Pulley Systems

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What are Pulleys

Pulleys are used to change the speed, direction of rotation, or turning force or torque.

pulley system consists of two pulley wheels each on a shaft, connected by a belt. This transmits rotary motion and force from the input, or driver shaft, to the output, or driven shaft.

A pulley system with one 40mm diameter pulley and a 120mm pulley, connected by a belt. The smaller pulley is rotating at 100rpm (http://www.bbc.co.uk/staticarchive/97fe6399a4d793223ee9826adee2b5017e17d385.gif)

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Velocity Ratio

If the pulley wheels are different sizes, the smaller one will spin faster than the larger one. The difference in speed is called the velocity ratio. This is calculated using the formula:

Velocity ratio = diameter of the driven pulley ÷ diameter of the driver pulley

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Output Speed and Output Torque

If you know the velocity ratio and the input speed of a pulley system, you can calculate the output speed using the formula:

Output speed = input speed ÷ velocity ratio

The velocity ratio of a pulley system also determines the amount of turning force or torque transmitted from the driver pulley to the driven pulley. The formula is:

output torque = input torque × velocity ratio.

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