Mechanisms and Mechanical Devices Sourcebook, Four

Asetofelastomericbearingsconstitutesaspringycouplingbetweenaspurgearandadriveshaft.Thegear,bearings,andshaftarepartsofasplit-drive(redundant)mechanicaltransmission,andthecompli-anceofthecouplinghelpstodistributetorquenearlyequallyalongtheloadpathsofthesplitdrive.Complianceisneces-sarybecausewithoutit,evenslightdevi-ationsinthedimensionsoftheredundantgearscancausegrosslyunequalsharingofloads.Indeed,intheabsenceofcom-pliantcoupling,thegearsalongoneloadpathcanassumetheentireloadwhilethosealonganotherloadpathcanfree-wheel.Thus,theadvantageofreducedloadsongearteethislost.Thefigureillustratesoneversionoftheshaft/bearing/gearassembly.Aninner,concentricelastomericbearingliesbetweenacentraldriveshaftandanextensionofaringspurgear.Asetofpadlikeouterelastomericbearingsjoinsoutwardprotrusionsonanextensionofthedriveshaftwithfacinginwardprotru-sionsontheringspurgear.Theinnerelastomericbearinghashighradialstiffnessandlowcircumfer-entialstiffness.Thisbearingcenterstheringspurgearontheaxisofthedriveshaftandprovidescomplianceinacir-cumferentialdirection.Inarepresenta-tivedesignofaredundanthelicoptertransmission,itshouldbeatleast0.5in.(1.27cm)thicksothatittransmitslittletorque.Theouterelastomericbearings,incontrast,havelowradialstiffnessandhighcircumferentialstiffness.Theythustransmittorqueeffectivelybetweenfac-ingprotrusions.Nevertheless,theyaresufficientlycompliantcircumferentially154ELASTOMERICGEARBEARINGSEQUALIZETORQUELOADSElastomericbearingsmaketorqueloadsmorenearlyequal.LewisResearchCenter,Cleveland,Ohiotoaccommodatethedesiredamountofcircumferentialdisplacement[upto1/16in.(1.6mm)inthehelicoptertransmis-sionapplication].Theprocessofassemblingthecom-pliantgearingbeginswiththepressingoftheinnerelastomericbearingontothedriveshaft.Then,withthehelpofanalignmenttool,theringspurgearispressedontotheinnerelastomericbear-ing.Theouterelastomericbearingsaregroundtofitthespacesbetweenthepro-trusionsandbondedinplaceonthepro-trusions.Asanalternativetobonding,theentireassemblycanbepottedinasoftmatrixthatholdstheouterbearingsinplacebutallowsrotationwithlittlerestraint.ThisworkwasdonebyC.IsabelleandJ.KishofUnitedTechnologiesCorp.forLewisResearchCenter.ElastomericBearingscoupleadriveshaftwitharingspurgear.Theinnerelastomericbearingisradiallystiffandcircumferentiallycompliant,whiletheouterelastomericbearingsarecircumferentiallystiffandradiallycompliant.Thecombinationaccommodatesminorvariationsinthedimensionsandplacementsofgears,shafts,andothercomponents.PreviousPageAnimprovedgearsystemintendedpri-marilyforuseinahelicoptertransmitstorquefromthehorizontalornearlyhor-izontalshaftsoftwoenginestotheverti-caloutputshaftthatsupportstherotor.Thesystemapportionstorquesequallyalongmultiple,redundantdrivepaths,therebyreducingthestressesonindivid-ualgearteeth,anditenablesoneenginetocontinuetoturntherotorwhentheotherenginefails.Theunderlyingdesignconceptcouldalsobeappliedtocoupletwoairplaneenginestoasetofpropellersinsuchawaythatbothpropellersturnaslongasatleastoneengineoperates.Thesystemexploitsthespecialadvantagesofthegeometryofthemesh-ingofaspur-gear-typepinionwithafacegear.Incomparisonwithothergeargeometriesthathavebeenusedinheli-coptertransmissions,thisoneismuchmoreforgivingof(1)errorsinmanufac-turingandalignmentand(2)thermalandvibrationalchangesinthesizesandposi-tionsofthemeshingcomponents.Oneofthebenefitsisareductionofgear-tooth-contactnoiseandvibration.Anotherben-efitisthepossibilityofachievingahigh(4)speed-reductionratioinasingle,efficientmesh,andtheconsequentpossi-bilityofreducingthenumberofparts,thesize,thecost,andtheweightofthegearsystem.Ofcourse,thereductionofthenumberofpartsconfersyetanotherbenefitbyincreasingthereliabilityofthesystem.Thesystemisshownschematicallyinthefigure.Theoutputofeachengineiscoupledbyapinionshafttoaspur-gear-typepinion.Eachpinionengagesan155REDUNDANTGEARINGINHELICOPTERTRANSMITSTORQUERedundantgearingtransmitstorquethroughanangleorangles.LewisResearchCenter,Cleveland,Ohioupperandalowerfacegear,andeachfacegeariscoupledbyaface-gearshafttoanupperspurgear.Theupperspurgearsfeedtorqueintoalargecombininggear.Thepinionendofeachpinionshaftislightlyspring-loadedinanominallat-eralpositionandisfreetoshiftlaterallythroughasmalldistancetotakeupslack,compensateformisalignments,andapportiontorquesequallytothetwofacegearswithwhichitisengaged.Thecombininggearissplinedtoashaftthatflaresoutwardlytoasungear.Thesungearoperatesinconjunctionwithplanetarygearsandastationaryouterringgear.Thetorqueiscoupledfromthesungearthroughtheplanetarygearstotheplanet-carrierring,whichismountedontheoutputshaft.ThisworkwasdonebyRobertB.Bossler,Jr.,ofLucasWestern,Inc.,forLewisResearchCenter.Torquefromeachengineissplitandtransmittedtothecombininggearalongtworedundantpaths.Shouldoneenginefail,theotherenginecouldstillturntheoutputshaft.Inaproposedworm-geartransmission,oilwouldbepumpedathighpressurethroughthemeshesbetweentheteethofthegearandthewormcoil(seeFigure1).Thepressureintheoilwouldseparatethemeshingsurfacesslightly,andtheoilwouldreducethefrictionbetweenthesesurfaces.