机械类外文.docx
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机械类外文.docx
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机械类外文
2.5TensionMembers
further462Foundations
refinementsinwhichtheAramidfibreisprocessedinsingleormultiplestagesto
achievethedesiredstructureoftwistedcord.
Example:
Acordconstructionofdtex1,6109193withadoubletwistZ/S150/
100meansthattheyarnhasanettensionmemberweightofapproximately1,610
g/10,000m.Thefirststepisasimpleright-handedtwistof150turns/mandthe
secondtwistingstageisatripleleft-handedtwistwith100turns/m.Thecordthus
hasatotaltitreof4,830dtex.
Carbon-fibretensionmembersarewell-knowninthemanufactureofcomposite
materials.Theirmajorapplicationsareinhighlystressedstructuralcomponentsfor
airandspacetechnology.Dependinguponthetypeoffibre,theelasticmodule
reachesfiguresof220–700Gpa(incomparisonsteelis210Gpa).Thefilament
diametersrangefrom5–11lm.Thisbasicfibrecannotsimplybeusedforthe
manufactureoftensionmembersbecauseofitspoortransverserigidity.Duetothe
developmentofspecialcoatingmaterials(surfacetreatments)afibreproductis
nowavailablewithgoodbendingabilityandtextileproperties.Therawmaterial
forbeltuseismadeofcarbonfilamentsinamulti-stagetwistedandmatrix
embeddedcable.Theadhesionagentprovidessuperiormechanicalcharacteristics
fortension,stiffnessandbendingability.Thefirsttimingbelttobeequippedwith
tensionmembersofthiskindwasfromGates[37]inthespringof2007,inthe
PolychainPCC-8MGTtimingbelt.Itisparticularlyworthmentioningthatthe
length/temperaturecoefficientofthistensionmemberisalmostzero,orevena
slightlynegativevalue.
Summary
Allthedescribedtensionmembers(steel,glass-fibre,Aramid,carbonfibre)exhibit
considerableservicelifeandoutstandinglengthconstancyundercontinuous
dynamicstressinthedirectionofload.Withforcetypestransversetothedirection
ofload,asoccurringduringshearingorbuckling,thebeltcompositestructurecan
provideverylittleresistance,howeverthissortofforcedirectionisvirtually
impossibleundernormaloperatingconditions.Itisconceivablethatfromthepoint
ofdispatchtoinstallationthathandlingdefectscouldhappen.Akinkedbeltcanbe
consideredsignificantlydamagedandsuchbeltsshouldnotbeacceptedforuse.
2.6ForcesinTimingBeltDrives
Atimingbeltexhibitsthesamebehaviourinitsinteractionwithamachine
regardlessoftypeorprofilegeometry.Figure2.12ashowsatimingbeltdriveat
standstilloridlingunderidealconditionsanditcanbeseenthatthepre-tensionFV
isequallydistributedthroughoutthebelt,asthereisnotorquetransfer.Figure
2.12bshowsthesamebelttransmittingpowerwiththeright-handpulleybeing
thedrivingpulleyandtheleft-handthedrivenpulley.Thedrivenpulleyisshown
2Foundations
Fig.2.12Forcesintimingbeltdrives:
aunloadedcondition,btransmittingpower,cgraph:
Span
forcecurveasafunctionofpower.FForce,FAAxialforce,FtTangentialforce,FVPre-tension,
F1Forceinloadedspan,F2Forceinunloadedspan,PPower
482Foundations
withabrakesymboltoindicatepoweruse.Whenthedriveisworkingandthe
drivenpulleyistransmittingpower,thenforceincreasesintheloadedspanandthe
resultingbeltelongationisseenontheidlesidewithacorrespondingforce
reduction.ThegraphinFig.2.12cshowstheforcecurveinthebeltunderboth
loadedandidleconditions.Wherepower=0,i.e.underno-loadconditions,
thestrandforcesF1andF2meetattheirstartingpointofFV.Themorepower
isrequired,thenthegreaterthedifferenceintensionbetweenthatdrivenandidle
strand.Thisdifferentialforceistheperipheralforce,alsoknownasthetangential
forceFt:
Ft?
F1_F2:
e2:
11T
Torqueandpoweristransmittedfromtherighttotheleft-handpulleyonlyby
thisperipheralforce.Thegraph,aswellastheobservationmodelforidleand
powertransfer,isoffundamentalimportanceforfurtherexaminationand,in
particular,theinitialpre-tensiontobeappliedisexaminedmoreclosely.
2.7Force-EffectMechanisms
Wecanseetheimportanceoftheappliedpre-tensionloadFVinFig.2.12a–c.
Reliablepowertransferisonlypossibleaslongasaresidualtensionremainsinthe
idlesideofthebelt.Thus,thetimingbeltshouldbepre-tensionedtoavalueof
FV_
1
2
Fte2:
12T
Herethesymbol‘‘C’’shouldbeunderstoodtobe‘‘equalorslightlylarger’’,so
asnottogiveanunnecessarilyhighpreloadinthebelt,shaftsandbearings.When
thepowerincreasesbeyondacriticalpoint,theslacksidewillrespondbysagging
andasaresultthereisacutedangerofbelttoothjump.Compactdriveswith
relativelyshortbeltswillbetterovercomeminoroverloadsbutdriveswithlong
beltsandlesspreloadtendtoseemoretoothjumping.Thetoothjumpingdanger
zoneisalwaysatthepointofentryofthebeltintothedrivenpulleywheretensile
forcesinthebeltareattheirlowest.
Inthiscontext,theworkofLotharKöster[67]isimportant,becauseitfirst
describedthemeshingbehaviourandthewedge-effectintheconvergenceofthe
beltandthedrivenpulley.Theformationofthiswedge-effect(Fig.2.13)occurs
whentheradialcomponentoftheexistingpre-tensionload,atthepointofentry,is
notsufficientforthebelttoothflanktofullymeshinthetoothgapagainstthe
frictionalresistanceofthemeshingteeth.Onlywithanincreaseofthetensileforce
inthearcofcontactcanthisresistancebegraduallyovercomeuntilthebeltteeth
finallyfullymeshintothecorrespondingtoothgaps.Stroboscopicstudieshave
shownthat,underrotation,thereisonlyagradualmeshingofthebelttakingplace
withthetoothgapsintheconvergencezoneofthedrivenpulley.Therebya
wedge-shapeddisengagementofthebeltformsonthepulleyandthebeltteeth
Fig.2.13Wedge-effectintheconvergencezoneofthebeltandthedrivenpulley
locatedinthisareawillonlycarrypartialloads.Thewedge-effectisallthemore
pronouncedthecloserthedriveisoperatedtoitspowerlimit,thegreatertheFt/F2
powerratiois,thelowerthepreloadFVandthegreatertheopposingfriction
resistanceofthemeshingprocess.
Incomparison,compactdriveswithrelativelyshortbeltlengthshavehigh
valuesofstiffness,makingthebeltelongationlow,andthereforethewedge-effect
isreduced.Accordingtomanufacturer’srecommendations[87],drivesofthiskind
canberunwithsignificantlylowerpre-tension(seethepowerdriveinFig.3.37in
Chapter3.9).
Additionally,therelationshipbetweenthebeltandpulleypitchimpactsonboth
thesmoothrunningofthebeltandtheformationofthewedge-effectatthedriven
pulley.Thebeltcanalsoundergosignificantload-elongation,whichleadstosmall
changesinpitch,dependingontheloadconditions.Thebeltsectionintheconvergence
zoneofthedrivenpulleywillexperienceadecreaseintractionwiththe
increasingsizeofthetorquetobetransferred;thebeltpitchintheconvergence
zoneisreduced.Therefore,theincomingtoothofthebeltcanclimbtheworking
flankofthepulley,withthebeltexperiencingtoothflankcompression,inorderto
transferthetangentialforcefromthebelttothepulley.Thebeltaccelerationalso
givesapitchincreaseattheteethonthedrivenpulley[80],sothatthebelt/pulley
meshingerrorscontinuetoincreaseuntilequilibriumbetweenthefrictionalforces
andthespantensionisestablished.
Extremepitchchangesmay,inconjunctionwiththeformationofthewedgeeffect,
alsocauseabnormalwearandteartothebelt.Frictionalforcesincreaseat
meshingduetonon-optimumtolerancesandlowpre-tensionhencethetendencyto
wear.
MetznerandKrausespecificallyexpandupontherelationshipofthepitch
consistencybetweenbeltandpulleyinachapteroftheirbookconcerningforceeffect
mechanisms[77].Acriticalassessmentofsmoothness,efficiencyandbelt
wearisderivedthereinfromtheloaddistributionontheteethintheangleofwrap.
Fig.2.13Wedge-effectintheconvergencezoneofthebeltandthedrivenpulley
502Foundations
Theoptimumsolutionwouldbeifeachsingletoothcouldenteritsrespectivetooth
gapwithoutloadorfriction,oratleastwithreducedfriction,whilethetooth
surfacepressureincreasesontheworkingflanksuptothebelttoothleavingthe
pulleytoothgap.Thetoothmeshingonthedriverpulleytakesplaceunderelevated
tensionwhileconverselyonthedrivenpulleyittakesplaceunderreduced
belttension.Theselocallyactingforcesresultinpitchchangesinthebeltand
couldbeoffsetbyincreasingthedrivingpulleyoutsidediameterandreducingthat
ofthedrivenpulley.Endurancetestsusingacorrectedoutsidediameter(ofabout
0.1–0.15mm)haveshownsignificantincreasesinperformance[77].Suchcorrections
oftheoutsidediametermeanbeingunabletousestandardpulleys,which
limitsinterchangeabilityandshouldonlybeappliedinveryexceptional
circumstances.
Theformationofthewedge-effectisaccompaniedbyanincreaseintheaxial
loadsasthebeltdoesnotrunonitsideallineduetotheneutrallineofthebeltbeing
movedoutwards.Thebeltisstretchedsostronglyandbuildssuchhightensileforces
thattheelongationloadalsoactsontheshafts.Thiseffectcanbeseeninthediagram
inFig.2.12cattheoperatingpointoftheloadborderand,inparticular,theslackside
forceF2increaseintotheoverloadrangeisclearlyvisible.
Thisknowledgeoftooth-jumpconditionscanbeusedbydrivefitterstotestrun
thedrivebycloselyobservingtheslacksideandthemeshingofthedrivenpulley.
Ifthebeltm
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