Compound Planetary Gears 1300:1

thingiverse

UPDATE: This old news is nothing new. It's the foundation for gearboxes like the High Torque 149.33:1 Compound Planetary Gear set. Check out the Solid Core Compound Planetary Gearbox for something fresh - a single piece sun gear version with similar high ratios. This print-in-place compound planetary gearbox has two planetary stages stacked vertically. The old trick of having one tooth fewer in one ring to emulate a cycloidal drive is taken further by noting that we don't need the same gear pitch (or rather number of planet teeth) in both stages. This way, we can achieve a fractional differential between the outer rings and absurdly high gear ratios. Consider this example: a ratio of 10/31 and 11/34 in the two stages gives a very close ratio of 1:340 relative to each other. Multiply this by the gear ratio of the first planetary gear with fixed ring, driven by the sun of 1:3.818, and we get a final gear ratio of 1:1298.18. Just make sure to separate the ring and sun gears before turning (there should be a one-layer gap after slicing). A sharpened spatula works well as long as there is enough space between planet gears to get a corner in. For a tightly packed arrangement, it may be possible to thread piano wire around the sun gears and crack them apart. Here's a quick example spreadsheet to calculate not quite matching ratios. (* It's probably been done before, but I can't find any examples)