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The Effects of a Selected Wheel Design and Caster Fixture Design
University of Wisconsin Milwaukee UWM Digital Commons Theses and Dissertations December 2014 The Effects of a Selected Wheel Design and Caster Fixture Design on Pushing Force When Pushing Four Wheeled Industrial Carts David Wein University of Wisconsin-Milwaukee Follow this and additional works at: http://dc.uwm.edu/etd Part of the Industrial Engineering Commons Recommended Citation Wein, David, "The Effects of a Selected Wheel Design and Caster Fixture Design on Pushing Force When Pushing Four Wheeled Industrial Carts" (2014). Theses and Dissertations. Paper 618. This Thesis is brought to you for free and open access by UWM Digital Commons. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of UWM Digital Commons. For more information, please contact [email protected]. THEEFFECTSOFASELECTEDWHEELDESIGNANDCASTERFIXTUREDESIGNON PUSHINGFORCEWHENPUSHINGFOURWHEELEDINDUSTRIALCARTS by DavidS.Wein AThesisSubmittedin PartialFulfillmentofthe RequirementsfortheDegreeof MasterofScience inEngineering at TheUniversityofWisconsin‐Milwaukee December2014 ABSTRACT THEAFFECTSOFASELECTEDWHEELDESIGNANDCASTERFIXTUREDESIGNSON PUSHINGFORCEWHENPUSHINGFOURWHEELEDINDUSTRIALCARTS by DavidS.Wein TheUniversityofWisconsin‐Milwaukee,2014 UndertheSupervisionofDr.WilkistarOtieno Manualmaterialhandlingstasks,manyofwhichrequirepushingandpulling arecommoninalmostallindustrialandservicesectorenvironments.Thesetasks exposeworkerstomusculoskeletalstressesaswellasotherrelatedslippingand trippinghazards.Thecompanysponsorofthisstudysoughttolowertheriskof injuryfrommanuallypushingandpullingcarts.Thecompanywantedtoevaluatea newerstyleofsplitwheelandalsoanoffsetpivotdualorbitalcaster,whichthe manufacturerstateswillreducepushingandpullingforce.Atotalofeight participants(4male,4female)wereincludedinthestudy.Participantswere requiredtopushafour‐wheeledcart16timesfor10meters.Thecartwaspushed8 timeswithatotalgrossweightof250lbs(113.4kgs)andanother8timeswith750 lbs(340.2kgs).Onlytherearwheelscouldswivelandweretestedboth perpendicularandinlinetothedirectionoftravel.Thesplitwheelwascomparedto asinglewheelandthedualorbitalcasterwascomparedtoastandardstyleofcaster. ii Allpossiblecombinationsweretested.Appliedforcewasmeasuredandanalysis wasconductedoninstantaneouspeakforce. Resultsshowedthatthecasterdesigndidnotsignificantlyaffecttheinitial appliedforce.However,thedualorbitalcasterwasconsistentintheamountof appliedforcewhenthewheelswereperpendicularoralignedtothedirectionof travel.Thedualorbitalcasterresultedinlowerinitialappliedforceswhenused togetherwiththesinglewheeldesign.Inaddition,thedualorbitalcastershowed markeddecreaseintheappliedpushforcewhenwheelswherepositioned perpendiculartothedirectionoftravelcomparedtothestandardcaster.These resultsstrengthentherecommendationforthecompanytoinvestinthedualorbital –alsoreferredtooffsetpivotcaster.Secondly,thoughthewheeltypesignificantly affectedtheappliedforcethemeanappliedforcedifferencebetweenthetwowheel typeswasnotpracticallysignificantenoughtowarrantachangeofthewheeltypein thecompany. iii ©CopyrightbyDavidS.Wein,2014 AllRightsReserved iv Thisthesisisdedicatedtomywonderfulwifewhoencouragedmethroughoutthe entireprogram.Itisalsodedicatedtomychildrenasareminderofthenecessityto perseveretotheend. v TABLE OF CONTENTS Contents TABLEOFCONTENTS.............................................................................................................................vi LISTOFFIGURES....................................................................................................................................viii LISTOFTABLES........................................................................................................................................ix ACKNOWLEDGEMENTS..........................................................................................................................x Chapter1:ResearchBackground......................................................................................................1 Chapter2:LiteratureReview..............................................................................................................3 2.1Force/LoadGuidelines..............................................................................................................................3 2.2RelatedPriorResearch..............................................................................................................................4 2.3FactorsAffectingPullandPushForces............................................................................................12 Weight................................................................................................................................................13 Friction/WheelHardness..........................................................................................................13 WheelPosition/Orientation.....................................................................................................14 WheelDiameter.............................................................................................................................17 Slope...................................................................................................................................................17 HandleHeight.................................................................................................................................18 HandleWidth..................................................................................................................................18 Chapter3:CurrentStudy....................................................................................................................18 3.1StudyVariables...........................................................................................................................................18 3.2ResearchQuestions..................................................................................................................................20 3.3ResearchHypotheses...............................................................................................................................21 Chapter4:MethodologyandDataAnalysis...............................................................................22 4.1ExperimentParticipants.........................................................................................................................22 4.2EquipmentandInstrumentation........................................................................................................24 CartDesign.......................................................................................................................................24 Handle................................................................................................................................................27 CasterFixtures...............................................................................................................................27 Wheels...............................................................................................................................................28 CartLoad...........................................................................................................................................29 Floor....................................................................................................................................................30 DataCollectionElectronics.......................................................................................................31 vi Calibration........................................................................................................................................31 RatingsofPerceivedExertionScale......................................................................................32 4.3StudyDesign................................................................................................................................................32 4.4ExperimentalProcedure.........................................................................................................................34 4.5StatisticalAnalysis....................................................................................................................................36 Chapter5:ResultsandDiscussions...............................................................................................37 5.1Results............................................................................................................................................................37 DataProcessing..............................................................................................................................37 AnalysisofPhysicalMeasures–PeakForce.....................................................................38 PsychophysicalSelfReportedExertionData....................................................................39 5.2ParticipantData/Information..............................................................................................................39 5.3PhysicalForceMeasurementAnalysis–InitialPushForce......................................................40 5.4PsychophysicalAnalysisofPerceivedExertion............................................................................49 Chapter6:Discussion............................................................................................................................58 6.1HypothesisDiscussion.............................................................................................................................58 6.2VariabilityBetweenSubjects................................................................................................................62 6.3PerceivedExertion....................................................................................................................................63 6.4ApplicationoftheResults/PsychophysicalApplication............................................................64 6.4.1Scenario1–ChangingFixtures....................................................................................64 6.5LimitationsoftheStudy..........................................................................................................................65 6.5.1InstantaneousPeakForce..............................................................................................65 6.5.2LateralForce........................................................................................................................65 6.6ContributiontotheBodyofKnowledge...........................................................................................65 6.7FutureStudies.............................................................................................................................................66 Chapter7:Conclusion...........................................................................................................................66 References..................................................................................................................................................68 Appendices................................................................................................................................................71 Appendix A – Pre Screening Form.................................................................................................................72 Appendix B – Participant Information..........................................................................................................73 Appendix C – Pain and Exertion Rating........................................................................................................74 vii LIST OF FIGURES Figure 1 ‐ Wheel Positions.....................................................................................................................15 Figure 2 ‐ Swivel Eaz Wheel...................................................................................................................19 Figure 3 ‐ Cart design...............................................................................................................................25 Figure 4 ‐ Handle Notches......................................................................................................................26 Figure 5 ‐ Swivel Eaz Pro Figure 6 ‐ Standard Swivel Caster (Fixture).........................28 Figure 7 ‐ Swivel Eaz Caster...................................................................................................................29 Figure 8 ‐ Single Wheel...........................................................................................................................29 Figure 9 ‐ Wheel/Fixture Orientation................................................................................................33 Figure 10 ‐ Example of Graph of Push Force...................................................................................38 Figure 11 ‐ Boxplot Gender Effect – Females versus Males.......................................................40 Figure 12 ‐ General Linear Model........................................................................................................41 Figure 13 ‐ ANOVA: Max Peak Initial Force vs Wheel Type & Wheel Position....................42 Figure 14 ‐ Main Effects Plots...............................................................................................................43 Figure 15 ‐ Interaction Plot....................................................................................................................43 Figure 16 ‐ ANOVA....................................................................................................................................45 Figure 17 ‐ Main Effects Plot.................................................................................................................46 Figure 18 ‐ Interaction Plot....................................................................................................................47 Figure 19 ‐ ANOVA, Fixture ‐ Wheel Type........................................................................................48 Figure 20 ‐ Main Effects Plot, Fixture ‐ Wheel Type ‐ Wheel Position....................................48 Figure 21 ‐ Interaction Plot; Fixture ‐ Wheel Type ‐ Wheel Position.......................................49 Figure 22 ‐ Pearson’s Correlation, Back – Shoulder ‐ Weight....................................................50 Figure 23 ‐ Normal Probability, Shoulder and Back......................................................................51 Figure 24 ‐ Box Cox ‐ Shoulder and Back Data Transformation................................................51 Figure 25 ‐ ANOVA General Linear Model ‐ Shoulders................................................................53 Figure 26 ‐ ANOVA General Linear Model – Back..........................................................................55 Figure 27 ‐ Boxplot ‐ Peak Initial Force..............................................................................................60 Figure 28 ‐ Summary of Box Plot of Combinations.......................................................................60 viii LIST OF TABLES Table 1 ‐ Related Research.......................................................................................................................5 Table 2 ‐ Additional Weight Added.....................................................................................................30 Table 3 ‐ Experiment Design Combinations.....................................................................................35 Table 4 ‐ Participant Information & Statistics.................................................................................39 Table 5 ‐ Applied Peak Force Range...................................................................................................63 ix ACKNOWLEDGEMENTS Iwouldliketothankthefollowingpeoplefortheirguidance,mentoring,andfor sharingtheirknowledgethroughouttheprocess:Dr.WilkistarOtieno,Dr.Jay Kapellusch,andProf.ArunGarg. IwouldalsoliketothankthefollowingOshkoshCorporationemployeesfortheir contributioninthedesignandbuildofthecartandassistancewiththedata collection:AndyGratton,RyanSchumacher,andJeffVolkman. x 1 Chapter 1: Research Background Manualmaterialhandlingtaskscanbefoundinmanyindustrialandservicesector environments.Muchfocusandattentioninresearchhasbeengiventoreducingrisk factorsassociatedwithliftingandloweringofparts,components,orboxesinthe workplace.TheNationalInstituteofOccupationalSafetyandHealth(NIOSH)first publishedtheirLiftingGuidein1981[2].Sincethattime,industryhasresponded byworkingtoreducetheamountofmanuallifting,lowering,andcarryingfoundin workplaces,oftenreplacingthosetaskswithpushingandpulling[3].Incertain industries,pushingandpullingmaneuverscanaccountforanestimated50%ofthe manualmaterialtasks[1]. Intheworkplace,overexertionfromanoutsidesourcewasrankedasthehighest causeofdisablinginjuries.Theseinjuriesincludedthoseinvolvingpushing,pulling, lifting,orthrowing.In2011,injuriesrelatedtomaterialhandlingcostbusiness $14.2billionindirectexpenses[4]. Itisestimatedthat9‐20%oflowerbackinjuriesintheindustrialenvironmentare associatedwithpushingorpullingtasks[5].InthestateofOhio,thetotalannual costforbackinjuriesisover$100milliondollarswhiletheaveragecostofaback injuryis$18,290peroccurrence[6].AccordingtothestateofTexasDepartmentof Insurance,theaveragecostforabackinjuryis$15,000.Backinjuriesalsoaccount for24%oftheworker’scompensationinjuriesinTexas[7].Inadditiontoincreasing 2 riskofinjurytotheback,pushingandpullinghasledtoincreaseddiscomfortinthe shouldersofworkerswhomanuallypushcartsonaregularbasis[8].Accordingto NIOSH,approximately600,000employeesareafflictedwithbackinjuriesannually, atacostofover$50billion[26].Anundocumentedbutbelievedtobesignificant portionoftheseinjuriesandcostsareattributabletopushingandpullingtasks. Thesponsorofthisstudy,amanufacturerofmediumandheavymilitarytrucks, tacticalwheeledvehicles,andcommercialvehicleshasbeenworkingtoreduce forcesofmanualpushingandpullingofcarts.Thecompanyseekstolowerinjuries byreducingpushingandpullingforce,apresumedriskfactorforinjury[1].Inan efforttoreducepushingandpullingforces,theyhavestartedpurchasingsplit wheeledcastersmanufacturedbyAubinIndustriesofTracy,California.Inaddition tothesplitwheelcasternamedSwivel‐Eaz™,AubinIndustriesandhasdevelopeda newstyleofcasterfixturecalledaSwivel‐Eaz™Pro.Thesponsorofthestudyseeks toverifythemanufacturer’sclaimsthatbothproductsreducetheforcerequiredto pushorpullacart. Thisresearchstudywasdesignedtorespondtothesponsoringcompany’sconcerns. Weseektodetermineifthesplitwheelandoffsetpivotcasterindividuallyor interactivelyhaveaneffectoneitherinitialorsustainedforcewhencomparedtoa standardsinglewheelandcasterdesign.Torealizethisgoal,wesoughttotestthe followingthreehypotheses,whichwillbediscussedindetailinChapter3ofthis thesis. 3 1.Split‐wheeldesignwillaffecttheinitialappliedpeakforcerequiredtomovea fourwheeledcart 2.Theoffsetpivotcastermountingwillaffecttheinitialpeakappliedforcerequired tomoveafourwheeledcart. 3.Thereisastatisticallysignificantinteractionbetweenwheeltypeandcastertype ontheinitialappliedpeakforcerequiredtomoveafourwheeledcart. Chapter 2: Literature Review 2.1 Force/Load Guidelines IntheUnitedStates,theOccupationalSafetyandHealthAdministration(OSHA)has neitherestablishedasafethresholdontheweightofacartanditscontents,northe maximumforceanindividualisallowedtopushorpull.Theoften‐usedguidelinein generalindustryforpushingandpullingcomesfromthepsychophysicalresearch thatwasconductedattheLibertyMutualResearchCenterbySnookandCirielloin 1991[9].TousethetablesdevelopedbySnookandCiriello,theusermustidentify thefollowing:task(pushingorpulling),gender,heightofthehandswhilepushing orpulling,frequencyofthetask,andthedistancetraveled.Thegoalistodesignthe tasksoitcanbesafelyaccomplishedby75%and99%ofthefemaleandmale workers,respectively[9]. 4 2.2 Related Prior Research Table1isachronologicalsummaryofpublishedresearchdirectlyorindirectly relatedtothisstudy. Table 1 ‐ Related Research 5 6 7 8 9 10 11 12 Thisstudyisdifferentfromothersthathavepreviouslybeenconductedasit comparesthenewersplitwheeldesigntoastandardsinglewheelwhichis commonlyusedinmanyindustries.Additionally,theSwivel‐Eaz™Proswivelcaster wasrecentlypatentedandisnewtothemarket;thereforeithasnotbeenstudiedin thepast. Thisstudywillcontributetothebodyofknowledgebyfillinginthedearthof researchthatspecificallystudiestheeffectofSwivelEaz™wheelsandSwivelEaz™ Procastersinthepushandpullforces. 2.3 Factors Affecting Pull and Push Forces Intheindustrialsetting,employeesmayberequiredtopushcartswithvarious loads.Thereareseveraldifferentfactorsthatcontributetotheactualforcerequired tomovethecartincluding:friction,wheelposition,wheeldiameter,wheelhardness, floorslope,andthecartandcontentweight.Initialforceisdefinedastheforce requiredtogetanobjectinmotionwhilesustainedforceistheforcerequiredto keepanobjectinmotion[9].Alldataanalysisinthisstudywasconductedonthe peakinstantaneousappliedforcerecordedduringtheinitialphasewhilethecart wasbeingpushed.Forconvenienceinidentification,thepeakinstantaneousforceis thehighestpointfoundinatenthofasecondintimeduringtheinitialpushing phase.Hereinafter,thepeakinstantaneousforceisreferredtoaspeakapplied forceorpeakforce. 13 Weight Inavehicleassemblyplant,workersmayberequiredtopushcartsthatarefullof metalpartswhicharerelativelyheavy;ortheycouldpushasmallcartoflightweight plasticcomponents.Attimes,workersmayberequiredtopushorpullloadsof variousweightsincludingthosewhichmayrequirenearmaximalstrengthtomove. Forinstance,Cirielloetal.(1999)analyzed25,291manualmaterialhandlingtasks ofwhich1,879requiredpushingand1,866requiredpulling.Theyfoundthat28% ofthepushingtasksrequiredforcesgreaterthan70lbs(311N)[15].Inyetanother study,Resnick(1996)foundthatthemeanstaticpeakhorizontalforcewas75lbs (335N)forwomenand139lbs(620N)formen.Thesepeakforceswerefound withahandleheightofabout80%ofshoulderheight[16].Loadweightsthatare transportedinmostindustriescanvaryupto3,300lbs(1,500kgs)whichfarexceed therecommendweightlimitof496lbs(225kgs)forfourwheeledcartsand251lbs (114kgs)fortwowheeledcarts[13]‐[14].Inthisstudy,twodifferentloadweights wereused250lbs(113kgs)and750lbs(340kgs). Friction/Wheel Hardness Frictionisdefinedasa“forcethatactstoresisttherelativemotion(orattempted motion)ofobjectsormaterialsthatareincontact”[27].Inwheeledcarts,friction betweenthewheelandtheaxleandrollingresistancebetweenthefloorandthe wheeldeterminetheamountofforcerequiredtomovethecart[1].Higherfriction contributestoincreasedrollingresistanceandthusrelativelyhigherpushingand pullingforces.Forthisreason,low‐frictionwheelbearings,andrelativelyhard wheelsarepreferredwhenpushingandpullingcartswithheavierloads[10]‐[13]. 14 Therelationshipbetweentherollingresistance,friction,andloadisgovernedbythe followingequation[28]‐[29]: F=fxW/R F=theforcerequiredtoovercometherollingfriction f=thecoefficientofrollingfriction(unitsmustmatchsameunitsasR(radius)) W=Loadonthewheel R=Radiusofthewheel Ittakeslessforcetopushorpullhardwheelsthansoftwheelsduetoalowerrolling friction[10].Softwheelsorpneumaticwheelstendtodevelopflatspotswhena cartloadedwithaheavyweightisstoredforalongperiodoftime[13]. Wheel Position/Orientation Whetherinthemanufacturingortheservicesectorenvironment,itiscommonto findacartwithfourcastershavingonepositionedoneachcorner.Thereareother casterconfigurationsthatcanbeusedwhendesigningacart,suchashavingasingle orpairoffixedcastersinthecenterandaswivelcasteroneachofthefourcorners whichallowsthecarttobepositionedandmaneuveredintotightspaces[12].Some cartshavetwofixedcastersandtwoswivels,whileothershavefourswivels.Three morecommonlyfoundwheelpositionsarelistedin(Figure1)where“R”represents aridgedorfixedcasterand“S”isaswivelcaster.Wheelsinthestudywere positionedasdepictedinFigure1b. 15 Figure 1 ‐ Wheel Positions (1a) R R (1b) S (1c) R R S S R R S R R Isthisstudy,twoswivelcastersweremountedononeendofthecartnearestthe handleandtwofixedcastersweremountedontheoppositeend. ThemanufacturerDarcorCastersandWheels,definesSwivelCasterasabasiccaster unitwiththeadditionofabearingthatallowsthecastertoswivelaboutavertical axis[30].Oneoftheproblemswithhavingasetofswivelwheelsonacartis occasionallythewheelsarenotalignedinthedirectionoftravel.Whenthis happens,additionalforceisrequiredtogetthecartwheelstoswivelaroundand becomeproperlyalignedinthedirectionoftravel.AccordingtoastudybyAl‐ Eisawietal.,whenfrontwheelsofacartwerealignedinthedirectionoftraveland rearwheelswereperpendiculartothedirectionoftravel,minimumpullforcewas 19%higherthantheminimumpushforce.Thissuggeststhatswivelwheelsshould beplacedintherearifthecartwillbeprimarilypushed[10]. Swivelwheelsaffecttheamountofforcerequiredwhenpushingorstoppingacart [10].Al‐Eisawietal.alsofoundthatwhenthereartwoswivelcastersarealigned perpendiculartothedirectionoftraveltheaverageforcewas13.1%higherthan whenallfourcasterswerealignedinthedirectionoftravel.Inaddition,theyfound 16 thatwhenallfourswivelcasterswerealignedperpendiculartothedirectionof traveltheaverageforcewas30.7%higherthanwhenallfourwerealignedtothe directionoftravel.Asaresultoftheirstudy,theyfoundthatthesmallestforce requiredtomovethecartwasrecordedwhenallfourwheelswerealignedinthe directionoftravel.Additionally,thegreatestforcewasrecordedwhenallfour wheelswerealignedperpendiculartothedirectionoftravel[10].Therefore,inthis studythefrontwheelswerefixedtothedirectionoftravel,whiletherearwheels werepositionedbothin‐lineandperpendiculartothedirectionoftravelaswillbe reportedinthedesignofexperiment. Afour‐wheeledcartneedstohaveatleasttwowheelsthatswiveltoallowittoturn. Cartswithfourswivelwheelsrequiremoreforcetooperate[10].Theextraforce requiredtooperateafour‐wheeledcartwithswivelwheelscouldberelatedtothe additionalforcethatisappliedtokeepitundercontrolinthelateraldirection[1]. Inthisstudy,lateralforcewasmeasuredtodetermineifitdecreasedwithanyofthe newcasterdesignsorcombinations. Asfour‐wheeledcartsarecommonlyfoundinthecompanysponsoringthisstudy,a four‐wheeledcartwasconstructedwithswivelwheelspositionedattherearclosest tothehandle. 17 Wheel Diameter Generally,theeffectofwheeldiameteronthepushandpullforceismodeledusing theequationdescribedintheprevioussectionentitledFriction;whereR=radius,W =loadonthewheel,f=coefficientofrollingfriction,anF=forcerequiredto overcomerollingfriction. F=fxW/R Thereforeifallotherfactorsareequal,bydoublingradiusofthewheel,pushand pullforcewilldecreasebyhalf.Increasingthediameterofawheelcanbean effectivemeasureinreducingtheforceneededtomoveacart.Inastudyofpushing floor‐basedpatientliftingdevices,itwasfoundthattherewerehighersheerforces intheuser’sbackwhenpushingthedevicewithsmallerdiameterwheelscompared toasimilarliftingdeviceequippedwithlargerdiameterwheels[1].Irrespectiveof thefloorsurface(carpetorconcrete),pullforcedecreasedwhenwheeldiameter wasincreased[10].However,pushandpullforceisgreaterwithlargerwheels whenswivelwheelsarenotalignedinthedirectionoftravel[10].Largerdiameter wheelsareabletocrossoverbumps,holes,andotherobstructionsinthefloormore effectivelythansmallerwheels[1].Inthisstudy,diametersofthewheelschosen were6in.(15.2cm)becausethisdimensionisprimarilyusedinthesponsoring company.Inaddition,withthisdiameter,anybumpsintheconcreteinthetesting facilitywouldnotinterferewiththedatacollectionprotocol. Slope Insomegeneraltaskssuchasmovingproductsusingtwo‐wheeledhandcarts,stairs andcurbscancreateanobstaclethatisdifficultforthecarttogetover.Anoption 18 maybetoinstallaramptohelpnavigatethestepsorcurb;althoughanincreasein slopewillcauseanincreaseinpushandpullforce.Itisrecommendedthatwhen usingaramptheslopeshouldbekepttolessthan3.5%(2∘)[15]. Handle Height Al‐Eisawiet.al(1999)reportednostatisticaldifferenceinpushforcewithaload weightof160lbs(73kgs)atthreedifferenthandleheights;knuckle,elbow, shoulder.However,thedifferencewassignificantataloadweightof399lbs(181 kgs).Pushforcerequiredwhenhandleheightwasattheshoulderlevelwas10% lowerthanatelbowheight,andelbowheightwas10%lowerthanknuckleheight [11].Thepreferredhandleheightforhorizontalpushingisataboutelbowheight [13].Inaddition,biomechanicalresearchhasfoundcompressionforceontheL5/S1 vertebraewaslowestwhenhandleheightissetatelbowheight[16]. Handle Width Handlewidthshouldbedesignednogreaterthan18in.(45.7cm).Widerhandles mayplacehigherloadsontheweakershouldermuscles[13].Inthisstudy,the handlewidthwasfixedat20in.(51cm). Chapter 3: Current Study 3.1 Study Variables Thefollowingsummarizedvariableswereconsideredinthisstudy:(1)loadweight (2)wheelalignment(3)casterfixturedesignand(4)wheeltype. 19 Onecastermanufacturer,AubinIndustrieslocatedinTracy,Californiahasdesigned awheelandcasterassemblywhichtheyclaim“reducesturningandrolling resistanceonswivel,ridgedandfixedaxlesystems[18].”Thewheelmanufactured byAubinIndustriesisasplitwheeldesignwherethewheelsrotateindependently andshareasinglehubassemblyasshownin(Figure2). Figure 2 ‐ Swivel Eaz Wheel Accordingtothepatentforthewheels: Thetwo‐wheelcasterofferedanimprovementoverthesinglewheelintwo importantregards.Theabilityofthewheelstorotateatdifferentratesorin oppositedirectionsatthesametimegreatlyenhancestheabilitytoturn abouttheverticalpivotaxis,makingachangeinoveralldirectionofthe objectmuchsmoother[20]. 20 ThecasterassemblyalsomanufacturedbyAubinIndustries“allowsthecasterto pivoteasilytoaccommodatethedirectionofthrustappliedtoanobjectsupported bythecaster[20].” Accordingtothepatentforthecaster: Thisadvantageousfeatureismadepossiblebyprovidingadualpivot assemblyinthecastermountingthatislaterallyoffset,wherebythecaster wheelsmaynotonlypivotaboutawheelpivotaxisthatextendsthroughthe planeofthecasterwheel,butalsorevolveorbitallyaboutamountingpivot axisthatislaterallyoffsetfromthewheelaxis.Asaresult,thecaster assemblyeasilymayassumetheproperorientationforanythrustappliedto thecaster‐supportedobject[20]. 3.2 Research Questions Thisstudysoughttoanswerthefollowingquestions: 1.Doesthesplitwheelhaveaneffectoneitherinitialorsustainedforcewhen comparedtoastandardsinglewheelinvariouscasterorientations? 2.Doestheoffset‐pivotfixture(caster)haveaneffectoneitherinitialorsustained forcewhencomparedtoastandardcasterfixtureinvariouswheelorientations? 3.Dothesefactors:wheeltype(splitversussingle),fixture(standardversusoffset), andwheelorientation(alignedversusmisaligned)haveaninteractiveeffectonthe pushforces? 21 3.3 Research Hypotheses TheoverarchinggoalofthisresearchistodeterminetheefficacyoftheSwivelEaz™ splitwheelandoffsetpivotalSwivelEaz™Procasterinreducingpushforcesthat occurwhenmanuallymovingacart.Torealizethisgoal,wesetupthreemainaims fromwhichwedefinedtheresearchhypothesisasfollows: 1.Split‐wheeldesignwillaffecttheinitialappliedpeakforcerequiredto moveafourwheeledcart. Hypothesis1a:Splitwheelswillreducetheinitialappliedpeakforcewhen therearwheelsarepositionedat90degreestothedirectionoftravel. Hypothesis1b:Splitwheelswillaffecttheinitialappliedpeakforcewhenthe rearwheelsarepositionedat0degrees(alignedwiththedirectionoftravel). 2.The“offsetpivot”castermountingwillaffecttheinitialpeakappliedforce requiredtomoveafourwheeledcart. Hypothesis2a:Theoffsetpivotcastermountingwillreducetheinitial appliedpeakforcerequiredtomovethecartwhentherearwheelsare positionedat90degreestothedirectionoftravel. Hypothesis2b:Theoffsetpivotcastermountingaffecttheinitialapplied peakforcewhentherearwheelsarepositionedat0degrees(alignedwith thedirectionoftravel). 3.Thereisastatisticallysignificantinteractionbetweenwheeltypeandcaster typeontheinitialappliedpeakforcerequiredtomoveafourwheeledcart. Hypothesis3a:Thereisastatisticallysignificantinteractionbetweenwheel typeandcastertypeontheinitialappliedpeakforcerequiredtomoveafour 22 wheeledcartwhentherearwheelsarepositionedat90degreestothe directionoftravel. Hypothesis3b:Thereisastatisticallysignificantinteractionbetweenwheel typeandcastertypeontheinitialappliedpeakforcerequiredtomoveafour wheeledcartwhentherearwheelsarepositionedat0degrees(alignedwith thedirectionoftravel). Chapter 4: Methodology and Data Analysis 4.1 Experiment Participants Inordertobeginconductingthestudyonhumansubjects,approvalwasobtained fromtheUniversityofWisconsin‐MilwaukeeInstitutionalReviewBoard(IRB). FinalapprovaltobeginthisstudyIRB#14.316wasgivenonAugust13,2014for oneyear.Whileeightparticipantswasthetargetgroupsize,theIRBalloweda totaloftwelvesubjectstoparticipateinthestudytoallowforpossibleparticipant withdrawal. Eightprofessionalworkers(4male,4female)ofvariousagesandoccupationswere recruitedtoparticipateinthestudy.Allofthesubjectswereemployeesoftheheavy truckandmilitaryvehiclemanufacturerthatsponsoredthestudyandacceptedto jointhestudyvoluntarily.Allofthesubjectsworkedinaprofessionaloffice environment. 23 Subjectscompletedthestudyduringtheirnormalworkday;therebyreceivingtheir standardwage.Noovertimewaspaidtoparticipantsnorweretheypaidanything beyondtheirnormalwage.Participantswhosuccessfullycompletedthestudy receiveda$40creditcard,whichwasapprovedbytheUWMIRBcommittee.Before subjectswereallowedtoparticipateinthestudytheycompletedapre‐screenform (AppendixA)aswasstipulatedbytheIRBinclusion/exclusionprotocol.Thepre‐ screenformaskedthepotentialparticipantiftheyhadanypreviousinjuriesintheir backorshouldersoriftheywereatthetimeexperiencinganypainordiscomfortin theirbackorshoulders.Iftheyansweredyestoanyofthesequestionstheywould beprecludedfromparticipatinginthestudy.Iftheparticipantsuccessfullypassed thescreeningprocesstheyweregiventheUWM‐IRBcommitteeapprovedconsent formforreviewandsignature.Thestudentprincipalinvestigatormetwith supervisorsofeachpotentialparticipanttoobtainapprovalfortheiremployeesto participateinthestudy.Testingwasconductedonsiteatthesponsoringcompany's testanddevelopmentfacility. Afterthesubjectreviewedandsignedtheconsentformtheywerereadyto participateinthestudy.Ashortdemographicandanthropometricformwas completedthatincludedheight,weight,genderandstandingelbowheight (AppendixB).Participantswereaskedtoprovidetheiroverallstature.Standing elbowheightwasmeasuredinthelab.Thesemeasurementsweretakenwiththeir shoeson. 24 Subjectswereaskedtopushthecartoncefor10metersforeachofthe16test combinations.Testingoccurredovertwoseparatesessionsforatotalof8trials eachsession.Tominimizetheeffectofmusclefatigue,subjectswereallowedtorest 2minutesbetweentrialsandweregivenupto5minutesbetweentrialsifnecessary. Inaddition,therewasalongerrest/recoveryperiodofatleast2daysbetweenthe initial8trialsandthelater8trials.Voltage,speed,andtimewerecollectedduring eachofthetrialsandtheexperimentcombinationrunswererandomizedusinga randomnumbergeneratoravailableonlineatrandom.org. Thefloorofthetestfacilityisapouredconcretepad,whichisrepresentativeofthe floorsurfacefoundinmanyfactories.Tomaximizeforwardforceandminimize downwardforce,handleheightwasadjustedtothesameheightastheparticipant’s standingelbowheight. 4.2 Equipment and Instrumentation Cart Design Toreducesetuptimebetweentrials,twocarts(hereinnamedcart1and2)were builttothesamedimensionsusingthesameconstructionmaterialsfoundinFigure 3. 25 Figure 3 ‐ Cart design Thecartswerebuiltusing1in.(2.5cm)rectangularsteelstockwhichwerewelded togetheratthejoints.Thetotaldimensionsofthecartplatformmeasure24in.(61 cm)x36in.(91cm).A24in.(61cm)x36in.(91cm)x¾in.(2cm)thicksheetof plywoodwasscrewedtothetopoftheplatformframetoprovideasolidsurfacefor theweights.Toallowforaquickersetuptimebetweentrials,thestandardcaster fixtureswereinstalledononecartandtheSwivelEaz™Procasterfixtureswere installedonthesecondcart.Therefore,toreducesetuptimebetweentrials,either theweightorthewheelshadtobechanged,nevertheentirefixture. 26 Thehandlewasbuiltusing¾in.(2cm)roundsteelcutandweldedtothe¾in.(2 cm)squaresteelstock.Overalldimensionofthehandleis20in.(51cm).The verticalsupportforthehandlewasbuiltusinga¾in.(2cm)x¾(2cm)steelsquare tubethatmeasures40in.(102cm)inlength.Theverticalsupportwasinsertedinto a1in.(2.5cm)x1(2.5cm)squaretubewhichmeasured14in.(35.6cm)intotal length.Holesweredrilledthroughthe1in.(2.5cm)x1in.(2.5cm)squaretube every1in.(2.5cm)andapinwasinsertedtosupporttheverticalhandlestructure. Thisallowedthehandletoadjustverticallytomatchstandingelbowheightofeach participant.Thehandlewasdesignedtobeeasilyremovedandwassharedbetween thetwocarts.Theverticalsupportforthehandlewasnotchedonall4edgesto allowsupporttoflexandactivatethestraingaugesasshowninFigure4. Figure 4 ‐ Handle Notches Weightswereusedtobringthetotalweightofthecartupto250lbs(113.4kgs)or 750lbs(340.1kgs)dependingonthetrial.Weightofthecartsvariedduetothe variabilityinweightofthedifferentwheelsandfixtures.Therefore,itwas 27 necessarytoincorporateadditionalsmallerweightstothecarttobringthetotal weight(cartandload)uptothetarget. Whilecartloadweightvariesinthesponsoringcompanymanufacturing environment,thesetwoweightlevelswereselectedastheyarerepresentativeof theloadweightrangethatcouldexistinaheavy‐vehiclemanufacturing environment.Preliminaryforcetestingwascompletedusingahandheldforce measurementdevice(ergoFET300manufacturedbyHogganHealth).Toestablish themaximumpushforcethattheparticipantsinthisstudycouldexperience,a preliminarytestwasconductedintheworst‐casescenariowithwheelsaligned perpendiculartothedirectionoftravelandthecartwaspushedtoahighvelocity. Thisinitialtestresultedinforcesthatdidnotexceed50lbs.(222N),whichwaswell belowthemeanstaticforceforbothwomenandmenreportedbyResnick,(1996) [16]. Handle Thehandlewasdesignedtobeadjustable,hencetoberaisedorloweredto accommodatethestandingelbowheightforeachsubject.Toaccommodate98%of theworkingpopulation[24]thehandlewasdesignedsothatitcouldbeloweredto 37in(94cm)andraisedto48in(122cm). Caster Fixtures AllofthecasterfixturesweremanufacturedbyAubinIndustries.Thedualoffset swivelfixturenamedtheSwivelEaz™Proandhastwoseparatepivotorswivel 28 points,thatallowsmultipleorientations(Figure5).TheSwivelEaz™Procasterwas comparedtoastandardswivelcasterthatistypicalofwhatiscurrentlybeingused bythestudysponsor(Figure6) Figure 6 ‐ Standard Swivel Caster (Fixture) Figure 5 ‐ Swivel Eaz Pro Wheels Wheelswereprovidedbytwodifferentmanufacturers.Boththesinglewheeland splitwheelare6in.(15.2cm)indiameterandwereselectedtoensuresimilar hardness.Thebearingsinbothtypesofwheelswereprecisionballbearingsof similardesign.The6in.(15.2cm)diametersplitwheelwasmanufacturedbyAubin IndustriesinTracy,California.Perthemanufacturer,thepolyurethaneSwivel‐Eaz wheelhasahardnessdurometerof70A[18].TheSwivel‐Eazwheelmeasuresa totalof2in.(5.1cm)wideacrossbothedgesandhasacrownedsurfaceasshownin Figure7. 29 Figure 7 ‐ Swivel Eaz Caster Thestandardstylesinglewheelwhichalsomeasures2in.(5.1cm)wideand6in. (15.2cm)indiameterismanufacturedbyArbcoIncorporated(Figure8).Itisa phenolicwheelandhasasimilarhardnessastheSwivel‐Eazsplitwheel. Figure 8 ‐ Single Wheel Cart Load Steel weights were used to bring the gross weight of the cart up to 250 lbs (113.4 kgs) or 750 lbs (340.2 kgs) depending on the trial. The weight of the carts varied due to the 30 variability in weight of the different wheels and fixtures. Therefore, each of the 4 unique caster/wheel combinations required different amounts of additional weight to achieve the gross weights of 250 lbs (113.4 kgs) and 750 lbs (340.2 kgs). A chart was created listing the possible caster/wheel combinations, their gross weight, and additional weight needed to achieve the target weight (Table 2). Table 2 ‐ Additional Weight Added Floor Thefloorofthetestfacilitywasalevelpouredconcretepad,whichisrepresentative ofthefloorsurfacefoundinmanyfactories. 31 Data Collection Electronics Thehandlesupportwasnotchedatallfouredgesat18in.(45.7cm)fromthe bottom.Straingaugesweremountedonallfoursidesofthehandletoallowfor measuringforceinthreeaxis:(X)laterally,(Y)longitudinally,(Z)andvertically. Thefourstraingaugesweremountedtotheverticalhandlewithtapeandusedto measuredeflectionofthehandleunderload.Thefourstraingaugeswerewiredtoa 5‐channelbridgecompletionmodule.Thebridgecompletionmodulewas connectedtotheVbox3andwassetuptorunatasamplingrateof10Hz.Datawas savedonacompactflashmemorycard.Aphotosensorwasmagneticallyattached tothefrontwheelandcapturedspeedbyemittinglightontherotatingwheeland countingthemarkers.Thewheelrotationdatawasalsorecordedandlater convertedtospeedinmilesperhour.Thephotosensorwasalsoconnectedtothe Vbox. Calibration Tocalibratethestraingaugesthecartwasaffixedtoabedplate.Anoverhead bridgecranewasusedtosupporta500lbloadcellwhichwasattachedtoaratchet device.TheratchetwasusedtopullagainstthecartinX,Y,Zorientations.The testswereconductedin10lbincrementsfrom0to100lbs.Bendingofthehandle wasmeasuredandrecorded.Loadversusstrainwasrecordedandusedtodevelop aregressiontoallowforthedataconversion.Thewheelopticalreaderwasalso calibrated. 32 Ratings of Perceived Exertion Scale Afterpushingthecart,theparticipantcompletedaperceivedphysicalexertion questionnaire(AppendixC).Perceivedexertionisaresponsevariableandrated usingtheBorgCR10scale.Attheendofeachtrial,participantswereaskedtorate thelevelofexertiontotheirshouldersandback,usingtheBorgCR10scale[22]. TheBorgCR10scaleisacategoricalpsychophysicalscalethatprovidesaratingof perceivedexertion(RPE).Thisscalehasbeenusedwidelyinavarietyof applicationssuchasrehabilitationandsportstrainingtoassesstheintensityofa givenphysicalprocedure[23].Additionally,theywereaskedtoratehowwellthey likedordislikedthewheelandfixturecombination,subjecttoeaseofoperation. 4.3 Study Design Thepurposeoftheresearchwastoinvestigatetheeffectsofthesewheelandfixture designsonpushingforceofafour‐wheeledcart.Weuseda24balancedfullfactorial experimentdesign,with8participants(fourmaleandfourfemale).Each participantwasaskedtopushthecartforeachofthecombinations(foratotalof 128experimentruns).Inbrief(detailswillbediscussedinthemethodology chapter),thevariablesthatwereconsideredinclude:(i)astandardcommonlyused 2in.(5.1cm)thick,6in.(15.2cm)diametersinglewheelversusa2in.(5.1cm) thick,6in(15.2cm)diametersplitwheel(Swivel‐Eaz™),(ii)astandardswivel casterversustheoffset‐pivotorbitalcaster(Swivel‐Eaz™Pro),(iii)representative loadweightlevelsof250lbs(113.4kgs)and750lbs(340.2kgs),and(iv)rearwheel position(0degreei.e.alignedtothedirectionoftravel)versus90degrees (perpendiculartothedirectionoftravel).Itmustbenotedthatallwheel,fixture, 33 andweightcombinationsweretestedwiththefrontwheelsfixedon0degreesto directionoftravel.Tokeepthestudyfocusedondeterminingpushforcewith wheelsalignedinthedirectionoftravelandalsopositionat90degreesor perpendiculartothedirectionoftravel,pushingthecartaroundacornerwasnot consideredinthisstudy. BecausetheSwivel‐Eaz™Prohastwodifferentpivotpoints,thecasterassemblycan beorientedinmultiplepositions.Inthisstudy,thefixturetransferplateandwheels werepositionedinlinetothedirectionoftravel(Figure9a),hereinreferredtoas F0R0throughoutthestudy.Thecombinationwasalsostudiedwiththetransfer plateandwheelsfullyextendedperpendiculartothecart(Figure9b),herein referredtoasF0R90. Figure 9 ‐ Wheel/Fixture Orientation (9a)–F0R0 (9b)–F0R90 Asmentionedearlier,testingoccurredatsponsoringcompany’sTestand DevelopmentfacilityinOshkosh,Wisconsin.Thesponsoringcorporation“isa 34 leadingmanufacturerandmarketerofaccessequipment,specialtyvehiclesand truckbodiesfortheprimarymarketsofdefense,concreteplacement,refusehauling, accessequipmentandfireandemergency[22].” 4.4 Experimental Procedure Subjectsparticipatedintwo60minutesessions.Aminimumof24hoursofrestwas providedbetweeneachsession.Atthebeginningofthefirstsession,eligible subjectscompletedashortdemographicandanthropometricform(AppendixC) thatincludedheight,weight,genderandstandingelbowheight(measuredinthelab withshoeson).Subjectsthencompletedthe16pushandpulltrials(Table3)in randomorder,with8trialsperformedineachsession. 35 Table 3 ‐ Experiment Design Combinations Combo # Cart Weight (lbs) Fixture Wheel Type Wheel Position 1 1 250 Standard Single F0, R0 2 1 250 Standard Single F0, R90 3 1 250 Standard Swivel Eaz (dual) F0, R0 4 1 250 Standard Swivel Eaz (dual) F0, R90 5 2 250 Offset Pivot Single F0, R0 6 2 250 Offset Pivot Single F0, R90 7 2 250 Offset Pivot Swivel Eaz (dual) F0, R0 8 2 250 Offset Pivot Swivel Eaz (dual) F0, R90 9 1 750 Standard Single F0, R0 10 1 750 Standard Single F0, R90 11 1 750 Standard Swivel Eaz (dual) F0, R0 12 1 750 Standard Swivel Eaz (dual) F0, R90 13 2 750 Offset Pivot Single F0, R0 14 2 750 Offset Pivot Single F0, R90 15 2 750 Offset Pivot Swivel Eaz (dual) F0, R0 16 2 750 Offset Pivot Swivel Eaz (dual) F0, R90 Tominimizetheeffectofmusclefatigue,subjectsrestedaminimumof2minutes betweenconsecutivetrials.Upto5minutesrestwasprovidedifthesubjectsfeltit wasnecessary. Priortoeachtrial,theheightofthecarthandlewassettomatchthesubject’s standingelbowheightandthecartwasequippedwiththeappropriatewheel,caster fixture,andweightcombination.Thecartwasmovedintopositionandthewheels weresetateither90degreestothedirectionoftravelorat0degrees(inline)with thedirectionoftravel.Participantswereinstructedtopushthecartatasteadybut comfortablepacefor10metersuntiltheyreachedapredeterminedstopline.The 36 Vboxdatacollectionunitwassettorecordtheforce(ie,calibratedvoltage),speed, andtimeduringthetrial. AftereachtrialthesubjectcompletedthePainandExertionratingform(Appendix C),andprovidedtheirsubjectivefeedbackabouthowwellthey“liked”the fixture/wheelcombination.Finally,theresearcherrecordedtheVBoxtrialnumber onthePainandExertionratingformandreturnedthecarttothestartline. 4.5 Statistical Analysis AppliedpeakforcedatawasprocessedinMicrosoftExcel2007.Handlebending datawasconvertedtopoundsofforceinlateralandlongitudinaldirections.Force wasalsorecordedintheverticaldirection;howeverduetoalimitationinthedesign ofthecarthandlethestraingaugewasnotabletorecordverticaltensionaccurately. Therefore,only2axisofdatawereused,lateral(sidetoside),andlongitudinal (fronttoback). Sincea24fullyrandomizedfactorialdesignwasused,thepeakforcemeasurements wereanalyzedwithageneralizedlinearmodelusingabackwardseliminationusing Minitabversion17.Minitabperformsastepwiseregressionwithbackward eliminationbystartingwithallpredictorsinthemodelandremovestheleast significantvariableforeachstepandeventuallystopswhenthep‐valueislessthan orequaltothespecified“Alpha‐to‐Removevalue”[31]. 37 PsychophysicalselfreportedexertiondatawasanalyzedusingaPearson’s Correlationcoefficient.Analysiswasconductedtounderstandrelationshipbetween thevariablesandperceivedexertionontheshoulderandback. Chapter 5: Results and Discussions 5.1 Results Data Processing Samplingdatawascollectedforeachtrialrunandsavedinitsrawformat.Datawas collectedfromeachofthe4straingauges,pulsedataforthewheelrotation,and time.Next,voltagefromthestraingaugeswasconvertedintolongitudinaland lateralbendingloadinpoundsofforce.Rootmeansquare(RMS)valuewas calculatedusingthelateralandlongitudinalbendingload.Verticalloadwas recordedanddiscardedandwasnotusedinthecalculation.Thedesignofthe handleandplacementofthestraingaugesdidnothaveenoughsensitivityto accuratelycaptureverticalloadinthedownwardorupwardmotion.Inthisstudy, verticalloadwasminimizedashandleheightwasadjustedtostandingelbowheight foreachparticipant. Pulsedatafromthewheelwasconvertedtorevolutionperminutethenultimately convertedtomilesperhour.Onceallofthedatawasconvertedintotherequired formatandunitsofmeasure,datawasplottedinMicrosoftExcelonascatterplot withstraightlinesasshowninFigure10.EvidentinFigure10,forcevaluesbecame 38 negativewhenthehandlewasbendingbacktowardstheparticipanttodecelerate thecart(longitudinalbending). Figure 10 ‐ Example of Graph of Push Force 3.0 50.0 40.0 2.5 2.0 20.0 RPM lbs.offorce 30.0 10.0 1.5 0.0 0 ‐10.0 5 10 15 Time(seconds) ‐20.0 ‐30.0 20 25 1.0 0.5 longitudin al bending loadlb(+ tofront) lateral bending loadlb(+ toright) totalload in horizontal planelb speedmph 0.0 Analysis of Physical Measures – Peak Force Sincea24randomizedfullfactorialdesignwasusedintheexperiment,peak– instantaneousinitialforcemeasurementswereanalyzedusingageneralizedlinear modelwithbackwardseliminationprocedure.Statisticalprocedureswerecarried inMinitabVersion17forMicrosoftWindows.Backwardseliminationwasusedto removetheleastsignificantvariablesforeachstepuntilallofthevariableshaveap‐ valuelessthanthespecifiedtypeIerror[30]. 39 Psychophysical Self Reported Exertion Data APearsoncorrelationtestwasrunonthepsychophysicalperceivedexertiondatato determinethepresenceorabsenceofcorrelationbetweentheperceivedexertion dataandtheinitialforce.Thiscorrelationwasvalidatedbyaddingweightasan additionalcorrelationvariable.“Thelargertheabsolutevalueofthecorrelation coefficient,thestrongerthelinearrelationshipbetweenthevariables[30]”. 5.2 Participant Data/Information Thefollowingchartprovidesanthropometricaswellasthedemographic informationforthe8participantsincludingmeanandstandarddeviation(Table4). Table 4 ‐ Participant Information & Statistics Subject Gender Age Weight 1 Male 29 195 StandingElbow Height 47” 2 Male 26 185 44” 3 Male 49 215 44” 4 Male 45 183 43” 5 Female 57 125 42” 6 Female 43 200 42” 7 Female 48 189 41.5” 8 Female 38 190 42.5” Range 26‐ 57 125‐ 215 MaleMean (std.dev) 37.3 (9.9) 194.5 (12.7) FemaleMean (std.dev) 46.5 (7.0) 176 (29.8) OverallMean (std.dev) 41.9 (9.8) 185.3 (24.7) 40 Theboxplotsummary(Figure11)formaximuminitialforcebygendershowsthat malesexertedmoreinitialforcewhenpushingthecartforboth250lbs(113.4kgs) and750lbs(340.4kgs). Figure 11 ‐ Boxplot Gender Effect – Females versus Males (Figure 11a) (Figure 11b) 250 lbs (113.4 kgs) 750 lbs (340.4 kgs) 5.3 Physical Force Measurement Analysis –Initial Push Force UsingMinitab17,ageneralizedlinearmodelwasfittedtothedatafollowingthe backwardeliminationprocedure,theresultsofwhicharesummarizedinFigure12. Inthismodelfixture,wheeltype,wheelposition,andweightweretreatedasthe mainfactorswhileparticipantsweretreatedasablockingfactortoaccommodate theexpectedvariabilitywithinsubjects. 41 Figure 12 ‐ General Linear Model GeneralLinearModel:MaxPeakForversusWeight(lbs),WheelType,... Method Factorcoding(‐1,0,+1) BackwardEliminationofTerms Candidateterms:Weight(lbs),WheelType,Fixture,WheelPosition,Participant ‐‐‐‐‐Step1‐‐‐‐ ‐‐‐‐‐Step2‐‐‐‐ CoefP CoefP Constant 31.316 31.317 Weight(lbs) ‐9.2490.000 ‐9.2500.000 WheelType ‐1.1480.068 ‐1.1480.067 Fixture ‐0.2230.721 WheelPosition ‐1.5450.014 ‐1.5450.014 Participant 12.300.000 12.310.000 S 7.03694 7.01067 R‐sq 76.96% 76.93% R‐sq(adj) 74.77% 74.96% R‐sq(pred) 71.95% 72.39% Mallows’Cp 12.00 10.13 αtoremove=0.1 Givena90%confidencelevel,itwasfoundthatonlywheeltype,wheelposition, weight,andparticipantsignificantlyaffectedtheinitialpushforce.Inthefollowing sectionwepresentanddiscusstheresultsoftheanalysisthatwerecarriedoutto testeachofthethreehypotheses. 42 Testofhypothesis1: 1.Split‐wheeldesignwillaffecttheinitialappliedpeakforcerequiredto moveafourwheeledcart. Hypothesis1a:Splitwheelswillreducetheinitialappliedpeakforcewhen therearwheelsarepositionedat90degreestothedirectionoftravel. Hypothesis1b:Splitwheelswillaffecttheinitialappliedpeakforcewhenthe rearwheelsarepositionedat0degrees(alignedwiththedirectionoftravel). Figure13indicatesthattheinteractionbetweenwheeltypeandpositionhasap‐ valueof0.947,henceitisnotsignificant.However,bothfactorsareindividually significant.Inthisanalysis,weightwastreatedasafixedfactor,whosesignificance wasexpected.Toaccountfortheexpectedvariabilitywithinparticipants,the participantvariablewastreatedasablockingfactor,whichalsoturnedouttobe significant. Figure 13 ‐ ANOVA: Max Peak Initial Force vs Wheel Type & Wheel Position 43 Figures14and15arethemaineffectsandinteractionplotsforwheeltypeand wheelposition,respectively. Figure 14 ‐ Main Effects Plots Figure 15 ‐ Interaction Plot 44 Testofhypothesis2: The“offsetpivot”castermountingwillaffecttheinitialpeakappliedforce requiredtomoveafourwheeledcart. Hypothesis2a:Theoffsetpivotcastermountingwillreducetheinitial appliedpeakforcerequiredtomovethecartwhentherearwheelsare positionedat90degreestothedirectionoftravel. Hypothesis2b:Theoffsetpivotcastermountingaffecttheinitialapplied peakforcewhentherearwheelsarepositionedat0degrees(alignedwith thedirectionoftravel). Thereisastatisticallysignificantinteractionbetweenwheeltypeandcastertypeon theinitialappliedpeakforcerequiredtomoveafourwheeledcart. FromtheANOVAinFigure16,theinteractionbetweenthefixtureandwheel positionissignificant,withap‐valueof0.046. 45 Figure 16 ‐ ANOVA Eventhoughfixtureasasinglemainfactorwasnotsignificant,theoffsetpivot fixtureregisteredlowerinitialforcesthusprovidingvalidationforconsidering investingintheoffsetpivotfixtureinplaceofthestandarddesignfixture(Figure 17). 46 Figure 17 ‐ Main Effects Plot InFigure18,theInteractionPlotforfixtureandwheelposition,itcanbeseenthat theoffsetpivotcasterremainsrelativelyconsistentinperformanceacrossthetwo positions.Thisvalidatestheneedtoinvestintheoffsetcastertype.Inaddition, withtherearwheelsperpendiculartothedirectionoftravelthestandardfixture registeredremarkablyhighinitialappliedforce. 47 Figure 18 ‐ Interaction Plot Testforhypothesis3: Quantifythesignificanceofthethreemainfactors(wheeltype,castertypeand wheelposition)onthepeakforcerequiredtomoveafourwheeledcart. Hypothesis3a:Thethreemainfactorswillhaveasignificantinteractive effectonthepeakforce. Hypothesis3b:Thethreemainfactorswillnothaveasignificantinteractive effectonthepeakforce. Figure19givesasummaryofthe3rdand4thstepsinthelinearmodelbuilding procedure.Itcanbeobservedthattheinteractionbetweenfixtureandwheeltypeis notsignificantwithap‐valueof0.142whichisnearthethresholdof0.1.The interactionbetweenfixtureandwheeltype(Figure21),thoughsignificantisnot dependentonwheelposition. 48 Figure 19 ‐ ANOVA, Fixture ‐ Wheel Type Whenusingtheoffsetfixtureitseemsadvisabletousethesinglewheel;however whenthedualwheeltypeisusedtheredoesnotseemtobeadifferencewhetherthe standardoroffsetpivotisused(Figure21).Itisadvisabletoinvestintheoffset pivotfixture;howeverasfarasthetypeofwheelitseemsthesinglewheelperforms better.Thoughnotconsideredinthisstudy,theremaybeothersetsoffactorsthat wouldqualifytheneedtoinvestintheSwivelEaz™(dualwheel). Figure 20 ‐ Main Effects Plot, Fixture ‐ Wheel Type ‐ Wheel Position 49 Inaddition,theinteractionplotbetweenfixtureandwheeltypeinFigure21shows thattheinitialpeakforceisreducedmostwhenthesinglewheeltypeis incorporatedintotheoffsetpivot(SwivelEaz™Pro)fixturewhichkeptinitialforce nearlyconsistentbetweenthetwowheelpositions. Figure 21 ‐ Interaction Plot; Fixture ‐ Wheel Type ‐ Wheel Position 5.4 Psychophysical Analysis of Perceived Exertion Totestthestrengthoftherelationshipsbetweenweight,force,andperceived exertionontheshouldersandback,aPearson’sCorrelationtestwasused.Itis evidentfromFigure22thatastrongpositivecorrelationexistsbetweenthesefour variables.Weightwasaddedasacorrelationfactortovalidatethepositive correlationbetweentheselfreportedperceivedexertionresponses.Astrong 50 relationshipexistsbetweenweightandperceivedexertionattheshoulderandback withaPearsoncorrelationof0.609and0.597,respectively(Figure22). Figure 22 ‐ Pearson’s Correlation, Back – Shoulder ‐ Weight 51 TheAnderson‐DarlingNormalitytestwasconductedtodetermineiftheperceived exertiondatawerenormallydistributed.EvidentinFigures23a‐b,bothperceived exertiondataarenotnormallydistributed. Figure 23 ‐ Normal Probability, Shoulder and Back (Figure 23a) (Figure 23b) ItwasthereforenecessarytotransformthedatausingtheBoxCoxpower transformationmodelasseenFigures24a‐b. Figure 24 ‐ Box Cox ‐ Shoulder and Back Data Transformation (Figure 24a) (Figure 24b) Forconsistencyofinterpretation,shouldersandbackBorgratingsofperceived exertionweregivenalogarithmictransformation.Generallinearmodelsusingthe 52 backwardeliminationprocedurewereconstructedforbothofthedependent variables.Weightwasaddedasacorrelationfactortovalidatethepositive correlationbetweentheselfreportedperceivedexertionresponses.Wheelposition wasasignificantfactorintheanalysisofperceivedphysicalexertionforthe shoulder(p=0.02)(Figure25). 53 Figure 25 ‐ ANOVA General Linear Model ‐ Shoulders Backward Elimination of Terms Candidate terms: Blocks, Fixture, Wheel Type, Weight (lbs), Wheel Position, Fixture*Wheel Type, Fixture*Weight (lbs), Fixture*Wheel Position, Wheel Type*Weight (lbs), Wheel Type*Wheel Position, Weight (lbs)*Wheel Position ------Step 1----- ------Step 2----Coef P Coef P Constant 0.1946 0.1946 Blocks -0.8011 0.000 -0.8011 0.000 Fixture 0.0313 0.355 0.0313 0.353 Wheel Type -0.0494 0.145 -0.0494 0.143 Weight (lbs) -0.4375 0.000 -0.4375 0.000 Wheel Position -0.0802 0.019 -0.0803 0.018 Fixture*Wheel Type 0.0153 0.651 0.0153 0.649 Fixture*Weight (lbs) -0.0663 0.051 -0.0663 0.050 Fixture*Wheel Position 0.0287 0.396 0.0287 0.392 Wheel Type*Weight (lbs) -0.0252 0.455 -0.0253 0.453 Wheel Type*Wheel Position -0.0303 0.370 -0.0302 0.369 Weight (lbs)*Wheel Position 0.0047 0.888 S R-sq R-sq(adj) R-sq(pred) Mallows’ Cp 0.376918 76.92% 73.28% 68.61% 18.00 0.375219 76.91% 73.52% 69.17% 16.02 ------Step 3----Coef P 0.1943 -0.8008 0.000 0.0317 0.346 -0.0491 0.144 -0.4379 0.000 -0.0803 0.018 ------Step 4----Coef P 0.1939 -0.8004 0.000 0.0322 0.336 -0.0486 0.147 -0.4384 0.000 -0.0804 0.017 Constant Blocks Fixture Wheel Type Weight (lbs) Wheel Position Fixture*Wheel Type Fixture*Weight (lbs) -0.0660 0.050 -0.0655 0.051 Fixture*Wheel Position 0.0287 0.391 0.0287 0.391 Wheel Type*Weight (lbs) -0.0249 0.458 Wheel Type*Wheel Position -0.0302 0.367 -0.0301 0.367 Weight (lbs)*Wheel Position S R-sq R-sq(adj) R-sq(pred) Mallows’ Cp 0.373867 76.87% 73.71% 69.64% 14.23 0.373117 76.75% 73.82% 70.05% 12.77 54 ------Step 5----Coef P 0.1939 -0.8004 0.000 0.0323 0.335 -0.0486 0.146 -0.4385 0.000 -0.0799 0.018 Constant Blocks Fixture Wheel Type Weight (lbs) Wheel Position Fixture*Wheel Type Fixture*Weight (lbs) -0.0655 0.051 Fixture*Wheel Position Wheel Type*Weight (lbs) Wheel Type*Wheel Position -0.0305 Weight (lbs)*Wheel Position S R-sq R-sq(adj) R-sq(pred) Mallows’ Cp ------Step 6----Coef P 0.1939 -0.8004 0.000 0.0323 0.333 -0.0486 0.146 -0.4386 0.000 -0.0804 0.017 -0.0655 0.360 0.372687 76.60% 73.88% 70.38% 11.50 0.372431 76.42% 73.91% 70.68% 10.33 ------Step 7----Coef P 0.1930 -0.7995 0.000 0.0332 0.323 Constant Blocks Fixture Wheel Type Weight (lbs) -0.4394 0.000 Wheel Position -0.0804 0.017 Fixture*Wheel Type Fixture*Weight (lbs) -0.0647 0.055 Fixture*Wheel Position Wheel Type*Weight (lbs) Wheel Type*Wheel Position Weight (lbs)*Wheel Position S R-sq R-sq(adj) R-sq(pred) Mallows’ Cp 0.374295 75.97% 73.65% 70.64% 10.42 α to remove = 0.1 0.051 55 Fixturewasasignificantfactorintheanalysisofperceivedphysicalexertionforthe Back(p=0.04)(Figure26). Figure 26 ‐ ANOVA General Linear Model – Back Backward Elimination of Terms Candidate terms: Blocks, Fixture, Wheel Type, Weight (lbs), Wheel Position, Fixture*Wheel Type, Fixture*Weight (lbs), Fixture*Wheel Position, Wheel Type*Weight (lbs), Wheel Type*Wheel Position, Weight (lbs)*Wheel Position ------Step 1----Coef P Constant 0.0542 Blocks -0.6607 0.000 Fixture 0.0690 0.052 Wheel Type -0.0451 0.200 Weight (lbs) -0.4366 0.000 Wheel Position -0.0274 0.436 Fixture*Wheel Type 0.0208 0.555 Fixture*Weight (lbs) -0.0855 0.016 Fixture*Wheel Position 0.0142 0.686 Wheel Type*Weight (lbs) -0.0236 0.502 Wheel Type*Wheel Position -0.0086 0.806 Weight (lbs)*Wheel Position 0.0095 0.787 S R-sq R-sq(adj) R-sq(pred) Mallows’ Cp 0.392417 75.68% 71.85% 66.90% 18.00 Constant Blocks Fixture Wheel Type Weight (lbs) Wheel Position Fixture*Wheel Type Fixture*Weight (lbs) Fixture*Wheel Position ------Step 3----Coef P 0.0542 -0.6607 0.000 0.0690 0.050 -0.0451 0.196 -0.4366 0.000 -0.0276 0.427 0.0208 0.551 -0.0855 0.015 0.0145 0.677 0.0093 0.789 0.390723 75.66% 72.09% 67.49% 16.06 ------Step 2----Coef P 0.0542 -0.6607 0.000 0.0690 0.051 -0.0451 0.198 -0.4366 0.000 -0.0275 0.431 0.0208 0.553 -0.0855 0.016 0.0143 0.682 -0.0236 0.500 ------Step 4----Coef P 0.0542 -0.6607 0.000 0.0691 0.049 -0.0451 0.195 -0.4367 0.000 -0.0274 0.429 0.0208 0.550 -0.0855 0.015 56 Wheel Type*Weight (lbs) -0.0236 Wheel Type*Wheel Position Weight (lbs)*Wheel Position S R-sq R-sq(adj) R-sq(pred) Mallows’ Cp 0.498 0.389071 75.65% 72.33% 68.04% 14.13 S R-sq R-sq(adj) R-sq(pred) Mallows’ Cp 0.015 0.015 0.384958 0.015 ------Step 8----Coef P 0.0526 -0.6591 0.000 0.0708 0.042 -0.4384 -0.0839 0.386073 -0.0847 0.385564 75.43% 72.82% 69.44% 9.09 ------Step 7----Coef P 0.0534 -0.6599 0.000 0.0700 0.044 -0.0443 0.199 -0.4376 0.000 S ------Step 6----Coef P 0.0534 -0.6599 0.000 0.0700 0.044 -0.0443 0.200 -0.4376 0.000 -0.0275 0.425 0.505 0.386509 75.53% 72.69% 69.02% 10.65 Constant Blocks Fixture Wheel Type Weight (lbs) Wheel Position Fixture*Wheel Type Fixture*Weight (lbs) -0.0847 Fixture*Wheel Position Wheel Type*Weight (lbs) Wheel Type*Wheel Position Weight (lbs)*Wheel Position 0.497 0.387620 75.61% 72.53% 68.58% 12.30 ------Step 5----Coef P 0.0538 -0.6603 0.000 0.0695 0.047 -0.0447 0.197 -0.4371 0.000 -0.0275 0.427 Constant Blocks Fixture Wheel Type Weight (lbs) Wheel Position Fixture*Wheel Type Fixture*Weight (lbs) -0.0851 Fixture*Wheel Position Wheel Type*Weight (lbs) -0.0231 Wheel Type*Wheel Position Weight (lbs)*Wheel Position -0.0236 0.000 0.016 57 R-sq R-sq(adj) R-sq(pred) Mallows’ Cp 75.29% 72.91% 69.81% 7.71 74.93% 72.75% 69.89% 7.31 α to remove = 0.1 Itisunclearwhythewheeldesignseemstosignificantlyaffecttheselfreported shoulderexertionrating.Similarly,itisalsonotclearwhythecasterdesignseems tosignificantlyaffecttheselfreportedbackexertionrating.Wethereforepropose forfurtherstudies,especiallythephysiologicalimpactofthewheelandcaster designsonthepushandpullactions. Theperceivedexertionratingmeanfortheshoulderswhilepushingthecart withagrossweightof750lbs(340.2kgs),usingoffsetpivotfixture,SwivelEaz (dual)wheels,andwheelspositionedperpendiculartothedirectionoftravel was2.0withastandarddeviation(s.d)of1.16.Theperceivedexertiononthe shoulders,usingaBorgCR10scale,wasratedasLight.Perceivedexertiononthe back,usingthesamecombination,wasratedwithameanof1.68ands.d.of1.19. A1.68ratingfallsbetweenVeryLightandLightonthescale. Theperceivedexertionratingmeanfortheshoulderswhilepushingthecart withagrossweightof750lbs(340.2kgs),usingastandardfixture,SwivelEaz (dual)wheels,andwheelspositionedperpendiculartothedirectionoftravel was1.87(VeryLight–Light)withas.d.of1.33.Perceivedexertionontheback, 58 usingthesamecombination,hadaratingmeanof1.62(VeryLight–Light)and s.d.of1.41.Allofthecombinationsandtheirperceivedexertionscanbefoundin AppendixC. Thestandardfixturewithasinglewheelpositionedperpendiculartothe directionoftravelwith750lbs(340.2kgs)wasactuallyratedlowestwhen comparedtotheothertwowheel‐fixturecombinations.Thisisattributedto thestatisticallysignificantinteractionbetweenthewheeltypeandthecaster design,wheretheleastinitialpushforcewasrequiredwhenusingacartwiththe singlewheelandoffsetpivotcastercombination.Thisclaimissupportedbythe datasummaryinFigures27and28. Chapter 6: Discussion Thepurposeofthisresearchwastoinvestigatetheeffectsofwheelandfixture designsonpushingforceofafour‐wheeledcart.Thefollowingdiscussionisa reviewofthehypothesisandthepracticaleffectivenessandapplication. 6.1 Hypothesis Discussion Split‐wheeldesignwilleffecttheinitialappliedpeakforcerequiredtomovea fourwheeledcart. 59 Whenthewheelswerepositionedperpendicularwithdirectionoftravel(F0R90) andtheSwivelEaz™dualwheelwasusedinthestandardfixture,thepeakapplied forcemeanwas34.6lbsandwas1.4%greaterthanthepeakappliedforcemeanof 34.1lbsattainedwhenthesinglewheelwasusedinthesamestandardfixture. Takingintoaccountpossiblestatisticalerror,thedifferenceof1.4%maynotbe significantenoughtodifferentiateonewheelasabetteroptionovertheother (Figures27–28). 60 Figure 27 ‐ Boxplot ‐ Peak Initial Force Figure 28 ‐ Summary of Box Plot of Combinations Wheel Position Fixture Type Wheel Type 90 deg Standard 90 deg 90 deg Standard Offset Pivot Offset Pivot Standard Swivel Eaz (dual) Single Swivel Eaz (dual) Single 90 deg 0 deg 0 deg 0 deg 0 deg Standard Offset Pivot Offset Pivot Mean Peak Force 34.6 .5 1.4 3.1 9.6 29.0 .5 1.7 28.5 32.7 3.9 11.9 34.1 32.4 29.3 Swivel Eaz (dual) Single Swivel Eaz (dual) Single 28.8 Difference % (lbs) Difference 61 Whenthewheelswerepositionedinlinetothedirectionoftravel(F0R0)andthe SwivelEaz™dualwheelwasusedinthestandardfixture,thepeakappliedforce meanwas34.1lbs(15.5kgs)andwas1.7%greaterthanthepeakappliedforce meanof28.5lbs(12.9kgs)attainedwhenthesinglewheelwasusedinthesame standardfixture.Again,thedifferencebetweenthetwocombinationsmaynotbe significantenoughtodifferentiateonewheelovertheotherasabetteroption (Figures27–28). The“offsetpivot”castermountingwilleffecttheinitialpeakappliedforce requiredtomoveafourwheeledcart. Whenthesinglewheelswerepositionedperpendiculartothedirectionoftravel (F0R90)andusedwiththeoffsetpivot(SwivelEaz™Pro)fixturetheappliedpeak forcemeanwas29.3lbs(13.3kgs).Thiswas16.4%lowerthantheappliedpeak forcemeanof29.3lbs(13.3kgs)whichwasattainedwhenasinglewheelwasused inastandardfixtureandalsopositionedperpendiculartothedirectionoftravel.In otherwords,whentherearwheelswerepositionedat90degreestothedirectionof travel,offsetpivotfixture(caster)had16.4%lowerappliedpeakforcemeanversus thesinglewheelinthestandardfixture. Quantifythesignificanceofthethreemainfactors(wheeltype,castertypeand wheelposition)onthepeakforcerequiredtomoveafourwheeledcart. 62 Whenusingtheoffsetfixtureitseemsadvisabletousethesinglewheel;however whenthedualwheeltypeisusedtheredoesnotseemtobeadifferencewhether standardoroffsetpivotisused.Itseamsadvisabletoinvestintheoffsetpivot fixture;howeverasfarasthetypeofwheelitseemsthesinglewheelperforms betterinthisparticularapplication.Thoughnotconsideredinthisstudy,theremay beothersetsoffactorsthatwouldqualifytheneedtoinvestintheSwivelEaz™ (dualwheel).Forexample,thisstudydidnotconsiderwheelsurfacedurabilityor wheelbearingwearovertime. 6.2 Variability Between Subjects Therewasvariationinthepeakforcerangesbetweenthe8differentsubjects.For example,incombination4whenthegrosscartweightwas250lbs(113.4kgs),rear wheelswerepositionedatF0R90,SwivelEaz(dual)wheelsweremountedonthe standardfixture,theminimumpeakappliedforcewas14.3lbs(6.5kgs).The maximumpeakforceappliedbyaparticipantwas33.2lbs(15.1kgs)thedifference intherange18.9lbs(8.6kgs)andthestandarddeviationwas6.69lbs(3.0kgs) (Table5).Incombination12,whenthegrosscartweightwas750lbs(340.2kgs), rearwheelswerepositionedatF0R90,SwivelEaz(dual)wheelsweremountedon thestandardfixture,theminimumpeakappliedforcewas27.2lbs(12.3kgs).The maximumpeakforceappliedbyaparticipantwas65.4lbs(29.7kgs)thedifference intherangeis38.2lbs(17.3kgs)andthestandarddeviationwas15.41(7.0kgs). Theoverallrangedifferencesforeachoftherangesineachcombinationvariedfrom 16.5–45.5. 63 Table 5 ‐ Applied Peak Force Range 6.3 Perceived Exertion Asexpected,therewasastrongrelationshipbetweentheamountofweightthe participanthadtopushandtheirlevelofperceivedexertion.Whatremainsunclear iswhywheeltypewasasignificantfactorinthelevelofperceivedexertionforthe shoulderandnotwiththeback.Inaddition,itisunclearwhythefixtureisthe significantfactorinperceivedlevelofexertionintheback.Participantsdidratethe levelofperceivedexertionslightlyhigheronboththeshoulderandthebackwhen pushingthe750lb(340.2kgs)cartwiththeoffsetpivot,SwivelEazwheel,and positionedperpendiculartothedirectionoftravelversusthesamecombinationand usingthestandardfixture,LightcomparedtoVeryLight‐Light,respectively. 64 6.4 Application of the Results/Psychophysical Application Manyoftheforcedifferencesobservedinthisstudyweremodest,andgiventhe largevariabilitybetweensubjectvariability,itcouldbearguedthatthereareno practicaldifferencesbetweenanyofthewheel‐fixturecombinations.However,the offsetpivotfixturecouldbeavaluableinterventioninsomecommonsituations.To illustratethis,considerthefollowingscenario. 6.4.1 Scenario 1 – Changing Fixtures Aworkerisrequiredtopushacartfor200feet(61m)twotimesperhourwiththe handleheightathiplevel.Theinitialforcerequiredtogetthecartintomotion whenthewheelsareperpendiculartothedirectionoftravelis40lbs(18.1kgs). Thesustainedforcerequiredtokeepthecartinmotionis19.0lbs.Accordingtothe SnookandCirellotables[9]‐[32],theinitialforceof40lbs(18.1kgs)isacceptableto 83%ofmalesand66%offemales.TherecommendeddesigngoalfromSnookand Cirelliowasforthetasktobeacceptableforatleast75%offemales[9]‐[32]. Ifacartdesignerwantedtoreducetheinitialforcewhenthewheelsarepositioned perpendiculartothedirectionoftravel,theycouldinstallasetofoffsetpivot (SwivelEaz™Pro)fixtureswithsinglewheels.Basedonresultsfromthisstudy,and ifallothervariableswereequal,theappliedpeakforcemeanwouldbeloweredby 16.1%.Inotherwords,thepeakforceof40lbs(18.1kgs)wouldbeloweredto33.6 lbs(15.2kgs).Thiswouldnowmakethejobacceptableto84%ofthefemalesand 90%ofthemales[9]‐[32].Itthiscaseitseemsadvisabletoimplementtheoffset pivotfixturewiththesinglewheel. 65 6.5 Limitations of the Study 6.5.1 Instantaneous Peak Force Although,forcedatawasrecordedfromwhenthecartstartedmotionuntilafterit stopped,thisstudyonlyfocusedonthepeakappliedinstantaneousforce.Thereisa possibilitythatthetenthofaseconddatapointwasnotrepresentativeoftheactual averageinitialforce.Themedianforceoverthebriefinitialperiodmaybeabetter representationofinitialforce.Henceforfutureanalysisinthisstudy,averageinitial forceinadditiontothesustainedpushforceswillbeusedasopposedtoinitialpeak force. 6.5.2 Lateral Force Lateralforcewasrecordedduringthestudybutitwasnotafocusforthisthesis.It seemsreasonabletoexpectthattheoffsetpivotfixturewouldsignificantlyreduce lateralmovementwhenthecartisinitiallystarted,especiallywhenthewheelsare positionedat90degreesorotherwiseoutofalignment. 6.6 Contribution to the Body of Knowledge Theimpactofthisstudyistointroducedataandstatisticalanalysisofanewstyleof wheelandfixtureofwhichnopublishedstudieswerefound.Thestudy demonstratedthatifanorganizationislookingtoreduceinitialforcewhenwheels aremisalignedwiththedirectionoftraveltheoffsetpivot(SwivelEaz™Pro)isa viablealternative. 66 6.7 Future Studies FurtherstudycouldbeconductedontheSwivelEaz™wheelstounderstandhow theyperformafterbeingin‐useforanextendedperiodoftime.Thewheelsand fixturesusedwerenewatthestartofthestudy.Thewheelscouldalsobetested overroughsurfacesasitseemsreasonabletoexpecttheSwivelEaz™dualwheels toperformbetteroversurfacesthatarenotflat.Asmentionedearlier,futurestudy andanalysiscouldlookattheaverageinitialforce,thesustainedforce,andlateral force. Chapter 7: Conclusion Thesponsorsoughttoinvestigatetheeffectsofthewheelandfixture(caster) designsonpushingforceofafour‐wheeledcart.Basedonthestudyfindingsand resultsfromthisanalysisthefollowingrecommendationsarebeingmadetothe sponsor: 1) Iflookingtoreducepushforcewhenthewheelsarepositioned perpendiculartothedirectionoftravel,considerutilizingtheSwivel Eaz™Profixturewiththesinglewheel;however,thecostoftheSwivel Eaz™Profixturemaynotbejustifiabletoonlygaina16%reductionin force. 2) Otherfactorsshouldbeconsideredwhendecidingwhethertopurchase theSwivelEaz™ProfixtureandSwivelEaz™dualwheelssuchas 67 longevityunderruggedconditionsandwhetherthewheelscantraverse bumpsorunevensurfacesmoreeasily. 3) Duringthestudy,differencesintheparticipant’sperceivedexertion ratingsmeansseemednearlyinconsequentialastheratingsfor750lbs (340.2kgs)rangedfrom1.38–2.00whichfallsbetween“VeryLight– Light”fortheshouldersand1.19–1.81“VeryLight–Light”fortheback. 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ThomasE.Bernard‐Ergonomics,(2014,November25)LibertyMutualManual MaterialsHandlingTables(Online). http://personal.health.usf.edu/tbernard/ergotools/ 71 Appendices 72 Appendix A – Pre Screening Form Pre – Screening Form Date: Age: Gender: Male Height: Female Weight: 1) Are you currently experiencing any back or shoulder pain? YES NO a. If you answered YES to question 1 then you are not allowed to participate in the study. b. If you answered NO then continue to question 2… 2) Have you had any back or shoulder pain lasting more than 24 hours in the past 12 months ? YES NO a. If you answered YES to question 2, do you feel this pain will prohibit you from safely completing the task? YES NO i. If you answered YES to question 2a and feel the pain will prohibit you from safely completing the study then you are not allowed to participate in the study. 3) If you answered NO to question 2 then you are allowed to participate in the study. Thank you for assistance in this project. 4) If you are under the care of a physician and have restrictions regarding pushing, pulling, or other material handling tasks then you are not allowed to participate. If you have any questions regarding this study, please contact the researcher David Wein at 920-216-3598 or [email protected]. Thank you. 73 Appendix B – Participant Information 74 Appendix C – Pain and Exertion Rating
