A pulley tool changes the direction of force and can handle weights beyond human capacity. There are three types: fixed, movable, and compound. They consist of a wheel, axle, and rope or belt. Pulleys can also multiply mechanical advantage. Fixed pulleys have a mechanical advantage of one, while movable pulleys have a mechanical advantage of two. Compound pulleys have a variable mechanical advantage greater than two.
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 they would normally be able to handle. There are three different types of pulleys: fixed, movable and compound. One fixed pulley is only used to change the direction of the applied force while the other two will change the direction and increase the pulling force.
Regardless of the purpose of the pulley tool, the design is generally very similar. In its most basic form, a pulley is a wheel, axle, and rope or belt. The axis is attached to a larger machine or fixed in place by a fixed object. The wheel is able to spin freely on the axle and the rope or strap rests on the wheel. Most pulleys have grooves in the wheel so that the rope or webbing will not slip out when a lateral force is applied during use.
The original purpose of the pulley tool was to create a change in the direction of force. A pulley will change the direction of the applied force by wrapping itself around the wheel. The string enters the system at one angle, but exits at 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 passing over a pulley, the force on the rope decreases, but the effect transforms to go up after being draped over the wheel.
The other, but probably even more important, use of a pulley tool is multiplying the mechanical advantage. With some pulleys, the force applied to the rope at one end is multiplied as it moves through the system. A pulley with a mechanical advantage of one provides no additional advantage; a two doubles the lead, a three triples, and so on. For example, when a force of 100 pounds (220 kg) is applied to a pulley tool with a lead of three mechanisms, the other end of the rope pulls with a force of 300 pounds (660 kg).
The different types of tools are a mix of common pulley properties. The fixed pulley tool has the advantage of one and is only used when changing direction, such as with a car’s engine belts. These pulleys have an axis that cannot move from a single fixed position.
The other two types of pulley tools can move, changing their orientation based on the direction of the applied force and providing a mechanical advantage. A movable pulley is as essential as a fixed pulley, apart from the freedom of movement of its axis. These pulleys provide a mechanical advantage of two. A compound pulley, also called a block and tackle, is a single line that moves across more than one moving 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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