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The fixed set of pulleys (called sheaves) located at the top of the derrick or mast, over which the drilling line is threaded.
位于重型井架或桅架式井架顶部的固定的滑轮组（称之为滑车轮槽轮），通过该滑轮组钻井钢丝绳进行穿绳。
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Four fixed rings need to be added on upper deck for derrick holds.
吊货舱上层甲板上应装4个固定环。
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