Scrapers lectures

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• A. J. Clark School of Engineering •Department of Civil and Environmental Engineering

Sixth EditionCHAPTER

7

Construction Planning, Equipment,and Methods

S R PERS R PERS

ByByDr. Ibrahim Assakkaf Dr. Ibrahim Assakkaf

ENCEENCE 420420 – – Construction Equipment and MethodsConstruction Equipment and Methods

Spring 2003Spring 2003Department of Civil and Environmental EngineeringDepartment of Civil and Environmental Engineering

University of Maryland, College ParkUniversity of Maryland, College Park

CHAPTER 7. SCRAPERSENCE 420 ©Assakkaf

Slide No. 1

TRACTORSTRACTORS --PULLEDPULLEDSCRAPERSSCRAPERS

Tractor Tractor --pulled scrapers arepulled scrapers aredesigned to load, haul, anddesigned to load, haul, and

dump loose materials.dump loose materials.The advantage of tractor The advantage of tractor --scraper combinations is theirscraper combinations is theirversatility.versatility.


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Slide No. 2

SCRAPERSSCRAPERS


Slide No. 3

Dozer : short haul, less than 300ft

Scraper : medium haul up to 3,000 ft

APPLICATIONSAPPLICATIONS


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Slide No. 4


They can be used in a wide rangeThey can be used in a wide rangeof material types (including shotof material types (including shotrock) and are economical over arock) and are economical over awide range of haul lengths and haulwide range of haul lengths and haulconditions.conditions.To the extent that they can self To the extent that they can self --load, they are not dependent onload, they are not dependent onother equipment.other equipment.


Slide No. 5


If one machine in the spreadIf one machine in the spreadexperience a temporary breakdownexperience a temporary breakdownit will not shut down the job, asit will not shut down the job, aswould be the case for a machinewould be the case for a machinewhich is used exclusively forwhich is used exclusively forloading. If the loader breaks down,loading. If the loader breaks down,the entire job must stop until repairsthe entire job must stop until repairscan be made.can be made.


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Slide No. 6


Scrapers are available with looseScrapers are available with loose --heaped capacities up to about 44 cu yd,heaped capacities up to about 44 cu yd,although in the past a few machines asalthough in the past a few machines aslarge as 100 cu yd have been offered.large as 100 cu yd have been offered.For off For off --highway situations having haulshighway situations having haulsof less than a mile, scrapers ability bothof less than a mile, scrapers ability bothto load and haul gives them anto load and haul gives them anadvantage.advantage.


Slide No. 7

TYPES OF SCRAPERSTYPES OF SCRAPERS

There are several types ofThere are several types ofscrapers, primarily classifiedscrapers, primarily classifiedaccording to the number ofaccording to the number of

powered axles or by the method ofpowered axles or by the method ofloading.loading.Scrapers are all wheelScrapers are all wheel --tractor tractor --pulled machines.pulled machines.


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Slide No. 8


Available types include:Available types include:Push-loaded (conventional)

Single-powered axleTandem-powered axles

Self-loadingPush-pull, tandem-powered axlesElevating

Auger


Slide No. 9

CONFIGURATIONSCONFIGURATIONSConventional (push-loaded)

Single engine


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Slide No. 10


Many models can achieve speeds up to 30Many models can achieve speeds up to 30mph when fully loaded. This extends themph when fully loaded. This extends theeconomic haul distance of the units.economic haul distance of the units.PushPush --loaded scrapers are at aloaded scrapers are at adisadvantage when it comes to individuallydisadvantage when it comes to individuallyproviding the highproviding the high tractivetractive effort requiredeffort requiredfor economical loading. For the singlefor economical loading. For the single --powered axle scraper only a portion, onpowered axle scraper only a portion, onthe order of 50the order of 50 --55% of the total loaded55% of the total loaded

weight, bears on the drive wheels.weight, bears on the drive wheels.


Slide No. 11


Conventional (push-loaded)single engine scrapers become

uneconomical when:

Haul grades > 5%

Return grades > 12%


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Slide No. 12

CONFIGUR TIONS

Elevating


Slide No. 13

CONFIGUR TIONS

Elevating scrapers aregood for short haulsand in favorablematerial.

• Can work alone in the cut.• Cost more initially & to operate• Elevator adds weight & takes power.


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Slide No. 14

CONFIGUR TIONS

Tandem poweredtwin engine


Slide No. 15


Tandem powered ( twinengine ) scrapers are goodfor jobs having adversegrades and poor footing.

Owning and operating costare about 25% higher.


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Slide No. 16

CONFIGUR TIONS

Push-Pull


Slide No. 17


Push-Pull scrapers can workas a team or can operateindividually with a pusher.

Tire wear will increase in rockor abrasive materials becauseof more slippage from thefour-wheel drive action.


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Slide No. 18

Auger

CONFIGUR TIONS


Slide No. 19

CONFIGUR TIONS

Auger


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Slide No. 20


Auger scrapers can self-load indifficult conditions, laminatedrock or granular materials.The auger adds weight to thescraper during travel and it ismore costly to own and operatethan a conventional scraper.


Slide No. 21

VOLUME OF A SCRAPER VOLUME OF A SCRAPER

The volumetric load of a scraperThe volumetric load of a scrapermay be specified as either themay be specified as either thestruck struck oror heaped heaped capacity of thecapacity of thebowl expressed in cubic yards.bowl expressed in cubic yards.TheThe struck capacity struck capacity is the volumeis the volumethat a scraper would hold if the topthat a scraper would hold if the topof the material were struck off evenof the material were struck off evenat the top of the bowl.at the top of the bowl.


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Slide No. 22

BOWL BOWL

ejector

The load-carrying part of a scraper.


Slide No. 23


In specifying theIn specifying the heaped capacity heaped capacity of a scraper, manufacturers usuallyof a scraper, manufacturers usuallyspecify the slope of the materialspecify the slope of the materialabove the sides of the bowl with theabove the sides of the bowl with thedesignationdesignation SAESAE . The Society of. The Society of

Automotive Engineers ( Automotive Engineers ( SAESAE ))specifies a repose slope of 1:1 forspecifies a repose slope of 1:1 forscrapers.scrapers.


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Slide No. 24


TheThe SAESAE standard for other haul unitsstandard for other haul unitsand loader buckets is 2:1.and loader buckets is 2:1.

Actual repose slope will vary with the Actual repose slope will vary with thetype of material handled.type of material handled.The capacity of a scraper, expressed inThe capacity of a scraper, expressed incubic yards bank measure (cubic yards bank measure ( bcybcy ), can be), can beapproximated by multiplying the looseapproximated by multiplying the loosevolume in the scraper by an appropriatevolume in the scraper by an appropriate

swell factor.swell factor.


Slide No. 25


Because of the compacting effectBecause of the compacting effecton the material in a pushon the material in a push --loadedloadedscraper, resulting from the pressurescraper, resulting from the pressure

required to force additional materialrequired to force additional materialinto the bowl, the swell is usuallyinto the bowl, the swell is usuallyless than that for material droppedless than that for material droppedinto a truck by a hoe or loader.into a truck by a hoe or loader.


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Slide No. 28

SWELL FACTORS FORSWELL FACTORS FORDIFFERENT TYPES OF SOILDIFFERENT TYPES OF SOIL

ExampleExampleIf a pushIf a push --loaded scraper hauls aloaded scraper hauls aheaped load measuring 22.5 cu yd andheaped load measuring 22.5 cu yd andthe swell factor from the previous tablethe swell factor from the previous table(Table 1) is 0.8, the calculated bank(Table 1) is 0.8, the calculated bankmeasure volume will bemeasure volume will be

22.5 cu yd x (0.8 x 1.1) = 19.822.5 cu yd x (0.8 x 1.1) = 19.8 bcybcyNote: 10% increase in for swell factor


Slide No. 29

SCRAPER OPERATIONSCRAPER OPERATION

A scraper is loaded A scraper is loaded by lowering the frontby lowering the frontend of the bowl until the cutting edge, whichend of the bowl until the cutting edge, whichis attached to and extends across the width ofis attached to and extends across the width ofthe bowl enters the ground. At the samethe bowl enters the ground. At the sametime, the front apron is raised to provide antime, the front apron is raised to provide anopen slot through which the earth can flowopen slot through which the earth can flowinto the bowl. As the scraper moves forward,into the bowl. As the scraper moves forward,a horizontal strip of material is forced into thea horizontal strip of material is forced into thebowl. This is continued until the bowl is filled,bowl. This is continued until the bowl is filled,at which point the cutting edge is raised andat which point the cutting edge is raised andthe apron is lowered to prevent spillagethe apron is lowered to prevent spillageduring the haul.during the haul.


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Slide No. 30

SCRAPER OPERATIONSCRAPER OPERATION

The dumping operationThe dumping operation consistsconsistsof lowering the cutting edge to theof lowering the cutting edge to thedesired height above the fill, raisingdesired height above the fill, raisingthe apron, and then forcing thethe apron, and then forcing thematerial out by means of a movablematerial out by means of a movableejector mounted at the rear of theejector mounted at the rear of thebowl.bowl.


Slide No. 31

CUTTING AND LOADING

For maximum production both single- and tandem-engine scrapers need the assistance of a push tractor.


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Slide No. 32

CUTTING AND LOADING

Dirt enters horizontally and rolls back to fill corners.Curved ejector top keeps load “boiling” to heap high.

Ejector


Slide No. 33

H ULING ULING

Apron lowered to capture the material.Keeping the bowl low enhances stability.


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Slide No. 34

SPREADING THE LOAD

Dumping and spreading is one continuous operation.


Slide No. 35

SCRAPER SELECTIONSCRAPER SELECTION

Three main factors:Three main factors:Job size, volume of material to

move and maneuver room.Scraper configuration.Job conditions, grades, rollingresistance and material type.


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Slide No. 36

SCRAPER SELECTIONSCRAPER SELECTION

COST:Cost must consider allhourly cost for theentire pusher-scraper

fleet.


Slide No. 37ECONOMIC ZONES OF

PPLIC TION

HAUL DISTANCE

S E V E R I T

Y O F

C O N D I T I O N S


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Slide No. 38

P E R F

O R M

A N C E

C H A R

T


Slide No. 39

CYCLE TIME FOR ACYCLE TIME FOR ASCRAPER SCRAPER

The cycle time for a scraper is thetime to load, haul, dump, turn,return, and turn back into positionto pick up another load:

T s = load t + haul t + dump t + turn t + return t + turn t (1)


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Slide No. 40

WORK CYCLE

LOAD

DUMP

HAUL

RETURN


Slide No. 41


Average load time for push Average load time for push --loaderloaderscrapers in common earth is 0.85scrapers in common earth is 0.85min.min.BothBoth haul haul andand returnreturn timestimesdepend on the distance traveleddepend on the distance traveledand the scraper speed.and the scraper speed.


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Slide No. 42


DumpDump times vary with scraper sizetimes vary with scraper sizebut project conditions will affect thebut project conditions will affect theduration. Average values for dumpduration. Average values for dumptime are presented in tables.time are presented in tables.The averageThe average turnturn time in the cut istime in the cut is0.300.30 min and on the fill the averagemin and on the fill the averagetime istime is 0.210.21 min.min.


Slide No. 43


Table 2. Scraper Dump Cycle Times (U.S. Department of Transportation, FHWA)Scraper Type

Scraper Size(cu yd)

Single Engine(min)

Tandem-powered(min)


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Slide No. 46

OPERATING EFFICIENCYOPERATING EFFICIENCYAND PRODUCTIONAND PRODUCTION

The volume of material hauled wouldThe volume of material hauled wouldequal the product of the number of tripsequal the product of the number of tripsand the average volume per load.and the average volume per load.Reality is different from this perfectReality is different from this perfectproduction. Delays occur on the job;production. Delays occur on the job;machines break down; men stop formachines break down; men stop forwater, or machines cluster causing awater, or machines cluster causing aqueue in the cut for the pusher.queue in the cut for the pusher.


Slide No. 47

EFFICIENCY F CTOR

• 50 min-hrefficiency


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Slide No. 48


Studies of scraper operations suggestStudies of scraper operations suggestthat thethat the average productive time is 69%average productive time is 69%of a perfect 60of a perfect 60 --min hour min hour (excluding(excludingweather delays to the job).weather delays to the job).The actual number of trips in the realThe actual number of trips in the realworld would beworld would be

10.9 X 0.69 = 7.5 trips per work hour


Slide No. 49


The estimator should always try toThe estimator should always try tovisualize the work site before applying avisualize the work site before applying afactor:factor:

If the pit will not be congested and if thedump area is wide open, a 55-min hour may be appropriate.If the cut involves a tight area, such asa ditch, the estimator should consider a45-min hour .


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Slide No. 50

PUSH TRACTORSPUSH TRACTORSREQUIREDREQUIRED

If pushIf push --loaded tractor scrapers areloaded tractor scrapers areto attain their volumetric capacities,to attain their volumetric capacities,they need the assistance of a pushthey need the assistance of a pushtractor during the loading operation.tractor during the loading operation.Push tractor assistance will reducePush tractor assistance will reducethe loadingthe loading --time duration, andtime duration, andthereby will reduce the total cyclethereby will reduce the total cycletime.time.


Slide No. 51

PUSH TRACTORSPUSH TRACTORSREQUIREDREQUIRED

When using push tractors, weWhen using push tractors, weshould match the number ofshould match the number ofpushers with the number ofpushers with the number ofscrapers available at a given time.scrapers available at a given time.If either the pusher or the scraperIf either the pusher or the scrapermust wait for the other, operatingmust wait for the other, operatingefficiency is lowered and productionefficiency is lowered and productioncosts are increased.costs are increased.


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Slide No. 52

PUSHER CYCLE TIMEPUSHER CYCLE TIME

Caterpillar recommends calculatingCaterpillar recommends calculatingbacktrack pushbacktrack push --tractor cycle time,tractor cycle time,T T p p , by the formula, by the formula

T p = 1.4 Lt + 0.25

where Lt is the scraper load time (pusher contact time).

(2)


Slide No. 53


The formula is based on theThe formula is based on theconcept that pusher cycle time is aconcept that pusher cycle time is afunction of four components:function of four components:

Load time of the scraper Boost time, time assisting the scraperout of the cut, 0.15 min.Maneuver time, 40% of load time.Positioning for contact time, 0.10 min.


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Slide No. 54


The number of scrapers that apush tractor can serve is theratio of the scraper cycle time tothe pusher cycle time:

where N is the number of scrapers per one pusher.

p

s

T

T N = (3)


Slide No. 55

ROLLING RESISTANCE ANDROLLING RESISTANCE ANDSCRAPER PRODUCTIONSCRAPER PRODUCTION

Rolling resistance varies with haulRolling resistance varies with haul --road conditions.road conditions.

A well maintained haul road permits A well maintained haul road permitsfaster travel speeds and reducesfaster travel speeds and reducesthe costs of maintenance andthe costs of maintenance andrepairs for the scrapers.repairs for the scrapers.


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Slide No. 56


The following figure is from fieldThe following figure is from fieldstudy of scraper haul times:study of scraper haul times:

Shaded area represents the range ofaverage travel times.Good haul roads are indicated by the

lower boundary.Poor haul roads are indicated by the

upper boundary.


Slide No. 57


Figure 1. Average Travel Times Single-powered Axle Scrapers,Capacity < 25 cy (U.S. Department of Transportation, FHWA)


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Slide No. 58

Example 2Example 2Based on the the scraper specified in the following figures (Figures 4-8,4-10, and 6-9 of the textbook), and for haul conditions as stated below, whatis the probable production of the scraper?Given that the total length of haul when moving from the cut to the fill is4,000 ft as follows:

1,200 +4% grade1,400 ft +2% grade1,400 ft -2% grade

Soil: Clay 3,100 lb per bcyRolling Resistance 80 lb per ton

Assumptions:average load time = 0.85 min

→

load of 96% of the capacity (Figure 2, or Figure 7.10 Textbook)


Slide No. 59

Example 2 (cont’d)Example 2 (cont’d)Figure 2 (for Example 2)


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Slide No. 60

Example 2 (cont’d)Example 2 (cont’d)Figure 3 (for Example 2)


Slide No. 61

Figure 4

(for Example 2)

Example 2 (cont’d)Example 2 (cont’d)


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Slide No. 62

Figure 5(for Example 2)



Slide No. 63


Actual Load = 0.96 X 31 = 29.8 lcy

Swell Factor = 0.74, clay (Table 1, or Table 4-1 in Textbook)

Load Volume in Bank Measure = 29.8 X (0.74 X 1.1) = 24.3 bcy

Weight of Load = 24.3 (3,100) = 75,330 lb

Weight of Scraper (empty) = 96,880 lb (see Figure 3)

Gross Weight = 172,210 lb

%42080

lb/ton20lb/toninResistanceRolling

ResistanceRolling ===


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Slide No. 64

Example 2 (cont’d)Example 2 (cont’d)Table 3. Haul and Return Speeds

Distance (ft) Grade (%)

Total

Resistance (%) R = rr + gr

Speed (mph) Travel Time (min)

Haul (172,210 lb): 300 acceleration 4 8 5 0.68 900 4 8 10 1.021,400 2 6 15 1.061,200 -2 2 30 0.45 200 deceleration -2 2 12 0.19

Total 3.40Return (96,880 lb): 200 acceleration 2 6 10 0.231,200 2 6 21 0.651,400 -2 2 32 0.50

1,000 -4 0 32 0.36 200 deceleration -4 0 12 0.19Total 1.93


Slide No. 65


Calculate the total time required for a single-engine31-cu-yd scraper to complete a 4,000-ft haul cycle.

T s = load t + haul t + dump t + turn t + return t + turn t

Load t = 0.85 min

Haul t = 3.40 min

Dump t = 0.37 31 cu yd (Table 2)

Turn t = 0.21 min @ fill

Return t = 1.93 min

Turn t = 0.30 min @ cut

Hence, Total Cycle Time = T s = 7.06 min


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Slide No. 66


hr bcy

843(24.3)34.7Production

contacts7.341.44

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scrapers59.41.447.06

RequiredScrapersof Number

min44.125.0)85.0(4.125.04.1ePusher tim

hr bcy 17224.3(7.08)hour min-50 perscraper perVolume

trips08.77.0650

hour min-50 pertripsof Number

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s

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T

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LT


Slide No. 67


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