POWER SHOVEL

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. Mechanical cable-operated shovels apply engine power to the base and to the attachment by means of clutches, gears, shafts, winch drums, and cable. Electric cable-operated shovels have several electric motors supplied with current by a power line, or more rarely, by a deck-mounted generator, replacing the engine and most of the clutches, gears, and shafts of the mechanical shovel. Hydraulic shovels have engine-driven pumps to provide pressure for rams and motors. Mechanical shovels may include some electrical or hydraulic functions. There are five principal types of front attachments, which may be interchangeable in small and medium-sized machines of the cable type. The dipper shovel, the oldest and most important excavating machine (rail-mounted, steam-powered units were in use in 1835), is for hard digging and truck loading. It consists of a heavy, relatively short boom and a dipper stick (a beam that pivots on the boom) with digging bucket attached. The bucket has a hinged bottom that can be opened by the operator. Forward and upward motion fills the bucket. If a long boom is unnecessary and the ground is hard and unyielding, the most suitable excavators are the dipper shovels. The hoe, or backhoe, attachment may replace the boom, stick, and bucket of the dipper shovel for digging below working level; it pulls the load toward the deck of the shovel instead of away from it. Modern hydraulically powered hoes may be mounted on any shovel carrier or on the rear of a tractor. A lifting crane can be attached in place of the digging shovel to raise heavy loads, move them laterally by swinging or travelling, and then lower them into new locations. Specially constructed lifting cranes are designed for hydraulic machines. The dragline excavator has a long boom of lattice design that extends from the cab at an angle of about 35 with the ground. From its end is suspended a hoist cable, at the end of which is the digging bucket. The dragline runs from the cab directly to the bucket; when the bucket is lowered, a pull on the dragline causes it to dig into the earth. The machine then swings the loaded bucket into a disposal area and tips its contents. On soft and watery land, particularly in building power dams, the long reach of the dragline is more effective than bulldozers and other surface-earth removers. The clamshell is a bucket with two hinged jaws carried by a crane suspended from the boom by two lines: one raises and lowers the bucket, and the other pulls the jaws together against gravity for digging action. It is used chiefly for deep, narrow excavations, as in well digging, for piling materials up high, and for rehandling loose material, such as sand or gravel, but it can do almost any type of digging.