpasssungearwhichsaw22.6N*m(200in°lb)inputfor135.6 N.m(1200in.lb)outputduringbothMOandmaindrive operation. Forthisreason, thelastpassgear facewidthwasincreased to12.7mm(0.5in),whiletheotherstagesuseda6.35-mm (0.25-in) facewidth. I1 iISLAI_TP SPINDLE I_JIJGG_.AI_ B_.AP]l,lg i Figure 5:BMAMainDriveGearCrossection I GearHeatTreat Pyrowear ®675wasarelatively newmaterial inindustry in1996.Carpenter Technologies haddeveloped basicmaterial information (mechanical properties, basicheattreat,etc)butonlyhaddirectexperience with24hourcarburization cycles(equivalent toa1mm(0.040in)effective casedepth). Inconversations withCarpenter, itwasapparent thatatleastonemajorturbine enginemanufacturer wasusingthis material, butalldatadeveloped bythemwasdeemed proprietary. SincetheBMAprogram required a lightcasedepthusingfinepitched gearing, thedetailed heattreatprocess needed tobedeveloped through experimentation. Pyrowear ®wasselected because ithadthesamebasiccorrosion properties of 440Cinitscarburized state,wouldallowhighsurface hardness tocarrysignificant contact stress,and obtainhigherimpactstrength through itslowerhardness, moreductilecore.Thislastpointwas considered important sincepitchdiameter crossections ontheorderof0.9mm(0.035in)withfullhard material wouldnotallowformuchenergyabsorption, shouldsudden startsorstopsbeencountered. ere-O_x. Pyrowear ®675requires apre-oxidation phaseasafirststeptocarburization. Thepre-oxcleansthe surface ofcontaminants andprepsthesurface forbettercarboninfiltration. Unfortunately, thepre-ox phasealsoeffectively ablates (oroxidizes) thesurface whichresultsinalossofmaterial. Typically, a carburized material growsinnetsection. Thisdidnotprovetobeamajorproblem forthisprogram since therewasnobacklash requirement fortheunit.Adecrease intooththickness increased backlash and 8O
Figure6:Actual Planetary System Parts decreased bending strength slightly. Theamount ofmaterial losswasdependent onthepre-ox temperature, whichhadanacceptable rangeof954°C(1750°F) to1010°C (1850°F) for60minutes. The lowendoftherangeresulted inlessmaterial loss(ontheorderof0.013mm(0.0005 in)persurface) but alsoresulted inlowersurface hardness duetolesscarboninfiltration duringcarburization. Conversely, the1010°C levelresulted insignificant material loss(ontheorderof0.04mm(0.0015 in)persurface), but betterfinalsurface hardness. Thescalecreated onthesurface caused bythepre-oxphasewasalso veryhardtoremove afterfinaltemper. Duetothefinepitchgearing, theuseofinternal gears(ringgears) andthelightcasedepth,postheattreatgrinding wasnotanoption. Itwasnecessary toaluminum oxide gritblastallpartstoremove thisscale.Eventhen,thesurface wouldnotalways cleanupsufficiently to becontinuous andsmooth. Oneoptionsuggested byCarpenter toobtainabettersurface hardness with lessoxidation wastogritblastthepartpriortoheattreat,andthenusealowerpre-Oxtemperature during processing. Thisavenue wasinvestigated butshowed littlebenefitinthefinalresult.Asaresultofthe increased backlash, nonoiselimitrequirements, relatively slowrotational outputandrelatively lowloads duringthemajority ofitsoperational life,theBMAdesignwasnotaffected byimperfect gearface geometry caused byoxidation scale. Case.Depth Aftersubstantial experimentation withtheheattreatprocess, aspecific routinewasdeveloped to consistently achieve ahighRockwell hardness atthesurface (Rc58-62)andmaintain aneffective case depthof0.1to0.2mm(0.004to0.008in)whichwasneeded formostparts.Acasedepthof0.3to0.4 mm(0.012to0.016in)wasusedonthewormshaftandplanetpinssincefinishgrinding wasrequired on theseparts.Thecorehardness obtained wasdependent onsection thickness butwasintherangeofRc 40-45. Onekeyfactdetermined wasthatstandard heattreatenvironments usedduringtheaustenitize stepresulted inasignificantly reduced surface hardness, andspecial precautions hadtobetakento prevent thisfromhappening. Theseprecautions included decreasing thecarburizing timeandusinga carbon atmosphere duringaustenitizing. Figure7showsatypicalpolished geartoothmounted and etchedwithpicralHCL.Twobandsofmicrohardness impressions arerepresented bythediamond shaped marks. Thedarkbandalongtheperimeter ofthetoothrepresents thezoneofcarbon infiltration 81