Fig 1: Two types of clamp pulley
A pulley wheel attached to a clamp.
Pulleys are essentially a wheel with a groove along its edge for holding a rope or string. They can be set up to reduce the amount of force required to move a load.
Often used in forces lessons where masses are moved using string, pulleys come in various shapes and sizes. Some come with steel wheels, others are made from plastic or nylon.
The type you choose will depend on the weight it has to support. Cheap plastic pulleys will contort under too much pressure whereas heavy metal pulleys will not turn with ease if a small force is applied.
Always ensure the pulley is well oiled to enable slick rotation of the wheel. Also make sure the clamp (if required) fits tightly to the desk or beam to which it is to be attached. If not secure, weights being lifted could fall.
In the above picture, the red clamp is intended for fixing to a desktop whereas the blue clamp is designed to attach to a retort stand.
Always make sure that the string or rope that goes out with these clamps is of the correct diameter to fit the groove in the wheel.
Technically speaking, there are three main types of pulley:
A fixed or class 1 pulley has a fixed axle which is anchored in place and does not move (see Fig 2).
A moveable or class 2 pulley has a moveable axle. This type has a mechanical advantage of two which means that if one end of the rope is anchored, pulling on the other end will apply a doubled force to the weight attached to the pulley (see Fig 3).
A compound pulley is a combination of the above two pulley systems.
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