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.
