Pulleys are one of the six simple machines. The other simple machines are the wheel and axle, the inclined plane, the wedge, the screw, and the lever. A machine is a tool used to make work easier, and the six simple machines were some of mankind’s earliest discoveries.
The simplest pulleys are made from a wheel and a rope or cord. The rim of the wheel is grooved and the rope, or cord, fits into that groove. When you pull either up or down on the rope, the wheel will turn. This makes moving the object easier, because pulling a rope downward, for instance, is easier than lifting a heavy object upward. Of course, a single pulley will not affect the amount of work necessary to move an object very much. For this reason pulleys are usually paired or combined to form systems of pulleys.
Fixed Pulley Systems
In a fixed pulley system, the pulleys are immobile and generally attached to a wall, ceiling or framework. The load moves up or down with the rope, while the pulleys remain stationary. There are advantages and disadvantages to the fixed pulley system. The fixed pulley changes the direction of force, but not the amount. In fact, the fixed pulley is the only one that requires more effort than the load itself. It requires far more effort to move the load than a movable or compound pulley system, but the pulley does not have to be pushed or pulled up and down.
Movable Pulley Systems
In a movable pulley system, the pulley is attached to the load rather than to a wall or framework. This allows the pulley to move with the load. The advantage of a movable pulley is that less work is required to move the load. However, the movable pulley has its disadvantages as well. In a movable pulley system, the pulley itself must be pushed or pulled up and down, adding its own weight to the existing load.
Compound Pulley Systems
A compound pulley system uses a combination of fixed and movable pulleys. Compound pulley systems are the most widely used of the three systems, and for good reason. The effort needed to move a load rigged to a compound pulley system is less than half of the original load. The disadvantage to the compound pulley system is that with each pulley added, the length of rope necessary and the distance that the rope must travel increase.