All About Draglines

Draglines are used to excavate earth and load it into hauling units, such as trucks or tractor-pulled wagons, or to deposit it in levees, dams, and spoil banks near the pits from which it is excavated. In general, a power shovel up to a capacity of 2.5 cubic yards can be converted into a dragline by replacing the boom of the shovel with a crane boom and substituting a dragline bucket for the shovel dipper.

For some projects either a power shovel or a dragline may be used to excavate materials, but for others the dragline will have a distinct advantage when compared to a power shovel. A dragline usually does not have to go into a pit or hole in order to excavate. It may operate on natural ground while excavating material from a pit with its bucket.

This will be very advantageous when earth is removed from a ditch, canal, or pit containing water. If the earth is hauled with trucks, they do not have to go into the pit and contend with mud. If the earth can be deposited along a canal or ditch or near a pit, it frequently is possible to use a dragline with a boom long enough to dispose of the earth in one operation, eliminating the need for hauling units, which will reduce the cost of handling the earth. Draglines are excellent units for excavating trenches when the sides are permitted to establish their angles of repose, without shoring.

One disadvantage in using a dragline compared with a power shovel is the reduced output of the dragline. A comparison of the ideal output of various sizes of draglines with the output of power shovels shows that a dragline will excavate approximately 80 percent as much earth as a power shovel of the same size.

Draglines come in three types. Crawler-mounted draglines can operate on surfaces which are too soft for wheel- or truck-mounted equipment, but their speeds are so slow, frequently less than 1 mph, that it may be necessary to use auxiliary hauling equipment to transport them from one job to another, especially if the distance is great. Wheel- and truck-mounted draglines may have travel speeds in excess of 30 mph.

The size of a dragline is indicated by the size of the bucket, expressed in cubic yards, which, in general, is the same size as the dipper of the power shovel into which it may be converted. However, most draglines may handle more than one size bucket, depending on the length of the boom and the class of material excavated.

Because the maximum lifting capacity of a dragline is limited by the force which will tilt the machine over, it is necessary to reduce the size of the bucket when a long boom is used or when the material has a high specific gravity. In practice the combined weight of the bucket and its load should produce a tilting force not greater than 75 percent of the force required to tilt the machine over. A longer boom, with a smaller bucket, should be used when it is necessary to increase the digging reach or the dumping radius.