device consisting of a rotating. grooved wheel between two side plates or cheeks. Used to change the direction of pull in a line. as a fair lead to guide a line or as part of a mechanical advantage system (compare to block).

Merriam-Webster Online Dictionary
pulley (noun)
a sheave or small wheel with a grooved rim and with or without the block in which it runs used singly with a rope or chain to change the direction and point of application of a force and in various combinations to increase the applied force especially for lifting weights - pulling
a pulley or with ropes to form a tackle that constitutes one of the simple machines - pulleys
a wheel used to transmit power by means of a band, belt, cord, rope, or chain passing over its rim
pulley (Wikipedia)
Pulleys on a ship. In this context, pulleys are normally known as blocks.
ClassificationSimple machine
IndustryConstruction, transportation

A pulley is a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt, or transfer of power between the shaft and cable or belt. In the case of a pulley supported by a frame or shell that does not transfer power to a shaft, but is used to guide the cable or exert a force, the supporting shell is called a block, and the pulley may be called a sheave.

A pulley may have a groove or grooves between flanges around its circumference to locate the cable or belt. The drive element of a pulley system can be a rope, cable, belt, or chain.

Hero of Alexandria identified the pulley as one of six simple machines used to lift weights. Pulleys are assembled to form a block and tackle in order to provide mechanical advantage to apply large forces. Pulleys are also assembled as part of belt and chain drives in order to transmit power from one rotating shaft to another.

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