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辽宁科技大学本科生毕业设计第1页ScreeningVibratingscreensPrinciples--Vibratingscreenssavespaceandweightandoperateonlittlepowerbecausethescreeningsurfacemaybeactuatedbyvibrating,gyratingorpulsatingmovementofsmallamplitude,butatfrequenciesthatnormallyexceed3,000/min.SelectionofProperVibrationSereen--Besurethescreensupplierknowsalldetailsoftheapplication.Thecentrifugalforcefactor,orcombinationoffrequencyofvibration(speed)andamplitude(throw),mayaffectperformanceofanyvibratingscreen.Also,acorrectcombinationofslopeanddirectionofmechanismrotationisvitalforinclinedscreens.Usually,thelargertheopening,thegreatertheamplitudeneededforascreen.Ifthethrowistoosmall,thematerialmayclogorwedgeintheopenings.Increasingthethrowbeyondwhatisrequiredtopreventblindingorpluggingdoesnotnecessarilyincreasethelifeofthebearingsandreducescreeningefficiency.Increasedrateoftravelpermitsmoretonnagetobepassedoverthescreenperunitoftime.Foragiventonnage,afasterrateoftravelresultsinathinnerbedofmaterialandhighscreeningefficiency.Maximumslopeisreachedwhenthematerialtravelstoofastforthefinestopenetratetheribbonofmaterialandreachtheaperturesinthescreencloth.Atthispointanexcessiveamountoffinematerialpassesoverthescreenwiththeoversize,resultinginpoorefficiency.Whenanexistingscreenistobeusedforanapplicationotherthanthatforwhichitwasoriginallyintended,checkwiththeSuppliertoseeifanyoftheoperatingcharacteristicsneedmodificationandifthethescreenisstructurallysuitableforthenewapplication.Theoperatorcangetthecorrectvibratingscreenbyprovidingthesupplierwiththefollowinginformation:Maximumtonsperhourtobescreened,includinganycirculatingloadoranysurgesinthefeedrate.Acompletesizeconsistorsieveanalysisofthematerialor,ifavailable,anestimatedanalysis.Typeofmaterialandweightpercubicfootinbrokenstate.Separationsdesiredoneachdeck.Surfacemoisturecarriedbythematerialifscreeningistobedryoramountofwaterwithfeedifwet.Specialoperatingrequirementsorconditionssuchastemperature,abrasiveness,corrosivenessorotherphysicalcharacteristicsofthefeed,辽宁科技大学本科生毕业设计第2页efficiencyorproductrequirementswhichdetermineselectionofscreeningsurface,orinstallationproblemswhichaffectscreensizeselectionorcapacity.GeneralTypes--Vibratingscreensmaybedividedintotwomainclasses:mechanically-vibratedandelectrially-vibrated.Theformercanbesubdividedintoclassesbasedonhowthevibrationisproduced--byeccentrics；byunbalancedweight；bycamsorbumpers.Theycanalsobesubdividedasinclinedandhorizontal.Sizes--Vibratingscreensaremadeinstandardsizesoffrom12in.to10ftwideandfrom21/2ftto28ftlong.Commonpracticedictatesthatthelengthofthescreenshouldbe2.5timesthewidthfordryscreening.Forwetscreening,widerandshorterscreenisbest.Screensforscalpingaheadofprimarycrushers,operateataslopeoffrom12degreesto18degreesandhaveopeningsaslargeas11-in.square.Theeccentricthrowforopeningsfrom5to11in.isusually1/2in.；foropeningsfrom3to5in.about3/8in.；andforsmalleropenings1/4in.Thescreeningsurfaceconsistsofaheavycastdesk,perforatedsteelwithorwithoutskidbarsweldedbetweentheholes,roddeck,etc.Themechanicalshakingscreencomprisesarectangularframe,withperforatedsteelorwireclothscreeningmedium.Itisusuallyinclinedandsuspendedonlooserodsorcables.Thesescreensnowareusedmainlyforspecialtasksofcoarsescreening,havinggivenwaytovibratingscreens.Electrically-HeatedScreens--Electricallyheatedscreenclothdeckshaveaffordedbetterscreeningandlessdust.Modernorupdatedscreeningoperations,nowhandlingclays,limestone,potash,salts,phosphatesandvarioushydroscopicmaterials,reportminimumdustlosswhenequippedwithheatedscreens.Thecontrollingfactorinthisimprovementistheelectrically-heatedscreendothdeck.Anyvibratoryscreenwithfineopening(lessthan1/2in.)canbeequippedwithlowvolt-age-highamperageresistanceheating.Theprincipleofelectricheatingisbasedonthefactthatsmalldiameterwireofscreencloth(especiallystainlesssteel)servesasaconductor,butoffersresistancetoahigh-amperagecurrent.Thisresistancecausesheatingofthewirewhenpowerfultransformerandspeciallydesignedbusbarsconnectedtoscreendothdeckspushupto6,000ampscurrentintoacircuit.Thecurrentissafeandshock-proofbecausevoltageislow,rangingfromapproximately11/2to16volts.Workerscandotheirjobsaroundelectrically-heatedscreenswithoutspecialprecautions.Heatedscreensareeffectiveinpreventingmoisturecontentofmaterialfromcausingbuildupandblinding.Thescreenwireiskeptat100degreesto130degreesFdependingoncharacterandtonnageofthematerialprocessed.Thistemperatureisnothighenoughtoweakenthewireclothorscreenstructure,nordoesitsufficetodriveoffmoistureheldinmaterial.Thewarmwirestaysdry,breakingthesurfacetensionthatotherwisewouldbinddampmaterialtocold,dampmetal.Thisdifferentialor辽宁科技大学本科生毕业设计第3页warm,dryscreenwireversuscold,dampmaterialcanbemaintainedeconomicallythroughtransformercontrolsettings.Witheveryopeningintheheatedscreenmeshprotectedagainstblinding,thereisnoguessworkaboutwhatsizeparticlewillbedelivered.Auniform,unvaryingqualitytomeettoughspecificationscomesthroughdayafterday,nomatterwhattheweather.(Highhumiditymakestroublewithunheatedscreens.)Plantsoperatingheatedscreenswillhavenotroublewithexcessiveamountsoffine,dustymaterialthatformerlysiftedthroughreducedmeshesoncloggedscreens.Servicelifeofscreenclothisgreatlyincreasedwhenelectricheatingendsthepunishmentofold-fashionedcleaningmethods,suchasroughpoundingorbrushing.Blowtorchflamesputtoomuchheatinonespotandbouncingchainsaddedtowearandtear.But,screenskeptcleanandopenwithelectricheatingreportedlyareseldommistreatedandlastuptoeighttimesaslong.Heateddecksendthethreatofpileupsandstrainsthatcanbreakthemesh.Economicaloperationofelectricheatingforscreensrequiresonetransformerforinstallationsofuptothreepanelsofscreencloth(maximumarea4-x12-ft).Twotransformersareusedonlongerdecks.Anydeck(top,centerorbottom)canbeheated.Electricheatingismostdesirableofallwhereascreensurfaceishardtogetat.Changingheatedscreensdoesnotinvolvemoreunboltingthanneededforordinaryscreens.Inspecifyingscreenstodeliverauniformparticlethroughheatedmesh,thefirstthingtoconsiderisweightpersquarefootofthewireclothneededtosetupappropriateresistance.Mostcalculationsarebasedonsquareopenings.Slottedopeningsmustbeidentifiedastowidthofclearopening,diameterofwireandnumberofwiresperinchbeforeweightpersquarefootcanbefoundinanyscreenclothmanufacturer'scatalog.Havingdeterminedtheusableweightpersquarefootofscreencloth,thenextstepistoselectthesizeofclearopeningneeded,makingnoallowanceforreductioninthissizeasformerlywasthecasewhenmaterialstucktothewires.Heatedwireswillmaintaintheclearopeningatalltimes,makingitpossibletoscreenfinerwithoutblinding.Fromawirecatalog,selectameshweighingnomorepersquarefoot(canweighless)thandeterminedusingthemethodabovewiththeclearopeningcharacteristicsdesired.Thisoftenturnsouttobeamoreefficientscreenbecausethewirediameterwillbesmallerandthepercentageoftheopenareawillbegreater.Forexample,wherea1/8-in.clearopeningwasdesired(butoftenblinded)onunheatedscreens,thewirediameterwas0.63andtheopenareawas44percent.Theclothweighed1.43lb/ft.2.Foraheated3-x12-ftscreen,theweightmustnotexceed辽宁科技大学本科生毕业设计第5页1.1lb/ft2.Twooptionswerefoundinclothwith1/8-in.clearopening:wirediameter.054weighing1.09lb/ft2providing48.8percentopenarea；andwirediameter.047weighing0.85lb/ft2witha52.8percentopenarea.Loadconditionsmaymakefinediametersofcarbonsteelwireinadequate.Insuchcases,stainlesssteelwireoflargerdiameterwithgreaterload-carryingcapacitywillhavethenecessaryresistanceforgoodheating.Lighterwirewithmoreopenareayieldshighertonnageandheatedmeshstaysopen100percent.Twoscreenclothsconnectedinaseriesmaybeofdifferentmeshsizesorclearopenings,aslongastheybothweighthesameperft2anddonotexceedtheweightallowedforthatparticularsizeofscreen.Withheatedscreens,majorsavingsinpollutioncontrolarepossible.Alsomoisturecontentofmaterialisreportedlyincreasedinarangeof5to8percent.Thismoistureisaddedduringoraftercrushing-grindingoperationsandholdsdowndustduringtransferofmaterialandpassageoverheatedscreens.Warmwirehandles5to8percentmoistureinstride.Thecostofheatingequipmentandmistsprayingreportedlyislessthanthecostofhavingbagtowersandprecipitators.振动筛的选择振动筛的选择原则：选择振动筛的原则是所选择的振动筛要节省空间、重量并且驱动的功率要小，因为筛选表面可以驱动并且发生振动。同时还要保证旋转或脉动运动振幅要小,但振动筛的振动频率在通常情况下是要超过三千次每分钟的。选择合适的振动显示屏：要确定显示屏供应商知道所有有关方面的使用细节。离心力或组合频率振动（速度）以及振幅（投掷）等因素都可能影响到任何振动筛的正常工作。此外,需要有一个正确的组合坡度和方向的轮换机制，这对于显示屏来说是至关重要的。通常情况下，振动筛需要大开放、大振幅的显示屏。如果投入太少的话，则这些材料可能出现木屐或楔形开口等情况。增加投入以后，需要知道如何防止堵塞因为堵塞是不必要的，而且会增加日常轴承的磨损以及和降低筛选的效率。

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