# Torque and rotation

1. Jun 28, 2011

### salph

Hello all. I am working on a project and am a little lost on the physics end. Basically, I have 6 circular platforms that need to be rotated in the same direction. I figured I would have a gear under each and a chain connecting them similar to a bicycle chain. I need to figure out what DC motor to get and how much torque I will need if each platform had 600 grams on it and needed to rotate at a speed of 12 RPM.

Other things to note: Power Source - would like to be d cell batteries. Was hoping for 2 maybe 4. Open to other options that are light and small.

Thanks for any help.
-Scott

2. Jun 28, 2011

### BobG

You need the dimensions of the platform, as well.

If you're speaking of circular platforms, 600 grams each, each platform's moment of inertia can be found by:

I = mr^2/2

You add them up to get the overall moment of inertia (or multiply by 6 in this case).

The torque you need will depend on how fast the platforms have to get up to speed. (Torque equals moment of inertia times angular acceleration). Once up to speed, the motor only has to overcome the friction of the system.

Either the gearing has to turn the motor's speed into 12 RPM (with each additional gear increasing your moment of inertia, etc), or you need to turn the motor off when the speed is too fast and turn it back on when the speed is too slow (i.e. - an oscillator that puts pulses out 12 times a minute and some electronics to turn the motor off when you get two consecutive pulses from the gear system and turn the motor on when you get two consecutive pulses from the oscillator).