Pulleys

A pulley is a grooved wheel, which is free to turn about an axle that is fixed in a frame.

Types of pulley;

1.       Single fixed pulley

2.       Single movable pulley

3.       The block and tackle system of pulley

Single fixed pulley

This is the type of pulley whereby effort is applied at one end of the tape in order to raise the load. Single fixed pulleys are used to raise small objects e.g. Flags.

Consider the diagram below:-

In single fixed pulley M.A is equal to V.R of machine.

SINGLE MOVABLE PULLEY

Single movable pulley is the one which load is multiple of Effort.

Load = 2E

Consider the following diagram.

When Effort moved a distance X

The block and tackle system of pulley

When two or more pulleys are fixed in a frame, a block is formed. The pulleys in each block are fixed

independently on separate axles. Consider the diagram below:-

Note;

 Velocity ratio of Block and tackle system is equal to the number of pulleys.

For example, in above pulley system the Velocity ratio = 4

Example
A block and tackle pulley system has a velocity ratio of 4. If a load of 200N is raised by using a force of 75N. Calculate the mechanical (i) Advantage of the system  (ii) efficiency of the system.
Solution;

Data given   
Velocity ratio (V.R) = 4
Load = 200N
Effort = 75N
M.A =?

Mechanical Advantage of the system = 2.67

The efficiency of the machine is 66.7%