# How to Measure Torque Required for motor to spin a rotating shaft?

• Scott 33
In summary, the speaker has connected a motor from a cordless drill to rotate a shaft through a belt, but the motor is drawing 15 Amps and the power supply cannot go higher. The speaker suspects that the torque required to maintain constant speed is causing the high current draw, but the shaft only has ball bearings and is situated vertically. The speaker needs to figure out how to measure the torque required to spin the shaft and the best way to measure the torque/current relationship for the motor. They may need to consider the deformation of the belt as a factor in the torque measurement. f

#### Scott 33

I've harvested a motor from a cordless drill and connected it to a belt which turns a rotating shaft. The motor pulley and the pulley on the other side of the belt are roughly the same size, which a fairly small radius (5 mm maybe?).

The issue I'm running into, which I don't fully understand, is that the motor is drawing 15 Amps while spinning the shaft at a constant 600 RPMs. My power supply won't go higher than 15 Amps, so I'm unable to increase the speed. This amount of current draw seems very high given that the shaft is rotating at constant speed?

Note that I measured the no load current of the motor to be about 2 Amps. Without load, I have no problem increasing the voltage to the motor, and thus increasing the speed.

This would lead me to believe that the torque required to maintain constant speed of the rotating shaft is causing the high current draw, but the shaft is only connected to ball bearings and seems fairly easy to turn when I turn it by hand. In addition, it's situated vertically, so friction of the ball bearings should be the only force slowing it down?

So, I need to figure out if the shaft really does take a lot of torque to spin or if the motor, for whatever reason, can't handle very much torque (which would be surprising since it came from a cordless drill).

1. How would I measure the torque required to keep the shaft spinning at constant speed through the belt?

And/Or:

2. What's the best way to measure the torque/current relationship for the motor? How would I apply different amounts of torque to the motor in order to take the necessary data points?

Thanks for the help!

Welcome to PF.

It sounds like you can just measure the torque with a torque wrench. If you are operating at a constant angular speed, then that's not much different from just turning the shaft with a torque wrench. You just need to figure out how to couple your torque wrench to the shaft to get the reading when you turn the shaft by hand with the wrench.

Keep in mind, though, that a cordless drill's bearings are designed to handle thrust forces, not radial forces (perpendicular to the axis of rotation):

BTW, can you post a drawing or a picture of the setup? You can use "Attach files" at the lower left of the Edit window to upload a PDF or JPEG file.

There is a good possibility most of the torque is being used to deform the belt, especially if the belt has any appreciable thickness. That 5mm radius is a pretty sharp bend for all but a urethane film belt, a small bead chain, or a small roller chain.

To measure the torque you can use a spring scale (or similiar) attached to the handle of the drill. Or maybe you can put a bathroom scale under the drill handle to measure the force. In any case, you will be measuring how much force is needed to keep the drill handle from spinning around.

You then measure the distance from the spin axis, or centerline, of the chuck to where the scale is attached. The torque is the product of the measured distance and the scale reading.

Cheers,
Tom

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