# Exact gear ratio for 120:1 Mini Plastic Gearmotor

#1

Hi,
I’m using the 120:1 Mini Plastic Gearmotor with 90° 3mm D-Shaft Output and Extended Motor Shaft together with Magnetic Encoder with 12 CPR. So I assumed one full rotation of the d-shaft should result in 120*12= 1440 counts. But I’m measuring something around 1437. So it seems the ratio is somewhere near 1:119.75. Is there any information available about the exact gear ratio of this motor?
-Mathias

#2

Hello, Mathias.

The exact gear ratio is:

(23 × 38 × 44 × 26 × 30) / (10 × 11 × 11 × 22 × 9) ≈ 125.20

If your encoder counts continue to not match what we have, could you tell me more about how you are testing your encoders?

Grant

EDIT: (22 × 38 × 44 × 26 × 30) / (10 × 11 × 11 × 22 × 9) ≈ 119.75
I have gone back and counted all the teeth in the plastic gearmotor. The first number should have been 22 not 23.

#3

Thank you for your reply. For testing I’ve attached a paper strip on top of the d-shaft and I’m using a light switch next to the shaft. The motor is rotating continuously while I save and dump encoder counts at start of light switch interruption caused by the paper strip. The differences between start of interruption of consecutive revolutions are most of the time around 1437. My measured differences vary about +/-1. I also measure the ends of interruptions, and there I get the same differences.

I see there are two version of this motor, a normal version and a HP version which look the same from the pictures. I’m not sure which version I had purchased. Would the version make a difference?
-Mathias

#4

Hello.

Thanks for additional information. The gear ratio between the regular and high power (HP) version does not change, just the motor.

Grant

#5

Could it be possible that the actual ratio is
(22 × 38 × 44 × 26 × 30) / (10 × 11 × 11 × 22 × 9) ?
This would pretty well match my measurement.

#6

I have gone back and counted all the teeth in the plastic gearmotor, and you are correct. That gear does have 22 teeth not 23. Thanks for catching that mistake.

Grant

#7

Thanks a lot.
-Mathias