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A block and tackle〔"Tackle" can be in this usage.〕 is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads. The pulleys are assembled together to form blocks and then blocks are paired so that one is fixed and one moves with the load. The rope is threaded, or rove, through the pulleys to provide mechanical advantage that amplifies that force applied to the rope.〔Ned Pelger, (ConstructionKnowledge.net )〕 Hero of Alexandria described cranes formed from assemblies of pulleys in the first century. Illustrated versions of Hero's "book on raising heavy weights" show early block and tackle systems.〔A. P. Usher, (''A History of Mechanical Inventions'' ), Harvard University Press, 1929 (Dover Publications, revised edition, 2011, ISBN 978-0486255934)〕 == Overview == A block is a set of pulleys or "sheaves" mounted on a single axle. The assembly of two blocks with a rope threaded through the pulleys is called tackle. A block and tackle system amplifies the tension force in the rope to lift heavy loads. They are common on boats and sailing ships, where tasks are often performed manually. In the diagram on the right, the number of rope sections of the tackles shown is as follows: * Gun Tackle: 2 * Luff Tackle: 3 * Double Tackle: 4 * Gyn Tackle: 5 * Threefold Purchase: 6 Note that the Gun tackle, Double tackle and Threefold purchase all have the same number of pulleys in both blocks (one, two and three, respectively) whereas the Luff tackle and Gyn tackle have mis-matched blocks with differing numbers of pulleys. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「block and tackle」の詳細全文を読む スポンサード リンク
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