The Engineer's Companion_Fastener Design Manual, Part Two
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12/8/2015 The Engineer's Companion/Fastener Design Manual, Part Two http://www.designnotes.com/companion/manual2.html 1/8 Figure 12. Types of Belleville washers. (a) smooth; (b) serrated Figure 13. Combinations of Belleville washers. (a) series; (b) parallel; (c) parallelseries Figure 14. Helical spring washers. Figure 15. Tooth lockwashers. (a) flat; (b) countersunk Figure 16. Selfaligning washer. Figure 17. Wire thread insert installation. The Engineer's Companion Home Ethics Failures Fasteners Innovation On Jobs Licensing PID Tuning Quality Rhetoric About Ron What Engineers Are Engineer as Hero Fastener Design Manual, Part Two Rich Barrett Washers Belleville Washers Belleville washers (Figure 12) are conical washers used more for maintaining a uniform tension load on a bolt than for locking. If they are not completely flattened out, they serve as a spring in the bolt joint. However, unless they have serrated surfaces, they have no significant locking capability. These surfaces, on the other hand, will damage the mating surfaces under them. These washers can be stacked in combinations as shown in Figure 13 to either increase the total spring length (Figure 13(a) and (c)) or increase the spring constant (Figure 13(b)). Lockwashers The typical helical spring washer shown in Figure 14 is made of slightly trapezoidal wire formed into a helix of one coil so that the free height is approximately twice the thickness of the washer cross section. Lockwashers are usually made of hardened carbon steel, but they are also available in aluminum, silicon, bronze, phosphorbronze alloy, stainless steel, and KMonel. The lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat when the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is useless for locking. Tooth (or Star) Lockwashers Tooth lockwashers (Figure 15) are used with screws and nuts for some spring action but mostly for locking action. The teeth are formed in a twisted configuration with sharp edges. One edge bites into the bolthead (or nut) while the other edge bites into the mating surface. Although this washer does provide some locking action, it damages the mating surfaces. These scratches can cause crack formation in highly stressed fasteners, in mating parts, or both, as well as decreased corrosion resistance. SelfAligning Washers A selfaligning washer is used with a mating nut that has conical faces as shown in Figure 16. Because there is both a weight penalty and a severe cost penalty for using this nut, it should be used only as a last resort. Maintaining parallel mating surfaces within acceptable limits (two feet, per SAE Handbook (ref. 4)) is normally the better alternative. Inserts An insert is a special type of device that is threaded on inside diameter and locked with threads or protrusions on outside diameter in a drilled, molded, or tapped hole. It is used to provide a strong, wearresistant tapped hole in a soft material (e.g. plastics, nonferrous metals), as well as to repair stripped threads in a tapped hole. The aerospace industry uses inserts in tapped holes in soft materials in order to take advantage of weight savings provided by small highstrength fasteners. The bigger external thread of the insert (nominally 1/8 in. bigger in diameter than the internal thread) gives, for example, a 10 32 bolt in an equivalent 5/1618 nut. In general, there are two types of inserts: those that are threaded externally, and those that are locked by some method other than threads (knurls, serrated surfaces, grooves, or interference fit). Within the threaded inserts there are three types: the wire thread, the selftapping, and the solid bushing. Threaded Inserts
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Transcript of The Engineer's Companion_Fastener Design Manual, Part Two
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]
