Roving.docx

Roving.docx

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Roving

Roving

Introduction

Followingthefinalpassageofdrawing（finisherdrawing）,thesliverproducedhastobeattenuatedtogivetherequiredyarncount.Themostcommonapproachistoattenuatedthesliverintoarovingandthentoattenuatetherovingduringspinningpriortotwistinsertion.Rovingproductionisthenthelastofthepreparatorystagestospinning.Arovingisalooselytwistedstranddraftedfromadrawnsliverandgivencohesionbyeitherinsertingasmallamountoftwist.Itisdraftedandtwistedtobespunintoayarn.Thetasksoftherovingproductionare:

（1）Draftingtheslivertoafineronewhoseweightperunitlengthismadetoallowadraftatheringframeintheapproximaterangeof15to40;

（2）Insertingfewtwiststotherovingtomakeitstrongenoughtosustainthetensioninfurtherprocessing;（3）Windingrovingintothebobbininproperway.

SummaryofProcess

Drawnsliverispresentedtotherovingframeinlargecans.Thesliversaredrawnfromthecansandareforwardedtothedraftingarrangement.Thedraftingarrangementattenuatesthesliverswithadraftofbetween5and20.Thedeliveredstrandistoothintoholditselftogetherandastrength-impartingstepisnecessaryimmediatelyattheexitfromthedraftingarrangement.Thisisperformedbyinsertingtwist.Thetwistsarecreatedbytherotatingflyer.Astheflyerrotates,twistisinsertedintothedraftedribbonissuingfromthefrontrollersofthedraftingsystem,therebyformingtheroving.Toenablewinding,thedrivenbobbinisgivenahigherrotationalspeedthantheflyerandthustherovingisdrawnofftheflyerleg.Therailwiththepackagesonitmovesupanddowncontinuouslytoformafullpackage.

ConceptofTwist

Instapleyarns,twistisessentialtoholdthefiberstogetherandtoimpartsomedegreeofcohesivenesstothestructure.Ontheotherhand,thefilamentsinamultifilamentyarnwouldfrayawayiftherewerenocohesiveforcesholdingthemtogether.Furthermore,theformationofpliedorcabledyarnsisalsoachievedbytwistingsingleorpliedyarnstogethertoproduceacoherentlinearstructure.Inotherwords,twistisameansbywhichabundleoffibers,filaments,oryarnsinapliedyarnisheldtogethersothattheultimatestructureismadecapableofwithstandingthestressesandstrainsofthemanyprocessesinvolvedinthemanufactureanduseoftextilefabrics.Whenanumberoffibersinacontinuousstrandaretwisted,radialforcesdevelopwhichinturnaffecttherelativepositionofthefibersintheyarnstructure,leadingtoaclosepackingofallfibersinagivencrosssection.Thustheinsertionoftwistinfiberassemblies（yarns）affects,inadditiontothetensileproperties,thediameterandthespecificvolume（inotherwords,softnessorhardness）ofyarns.

Whentwoendsofastraightstrandarerotatedrelativetooneanother,thefibersonthesurfaceoftheyarnlieinhelicesabouttheyarnaxis.Inotherwords,ayarnistwistedwhenfibersonthesurface,whichwereoriginallyparalleltotheaxisoftheyarn,arenowdeformed（rotated）sothattheymakeananglewiththeaxis,andtheamountoftwistisafunctionofthisangle.Thefollowingfourparametersareofimportancewhendiscussingtwistinyarns:

Directionoftwist;Twistangle;Degreeoftwist（twistlevel）;Twistmultiplier.

DirectionandAngleofTwist

Fromyarnandrovingtwistgeometry,thespiraldirectionandangleofthesurfacehelix,representingtheyarnsurfacefibers,arebyconventionthedirectionandtwistangleoftheyarn.InFigure6-1,thetwistangleisβ.Twistisinsertedbyrotatingfiberstrandsineitheraclockwiseorcounterclockwisedirection.ThedirectionoftwistisdefinedbythelettersSorZ.AnS-twistyarnisoneinwhichthespiraloftheyarnisparalleltothecenterbaroftheletterS;likewiseforaZ-twistyarn.AspindlerotatingcounterclockwiseproducesanStwist;aspindlerotatingclockwiseproducesaZtwist.MostyarnsspunoncottonsystemshaveaZtwist.Becausemostofthepeoplewhooperateandrepairspinningframesareright-handed,theequipmentisdesignedtospinZ-twistyarnstomaximizetheirefficiency.

DegreeofTwist

Thetwistlevelorthedegreeoftwistinayarnisthenumberofturnsoftwistperunitlength.Inimperialunits,werefertotheturnsperinch（tpi）,whereas,inmetricandSIunits,wespeakofturnspermeter（tpm）.Turnspercentimeter（tpcm）isalsoused.

Ifthereisachangeofcountbutthetwistangle,andthereforethetpi（ortpm）changes.Fineyarnsrequireahighertpithancoarseyarns.Toachievesimilarpropertiesinyarnsofvariouscounts,theconceptoftwistmultiple,ortwistmultiplier（TM）,isused.TheTMistheratioofthetpitothesquarerootofthecottoncountoftheyarn,statedasformula:

TwistInsertion,RealTwist,andFalseTwist

Thesimplestwaytoinserttwistintoastrandoffibersistoholdoneendofthestrandwhilethestrandlengthismadetorotateonitsaxis.Figure6-2illustratesasituationofrealtwistinsertion.ThestrandisnippedatpointAbetweenapairofstationaryrollers.Cisattachedtoabobbinplacedonarotatingspindle.ThenippedpointAisinlinewiththespindleaxisofrotation.Asthespindlerotatesatarotationalspeedn,thelengthABismadetorotatewiththespindle,andBcirculatesthespindleaxis.EachrotationofBwillcausethestrandtoalsorotateonitsownaxis,therebyinsertingtwist.Thetwistinsertedwillremaininthestrandandisthereforecalledrealtwist.Whentherollersaredeliveringthestrandtothetwistingzoneatadeliveryspeed,V,inthissituation,thetwistinsertedwouldbegivenbythegeneralformula,

.

WhenastationarystrandoffibersheldatACisrotatedatB，thenStwistisinsertedbetweenAB,andZtwistisinsertedbetweenBC（Figure6-3（a））.AsshowninFigure6-3（b）,underthedynamicconditionswhicharisewhenthesliverismoving,thesituationismodified.Attheright-handsideofthepointofrotationE,theturnsofZtwistinitiallyinsertedwillbelostfromthesystem.Afterthisinitiallossoftwist,thesystemwillsettleintoanequilibriumstate.Z-twistwillbeobservedintheDEzoneandnotwistwillbeseentheEFzone.Thistwistingactioniscalledfalse-twistingbecause,underdynamicequilibrium,thestrand,althoughbeingtwisted,hasnotwistwhenitleavesthetwistingdevice.

Fig.6-3Falsetwistinsertion.（a）stationarystrandoffibers（b）strandmovingfromlefttoright

RovingProduction

Drafting

Thedraftingsystemontherovingframetypicallyconsistsofa3-over-3double-aprondraftingarrangement.Thebottomrollersareflutedsteelandthetoprollersarecoveredwithsyntheticrubber.Whenthetoppendulumislatcheddown,springsarecompressedtoapplypressureonthetoprollersforgood,strongcontactwiththebottomrollers.Locatedinthefrontdraftzonearetopandbottomaprons,necessarytosuppressdraftingwavescausedbyrollerdrafting.

Illustrationsof4-roller-2-apronand3-roller-2-aprondraftingsystemsforarovingmachine.（ZenserGMBH）

Draftingrollerspacingisthedistancebetweentheniplinesofsuccessiverollerpairs.Thisdistance,aswithallrollerdraftingsystems,isadjustabletoaccommodatethefiberlengthtobedrafted.Ageneralguidetosettingtherollerspacing（ratch）istosetthespacejustgreaterthanthelongestfibersinthematerialtobeprocessed.Totaldraftisdeterminedbythesurfacespeedsofthebackandfrontrollerpairs.Attimestherecanbeaslightdifferencebetweenthe“mechanicaldraft”（rationofrollerspeeds）andthe“actualdraft”（rationofinputtooutputstrandweightsperunitlength）.Astheactualoreffectivedraftistheprimaryobjective,thedraftshouldalwaysbesettogivethecorrectactualdraft.Anotherimportantconsiderationforthedraftzoneis“draftdistribution”;thisistheamountofdraftinthebackzoneandtheamountofdraftinthefrontzone.Thedraftinthebackzoneisreferredtoasbreakdraft,anditshouldbeaslowasfeasiblesothatthelargestportionofthetotaldraftwilltakeplaceinthefrontzonewhereaproncontroloffibermovementislocated.Mostrovingframesaredesignedtoallowalargerangeintotaldraftfrom4to20andabove.Amorepracticalrangeof5to12hasbeenfoundtoyieldabetterqualityproduct.

Totaldraftissettoyieldthedesiredweightofroving.Breakdraftsandrollerspacingsaresetaccordingtothetypeofdraftingsystem.TakeSKF1500forexample:

Rollerzone

Front-to-middle:

Cotton50.8mm

Synthetic50.8mm

Middle-to-back:

Cotton50.8-57.2mm

Synthetic69.9mm

Breakdraft

Cotton1.10

Synthetic1.15

TwistInsertionandWindingon

Fig.6-4Basicfeaturesofatwist-and-bobbinbuilddevice

Inrovingproduction,acombinedtwistinsertionandbobbinbuilddeviceisusedtoinserttwiststotheattenuatedstrand,andwindtherovingintothebobbin.AsshowninFigure6-4,thedevicehasacentralspindlethatpassesthroughasecond,buthollow,spindle.Botharedrivenindependentlyandpassfreelythroughtherail,whichtraversesupanddownasetdistancealongthelengthofthehollowspindle.Abobbintube,ontowhichtherovingiswound,sheathesthehollowspindleandrestsonamountfittedtotherail.Themotionoftherailmovesthebobbinupanddownthehollowspindlelength.Mountedonthetopendofthecentralspindleisacomponentknownastheflyer.Thishasahollowlegthroughwhichtherovingtravelstothebobbin.Locatedattheexitisapresserarmpaddlearoundwhichtherovingslidesalongtothebobbin.Duringrotationoftheflyer,thepresserarmguidestherovingontoth