In most cases, the bridge itself can move along a pair of parallel rails, so that the crane can serve a large rectangular area. A simple pivoting hand-operated jib crane is depicted in Figure 1.īridge cranes comprise another important class of cranes in which the pulley system is suspended from a trolley that moves on tracks along one or two horizontal beams, called the bridge, that are supported at both ends.
The loads may also be moved from side to side by having the jib pivot, or rotate, on its base around the mast. The cable dropping from the top of the jib is attached to loads and lifts them vertically. Along the entire length of the jib runs a pulley system whose cables or chains are wound and unwound around a drum, or cylinder, that is placed at the jib’s base and is turned by a motor. The jib is supported or held aloft by guy wires running from its top to a vertical mast, or pillar, that is itself stiffly braced the guy wires set the angle at which the jib leans. The most prominent component of that class of cranes known as derrick cranes is the jib, or boom this is a long beam that is structurally reinforced so that it will not bend. Cranes have come into their present widespread application only since the introduction of steam engines, internal-combustion engines, and electric motors, beginning in the 19th century. Cranes are distinct from hoists, passenger elevators, and other devices intended solely or primarily for vertical lifting and from conveyors, which continuously lift or carry bulk materials such as grain or coal.
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