Scraper




It is the equipment commonly used for scraping, loading, hauling and discharging including spreading large quantities of earth over long distances, say around three Km. It can scrape soils in layers of 15 cm to 30 cm in depth. Basically, a scraper has a soil container or bowl mounted on two wheels. It digs into the earth after the forward portion of the container is lowered, and it collects the earth as the scraper moves forward. Unloading and spreading takes place in controlled layers in the discharge area with the aid of a tractor plate while the unit keeps on moving. Scrapers come in many sizes varying from 8 m3 to 50 m3. There are two main categories of scrapers :

·        Towed scrapers

·        Motorized scrapers


Towed scrapers

These are pulled by a tractor or a bulldozer capable of 300 HP or more. Although the loading cycle may take hardly two in three minutes, its travelling speed is slow. Its main advantage over the motorized scraper is that it can operate in small areas and can scrape in heavy soil areas. Towed scrapers are best suited for medium distances up to 400 meter. Towed scrapers range from 8 m3 to 30 m3.


Motorized scrapers

Several types of motorized scrapers with heaped capacity ranging from 15 m3 to 50 m3 are available to suit varying job requirements. These include single engine scraper, double engine scraper and elevating scraper.

A. Single engine scraper requires a pusher bulldozer to provide the necessary tractive force. Generally one medium-sized crawler tractor is sufficient to serve four to five scrapers.

Scrapers per pusher = Cycle time of each scraper / Cycle time of pusher

Example : Cycle time of scraper is 6 minutes and a pusher to fill a scraper is 1.5 minutes. Calculate the number of scrapers which a pusher can serve. Determine the number of pushers to serve 10 scrapers.

Solution : Number of scrapers per pusher = 6.0/1.5 = 4

Number of pushers for 10 scrapers = No. of scrapers / No. served by one pusher
= 10/4
=3

B. Double-engine scrapers are fitted with two engine, one in the front and the other in the rear axle. For scrapers having capacity 35 m3 and above, two engines are preferred instead of one very large equivalent engine. Although the engine in the rear provides a four-wheel drive, double-engine scrapers do require a pusher specially in hard soil excavation. In the push-pull method, two double-engine scrapers are used to mutually load each other in turn, without the aid of a pusher.

C.   Elevating scrapers are fitted with an elevating mechanism for self-loading. Due to their heavy weight, they are at a disadvantage over long hauls.


Scraper output planning

The ideal indicative output of towed scraper and motorized scraper are reflected as:

Volume Conversion of Soil into its Three States
Name of soil
Bank volume
Loose volume
Compacted volume
Common earth
1.00
1.25
0.90
Sand
1.00
1.12
0.95
Clay
1.00
1.27
0.90
Rock (blasted)
1.00
1.50
1.30

This ideal output is for good haulage by road, and this has to be modified by taking into consideration various corrections and efficiency factors discussed above.
Output/h in Lm3 = Ideal output x correction factor x performance factor

Example : A new motorized scraper, working under average conditions, employed in spreading of excavated materials along a road alignment 1.5 km long using the following data:

Scraper capacity
= 16/23.7 (struck/heap in loose soil m3)
Gross vehicle weight (empty)
= 36 tons
Maximum pay load
= 34020 kg
Material density
= 1500 kg Lm3
Job efficiency
= 50 min/h
Rated power of engine
= 450 HP
Correction factor
= 0.91 (estimated from equipment manual)

Evaluate the output per hour after making adjustment for various factors affecting production.

Solution
Production per hour in Lm3 = Load per cycle (m3) x 60 min x Operation efficiency + Cycle time in minutes
1.      Load per cycle (Lm3)
= Weight of heaped capacity
= 23.7 m3 x 1500 kg/m3
= 35.550 kg.

But this weight exceeds pay load of 34,020 kg.
Therefore maximum load carriage capacity
= 34020 kg

=34020/1500 Lm3

= 22.68 Lm3, and not 23.7 Lm3

2.    Approximate output from above table work out as under:
Ideal production per hour for 16 m3 (struck capacity scraper) with 23.7 Lm3 heaped capacity
= 150 m3 (approximate)

Correction factor = 0.91
Performance factor = 0.84

Load carriage capacity factor = Maximum load carrying capacity / Scraper heaped capacity

= 22.68/23.7
= 0.96

Therefore scraper output = 150 x 0.91 x 0.84 x 0.96

= 110 LCM/Hour.


3.    Note: The Gross Vehicle Weight-(GVW), the effective grade (%), the maximum travel speed and the rimpull (or the drawbar pull) can be determined from the equipment performance charts which are provided by the manufacturer. This information aids in determining the cycle time, output data and usable rimpull (i.e. adjusted rimpull) necessary to overcome traction of the haulage road.