# Simple question on motor rotation and pulley

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1. Apr 10, 2015

### jsmith24

Hi all,

I have a small DC motor which turns at 20 rpm. The shaft diameter is 6mm, giving a circumference of 18.85 mm. What I want to do is get an effective rate of 30 rpm. Can I do this by simply attaching a spool or spindle to the motor output shaft with a larger diameter? If so, would it be as simple as increasing the diameter by a factor of 1.5?

Thanks,
Jack

2. Apr 10, 2015

### Hesch

20 rpm! That was not much. Is a gear built-in the motor? Can you change the gear?

But yes: Increasing the diameter of the pulley, you will speed up the velocity of the belt.

Other possibility: Increase the voltage to the motor.

3. Apr 10, 2015

### jsmith24

Yes, it's a geared motor, sorry! High torque. I'm limited to 12 volts DC, so that's out. I thought it was as simple as increasing the diameter of the shaft (pulley), which would increase the circumference, thereby moving more belt per revolution, effectively increasing the rpm. Thanks!

Jack

4. Apr 10, 2015

### Hesch

By increasing the diameter of the pulley (on the motor axis), you are mechanically gearing up the motor. An alternative is to decrease the diameter of the pulley on the load.

5. Apr 10, 2015

### jsmith24

There's no pulley on the load. It's a simple guillotine-style door. The motor will have a spindle on it, a cord attached to the spindle which will wind up when the motor is forward energized, which, in turn, raises a flat door in a track. When the motor is reverse energized, the spindle reverses, the cord unwinds, the door descends. The problem is that I find 20 rpm (or rather, the amount of cord travel per revolution) too slow. As this was the only motor I found handy, I'm trying to make do. The motor already has plenty of torque to raise the door (which is rather light), so gearing up a bit shouldn't be an issue.

Oh! I forgot to answer part of your question: No, I don't think I can change the internal gearing of the motor.

Thanks!

6. Apr 10, 2015

### Hesch

Well, then just increase the diameter of the spool as you suggested in #1.