Block and Tackle

A block and tackle (pronounced "block and take-el") is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads. Although used in many situations, they are especially common on boats and sailing ships, where motorized aids are usually not available, and the task must be performed manually. The block and tackle pulley was invented by Archimedes.

The most common arrangement is to have a set of fixed pulleys or "sheaves" all mounted on a single axle, and another set left to move. Each set is called a "block" (the whole assembly, with the rope, is the "tackle"). The mechanical advantage of a block and tackle is equal to the number of lines running between the two blocks. For example, a block and tackle consisting of three fixed and three moving pulleys has six lines going between the pulleys, which gives it a mechanical advantage of 6. This enables the lifting of weights six times heavier than would be possible otherwise.

The increased force produced by a block and tackle is offset by the increased length of rope needed. In order to raise a block and tackle with a mechanical advantage of 6 a distance of 1 metre, it is necessary to pull 6 metres of rope through the blocks.

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