两级门架(Two-stage Mast)
两级或双工桅杆是一种伸缩式桅杆，具有固定的外部框架和内部可移动框架。抬起叉车时，它会横穿固定门架的整个长度（自由举升），然后延伸内部门架通道。
英文介绍
Two-stage or Duplex mast, is a telescopic mast with a stationary outer frame and an inner moveable frame. When lifting the fork carriage traverses the entire length of the stationary mast (free lift) and then extends the inner mast channel.