25D 4.4 design flaw?

I’m encountering meshing problems with the gear drive on the 4.4:1 Metal Gearmotor 25Dx48L mm HP 12V with 48 CPR Encoder. When I took the gear drive apart, I discovered that the gear drive output shaft is offset so that only about 1 mm (axial runout) meshes with the pinion gear. Because the runout is larger than 1 mm, the pinion easily becomes disengaged. Has anyone else had these problems?

The drive is set up in this order: Motor output (small pinion) to 1st drive pinion which is mounted on a pin. A plastic bushing provides the spacing for the drive pinion. The drive pinion meshes with the output shaft. Bronze bushings hold the output shaft in place. These are press fit with collars, so they cannot be adjusted axially to change the mesh.

Is there another alternative? I’m looking for a quiet 12v motor, with or without the gearbox, that has similar specifications: 2250 RPM and 300 mA free-run, 23 oz-in (1.7 kg-cm) and 5.6 A stall.

[edit] I discovered that the middle gear drive collar is a little too thick, so I chucked it into a lathe and trimed some metal back from both sides, then cut the inner bushing back by a millimeter. This added about a millimeter to the gear mesh. Not a lot, but hopefully enough to solve the disengagement problems. Still, the pinion barely meshes with the output shaft. Without design changes, it is a fairly weak gearbox.


Thank you for the feedback and information. We are looking into the issue.


Thanks for checking into the gear reduction mesh offsets. The gear body modification that I suggested has worked fine. I’ve run the motor for about 14 hours on the back of a vintage automobile speedometer head, at various RPMs, without any more problems. It is a light load for the motor, so the small overlap between the output pin and the drive pinion is probably not going to be an issue here.

BTW, is a cover available for the encoder?

Thanks for the additional information about how your modification worked out; we are continuing to look into how we can improve the way the pinion gear meshes with the next gear. Unfortunately, we do not have a cover for the encoder.


This thread came up on Twitter today. This issue was addressed back in 2017. We’d like to remind people that if they ever have problems with Pololu products, they should just contact us directly with their order information at support@pololu.com.

Here’s what the gearbox looks like now:

Separately, versions with plastic caps over the encoders will be available soon.