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Originalscientificpaper

Croat. j. for. eng. 37(2016)1 139

Modelling of Downhill Timber Skidding:

Bigger Load – Bigger Slope

Andreja Đuka, Tibor Pentek, Dubravko Horvat, Tomislav Poršinsky

Abstract

Skidder mobility during timber extraction is defined by: 1) basic dimensional features of the vehicle, 2) ability to overcome obstacles during movement, 3) traction performance and 4) environmental soundness. Traction performance depends on the ground conditions (soil bear-ing capacity) and the total effect of all forces on the vehicle. In downhill skidding, the skidder is under great influence of parallel component of forces, adhesion weight and longitudinal terrain slope, which combined result in negative traction force, torque and thrust force. When the horizontal component of rope force is equal to zero i.e. the moment when the weight of the load and resistance to traction are in equilibrium, the slope angle α is a function of load mass distribution factor and skidding resistance factor. This is a »turning point« that can be defined as a critical slope because the load starts to push the vehicle downhill, which results in negative horizontal component of rope force. Depending on skidder Ecotrac 120V dimensional features, centre of gravity, load mass distribution factor, skidding resistance factor of previous research, five different loads were analyzed (1 to 5 tonnes) in order to define the critical slope angle for each of them. Critical slope for downhill skidding of 1 tonne timber is on longitudinal slope of –26%, for 2 tonne timber on –30%, 3 tonne timber on –34%, 4 timber on –38% and for 5 tonne timber on –43% of terrain longitudinal slope. Even though skidding bigger load in-creases vehicle mobility to even greater slope angles, the most important in downhill skidding, is to avoid blocking of the wheels, which will lead to a complete vehicle slippage and the driver must be constantly aware of that fact. The general recommendation should be that skid-ding small loads (1 to 3 tonnes) downhill is suitable for smaller longitudinal terrain slopes (up to maximum –34%), while the heavier the load, the further down the slope the skidder can go. The load of 5 tonnes »anchors« the skidder better and therefore it can go on terrain slopes up to –43%, during which less traction force is used (torque is used for braking) and skidder pulls the load by its own weight. It can be concluded that extending the operating range of skidder onto steeper slopes with heavier loads has the potential to decrease harvesting costs and increase productivity.

Keywords: skidder, downhill timber extraction, rope force, critical slope

conflict«withthesamemacro-topographicconditions.SkiddermobilityisitsabilitytomovefrompointAtopointBwhileachievingitsprimalgoal–timbertrans-port.Intimberextraction,vehiclemobilitycanbecon-sideredfromtwodifferentaspects:1)extractiononsoilsoflimitedbearingcapacity(forexamplelowlandforestsongleysoils)and2)extraction inhillyandmountainousforests,whereslopeandgroundobsta-clesdefineconditionsforapplicationofspecialisedforestryvehicles.Manyparametersdefinevehiclemo-bilityduringtimberextraction(Šušnjar2005,Šušnjar

1. IntroductionTerraintrafficabilityisaterrainpropertythatal-

lowsvehiclemobility,duringwhichvariousterrainfactors(slope,groundobstacles,soilbearingcapacity)showtheirinfluence(JanosiandGreen1968,Eichrodt2003,Suvinen2006,Lubello2008).Fromthestand-pointoftimberharvestingandforestopening,terrainslopeisthemostimportantterrainfactoraffectingthechoiceofaharvestingsystem.Terrainslopeaffectsve-hiclestabilitybecauseallwheels(i.e.tracks)are»in

A. Đuka et al. Modelling of Downhill Timber Skidding: Bigger Load – Bigger Slope (139–150)

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

etal.2010,Poršinskyetal.2012),ofwhichthesefourarethemostimportantones:1)basicdimensionalfea-turesofthevehicle(dimensions,turningradius,mass,centreofgravity,longitudinalandlateralangleofsta-bility,clearance,frameandaxleoscillation,unloadingofthefrontaxle,payloadofrearaxle,tyresloadcapac-ity),2)theabilitytoovercomeobstaclesduringmove-ment(groundclearanceandlateralvehiclestability),3)tractionperformance(dependenceofslip,tractionpowerandspeedtotractionforceandsoilbearingca-pacity) and 4) environmental soundness (nominalgroundpressureandminimalconeindex).Manyscientistsdeterminedcriticalterrainslopes

foraskidderbetween30%and50%,regardlessofex-tractiondirection(MacDonald1999,Heinimann1999),whileothersdifferentiatebetweendownhillandup-hillskidding.So,criticalslopeindownhillskiddingrangesfrom23%to50%(Rowan1977,InoueandTsu-ji2003,Lubello2008)andinuphillskiddingfrom18%to30%(Rowan1977,InoueandTsuji2003,Lubello2008).SomehighlighttheimportanceofloadsizesuchasHippolitiandPiegai(2000),asquotedbyLubello(2008),whoreportedthatanunloadedskiddercanovercomethemaximumgradientof40%,butloadedonlyupto20%regardlessofslopedirection.EgerandKiencke (2003) reported that theeffectofdynamicchangesinloadshouldbealsoconsideredaskeyfac-torsthataffectmachinestability.SarlesandLuppold(1986)statethatwhenskiddinguptheslope,foranyincreaseintheterrainslopeof1%(abovetheterraininclinationof 10%), thequantityofhooked timbershouldbereducedby2.5%.Otherscientistsemphasisetheimportanceofsecondaryforestnetwork.Accord-ingtoHeinimann(1999)ifskidderisextractingtimberonterrainslopeshigherthan35%,itshouldmoveonlyonsecondaryforestroadnetwork.HippolitiandPie-gai(2000)notethepossibilityofskiddingtimberdowntheslopeof60%,butonlyinthecaseofwell-designedandbuiltstriproads.ImportanceofgroundobstaclesandsoilbearingcapacityofforeststandduringtimberextractionbygroundbasedvehiclesishighlightedbyKühmaierandStampfer(2010).Tendencyofanchor-ingvehiclesfortimberextraction,andthusmovingcriticalterrainslopestoevenhigherextents,hasbe-comemoreandmorepopularinthepastcoupleofyears.Sauteretal.(2012)definecriticalterrainslopeas55%fortheskidderwithacraneequippedwiththeadditionalwinchforanchoringthevehicle,andCa-valli (2015) surmised thatwheeledmachineswithchainsorbandsmighthaveanupperlimitof45%,integraltrackmachinesupto60%,andthattetheredmachinesshouldbeabletooperateuptoarangeof75to85%terrainlongitudinalslope.

BesidesdimensionalcharacteristicsdefinedinISOstandard13861 (2000), someauthors (Bekker1969,JanosiandGreen1968,SeverandHorvat1985,USACodeofFederalregulations49CFR523.2)giveaddi-tionalcharacteristicsthatallowbypassingandover-ridingofmacro(slope)andmicro(groundobstacles)terrainpropertiesduringvehicleoff-roadmovement:1)approachangle(thesmallestangle,inaplanesideviewof a vehicle, formedby the level surface onwhichthevehicleisstandingandalinetangenttothefronttyrestaticloadedradiusarcandtouchingtheundersideofthevehicleforwardofthefronttyre),2)departureangle(thesmallestangle,inaplanesideviewof a vehicle, formedby the level surface onwhichthevehicleisstandingandalinetangenttothereartyrestaticloadedradiusarcandtouchingtheundersideofthevehiclerearwardofthereartyre),3)break-overangle(meansthesupplementofthelarg-estangle,intheplansideviewofavehiclethatcanbeformedbytwolinestangenttothefrontandrearstaticloadedradiiarcsandintersectingatapointontheundersideofthevehicle),4)longitudinalclearancediameter(diameterofacirclethattouchestheinnersideofthetyresfromeachaxleandthelowesthangingpointunderavehicle),5)transverseclearancediameter(diameterofacirclethattouchestheinnersideofthetyresandthelowesthangingpointunderavehicle,usuallyadifferential)and6)centreofgravityposition(heightfromground,distancefromfrontandrearaxles),whichisanimportantconstructionalparameterthatinfluencesloaddistributiononaxlesdependingonter-rainslopeduringtimberextraction.VisserandBerkett(2015)statethat,accordingto

Bell(2002),McMahon(2006)andRaymond(2010),ex-tendingtheoperatingrangeofground-basedmachin-eryontosteepslopeshas thepotential todecreaseharvestcostsandimprovesafety.Thesameauthorsconcludeintheirstudyof22machinesandeffectofterrain steepnessduringharvesting, thatmachinesexceedslopelimitscommonlyassociatedwithhar-vestingoperations,andexceed themoftenand forlongerperiodsoftime,whichisinaccordancewithVisserandStampfer(2015),whoclaimthattodaythereisnoguidanceonslopelimits,basedoneitherscienceorexperience.Authorsconcludethatmanyguidelinesrefertomanufacturer’sspecifications,yetfewofthemajor forestry equipment manufacturers provideslopeand/oroperatinglimitsfortheirpurposebuiltmachinery.The goal of defining limiting terrain slopes for

downhilltimberextractionofcableskiddersshouldbeconsideredasguidelinesforoperatorsandplan-ners,whocanthen,dependingonloadsizeandterrain

Modelling of Downhill Timber Skidding: Bigger Load – Bigger Slope (139–150) A. Đuka et al.

Croat. j. for. eng. 37(2016)1 141

Fig. 1 Distribution of forces during timber skidding


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

macrocharacteristics(slope),definebetterroutesforskidderoff-roadmovementprovidingbettercontrolandmanoeuvrabilityofvehicles.

2. Theoretical ApproachDuringskidding,timberispartiallysuspendedon

thevehiclei.e.onepartoftheloadisliftedabovegroundlevelandhangedbyropetotherearendoftheskidder,whiletheotherpartisdragged(trailed)ontheground.Sinceapartoftheloadisonground,onlyapartoftheloadweight isactuallycarriedby theskidderrope.Whileskidding,theforceintheropethatcarriesapartofthetimberweightisthesocalledverticalcomponentofropeforce(V),andforcethatmustovercometractiveresistanceoftimberthatisonthegroundiscalledhor-izontalcomponentofropeforce(H).Duringskidding,theadhesionweightoftheskidderisgreaterthanitsstaticweightastherearaxleofthevehicleisunderad-ditionalinfluenceoftheload,whiletheverticalcompo-nentofropeforceshowsitseffect.Theoreticalapproachtodistributionofforcesdur-

ingskiddingwasestablishedbyBennet(1962),whodifferentiatedhorizontal,verticalandfrictionalforcesinvolvedintimberskiddingofdifferentloads,andsincethenmanyscientistsusedthemintheirownre-

search (Calvert andGarlicki 1967,RichardsonandCooper1970,Hassan1977,Perumpraletal.1977,Sev-er1980,MatthesandWatson1981,HassanandSirois1983,HassanandGustafson1983,Iffetal.1984,Horvat1990,SeverandHorvat1995,ŠušnjarandHorvat2006,Tomašićetal.2007,Tomašićetal.2009,Šušnjaretal.2010,Poršinskyetal.2013).Skiddingtimberonflatterrainbeginsinthemo-

mentwhenthrustforce(broughtbytransmissionsys-tem to thewheels) begins to overcome resistanceforces(Fig.1A):1)skidderrolling,2)rollingofhookedtimberand3)frictionoftimberontheground.Duringskiddinguptheslope(Fig.1C),loaddistri-

butionbecomesmorecomplexand tractionbeginswhenthrustforceovercomesresistanceforces:1)skid-derrolling,2)terrainslope,3)rollingofhookedtim-ber,4)overcomingterrainslopeofhookedtimber,5)frictionoftimberonthegroundand6)overcomingterrainslopeoftimberontheground.Whileskiddingtimberdowntheslope(Fig.1B),

thrustforceovercomesthesameresistanceasforskid-dingtimberuptheslope,onlyresultantsofthethreeforcesofresistance(terrainslope,overcomingterrainslopeofhookedtimber,overcomingterrainslopeoftimberontheground)arenowintheoppositedirec-tion,i.e.directionofthevehiclemovement.

Table 1 Equations of some parameters of downhill and uphill timber skidding

Downhillskidding Uphillskidding

Adhesiveweight

a cosG G Va= ⋅ + (1)

Verticalcomponentofropeforce

cosV k Q a= ⋅ ⋅ (2)

Horizontalcomponentofropeforce

p(1 ) cos sinH Q k Qa m a= ⋅ − ⋅ ⋅ − ⋅ (3) p(1 ) cos sinH Q k Qa m a= ⋅ − ⋅ ⋅ + ⋅ (4)

Frontaxleload

t1

cos sinG a G h H d V cG

La a⋅ ⋅ + ⋅ ⋅ − ⋅ − ⋅

= (5) t1

cos sinG a G h H d V cG

La a⋅ ⋅ − ⋅ ⋅ − ⋅ − ⋅

= (6)

Rearaxleload

t2

cos sin ( )G b G h H d V L cG

La a⋅ ⋅ − ⋅ ⋅ + ⋅ + ⋅ +

= (7) t2

cos sin ( )G b G h H d V L cG

La a⋅ ⋅ + ⋅ ⋅ + ⋅ + ⋅ +

= (8)

Drawbarpull

a= − ⋅v a sinF H G (9) a= + ⋅v a sinF H G (10)


Croat. j. for. eng. 37(2016)1 143

Sinceskiddermovementdynamicsisconsiderablydifferentdependingonextractiondirection,forcesdis-tributionandrelatingequationsarepresentedinTable1 for:adhesiveweight,vertical componentof ropeforce,horizontalcomponentofropeforce,frontaxleload,rearaxleloadanddrawbarpull(tractionforce).Loadmassdistributionfactor(k)showshowmuch

loadmassisliftedfromtheground(hookedontherope)andhowmuchispulledonthegroundsurface(Eq.11).Iftheloadmassdistributionfactoris0.5,thismeansthatthesamepartofthetimbermassishookedbyropeasitispulledontheground.Authors(Sever1980,HassanandGustafson1983,HassanandSirois1983,Iffetal.1984,Horvat1987,Šušnjar2005,Tomašić2007,Poršinskyetal.2012)reportedthatthenatureofloadingandloadmassdistributionfactordependonthesevariables:treediameterandslendernessratio,numberoftreesperload,heightofsuspendedbuttaboveground,treeform(methodoftimberprocess-ing), timber orientation (thinner or thicker end isaboveground).Iftheloadincreases,theportionofitsweightsupportedbythegroundincreasesathigherpercentage.Thisincreaseisalsoattributedtothebuttheightaboveground,whichtendstodecreaseasthenumberoftreesintheloadincreases.Treeweightongroundcontactlengthdecreasesandloadmassdistri-butionfactorincreaseswiththeincreaseintreesemi-suspensionheightaboveground.Loadmassdistribu-tionfactorisunaffectedbytreelengthofupto20m.

cosVk

Q a=

⋅ (11)

Skiddingresistanceoccursduetotheeffectofloadweightpulledonthegroundandskiddingresistancefactor–μp(Hassan1977,Perumpraletal.1977,Sever1980,HassanandGustafson1983,HassanandSirois1983,Samset1985,Šušnjar2005,Tomašić2007).Samset(1975)accordingtoMegille(1954)statedthatskiddingresistancefactordependsonsoiltypeandmoisturelevel, and Samset (1975) according toDahl (1973)claimed that italsodependsonorientationof sus-pendedtimber(thinnerorthickerendisaboveground)andontimberprocessingmethod(full-tree,half-tree,etc.).Thehorizontalcomponentofropeforceover-comestheskiddingresistancebetweentheloadandforestsoilandaccordingtoknownvaluesofforce,weight,loadmassdistributionfactorandterrainslope,skiddingresistancefactorcanbedetermined(Eq.12).

psin

(1 ) cosH Q

Q ka

ma

± ⋅=⋅ − ⋅

(12)

Inexploringskiddertractionfeaturesduringskid-dingdowntheslope,Šušnjaretal.(2010)givesome

limitationsidentifiedthroughtwo»turningpoints«ofterrainslope.Thefirst»turningpoint«isdeterminedbythean-

gleofinclinationoftheterraininwhichvehiclenolongerachievespositivetractionandbreakingforcei.e.thrustforceisequaltozero(Eq.13).

p(1 )tg

G f Q k f k QG Q

ma

⋅ + ⋅ ⋅ + − ⋅ ⋅=

+ (13)

Thesecond»turningpoint«isdeterminedbytheangleofterraininclinationinwhichhookedtimberstarts topushtheskidderdowntheslope(Eq.14),whichoccursatthetimewhenthehorizontalcompo-nentoftheropeforceintheropeisequaltozero(H=0),orwhentheweightoftheload(Q sin α)andtractionresistanceareinbalance.

ptg (1 )ka m= − ⋅ (14)

3. Materials and MethodsValidmodelofskidder–terraininteractionwillper-

mitforestryresearcherstostudyandanalysemanyissuesandproblemsrelatedtoskidderperformanceunder awide rangeof conditions (different loads,variousterraincharacteristics,etc.).Thisway,skidderoptimisationandimprovementofitsoperationalpa-rameters canbe expected. Significanceof skiddersparametersthataffectitsoff-roadperformancecanbeidentifiedwithoutexpensivefieldtesting.Theresultswillnotonlyhelpforestryplannersinbetterforestmanagement,butalsopractitionersinreal-lifesitua-tionsofaskidderoff-roadlocomotion.AnalysiswasdonebasedonskidderEcotrac120V

dimensionsandcentreofgravity(Šušnjar2005),de-pendenceofskidderEcotrac120Vloadmassdistribu-tionfactorandskiddingresistancefactortoaffectingparameters(Poršinskyetal.2012),loaddistributionduringtimberextractionondifferentterrainslopesandfivedifferentloads(from1to5tonnes).Loadmassdistributionfactor(Eq.15)isafunctionof(statisticallyand inversely correlated) loadmass, loadweight,numberoflogsperload,loadvolume.Skiddingresis-tancefactor(Eq.16)isafunction(statisticallyandin-verselycorrelated)ofterrainslopeanddirectionoftimberextractioni.e.uphillordownhillskidding.

0.62017 0.0476k Q= − ⋅ (15)

p 0.50529 0.042m a= − ⋅ (16)Where:Q–Loadmass,tα–Longitudinalterrainslope,%(+,–indicatedi-rectionofskidding)


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

Fig.

2 Te

chnic

al da

ta o

f Eco

trac

120V

skid

der


Croat. j. for. eng. 37(2016)1 145

Loaddistributionwasdeterminedbycalculationandanalysesofthefollowingparameters:1)adhesionweight(Eq.1),2)verticalcomponentofropeforce(Eq.2),3)horizontalcomponentofropeforce(Eq.3),4)loaddistributiononfrontaxle(Eq.5),5)loaddistribu-tiononrearaxle(Eq.7),6)angleofterraininclinationinwhichhookedtimberstarts topush theskidderdowntheslope(Eq.14),and7)tractionforce(Eq.9).SkidderEcotrac120Visafour-wheeled(4×4)ar-

ticulatedforestryvehicle,equippedwithahydraulic

forestwinchHittner2×80,ofthenominaltractiveforceof80kN.Itisdrivenbya6cylinderdieselDEUTZen-ginewiththenominalpowerof84kWat2300min–1andmaximumtorqueof400Nmat1500min–1.Basictechni-caldataofEcotrac120VskidderisgiveninFig.2.

4. Results and DiscussionSkiddertractionperformanceandforcedistribu-

tionduringtimberextractiondependsongainedforc-

Fig. 3 Slope and load influence on skidder adhesion weight and its distribution on both axles


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

esonwheelsandforcesresistingthem,whereadhe-sionweightisaveryimportantparameter.Itactuallyrepresentsthesumofverticalloadsondrivingwheelsduringskidding(Fig.3A).Adhesionweightdependsonskidderweight(G),longitudinalterrainslope(α)andthesizeoftheverticalropeforcecomponent(V),whichisdirectlyinfluencedbyloadweight(Q).Adhe-sionweightisdifferentthanemptyskidderweight(G)becauseskidderrearaxleisadditionallyloadedwiththefullamountoftheverticalropeforcecomponent(V)thatisdispersedtorearwheelsthroughhorizontalrollersofthewinch.Resultsofmodellingloaddistributiononskidder

axles,ontheexampleofskidderEcotrac120V,pointedoutthatloaddistributionvariesduetotheamount(mass)ofhookedtimber,timberextractiondirection(uphillordownhill)andduetolongitudinalterrainslope(Fig.3Band3C).Byincreasinglongitudinalterrainslopeandload

massduringuphilltimberskidding,thereisanin-creaseofloadonrearskidderaxleduetothegrowthofthehorizontalcomponentofskidderweight(G sin α),whichactsagainstthedirectionofvehiclemove-ment,andduetothegrowthofthehorizontalcompo-nentofropeforce(H).Axleloaddistributionoftheskidder,duringuphill

timberextraction,isrelatedtomanycriteria(limits)derivedfrompreviousresearch:1)Unloadingofthe

frontaxle(WeiseandNick2003),whereatleast10%ofthetotaldynamicloadshouldremainonthefrontaxle(G1 >0.1Ga)toretaincontrol;2)Overloadingoftherearaxle(Horvat1990),wherebytheloadoftheskidderrearaxlemustnotexceedthetotalweightoftheskidder(G2 < G);3)Longitudinalskidderstability(Sever1980),whichisdefinedastheminimumratioofloadonfrontandrearaxles(G1:G2>1:3.5),afterwhichlongitudinalstabilityofthevehiclebecomesanissue;4)Permittedtyresloadcapacity,withregardtotheairpressurerecommendedbythemanufacturer(Đuka2014).Indownhillskidding,theloadistransferredfrom

thereartothefrontaxleofaskidder.Increasingter-rainslopeleadstothegrowthofloadontheskidderfrontaxleduetoanincreaseinthehorizontalcompo-nentofskidderweight(Gsinα),whichactsinthedirectionofskiddermovement.Increasingthequan-tity(mass)ofhookedtimberindownhillskiddingwillleadtothereductionoftheloadonthefrontskidderaxle,becauseoftheincreaseoftheverticalcomponentofropeforce(V).Itishardtounderstandthedynamicsofloaddis-

tributiononskidderaxlesregardingweight(mass)ofhookedtimber,directionofskidding(uphill/downhill)andslopeinclination(Fig.3Band3C)withoutknow-ingtheeffectofropeforcei.e.itsverticalcomponent(V) thatcarries thehookedload,anditshorizontal

Fig. 4 Load and slope influence on horizontal and vertical components of rope force


Croat. j. for. eng. 37(2016)1 147

component(H)whichovercomestractiveresistanceoftheloadontheground.Theanalysisofhorizontalandverticalcomponentsofropeforceaccordingtolongi-tudinal terrain slope, skidding direction and loadmassisshowninFig.4.Duringdownhillextractionoftimber,thehorizon-

talcomponentofropeforce(Fig.4A)decreaseswiththeincreaseofterrainslopeandloadmass,whiletheverticalcomponentofropeforce(Fig.4B)increasesonlybyincreaseofloadmassi.e.slightlydecreaseswiththeincreaseofterrainslope(duetoreductionoftheloadthatishookedbywinchropeandincreaseoftheloadoftimberontheground).Duringdownhillskidding,thehorizontalcomponentofropeforceisgreaterthantheverticalcomponentofropeforceforloadmassof1tandterrainslopehigherthan45%,forloadmassof2tandterrainslopehigherthan36%,forloadmassof3tandterrainslopehigherthan27%,forloadmassof4tandterrainslopehigherthan19%,forloadmassof5tandterrainslopehigherthan10%.Throughout downhill skidding, the horizontal

componentofropeforcedecreaseswiththeincreaseof terrainslopeandwith thereductionof load,bywhichtheverticalcomponentofropeforceisalwaysgreaterthanthehorizontalcomponent(Fig.4).Thehorizontalcomponentofropeforcedecreasesduringdownhillskiddingbecauseloadtendstogetclosertorearendoftheskidder,whichmakestheverticalcom-ponentofropeforcemoreimportantbecauseitholdstheloadabovetheground.Therefore,thehorizontal

componentofropeforceissmallerbecauselessloadweightispulledontheground.Animportantcriteriaindownhillskiddingister-

rainslopeinclination(α)whentheloadstartstopushtheskidderi.e.themomentwhenthehorizontalcom-ponent of rope force is zero (H=0).When the loadpushesthevehicledowntheslope,duetotheconstantthrustofthetimberatthebackendofaskidder,itcanbeconcludedthat,induetime,suchperformancewillresultinfatigueofthematerialandearlydamagetothe vehicle (according to FAOoperatinghours forwheeledskidder it isbetween8,000and12,000de-pendingonoperationconditions). Itwillalsohavenegative influence on psycho-physical state of thedriver(asconformedinpatentEP2711226A1(Eskil-sons2014),inthevehicle-driverinteractions,itises-sentialthatthevehiclecarriesoutthedriver’scom-mandsinthemannerbelievedtobedesiredbythedriver).Theturningpointwhenskiddingisnolongerrecommendedforskiddingloadsupto1tisonterrainwithlongitudinalslopeof–26%,forskiddingloadsupto2tonterrainwithlongitudinalslopeof–30%,forskiddingloadsupto3tonterrainwithlongitudinalslopeof–34%,forskiddingloadsupto4tonterrainwithlongitudinalslopeof–38%andforskiddingloadsupto5tonterrainwithlongitudinalslopeof–43%.Inuphillskidding,tractionforceneedstoovercome

theresistanceoftheloadontheground(H),butalsotheresistance of the horizontal component of skidderweight(G sin α),whichpullsthevehicleintheoppositedirection.Withthegrowthoftheinclinationangle,trac-tionforcegrowswiththeincreaseofloadweight,duetoanincreaseofthehorizontalcomponentofropeforce(tractionresistance)andtheweightoftheskidderthatneedstoovercometractionforce(Fig.5).Indownhillskidding,thehorizontalcomponentof

theskidderweight(G sin α)actsinthedirectionoftheskidderandduetoitsactiontheskidderovercomestraction resistance of the load on the ground (H),whichcausestheappearanceofnegativetractionforce(Fig.5)i.e.appearanceofbrakingforce.Resultsofmodellingloaddistributiononskidder

frontandrearaxles,horizontalandverticalcomponentsofrope force,basedondimensioncharacteristicsofskidderEcotrac120V(centreofgravity),knowingloaddistributionandskidderresistancefactors,consideringdifferentquantity(mass)ofhookedtimber,extractiondirection(uphillanddownhillextraction)andlongitu-dinal terrainslope,are inaccordancewithpreviousresearchthatwerebasedonfieldtesting(Šušnjar2005,ŠušnjarandHorvat2006,Tomašić2007,Tomašićetal.2007,Tomašićetal.2009,Šušnjaretal.2010).

Fig. 5 Load and slope influence on traction force


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

5. ConclusionsItcanbestatedthatduringdownhillskiddingno

realtractionforcecanbeachieved(torqueisusedforbraking),becausetheskidderpullstheloadbyitsownweight,andalsothetransferofpowerfromthemotortothewheelsisusedforbrakingduetothelargeim-pactofparallelcomponentoftheskidderweight.Eventhoughskiddingispossibleonevengreater

slopeanglesthanstatedabove,themostimportantindownhillskiddingistoavoidblockingofthewheels,whichwillleadtoacompletevehicleslippage.Whentheloadpushesthevehicledowntheslope,duetotheconstantthrustofthetimberatthebackendofaskid-der,itcanbeconcludedthat,induetime,suchperfor-mancewillresultinfatigueofthematerialandearlydamagetothevehicleaswellasinnegativeinfluenceonthedriver.Thegeneralrecommendationshouldbethatskid-

dingsmallloads(1to3tonne)downhillissuitableforsmallerlongitudinalterrainslopes(uptomaximum–34%),whiletheheaviertheload,thefurtherdowntheslopeskiddercango.Theloadof5tonnes»an-chors«theskidderbetterand,therefore,itcangoonterrainslopesupto–43%,duringwhichlesstractionforceisused(torqueisusedforbraking)andskidderpullstheloadbyitsownweight.Itcanbeconcludedthatextendingtheoperating

range of skidder onto steeper slopeswith heavierloadshasthepotentialtodecreaseharvestingcostsandincreaseproductivity.

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

Received:June9,2015Accepted:June18,2015

Authors’address:

AndrejaĐuka,PhD.e-mail:[email protected],PhD.e-mail:[email protected],PhD.e-mail:[email protected]šinsky,PhD.*e-mail:[email protected],FacultyofForestryUniversityofZagrebSvetošimunska2510002ZagrebCROATIA

*Correspondingauthor

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