Drag line excavator

From Academic Kids

Dragline excavation systems are heavy equipment used in civil engineering and surface mining. In civil engineering the smaller types are used for road and port construction. The larger types are used in strip-mining operations to extract coal and these are amongst the largest mobile equipment (not water-borne), and weigh in the vinicity of 2000 metric tonnes.

A dragline bucket system consists of a large bucket which is suspended from a boom (A large truss like structure). The bucket is maneuvered by means of a number of ropes and chains. The hoistrope, powered by large diesel or electric motors, supports the bucket and hoist-coupler assembly from the boom. The dragrope is used to draw the bucket assembly horizontally. By skillful maneuver of the hoist and the dragropes the bucket is controlled for various operations. A schematic of a large dragline bucket system is shown below.

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Contents

Operation

In a typical cycle of excavation, the bucket is positioned above the material overburden to be excavated. The bucket is then lowered and the dragrope is then drawn so that the bucket is dragged along the surface of the material. The bucket is then lifted by using the hoist rope. A swing operation is then performed to move the bucket to the place where the material is to be dumped. The dragrope is then released causing the bucket to tilt making the material in the bucket to fall down. This is called a dump operation.

Draglines in mining

A large dragline system used in the open pit mining industry costs approximately US$20-50 million. A typical bucket has a volume ranging from 30 to 60 cubic metres (Extremely large buckets range up to 150 cubic metres). The length of the boom ranges from 45 to 100 metres. In a single cycle it can move up to 450 metric tonnes of material.

A notable feature of mining draglines is that they are not fuel powered like most other mining equipment. Their power consumption is so great that they have a direct connection to the high voltage grid at 11 kV. Many (possibly anecdotal) stories have been told about the blackout-causing effects of mining draglines. For instance, there is a long-lived story that, back in the 1970s, if all 7 of Peak Downs (a very large coal mine in central Queensland, Australia) draglines turned simultaneously, they would black-out all of North Queensland.

In all but the smallest of draglines, movement is accomplised by 'walking' using pontoons, as caterpillar tracks place too much pressure on the ground, and have great difficultly under the immense weight of the dragline. Maximum speed is only up to a few hundred metres per hour. If travelling medium distances, (about 30-100 km), a special dragline carrier can be brought in to transport the dragline. Above this distance, disassembly is generally required.

Limitations

The primary limitations of draglines are their boom height and boom length. This limits where the dragline can dump the waste material. They do not have great accuracy when it comes to vertical waste removal, so the dragline will never be used for coal removal, so as to minimise coal loss and dilution. Inherent with their construction, a dragline is most efficient excavating material below the level of their tracks, a dragline is not suitable to load piled up material (like a wheel loader can).

Despite their limitations, and their extreme capital cost, draglines remain popular with many mines, due to their reliability, and extremely low waste removal cost.


Examples

The British firm of Ransome and Rapier produced a few large (1400-1800 ton) excavators, the largest in Europe at the time. Power was from internal combustion engines driving generators. One, named SUNDEW, was used in a quarry from 1957 to 1974. After its working life at the first site in Rutland was finished it walked 13 miles to a new life at Corby, the walk took 9 months.


References

K. Pathak, K. Dasgupta, A. Chattopadhyay, "Determination of the working zone of a dragline bucket - A graphical approach", Doncaster, The Institution of mining engineers, 1992.

Peter Ridley, Peter Corke, "Calculation of Dragline bucket pose under gravity loading", Mechanism and machine theory, Vol. 35, 2000.

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