Drag line
A drag line is a machine used for excavating earth, sand or soft rock and consists essentially of a revolving deck, a long light boom, crawler chains, and a special type of bucket held in position and controlled by cables (Fig. 5.13). The bucket, when it has to be loaded, is lowered in the earth or loose rock by manipulation of the cables and is dragged by them. As it is dragged it gets loaded. Hence the name dragline. A dragline is operated by diesel engine or a motor which is supplied power at high voltage from external source through a trailing cable. The depth to which a dragline digs is limited by the capacity of the drums to hold the hoist cable. When digging, the bucket, after it is loaded, is hoisted up, the boom given a swing through nearly 90° and the contents then unloaded by manipulation of the cable.
A dragline is suitable for digging alluvium, sandy soil, unconsolidated rock or blasted coal/rock. It digs below the level, at which it stands and from one position can dig over a wide working place and cast the earth over a wide area within the reach of the boom. It is generally not employed to load dumpers as the accurate positioning of the dragline bucket over the limited area of the dumper delays the cycle of operations and the common application is for dumping overburden. It is suitable for working a quarry with watery conditions as the dragline works from a higher and, therefore, dry position.
Working ranges of a dragline
A—Digging depth. C—Dumping and digging radii, D—Dumping height
(Above) A dragline showing the bucket in various stages
1-Bucket empty, II-Bucket starts digging, Ill-Bucket being dragged and getting loaded, IV-After a swing of the boom bucket unloaded on 9 spoil heap.
Loading capacity of a dragline
A dragline is capable of dealing with the following quantities of rock/earth (solid) in a year (12 to 14 hours work daily)
Bucket size
4.5 to 7.5 m3 (6 to 10 cyd)
11.5 to 15 m3 (15 to 20 cyd)
23 to 30 m3 (30 to 40 cyd)
Million m3
0.25 to 0.75
1.5 to 1.7
3 to 4.5.
A drag line may be crawler mounted or of walking type. Crawler mounted machines have travelling speeds of 0.25 to 5 km/hr. A walking dragline has a travelling speed of 0.18 to 0.6 km/hr.
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