A pulley is a mechanical device consisting of a wheel mounted on an axle or shaft, designed to facilitate the movement and redirection of a taut cable or belt, or to transfer power between the shaft and the cable or belt. Its versatile design enables it to serve various functions, from supporting loads to guiding cables or exerting forces in different applications.

In instances where a pulley is supported by a frame or shell that guides the cable or exerts a force without directly transferring power to a shaft, it's often referred to as a block. The pulley itself may then be termed a sheave. This distinction is crucial in systems where the primary function is to guide or redirect rather than to transmit power.

To ensure smooth and efficient operation, pulleys are often equipped with grooves between flanges around their circumference. These grooves serve to securely hold the cable or belt in place, preventing slippage and ensuring proper alignment and engagement between the pulley and the drive element.

The drive element of a pulley system can vary depending on the specific application and requirements. It may consist of a rope, cable, belt, or chain, each chosen for its suitability in transmitting power or facilitating movement with minimal friction and wear.

History of the Pulley

The pulley, a simple yet ingenious device, has played a crucial role in human civilization for thousands of years. From its ancient origins to its modern applications, the pulley has been instrumental in lifting heavy loads, facilitating construction, and powering machinery.

Throughout history, the pulley has exemplified the principles of simplicity, efficiency, and mechanical advantage. Its ability to multiply force and redirect motion has enabled humans to overcome physical limitations and accomplish tasks of immense complexity. As a timeless symbol of engineering ingenuity, the pulley continues to shape the course of human progress and innovation in the modern world.

Mechanical Advantage (MA)

The powerful mechanical advantage of a pulley is in using many pulleys at once. Combining multiple pulleys decreases the amount of force necessary to move an object by increasing the amount of rope used to raise the object. The amount of rope can be found by rope = original amount of rope x the number of pulleys. The mechanical advantage (MA) of a pulley system is equal to the number of ropes supporting the movable load.

The most commonly understood concept of a pulley is that it is a simple machine that redirects force. This means that by looping a rope around a pulley and attaching the rope to an object, one pulls down on the rope to raise the object, instead of having to lift the object. Although this is a helpful and convenient use for pulleys, it has a major limitation: the force you must apply to lift the object is the same amount as if you were just lifting the object without the pulley (which is acceptable for raising a flag, but not helpful enough if trying to raise a pyramid stone). This means that a fixed pulley does not give any mechanical advantage. A fixed pulley configuration is useful for raising an object to a level above your head. 

Using this type of pulley also enables you to take advantage of gravity. And, by attaching weights to the end of the rope that you pull, you can lessen the amount of force you must apply. This type of pulley can also be used to balance an object, by attaching objects of equal weight to both sides of the rope, neither object moves. Once a force is applied to either side, the system continues moving in that direction. This kind of pulley system is used in some elevators. The elevator has a cable attached to it that goes up, around a pulley, then comes down and attaches to a counterweight. The motor that moves the elevator car uses much less power since the counterweight keeps the elevator balanced.

Another type of pulley is a movable pulley. In a movable pulley system, the rope is attached to a fixed point, the pulley is attached to the object that you want to move and the other end of the rope is left free. By pulling on the rope, the pulley moves and the object raises. This type of system is good if you are trying to raise an object located below you to your level. Using a system of pulleys can be much more complex and provide a powerful mechanical advantage — greatly reducing the amount of force required to move an object. If one movable pulley is used, the amount of force required to raise the object attached to the movable pulley is cut in half. 

The pulley system seen in the image to the left does not change the mechanical advantage from image to the right. However, it does change the direction of the necessary force. The trade-off is that the amount of rope required increases and the amount of rope that you must pull to raise the object is also increased. 

If two fixed pulleys are added to the system and a second movable pulley is attached to the object, the amount of force needed to raise the object becomes one-fourth of the object's weight, and four times as much rope is required. This pattern can continue with as many pulleys as possible to continue increasing the Mechanical Advantage. You will just need more rope!

Common Uses

Pulleys are used all over the place, oftentimes without us giving it a second thought. Pulleys were once used and still used to raise a bucket of water from a well, they are used in elevators, window blinds, workout equipment, cranes and other construction equipment, boats, and more! Pulleys are used any time you want to raise something high or lower something down and they are used anytime you need an easy way to create a mechanical advantage for yourself.