Pulley Examples


A pulley is a wheel on an axel or shaft that is designed to support movement and change of direction. Pulleys can also be machines consisting of a wheel over which a pulled rope or chain runs to change direction or lift a load. Pulleys are examples of what scientists call simple machines. If you want to lift a really heavy weight, there's only so much force your muscles can supply. Using a simple machine such as a pulley can effectively multiply the force your body produces. Pulleys are used in a variety of ways to lift loads, apply forces, and to transmit power.

A pulley may also be called a sheave or drum and may have a groove or grooves between two flanges around its circumference. The element used to drive a pulley system can be a rope, cable, belt or chain that runs over the pulley inside the groove or grooves.

The simplest theory of the operation of a pulley system assumes that the pulleys and lines are weightless, and that there is no energy loss due to friction. It is assumed that the lines do not stretch. The load on the moving pulley is balanced by the tension in two parts of the rope supporting the pulley.

The more wheels you have, and the more times you loop the rope around them, the more you can lift. If you have a single wheel and a rope, a pulley helps you reverse the direction of your lifting force. So you pull the rope down to lift the weight up. If you want to lift something that weighs 100 lbs, you have to pull down with a force equivalent to 100lbs. If you want to raise the weight 1foot into the air, you have to pull the loose end of the rope a total distance of 1foot at the other end.

Now if you add more wheels, and loop the rope around them, you can reduce the effort you need to lift the weight. Suppose you have two wheels and a rope looped around them, a 100lb mass is now effectively supported by two sections of the same rope instead of just a single rope. This means you can lift it by pulling with a force half as much. If four pulleys are used, then you are distributing the force between all four pulleys and it only takes a quarter of the force needed to pull the 100lb weight.

Examples of Pulley:

1. Fixed pulleys

It has an axle mounted in bearings attached to a supporting structure. A fixed pulley changes the direction of the force on a rope or belt that moves along its circumference. Mechanical advantage is gained by combining a fixed pulley with a movable pulley or another fixed pulley of a different diameter. An example would be the grooved wheel on a bicycle where the chain is wrapped. As you apply force the pulley moves to produce energy which propels the bicycle forward.

2. Movable pulleys

They have an axle in a movable block. A single movable pulley is supported by two parts of the same rope and has a mechanical advantage of two. Examples of movable pulleys include construction cranes, modern elevators, and some types of weight lifting machines at the gym.

3. Combination pulleys:

They form a block and tackle. A block and tackle can have several pulleys mounted on the fixed and moving axles, further increasing the mechanical advantage.

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