[John Neun]

These are great drives because you get tremendous turndown (torque multiplication) in a very compact package. There is a company called Harmonic (at least that is what they used to be called) that makes a similar drive but very elegant. The inner "gear" is actually flexible, such that it is deformed into an ellipse that meshes the ring gear in two places. The teeth are very fine pitch. Therefore, the one tooth difference between inner and outer gears makes for a huge turndown (something like 75:1 or better in a transmission not much larger than a coupling), and the drive is incredibly smooth. The biggest advantage in our application was the lack of any perceptible gear noise, as the drive was for cooling rolls on a polycarbonate sheet line. Any ripple would have produced an optically perceptible imperfection in the lexan. Very cool, but expensive.