Block and tackle

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This block and tackle on a davit of the Mercator is used to help lower a boat.

A block and tackle"Tackle is pronounced "take-el" with a long "a". The source for this is said to be the Royal Navy 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 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 tackle is offset by both the increased length of rope needed and the friction in the system. 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. Frictional losses also mean there is a practical point at which the benefit of adding a further sheave is offset by the friction. This is revealed by either the tackle not allowing the load to be released easilyFriction may mean that the rope in a tackle "bunches" and jams when the force is released if the tackle has too much friction for the load to balance, or that the tackle does not "lower" the load, or by the reduction in force needed to move the load being judged insufficient because undue friction has to be overcome as well.

A tackle may be

• "Rigged to advantage" - where the pull on the rope is in the same direction as that in which the load is to be moved
• "Rigged to disadvantage" - where the pull on the rope is in the opposite direction to that in which the load is to be moved

See also

• Crane (machine)
• Tripod
• Two six heave
• Winch

Notes