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HowstuffworksInthiscaseGearUnits AllinformationaretakenfromtheInternetpage TECHNICALINFORMATION GEARS HOWDIFFERENTGEARSWORK SPURGEARisusedwhenshafts white mustrotateinthesameplane Theteetharestraightandparalleltotheshaft Thepinion yellow isthesmallerofagearpair ThedifferentkindofGears INTERNALGEARallowsthepinion yellow torotateontheinsideofaringtypegearwithstraightteethonitsinnersurface Thisisatypeofspurgearinwhichboththepinionandthegearturninthesamedirection unlikeordinaryspurgears ThedifferentkindofGears HELICALGEARhascurvedteethatanangletotheshaft Theseteethgripwithlessnoisethanstraightteeth especiallywhentheyareturningathighspeeds ThedifferentkindofGears HERRINGBONEGEARhasteeththatareV shaped likeadoublehelicalgear Theshaftsrotateinoppositedirectionsasinanordinaryspurgear ThedifferentkindofGears BEVELGEARhasteeththatslopealongonesurfaceofthedisc Thepinion yellow rollsatanangletothetopofthegear notalongitsedge Thistypeofgearisusedwhentheshaftstobeturnedmeetatanangle ThedifferentkindofGears WORMGEARhasascrewpinion theworm whichturnsalongaspurgear Motioncanbetransmittedbetweenshaftsthatareatrightangles ThedifferentkindofGears PLANETARYGEARisasystemwherebyPLANETspurgearsturnonacentralSUNgearandaninternalringgear Thissystemisusedinautomaticcars ThedifferentkindofGears Youseegearsinjustabouteverythingthathasspinningparts Carenginesandtransmissionscontainlotsofgears IfyoueveropenupaVCRandlookinside youwillseeitisfullofgears Wind up grandfatherandpendulumclockscontainplentyofgears especiallyiftheyhavebellsorchimes Youprobablyhaveapowermeteronthesideofyourhouse andifithasasee throughcoveryoucanseethatitcontains10or15gears Gearsareeverywherewherethereareenginesandmotorsproducingrotationalmotion PuttingGearstoWorkGearsaregenerallyusedforoneoffourdifferentreasons ToreversethedirectionofrotationToincreaseordecreasethespeedofrotationTomoverotationalmotiontoadifferentaxisTokeeptherotationoftwoaxessynchronized Youcanseeeffects1 2and3inthefigureabove Inthisfigure youcanseethatthetwogearsarerotatinginoppositedirections thatthesmallergearisspinningtwiceasfastasthelargergear andthattheaxisofrotationofthesmallergearistotherightoftheaxisofrotationofthelargergear Thefactthatonegearisspinningtwiceasfastastheotherisbecauseoftheratiobetweenthegears thegearratio Inthisfigure thediameterofthegearontheleftistwicethatofthegearontheright Thegearratioistherefore2 1 pronounced twotoone Ifyouwatchthefigure youcanseetheratio Everytimethelargergeargoesaroundonce thesmallergeargoesaroundtwice Ifbothgearshadthesamediameter theywouldrotateatthesamespeedbutinoppositedirections UnderstandingtheConceptofGearRatioUnderstandingtheconceptofthegearratioiseasyifyouunderstandtheconceptofthecircumferenceofacircle KeepinmindthatthecircumferenceofacircleisequaltothediameterofthecirclemultipliedbyPi Piisequalto3 14159 Therefore ifyouhaveacircleoragearwithadiameterof1inch thecircumferenceofthatcircleis3 14159inches Thefollowingfigureshowshowthecircumferenceofacirclewithadiameterof1 27inchesisequaltoalineardistanceof4inches Let ssaythatyouhaveanothercirclewhosediameteris0 635inches 1 27inches 2 andyourollitinthesamewayasinthisfigure You llfindthat becauseitsdiameterishalfofthecircle sinthefigure ithastocompletetwofullrotationstocoverthesame4 inchline Thisexplainswhytwogears onehalfasbigastheother haveagearratioof2 1 Thesmallergearhastospintwicetocoverthesamedistancecoveredwhenthelargergearspinsonce Mostgearsthatyouseeinreallifehaveteeth Theteethhavethreeadvantages Theypreventslippagebetweenthegears Therefore axlesconnectedbygearsarealwayssynchronizedexactlywithoneanother Theymakeitpossibletodetermineexactgearratios Youjustcountthenumberofteethinthetwogearsanddivide Soifonegearhas60teethandanotherhas20 thegearratiowhenthesetwogearsareconnectedtogetheris3 1 Theymakeitsothatslightimperfectionsintheactualdiameterandcircumferenceoftwogearsdon tmatter Thegearratioiscontrolledbythenumberofteethevenifthediametersareabitoff DetailsonInvoluteGearProfilesOnaninvoluteprofilegeartooth thecontactpointstartsclosertoonegear andasthegearspins thecontactpointmovesawayfromthatgearandtowardtheother Ifyouweretofollowthecontactpoint itwoulddescribeastraightlinethatstartsnearonegearandendsupneartheother Thismeansthattheradiusofthecontactpointgetslargerastheteethengage Thepointofcontactdescribesastraightlinethatstartsnearonegearandendsupneartheotherfrombasecircletobasecircle 341 278 303 408 446 316 473 325 535 323 534 366 596 349 DetailsofanInvoluteGearProfile1 Onaninvoluteprofilegeartooth thecontactpointstartsclosertoonegear andasthegearspins thecontactpointmovesawayfromthatgearandtowardtheother Ifyouweretofollowthecontactpoint itwoulddescribeastraightlinethatstartsnearonegearandendsupneartheother Thismeansthattheradiusofthecontactpointgetslargerastheteethengage 341 278 303 408 446 316 473 325 535 323 534 366 596 349 2 Thebaselinediameteristheeffectivecontactdiameter Sincethecontactdiameterisnotconstant thepitchdiameterisreallytheaveragecontactdistance Astheteethfirststarttoengage thetopgeartoothcontactsthebottomgeartoothinsidethepitchdiameter Butnoticethatthepartofthetopgeartooththatcontactsthebottomgeartoothisveryskinnyatthispoint 341 278 303 408 446 316 473 325 535 323 534 366 596 349 3 Asthegearsturn thecontactpointslidesupontothethickerpartofthetopgeartooth Thispushesthetopgearahead soitcompensatesfortheslightlysmallercontactdiameter Astheteethcontinuetorotate thecontactpointmovesevenfurtheraway goingoutsidethepitchdiameter buttheprofileofthebottomtoothcompensatesforthismovement GearTrainsTocreatelargegearratios gearsareoftenconnectedtogetheringeartrains asshownhere Theright hand purple gearinthetrainisactuallymadeintwoparts asshownabove Asmallgearandalargergearareconnectedtogether oneontopoftheother Geartrainsoftenconsistofmultiplegearsinthetrain asshownonthenexttwopages Inthecaseabove thepurplegearturnsataratetwicethatofthebluegear Thegreengearturnsattwicetherateofthepurplegear Theredgearturnsattwicetherateasthegreengear Thegeartrainshownbelowhasahighergearratio Inthistrain thesmallergearsareone fifththesizeofthelargergears Thatmeansthatifyouconnectthepurplegeartoamotorspinningat100revolutionsperminute rpm thegreengearwillturnatarateof500rpmandtheredgearwillturnatarateof2 500rpm Inthesameway youcouldattacha2 500 rpmmotortotheredgeartoget100rpmonthepurplegear Ifyoucanseeinsideyourpowermeterandit softheolderstylewithfivemechanicaldials youwillseethatthefivedialsareconnectedtooneanotherthroughageartrainlikethis withthegearshavingaratioof10 1 Becausethedialsaredirectlyconnectedtooneanother theyspininoppositedirections youwillseethatthenumbersarereversedondialsnexttooneanother PlanetaryGearsTherearemanyotherwaystousegears Onespecializedgeartrainiscalledaplanetarygeartrain Planetarygearssolvethefollowingproblem Let ssayyouwantagearratioof6 1withtheinputturninginthesamedirectionastheoutput Onewaytocreatethatratioiswiththefollowingthree geartrain PlanetaryGearsets GearRatiosAnyplanetarygearsethasthreemaincomponents ThesungearTheplanetgearsandtheplanetgears carrierTheringgearEachofthesethreecomponentscanbetheinput theoutputorcanbeheldstationary Choosingwhichpieceplayswhichroledeterminesthegearratioforthegearset Let stakealookatasingleplanetarygearset Oneoftheplanetarygearsetsfromourtransmissionhasaringgearwith72teethandasungearwith30teeth Wecangetlotsofdifferentgearratiosoutofthisgearset Inthistrain thebluegearhassixtimesthediameteroftheyellowgear givinga6 1ratio Thesizeoftheredgearisnotimportantbecauseitisjusttheretoreversethedirectionofrotationsothattheblueandyellowgearsturnthesameway However imaginethatyouwanttheaxisoftheoutputgeartobethesameasthatoftheinputgear Acommonplacewherethissame axiscapabilityisneededisinanelectricscrewdriver Inthatcase youcanuseaplanetarygearsystem asshownhere Inthisgearsystem theyellowgear thesun engagesallthreeredgears theplanets simultaneously Allthreeareattachedtoaplate theplanetcarrier andtheyengagetheinsideofthebluegear thering insteadoftheoutside Becausetherearethreeredgearsinsteadofone thisgeartrainisextremelyrugged Theoutputshaftisattachedtotheblueringgear andtheplanetcarrierisheldstationary thisgivesthesame6 1gearratio Youcanseeapictureofatwo stageplanetarygearsystemontheelectricscrewdriverpage andathree stageplanetarygearsystemofthesprinklerpage Youalsofindplanetarygearsystemsinsideautomatictransmissions Also lockinganytwoofthethreecomponentstogetherwilllockupthewholedeviceata1 1gearreduction Noticethatthefirstgearratiolistedaboveisareduction theoutputspeedisslowerthantheinputspeed Thesecondisanoverdrive theoutputspeedisfasterthantheinputspeed Thelastisareductionagain buttheoutputdirectionisreversed Thereareseveralotherratiosthatcanbegottenoutofthisplanetarygearset butthesearetheonesthatarerelevanttoourautomatictransmission Youcantrytheseoutintheanimationbelow AnimationofthedifferentgearratiosrelatedtoautomatictransmissionsClickonthebuttonsontheleftinthetableabove Sothisonesetofgearscanproduceallofthesedifferentgearratioswithouthavingtoengageordisengageanyothergears Withtwoofthesegearsetsinarow wecangetthefourforwardgearsandonereversegearourtransmissionneeds We llputthetwosetsofgearstogetherinthenextsection WhatisTorque Torqueisaforcethattendstorotateorturnthings Yougenerateatorqueanytimeyouapplyaforceusingawrench Tighteningthelugnutsonyourwheelsisagoodexample Whenyouuseawrench youapplyaforcetothehandle Thisforcecreatesatorqueonthelugnut whichtendstoturnthelugnut Englishunitsoftorquearepound inchesorpound feet theSIunitistheNewton meter Noticethatthetorqueunitsco