| 摘要 |
<p>The portal crane comprises two columns and a gantry, and its clear width is variable, typically by sliding the gantry in relation to one or both columns. There can be two or more counterweights sliding freely on the gantry, their movement being controlled by the tipping moment. The crab can be arranged to travel on the gantry beyond the columns. The gantry can be telescopic, or divisible into two halves. It can be formed by joining together end-to-end two main or tailweight jibs of tower cranes rigidly so that they can slide as a whole in relation to the columns. The latter can be equipped with climbing mechanisms.</p> |
