Vibration Exposure in Forwarder Work: Effects of Work

Originalscientificpaper

Croat. j. for. eng. 37(2016)1 107

Vibration Exposure in Forwarder Work:

Effects of Work Element and Grapple Type

Carola Häggström, Mikael Öhman, Lage Burström, Tomas Nordfjell, Ola Lindroos

Abstract

Exposure to whole body vibration (WBV) is a major concern in mechanized forestry work because its adverse effects may become exacerbated by repetitive hand and arm movements, and non-neutral body postures. Moreover, shock-type vibrations have recently been suggested as a pos-sible agent behind pains in the neck and shoulders of forest machine operators. Shocks have been identified in forwarders during loading, but the effects of crane work in forwarders have, to the best of our knowledge, not been studied. Thus, the aim of this study was to assess contributions of crane work elements, and potential effects of the use of three grapple and brake-link combina-tions, to vibration exposure levels in a small forwarder. Repeated measurements of cabin WBV were acquired, and work elements timed, as a single experienced operator forwarded wood piles on a standardized track in northern Sweden, using the same forwarder and work procedures with each grapple and brake-link combination. The studied crane equipment was found to have little or no effect on the daily WBV exposure. Furthermore, exposure to shock-type vibrations while loading appears to be due to driving rather than crane work. However, there were fewer collisions with remaining trees while using the tilt grapple with brake link, suggesting its use provides a more relaxed and comfortable work environment for forwarder operators and financial benefits for the forest owner by reducing damage in the remaining stand.

Keywords: crane work, forestry, forest machine, seated health, whole body vibration, work elements, work environment

takentoreducetheimpactofWBViftheA(8)valueexceeds 0.5 m/s2 or VDV exceeds 9.1 m/s1.75.Amoregeneralguidelineistoalwaysminimizeoccupationalvibrationexposure(Burströmetal.2014).

WBV isamajorconcerninmechanizedforestryworksinceitsadverseeffectsareexacerbatedbyre-petitivehandandarmmovements,non-neutralbodypostures,andmanuallifting(PunnettandWegman2004,Okunribidoetal.2006,Lisetal.2007,Burströmetal.2014).Operatorsofforestmachineshaveahighprevalenceofmusculoskeletalsymptomsinthelowerback,neckandshoulders(Rehnetal.2002,JackandOliver2008),whichmaybeatleastpartlylinkedtotheir WBVexposure,althoughtheassociationbetweenWBVexposureandneckandarmpainhasnotbeenclearlyestablished(Rehnetal.2009).However,itissuggestedthatthehighprevalenceofneckpainamongforestmachineoperatorsisassociatedwithexposuretoshock-typevibration(Rehnetal.2009).However,

1. IntroductionWholebodyvibration(WBV)isrelatedtonumer-

oushealthproblems,interaliavariousmusculoskel-etal,digestiveandreproductivedisorders,lowbackpain(SeidelandHeide1986,BovenziandHulshof1999,PunnettandWegman2004,Burströmetal.2014),andmoreinstanteffectsincludingmotionsickness,sightimpairmentandfatigue(ISO1997).Inadditiontohealtheffects,WBVhasbeenshowntoimpairper-formance(Conwayetal.2006),especiallyinaccuracybasedtasks,whicharetypicalforcraneworkduringforestryoperations.Itisimportanttorestrictvibrationexposureandmonitor its effectson thoseexposedsinceadoseresponserelationshipisyettobeestab-lished(Popeetal.2002).Thus,forinstance,EUDirec-tive2002/44/ECrestrictsdailyexposurenormalisedtoaneight-hourreferenceperiod,designatedA(8), to1.15 m/s2orafourthpowervibrationdosevalue(VDV)of21m/s1.75,andstipulatesthatmeasuresshouldbe

C. Häggström et al. Vibration Exposure in Forwarder Work: Effects of Work Element and Grapple Type (107–118)

108 Croat. j. for. eng. 37(2016)1

shocksmayalsobemoreimportantthansinusoidalvibrationwithregardstolowbackpains(Okunribidoetal.2006).Hence,reducingWBVshouldimprovetheworkenvironmentinforestry.Furthermore,reducingvibrationsmayalsoreducemachinewearanddamagetotheground(Rieppoetal.2002).Dueinlargeparttotheergonomicproblems,nu-

merousaspectsofWBVinforestryworkhavebeenintensivelyresearched.Theseaspectsincludeeffectsofdampeningsystemsforforestryvehicles(Geller-stedt1998,Sherwinetal.2004,Baes2008)andbothchairsandcushioning(BoileauandRakheja1990,San-karandAfonso1993,Mansfieldetal.2002,Cationetal.2008,Jietal.2015).Vibrationsassociatedwithdif-ferentforestmachinesandmachinesystemshavealsobeenexamined(Rehnetal.2005b,GerasimovandSo-kolov2009),andattemptshavebeenmadetoestablishdoseresponserelationships (Rehnetal.2009),andstandardize measurement techniques (Rehn et al.2005a,Burströmetal.2006).Duringworkstudies,forwardingisnormallydi-

videdintotheworkelements(WEs)driving(emptyorloaded),loadingandunloading.Vibrationexposureduring these WEs hasbeenevaluated,drivinghasbeenidentifiedasthemajorsourceofWBV,andtheoperatorisexposedtohighervibrationlevelswhendrivingemptythanwhendrivingloaded(Hansson1990,Rehnetal.2005a).Oneoffewstudiesreportingbothr.m.sandshocksensitiveVDVvaluesfoundex-posuretoshock-typevibrationstobecommonwhileloading,buttheshocksarebelievedtomainlyorigi-natefromsimultaneousdrivingbetweenpilesinun-even terrain (Rehn et al. 2005a). However, to ourknowledge,nopreviousstudieshaveexaminedWBV exposurelevelsinsufficientdetailtoevaluateexpo-sure during crane WEs.Furthermore,mostpreviousstudieshavefocused

onvibrationsassociatedwithlargeforestmachines(10to20tonnes),whicharealmostexclusivelyusedinin-dustrialapplications.Thus,thereisalackofinforma-tion on WBVinsmallforwarders(lighterthan4tonnes),which areusedbybothprofessionals and self-em-ployednon-industrialprivateforestowners(cf.Nordfjelletal.2003,Lindroosetal.2005).Thereareseriouscon-cernsaboutbothofthesegroups.Professionalscon-tinuouslyusethemachineswhenworking(cf.PassicotandMurphy2013),sotheyarehighlysensitivetovari-ationsinthemachinedesign,whilethelatterareoc-casionaluserswhoareheavilyrepresentedinaccidentstatistics,butdifficulttoinformaboutpreventiveac-tions (LindroosandBurström2010).Thus, forbothgroupsitseemshighlyimportanttoidentifyandimple-mentmodificationsthatminimizevibrations.

Vibrationexposureinvehiclesmaybeaffectedbynotonlydriverseatsanddampeningsystems,butalsoworkingtechniques,whichareinfluencedbytheop-erators’experienceandequipment.Forexample,vi-brationexposureofprofessionaltaxidriversandtrainoperatorsreportedlydeclinesastheirworkexperienceincreases(Chenetal.2003),andforwarderoperators’workingtechniquesreportedlyinfluenceWBVlevelswhiledrivingloaded(Rehnetal.2005a).Brakelinksandtiltgrapplesareequipmentthatmayalterworktechniquesduringforwardercranework.Brake-links,placedbetweenthecranetipandgrapple(orothertool)arecommonequipmentonlargeforestmachinesandhelptoincreaseprecisionbyreducingswingingmovementsofthegrapple.Standardbrake-linksarestatic,butanactivebrake-link that canbeused tobrakewhendesiredhaspotentialcapacitytofurtherincreasetheprecisionofmovements.Atiltgrappleprovidesnotonlythefeaturesofanactivebrake-linkbutalsothepossibilityofpreciselytiltingthegrappleandthegrippedlogs.Theuseoftiltgrappleshasbeenfoundtoincreaseproductivityinforwarding,aswellasreducingdamagetostands(Fogdestam2010,Nils-son2013),buttherehavebeennodetailedstudiesontheireffectsonvibrations.Thus,therearegapsofinformationonWBVexpo-

sureinsmallforwarders,thevariationbetweenWEs andtheeffectofcraneequipment.Therefore,theaimofthisstudywastoassesscontributionsofspecificcrane WEstotheoverallvibrationexposureinsmallforwardersandpossibleeffectsofthreegrappleandbrake-linkcombinationsontheWBVexposure.

2. Material and Methods

2.1 Experimental designRepeatedfieldmeasurementsofcabinWBV in a

forwarderwereacquired,whileasingleoperatorwasforwardingstandardizedwoodpilesonastandard-izedtrack,usingthreetypesofcraneequipment.Eachmonitoredworkcycle(observation)correspondedtooneroundonthestandardizedtrack,beginningwithloadingtheemptybunkandendingwhenthelastlogwasunloaded.Throughtimestudies,eachworkcyclewassplitintoWEs and WBV were analyzed within and overWEs.Thus,thedesignconsistedoftwofixedfac-tors(CraneEquipmentandWE)withinsetsofrepeti-tions(blocks).Intotaltherewerefiveblocks.Thethreetypesofcraneequipmentwererandomlyassignedwithinblockstominimizepossibilitiesoforderandcarry-overeffectsconfoundingtheresults.Thefield studywas conductedduringOctober

7–22,2013,withonetrial(workcycle)perdayforthe

Vibration Exposure in Forwarder Work: Effects of Work Element and Grapple Type (107–118) C. Häggström et al.

Croat. j. for. eng. 37(2016)1 109

firstblock,andsubsequentlytwotrialsperday.Onedesignatedresearcherfilmedallthetrialsandmadeallthemeasurements.Duringthestudythetempera-turewascirca0°C.

2.2 The standardized trackThestudywasconductedinaforeststandinthe

northernpartofSwedenthatwasselectedtorepresentatypicaldensestandthathadjustbeensubjectedtoafirstthinning(seee.g.ErikssonandLindroos(2014)fortypicalSwedishconditions).Thestandcontainedonly

Scotspine(Pinus sylvestrisL.)trees,withabasalareaweightedmeanageof 46years.The standdensity,basalareaatbreastheightandmeantreevolumewere1370stemsperha,21m2/ha and 0.1 m3ofsolidwoodoverbark(m3sob),respectively.Thegroundwasex-tremelyflat,sandyandhadgoodcarryingcapacity(class1–1–1accordingto»Terrainclassificationforfor-estrywork«Berg1992).Thus,itwassuitableforthetestssincerisksofconfoundingthemeasuresofcraneequipmentinducedvibrationwithvibrationsduetoterrainstructurewereminimaldespitepossibilitiesof

Fig. 1 Scaled map of the 114 m long standardized track. Positions and sizes (numbers of logs) in the even (e) and uneven (ue) piles are marked by gray lines, trees in the stand by gray dots and the landing area by rectangle


110 Croat. j. for. eng. 37(2016)1

simultaneouscraneworkanddriving.Thestandcon-taineda144mlongroughlycirculartrack(a2.9mwidestriproad),alongwhich95pinepulpwoodlogsweredistributedinto38pilesinastandardizedmannerforeachtrial,withaspurleadingtoalanding(Fig.1).Thevolumeofthelogswasequivalenttoonefullloadforthestudiedforwarder(3.3m3sob).Themeanlengthandtopdiameterofthelogswere4.4withastandarddeviation(SD)of0.3and0.078(SD=0.014)m,respec-tively.Themeanwooddensity,basedonasampleoffivelogs,was997kg/m3sob.Thesamelogswereused,andthenumber,positionsandsizes(i.e.numbersoflogs)ofthepileswerekeptconstantduringthetrials.However,givenlogswerenotalwaysplacedingivenpiles,thusthevolumesofthepilesmayhavevariedslightlybetweenrepetitions.Eachpilecontained1–5logs,andcouldbehandledwithonegripofthegrap-ple.Thecenterofeachpilewasplacedatafixeddistancebetween1.5and5.0mfromthecenterofthestriproad.Twenty-onepileswereplacedtotheoutersideofthecircularroadandslightlyfewer(17)totheinnerside(duetospatiallimitations).Thepileswerealwaysplacedatthesameanglewithrespecttothestriproad.Ineachofthe38piles,allbuttendswereorientedinthesamedirection.For20ofthepiles,logswereplacedsothatthe

buttendsurfaceswereverticallylevelwitheachother(evenpiles),whilefortheother18pilestheirverticalpositionswerevariedbyupto0.7–1m(unevenpiles).Duringeachtrial,theloadingstartedatthebegin-

ningofthetrack(sotherewasnodrivingempty),andthelast36mofthetrackwasdrivenwithafullload(sotherewas108mofdrivingwhile loading).Atthelanding,logswereunloadedontoapre-markedareaforroadside-piles.

2.3 Base machine and crane equipmentAstandard3.5tonnesVimek608.2BioCombifor-

warderwasusedinthestudy(VimekAB,Vindeln,Sweden).Theforwarderwasequippedwithastan-dardcranewithareachof5.2mandaliftingtorqueofabout20kNm.Thethreestudiedtypesofcraneequip-ment(grappleandbrake-linkcombinations)were:aVimektiltgrapplewithaVimekdynamicbrake-link(brakedtiltgrapple,Fig.2a);aVimekstandardgrapplewithVimekdynamicbrake-link(brake-linkgrapple,Fig.2b);andaVimekstandardgrapplewithnobrake-link(standardgrapple,Fig.2c).Thegrippingareawasthesameforallgrapples(0.16m2).Thetiltingcapacityofthebrakedtiltgrapplewas1.3kNm.Theweightsofthebrakedtiltgrapple,brake-linkgrappleandstan-

Fig. 2 Vimek grapples and brake-links


Croat. j. for. eng. 37(2016)1 111

dardgrapplewere127,101and81kg,respectively.Thus,themassofasinglepilewasnotalimitingfactorforthecraneoranyofthestudiedgrapples.

2.4 Operator and work instructionsInordertoavoiderrors betweensubjectvariations

(Lindroos2010,Häggströmetal.2015),asingleopera-,asingleopera-torwithpreviousexperienceofforestrytimestudiesoperatedtheforwarderthroughoutthestudy.Theop-eratorwasmale,68yearsold,familiarwiththefor-warderusedinthestudyandhad30yearsofexperi-ence inforwarding.Beforethestudy,hehadexperiencewithallthestudiedtypesofgrapplesandbrake-links,butlittleexperiencewiththebrakedtiltgrapple.Theoperatorfirsthada trainingsessionofone

workcyclewitheachofthegrappleandbrake-linkcombinations,duringwhichhewasinstructedtofindapreferredworkingmethodforallthreecraneequip-menttypes.Hewastheninstructedtousetheselectedworkpatternsthroughoutthestudy.Theendsurfacesofgrippedlogsweretobealignedbeforeloadingonlywhen the operator considered it necessary.Whenaligningendsurfaces,theoperatorwasinstructedtodo itagainst theheadboardwiththestandardandbrake-linkgrapplesandverticallyagainstthegroundwiththebrakedtiltgrapple.Betweentrials,theoperatorhadthechancetoget

to know theequipmenttobeusedinthefollowingtrial while re-arranging the logs along the track.

2.5 Time studyALEGRIAHFS200highdefinitionvideocamera

(Canon Inc.,Tokyo, Japan)wasused to record theworkineachtrial.ProTimeEstimationsoftware(Pro-

planner,Ames,USA)wasthenusedtomeasuretimesofthesevendefined,non-overlappingworkelements(WE) described in Table 1. Collisions between thegrappleorliftedlogs,andtreesorthebasemachinewere also counted.

2.6 Vibration measurementsVibrationsweremeasuredinthreeorthogonalaxes

accordingtoISO2631-1(ISO1997)usingaMTi-Gtri-axial accelerometer (Xsens,Enschede,TheNether-lands)placedonthefloorclosetothecenterofthecabin,infrontofthechair.Theplacementensuredthattheoperator’sweight,heightandthechairdampeningwouldnotaffectthemeasures.Samplesweretakenatafrequencyof100Hzduringeach,approximately40minutelong,workcycle,usingaXKFScenario»2.7Automotive unit« (Xsens, Enschede, The Nether-lands).ThemeasuringequipmentwascheckedusingaBrüel&Kjær4294calibratorafterthemeasurements.

2.7 Data analysesAlldataprocessingwasperformedofflineusinga

commercialsoftwarepackage(MATLABR2014a8.3,TheMathWorksInc.,Natick,USA)withthe»Continu-ousSoundandVibrationAnalysis«program(Zech-mann2013).The accelerationdatawere convertedfromtherecordedtimedomaintofrequencydomainwithafrequencyrangeupto50Hz,i.e.themaximumfrequencyrangethatcanbecalculatedfrom100Hzoutput.Intheanalyses,1/3octavebandvalueswerecalculatedfrom0.1to50Hz.Theresultingdatawerethenusedtocalculatefrequencyweightedr.m.s.ac-celeration and VDVvalueswithrespecttohealthef-fectsonaseateddriverinaccordancewithISO2631-1

Table 1 Definitions of time study work elements

Work element Definition Priority

Crane out1 Begins when the crane starts moving towards a pile on the ground and stops when grip begins 1

Grip1 Begins when the grapple is placed against the pile and stops when all logs are gripped and crane in begins 1

Crane in1 Begins when the grapple is loaded and the crane starts moving towards the bunk and stops with release 1

Release & reorganise1 The sum of release (which begins when the grapple is inside the supports above the bunk and ends when no log has contact with the grapple) and reorganise (the time the operator spends reorganizing logs on the bunk)

1

Unloading1 Begins when the crane starts moving for unloading on the roadside landing and stops when all logs are unloaded 1

DrivingBegins when the forwarder wheels start to move without the crane being active and stops when the wheels stop or crane movements are initiated, whichever comes first

2

Other working time All time that is not covered by any of the definitions above, including disruptions 3

Note: If multiple work elements were performed simultaneously, time consumption was recorded for the work element with the highest order of priority (lowest number)1 The WE crane work used in the analysis includes all the crane activities pooled


112 Croat. j. for. eng. 37(2016)1

(ISO1997).Theweightedr.m.s.valueswerecalculatedwithrespecttoallthreeorthogonalaxes,awx(backandforth),awy(lateral)andawz(vertical),andtheirsumvec-tor(av).Similarly,VDVwascalculatedforallthreeor-thogonalaxes(VDVz,VDVy and VDVz)andthevectorvalue(VDVv)overeachmeasurementperiod.Further-more,crestfactorswerecalculatedforallorthogonalaxesaswellasthe8hourequivalent,A(8),valueovereachmeasurementperiod.

2.8 Statistical analysisDatawereanalyzedusingMinitab16(MinitabLtd,

StateCollege,PA,USA).AnalysisofVariance(ANO-VA)wasusedtoanalyzethefixedeffectsofWE and CraneEquipmenttype,andthefixedinteractionbe-tweenthem,onthevibrationmeasures.TheANOVAmodelsalsoincludedtherandomblockeffect.Agen-erallinearmodel(GLM)wasappliedwhenanalyzingtheANOVAmodels,andTukey’sHonestSignificantDifference(HSD)testofmeanswasusedforpairwisecomparisons.

ForWE,twosetsoftreatmentwereanalyzed.Thefirst set (denotedWorkCycle) included two levels(cranework,i.e.thesumofallcraneWEs,versusdriv-ing,)andthesecondset(denotedCraneActivity)in-cludedfivelevels(release&reorganize,grip,cranein,craneoutandunloading,).Otherworkingtimewasexcluded fromanalyses.The same levels ofCraneEquipmentwereusedinbothanalyses.Inathirdsetofanalyses,effectsofCraneEquip-

mentwereanalyzedusingasinglepooledWE(cranework).Alignmentsofendsurfaces,collisionswithre-sidual trees and collisions with the machine were in-cludedascovariatestoinvestigatetherelationshipsbetweenvibrationmeasuresandcollisions.Inafourthsetofanalyses,effectsofCraneEquipmenttypeonthenumberofcollisions,andalignments,wereanalyzedwith a GLMincludingblockasarandomfactor.ANOVAassumptionsofindependence,homosce-

dasticityandnormalityofresidualswerenotsuffi-cientlyviolatedtorequiretransformationofthedata,accordingtoocularinspectionofresidualplots.Inallanalyses,thesignificancelevelwassetto5%.

3. ResultsEachof the15observations (fiverepeatedtrials

witheachofthethreeequipmenttypes)lastedabout40minutes,providing9hoursand22minutesofre-cordingsintotal.Ofthattime,5%wasclassifiedasotherworkingtimewithameandurationperobserva-tionof105(SD=83)s,whichwasexcludedfromfurtheranalysis.So,theaveragedurationofworkcycleswas2122(SD=152),2077(SD=101)and2229(SD=65)s,re-spectively,foroperationswiththebrakedtiltgrapple,break-link grapple and standard grapple.Missing

Table 2 Assumptions made during calculation of the time distribu-tion for each work element (WE) during one full day (8 hours) of work with the forwarder

Parameter Value

Daily work hours, h 8

Technical utility1, % 88–100

Conversion constant PMh15 to PMh02 0.9

Proportion of crane work during loading3, % 50–90

Work cycle (including driving empty), %

Proportion of loading4 45

Proportion of unloading4 15

Proportion of driving empty4 24

Proportion of driving loaded4 16

Crane Activity (without unloading), %

Proportion of Release & Reorganise5 32

Proportion of grip5 14

Proportion of crane in5 34

Proportion of crane out5 19

1 Based on Nordfjell et al. (2010)2 Based on unpublished material, Skogforsk3 Based on Manner et al. (2013)4 Based on Rehn et al. (2005a)5 Based on observed time distribution in the present study

Table 3 Ranges of the work elements’ estimated contributions (%) to the total daily WBV dose during the studied forwarding opera-tions, based on the average and maximal measured awz and time distributions presented in Table 2

Type of work Work element Contribution, %

Work cycleCrane work1 33–59

Driving 41–67

Crane activity

Crane in 7–15

Crane out 3–6

Grip 2–5

Release & Reorganise 7–14

Unloading 15–20


Croat. j. for. eng. 37(2016)1 113

datafor4,10and4%oftheworkcycleswiththere-spectivegrappleswerenotincludedinthecalculationofthetotaltime.

BasedontheassumptionsinTable2,thedailytotalvibrationexposuredosewasonaverage0.3m/s2 and theestimatedmaximumdosewas0.38m/s2.Gener-

Fig. 3 Mean values of vibrations measured at the floor of the for-warder as a function of frequency (1/3-octave bands) for each in-dicated work element (WE) during driving and crane work (means of 15 observations, i.e. pooled data for trials with all types of crane equipment)

Fig. 4 Mean values of vibrations measured at the floor of the for-warder as a function of frequency (1/3-octave bands) for the pooled crane work in all three directions (ax, ay, az) and the sum vector (av) (mean of 15 observations, i.e. pooled data for trials with all types of crane equipment)

Table 4 Frequency weighted acceleration in the three orthogonal axes (x, y and z), the sum vector (v) and the A(8) value for indicated work elements (based on pooled data for trials with all types of crane equipment) according to »health« in ISO 2631-1. Measurements were taken at the feet

Type of work WE N

Duration awx awy awz av A(8) VDVv

sm/s2 m/s1,75

Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD

Work cycleCrane work1 15 1537–1887 0.15B 0.01 0.21B 0.01 0.32B 0.08 0.41B 0.06 0.079A 0.019 4.24A 0.37

Driving 15 335–502 0.31A 0.03 0.33A 0.03 0.35A 0.04 0.57A 0.05 0.042B 0.003 4.42A 0.37

Crane activity

Crane in 15 380–638 0.14b 0.01 0.23a 0.02 0.32bc 0.08 0.42bc 0.06 0.042a 0.010 3.06a 0.37

Crane out 15 262–309 0.11c 0.01 0.16c 0.02 0.28d 0.07 0.34d 0.05 0.028c 0.007 2.20b 0.35

Grip 15 157–313 0.11c 0.01 0.22a 0.02 0.30cd 0.07 0.39c 0.06 0.026c 0.005 2.21b 0.29

Release & Reorganize 15 343–628 0.17a 0.02 0.19b 0.01 0.34ab 0.07 0.43ab 0.05 0.043a 0.010 3.01a 0.28

Unloading 15 256–345 0.16a 0.02 0.23a 0.03 0.34a 0.09 0.45a 0.08 0.035b 0.009 3.02a 0.44

Note: Mean values within columns and type of work with different superscript letters (A–B for the full work cycle and a–d for crane activities) are significantly differ-ent (p<0.05, Turkey’s HSD). WE = Work Element; SD = Standard Deviation1 Crane Work includes all crane activities pooled


114 Croat. j. for. eng. 37(2016)1

ally,drivingcontributedsomewhatmorethancraneworktothedailydose(Table3).Low-frequencyvibrationsweremoreintensedur-

ingdrivingthanduringcranework(Fig.3).However,therewerenovisibledifferencesinthefrequencyspec-traofvibrationsinthevertical(z)directionbetweenthe cranework (Fig. 4) anddrivingWEs (datanotshown).Accelerationsinthehorizontaldirections(x and y)werehighestinthefrequencyrange1.25–4Hzduringcranework(Fig.4)and0.25–5Hzduringdriv-ing.Thefrequencydistributions for thegivenWEs weresimilarwhenusingallCraneEquipmenttypes.ThereweresignificantmaineffectsofWorkCycle

onallvibrationmeasuresexceptVDVv.Meanav was significantlyhigherduringdrivingthanduringcraneworkaccordingtothevarianceanalysis.Furthermore,vibrationaccelerationmagnitudes (meanweightedr.m.s)inthepredominantverticalz-direction were also highestduringdriving(Table4).However,forthetimeweightedr.m.s.value,A(8),therelationshipwasre-versed(Table4).AnadditionalsetofANOVAsshowedthatthisrelationshipbetweenazandA(8)alsoheldfordrivingversusalltheCraneActivityWEs(datanotshown).Onaverage,morethanfourtimesasmuchtimewasspentoncraneworkthanondriving.Ahighcrestfactorinthex-direction(mean12,max15)indi-catedoccurrencesofshocksduringcranework.Nev-ertheless,VDVx,andVDVywerehigherduringdrivingthanduringcranework.Incontrast,VDVz was higher duringcraneworkthanduringdriving.Consequently,theoverallvector(VDVv)wasnotaffectedbyWE.CraneActivitiessignificantlyaffectedallvibration

measures,buttheinteractionbetweenCraneActivitiesandCraneEquipmentwasnon-significant.Craneinandrelease&reorganizewereboththemosttimecon-sumingCraneActivitiesandtheWEs with the highest averagevectorvibrations.However,theydifferedinthatcraneinhadhighvaluesinthe y-direction while

release&reorganizehadhighvaluesinthex-direction (Table4).NoeffectofCraneEquipmenttypeonanyvibra-

tionmeasurewasfoundduringcranework(Table5).However,thereweresignificantdifferencesbetweenCraneEquipmenttypesinfrequenciesofcollisionswithresidualtrees.Fewesttreeswerehitwhenusingthebrakedtiltgrappleandmosttreeswerehitwhenusingthestandardgrapple(Fig.5).However,apply-ing collisions and alignments as covariates in theANOVAdidnotrevealanysignificantrelationshipbetweencollisionsoralignmentsandvibrationlevelsinanydirection,norforthesumvectorforanyofthevibrationmeasures.

Table 5 Frequency weighted acceleration in the three orthogonal axes (x, y and z), the sum vector (v) and the A(8) value for crane work with the indicated crane equipment types according to »health« in ISO 2631-1. Measurements were taken at the feet

Crane equipment N

Duration awx awy awz av A(8) VDVx VDVy VDVz VDVv

s10-2m/s2 10-2m/s1,75

M SD M SD M SD M SD M SD M SD M SD M SD M SD

Standard 5 1784–1887 14 1 20 2 32 8 41 7 8.2 2.2 171 15 241 32 301 56 424 54

Brakelink 5 1565–1781 15 1 21 1 33 9 42 6 7.9 2.1 189 16 242 24 313 50 441 22

Braked tilt grapple 5 1537–1846 15 1 21 1 31 8 41 6 7.6 1.6 178 23 222 8 286 50 406 27

Fig. 5 Average numbers of collisions – with residual trees (Trees), the Machine, or both (T&M) – and alignments of end surfaces of the logs per work cycle with each type of crane equipment. Means within categories with different letters are significantly different (Turkey’s HSD p<0.05)


Croat. j. for. eng. 37(2016)1 115

4. DiscussionPreviousstudies(Hansson1990,Rehnetal.2005a)

haveshownthattheterrainsignificantlyinfluencesvibrationlevelsandthatWBVexposureishighestdur-ingdrivinginforwarderoperations.Shock-typevibra-tionshavealsobeendetectedduringforwarderload-ing (cf. Rehn et al. 2005a).Nevertheless, effects offorwardercraneworkhavebeenseldomaddressed,althoughitaccountsforahighproportionofforward-ingwork:50–90%ofloadingandunloadingworktimedependingonextractiondistance(Manneretal.2013),and about 80%of the totalmonitored time in ourstudy,reflectedinhigherA(8)valuesforcraneworkthanfordriving(Table4).However,craneworkisof-tendonesimultaneouslywithdriving,sovibrationsoriginatingfromdrivingconfoundthosefromcranework.Therefore,ourstudywasconductedonaveryflat,firmandevenstandardizedtracktominimizetheinfluenceofdriving.Unstructuredobservationsbythedesignatedresearcherrevealedalmostnooccurrencesofsimultaneouscraneandvehiclemovementsduringthestudy.Thisindicatesthatourattempttoreducedrivingvibrationswassuccessful.Nevertheless,de-spiteoperatingonaneventrack,theinstantaneousvibrationlevels(ax-z and av)werestillhigherduringdrivingthanduringanytypeofcraneworkexaminedin the study.We investigated theeffectsof sixdefinedcrane

WEs on WBV,andobtainedaccelerationvaluesrang-ingfrom0.34to0.45m/s2.PreviousanalysesofcraneWEsduringoperationsofasingle-gripharvesterhavereportedgenerallyca.0.1m/s2lowervibrationmagni-tudes(measuredatthecabinfloor),rangingfrom0.20to0.34m/s2,duringbothdelimbingandfelling(Bur-strömetal.2006).Itshouldbenotedthatvibrationmagnitudesarenormallyloweratthechair,wheremostvibrationsaretransmittedtotheoperator.In-deed.Burströmetal.(2006)foundthatthevibrationstransmitted to the seatwere lower than 0.04m/s2 (0–22%ofthevibrationsatthefloorinthex,y and z-directions).Thus,thecombinedWBVstheoperatorwasexposedtothroughtheseatinthestudiedsmallforwarderwereprobablyconsiderablyweakerthanthe floor-level values presented here. However, itshouldalsobenotedthatchairscharacteristicsstrong-lyinfluencevibrationtransmissions(PaddanandGrif-fin2002),andevaluationorcomparisonofchairswasbeyondthescopeofthisstudy.ThecraneWEsassociatedwiththehighestvibra-

tions in our study were associated with handling logs (i.e.craneinandrelease&reorganize).Thisimpliesthattheweightandbalanceatthecranetipinfluenced

WBVmagnitudes.Nevertheless,despitenoticeableshockscausedbyimpactsthatweretransmittedasvibrationsthroughthecranetothecabin,nocorrela-tionwasfoundbetweenWBVexposureduringthepooledcraneworkandgrapplecollisionswithstand-ingtreesorthemachine.Thus,thesefindings,incom-binationwiththenon-significanteffectofcraneequip-menttype(Table5)andthepredominanceofvibrationsin the z-direction(Table4),implythatmodificationsthatincreasethestabilityofthebasemachineshouldbeconsideredinattemptstoreducetheoperators’ex-posuretocraneworkinducedWBVs.Thisrecommen-dation issupportedbyfindings thatvibrationsarenegativelycorrelatedwithmachineweight(Rehnetal.2005a).However,othermeasures,forexampleim-provinghydraulicsandcranecontrolsystems,mayalso smoothoperationsandreducecranework-in-ducedvibrations(HanssonandServin2010).Asmentionedabove,shock-typevibrationexpo-

sureiscommonduringloading(Rehnetal.2005a),butwefoundnoassociationbetweeneithervibrationsofthistypeorimpactsduringcranework.NoneoftheVDVvaluesassociatedwithanyCraneActivitywerehigher than the unloading values either (Table 4),whichwouldalsohaveindicatedhighfrequenciesofshocksduringthoseactivities(cf.Rehnetal.2005a).Thus,itishighlylikelythathighWBVexposurewhileloadingisduetodrivingbetweenpiles.Nevertheless,thedifferencesincollisionfrequenciesbetweencraneequipmenttypesobservedinthisstudywouldbeofinterestwhenselectingthinningequipmenttomini-mizedamagetoresidualtrees(Sirénetal.2013).Anexperimentalsetupwasused,which iscom-

monlyusedwithin forestengineeringworkstudies(Koširetal.2015)toenablecomparisonoffactorsofinterest.However,experimentalresultsmightbediffi-culttogeneralizetootherconditions.Sincethesmooth-nessofoperationsalsoaffectsvibrationlevels(HanssonandServin2010),theseresultsmaynotbereadilyap-pliedtodriverswithotherexperiencelevels,griporworkingtechniquepreferences.Indeed,therankingsofcraneequipmenttypesintermsofassociatedvibrationsmaydifferforotheroperatorsunderthesamecondi-tions (cf.Chen et al. 2003, Purfürst andErler 2006,Lindroos2010).Nevertheless,theobtainedresultsre-gardingcraneequipmentareconsistentwithindica-tionsofvibrationeffectsfromapreviousstudy(Nilsson2013)andtherewerenoindicationsofdifferencesinvibrationexposurebetweenforwarderoperatorsdur-ingcranework(Rehnetal.2005a).Furthermore,thevariationincraneequipmenttypesandassociateddif-ferencesinworkingtechniquesdidnotaffecttheWBV


116 Croat. j. for. eng. 37(2016)1

exposureduringeithercraneworkoverallorthede-finedcraneWEs in this study.Theupperlimitofthemeasuredfrequencyinthis

studywas100Hz.Thustheresultsshouldbeinter-pretedcautiously.Nevertheless,mostvibrationwasoflowerfrequencythan50Hz(Fig.3andFig.4).Thus,thelimitationsinmeasurementsshouldnothavehadamajorimpactonthecalculatedexposure,andtherelativelevelsarefullycomparable.Moreover,vibra-tionsduringforwarderworkdependonnumerousfactorsandmeasuredvaluesareonlyvalidundertheprevailingconditionsduringthestudy.Moreresearchishenceneededtofullygeneralizeforwarderopera-tionswithotherweight,sizeandwithotherdampen-ingsystems.Nevertheless,asnosignificanteffectofCraneEquipmentwasfound,itisunlikelythattheaction value for thedaily exposurewould be sur-passedduringforwarderworkduetodifferencesincraneequipmentor(crane)workingtechnique.

5. ConclusionsThestudiedcraneworkingtechniquesandcrane

equipmenttypeswerefoundtohavelittleornoeffecton the daily WBV exposurewith respect to seatedhealth.WefoundnoindicationthatanycraneWE or impactsfrommakingpilesshouldcontributesignifi-cantlytoshock-typevibrationsassumedtobeassoci-atedwithneckandarmpains.Thus,thehypothesisthathighlevelsofshock-typevibrationsduringload-ingoriginatefromdrivinginanuneventerrain(cf.Rehnetal.2005a)seemstohold.However,duetobet-ter controllability, therewere fewer collisionswithtreesandthemachinewhenusingthebrakedtiltgrap-ple.Thusitsuseshouldmaketheoperator’sworken-vironmentmorerelaxedandcomfortable,andprovidefinancialbenefitsforthelandownerbyreducingdam-age to the remaining stand.

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118 Croat. j. for. eng. 37(2016)1

Received:October24,2014Accepted:December04,2014

Authors’addresses:

CarolaHäggström,PhD.*e-mail:[email protected]Öhman,MSc.e-mail:[email protected],PhD.e-mail:[email protected],PhD.e-mail:ola.lindroos@slu.seSwedishUniversityofAgriculturalSciencesDepartmentofForestBiomaterialsandTechnologySE-90183UMEÅSWEDEN

LageBurström,PhD.e-mail:[email protected]åUniversityDepartmentofPublicHealthandClinicalMedicineSE-90187UMEÅSWEDEN

*Correspondingauthor

Norwayspruceinunfrozensoil.InternationalJournalofFor-estEngineering24(1):60–75.

Zechmann,E.,2013:ContinuousSoundandVibrationAnaly-sis.MatlabCentralFileExchange[Online].Available:http://

www.mathworks.com/matlabcentral/fileexchange/21384-continuous-sound-and-vibration-analysis[AccessedApril112014].

Documents

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