# Calculate RPM when changing pads

Written by: Vinnie van Rooij

In some cases, you feel comfortable with a certain speed (rpm), and feel that this works best with a certain product. However, not all your pads are the same size and you want to know what speed setting you need to maintain the same speed.

## Why maintaining the speed

Not all pads are the same size, which means that the outside of the pads does not always spin with the same speed. We can demonstrate this with a simple example.

Let’s say that we have a wheel that makes 20 turns every minute. The wheel has a diameter of 20 inches.

The circumference of this wheel is (diameter * π) 20 * 3.14 = 62.8 inch. Which basically means that the wheel would travel a distance of 62.8 inch when it spins 1 time. Seeing as the wheel spins 20 times a minute, we can calculate that it would travel 20 * 62.8 a minute. This comes to: 1256 inch (104 feet 8 inches). The outside of this wheel spins with a speed of 1256 inch per minute.

Now we take a wheel that spins with the same speed, but it has a diameter of 10 inches.

The circumference of this wheel is 10 * 3.14 = 31.4 inch. If you add the amount of spins per minute to this, it comes to (20 * 31.4) 628 inch (52 feet 4 inches). We can therefore conclude that the wheel spins with a speed of 628 inch per minute. Even though the wheel makes the same amount of spins per minute, the speed on the edge is very different.

## Calculate the speed of your pad

Lets say that you are using a wool pad that has a diameter of 16 centimeter. You want to use another foam pad that is 12 centimeter in diameter. You are going to use the wool pad with a speed of 1800 rpm (read the manual of your polisher to see what setting matches what rpm).

The formula would be:

rpm * diameter = speed

For the wool pad this comes to: 1800 * 16 = 28800 centimeter per minute.

For the foam pad, the formula would look a bit different, seeing as we only know we want to keep the same speed. So it would look a bit like: ??? * 12 = 28800 centimeter per minute. To fill in the ??? we need to turn the formula around, which would become: 28800 / 12 = ???. The answer would be: 2400.

To use the foam pad with the same effective speed in comparison to the foam pad, the rpm should be set to 2400 when using the foam pad.