# RPM question

Discussion in 'General Electronics Chat' started by C44play, Nov 8, 2013.

1. ### C44play Thread Starter New Member

Oct 8, 2013
10
0
If you have a gear motor with an rpm of 100 and a ratio of 25:1 what does that mean?

Jul 18, 2013
10,548
2,373
It means the motor must run at 2500 rpm to attain the 100rpm out.
Max.

3. ### Stuntman Active Member

Mar 28, 2011
181
47
Additionally, realize this reduction in RPM's means the same ratio of increase in effective torque, which is often desirable.

4. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
Minus losses, those cheap little 100:1 gearboxes might be as poor as 40-50% efficient. So it gives;
torque *100 *0.50

5. ### WBahn Moderator

Mar 31, 2012
17,748
4,796
How do you figure that? Why are you multiplying the torque by 100 when the gear ratio is 25:1?

6. ### killivolt Active Member

Jan 10, 2010
367
317

I think you mis-read his post?

Edit: Maybe I'm the one mis-reading yours

Last edited: Nov 9, 2013
7. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
Temporary stupidity.

It should have been *25 as I mixed up the 100 RPM and 25:1 gearbox.

8. ### WBahn Moderator

Mar 31, 2012
17,748
4,796
I didn't even spot that you specified a 100:1 box in your post because I was expecting it to be 100rpm, so your post was consistent with the box you were talking about.

We all tend to see what we expect to see, don't we? I guess it's a tendency that often serves us well in lots of situations. But when details count, it's a pain in the ass.