Cranes
Cranes are tower or derrick equipped with cables and pulleys that can be used both to lift and lower materials and to shift them horizontally. Cranes are commonly employed in the construction industry and in manufacturing heavy equipment. Construction cranes are usually temporary structures, either fixed to the ground or mounted on a purpose-built vehicle. Cranes may either be controlled from an operator in a cab that travels with the crane, by a pushbutton pendant control station, or by infrared or radio control.
Where a cab operator is employed, workers on the ground will communicate with the operator through a system of standardised hand-signals or, in larger installations, radio systems; an experienced crew can position loads with great precision using only these signals.
Design and Structures
Crane is a hoisting machine for lifting heavy loads and transferring them from one place to another, ordinarily over distances of not more than 200 ft (60 m). Cranes have a long reach and can lift loads to great heights. Powered by manual or animal power, cranes have been in use from early times. Modern cranes are of varied types and sizes; they may be actuated by steam, electricity, diesel, or hydraulic power as well as by manual power, and they are indispensable in industries where heavy materials are handled constantly. The overhead traveling crane, a type of bridge crane, is used inside buildings or in outdoor storage yards. Two or more parallel girders span its working area. Another girder, called the bridge, stretches between them and rolls along them on wheels; this girder, in turn, supports a carriage from which a lifting attachment is lowered by pulleys. On a stacking crane the pulleys are replaced by a stiff, rotating column on which a pair of forks ride up and down. The gantry crane, another type of bridge crane, has a bridge supported by vertical structures that move along tracks. Gantries are used on piers or in shipyards. The jib crane has a horizontal load-supporting boom fastened to a rotating vertical column, either attached to a wall or extending from floor to ceiling; when the column is held only at the bottom it is called a pillar crane. The derrick is a crane equipped either with a vertical mast held by struts, as on barges, or with guy wires, as in building construction. The boom is attached to the bottom of the mast by a pivot and is raised and lowered by a cable reaching from the top of the mast to the end of the boom. A crawler crane is a self-propelled crane that moves on caterpillar treads.
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