Planetary Roller Robot Actuator 44:1

Work in progress (awaiting parts - some of which have arrived to rekindle my interest). Not ready for printing as I just realized the "ring" teeth won't clear the center spacers of the "planets" of the top and bottom layers. Was thinking of ditching that anyway and just running a pin through the center supported by bearings on the carrier.

An idea for a robot actuator I'm cooking up. Split-ring compound planetary gear system which uses bearings instead of gear teeth to eliminate the source of the huge inefficiency typical of these gearboxes.

Print the ring gears in assembled positions (in place) otherwise you won't be able to assemble them due to the integrated plain bearing surface. Using 0.2mm layer height, a one-layer gap should separate them and be easily de-laminated post printing using a sharp edge. The outer arm could easily be split down the middle for assembly with screws but I'm trying to minimize hardware for now.

HK0306 Bearings and M3 steel dowel pins of 12mm, 18mm and 42mm lengths, some screws and a NEMA 17 stepper motor or two.

https://youtu.be/hEM9N223gk8
https://youtu.be/D-CN17TTPPo
https://imgur.com/a/8dUUPnR
https://imgur.com/a/gTzpcBx
https://thangs.com/thang/46107

Design notes:
I think we're going to need a planet carrier. At various stages as the "teeth" engage and disengage there doesn't appear to be enough points of contact to fully constrain the planets in the naive radial pattern the gears were cut. Typically floating gears are helical or herringbone where this is not a concern as the teeth are always fully engaged somewhere provided the helix angle is steep enough to span the tooth to tooth gap.
This periodic slop in the planets would otherwise introduce some nasty backlash and ka-chunk-ka-chunk, I suspect. Tweaking the profile for possible non-radial planet motion to be explored in another version (ie. walking it around the ring).

Found some examples of gears using rolling contact, notably Trochoid Cam Gears (I would've said cyclocycloid). US6902507B2 describes something quite similar to a cycloidal gearbox with rollers which we could arrive here by shrinking and duplicating the internal gear into multiple planets, incorporating a sun gear to mesh with the very same rollers.

Update (4/20/22): Some parts never arrived via AliExpress, don't think it was a scam just multiple orders from a single seller got overlooked and they only sent one. Might just order some more since it's only small change. In the mean time, thinking about friction losses of hard rollers getting bogged in soft plastic and the effect of impact stresses on bearings...

Update (8/27/22): Decided to dust this one off and sized my own dowel pins with a Dremel cutoff disk. Assembly is a bit of a challenge, a bit like a ship in a bottle. First roller is OK, but the rest need to be assembled in place within the ring gears, layer by layer. I might need to split one of the elbow pieces in two to make it easier (or even possible).