The gantry crane (also known as the Longmen crane) is a bridge type crane which is supported by the two legs of the bridge on the ground track. The structure is composed of the door frame, the running mechanism of the big car, the lifting trolley and the electrical part, etc. Some gantry cranes only have legs on one side, and the other side is supported on the factory or Zhanqiao. It is called a half door type crane. The upper frame of the portal crane is composed of the upper frame (including the main beam and end beam), the leg, the lower cross beam and so on. In order to enlarge the operation range of the crane, the main beam can extend the leg to one side or both sides to form a cantilever. The lifting car with arm frame can also be used to expand the operation range of the crane through the pitching and rotation of the arm frame.