The gantry crane is a bridge crane supported on ground tracks by two side legs. Structurally, it consists of the gantry, trolley running mechanism, hoisting trolley, and electrical parts. Some gantry cranes have legs only on one side, supported on the factory or pier on the other, and are called semi-gantry cranes. The upper bridge girder (including the main beam and end beam), legs, and lower transverse beam of the gantry crane form its structure. To expand the crane's working range, the main beam can extend beyond the legs to one or both sides, creating an overhang. A crane trolley with an arm can also be used to extend the working range through the tilting and rotation of the arm.