Pulleys provide a mechanical advantage when lifting heavy loads. A pulley system consists or either a rope, belt or a chain wrapped around a wheel. A pulley's main function is to change the direction of the force or transmit rotational motion, making it easier to lift things. If you are working with pulleys, it is important that you know the different types.
A fixed pulley has an axle that is anchored in place. This type of pulley is used to change the direction of the force on a belt or rope. When using this pulley, you don't have to push or pull it down. The force that is used to lift the load is even on both sides.
A movable pulley has a free-moving axle, meaning that it is free to move in space. This lessens the effort to move the load. With a movable pulley, if you anchor one end of the rope then pull on the other end, this will apply a doubled force to the object attached to the pulley.
The combined, or compound, pulley is basically a combination of both the fixed and movable pulley system. This type of pulley is easier to handle as you use even less effort to lift the load. The advantage of using the combined pulley is that the effort expended reduces the weight of the load by half.