What is a pulley?
The pulley is considered one of the simplest mechanisms used to transmit a force through a rope in order to lift an object that is placed at a point other than the application of force, also to transmit it from an axis to other.
These types of devices that are simple and capable of changing the direction of force are known as simple machines.
Parts of a pulley
These mechanisms are quite simple since they have few parts, but each one does an indispensable job for its operation. Its parts are:
- Body: It has a disc shape, either totally solid, or with some vents that are used for air flow and to reduce the weight of the pulley.
- Hub: It is the central part of the disc, it contains a hole in which an axis is placed to keep the pulley fixed.
- Throat or channel: It is a small groove or groove that is used to house the band or the rope.
- Structure : It is the part where the body is placed, it is used to fix the pulley to some structure, wall or ceiling. They usually have a hook for easy attachment.
- Force transmission element: Ropes, bands or chains are usually used.
How does a pulley work?
The operation of a pulley is quite simple since you have to put a rope that surrounds the slot of the disc, then the load or object to be lifted is attached to one end and as a final step the other end is taken and exerts a downward force to be able to lift.
Types of pulleys
There are different types of classifying them, either by how they are used, by the shape of their grooves, or by their physical shape.
Due to its use
There are several ways to use the pulleys, either as a whole or individually and depending on each way there is a classification as simple, mobile or hoists.
This is the easiest way to use them, since it consists of using only a fixed pulley, it has no mechanical gain, that is, if an object exerts 50 Newtons of force towards the ground due to the force of gravity and its mass, the same 50 Newtons to lift it.
In this way, a single mobile pulley or two pulleys can be used, one of them fixed and the other is mobile, they have the ability to move up or down depending on the moment the load is. The force necessary to lift an object is less than that generated by gravity, since to lift an object it takes half the force exerted by it on the ground, there is a small disadvantage and this is that the distance it has that running the rope is twice the desired height.
Hoists are the combination of fixed and mobile pulleys covered by a single rope, these systems allow lifting objects that have high loads since with each pulley that is coupled the force required to lift it is decreased.
Within the hoists we can find two different ones:
- Potential rigging: It is composed of a fixed pulley and 2 or more mobile pulleys, although this type is very useful it has a disadvantage and this is that the maximum lifting height is given by the distance of the first two pulleys (the fixed and first mobile).? (figure above)
- Factorial rig: It has an even number of pulleys, half are fixed and the other half are mobile, the force to be used depends on how many mobile pulleys are placed. (Figure below)
By the shape of its grooves
This classification is quite simple, since they are designed for two different transmission objects, for ropes and bands.
- For ropes: There is only one type of groove and it is semicircular
- For bands: We found three different types of grooves: grooved, flat and trapezoid. Each one of these designed to fit correctly to the different shapes of the bands that are used.
This type of pulleys are mainly used for machine transmissions in order to vary the speed due to the difference in diameters.
Double, Triple and Quadruple
Basically they are some pulleys placed in parallel on the same structure, we can find from two, three and up to four pulleys placed simultaneously. These are regularly used to make an arrangement of a hoist in a simpler way.
This type of pulley can only be used to raise loads since once the rope is pulled it is impossible to return or lower it since it has a mechanism that blocks that part of the pulley.
Usually they are used for large loads in transmission systems, within this type we can find pulleys with two or three grooves and this is because the load is distributed among the number of grooves. For example, in a system where a great force is needed to move, if we only have one band in place, it slips because it is not capable of supporting the load.
This type of pulley is designed to be used in transmission systems where there can be no slippage of the belt, they are also used when two axles are separated at such a distance that it is not possible to economically or technically place a transmission with gears.
This type of pulleys are for more specific and personalized applications since depending on the position in which this pulley is found, it will be necessary to apply a greater or lesser force than the resistance exerted by the object.