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:


      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.


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.

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