The fixed set of pulleys (called sheaves) located at the top of the derrick or mast, over which the drilling line is threaded.是什么意思_The fixed set of pulleys (called sheaves) located at the top of the derrick or mast, over which the drilling line is threaded.在線(xiàn)翻譯_英語(yǔ)_讀音_用法_例句