# Testing torque for motor selection

1. Oct 16, 2011

### jloogoons

I need to drag a ring across a surface by pulling it with a string attached to a motor. The motor will simply coil the string, or the motor will be attached to a pulley and the ring will be tied on the rope. My key problem: the surface is not conventional so there is no friction coefficient.

Basically, I know very little about physics but I've done a lot of research on this topic so I hope someone can follow my thought process and point me in the right direction. Thanks in advance.

At first, I was comparing the strength of motors based on calculating the watts. I have a hand mixer that will pull the ring across with no problem, but I have a 5 hobby motors that failed. There seemed to be no correlation to the amount of watts used by the motor. After some research, I realized torque was the missing element. But without a friction coefficient I'm not sure how to determine my required torque.

Since I need to buy the cheapest motor that will do the job, it would be nice to know the exact amount of torque required to drag this ring across the surface. I pulled out a luggage scale and measured the resistance of the ring to the surface. This involved tying one end of the rope to the scale and the other to the ring. Then, I held the surface with my left hand and the scale with my right and simply pulled the ring across. The scale showed 2 pounds of resistance.

I then measured the hand mixer that had worked so well and the scale showed 5 pounds. The hobby motor that came closest to working provides .25 inch pounds of torque according to the manufacturer.

From what I understand about measuring torque, it is simply the length of the lever multiplied by the force. Since I didn't have a lever for this exercise, I need a motor with 2 inch pounds of torque, and my hand mixer has 5 inch pounds of torque. In other words, whatever was on the luggage scale is my required torque. So I need to look for a motor with more than 2 inch pounds of torque, correct?

I hope I've provided enough information. I would really appreciate any help you can provide on this topic. Thank you.