Harmonic Drive

This is a 3D printable version of a harmonic drive. Harmonic drives are mechanical gear systems that have some very nice properties such as no backlash, high gear ratios, and being able to hold their position without power :

http://en.wikipedia.org/wiki/Harmonic_drive

Harmonic drives have an elliptically deformable inner gear (the flex spline) that is meshed into a ridged outer gear (the circular spline) using an elliptical horn ( the wave generator) that attaches to a motor. As the wave generator moves it deforms the flex spline and changes its contact points with the circular spline. Every full rotation of the wave generator moves the flex spline by only by the difference in the number of teeth in the flex spline and the circular spline. This design has 32 teeth in the flex spline and 34 teeth in the circular spline. See more details:

http://www.projectbiped.com/projects/harmonic-drive

Check out the video of the drive in action:

http://www.youtube.com/watch?v=vF3HHH-Et4c