Ball caster for 32mm wheel


I want to use some of Pololu’s great products for a swarm of robots that I am planning to build for my final year project. I want to use Pololu’s smallest wheels (32x7mm) with micro metal gearmotors mounted on a PCB. If I understand correctly, pololu item #957 (“Replacement 3pi Ball Caster with 1/2” Plastic Ball") is the apt ball caster to use for this configuration. However, it is vague from the description how the ball caster is being kept in place. If I understand correctly, the ball caster has to be placed through a 600 mils hole in the PCB, but what prevents it from falling through? Does the ball increase the diameter of the holder to prevent it from falling through? Also, in the description of the product, what is meant by “[it] has a side that is flattened by 30 mils”?

With a standard PCB thickness, Pololu’s micro gear motors and 32mm wheels, what will be the distance between the bottom of the PCB and the ground? Some rough calculations led me to about 9mm.

Are two 50:1 micro MP motors excited at 6V a suitable choice to drive a small (3pi’s size or even smaller) robot at a sane speed?

Thanks for helping!


On the 3pi, that ball caster is held from falling through the PCB by the two small extensions on the top. To prevent it from being pushed back up through the hole, the 3pi’s battery pack sits on top of it.

The hole for the ball caster on the 3pi’s PCB is not perfectly circular; it has one flat side that is indented by 30mils; this keeps the ball caster housing from rotating in the hole.

You might also consider using this ball caster, which uses mounting screws, and might be easier to mount.

I don’t know enough about your setup to estimate your ground clearance or motor speed. For comparison, the 3pi robot’s PCB is about 9mm off the ground, but you could always mount the motors on the other side of the chassis to get more clearance between the chassis and the ground. Also, since the 30:1 micro metal gearmotors used on the 3pi have a free-run speed of 660RPM at 9V (the voltage supplied by the 3pi) and the 50:1 MP motors you suggested have a free-run speed of 420RPM at 6V, I would expect your proposed robot to be slower than the 3pi.

  • Grant