According to crane girders, gantry crane can be divided into three types, single girder gantry crane, double girder crane, semi gantry crane. Both single girder crane and semi gantry crane belong to light and small gantry crane, they are widely used for frequent lifting and transporting work in assembly field, warehouse yard etc. Double girder cranes belong to heavy duty gantry crane, applied for general loading work in construction site, hydropower station, industrial and mining enterprise, railway engineering etc. In fact, semi gantry crane also has single girder type and double girder type. For single girder crane, the hoist travels on the main girder, for double girder crane, the trolley travels between two girders.