Dumpers or tippers
These are heavy duty trucks with a container-body of steel open at the top for receiving material loaded mechanically by tractor shovel, dipper shovel, dragline, etc. All dumpers/tippers are provided with arrangements to lift the loaded body by utilising hydraulic-pressure to force a ram out. The body swings from its horizontal position round a fulcrum through nearly 70° to dump its load and the hydraulic system also functions to pull the body back on its seat i.e. the chassis. Fig. 5.11. shows a typical hydraulic system layout for the tipping gear of a dumper. From an oil tank oil flows by gravity to hydraulic pump. When the driver engages the power take off (P.T.O) control lever, power from the engine is transmitted from the transmission countershaft to the power take off which drives the pump. The oil under high pressure from the pump goes to the control valve whose lever can be manipulated for 4 different positions.
(a) Raise position: High pressure oil goes through the hose pipes to the bottom of the hoist cylinder and the ram is then forced out. Oil at the top of the hoist travels back to the control valve through the hose connected to the piston rod.
Fig.5.11 The tipping gear for dumpers
(b) Hold position: Both the passages between the control valve and hoist arc closed so that oil at the bottom and top of the hoist is at a stand still and the latter is unable to move in either direction.
(c) Float position: Both hose passages between the control valve and hoist are open so that oil at either end of the hoist can flow either way. The hoist can then travel in either direction depending upon the direction in which the force is applied.
(d) Lower position: High pressure oil goes to the top of the hoist which then telescopes itself by the oil pressure and the oil at the bottom of the hoist travels back to the tank via the control valve. The body is thereby lowered on to the chassis.
Steering on all the heavy duty dumpers is mechanical but assisted by hydraulic power, generated by the engine. The dumper operator's exertion is thereby considerably reduced.
Mechanical transmission from the engine to the rear wheels is the standard practice now-a-days, though for some years the rear wheels were driven by individual electric motors controlled from operator's cabin. Medium sized mechanised quarries employ dumpers of 25-50 te carrying capacity. 50-60 te coal haulers are on the manufacturing line of B.E.M.L. and Hindusthan Motors. Future planning of large projects is for employment of 100-150 te dumpers which will be fed by shovel of 8-10 m3 capacity. Bottom discharging coal haulers of 55 te payload, 43 m3 struck capacity (model GB 60C) are manufactured by BEML
Brakes on dumpers are operated bycompressed air (Fig. 5.12). Some dumpers are equipped with hydraulic retarder (hydrotarder). This is a device used on some trucks and dumpers to prevent the speed from exceeding certain limits when travelling a steep down-slope and also to produce a braking action on the vehicle. In a way, it acts as a governor. It uses the hydraulic friction to produce the braking action. Unlike the regular brakes, the hydrotarder will not completely stop the vehicle but will slow it down preparatory to stopping with the familiar friction brakes, operated by compressed air or hydraulic pressure.
The retarder essentially consists of a vane type rotor turned by the drive shaft, a fixed casing or stator fitted with vanes and an oil circulation system.
The machines deployed in the opencast mines, at the crushing and ore preparation plant have to be of matching capacities. At Kudremukh Iron Ore Project, one of the largest opencast mines in India, the capacities of some of the machines are: shovels 10.7 m3, production trucks 108 te, front end loaders 10m3, electric drills for 310 mm dia. blast holes, gyratory crushers 4000 te/hr.
No comments:
Post a Comment