## What is Pulley?

• A Pulley is a wheel mounted on the shaft which can move a belt along it's circumference.
• It can be used to lift loads and transmit the motion.

• Pulleys are used to increase or decrease the speed of rotary motion.
• There are various times of timing pulleys like Flat Pulley, V Pulley, Timing pulley, etc....
• Below are useful calculations which are used in belt and pulley transmission.

• In above image two pulleys are shown: Driver pulley is mounted on the shaft which is connected with motor or any other source and Driven pulley is which is driven by driver pulley.
• In above example the diameter of mm and rotating at  100 revolutions per minute (rpm).
• The driven pulley has a diameter of 120 mm.
• If we want to calculate the rpm of the driven pulley there are two options:

### Option-1:

We can use the standard equation N1D1=N2D2
N1= rpm of driver pulley
D1= Diameter of the driver pulley
N2= rpm of driven pulley
D2= Diameter of the driven pulley
Hence, N2 = (N1D1)/D2
= (100x40)/120
= 33.33 rpm

Instead of flat pulley and belt if timing pulley and belt are used the equation will be N1T1=N2T2,
Where T1 and T2 indicates the number of teeth of driver and driven pulley and N1 and N2 indicates the rpm of driver and driven pulley.

### Option-2:

Calculate the velocity ratio
Velocity ratio =  Diameter of  driven pulley / Diameter of driver pulley
Velocity ratio = 120/40 = 3
The output speed of a pulley system is calculated as follows.
Output speed =Input Speed / Velocity ratio
output speed = 100 3 = 33.3 rpm.

By above options you can calculate unknown diameter of rpm if you have  factors.