Gantry cranes are overhead bridge cranes supported on ground tracks by legs on both sides. Structurally, they consist of the gantry, trolley running mechanism, hoist trolley, and electrical components. Some gantry cranes have legs only on one side, supported on the factory or pier on the other, known as semi-gantry cranes. The upper bridge girder (including the main beam and end beam), legs, lower cross beam, and other parts make up the gantry. To expand the crane's working range, the main beam can extend beyond the legs on one or both sides, forming a cantilever. A crane trolley with a boom can also be used to extend the working range by tilting and rotating the boom.