3. How to calculate the size of Pulleys for a machine using 2 sets

As we calculated size of a Pulley required for a machine for 84 RPM and a motor of 2800 RPM. Now if we don’t want larger pulleys in case we don’t have a space for a 14 inch pulley to be used on machine. We can reduce the size of pulley by introducing another set of coupled pulley. As below:

We can calculate the RPM at 1^st pulley;

RPM at 1^st Pulley = Motor Pulley  x  Motor RPM

1^st Pulley

RPM at 1^st Pulley = 2 x 2800  =  700 RPM

8

Now we will find the RPM at 2^nd pulley;

RPM at Machine  =     2^nd Pulley  x  RPM at 2^nd Pulley

Machine Pulley

RPM at Machine  = 3 x RPM at 2^nd Pulley  =  84

8

=>   RPM at 2^nd Pulley  =  84 x 8 / 3  =  224 RPM

Now we can find out the 1^st pulley ‘r’ as we know:

RPM at 2^nd Pulley  =     1^st Pulley  x  RPM at 1^st Pulley

2^nd Pulley

RPM at 2^nd Pulley  =  r  x  700  =  224 RPM

8

=>                    r  =  224  x  8/700  =  2.56 inch

Now we have calculated all the required components.