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SNAP-FITSFORASSEMBLYANDDISASSEMBLYPresentedby:TimSpahrTechnicalServiceEngineerNovember1991(Revised1/00)TICONATTiiccoonnaaAAbbuussiinneessssooffCCeellaanneesseeAAGG11GENERALDISCUSSIONAneconomicalandquickmethodofjoiningplasticpartsisbyasnap-fitjoint.Asnap-fitjointcanbedesignedsoitiseasilyseparatedorsothatitisinseparable,withoutbreakingoneofitscomponents.Thestrengthofthesnap-fitjointdependsonthematerialused,itsgeometryandtheforcesactingonthejoint.Mostallsnap-fitjointdesignssharethecommondesignfeaturesofaprotrudingledgeandasnapfoot.Whetherthesnapjointisacantileveroracylindricalfit,theybothfunctionsimilarly.Whensnap-fitjointsarebeingdesigned,itisimportanttoknowthemechanicalstressestobeappliedtothesnapbeamsafterassembly,therequiredmechanicalstressesorstrainsonthesnapbeamsduringassembly,thenumberoftimesthesnapjointwillbeengagedanddisengaged,andthemechanicallimitsofthematerial(s)tobeusedinthedesign.ReasonstoUseSnap-Fits:·Reducesassemblycosts.·Aretypicallydesignedforeaseofassemblyandareofteneasilyautomated.·Replacesscrews,nuts,andwashers.·Aremoldedasanintegralcomponentoftheplasticpart.·Noweldingoradhesivesarerequired.·Theycanbeengagedanddisengaged.ThingsToBeAwareofWhenUsingSnap-Fits:·Somedesignsrequirehighertoolingcost.·Theyaresusceptibletobreakageduetomishandlingandabusepriortoassembly.·Snap-fitsthatareassembledunderstresswillcreep.·Itisdifficulttodesignsnap-fitswithhermeticseals.Ifthebeamand/orledgerelaxes,itcoulddecreasetheeffectivenessoftheseal.TYPESOFSNAP-FITJOINTSThereareawiderangeofsnap-fitjointdesigns.Intheirbasicform,themostoftenusedarethecantileverbeam(snapleg),Figure1,andthecylindricalsnap-fitjoint,Figure2.Forthisreason,thesetwodesignsanddesignsderivedfromthesebasicsarecoveredinthistext.Figure1CantileverSnapFigure2CylindrialSnap22CantileverSnapBeamsUsingthestandardbeamequations,wecancalculatethestressandstrainduringassemblyofthesnapbeam.Ifwestaybelowtheelasticlimitofthematerial,weknowtheflexingbeamwillreturntoitsoriginalposition.However,forsuchdesigns,thereisusuallynotenoughholdingpowerwiththelowforcesorsmalldeflectionsinvolved.LbhYs=FLZEquation1YFLEI=33Equation2se=EEquation3Where:σ=MaximumstressonbeamF=ForceonthebeamL=LengthofthebeamTherefore,muchhigherdeformationsaregenerallyused.Withmostplasticmaterials,thebendingstresscalculatedbyusingsimplelinearbendingmethods(Equations1-3)canfarexceedtherecognizedyieldstrengthofthematerial.Thisisparticularlytruewhenlargedeflectionsareusedandwhentheassemblyoccursrapidly.Therefore,itoftenappearsasifthebeammomentarilypassesthroughthemaximumdeflectionorstrain,greatlyexceedingitsyieldstrengthwhileshowingnoilleffectsfromtheevent.Whatactuallyhappensisdescribedlater.Forthepresent,simplynotethatthesnapbeamsareusuallydesignedtoastainratherthanastress.Z=Ic,SectionModulusc=d2=HalfthebeamheightI=bd312,Momentofinertiah=Beamheightb=Beamwidthε=MaximumstrainonbeamY=BeamdeflectionThestrainshouldnotexceedtheallowabledynamicstrainfortheparticularmaterialbeingused.BycombiningEquations1-3,thedesignequation(Equation4)canbeproduced.Notethatthestrainiswrittenintermsoftheheight,length,anddeflectionofthebeam.e=322YHLEquation4Figure333StrainGuidelinesFigure4Figure5Generallyspeaking,anunfilledmaterialcanwithstandastrainlevelofaround6%andafilledmaterialofaround1.5%.Asareference,a6%strainlevelcouldbeabeamwithathicknessthatisequalto20%ofitslength(a5:1L/ho)andadeflectionthatisalsoequalto20%ofitslength(seeFigure5).A1.5%strainlevelcouldbeabeamwithathicknessthatisequalto10%ofitslength(10:1L/ho)andadeflectionthatisequalto10%ofitslength(seeFigure4).Ifusingabeamthatistaperedsothethicknessatthebaseofthesnapfootis50%thatofthebaseofthebeam,thelengthofthebeamwillapproximately78%(0.7819346calculated)thelengthofthe6%and1.5%beamswithuniformthickness.Amoreaccurateguidelinefortheallowabledynamicstraincurveofthematerialmaybeobtainedfromthematerial'sstressstraincurve.Theallowabledynamicstrain,formostthermoplasticsmaterialswithadefiniteyieldpoint,maybeashighas70%oftheyieldpointstrain(seeFigure7).Forothermaterials,thatbreakatlowelongationswithoutyielding,astrainlimitashighas50%ofthestrainatbreakmaybeused(seeFigure6).Ifthesnapjointisrequiredtobeengagedanddisengagedmorethanonce,thebeamshouldbedesignedto60%oftheaboverecommendedstrainlevels.However,thebestsourceforallowabledynamicstrainisthematerialsupplier.Figure6Figure744ModulusofElasticityvs.SecantModulusFigure8Beforegoingfurther,weneedtoexaminetheactualstressesandforcesdevelopedinasnapfinger.Figure8showsastressstraincurveforabrittlethermoplasticmaterial.Thestraightlineportionofthecurveistheregionwherestressisproportionaltostrain.LineAisdrawntangenttothisregionTheslopeofLineAisgenerallyreportedasthemodulusofelasticity(Young'smodulusorinitialmodulus)ofthematerial.Manyplasticsdonotpossessthisstraight-lineregion.Forthesematerials,LineAisconstructedtangentattheorigintoobtainthemodulusofelasticity.Ifwedesignedasnapbeamat1.5%strainforthismaterialinFigure8usingEquations1-4andamodulusofelasticityof1.6x106psi(givenbythematerial)asdeterminedfromLineA,theresultingstresswouldbe24