A power shovel is a digging and loading machine consisting of a revolving deck with a power plant, driving and controlling mechanisms, sometimes a counterweight, and a front attachment, such as a boom or crane, supporting a handle with a digger at the end. The whole mechanism is mounted on a base platform with tracks or wheels. Power shovels are used principally for excavation and removal of debris.
Power shovels use a dipper and a power plant set on a turntable, which, in turn, is mounted on a caterpillar tread. With the turntable, the operator can dig in one spot, lift the dug up material, swing the scoop, and dump the material into a waiting truck or railroad car.
The capacity of dippers normally ranges from 1 to 20 cubic yards (0.75 to 15 cubic meters), though much larger units with a capacity of up to 200 cubic yards (150 cubic meters) are used in mining operations. Power shovels driven by gasoline or diesel engines or by electric motors have almost entirely replaced earlier steam shovels.
A specialized machine called a ladder trencher, or ditcher, is effective for opening trenches for utility lines, telephone cables, drainage and sewer lines, and the like. It consists of two main components: a crawler, or rubber-tired carrier, and a digging mechanism. Depending on the soil condition and the depth of the trench to be excavated, the trencher can move at a speed of up to 2 miles (3.2 kilometers) per hour.
A ladder trencher has a long boom, and an endless chain rotates along the boom. Buckets or clamshells attached to this chain dig at the bottom of the trench, bring up excavated material, and unload it onto a conveyer while the machine moves slowly along. The width of the excavated trench ranges between 18 and 60 inches (46 and 152 centimeters). Ladder trenches can be used for digging depths of up to 20 feet (6 meters).
Another machine with a wide radius of action uses a dragline, which is a lifting crane with a drag bucket. A long boom flings out the bucket on the line and drops it into the material to be excavated. The machine then pulls in the line, drawing in the bucket until it is filled, lifts it, and swings it with the crane boom to dump the material wherever desired. The size of the scoop is normally between 1/2 and 3 cubic yards (0.38 and 2.3 cubic meters).
A clamshell is a modified lift crane with the hook replaced by a clam bucket. It is suitable for raising loose material such as the removal of silt from bottoms of rivers, lakes, or harbors. It is also used to unload ore, coal, and crushed stone (see Coal). Draglines and clamshells are land-based machines that are similar to dipper and clamshell dredges.
Devices that carry buckets on an endless chain are used to unload sand or gravel from ships and barges. The unloading of grain is normally carried out by large suction heads or dustpans. The loading of grain silos and the unloading of ships' holds is accomplished with these giant vacuum cleaners.
