15.5D gearmotor with radial load (tip:get thrust bearings)


I recently was using a Pololu 15.5D 1:32 gearmotor for a radial load application. That is, our application applied a force towards the motor by the perspective of facing the circle face of the motor.

To be specific, we were using it for a small linear actuator that turned a sealed acme rod and pushed a cyclinder forward.

After several days, we found the strength was declining. We almost replaced the 1:32 gearbox with a 1:172 gearbox, thinking the motor was simply not geared enough. As we took apart the actuator, though, someone noticed that it was only when you pushed down on the output shaft, that it was running slow. Taking apart the gearbox it became clear that the gears would bind against each other.

So our solution was to add a thrust bearing that would distribute the load on the outside of the casing. A thrust bearing is basically two washers with tiny balls between them. Imagine making two fists with your hands, putting the thumbs next to each other, pushing hard towards each other, and twisting. That shearing motion as you twist is the one that you’re helping reduce by adding ball bearings. It is also the friction that you introduce by pressing your coupling shaft hard onto the gearbox casing, because you’re trying to make sure the force is transferred through the casing, and not the shaft. I have no idea how much you (said reader, reading) are familiar with this, I only just realized it so I am trying to write in a clear fashion. Maybe I just took a while, and didn’t make much sense :p.

Anyway! It was hard to find super tiny thrust bearings. Someone in our group is an RC helicopter enthusiast, and it turns out a lot of these tiny $30-100 dollar helicopters have tiny thrust bearings in the tail rotors. We found one of those fit perfectly! you can get them as replacement parts, without buying the whole helicopter.

I have no idea how common radial load applications are for people using these motors. But it seems most likely someone might do something like that, as we did, for a linear actuator. In which case, be aware these motors aren’t good out-of-the-box for radial loads, but it does work right now after we added a thrust bearing.

-The 15.5D thrust bearings have gearboxes without any thrust bearing.
-If you are using a radial load application, add a thrust bearing.
-High quality miniature RC helicopters (in the tail rotor) are a great place to get these bearings from.