| 摘要 |
A drilling or service rig includes a worm drive disposed within and rotatabl y supported within a mast having one open side. The worm drive defines a helical groove, and a collar is disposed on the worm drive and engages the helical groove. Rotation of the worm drive causes movement of the collar upwards or downwards along the worm drive. The collar may support a block and top drive assembly.
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