DC Gearmotor selection for balancing robot

Hello everyone,

i’m currently designing a balancing robot and i am using the pololu 20.4:1 Metal Gearmotor (25Dx50L). The issue i’m having with this particular motor is the high amount of backlash on the shaft output. It is causing the controller to oscillate a lot when trying to stabilize the robot. On a previous design i used the 30:1 Metal Gearmotor 37Dx68L and noticed it had lower backlash, what simplified the controller design a lot. Is there any motor from the 25D type (i was wondering about using the 34:1 Metal Gearmotor 25Dx52L) that ensures a lower backlash, which doesn’t affect the stabilizing controller that much? Or can someone recommend me another motor. My requirements are:

Voltage: 12 v
Max. current: 6 A
Min. torque: 650 mNm
Min. speed (no load): 200 rpm

Thanks in advance!

Hello, Emmanuel.

We have not characterized the backlash of our gearmotors, and we do not carry motors specifically optimized for low backlash, so unfortunately, I do not have any particular recommendations for you. In general, I do not expect the backlash to vary that much across a family of gearmotors, but the general trend will probably be that it gets worse as the number of stages in the gearbox increases. For example, if you switched from the 20.4:1 to the 34:1 gear ratio, that increases the total number of stages in the gearbox from four to five, which would probably result in a larger backlash.

Out of curiosity, what kind of backlash are you seeing from your motors? Was it always high, or is it possible your application has damaged or loosened something in the gearbox?


Hi Jon,

thanks for the quick reply. Yes i thought that a higher gear ratio should have higher backlash, but comparing the one i used before with a 30:1 ratio to the one i have now a 20.4:1 the backlash is much higher on the last ones i used. Unfortunately i cannot tell you if the behaviour was like that from the beginning, as i thought it may have been some software problem with the controller and didn’t pay attention on comparing the amount of backlash. But at the beginning i noticed i runned the motors with the gearbox screws being loose. Do u think it may have damaged the gears? As u said i would have expected less backlash on the 20.4:1 ratio as on the 30:1. Do u think it would be wise going back to the initial design or just ordering a new pair of the same one i’m using at the moment?


The gearboxes of the 20.4:1 25D mm gearmotors have four stages and the gearboxes of the 30:1 37D mm gearmotors have three stages, so it is not entirely unreasonable that your 20.4:1 has larger backlash. If the gearbox screws are loose enough, the pinion gear would mesh poorly and be more susceptible to chipping or other damage from excessive rattling. Damage like that can often be heard as the gearmotor operates, so if you have not noticed anything yet, I suspect that the gearmotor has not been damaged. However, if you are concerned about it, you can unscrew the gearbox and inspect the pinion gear to see if any teeth are broken or missing. Also, if you have not already, you should definitely tighten the screws down.

If there is no damage, it seems like there are no obvious reasons to expect a new 20.4:1 gearmotor to work any better for your balancing robot. So, it might be reasonable to revert back to your previous design using the 37D mm gearmotors, since it seems like you can expect that to continue to work well.