Chapter09-Springs-機械零件設(shè)計英文全套教案

Chapter7PowerScrewAssemblies

7.1UsesandCharacteristicsofPowerScrews7.2PotentialFailureModes7.3Materials7.4TheMechanicsofPowerScrews7.5SuggestedPowerScrewDesignProcedureChapter7PowerScrewAssembl17.1UsesandCharacteristicsofPowerScrewsPowerscrews,aremachineelementsthattransformrotarymotionintotranslationalmotion,oramplifyasmalltangentialforcemovingthroughasmalldistance.Geometrically,apowerscrewisathreadedshaftwithanattachedthrustcollaratoneend,engagedintoamatingnut.Withsuitableconstraint,eitherthenutmayberotatedtocauseaxialtranslationofthethreadedshaft(screw),orthescrewmayberotatedtocauseaxialtranslationofthenut.7.1UsesandCharacteristicso2Basicconfigurationofload-raisingpowerscrew

Basicconfigurationofload-ra3Chapter09-Springs-機械零件設(shè)計英文PPT全套教案4Chapter09-Springs-機械零件設(shè)計英文PPT全套教案5Ifnisthenumberofparallelthreads,

l=npBy“developing”onefullturnofathreadontoaplane,itmaybededucedthat

Right-handthreadsareoftenusedinpractice,butforspecialapplicationsleft-handthreadsmaybeused.Ifnisthenumberofparallel6CommonthreadformsforpowerscrewsBecauselargemechanicaladvantagesarepossible(thethreadsareessentiallyinclinedplanes,wrappedhelicallyaroundtheshaft),threadformsarechosentomaximizeaxialload-carryingcapacityandtominimizefrictionaldrag.Themostusefulthreadformsarethesquarethread,themodifiedsquarethread,theAcmethread,and(forunidirectionalloads)thebuttressthread.Commonthreadformsfor7Chapter09-Springs-機械零件設(shè)計英文PPT全套教案8TypesofthreadformsandcharacteristicsThesquarethreadprovidesthebeststrengthandefficiencybutisdifficulttomanufacture.The5°threadangle,θ,(10°includedangle)ofthemodifiedsquarethreadimprovesitsmanufacturability.TheAcmethread,withits29°includedangle(θ=14-1/2°)iseasytomanufactureandallowstheuseofasplitnutthatcanbesqueezedradiallytoadjustforwear.Thebuttressthreadprovidesgreaterstrengthforunidirectionalloads.Powerscrewthreadstandardshavebeendevelopedformostpracticalthreadforms.Typesofthreadformsandch9Ballscrews

Insomespecialapplications,whereitisveryimportanttominimizethread-frictiondrag,theslidingfrictionbetweenscrewandnutthreadsmaybereplacedbyrollingfrictionthroughuseofaballscrew,Inballscrewsacontinuousstreamofballsflowsalongbetweenthreadlikenominallysemicirculargroovesinthescrewandnut.Eachball,afterrollingtotheendofthenutgroove,entersareturntubeontheoutsideofthenuttobefedbackintothecircuitattheentrypoint.BallscrewsIn107.2PotentialFailureModesSincepowerscrewsembodycertaincharacteristicsofslidingbearings,powertransmissionshafting,androllingelementbearings(forballscrewversions),thepotentialfailuremodesforapowerscrewassemblythereforeincludecandidatesfromeachoftheseapplications,includingfailurebyyielding,brinnelling,fatigue,corrosion,surfacefatiguewear,adhesivewear,abrasivewear,corrosivewear,frettingwear,galling,seizure,andpossibly,forscrewswithlongunsupportedspans,failurebybuckingorelasticinstability.7.2PotentialFailureModesSin11DesignfromaperspectiveoffailurepreventionThedesignerneedstoassesswhichofthepotentialfailuremodesmustbeconsideredineachparticularcase,thenmakethenecessaryjudgmentsandsimplifyassumptionsneededtodesigntheassemblyandavoidfailure.Inaccuraciesinthreadprofileandspacingcausevirtuallyalloftheloadtobecarriedbythefirstfewthreads.Designiscomplicatedbytheindeterminatenatureofthethreadcontactregionbetweenthescrewandthenut.Thisuncertaintyaffectsthedeterminationofproperthread-bearingcontactarea,andselectionofacceptablelubricants,andfailuremodesofwear,galling,orseizure,fatigue,yielding,orbrinnelling.

Designfromaperspectiveoff12

7.3MaterialsCandidatematerialpairsforpowerscrewsandmatingnutsshouldhavegoodcompressivestrength,goodfatiguestrength,goodductility,goodthermalconductivity,andgoodmaterialcompatibilitybetweenmatingsurfacesofthescrewandnut.Forthecaseofballscrews,materialsurfacesshouldhavehighsurfacehardness,usuallyahardfatigue-resistantcaseoveratoughductilecore.7.3MaterialsCandidatem13Inmostcircumstances,athreadedpowerscrewwouldbemanufacturedfromlow-carboncarburizing-gradealloysteel,casecarburizedandheattreated(Steel10,12CrNi3A).Thematingnutwouldtypicallybemadeofasoftductilematerialsuchasbronze,leadedbronze,oraluminumbronze,orsinteredporousversionsofthesematerials,impregnatedwithlubricant.ThrustcollarmaterialselectionwouldfollowtheguidelinesforplainbearingsinChapter5.Inmostcircumstances,athrea14

7.4TheMechanicsofPowerScrewsForasquare-threadedpowerscrewwithsinglethreadhavingapitchdiameterdp,apitchp,aleadangle,andahelixangleisloadedbytheaxialcompressiveforceF.Thetorquerequiredtoraisethisload:(7-3)Thetorquerequiredtolowertheload.(7-4)7.4TheMechanicsofPo15Thisisthetorquerequiredtoovercomeapartofthefrictioninloweringtheload.Whenapositivetorqueisobtainedfromthisequation,thescrewissaidtobeself-locking.Whentheleadislargeorthefrictionislow,thattheloadwillloweritselfbycausingthescrewtospinwithoutanyexternaleffort.Insuchcases,thetorqueTfromaboveEq.willbenegativeoraero.Thustheconditionforself-lockingis(7-5)(7-4)Thisisthetorquerequiredto16

EfficiencyinraisingtheloadThetorquerequiredtoraisetheloadwhileignoringthreadfriction,isTheefficiency,therefore,is

Efficiencyinraisin17InthecaseofAcmeorotherthreads,thenormalthreadloadisinclinedtotheaxisbecauseofthethreadangleandtheleadangle.Sinceleadanglesaresmall,thisinclinationcanbeneglectedandonlytheeffectofthethreadangleconsidered.Theeffectoftheangle

istoincreasethefrictionalforcebythewedgingactionofthethreads.EffectsofthreadformsInthecaseofAcmeorothert18ThereforethefrictionaltermsinEq.(7-3)mustbedividedbycosθ.Forraisingtheload,orfortighteningascreworbolt,thisyieldsThereforethefrictionalterms19CollarbearingeffectWhenthescrewisloadedaxially,athrustorcollarbearingmustbeemployedbetweentherotatingandstationarymembersinordertotakeouttheaxialcomponent.Theloadisassumedtobeconcentratedatthemeancollardiameter.Thetorquerequiredis

(7-8)Collarbearingeffe20Efficiencyasfunctionaleadangle

andfrictioncoefficientEfficienciesareverylowforleadanglesapproaching0°or90.Near-maximumefficienciesareachievedusingleadanglesintherangeof30°to60°andlowthreadfrictioncoefficients.Efficiencyasfunctio217.5SuggestedPowerScrewDesignProcedureTentativelyselectthematerialsforscrewandnut,andselectthedesiredthreadform,thetypeofthrustcollarbearing.Iftheselecteddes