Simple Machines: Pulley Systems

A simple machine is a mechanical device, that makes it easier for us to lift or move a load, by changing the direction or increasing the magnitude of the force applied on a load. In simpler terms, simple machines are tools that make our work easier. A simple machine uses a single force that is applied to work against a single load. A number of simple machines when put together forms a complex machine, that further reduces the effort which is required to do 'work' on a load, i.e. to lift or drift a load.

Classification of Simple Machines
These machines can be classified into the following categories:

• Lever
• Pulley
• Wheel and Axle
• Inclined Plane
• Wedge
• Screw

The Pulley System
A pulley is a simple machine which consists of a wheel with a groove, running along its circumference between the rims of the wheel. A rope or cable passes through the groove which is attached to the load on one end and effort is applied to do the 'work', on the other end.

Types of Pulley Systems
There are three different types of pulley systems. They are:

1. Fixed or class 1 Pulley System: In this system the axle of the pulley is fixed at one place. While the load is tied to the one end of the rope, force is applied on the other end to lift the load. A fixed pulley only changes the direction of the force applied to do 'work' on the load. The mechanical advantage in this system is equal to 1 which means that the force applied on one end of the rope is the same as that being applied on the load to lift it.
2. Movable or class 2 Pulley System: In this system the axle of the pulley is free to move in space. While one end of the load is attached to some fixed object, force is applied at the other end. Hence when force is applied at one end, the load gets lifted or drifted as the pulley rolls over the length of the rope. The mechanical advantage of a movable pulley is equal to 2 which means that the force applied on the free end of the pulley doubles the force being applied on the load.
3. Compound Pulley: A compound pulley is a combination of fixed and movable systems. A compound pulley system has mechanical advantage greater than 2. A good example of a compound pulley is the Block and Tackle system. The 'block' refers to the case that contains the pulleys side by side, and also holds the axle in place. The 'tackle' is the rope that are used with these pulleys to lift loads. In block and tackle system the mechanical advantage is greater than 2.

Uses
In real world, the pulley system is used to lift loads to heights. Compound pulleys are used by cranes to lift loads. A simple pulley is used in a flagpole, to hoist a flag. block and tackle systems are used on ships, to lift heavy sails. Sometimes one block and tackle system is connected to another block and tackle system to increase the mechanical advantage by manifolds. The first block and tackle system is believed to have been used by Archimedes, who is said to have moved a warship full of soldiers with a compound system of pulleys.

Ancient people throwing vines over branches of trees to lift weights is not a difficult scenario for us to imagine. Pulleys have been modified from simple system to compound system, to help human being lift heavier loads. Nowadays, block and tackle systems are attached to powerful motors in cranes at construction sites and shipyards, to lift heavy loads.