manual work design - eskisehir.edu.tr 301/icerik/manual_work_design.pdfineffective therbligs end 202...
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Manualwork design
• The design of manual work was introduced by theGilbreths through motion study and the principlesof motion economy
• Theprinciplesarebrokendowninto3basicsubdivisions:– Theuseofthehuman body– Thearrangementandconditionsofthe workplace– Thedesignoftoolsand equipments
• Theprinciplesarebasedonanatomical,biomechanicalandphysiologicalprinciplesofthehumanbody.Theyformthescientificbasis forergonomicsandwork design.
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Musculoskeletal system
Musculoskeletalsystemofthe arm
• Thehumanbodyisabletoproducemovementsbecauseofacomplexsystemofmusclesand bones
• Themusclesareattachedtothebonesbya joint• Agonistsactastheprimeactivatorsofthe motion• Antagonistscounteracttheagonistsandopposethe
motion
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Musculoskeletal system
• Thereare3typesofmusclesinthehumanbody– Skeletal:attachedtothebones,approx.500inthebody
– Cardiacmuscles:found inthe heart
– Smoothmuscle:foundintheinternalorgansandthewallsoftheblood vessels
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Musculoskeletal system
Skeletal muscles• Myofibrilsaresubdividedintomyofilaments
• Myofilaments:thick(myosine)andthinfilaments (actine)
• Filaments slide over oneanother to contract andextendmuscles– Completecontraction=50% oftheresting length
– Completeextension=180%oftheresting length
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Principlesofworkdesign: Humancapabilitiesandmotion economy
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Principlesofwork designForce-lengthrelationshipofskeletal muscle
Ataskrequiringanimportantmuscleforceshouldbeperformedattheoptimum position.
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Principlesofwork design
Maximumforce state• restinglength:optimalbondingbetweenthick(myosin)
andthin(actin) filaments• stretchedstate:minimaloverlap,decreasedmuscle force• contractedstate:interferencebetweenfilaments,
decreasemuscle force
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Principlesofwork design
1. Achieve the maximummuscle strength at themidrange of motion
Midrangeof motion(relaxed posture)
Thepostureassumedbyanastronautinweightlessconditionswhenbothagonistandantagonistmusclesarerelaxed.
Typicalrelaxed posture
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Principlesofwork design
Force-velocityrelationship
2.Achievethemaximummusclestrengthwithslowmovements
• Thefasterthemolecularbondsareformed,broken,reformed,thelesseffectiveisthebondingandthelessmuscularforceis produced
• Slowmovementsare themost efficient
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Principlesofwork design
3. Use momentum to assist workers whereverpossible; minimize it if it is counteracted bymuscular effort
• Fastermovementsproducehighermomentumandhigherimpactforcesinthecaseof blows
• Downward motions are moreeffective than upwardmotionsbecauseoftheassistancefrom gravity
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Principlesofwork design
4.Designtaskstooptimizehumanstrength capability
• Humanstrengthcapabilitydepends on– thetypeof strength:• Dynamicstrength (isotonic)• Staticstrength (isometric)• Psychophysicalstrength(extended time)
– themuscleorjointmotionbeing utilized– posture
• Maximumacceptableloadis40to50percentlessthanaone-timestaticexertion
• Extensive tables for psychophysicalstrength of various frequencies andpostures (pg 134, Tables 4.2,4.3,4.4)
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Staticstrength positions
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Principlesofwork design
5.Uselargemusclesfortasksrequiring strength
• Musclestrengthisdirectlyproportionaltothesizeofthemuscle
• Forexample,legandtrunkmusclesshouldbeusedinheavyloadlifting,rather thanweakarmmuscles
Staticstrength positions
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Principlesofwork design
6.Staybelow15%(evenbelow10%or5%)ofmaximumvoluntary force
• Thenon-linear relationship:– atamaximalcontractionaveryshort
endurancetime(6 sec)– indefiniteendurancetimeatapprox.
15%ofamaximal contraction
T = 1.2 /( f - 0.15)0,618-1.21T : Endurance time (min)f : Required force, expressed as a fraction
of maximum static strength
Ex:T=1.09minforf= 50%
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Staticmuscleendurance-exertionlevel relationship
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Principlesofwork design
7.Useshort,frequent,intermittent,work/restcycles
• Afastinitialrecoveryperiod,whichthentendstoleveloffwithincreasing time
• Mostofthebenefitisgainedinarelativelyshort period
• Ahigher%ofmaxstrengthcan bemaintainedifthestrengthisexertedasaseriesof repetitivecontractionsratherthan onesustainedstatic contraction Percentageofmaxstaticstrengththatcanbemaintained
inasteadystateduringrhythmic contractions
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Principlesofwork design
8. Designtaskssothatmostworkerscando them• Individualfactorsaffectstrengthperformance:gender,age,
handedness,and fitness/training• Musclestrengthappearstopeakinthemid-20s,thendecreases
linearlybythe mid-60s.• Handedness:non-dominanthandproducesabout90%ofdominant
hand’s grip
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Principlesofwork design
9. Useoflowforceforprecisemovementsorfinemotorcontrol
– Whentheforceincrease,musclecontrol decreases
10. Donotattemptprecisemovementsorfinecontrolimmediatelyafterheavy work
– Ex:Assemblyoperationsafterliftingheavyparts.Usedifferentworkersfor lifting.
11. Beginandendmotionswithbothhands simultaneously– Workstationscanbedesignedtodo“twoata time”
12. Movethehandssymmetricallyandsimultaneouslytoandfromthecenterofthe body
– Deviationsfromsymmetryresultinslow,awkward movements
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Principlesofwork design
13.Usethenaturalrhythmsofthe body
• Optimumworktempoforsome tasks:– Filingmetal:60-78strokesper minute– Chiseling:60strokesper minute– Armcranking:35rpm(numberoffullrotationsover1 min)– Shoveling:14-17tossesper min
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Principlesofwork design
14.Usecontinuouscurvedmotions
• Straight-linemotionsinvolvingsuddenandsharpchangesindirectionrequiremoretimeandareless accurate
• Continuouscurvedmotionsdo notrequiredecelerationtomakeadirectionalchangeandperformedfasterperunitof distance
• Pivotaroundajoint (elbow)Forearmmotion is bestwhilepivotingon elbow
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Principlesofwork design
15.Usethelowestpracticalclassificationof movement
• Alwaysutilizethelowestpossible: finger→wrist→forearm→shoulder→ body
• Additionaltimeisrequiredforthecentralnervoussystemtoprocessadditionaljointsand muscles
16.Workwithbothhandsandfeet simultaneously
• Relievethehandsofworkthatcanbedonebythefeetifthisworkisperformedwhilethehandsare occupied Classificationsof movements
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Principlesofwork design
17.Minimizeeye fixations• Thelocationoftheprimaryvisualtargetsshouldbeoptimizedwith
respecttothe operator• Withintheareainthefigurenoheadmovementsareneededand
eyefatigueis minimized
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Principlesofworkdesign– Summary
Humancapabilitiesandmotion economy1.2.3.4.5.6.7.
AchievethemaximummusclestrengthatthemidrangeofmotionAchievethemaximummusclestrengthwithslow movementsUsemomentumtoassistworkerswhereverpossible;minimizeitifitiscounteractedbymusculareffortDesigntaskstooptimizehumanstrength capabilityUselargemusclesfortasksrequiring strengthStaybelow15%(evenbelow10%or5%)ofmaximumvoluntaryforceUseshort,frequent,intermittent,work/rest cycles
8. Designtaskssothatmostworkerscando them9. Useoflowforceforprecisemovementsorfinemotor control10. Donotattemptprecisemovementsorfinecontrolimmediatelyafterheavy work11. Beginandendmotionswithbothhands simultaneously12. Movethehandssymmetricallyandsimultaneouslytoandfromthecenterofthe body13. Usethenaturalrhythmsofthe body14. Usecontinuouscurved motions15. Usethelowestpracticalclassificationof movement16. Workwithbothhandsandfeet simultaneously17. Minimizeeyefixations
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Principlesofworkdesign– Summary
• Theprinciplesofhumancapabilitiesandmotioneconomyarebasedonanelementaryunderstandingofhuman physiology
• Theanalystneednotbeanexpertinhumananatomy
• Themotioneconomychecklistsummarizesmostoftheprinciplesinaquestionnaireformat(Figure4.16,pg146:Coursewebpage )
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Motion Study
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Motion study
• Motionstudyisthecarefulanalysisofbodymotionsemployedindoinga job
• Thepurposeistoeliminateorreduceineffectivemovementsandfacilitateandspeedeffectivemovements
• Gilbrethspioneeredthestudyofmanualmotion anddevelopedbasiclawsofmotion economy
• Gilbrethsconcludedthatallwork,productiveornon-productive,isdonebyusingcombinationsof17 basicmotionscalled therbligs:– Effectivetherbligs:directlyadvancetheworkprogress, theycanbeshortenedbutnotcompletely eliminated
– Ineffectivetherbligs:donotadvancetheworkprogress,shouldbe eliminated
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Effective therbligs
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Ineffective therbligs
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Thetwo-handprocess chart
• Thetwo-handprocesschart(operatorprocesschart)showsallmovementsanddelaysmadebytherightandlefthands,andtherelationshipsbetweenthem
• Analystcandeterminewhatimprovementscanbe introduced
• Therbliganalysischecklist(Figure4.18,pg151:Coursewebpage)
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Manualworkanddesign guidelines
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Manualworkanddesign guidelines
• Energyexpenditureandworkload guidelines• Heartrate guidelines• NIOSHlifting guidelines• Multitasklifting guidelines• Generalguidelines:Manual lifting
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Energyexpenditureandworkload guidelines
• Energyisrequiredformuscle contraction• ATP(adenosinetriphosphate)molecule:immediate
energysourcebutverylimited,lastingonlyfew seconds• ATPmustbereplenishedfromCP(creatinephosphate)
molecule,lastingforlessthan1 min• CPisregeneratedfromthebasicfoods:carbohydrates,
fats, proteins• TwodifferentmodesforCP generation:
– Aerobic(requiresoxygen):muchmoreefficient,generates38ATPsforeachglucosemolecule,butitis slow
– Anaerobic(withoutoxygen):veryinefficient,generatesonly2ATPsforeachglucosemolecule,butmuch quicker.(Glucosemoleculeisonlypartiallybrokendowninto2lactatemolecules,inwateryenvironmentofthebodytheyformlacticacidwhichcauses fatigue)
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Energyexpenditureandworkload guidelines
Sourcesofenergyduringthefirstfewminutesofmoderatelyheavy work
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Energyexpenditureandworkload guidelines
• Bywarmingupandstartingheavyworkslowly,theworkercanminimizetheamountofanaerobicmetabolismandthebuildupoflacticacidassociatedwithfeelingsof fatigue.
• Thedelayoffullaerobicmetabolismistermedoxygendeficit
• Theenergyexpendedonataskcanbeestimatedbyassumingthattheenergyisproducedthroughaerobicmetabolismandmeasuringtheamountofoxygenconsumedbythe worker
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Energyexpenditureandworkload guidelines
• Theamountofinspiredairisassumedtocontain21%oxygen
• Typically4.9kcalofenergyisproducedforeachliterofoxygenusedin metabolism
• Energy expenditure:E(kcal/min)=4.9xoxygen consumption
=4.9*V*(0.21– EO2)– V=volumeofairinspired,L/min(canbemeasuredbya flowmeter)– EO2=fractionofoxygeninexpiredair(canbemeasuredbyan
oxygenmeter)
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Energyexpenditureandworkload guidelines
• Theenergyexpendedonataskvariesbythetypeoftask,thepostureandtheload carriage.
• Dataonseveralhundreddifferenttypesoftaskshavebeen collected
• Formanualmaterialhandling,themannerinwhichtheloadiscarriedismost critical
• Balancedloadscarriedclosesttothecenterofgravityofthebodydemandlowest energy– Ex:backpacksupportedbythetruckmusclesis easier than
twosuitcasesofequalweightineach arm• Posturealsoplaysanimportantrole,withlessenergy
forsupported postures
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Energyexpenditureandworkload guidelines
• 5.33kcal/ministhelimitofacceptableenergyexpenditureforan8hworkdayfora man,– Thisnumbercorrespondsto1/3themaximumenergyexpenditureoftheaverageU.S. Male
– 16 kcal/min x1/3=5.33 kcal/min– Forfemales: 12 kcal/min x1/3=4 kcal/min
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Energyexpenditureandworkload guidelines
Examplesof energycostsofvarioustypesofhumanactivity (kcal/min)
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Energyexpenditureandworkload guidelines
• Iftheoverallworkloadisexceeded(morethan5.33kcal/minformalesand4kcal/minforfemales)restisrequiredtoallowthebodytorecoverfromfatigueandrecyclethelactic acid
• Guidelineforrestallocation:
R=(W– 5.33)/(W– 1.33)– R:timerequiredforrest,aspercentoftotal time– W:averageenergyexpenditureduringwork, kcal/min– 1.33kcal/min:energyexpenditureduring rest
• Example:ShovelingcoalW=9.33 kcal/min R= 0.5Foran8hofworkspend4hfor resting
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Energyexpenditureandworkload guidelines
• Thedurationoftheworkcycleisimportantforanoptimal recovery• Forheavyworks,withoutresting,therecoveryprocesstendstobe
exponential• Shortburstsofheavyworks(1/2to1min)followedbyshortrest
periodsprovidemaximum benefit• Micropausesof1to3sarealso useful• Activebreaks,duringwhichtheworkeralternateshandsoruses
othermuscles,relievesfatiguedmuscles• Itisbestforworkerstodecidewhentotakebreaks,wheneverthey
feeltheneedforrestasopposedtoprescribed breaks• Theuseoffrequentshortwork/restcyclesishighly recommended
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Heartrate guidelines
• Themeasurementofoxygenconsumptioncanbecumbersomeandexpensive(equipmentcostsseveralthousanddollarsandinterfereswiththe worker)
• Analternativewaytomeasuretheenergyexpenditureistheheartrate level
• Thehighertheenergyexpenditurethehighertheheart rate• Lessexpensive(100$foravisualreadout,several hundred
dollarsforaPC interface)
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Heartrate guidelines
Theanalystmustbe careful:• Thismeasurementismostappropriatefordynamic work
• Canvaryconsiderablybetweenindividuals,dependingontheirfitnesslevelsand age
• Canbeconfoundedbyotherstressorsincludingheat,humidity,emotionallevels,andmentalstress
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Heartrate guidelines
• Averageworkingheartrate=40beats/min+restingaverageheartrate(≈72 beats/min)
• averageincreaseinheartrate/increaseinenergyexpenditure(slope)=10beats/minper1 kcal/min
• A5.33kcal/minworkload(4kcal/minabovetherestinglevel of1.33kcal/min)producesa40beats/minincreaseinheart rate
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Heartrate guidelines
• Heartratebetween1/2to 1minaftercessation: HR1
• Heartratebetween2.5to3minaftercessation:HR2
• Acceptableheartraterecovery:– HR1≤110 beats/min– HR2– HR1≥20 beats/min
• Theincreaseintheheart rateduringsteady-statework,calledheartratecreep,indicatesanincreasingbuildupoffatigueandinsufficientrecoveryduringrestpausesandmustbe avoided
Averageheartratemeasurementfortwodifferent workloads
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Heartrate guidelines
Subjectiveratingsofperceived exertion
• ThescaledevelopedbyBorg (1967)
• Ratingsthrough6to 20correspondtotheheartratesdividedby 10
• Theratingsshouldbeusedwithcaution andnormalizedtoeachindividual’smaximumrating
BorgRatingofPerceivedExertion (RPE)
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Lowbackcompressive forces
• 40%ofworkaccidentsarecausedduringmaterialhandling
• 70%ofmanualmaterialhandlingaccidentsconcernlow back
• Lowbackaccidentsandillnessesconsists25%ofthetotal compensations
• Theaveragecostoflowbackaccidentsis 60,000$
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Lowbackcompressive forces
• Vertebraeisdividedinto:cervical,thoracic,lumbarandsacrum regions
• Spinalcordisprotectedbyvertebrae,spinalnerverootsareseparatedfromthespinalcordtoachieveinternalorgansand extremities
• Vertebralbonesareseparatedbyasoftertissue:intervertebral disks.
• Theyserveasjoints,allowingalargerangeofmotion,andcushionsbetween vertebral
• Mosttrunkflexionoccursinthetwolowestjoints:L5/S1ve L4/L5
Anatomyofthehuman spine
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Lowbackcompressive forces
Lowback problems• Aging,heavymanualworkexposure:thediskscan
weaken• Cartilageendplatecansuffermicrofractures,releasing
somegelatinousmaterial,thecenterstartsdrying up• Thediskspacenarrows,vertebralbonescomecloser
andtouch:irritationandpain,motor impairments• Diskherniation(slipped disk)• Softtissueinjuries(ligaments,muscles, tendons)
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Lowbackcompressive forces
(a) Normal state(b) Narrowingofthediskspace,allowing
thenerveroottobe pinched(c) Herniateddisk,allowingthegel
materialtoextrudeandimpingeuponthenerve root
Anatomyofavertebraandtheprocessofdisk regeneration
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Lowbackcompressive forces
Somecausesforlow-back problems• Heavy work
– Frequentliftingoflarge loads– Forward-bendingtrunkposturesforlong periods
• Longperiodsofimmobility,eveninsitting postures• Wholebody vibration• Geneticpredisposition(weakerconnectivetissues,disks,
ligaments, etc.)• Personallifestyleconditions(smoking,obesity, etc.)
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Lowbackcompressive forces
AnalogybetweenL5/S1diskandfirst-class lever
• Centerofthediskactsasthe fulcrum• FindthemuscleforceFMandthetotalcompressiveforce Fcomp
2xFM=30x50=>FM=750lb(341kg)Fcomp=750+50=800lb (364kg)
• Acompressiveforceof770lb(350kg)isconsideredthedangerthreshold
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Lowbackcompressive forces
Moreaccuratevaluesforvariousloadsandhorizontal distances
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NIOSHlifting guidelines
• NIOSH:NationalInstituteforOccupationalSafetyand Health• Keyoutputistherecommendedweightlimit (RWL)• RWLisbasedontheoptimumweight,withadjustmentsfor
variousfactorsrelatedtotask variables• RWListheloadthatcanbehandledbymost workers
– 350kgcompressionforcecreatedbyRWLcanbetoleratedbymostyoung,healthy workers
– Over75%ofwomenand99%ofmenhavethestrengthcapabilitytoliftaloaddescribedby RWL
– Maximumresultingenergyexpendituresof4.7kcal/minwillnotexceedrecommended limits
• IfLoad>RWL,thentheinjuriesandlowbackproblemsincrease
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NIOSHlifting guidelines
• TheformulationforRWLisbasedonamaximumloadthatcanbehandledinanoptimum posture
RWL=LCxHMxVMxDMxAMxFMx CM– LC=LoadConstant=51 lbs– HM=HorizontalMultiplier= 10/H
H:Horizontallocationoftheloadforwardofthemidpointbetweentheankles,10≤H≤25in
– VM=VerticalMultiplier=1– 0.0075|V-30|V:Verticallocationoftheload,0≤V≤70 in
– DM=DistanceMultiplier=0.82+ 1.8/DD:Verticaltraveldistancebetweenoriginanddestinationoflift,10≤D≤70 in
– AM=AsymmetryMultiplier=1– 0.0032*AA:Angleofsymmetrybetweenthehandsandfeet,0≤A≤135 derece
– FM=FrequecnyMultiplier=Table4.7– CM=CouplingMultiplier=Table 4.8
• Thesemultipliersrangefromaminimumvalueof0forextremeposturestoamaximumvalueof1foranoptimalpostureor conditions
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NIOSHlifting guidelines
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NIOSHlifting guidelines
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NIOSHlifting guidelinesCouplingmultiplier
Coupling Type
V < 30 in (75 cm)
V ≥ 30 in (75 cm)
Good 1.00 1.00Fair 0.95 1.00Poor 0.90 0.90
• OptimalContainer:Boxesandcrateswithwell-definedhandlesorhand-holdcutouts
• Optimalhandle:cylindrical,withasmooth,nonslip surface
• Forlooseobjects,agoodcouplingwouldconsistofacomfortablegripinwhichthehandcancomfortablywraparoundtheobjectwithoutanylargewrist deviations
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NIOSHlifting guidelines
LiftingIndex (LI):LI=loadweight/ RWL
• Toprovideasimpleestimateofthehazardlevelofliftingagiven load– Valuesexceeding1.0deemedtobe hazardous
• Usefulinprioritizingjobsforergonomic design• Controlthehazardbyredesigningjobsand workplaces
– Avoidhighandlow locations– Useliftandtilttables– Usehandlesorspecializedcontainersforhandling loads– Reducethehorizontal distance
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NIOSHlifting guidelines
Example:Liftingaboxintothetrunkofa car
• 30lbboxfromgroundintothe trunk• Twist90° topickup→A= 90• Groundlevel→Vorigin= 0• Trunk→Vdest=25,D= 25• Horigin=10andHdest= 25• One-timelift,FM= 1• Boxisfairlysmallandcompactbuthas no
handles→CM= 0.95
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NIOSHlifting guidelines
Example:Liftingaboxintothetrunkofa carRWL=51(10/H)(1-0.0075|V-30|)(0.82+1.8/D)(1-0.0032A)xFMxCM
RWLorigin=51(10/10)(1-0.0075|0-30|)(0.82+1.8/25) (1-0.0032*90)(1)(0.95)
=23.8 lbRWLdest=51(10/25)(1-0.0075|25-30|)(0.82+1.8/25)(1-0.0032*0) (1)(0.95)
=16.6 lbLI=loadweight/RWL→30/16.6= 1.8
• Only16.6lbcouldbeliftedsafelyandthe30lbboxwouldcreateahazardalmosttwicetheacceptable level
• Decreasethehorizontaldistanceto10in→RWLdest=41.5 lb• Movethefeetandeliminatethetwist→RWLorigin=33.4 lb
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Multitasklifting guidelines
• Usedforjobswithavarietyoflifting tasks• Overallphysical/metabolicloadisincreasedcomparedtothe
single lifting• ThisisreflectedinadecreasedRWLandanincreased LI• Compositeliftingindex(CLI)representsthecollective
demandsofthe job• CLIequalsthelargestsingle-taskliftingindex(STLI)and
increasesincrementallyforeachsubsequent task• CalculationsforCLIandSTLIarepresentinthebook(pg 166)• Whenthenumberoftasksexceedsthreeorfouravarietyof
softwareprogramscanbeused(Design Tools)
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Safelifting procedure
a. Planthe lift
b. Determinethebestlifting technique
c. Getasecure grip
d. Pulltheloadinclosetoyour body
e. Alternateliftingandlightwork tasks
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Generalpostureandtaskevaluation checklist
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Manualworkdesign– key points
• Designworkaccordingtohumancapabilitiesandlimitations
• Formanipulative tasks– Usedynamicmotionsratherthanstatic ones– Keepthestrengthrequirementbelow15%of maximum– Avoidextremerangesof motion– Usethesmallestmusclesforspeedand precision– Usethelargestmusclesfor strength
• Forliftingandotherheavymanual work– Keepworkloadsbelowone-thirdofthemaximumwork
capacity– Minimizehorizontalload distances– Avoid twisting– Usefrequent,shortwork/rest cycles
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