Shovels
The field of application of power shovels (see figure) extends from the removal of shallow overburden prior to opencast mining, to stripping and mining deposits of large area and tonnages which are fairly uniform in character, and shovelling material broken down by explosives.
Power shovel
Crawler mounted face shovel is the machine most capable of handling hard, heavy, blasted ground by virtue of its positive crowd action and high break-out force. It can accurately spot for loading dump trucks, rail wagons, etc.. the larger units are used for direct casting. The shovels dig the material above the base level and have little ability to dig below base level. For the operation of the shovels a competent floor is required to allow positive traction by crawlers, without the possibility of damage or ‘bogging down‘ and the surface area of the crawler must provide a bearing pressure to match that can be accepted by the ground. The level width must be sufficient so as to preferably allow trucks to pull it either side of the shovel to minimise track spotting delays.
Shovels may be powered electrically or by diesel engine. The power consumption is approximately 0.26 to 0.6 KWh per bank m3.
Shovels may be broadly put in two classes:
Stripping shovel The stripping shovels are used for the removal of overburden. They can handle all types of material including large blocky material. However, it is restricted to rigid operating conditions and has a limited ability to spoil. The shovel digs material from its operating base upwards.
Face-loading shovels It is similar to stripper shovel but smaller in size. The shorter boom of the loading shovel is equipped with cither a bottom-dump or a rigidly attached side-dump bucket. The loading shovel is widely used for coal loading in surface mines, but it is not very mobile. It is being replaced at many mines with front-end loaders which can perform better than shovels especially in contour mining. The loading shovel can load above the grade and over the end of the truck; can handle coarser material than the front-end loader; has a large life and better availability; and can load selectively under various mine conditions.
Bulldozer
A bulldozer is often referred to simply as a dozer. It is a tractor with a pusher blades attached to the front portion. The tractor is the diesel-operated power unit equipped with either crawler chains or rubber tyred wheels for lifting. The pusher blade can be raised or lowered or tilted through small angles horizontally by rams operated through hydraulic pressure or by ropes. The dozer blade is used for pushing loose material or for digging in earth, sand and soft weathered rock. The machine is also engaged for levelling or spreading earth, for levelling of rock spoil in the dumping yard, grading and compacting temporary roads, pushing mineral into sub-ground level bunkers through grizzly, for towing dumpers, etc.
It also serves the purpose of pushing boulders, pulling down trees, and is an essential equipment to push scrapers. A dozer equipped with a fork like attachment is known as ripper and operates like a plough to loosen moderately hard rock. The loosened rock may be loaded by a scraper. A dozer can dig 1.2 m to 1.5 m below ground in earth or weathered rock.
Example
Dozer blade has the dimension of 4500 mm x 2000 mm operates on level ground. The output capacity of the dozer is ____.
Solution
Consider following figure.
The output of the dozer blade = (1/2) x h2 x L
= 0.5 x 2 x 2 x 4.5 = 9.0 m3
Consider a 20% bulging of the material in front of the blade.
The output of the dozer blade = 9 x 1.2 = 10.8 m3
Example
A surface coal mine is worked by 10 m3 shovel – 85 tonne dumper combination. If the shovel bucket fill factor is 0.8, the material swell factor is 0.75 and the average in-situ specific gravity of the material is 3.5 tonne/m3, the number of shovel bucket loads required to fill a dumper will be nearly____.
Solution
Bucket loose volume = 10 x 0.8 = 8 m3
This is Volume of material as it sits in the bucket (before swelling).
In-situ weight of material per bucket
= 8 x 3.5 = 28 tonnes
Number of loads = dumper capacity/(swell factor x in-situ weight)
= 85/(0.75 x 28) ≈ 4
The swell factor usually means that the loose volume taken by the shovel is less than the in-situ volume.
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