Construction Methods - Chapter04.pdf - po ang - Januszek66

LoadingandHauling

4–1ESTIMATINGEQUIPMENTTRAVELTIME

Incalculatingthetimerequiredforahaulunittomakeonecompletecycle,itiscustomary

tobreakthecycledownintofixedandvariablecomponents.

Cycletime % Fixedtime+Variabletime (4–1)

Fixedtime representsthosecomponentsofcycletimeotherthantraveltime.Itincludes

spottime(movingtheunitintopositiontobeginloading),loadtime,maneuvertime,and

dumptime.Fixedtimecanusuallybecloselyestimatedforaparticulartypeofoperation.

Variabletime representsthetraveltimerequiredforaunittohaulmaterialtotheun-

loadingsiteandreturn.Asyouwouldexpect,traveltimewilldependonthevehicle’s

weightandpower,theconditionofthehaulroad,thegradesencountered,andthealtitude

abovesealevel.Thissectionpresentsmethodsforcalculatingavehicle’sresistanceto

movement,itsmaximumspeed,anditstraveltime.Methodsforestimatingfixedtimesare

giveninSections4–2to4–5,whichdescribespecifictypesofhaulingequipment.

RollingResistance

Todeterminethemaximumspeedofavehicleinaspecificsituation,itisnecessarytode-

terminethetotalresistancetomovementofthevehicle.Theresistancethatavehicleen-

countersintravelingoverasurfaceismadeupoftwocomponents,rollingresistanceand

graderesistance.

Totalresistance % Graderesistance+Rollingresistance (4–2)

Resistancemaybeexpressedineitherpoundspertonofvehicleweight(kilogramspermet-

ricton)orinpounds(kilograms).Toavoidconfusion,theterm resistancefactor willbeused

inthischaptertodenoteresistanceinlb/ton(kg/t). Rollingresistance isprimarilyduetotire

flexingandpenetrationofthetravelsurface.Therollingresistancefactorforarubber-tired

vehicleequippedwithconventionaltiresmovingoverahard,smooth,levelsurfacehasbeen

foundtobeabout40lb/tonofvehicleweight(20kg/t).Forvehiclesequippedwithradial

tires,therollingresistancefactormaybeaslowas30lb/ton(15kg/t).Ithasbeenfoundthat

CHAPTER4

Table4–1 Typicalvaluesofrollingresistancefactor

RollingResistanceFactor

TypeofSurface

lb/ton kg/t

Concreteorasphalt 40(30)* 20(15)

Firm,smooth,flexingslightlyunderload 64(52) 32(26)

Rutteddirtroadway,1–2in.penetration 100 50

Soft,rutteddirt,3–4in.penetration

150 75

Loosesandorgravel

200 100

Soft,muddy,deeplyrutted

300–400 150–200

*Valuesinparenthesesareforradialtires.

therollingresistancefactorincreasesabout30lb/ton(15kg/t)foreachinch(2.5cm)oftire

penetration.Thisleadstothefollowingequationforestimatingrollingresistancefactors:

Rollingresistancefactor(lb/ton) % 40+(30 $ in.penetration) (4–3 A

Rollingresistancefactor(kg/t) % 20+(6 $ cmpenetration) (4–3 B

Therollingresistanceinpounds(kilograms)maybefoundbymultiplyingtherollingre-

sistancefactorbythevehicle’sweightintons(metrictons).Table4–1providestypicalval-

uesfortherollingresistancefactorinconstructionsituations.

Crawlertractorsmaybethoughtofastravelingoveraroadcreatedbytheirown

tracks.Asaresult,crawlertractorsareusuallyconsideredtohavenorollingresistance

whencalculatingvehicleresistanceandperformance.Actually,ofcourse,therollingre-

sistanceofcrawlertractorsdoesvarysomewhatbetweendifferentsurfaces.However,the

standardmethodforratingcrawlertractorpower(drawbarhorsepower)measures

thepoweractuallyproducedatthehitchwhenoperatingonastandardsurface.Thusthe

rollingresistanceofthetractoroverthestandardsurfacehasalreadybeensubtractedfrom

thetractor’sperformance.Althoughacrawlertractorisconsideredtohavenorollingre-

sistance,whenittowsawheeledvehicle(suchasascraperorcompactor)therollingre-

sistanceofthetowedvehiclemustbeconsideredincalculatingthetotalresistanceofthe

combination.

GradeResistance

Graderesistance representsthatcomponentofvehicleweightwhichactsparalleltoanin-

clinedsurface.Whenthevehicleistravelingupagrade,graderesistanceispositive.When

travelingdownhill,graderesistanceisnegative.Theexactvalueofgraderesistancemaybe

foundbymultiplyingthevehicle’sweightbythesineoftheanglethattheroadsurface

makeswiththehorizontal.However,forthegradesusuallyencounteredinconstruction,it

issufficientlyaccuratetousetheapproximationofEquation4–4.Thatis,a1%grade(rep-

resentingariseof1unitin100unitsofhorizontaldistance)isconsideredtohaveagrade

resistanceequalto1%ofthevehicle’sweight.Thiscorrespondstoagraderesistancefac-

torof20lb/ton(10kg/t)foreach1%ofgrade.

LOADINGANDHAULING

Graderesistancefactor(lb/ton) % 20 $ grade(%)

(4–4 A

Graderesistancefactor(kg/t) % 10 $ grade(%)

(4–4 B

Graderesistance(lborkg)maybecalculatedusingEquation4–5or4–6.

Graderesistance(lb) % Vehicleweight(tons) $ Grade

resistancefactor(lb/ton)

(4–5 A

Graderesistance(kg) % Vehicleweight(t) $ Graderesistancefactor(kg/t) (4–5 B

Graderesistance(lb) % Vehicleweight(lb) $ Grade

(4–6 A

Graderesistance(kg) % Vehicleweight(kg) $ Grade

(4–6 B

EffectiveGrade

Thetotalresistancetomovementofavehicle(thesumofitsrollingresistanceandgrade

resistance)maybeexpressedinpoundsorkilograms.However,asomewhatsimpler

methodforexpressingtotalresistanceistostateitasagrade(%),whichwouldhaveagrade

resistanceequivalenttothetotalresistanceactuallyencountered.Thismethodofexpress-

ingtotalresistanceisreferredtoas effectivegrade,equivalentgrade, or percenttotalre-

sistance andisoftenusedinmanufacturers’performancecharts.Effectivegrademaybe

easilycalculatedbyuseofEquation4–7.

" Rollingresistancefactor 1 lb > ton 2

Effectivegrade 1

% Grade 1

(4–7 A

" Rollingresistancefactor 1 kg >

Effectivegrade 1

% Grade 1

(4–7 B

EXAMPLE4–1

Awheeltractor-scraperweighing100tons(91t)isbeingoperatedonahaulroadwithatire

penetrationof2in.(5cm).Whatisthetotalresistance(lbandkg)andeffectivegradewhen

(a)thescraperisascendingaslopeof5%;(b)thescraperisdescendingaslopeof5%?

SOLUTION

Rollingresistancefactor % 40+(30 $ 2) % 100lb/ton (Eq4–3

% 20 "

% 50 1 kg > t 24

(Eq4–3

Rollingresistance % 100 1 lb > ton 2

$ 100 1 tons 2

% 10,000lb

% 50 1 kg >

$ 91 1

% 4550kg 4

2 Graderesistance % 100 1 tons 2

$ 2000 1 lb > ton 2

$ 0.05

(Eq4–6

% 10,000lb

3 % 91 1

$ 1000 1 kg >

$ 0.05 % 4550kg 4

(Eq4–6

Totalresistance % 10,000lb " 10,000lb % 20,000lb

(Eq4–2)

3 % 4550kg " 4550kg % 9100kg 4

CHAPTER4

" 100

Effectivegrade %

20 % 10%

(Eq4–7 A

2 Graderesistance % 100 1 tons 2

$ 2000 1 lb > ton 2

1 # 0.05 2

(Eq4–6 A

%# 10,000lb

3 % 91 1

$ 1000 1 kg >

1 # 0.05 2 %# 4550kg 4

(Eq4–6 B

Totalresistance %# 10,000lb " 10,000lb % 0lb

3 %# 4550kg " 4550kg % 0kg 4

(Eq4–2)

" 100

Effectivegrade %# 5

20 %

(Eq4–7 A

" 50

B%# 5

10 %

(Eq4–7 B

EXAMPLE4–2

Acrawlertractorweighing80,000lb(36t)istowingarubber-tiredscraperweighing

100,000lb(45.5t)upagradeof4%.Whatisthetotalresistance(lbandkg)ofthecombi-

nationiftherollingresistancefactoris100lb/ton(50kg/t)?

SOLUTION

% 100,000 1 lb 2

2000 1 lb > ton 2

Rollingresistance 1 neglectcrawler 2

$ 100 1 lb > ton 2

% 5000lb

3 % 45.5 1

$ 50 1 kg >

% 2275kg 4

Graderesistance % 180,000 $ 0.04 % 7200lb

(Eq4–6 A

3 % 81.5 $ 1000kg >

$ 0.04 % 3260kg 4

(Eq4– B

Totalresistance % 5000 " 7200 % 12,200lb

(Eq4–2)

3 % 2275 " 3260 % 5535kg 4

EffectofAltitude

Allinternalcombustionengineslosepowerastheirelevationabovesealevelincreasesbe-

causeofthedecreaseddensityofairathigherelevations.Thereissomevariationintheper-

formanceoftwo-cycleandfour-cyclenaturallyaspiratedandturbochargeddieselengines.

However,enginepowerdecreasesapproximately3%foreach1000ft(305m)increasein

altitudeabovethemaximumaltitudeatwhichfullratedpowerisdelivered.Turbocharged

enginesaremoreefficientathigheraltitudethanarenaturallyaspiratedenginesandmay

deliverfullratedpoweruptoanaltitudeof10,000ft(3050m)ormore.

Manufacturersusea deratingfactor toexpresspercentageofreductioninratedve-

hiclepoweratvariousaltitudes.Wheneverpossible,usethemanufacturer’sderatingtable

forestimatingvehicleperformance.However,whenderatingtablesarenotavailable,the

LOADINGANDHAULING

deratingfactorobtainedbytheuseofEquation4–8issufficientlyaccurateforestimating

theperformanceofnaturallyaspiratedengines.

B Altitude 1 ft 2 # 3000 *

Deratingfactor 1

(4–8 A

1000 R

# 915 *

% Altitude 1

Deratingfactor 1

(4–8 B

102

Thepercentageofratedpoweravailableis,ofcourse,100minusthederatingfactor.Theuse

ofderatingfactorsindeterminingmaximumvehiclepowerisillustratedinExample4–3.

EffectofTraction

Thepoweravailabletomoveavehicleanditsloadisexpressedas rimpull forwheelvehicles

and drawbarpull forcrawlertractors.Rimpullisthepullavailableattherimofthedriving

wheelsunderratedconditions.Sinceitisassumedthatnoslippageofthetiresontherimswill

occur,thisisalsothepoweravailableatthesurfaceofthetires.Drawbarpullisthepower

availableatthehitchofacrawlertractoroperatingunderstandardconditions.Operationatin-

creasedaltitudemayreducethemaximumpullofavehicle,asexplainedinthepreviouspara-

graph.Anotherfactorlimitingtheusablepowerofavehicleisthemaximumtractionthatcan

bedevelopedbetweenthedrivingwheelsortracksandtheroadsurface.Tractiondependson

thecoefficientoftractionandtheweightonthedriversasexpressedbyEquation4–9.This

representsthemaximumpullthatavehiclecandevelop,regardlessofvehiclehorsepower.

Maximumusablepull % Coefficientoftraction $ Weightondrivers (4–9)

Forcrawlertractorsandall-wheel-driverubber-tiredequipment,theweightonthedrivers

isthetotalvehicleweight.Forothertypesofvehicles,consultthemanufacturer’sspecifi-

cationstodeterminetheweightonthedrivers.Typicalvaluesofcoefficientoftractionfor

commonsurfacesaregiveninTable4–2.

Table4–2 Typicalvaluesofcoefficientoftraction

Rubber

TypeofSurface Tires Tracks

Concrete,dry 0 .90 0.45

Concrete,wet 0 .80 0.45

Earthorclayloam,dry 0 .60 0.90

Earthorclayloam,wet 0 .45 0.70

Gravel,loose 0.35 0.50

Quarrypit 0.65 0.55

Sand,dry,loose 0.25 0.30

Sand,wet

0 .40 0.50

Snow,packed

0 .20 0.25

Ice

0.10 0.15

*Substitutemaximumaltitudeforratedperformance,ifknown.

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