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A pulley tool is a simple machine that allows a worker to change the direction of applied force and possibly handle weights that far exceed what he would normally be able to maneuver. There are three different types of pulleys: fixed, movable and compound. A fixed pulley is used to change the direction of applied force only while the other two will change the direction and increase pull strength.
Regardless of the purpose of the pulley tool, the design is generally very similar. At its most basic, a pulley is a wheel, axle and rope or strap. The axle is connected to a larger machine or secured in place using a stationary object. The wheel is able to spin freely on the axle and the rope or strap sits against the wheel. Most pulleys have grooves in the wheel so the rope or strap doesn’t slide off when lateral force is applied during use.
The original purpose of the pulley tool was to create a change in force direction. A pulley will change the direction of applied force by wrapping around the wheel. The rope goes into the system at one angle, but comes out another. The difference between these two directions is the degree of change provided by the tool. For example, if a person pulls down on a rope that goes over a pulley, the force on the rope is down, but the effect is transformed to go up after draping over the wheel.
The other, but arguably even more important, use of a pulley tool is multiplying mechanical advantage. With some pulleys, the force applied to the rope on one end in multiplied while moving through the system. A pulley with a mechanical advantage of one provides no additional benefit; a two rating doubles the benefit, a three triples and so on. For instance, when 100 pounds (220 kg) of force are applied to a pulley tool with a three-mechanical advantage, the other end of the rope pulls with 300 pounds (660 kg) of force.
The different types of tools are a mix of the common pulley properties. The fixed pulley tool has an advantage of one and only is used when changing direction, such as with the belts in a car engine. These pulleys have an axle that can’t move from a single fixed position.
The other two types of pulley tools can move around, changing their orientation based on the direction of applied force and providing mechanical advantage. A movable pulley is just as basic as a fixed pulley other than the freedom of its axle to move. These pulleys provide a mechanical advantage of two. A compound pulley, also called a block-and-tackle, is a single line that moves through more than one movable pulley. Each additional pulley provides slightly less mechanical advantage than the one before it so the final value is variable, but this style will always provide an advantage greater than two.
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