| 摘要 |
An extensible boom, which comprises a first part which is pivotally anchored to a rotatable pedestal so that the boom may be raised or lowered. A second part of smaller lateral dimension is adapted to slide within the first part. The outer end of the second part carries a crown, with at least two crown sheaves. On the inner end of the second part there are two pulleys having transverse vertical pins about which they rotate. On the outer end of the first part there is a third pulley having a shaft substantially horizontal. A conventional cable drum is supported on the pedestal with conventional drive means. The cable spools from the drum over one of the crown shafts down to a travelling block and hook and is arranged over said pulleys in a manner such that when there is a weight hanging on the hook, there will be a force tending to extrude the second part out of the first part, unless it is held back by a tension member. This tension member comprises a long threaded rod, which is threaded through a nut on the inner end of the second part. The extension or withdrawal of the second part from the first part of the boom, is controlled by rotating the threaded rod in the nut.
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