To determine the input or output speed or the pulley and cog sizes needed in a drive you may use a simple ratio. Given any three items you can solve for the fourth. With a V-Belt drive the answer is theoretical, due to belt slippage, and with a timing belt or a chain drive not every answer provided is an available combination. You may use this basic formula for more than just a two pulley setup. The RPM of the output pulley becomes the input RPM for the next stage of the drive. Done this way you can use this formula to solve for a three or more pulley drive. Just remember that idlers do not change the output of the drive system. They are used to provide tension or to route the belt. Below is the basic formula, an example, and a Pulley Speed Calculator for your use. Please remember that there are more factors in a good drive setup than just if the pulleys fit.
Ratio:
| RPM1 | D1 |
| RPM2 | D2 |
RPM1 = RPM of Driver - RPM2 = RPM of Driven
D1 = Diameter or tooth count of Driver - D2 = Diameter or tooth count of Driven
Example:
| 1750 RPM | 3.5 INCHES |
| 1000 RPM | D2 |
1750 RPM ÷ 1000 RPM × 3.5 INCHES = D2
D2 = 6.125 INCHES
Pulley and Speed Calculator:
Below is a small calculator that will solve the ratio for you. Check the circle next to the item you are solving for and enter the remain three items in the spaces provided. If you have any problems with the pulley calculator please email us.
