Crane (machine)

Crane (machine), machine for moving heavy objects both vertically and horizontally. Cranes range in capacity from a few hundred pounds to several hundred tons; motive power may be furnished by hand, by steam or internal-combustion engines, or by electric motors. In form, cranes are classified as jib, derrick, or bridge. A jib crane carries a horizontal jib or beam at the top of a vertical pillar. Horizontal motion of the load is obtained by rotation of the jib or of the entire crane or by carrying the hoisting tackle over a movable trolley running on the beam. The derrick crane, or derrick, is of cantilever design and consists of a boom, hinged at the base to the bottom of a vertical mast and supported at the tip by tackle from the top of the mast; the mast is braced or counterweighted to keep it from collapsing toward the boom. Horizontal motion is obtained by rotating the derrick and by luffing, that is raising or lowering, the boom. A bridge crane consists of a horizontal beam, running on tracks at both ends for longitudinal motion and carrying a trolley running on the beam for lateral motion. The ordinary bridge crane runs on elevated tracks, while the gantry crane is mounted on legs that run on tracks at ground level.