Gantry crane (also known as Longmen crane) is a bridge crane with two legs supported on the ground track. In structure, it consists of gantry, cart operation mechanism, lifting trolley and electric parts. Some gantry cranes only have one leg on one side, and the other side supports the workshop or the Zhanqiao. It is called semi gantry crane. Gantry crane upper frame (including main beam and end beam), outrigger, lower beam and other parts. In order to expand the scope of crane operation, the main girder can extend to one side or both sides to extend the leg to form a cantilever. It is also possible to extend the operation scope of the crane by lifting and lifting the boom with the boom.