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DynamicsandscreeningcharacteristicsofavibratingscreenwithvariableellipticaltraceHEXiao-mei,LIUChu-shengSchoolofMechanicalandElectricalEngineering,ChinaUniversityofMining&Technology,Xuzhou,Jiangsu221116,ChinaAbstract:theidealmotioncharacteristicsforthevibratingscreenwaspresented,vibratingscreenwithvariableellipticaltracewasproposed.Anaccuratemechanicalmodelwasconstructedaccordingtotherequiredstructuralmotionfeatures.Applyingmulti-degree-of-freedomvibrationtheory,characteristicsofthevibratingscreenwasanalyzed.Kinematicsparametersofthevibratingscreenwhichmotiontraceswerelinear,circularorellipticalwereobtained.Thestablesolutionsofthedynamicequationsgavethemotionsofthevibratingscreenbymeansofcomputersimulations.Technologicalparameters,includingamplitude,movementvelocityandthrowingindex,offivespecificpointsalongthescreensurfaceweregainedbytheoreticalcalculation.Theresultsshowthatthetracesofthenewdesignedvibratingscreenfollowtheidealscreeningmotion.Thescreeningefficiencyandprocessingcapacitymaythusbeeffectivelyimproved.Keywords:variableellipticaltrace;screeningprocesswithconstantbedthickness;dynamicmodel;motioncharacteristic;screeningcharacteristics1IntroductionScreeningoperationsareanimportantpartofcoalprocessing.Thevibratingscreenisoneofthemostextensivelyusedscreeningtools.Vibratingscreens,suchaslinearvibratingscreen,circularvibratingscreenorellipticalvibratingscreen,haveasimpletranslationalmotion.Themotionfollowsthesamepatheverywhereonthescreenandsothescreenhasconstanttransportvelocityandthrowingindex,whichleadstolowscreeningefficiency.Augmentingthethrowingindextoimprovebreakstheexcitingmotorsprocessingcapacitylowerstheworking.Inthispaper,wereportonthedesignofanewvibratingscreenwithvariablemotiontracesthatisbasedontheprincipleofscreeningprocesswithconstantbedthickness[3–4].Differentpartsofthevibratingscreentraversedifferentellipticaltracesandtheresultingmotionagreeswellwiththeidealmotion.Thusthescreenprocessingcapacityandefficiencycanbothbeimproved.2Idealmotionforascreensurfaceandtheproposalofavibratingscreenwithvariableellipticaltrace2.1ScreeningcharacteristicsofcommonvibratingscreensVibratingscreenscommonlyworkatafixedvibrationintensity.Materialonthescreensurfacemovesbythrowing,rollingorslidingmotions.Forcommonscreeners,materialgranularityiswidelydistributedatthefeedend.Theenergyimpartedtothematerialparticlesfromthevibratingscreenisseverelydissipated.Consequently,alargenumberofparticlesbecomelaminatedonlyashortdistancefromthefeedend.Thematerialpenetratesthescreenwithinthefirst1/4to1/2ofthescreen,whichaffectsscreeningandlowersprocessingcapacity[5].Thedecreaseoffine-grainedmaterialcausestheratioofparticlescloseinsizeto,orlargerthan,themeshtoincrease.Thus,thescreeningefficiencydeclinesdramatically.Thematerialgranularitysimultaneouslybecomesuniformandtheenergyimpartedfromthevibrationstothematerialsufferslittleloss.Hence,theamplitudeandvelocityofthematerialparticlesincrease.ThiscausesthematerialbeddepthatthefeedendtobethickwhileatthedischargeenditisThin.Thiskindofmotionleadstoanasymmetricalpenetrationalongthescreensurface,whichinfluencesthescreeningefficiencyandprocessingcapability[6].CommonscreeningcharacteristicsareshowninFig.1.2.2IdealmotionforscreensurfaceandimplementingschemeTheidealmotionforscreensurfaceisdescribedbelow,accordingtotheprincipleofscreeningprocesswithconstantbedthickness.Thefeedendofthescreenhasabiggerthrowingindexandahighermaterialdeliveryvelocity,whichmakesbulkmaterialquicklypenetrateandcausesrapiddelaminating.Earlierlaminationofmaterialincreasestheprobabilityoffine-grainedmaterialpassingthroughthemesh.Thescreenhasanappropriatethrowingindexandalittlehighermaterialdeliveryvelocityinitsmiddlepart.Thisisofbenefitforstabilizingfine-grainedmaterialsandforpenetratinguniformlyalongthescreenlength.Alowerthrowingindexandmaterialdeliveryvelocitynearthedischargeendcausesthematerialtostaylongeronthescreenandencouragesmorecompletepenetrationofthemesh.Twomethodsarecurrentlyusedtoimprovescreeningefficiency[7–8].Thefirstistoaddmaterialtothescreenfrommultiplefeedports.Thisistroublesomeinpracticaluseespeciallyintermsofcontrollingthedistributionofdifferentlygranulatedmaterials.Henceitisrarelyusedinpracticalproduction.Thesecondwayistoadoptnewscreeningequipmentlike,forexample,aconstantthicknessscreen.Themotionofthenewscreensurfacecausesmaterialtomaintainthesame,oranincreased,thickness.Itachievesarathermoreidealmotion.Themainproblemwiththeconstantthicknessscreenisthatitcoversabiggerareaandthatthestructureiscomplicatedandhardtomaintain.Asimplestructurewithgoodscreeningefficiencyisstillanecessity.Wehavedesignedanewvibrationscreenwithavariableellipticaltracethatisbaseduponanidealscreenmotionforuseinrawcoalclassification.Thesizeofthevibratingscreenis3.6m×7.5m,thefeedgranularityis0to50mmandtheclassificationgranularityis6mm.Ellipticallyvibratingscreenscombinethebasicadvantagesofbothcircularandlinearvibratingscreens[9–10].Thelongaxisoftheellipsedeterminesmaterialdeliveryandtheshortaxisinfluencesmaterialloosening,tobeexac