Part of exhibit technical design is crushing the joy of those who visit museums. Nothing is more fun, apparently, than spinning large plastic wheels with images of planets and moons on them as fast as possible for as long as you heart holds out.

In order to get the visitor to actually do the intended activity, I decided to put a brake on the wheels so that they were easy to position, but would not spin easily. I first tried attaching the axle to a shorted-out stepper motor. This worked, but the wheels still spun a bit too freely. So I went with a mechanical brake. Here is a picture:

A piece of 3/4 Delrin has a notch that precisely fits on a drum mounted on the axle that supports the wheel. A spring is connected to the end of the brake and is attached to an adjustable screw that runs through a hole at the bottom. Brake tension is set by adjusting this screw. This brake can be adjusted through the side panels in the cabinet without having to remove the front of the exhibit. As the Delrin wears, the braking force can be maintained by simply tightening the screw.