Lesson-7-Fasteners-and-Springs

Lesson7FastenersandSpringsFastenersaredeviceswhichpermitoneparttobejoinedtoasecondpartand,hence,theyareinvolvedinalmostalldesigns.Theacceptabilityofanyproductdependsnotonlyontheselectedcomponents,butalsoonthemeansbywhichtheyarefastenedtogether.Theprincipalpurposesoffastenersaretoprovidethefollowingdesignfeatures:(1)Disassemblyforinspectionandrepair;(2)Modulardesignwhereaproductconsistsofanumberofsubassemblies.Modulardesignaidsmanufacturingaswellastransportation.Therearethreemainclassificationsoffasteners,whicharedescribedasfollows:(1)Removable.Thistypepermitsthepartstobereadilydisconnectedwithoutdamagingthefastener.Anexampleistheordinarynut-and-boltfastener.(2)Semipermanent.Forthistype,thepartscanbedisconnected,butsomedamageusuassyoccurstothefastener.Onesuchexampleisacotterpin.(3)Permanent.Whenthistypeoffastenerisused,itisintendedthatthepartswillneverbedisassembled.Examplesarerivetedandweldedjoints.Thefollowingfactorsshouldbetakenintoaccountwhenselectingfastenersforafivenapplication:(1)Primaryfunction;(2)Appearance;(3)Alargenumberofsmallsizefastenersversusasmallnumberoflargesizefasteners(anexampleidbolts);(4)Operatingconditionssuchasvibration,loadsandtemperature;(5)Frequencyofdisassembly;(6)Adjustabilityinthelocationofparts;(7)Typesofmaterialstobejoined;(8)Consequencesoffailureorlooseningofthefastener.Theimportanceoffastenerscanberealizedwhenreferringtoanycomplexproduct.Inthecaseoftheautomobile,thereareliterallythousandsofpartswhicharefastenedtogethertoproducethetotalproduct.Thefailureorlooseningofasinglefastenercouldresultinasimplenu9isancesuchasadoorrattleorinaserioussituationsuchasawheelcomingoff.Suchpossibilitiesmustbetakenintoaccountintheselectionofthetypeoffastenerforthespecificapplication.Springsaremechanicalmemberswhicharedesignedtogivearelativelylargeamountofelasticdeflectingundertheactionofanexternallyappliedload.Hooke’Law,whichstatesthatdeflectionisproportionaltoload,isthebasisofbehaviorofsprings.However,somespringsaredesignedtoproduceanonlinearrelationshipbetweenloadanddeflection.Thefollowingisalistoftheimportantpurposesandapplicationsofsprings:(1)Controlofmotioninmachines.Thiscategoryrepresentsthemajorityofspringapplicationssuchasoperatingforcesinclutchesandbrakes.Also,springsareusedtomaintaincontactbetweentwomemberssuchadacamanditsfollower.(2)Reductionoftransmittedforcesasaresultofimpactorshockloading.Pplicationshereincludeautomotivesuspensionsystemspringsandbumpersprings.(3)Storageofenergy.Applicationsinthiscategoryarefoundinclocks,moviecamerasandlawnmowers.(4)Measurementofforce.Scalesusedtoweighpeopleisaverycommonapplicationforthiscategory.Thethreemajorclassificationsofspringsarecompression,extension,antorsion.SeeFig.7.2.Compressionandextensionspringsarethespringsmostoftenused.Deflectingislinearinthesetypes.Torsionspringsarecharacterizedbyangularinsteadoflineardeflection.Leafspringsareofthesimplebeamorcantilevertype.Theirdeflectionislinear.Mostspringsaremadeofsteel,althoughsiliconbronze,brassandberylliumcopperarealsoused.Springsareuniversallymadebycompanieswhichspecializeinthemanufactureofsprings.Thecylindricalhelicalspringisthemostpopulartypeofspring;torsionbarspringsandleafspringsarealsowidelyused.Springsareuniversallymadebycompanieswhichspecializeinthemanufactureofsprings.Thecylindricalhelicalspringsthemostpopulartypeofspringtorsionbarspringsandleafspringsarealsowidelyused.Ifthewirediameter(assumingacylindricalhelicalspring)islessthan3mm,thespringwillnormallybecold-woundfromhard-drawnoroil-temperedwire.Forlargerdiameters,springsareformedusinghot-rolledbar.Itisgoodpracticetoconsultwithaspingmanufacturingcompanywhenselectingaspringdesign,especiallyifhighloadsortemperaturesaretobeencountered,orifstressreversalsoccurorcorrosionresistanceisrequired.Toproperlyselectaspring,acompletestudyofthespringrequirements,includingspacelimitations,mustbeundertaken.Manydifferenttypesofspecialspringsareavailabletosatisfyunusualrequirementsorapplications.