Cycloidal drives are a type of speed reducer that are significantly more compact than gearboxes, but they still come with a fair number of components. In comparison, the harmonic pin-ring drive that [Raph] recently came across as used in some TQ electric bicycles manages to significantly reduce the number of parts to just two discs. Naturally he had to 3D model his own version for printing a physical model to play with.
How exactly this pin-ring cycloidal drive works is explained well in the referenced [Pinkbike] article. Traditional cycloidal drives use load pins that help deal with the rather wobbly rotation from the eccentric input, but this makes for bulkier package that’s harder to shrink down. The change here is that the input force is transferred via two teethed discs that are 180° out of sync, thus not only cancelling out the wobble, but also being much more compact.
It appears to be a kind of strain wave gearing, which was first patented in 1957 by C.W. Musser and became famous under the Harmonic Drive name, seeing use by NASA in the Lunar Rover and beyond. Although not new technology by any means, having it get some more well-deserved attention is always worth it. If you want to play with the 3D model yourself, files are available both on GitHub and on MakerWorld.
Looks to be a dual nested cycloidal rather than a strain wave design, as I don’t see anything that’s flexing. Darn clever to get rid of all those load pins, which I’m using currently. After my head stops hurting I’m going to have to build one.
Have a look at the Nabtesco variant. It’s a bit of a hybrid between a planetary gear and a cycloidal gear. Nabtesco has put excentric shafts on the place of the load pins (Which now have normal bearings, instead of the weird sliding contact). And they are driven like the planets in a planetary gear. It’s a really nice combination. Nabtesco makes a whole lot of gear boxes for robot arms.
next step, embed coils in the casing and magnets in the blue ring and eliminate the input shaft.