What is Pulley: Types, Formula, Working & Examples

A pulley is a wheel containing a flexible rope, cord, cable, chain, or belt hanging on its rim.

• Pulleys can be used singly or in combination to transmit energy and motion.
• Pulley makes our work simpler and knowing its mechanism we can use the pulley for our benefit.
• Block and tackle is a combination of pulleys, blocks, rope, and other flexible material on which the shafts of the pulleys turn.
• A pulley is used to lift weights. A simple example of a pulley is the device used to raise a flag.
• Compound pulleys are said to have been used by the Greek mathematician Archimedes (3rd century BCE) to pull a ship onto dry land.

Key Terms: Pulley, Pulley Formula, Mechanical Advantage, Pulley System, Friction, Motion, Force, Tension, Effort, Types of pulleys, Lift

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What is a Pulley?

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A pulley is a mechanical device consisting of a wheel and a rope that can be used to lift objects. A pulley can be of different sizes. There are different sizes of pulley available in the market.

• A pulley rotates freely about an axis passing through its center.
• With the help of a pulley, we can easily lift heavy objects.
• The best and most simple example of a pulley can be seen in the wells of most villages.
• In this type of pulleys, a bucket is attached to one end of the pulley which goes down into the well, and the upper part of the rope is pulled by hands.

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Types of Pulley

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In today’s world, out of all the types of pulleys, the three most common and prominent categories are given below:

1. Fixed Pulley
2. Movable Pulley
3. Compound Pulley

Fixed Pulley

Fixed Pulleys are linked to a rigid structure that’s why they are not movable. These types of pulleys can be usually seen in ceilings, walls, or floors.

• Therefore the system is stationary. However, the rope can move freely.
• The number of segments pulling up in this system is one.
• Ideal Mechanical advantage is one and there is a change in the direction of a force.
• This particular system provides us with the option to alter the direction of the implemented force.
• Apart from this, it reduces the need for a person to lift a heavy object off the ground.
• Some of the examples are the ones mounted on a deep well. 

Movable Pulley

A Movable Pulley is not linked to a rigid structure. One of the ends of the rope is connected to a rigid structure.

• In this mechanism, wheels carry the load instead of ropes.
• If one end of the rope is pulled, the load shifts from one position to another.
• A small amount of work is required in this type of pulley.

Compound Pulley

It is the combination of the functions of both fixed and moving pulleys.

• In this case, the movable pulley connects with the rope attached to a fixed one.
• Accordingly, one can raise the weight on the movable wheel to shift its position.
• Based on one’s requirements, it helps one to redirect force while also altering the overall workload of that force.

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Mechanical Advantage of Pulleys

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To measure the effectiveness of the pulleys mechanical advantage is used. The ideal mechanical advantage of a machine can be calculated in the absence of friction.

• All the machines have some friction, but somehow if all the machines overcome friction, the mechanical advantage is always somewhat greater than the mechanical advantage in the real world.
• The mechanical advantage of a pulley system is the ratio of the force utilized in the work to the force applied in the work.
• During the calculation of the mechanical advantage of the pulley and rope system, we make assumptions that the weights of the pulley and ropes are negligible and no loss of energy occurs due to the friction between the wheel and ropes.
• It is also assumed that ropes used during the operation do not elongate during the process.

The mechanical advantage of a pulley is directly proportional to the number of loops in the rope. In a single-loop system, the effort required to lift the weight is equal to the weight of the body itself.

• When there are two loops (attaching another wheel with additional rope) the effort of pulling the weight becomes half.
• This is a case where mechanical advantage becomes 2.
• Again when another loop is added to the system, it reduces the effort used in pulling the weight of the body by one-third.
• In this case, the mechanical advantage becomes 3.
• In simple words, if you want to increase the mechanical advantage of the system, you can add more pulleys.
• But after adding a certain number of pulleys, the mechanical advantage doesn't change at all and remains the same.

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Pulley Formula

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The following are the formulas used in solving the problems regarding pulley

Mechanical Advantage = Load/Effort

We can also write the above formula as

• Load = Mechanical advantage x Effort
• Effort = (Load/Mechanical advantage)

Velocity ratio is defined as the ratio of the distance moved by effort and load.

Velocity Ratio = Distance of effort/Distance of Load

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Advantages of Pulley

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The following are the advantages of pulley

• While using a pulley the effort becomes less to lift a weight as compared to normal lifting.
• It mainly reduces the actual amount of force required to lift heavy objects.
• While using a pulley, the actual direction of force also changes.
• Change in the direction of force and reducing the amount of it provides a mechanical advantage.
• There is also an advantage regarding the distance between the operator and the weight.
• An operator can do its work from a distance.
• There will be a safe distance while lifting the weight which can avoid any disaster.
• Another advantage of the pulley system is that it is easily assembled and cost-effective.
• If we put a number of different directional pulleys it can reduce a lot of effort to lift the weight.
• There are some parts of the pulley that after installation require less or no lubrication.

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Disadvantages of Pulley

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After going through all the above advantages of the pulley system, it also comes with some disadvantages.

• The first disadvantage is that a large space is required to install and use a pulley.
• We can also use a high number of pulleys for mechanical advantage but it will need a large amount of space too.
• There are chances of slipping while using ropes or belts over the wheels with no grooves.
• If the pulleys are operated for a long time, it requires some maintenance and regular check-up of the cables/ropes/belts.
• This is because friction causes wear and tear between the rope and the wheel.
• Continuous use of rope makes the ropes weak.
• The rope can break sometimes while used by the operator causing trouble sometimes.

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Things to Remember

• A pulley is a machine simply consisting of a rope wrapped around a grooved wheel. It is mainly used to lift a load.
• Three types of pulleys are used: Fixed Pulleys, Movable Pulleys, and Compound Pulleys.
• The ideal mechanical advantage of the pulley system is equal to the number of loops of the rope pulling the load.
• If we take a perfectly efficient pulley then its mechanical advantage and velocity ratio are both equal to the number of pulleys taken.
• Mechanical advantages are the ratio of load to effort.
• Velocity ratio is defined as the ratio of the distance moved by effort and load.
• We can also use a high number of pulleys for mechanical advantage but it will need a large amount of space too.
• There are chances of slipping while using ropes or belts over the wheels with no grooves.

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Previous Year Questions

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