# Magnetic decoupling torque

I have a system consisting of 4 magnets rotating at speeds between 200 and 6,000rpm, which are facing 4 other magnets with a 0.5mm gap between the two sets. The set-up looks like this:

Both sets of magnets have their noth pole facing out so that they repel each other (the 0.5mm gap is maintained by other means). The resulting effect is that when the bottom set starts spinning, the top set follows because the top magnets naturally fall (radially) between the gaps of the bottom ones.

What I'm trying to do is work out what the decoupling torque provided by this set up is. In other words, if the bottom part is spinning, how much torque do I need to provide to stop the top part from rotating?

The diameter between the centres of the magnets is 17mm, they are 90 degrees apart, and I have the following (very basic) information on the magnets themselves:

• Diameter: 6mm
• Performance: 5,900 Gauss
• Vertical pull: 1.4 kg
• Slide resistance: 0.28kg

Can anybody point me in the right direction? I have made the following attempt, but I don't think that's right:

T = 4 * (8.5e-3 * 0.28 * 9.81) = 0.093 Nm

• My hunch would be to go look at motor slip, especially in the case of a stepper motor. I would imagine whatever equations are used there would be very similar to the scenario you describe, as stepper motors work by a very similar principle, but they use electromagnets instead of permanent magnets (so they can alter which sets are "active" and thus drive/step the output shaft). – Chuck Nov 15 '16 at 20:30