[DIY] Split-Ring Hypocyclic Compound Gearbox

Continuing my search for a gearbox that can be 3D printed and handle a 40Nm load (~40:1 ratio, ~120x120x80mm volume), I toy-modeled this Carrier-Driven Split-Ring Hypocyclic Compound gearbox to get familiar with the 1-tooth-off compound principle.

It's inspired from David Hartkop's Carrier-Driven Split-Sun Epicyclic Compound gearbox

However, the sun gear of most planetary designs is the weakest link when handling high load torque. Because of the force concentration on this very small single gear, its axle usually shears off or its teeth break.

So, instead, I replaced the suns by large outside rings, positioned the planets inside (ie hypocyclic), and drove the gearbox via the disc that carries the planets instead of via a sun gear. That considerably lowers the input forces on the teeth and allows to spread the torque between many planets.

This is just a concept toy design but, seeing the potential, I might decide to design from scratch a full-blown sturdy version with bearings and Nylon, and put it to the test.

The Fusion360 CAD (very messy), STEP and 3MF files are on Github and Makerworld

Feel free to copy, remix, whatever (non-commercially)

Happy 3D printing to all !

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How It Works

The key principle behind this split-ring compound gearbox is the 2 rings that are off by 1 tooth. As the planets revolve around the fixed stator ring, they also make the output ring rotate because of this 1 tooth difference. After a full rotation by the planets around the stator, the ouput ring has now rotated by just 1 tooth. So that stage's reduction ratio is 60:1. Mind-boggling...

Watch David's video to see the 1-tooth-over-360-rotation principle in action.

The first stage is just a regular spur gear drive, for giggles, with a reduction ratio of 76 / 12 = 6.3. So this gearbox yields a total ratio of 60 x 6.3 = 380.

Note that the number of teeth for the planets doesn't matter. What's crucial is that each planet but one gets its output teeth rotated compared to its stator teeth, depending on its polar location on the carrier disc.

Here, since there are only 2 planets, the one at the top of the pic is straight, no rotation, while the bottom one shows a rotation of half a tooth:

That's because at the 180° point opposite the first planet, the output ring itself is now half a tooth off. So that planet's rotation angle is 360 / 2 / 12 = 15°.

If 2 more planets were added between the existing ones, ie at 90°, they'd be rotated by 7.5° and 22.5° respectively. Etc.

The massive advantage of this design is that the load torque (output) can be spread across many planets, and that each planet transfers the torque via its own axle to the first stage. Nowhere is the resulting torque transfered to a unique itty-bitty sun gear like in a regular planetary gearbox.

Manual rotation in action:

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