Quick change pager motors

Martin Newell

August 2004

Pager motors are wonderful pieces of engineering, but we abuse them by driving them at about 4 times rated voltage which imposes 16 times the rated power. It is amazing that they last as long as they do, but they do fail. Also, not all motors are created equal - some seem to run quieter and more efficiently than others.

This means that it can be a frequent chore to have to replace a motor in a gearbox. Here I show a method for quick and easy replacement of pager motors. Using it I can take a plane with a failed motor and replace the motor ready for flying again in about 1 minute, with no gluing involved.

The idea is to replace the motor with a piece of tube into which the motor fits snugly. Then to change a motor you just push the old motor out the back of the tube, transfer the pinion to the new motor, and push the new motor into the mounting tube. To complete the change of the motor in the plane you need to use the push-on motor mount described in another article, or some other method for removing the gearbox, or arrange enough room behind the gearbox to be able to push the motor out.

For a 6 mm motor an easy way to make a snugly-fitting mounting tube is to take a piece of 1/4" shrink tubing the same length as the motor, and shrink it onto a motor case using a small flame. You might want to use an old motor if you are concerned about the heat, but I don't think it is an issue. Build the gearbox using the motor still in the tube, to provide rigidity. Then push out the motor when all the glue has hardened.

Sometimes it can be a little difficult to push out the old motor. I use a piece of 9/32" brass tubing to support the back of the mounting tube, and a narrow stick to push the motor from the front. The motor then slides back into the brass tube. Depending on the wall thickness of the shrink tubing, the mounting tube weighs about 0.1g.

The thermal insulation of the tube probably makes the motor run a little hotter, but this hasn't been an issue in practice. I have motors that have 2 hours of flight time on them and they are still running well. Cooling holes could be cut in the tube without losing rigidity, though I have never tried this.

The pictures show examples of this method for single bearing and double ball bearing gearboxes.