Dragline excavator

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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, though specimens weighing up to 13,000 metric tonnes have also been constructed.

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.

Contents

1 Operation
2 Draglines in mining
3 Limitations
4 Examples
5 History
6 References
7 See also

Operation

In a typical cycle of excavation, the bucket is positioned above the material 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.

On smaller draglines the bucket was 'thrown' by winding up to the jib and then releasing a clutch on the drag cable. This would then swing the bucket like a pendulum. Once the bucket had past the vertical the hoist cable would be released throwing the bucket. A skilled operator could make the bucket land about 1/2 the length of the jib further away then if it had just been dropped.

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, though extremely large buckets have ranged up to 168 cubic metres.[link] 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 seven of Peak Downs (a very large coal mine in central Queensland, Australia) draglines turned simultaneously, they would blackout all of North Queensland.

In all but the smallest of draglines, movement is accomplished 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. 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.

Draglines have different cutting sequences. The first is the side cast method using offset benches; this involves throwing the overburden sideways onto blasted material to make a bench. The second is a key pass. This pass cuts a key at the toe of the new highwall and also shifts the bench further towards the low-wall. This may also require a chop pass if the wall is blocky. A chop pass involves the bucket being dropped down onto an angled highwall to scale the surface. The next sequence is the slowest operation, the blocks pass. However, this pass moves most of the material. It involves using the key to access to bottom of the material to lift it up to spoil or to an elevated bench level. The final cut if required is a pull back, pulling material back further to the low-wall side.

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 weeks.

Smaller draglines were also commonly used before hydraulic machines become widespread. Firms such as Ruston and Bucyrus made models such as the RB10 which were popular for small building works and drainage work. Several of these can still be seen in the English Fens of Cambridgeshire, Lincolnshire and parts of Norfolk, usually associated with drainage pumping engines.

The coal mining dragline known as Big Muskie, owned by the Central Ohio Coal Company (a division of American Electric Power), was the world's largest mobile earth-moving machine, weighing nearly 13,000 metric tons and standing nearly 22 stories tall.[link] It operated in Guernsey County, in the U.S. state of Ohio from 1969 to 1991, and was powered by 13,800 volts of electricity. It was dismantled in 1999.[link] The GEM of Egypt (GEM standing for "giant excavating machine" and Egypt referring to the Egypt Valley in Belmont County, eastern Ohio where it was first put to use), which operated from 1967 to 1988, was of comparable size.[link]

Big Brutus, 160 feet tall and weighing 5,500 tons, was at one time the second largest dragline shovel, and remains on display at the Big Brutus museum in West Mineral, Kansas.

History

The first dragline was built by the "Page & Schnable Contracting" in 1904, for use digging a chicago canal. The compamy became the "Page Engineering Company" in 1912.

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.