Scrapers Machines

Scrapers

Mansoor Azam QureshiNUST

Islamabad

April 10, 2023 mansoor Azam Qureshi 2

Scrapers Scrapers are designed to load, haul, and dump

loose material. The greatest advantage is their versatility. They can be used for a wide variety of material types and are economical for a range of haul distances and conditions. They are a compromise between a bulldozer, and excavator, and a dump truck.

Scrapers are articulated, tractor powered, and pull a bowl that holds the soil. A blade is mounted on the bottom of the bowl that cuts into the travel surface and the disturbed soil flows into the bowl as the scraper moves forward.

Scrapers can self-load or be assisted by another scraper or a bulldozer.


Scraper components


Scraper TypesSingle engine: A tractor

pulling a bowl that can operate under its own power or be push-assisted. This is the most common type of scraper on large earthmoving jobs.

Tandem or twin engine: This type has a second engine mounted in the rear and can develop greater power. This is ideal for steeper hauls at greater speeds


Push–pull scraper: This type is designed with a push block mounted on the rear and a bail mounted on the front to assist other scrapers or be pushed by other scrapers. They are ideal for dense soil-excavating projects when a dozer is not utilized for pushing.

Elevating: These are self-contained loading and hauling units. The chain elevator serves as a loading mechanism. The extra weight of the loading mechanism is a disadvantage during the haul cycle

Scraper Types


Auger Scrapers This is another completely self-contained loading and haulmg scraper. Auger scrapers can self-load in difficult conditions, such as laminated rock, granular materials, or frozen material. An independent hydrostatic system powers the auger that is located in the center of the bowl. The rotating auger lifts the material off the scraper cutting edge and carries it to the top of the load, creating a void that enables new material to easily enter the bowl.

Scraper Types


Scraper Production Cycle A scraper is loaded by

lowering the front end of the bowl until the cutting edge enters the ground. At the same time, the front apron is raised to provide an open slot. Hauling load to dump site.

The dumping operation consists of lowering the cutting edge to the desired height above the fill, raising the apron, and then forcing the material out· by means of the movable ejector mounted at the rear of the bowl.

Turn back Return to loading site Turn to load


Estimating ProductionBowl capacity of scraperSoil typeEmpty weight of scraperLoaded weight of scraperHaulage route grades and rolling resistance.


Loading Scraper Bowl Loose volume

capacity of bowl in heaped or struck level is specified.

Optimum load is smaller than max capacity. Loading beyond one min may not be feasible.

Loading time may remain within 0.5 to 1.0 min typically 0.8 min is taken.


Haulage/ Travel TimeBoth the haul and return time depend on the

distance traveled and scraper speed. Hauling and returning are usually at different speed ranges.

If the haul road has multiple grade or roIling resistance conditions, a speed should be calculated for each segment of the route.

When using the charts, always consider the human element; do not blindly accept chart speeds.

Manufacturer provided average speed and grade charts may be used for calculation of travel time.


Travel Time, Distance and grade chart


ExampleEstimate the production of a wheel tractor-scraper given the job information below. Use the travel time curve for scraper.Scraper type: single-engine overhungLoading method: single pusherMaterial: sandy clay, 3,200 IblBCY 2,560 IblLCY Rolling resistance = 100 lb/ton Scraper capacity: rated payload = 72,000 lb heaped volume = 31 LCY Altitude derating factor = 4%Job efficien cy factor = 0.83Haul route :Section 1. Level loading area.Section 2. Down a 4% grade , 2,000 ft Section 3. Level dumping areaSection 4. Up a 4% grade , 2,000 ft Section 5. Level turnaround, 600 ft


Travel Time Curve for Scraper

April 10, 2023 14

Solution Bucket Capacity= 31lcy Weight of material

= 31x2560 = 79360 lcy Rated payload of bucket

= 72000 lbs Volume of material to be

carried= 72000/3200

= 22.5 bcy

Load ratio=22.5/31

=0.73

Effective Grade◦ Haul = -4.0+100/20 = 1.0%◦ Return= 4.0+100/20=9.0%◦ Turnaround=0+100/20=5%

Travel Time◦ Section 2 = 1.0 min◦ Section 4 = 1.6 min◦ Section 5 = 0.45 min◦ Total Time=3.05 min

Adjusted for altitude =3.05x1.04 = 3.2 min

mansoor Azam Qureshi


Loading Time=0.6 min

Dump and maneuver time = 0.7 min

Turning and spot=0.3 min

CycleTime =3.2+0.6+0.7+0.3 =4.8 min

Production◦ = 22.5x60/4.8◦ =281.25 lcy◦ =281.25x2560/32

00◦ =225 bcy

Solution


Hints for Efficient Production

Loading Hints.

Use downhill loading whenever possible.

Shuttle or chain load if possible.

Straddle loading may be used to speed up loading. In this technique.

The scraper should leave the cut as fast as possible,

Adjust loading techniques according to the soil type encountered. Use rippers or scarifiers to loosen hard soils before loading.

Normally, the operator should maintain a depth of cut that gives a constant loading speed and leaves a smooth floor.

However, pumping the bowl up and down during loading may be required when loose sand is being loaded.

Pushers should provide an adequate boost to accelerate scrapers leaving the cut.

Maintain a loading time as close to optimum as possible and provide adequate pushers. Tandem or dual pushers may be required for adequate power.

Maintain the cut area with idle pushers


Hauling Maintain the haul road in

top condition; use graders and water sprinklers as required .

Keep haul units uniformly spaced. Unless passing room is provided, hauling speed is limited to that of the slowest unit.

Use one-way haul roads if possible. When two-way traffic is necessary, provide grade separation on sharp curves to reduce the danger of collision.

Haul from two cuts to a common fill or haul from one cut to two fills whenever possible. Either procedure reduces time lost in turning, which typically amounts to about 0.25 min per turn.

Use sprinkler s to reduce dust on the haul road. Dust increases equipment wear and reduces operator visibility. Do not overwater because overwatering may cause loss of traction and be a greater hazard than dust.



Spreading

Spread the first load at the beginning of each fill lane and work down the fill so that the loaded scrapers compact the material spread previously.

Keep the fill high on the outside for safety and to help maintain an accurate slope. However, drainage must be provided for the fill surface.

Provide adequate spreading and compacting equipment and give scrapers the right-of-way on the fill.

Vary spreading procedures according to the material involved. Spread loose sand in thin layers. wet and sticky material is best spread by moving the tailgate forward about 12 m. (305 mm) and then returning it about 6 in. (152 mm) before repeating the procedure. When using this procedure, be sure to keep the bowl high enough to allow the dumped material to pass under the scraper without clogging.

Keep the fill smooth enough to allow fast dumping and return to the haul road.

Use boosting on the fill if scrapers require extra power for fast, smooth spreading.


Scrapers Machines

Education

Transcript of Scrapers Machines