Split-Ring Planetary Reduction Gear

This is a remix of https://www.thingiverse.com/thing:53451, although the remix search box seems not to be able to find it.

It's a reduction gear based on a planetary gear arrangement that can get fairly high ratios in a fairly compact space, inspired by an enigmatic throwaway comment on the Wikipedia page about planetary gears:

During World War II, a special variation of epicyclic gearing was developed for portable radar gear, where a very high reduction ratio in a small package was needed. This had two outer ring gears, each half the thickness of the other gears. One of these two ring gears was held fixed and had one tooth fewer than did the other. Therefore, several turns of the "sun" gear made the "planet" gears complete a single revolution, which in turn made the rotating ring gear rotate by a single tooth like a Cycloidal drive.[citation needed]

I couldn't find a citation for it, so I knocked one up in openscad.

How I Designed This

It's pretty much as described, only for simplicity I made the lower-toothed ring have one less tooth per planetary gear than the higher, which means the upper half of the planet gear is in phase with the lower. If that wasn't the case, you'd get 3x the reduction at the cost of much more complex assembly. As configured, from memory this file has a 63:1 reduction ratio between the sun and the ring. You could use probably plug an electric motor into it and drive a wheel with it directly. The sun:planet gearing is 1:1 in this version, so you could multiply that 63:1 up quite easily by changing their ratio. I've not tested much in that direction, though.

Because of the two rings, the planet and sun gears are double-herringbone, not single, to hold everything in place. It doesn't need a separate cage.

The split ring means this isn't print-in-place. It's print-in-two-places. One of the rings needs to be printed separately and squeezed on after the fact. The one I printed in PETG took a little effort to fit but wasn't too hard to get right.

Otherwise print settings should be as in the original.