What is a pulley?

We explain what a pulley is and what is the history of this machine. In addition, the types of pulleys that exist and the parts that compose it.

1. What is a pulley?

It is known as a simple machine pulley designed to transmit force and operate as a traction mechanism , reducing the amount of force needed to move or suspend a weight in the air . It consists of a wheel that turns on a central axis, and provided with a channel in its periphery where a rope passes.

The pulley can also be defined as the fulcrum of a rope that moves around it without giving it a full turn ; Such is the definition of the French scientist Hatón de la Goupillière. Thus, in one of the ends of said rope a resistance or weight acts, while in the other a power or force.

The use of pulleys  is very frequent in the fields of construction , loading or unloading of vehicles and many others, in which a rig is required to mobilize large weights with considerably less force.

For example, the mechanism designed to extract water from a deep well, so common in films and the medieval imaginary, consisted of a bucket tied to a rope that passes through a pulley. Thus, by pulling the free end, you can raise the bucket full of water (and considerably heavy) to the edge of the well.

2. Pulley history

Not much is known about the invention of the pulley. The only note in this regard in historical literature accuses Archimedes as its inventor , although he may well have been hardly a scholar and enthusiast of its use.

Plutarch tells in his book  Parallel Lives  (100 BC) that the Greek Archimedes affirmed the King of Syracuse, Hieron, to whom he joined a friendship loop , which given a strength and a fulcrum, could move any weight

 , even that of a Whole earth. To which his friend demanded a practical demonstration: he filled a ship of the royal army with cargo and passengers and asked the philosopher to move him to a dry dock.

After designing the appropriate pulley system, Archimedes sat at a distance and pulled almost effortlessly from a rope, causing the ship to rise and move so steadily that it still seemed to remain in the water .

3. Types of pulley

There are two ways to classify pulleys:

• According to its  displacement . You can talk about fixed pulleys   when they are suspended from a fixed point; or  mobile  when it comes to a set of two pulleys: one fixed and one mobile.
• According to your number . Depending on whether it is a single pulley acting alone or an interconnected set of them, we can talk about  simple  pulleys  or combined or  compound pulleys  , respectively.

4. Parts of a pulley

Every pulley consists of four fundamental parts:

• Axis . The fixed portion around which the pulley is inserted or suspended and which allows its free rotation. It is the motionless and central part.
• Tire . External area of ​​the pulley, where the throat is where the rope passes.
• Body . This is what is called the middle part of the pulley, between the hub and the tire, designed to rotate before the action of the force, provided with arms or nerves to facilitate its movement .
• Cubo . It is the inner part of the pulley, it is the cylindrical hole to which the shaft is attached.