Chapter 2 -Scrapers.pdf

Scrapers

1. SCRAPERS

2. Thescraperisanearth-movingmachinedesignedforlayer-by-layerseparationofsoil
fromthemassif,itscapture,transportationandlayingwithalayerofagiventhickness.
Inengineering,machineformovingearthovershortdistances(uptoabouttwomiles)over
relativelysmoothareas.Eitherself-propelledortowed,itconsistsofawagonwithagate
havingabladedbottom.Thebladescrapesupearthasthewagonpushesforwardandforces
theexcavatedmaterialintothewagon.Whenthewagonisfilled,thegateisclosed,andthe
materialiscarriedtotheplaceofdisposal.Thescraperisthedominanttoolinhighway
construction.
Equipped with a large bowl with a cutting edge attached to the bottom, scrapers can cut, self-load, haul, and spread a
great amount of material. They are most efficient when operated in light and medium materials that are nearly free of
roots, stumps, and boulders. Heavy or consolidated materials require ripper-equipped dozers to rip open the surface
and assist loading operations by pushing the scraper through the cut to achieve maximum loading. A dozer pushing the
scraper is referred to as a push cat.

3. COMPROMISE BETWEEN BEST LOADING AND BEST HAULING MACHINES
UNIQUE FOR LONG DISTANCE HAULING
IT ITSELF DIGS, HAULS, DEPOSIT THE MATERIALS IN UNIFORM THICK
LAYERS
TYPES:
•CRAWLER TRACTOR PULLED
•WHEEL TRACTOR PULLED
BASIC PARTS:
•BOWL, APRON, EJECTOR (TAIL GATE).
CAPACITY:
•HEAPED CAPACITY OF THE BOWL.
OPERATION:
•WHILE DIGGING CUTTING EDGE ENTERS THE GROUND BY LOWERING THE
BOWL, THE FRONT APRON IS RAISED
•EDGE IS RAISED, APRON IS LOWED WHILE HAULING.
•WHILE DUMPING EJECTOR IS USED

4. Trailed scraper workflow

5. Self-propelled scraperCATERPILLAR651

6. Appointment
-building of embankments of roadbed from lateral reserves or earth pits;
-the formation of excavations with the removal of soil in the embankment or cavalier;
-layout of construction sites;
-cutting off the vegetation layer of the soil in the road right-of-way.
Scraper work cycle
loading Hauling Unloading Return to the cut
Loading
Whenloading,enterthecutwiththeejectorpositionedattherearofthebowl,opentheapronenoughtoallow
materialtoenterthebowl(normally4to8inchesabovetheleadingedgeofthebowl),andthenlowerthebowlto
cutadepthof1to11/2inches.Thegearthatthetransmissionisengagedindependsonthenatureofthe
materialbeingcut.Forlight,loosematerial,arelativelyhighgearcanbeused.Forheavy,compactedmaterial,
alowgearisused; however,toobtainafullload,usealowergear,eveninloosematerial.Asthescraper
proceedsthroughthecut,thematerialisloosenedbythescrapercuttingedgesandforcedintothebowlbythe
forwardmotionofthescraper,asshowninFigure1
Figure 1-Entering the cut

7. Figure 2- Loading the bowl
Figure 3 -Loaded bowl
Hauling
After the scraper is fully loaded and has reached the haul road, raise the bowl to travel height and proceed to
the fill or dump area in the highest gear range practicable. The bowl travel height should be no higher than
needed to clear any obstacles on the haul road. A low bowl height allows better control of the scraper by
keeping the center of gravity low and preventing the loss of time needed to lower the bowl as the scraper
approaches the fill area. The best bowl height is the height at which the bowl must be when the load is spread.
When hauling down steep grades, lower the bowl until the cutting edge drags to slow the scraper down. When
traveling over a slippery haul road, keep the bowl as low as possible to allow for a fast emergency stop by
dropping the bowl.
When traveling over haul roads, avoid holes and large obstacles that may damage the scraper tires. When
making sharp turns, allow enough clearance for the length and width of the scraper to keep the scraper wheels
on the road.

8. SpreadingorUnloading
Whenapproachingthefillarea,lowerorraisethescraperbowltothedepthoffilldesired.Alsoadjustthespeed
ofthescraperforthisdepth,suchasahighspeedforathinspreadoraslowerspeedforadeepspread.
Tostartspreading,raisetheapronbyengagingtheaproncontrollevertoallowthematerialtofalloutofthe
bowl.Thesizeoftheapronopeningdependsmainlyuponthedepthofthespreadandtypeofmaterialbeing
spread,forexample,athinlayeroffree-flowingsandneedsafairlysmallapronopeningandahightravel
speed,whileathinspreadofwetclaywillneedalargeropeningandaslowertravelspeed.
Aftertheapronopeninghasbeenadjustedandthedirtflowingthroughtheopeninglessens,engagetheejector
levertofinishunloadingthescraperbowl.Whenthescraperisempty,engagetheejectorlevertoreturnthe
ejectortotherearofthebowlandlowertheapron.
Unloadingtechniquesareasfollows:
Keepthescrapermovingwhileunloading.Stoppingwhenunloadingonsoftfillcostsproductiontimeby
needlessshiftingandthepossiblemiringdownofthescraper.
Alwaysmakeanevenspread,sothenexttripwillnotberough.
Ifpossible,whentravelingoutofafill,passbackovertheareayouhavejustfilledtocompactitwiththelarge
scrapertires.
ReturningtotheCut
Afterunloadingthescraperandreachingthehaulroad,returntothecutassoonaspossible.Whenreturning
tothecut,carrythescraperbowlhighenoughtoavoidanyhaulroadobstacles,yetlowenoughforsafe
handlingofthescraper.Carryingthebowllowallowsforquickloweringofthebowltostopthescraperinthe
eventofanemergency.Allowplentyofroomfortherearwheelofthescrapertoavoidobstacleswhenmaking
tightturns,andmaintainasafespeedfortheconditionofthehaulroad.
UsingaPushCat
Duringloadingoperations,whenapushcatisusedtosupplyextrapowertothescraper,itshouldbe
positionedabout45degreesoffthelanetobecut.Thescrapershouldstartloadingbeforethepushcatmakes
asmoothcontactwiththerearpushblock.

9. PHASESOF A CYCLE

10. MAJOR COMPONENTS
Depending on make, model, and configuration, major components on scrapers may
vary; therefore, you are responsible for reading the operator’s manual for specific
information. For a general understanding, the major components of a Caterpillar 621Â
single- engine scraper are shown in Figure .
Fig. MAJOR COMPONENTS

13. TractorUnit
Thetractorunitcontainstheengine,thedrivetrainandwheels,thehydraulicpumps,andtheoperator’scab.Thetractoris
connectedtothescraperbyaverticalkingpinswivelconnection,usuallyintwopartswithupperandlowerpins.When
steering,thisconnectionpermitsturnsof85to90degreestoeachsideofthecenterlineofthescraper.Thereisalsoa
longitudinalhorizontalhingethatpermitsthetwounitstomoveindependentlyfromsidetoside,asshowninFigure1.
Gooseneck
Thegooseneckcouplesthetractorandscraperunit.By
archingup,itprovidesspaceforthetractorwheelstomove
underit.Thegooseneckthenwidensintoaverymassive
crossbeambecomingapairofliftarms,alsoknownasbowl
stiffeners,whichextendbacktothetrunnionfasteningson
thesidesofthescraperbowl.
The gooseneck supports the steering cylinders as well as the
cylinders that raise and lower the bowl and apron.
Scraper Unit
The scraper unit has three basic operating parts: the bowl, apron,
and ejector.
Bowl
The bowl is a box with rigid sides. Bolted to its bottom is a cutting edge consisting of three blades made of wear-resistant steel.
The bottom front sides of the bowl usually have bolted-on wear plates called side cutters. The side cutters normally receive less
wear than the cutting edge. By use of a lever inside the operator’s cab, the bowl can be hydraulically lowered or raised. As the
scraper travels forward, the bowl is lowered to cut into the material. The cut material is then forced into the bowl. Once the bowl is completely loaded, it is then raised. The material is hauled to a designated location where it is unloaded or spread.Conveniently, the top of the bowl is open so that spread material can be loaded by crane clam shell, conveyor, or front-end loader.
Apron
The apron forms the forward section and a variable amount of the bottom of the bowl assembly. The apron is also hydraulicallyraised or lowered by use of a lever inside the operator’s cab. When lowered (closed), it rests at the cutting edge to preventspillage. When raised, it leaves the whole front of the bowl open to accept material.
Ejector
The ejector is the rear wall of the bowl. The most common ejector is hydraulically controlled to move forward horizontally toforce
the load out of the bowl. It is supported by rollers riding on the floor and on tracks welded to the sides of the bowl.
Push Block
The push block extends past the scraper’s rear tires. It is used during loading operations by the push cat while providing extra
power.
Figure 1 - Articulation

14. Classification
1) By bowl capacity:
-small capacity (up to 6 m³);
-medium capacity (6-12 m³);
-large capacity (15-25 m³);
-especially large capacity (over 25 m³).
2) By the method of aggregation with a tractor:
-Pull scraper;
-Semi-pull scraper;
-Engine scraper.
3) By bowl loading method:
-with free loading (under the action of
traction);
-with forced loading (scrapers),
elevator loading, auger, etc.

15. 4) By the way of unloading the bowl:
-free "dump truck" (lifted by one side and poured out), tilted forward, tilted back,
bucket forward;
semi-compulsory (forward tilt of the bottom and rear wall)
forced (forward movement of the rear wall or movement of the front flap back
5) By the design of the running equipment (number of axles):
-uniaxial (trailed) low capacity;
-biaxial (trailed, self-propelled);
-three-axle (semi -trailed).
6) By the number of engines (ICE):
-single-engine -front axle drive (front driving wheels);
-twin-engine -front and rear axle drive.

16. Single-Engine Scraper
The single-engine scraper, like the one shown in Figure 1, is essentially a tractor with one engine coupled to a
scraper unit. During loading operations, its single-engine does not provide sufficient power or traction to completely
load the bowl; therefore, the assistance of a push cat is often required, as shown in Figure 2.
Fig. 1
Fig. 2-Push cat pushing scraper

17. Push cat pushing scraper

18. Fig. 3
Fig. 4
Thetwin-enginescraperisalsoreferredtoasthepush- pullscraperbecauseithastwoengines.
Theengineinthetractorunitpullsthemachine,whiletheotherengineattherearofthescraper
unitpushesthemachine,asshowninFigure3.Undercertainloadingconditions,twotwin-
enginescrapersmayneedtoworkintandem.Thescrapersarejoinedtogetherbyarear-
mountedhookandafronttriangularbail,asshowninFigure4.

19. PADDLEWHEEL SCRAPER
Anothertypeofscraperisthepaddlewheelscraper,showninFigure5,alsocalledtheelevatingscraper.Thistype
ofscraperisidealforsmall-tomedium-sizeearthmovingjobs,andonmostprojectsdoesnotrequireapushcat.
Afterthematerialhasbeencutbythecuttingedge,thematerialisnotforcedtothetopasonotherscrapers;
instead,thescraperloadsitsbowlfromthetopbymeansofapaddlewheelelevator.Thepaddlealsoaidsin
breakingupthematerial,creatingamoreconsistentload.
The bowl can be unloaded by placing the paddle wheel in reverse. This gives the operator more control of the
desired depth of fill. For increased productivity, the operator is also able to control the speed of the elevator to
match the material condition.
Fig. 5

20. ATTACHMENTS
Unlikeotherpiecesofequipment,thereareveryfewattachmentsusedwithoronscrapers;
however,therearethreecommonlyusedcuttingedgesthatservevariouspurposes.Theyarethe
straight,curvedandthree- piececuttingedges,showninFigure1.Thesecuttingedgesare
availableinvariousthicknesses.Thinneredgesprovidegreaterpenetration,buthavelesswear
materialandimpactresistancecomparedtothickeredges
1.StraightCuttingEdge
Thestraightcuttingedgeisthemost
efficientforsmoothfinishgrading.
2.CurvedCuttingEdge
Thecurvedcuttingedgepenetratesmore
thanthestraightedge.
3.Three-PieceCuttingEdge
Thethree-piece cuttingedgehasa
centerbladepositionedaheadofthe
othertwobladesfordeeperpenetration.
Thiscenterbladeisreferredtoasthe
stinger. Figure 1

21. Determination of scraper productivity
Operationalproductivityiscalculatedbytheformula,m
3
/h:
q–geometricalbowlcapacity,m
3
;
К
н–bowlfillingcoefficient;
К
в–коэффициентиспользованияскрепераповремени(К
в=0,8…0,9);
Т
ц–продолжительностьрабочегоцикла,с;
К
р–коэффициентразрыхлениягрунта.