门式起重机（Gantry Crane），又叫龙门吊，是桥式起重机的一种变形，主要用于室外的货场、料场货、散货的装卸作业。其金属结构像门形框架，承载主梁下安装两条支脚，可以直接在地面的轨道上行走，主梁两端可以具有外伸悬臂梁，具有场地利用率高、作业范围大、适应面广、通用性强等特点。门式起重机(又称龙门起重机)是桥架通过两侧支腿支撑在地面轨道上的桥架型起重机。在结构上由门架、大车运行机构、起重小车和电气部分等组成。有的门式起重机只在一侧有支腿，另一侧支撑在厂房或栈桥上运行，称作半门式起重机。门式起重机的门架上部桥架(含主梁和端梁)、支腿、下横梁等部分构成。为了扩大起重机作业范围，主梁可以向一侧或两侧伸出支腿以外，形成悬臂。也可采用带臂架的起重小车，通过臂架的俯仰和旋转扩大起重机作业范围。Gantry crane, also called gantry crane, is a kind of deformation of bridge crane, which is mainly used for loading and unloading of outdoor freight yard, stock yard and bulk cargo. Its metal structure is like a door-shaped frame, carrying two support legs installed under the main beam, which can directly walk on the ground track. The two ends of the main beam can have extended cantilever beams, which have the characteristics of high site utilization, large operating range, wide adaptability, and strong universality. Gantry crane (also known as gantry crane) is a bridge type crane whose bridge is supported on the ground track through the legs on both sides. Structurally, it consists of a gantry, a crane operating mechanism, a lifting trolley, and electrical components. Some gantry crane only have legs on one side, and the other side is supported on the powerhouse or trestle for operation. They are called semi gantry crane. The upper bridge (including main beam and end beam), legs, lower beam and other parts of the gantry of the gantry crane are composed. In order to expand the operating range of the crane, the main beam can extend beyond the support legs to one or both sides, forming a cantilever. A lifting trolley with a boom can also be used to expand the operating range of the crane through the pitch and rotation of the boom.