[Richard Wallace]

Originally Posted by 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.

Thanks, John. For many years I've been using a Harmonic Drive Model PCR-5C 100:1 reducer as part of low-speed motor tester. [Note: this reducer is now obsolete, replaced by the CSG series, I think.] We use this tester to measure cogging, ripple, and frictional torques developed by brushless permanent magnet motors at 2 RPM. The Harmonic Drive unit operates with negligible backlash, is very quiet, and as you pointed out it provides very high reduction ratio in a compact package. It is also not cheap.

I did not recognize the similarity of the harmonic drive to the low-cost lugnut remover gear set.