| 摘要 |
Mounted in the upper part of an oil well drilling derrick are horizontal crown beams having end portions projecting beyond the opposite sides of the derrick. A fast line sheave and a dead line sheave are mounted on those beams near the opposite sides of the derrick, and a crown block is mounted on the beams between those sheaves. The sheaves and the crown block are adjustable along the beams. The fast line sheave is movable outwardly into an inactive position on the adjacent projecting end portions of the beams when the crown block is in a position suitable for supporting a fast line directly, and the dead line sheave is movable outwardly into an inactive position on the adjacent projecting portions of the beams when the crown block is in a position suitable for supporting a dead line directly. When both sheaves are in operating positions, the crown block can be placed in any desired location between them.
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