12/8/2015 TheEngineer' sCompanion/FastenerDesignManual,PartTwohttp://www.designnotes.com/companion/manual2.html 1/8Figure12.TypesofBellevillewashers.(a)smooth(b)serratedFigure13.CombinationsofBellevillewashers.(a)series(b)parallel(c)parallelseriesFigure14.Helicalspringwashers.Figure15.Toothlockwashers.(a)flat(b)countersunkFigure16.Selfaligningwasher.Figure17.Wirethreadinsertinstallation.TheEngineer'sCompanionHome Ethics Failures Fasteners Innovation OnJobs Licensing PIDTuning Quality Rhetoric AboutRon WhatEngineersAre EngineerasHeroFastenerDesignManual,PartTwoRichBarrettWashersBellevilleWashersBellevillewashers(Figure12)areconicalwashersusedmoreformaintainingauniformtensionloadonaboltthanforlocking.Iftheyarenotcompletelyflattenedout,theyserveasaspringintheboltjoint.However,unlesstheyhaveserratedsurfaces,theyhavenosignificantlockingcapability.Thesesurfaces,ontheotherhand,willdamagethematingsurfacesunderthem.ThesewasherscanbestackedincombinationsasshowninFigure13toeitherincreasethetotalspringlength(Figure13(a)and(c))orincreasethespringconstant(Figure13(b)).LockwashersThetypicalhelicalspringwashershowninFigure14ismadeofslightlytrapezoidalwireformedintoahelixofonecoilsothatthefreeheightisapproximatelytwicethethicknessofthewashercrosssection.Lockwashersareusuallymadeofhardenedcarbonsteel,buttheyarealsoavailableinaluminum,silicon,bronze,phosphorbronzealloy,stainlesssteel,andKMonel.Thelockwasherservesasaspringwhiletheboltisbeingtightened.However,thewasherisnormallyflatwhentheboltisfullytorqued.Atthistimeitisequivalenttoasolidflatwasher,anditslockingabilityisnonexistent.Insummary,alockwasherofthistypeisuselessforlocking.Tooth(orStar)LockwashersToothlockwashers(Figure15)areusedwithscrewsandnutsforsomespringactionbutmostlyforlockingaction.Theteethareformedinatwistedconfigurationwithsharpedges.Oneedgebitesintothebolthead(ornut)whiletheotheredgebitesintothematingsurface.Althoughthiswasherdoesprovidesomelockingaction,itdamagesthematingsurfaces.Thesescratchescancausecrackformationinhighlystressedfasteners,inmatingparts,orboth,aswellasdecreasedcorrosionresistance.SelfAligningWashersAselfaligningwasherisusedwithamatingnutthathasconicalfacesasshowninFigure16.Becausethereisbothaweightpenaltyandaseverecostpenaltyforusingthisnut,itshouldbeusedonlyasalastresort.Maintainingparallelmatingsurfaceswithinacceptablelimits(twofeet,perSAEHandbook(ref.4))isnormallythebetteralternative.InsertsAninsertisaspecialtypeofdevicethatisthreadedoninsidediameterandlockedwiththreadsorprotrusionsonoutsidediameterinadrilled,molded,ortappedhole.Itisusedtoprovideastrong,wearresistanttappedholeinasoftmaterial(e.g.plastics,nonferrousmetals),aswellastorepairstrippedthreadsinatappedhole.Theaerospaceindustryusesinsertsintappedholesinsoftmaterialsinordertotakeadvantageofweightsavingsprovidedbysmallhighstrengthfasteners.Thebiggerexternalthreadoftheinsert(nominally1/8in.biggerindiameterthantheinternalthread)gives,forexample,a1032boltinanequivalent5/1618nut.Ingeneral,therearetwotypesofinserts:thosethatarethreadedexternally,andthosethatarelockedbysomemethodotherthanthreads(knurls,serratedsurfaces,grooves,orinterferencefit).Withinthethreadedinsertstherearethreetypes:thewirethread,theselftapping,andthesolidbushing.ThreadedInserts12/8/2015 TheEngineer' sCompanion/FastenerDesignManual,PartTwohttp://www.designnotes.com/companion/manual2.html 2/8Figure18.Wirethreadinserttypes.(a)freerunning(b)lockingFigure19.Selftappinginserts.(a)Slotted(b)NylokFigure20.Speedsert.Figure21.Keensert.Figure22.Plasticexpandableinsert.Figure23.Moldedinplaceinsert.Figure24.Ultrasonicinserts.Wirethread.Thewirethreadtypeofinsert(Helicoil10)(a)isaprecisioncoilofdiamondshapedCRESwirethatformsbothexternalandinternalthreadsasshowninFigure17.Thecoilismadeslightlyoversizesothatitwillhaveaninterferencefitinthetappedhole.Inaddition,thisinsertisavailablewithadeformedcoil(Figure18)foradditionallocking.Thetangisbrokenoffatthenotchafterinstallation.Thewirethreadinsertisthemostpopulartypeforrepairofatappedholewithstrippedthreads,sinceitrequirestheleastamountofholeenlargement.However,thesolidbushinginsertispreferredifspacepermits.Selftapping.Mostoftheselftappinginsertsarethesolidbushingtypemadewithataperedexternalthreadsimilartoaselftappingscrew(Figure19).Thereareseveraldifferentlockingcombinations,suchastheNylokplug(Figure19(b))orthethreadformingSpeedsertdeformedthread(Figure20).Anadditionaladvantageofthethreadforminginsertisthatitgeneratesnocuttingchips,sinceitdoesnotcutthethreads.Itcanonlybeused,however,insoftermaterials.[RexnordSpecialtyFastenersDivision,Torrance,California.]Solidbushing.Solidbushinginsertshaveconventionalthreadsbothinternallyandexternally.ApopulartypeistheKeensert[Rexnord]showninFigure21.Thelockingkeysaredriveninaftertheinsertisinplace.Anothermanufacturerusesatwoprongringforlocking.TheseinsertsarealsoavailablewithdistortedexternalthreadorNylokplugsforlocking.NonthreadedInsertsPlasticexpandable.Themostfamiliarofthenonthreadedinsertsistheplasticexpandabletypeshowninfigure22.Thisinserthasbarbsontheoutsideandlongitudinalslitsthatallowittoexpandoutwardasthethreadedfastenerisinstalled,pushingthebarbsintothewallofthedrilledhole.(Seeref.5.)Moldedinplace.Thistypeofinsert(Figure23)isknurledorseffatedtoresistbothpulloutandrotation.Itiscommonlyusedwithceramics,rubber,andplastics,sinceitcandevelophigherresistancetobothpulloutandrotationinthesematerialsthanselftappingorconventionallythreadedinserts.(Seeref.5.)Ultrasonic.Ultrasonicinserts(Figure24)havegroovesinvariousdirectionstogivethemlockingstrength.Theyareinstalledinapreparedholebypushingtheminwhiletheyarebeingultrasonicallyvibrated.Theultrasonicvibrationmeltsthewalloftheholelocallysothattheinsertgroovesare"welded"inplace.Sincetheareameltedissmall,theseinsertsdonothavetheholdingpowerofthosethataremoldedinplace.Ultrasonicinsertsarelimitedtouseinthermoplastics.(Seeref.5.)ThreadsTypesofThreadsSincecompleteinformationonmostthreadscanbefoundintheANSIstandards(ref.6),theSAEHandbook(ref.4),andtheNationalInstituteofStandardsandTechnologyHandbookH28(ref.7)nothreadstandardswillbeincludedhere.Thegoalhereistoexplainthecommonthreadtypes,alongwiththeiradvantagesanddisadvantages.Thecommonthreadtypesareunifiednationalcoarse(UNC),unifiednationalfine(UNF),unifiednationalextrafine(UNEF),UNJC,UNJF,UNR,UNK,andconstantpitchthreads.Unifiednationalcoarse.UNCisthemostcommonlyusedthreadongeneralpurposefasteners.Coarsethreadsaredeeperthanfinethreadsandareeasiertoassemblewithoutcrossthreading.Themanufacturingtolerancescanbelargerthanforfinerthreads,allowingforhigherplatingtolerances.UNCthreadsarenormallyeasiertoremovewhencorroded,owingtotheirsloppyfit.However,aUNCfastenercanbeprocuredwithaclass3(tighter)fitifneeded(classestobecoveredlater).Unifiednationalfine.UNFthreadhasalargerminordiameterthanUNCthread,whichgivesUNFfastenersslightlyhigherloadcarryingandbettertorquelockingcapabilitiesthanUNCfastenersofthesameidenticalmaterialandoutsidediameter.ThefinethreadshavetightermanufacturingtolerancesthanUNCthreads,andthesmallerleadangleallowsforfinertensionadjustment.UNFthreadsarethemostwidelyusedthreadsintheaerospaceindustry.Unifiednationalextrafine.UNEFisastillfinertypeofthreadthanUNFandiscommontotheaerospacefield.Thisthreadisparticularlyadvantageousfortappedholesinhardmaterialsandforthinthreadedwalls,aswellasfortappedholesinthinmaterials.UNJCandUNJFthreads."J"threadsaremadeinbothexternalandinternalforms.TheexternalthreadhasamuchlargerrootradiusthanthecorrespondingUNC,UNR,UNK,orUNFthreads.Thisradiusismandatoryanditsinspectionisrequired,whereasnorootradiusisrequiredonUNC,UNF,orUNEFthreads.Sincethelargerrootradiusincreasestheminordiameter,aUNJForUNJCfastenerhasalargernettensileareathanacorrespondingUNForUNCfastener.Thisrootradiusalsogivesasmallerstressconcentrationfactorinthethreadedsection.Therefore,highstrength(180ksiormore)boltsusuallyhave"J"threads.12/8/2015 TheEngineer' sCompanion/FastenerDesignManual,PartTwohttp://www.designnotes.com/companion/manual2.html 3/8UNRthreads.TheUNRexternalthreadisarolledUNthreadinallrespectsexceptthattherootradiusmustberounded.However,therootradiusandtheminordiameterarenotcheckedortoleranced.ThereisnointernalUNRthread.UNKthreads.TheUNKexternalthreadissimilartoUNR,exceptthattherootradiusandtheminordiameteraretolerancedandinspected.ThereisnointernalUNKthread.AccordingtoasurveyofmanufacturersconductedbytheIndustrialFastenersInstitute,nearlyallmanufacturersofexternallythreadedfastenersmakeUNRrolledthreadsratherthanplainUN.Theonlyexceptionisforgroundorcutthreads.Constantpitchthreads.Thesethreadsofferaselectionofpitchesthatcanbematchedwithvariousdiameterstofitaparticulardesign.Thisisacommonpracticeforboltsof1in.diameterandabove,withpitchesof8,12,or16threadsperinchbeingthemostcommon.AgraphicalandtabularexplanationOfUN,UNR,UNK,andUNJthreadsisgivenonpageM6ofreference8.Acopy(Figure25)isenclosedhereforreference.AccordingtotheIndustrialFastenersInstitute,"[thefollowing]isnotascrewthreadstandard,shouldnotbeusedasaworkingsheet,andshouldonlyreferthereadertotheproperANSIStandardsdocumentwhereinthefullthreaddetailsonworkingdataarecontained."12/8/2015 TheEngineer' sCompanion/FastenerDesignManual,PartTwohttp://www.designnotes.com/companion/manual2.html 4/8Figure25.ExplanationofUN,UNR,UNK,andUNJthreads.ReprintedwithpermissionofIndustrialFastenersInstitute.ClassesofThreadsThreadclassesaredistinguishedfromeachotherbytheamountsoftoleranceandallowance.ThedesignationsrunfromIAto3AandIBto3Bforexternalandinternalthreads,respectively.AclassIisalooserfitting,generalpurposethreadaclass3istheaerospacestandardthread,andhasatightertolerance.(TheindividualtolerancesandsizesforthevariousclassesaregivenintheSAEHandbook(ref4).)FormingofThreadsThreadsmaybecut,hotrolled,orcoldrolled.Themostcommonmanufacturingmethodistocoldformboththeheadandthethreadsforboltsuptooneinchindiameter.Forboltsoflargerdiameterandhighstrengthsmallerbolts,theheadsarehotforged.Thethreadsarestillcoldrolleduntiltheboltsizeprohibitsthematerialdisplacementnecessarytoformthethreads(uptoaconstantpitchofeightthreadsperinch).Threadsarecutonlyatassemblywithtapsanddiesorbylathecutting.Coldrollinghastheadditionaladvantageofincreasingthestrengthoftheboltthreadsthroughthehighcompressivesurfacestresses,similartotheeffectsofshotpeening.Thisprocessmakesthethreadsmoreresistanttofatiguecracking.FatigueResistantBoltsIfaboltiscycledintension,itwillnormallybreakneartheendofthethreadedportionbecausethisistheareaofmaximumstressconcentration.Inordertolessenthestressconcentrationfactor,theboltshankcanbemachineddowntotherootdiameterofthethreads.Thenitwillsurvivetensilecyclicloadingmuchlongerthanastandardboltwiththeshankdiameterequaltothethreadoutsidediameter.Fatigue(Cyclic)LoadingofBolts12/8/2015 TheEngineer' sCompanion/FastenerDesignManual,PartTwohttp://www.designnotes.com/companion/manual2.html 5/8Figure26.Fatigueloadingofbolts.(a)boltedflangeswithexternalload(b)freebodywithnoexternalload(c)freebodywithexternalloadTheboltedjointinFigure26(fromref.9)ispreloadedwithaninitialloadFi,whichequalstheclampingloadFCbeforetheexternalloadFEisapplied.Theequation(fromref.11)forthisassemblyisFB=Fi+FEKB/(KB+KC)whereFBisthetotalboltload.InthisequationKBisthespringconstantoftheboltandKCisthespringconstantoftheclampedfaces.Toseetheeffectsoftherelativespringconstants,letR=KC/KB.Then(fromref.10)FB=Fi+FE/(1+R)InanormalclampedjointKCismuchlargerthanKB(R=5.0forsteelboltandflanges),sothattheboltloaddoesnotincreasemuchastheinitialexternalloadisapplied.(NotethattheboltloaddoesnotincreasesignificantlyuntilFEexceedsFi.)Aseriesoftriangulardiagrams(Figure27,fromref.11)canbeusedtoclarifytheeffectofexternallyappliedloads.TriangleOABisidenticalinallfourdiagrams.TheslopeofOArepresentstheboltstiffnesstheslopeofABrepresentsthejointstiffness(jointisstifferthanboltbyratioOC/CB.)InFigure27(a)theexternallyappliedloadFE(a)doesnotloadthebolttoitsyieldpoint.InFigure27(b)theboltisloadedbyFE(b)toitsyieldpoint,withthecorrespondingdecreaseinclampingloadtoFCL.InFigure27(c)theexternalload(c)hascausedthebolttotakeapermanentelongationsuchthattheclampingforcewillbelessthanFiwhenFE(c)isremoved.InFigure27(d)thejointhascompletelyseparatedonitswaytoboltfailure.NotethattheflattertheslopeofOA(orthelargertheratioOC/OBbecomes),thesmallertheeffectFihasonboltload.Therefore,usingmoresmalldiameterfastenersratherthanafewlargediameterfastenerswillgiveamorefatigueresistantjoint.ReferringtoFigure27(a),notethatthecyclic(alternating)loadisthatportionaboveFi.Thisisthealternatingload(stress)tobeusedonastressversusloadcyclesdiagramoftheboltmaterialtopredictthefatiguelifeofthebolts.NotethataninitialpreloadFineartheboltyieldsminimizescyclicloading.Figure27.Boltexternalloading.ThermalCyclicLoadingofBoltsIftheboltandjointareofdifferentmaterials,anoperatingtemperaturehigherorlowerthantheinstallationtemperaturecancauseproblems.Differentialcontractioncancausethejointtounload(orseparate)differentialexpansioncancauseoverloadingofthefasteners.Inthesecasesitiscommonpracticetouseconicalwashers(seewashersectionofthismanual)togiveadditionaladjustmentsinfastenerandjointloading.12/8/2015 TheEngineer' sCompanion/FastenerDesignManual,PartTwohttp://www.designnotes.com/companion/manual2.html 6/812/8/2015 TheEngineer' sCompanion/FastenerDesignManual,PartTwohttp://www.designnotes.com/companion/manual2.html 7/8Table4.Coefficientsofstaticandslidingfriction.Oncoefficientsofstaticandslidingfriction(fromreference12)1. Campbell,Trans.ASME,1939.2. Clarke,Lincoln,andSterrett,Proc.API,1935.3. BeareandBowden,Phil.Trans.Roy.Soc.,1985.4. Dokos,Trans.ASME,1946.5. BoydandRobertson,Trans.ASME,1945.6. Sachs,zeitf.angew.Math.AndMech.,1924.7. HondaandYamada,Jour.IofM,1925.8. Tomlinson,Phil.Mag.,1929.9. Morin,Acad.Roy.desSciences,1838.10. Claypoole,Trans.ASME,1943.11. Tabor,Jour.AppliedPhys.,1945.12. Eyssen,GeneralDiscussiononLubrication,ASME,1937.13. BrazierandHollandBowyer,GeneralDiscussiononLubrication,ASME,1937.14. Burwell,Jour.SAE,1942.15. Stanton,"Friction",Longmans.16. ErnstandMerchant,ConferenceonFrictionandSurfaceFinish,MIT,1940.17. Gongwer,ConferenceonFrictionandSurfaceFinish,MIT,1940.18. HardyandBircumshaw,Proc.Roy.Soc.,1925.19. HardyandHardy,Phil.Mag.,1919.20. BowdenandYoung,Proc.Roy.Soc.,1951.21. HardyandDoubleday,Proc.Roy.Soc.,1923.22. BowdenandTabor,"TheFrictionandLubricationofSolids,"Oxford.a. Oleicacidb. Atlanticspindleoil(lightmineral)c. Castoroild. Lardoile. Atlanticspindleoilplus2%oleicacidf. Mediummineraloilg. Mediummineraloilplus0.5%oleicacidh. Stearicacidi. Grease(zincoxidebase)j. Graphitek. Turbineoilplus1%graphitel. Turbineoilplus1%stearicacidm. Turbineoil(mediummineral)n. Oliveoilo. p. Palmiticacidq. Ricinoleicacidr. Drysoaps. Lardt. Wateru. Rapeoilv. 3in1Oilw. Octylalcohol12/8/2015 TheEngineer' sCompanion/FastenerDesignManual,PartTwohttp://www.designnotes.com/companion/manual2.html 8/823. Shooter,Research,4,1951. x. Trioleiny. 1%lauricacidinparaffinoilFastenerTorqueDeterminingthepropertorqueforafasteneristhebiggestprobleminfastenerinstallation.Someofthemanyvariablescausingproblemsare1. Thecoefficientoffrictionbetweenmatingthreads2. Thecoefficientoffrictionbetweenthebolthead(ornut)anditsmatingsurface3. Theeffectofboltcoatingsandlubricantsonthefrictioncoefficients4. Thepercentageofbolttensilestrengthtobeusedforpreload5. Onceagreementisreachedonitem4,howtoaccuratelydeterminethisvalue6. Relativespringratesofthestructureandthebolts7. Interactionformulastobeusedforcombiningsimultaneousshearandtensionloadsonabolt(shouldfrictionloadsduetoboltclampingactionbeincludedintheinteractioncalculations?)8. Whether"runningtorque"foralockingdeviceshouldbeaddedtothenormaltorqueGobacktopartone.Theconscientious,effectiveengineerisavirtuousengineer.SamuelFlorman,TheCivilizedEngineerTheEngineer'sCompanionisCopyright1995presentbyRonGrahamLastupdated11/14/200817:38:[email protected]
