A Planetary/Harmonic Hybrid Gearbox

I recently saw this amazing idea from Darren Schwenke on Hackaday.io:

https://hackaday.io/project/164732-mprt-modified-planetary-robotics-transmission

Which is (so far as I know) a brand new type of gearbox, inspired by a well known concept called a  “Harmonic-Drive“. Harmonic drives have been around for years and were used whenever light weight or small size was required (on the moon rover, for example). They work via the deformation of a flexible ‘strain wave gear’ to enforce the meshing between two gears with nearly matching numbers of teeth. This allows very small reduction ratios, in a compact space:

240px-HarmonicDriveAni.gif

Image from Wikimedia Commons, here

The red strain wave gear is where output goes. The downside is that a strong and really flexible gear like that is hard to make, and also difficult to couple the output from easily. Most designs I’ve seen require custom spring steel ‘cups’ which are precision manufactured via electro discharge machining or similar. (Although there are several awesome 3D printed versions out there. )

Darren’s idea with the MPRT gearbox is to take the basic concept of the harmonic drive, but remove the complicated strain wave gear and instead substitute ordinary planetary gears, which then do the same job of enforcing the meshing of the two outer rings at several key points:

Gear sketch v01.jpeg

The final gear ratio for this is around 66:1 reduction, which is amazing for a single stage:

IMG_2322.JPG

I liked his design, but didn’t want to wait a long time to 3D print it, so I drew up this one for my laser. I created the gears just using the involute generator in inkscape, then added all the bits and bobs to hold it on the motor and mount reliably. I laser cut the pieces out of bamboo ply, and screwed them together with M3 fasteners.

A NEMA 17 stepper motor sits underneath and drives the whole assembly:

IMG_2323.JPG

Also, I’d like to mention one of my favourite construction techniques, using M3 nylon standoffs as thumbscrews. I’ve had to put various bits of the gearbox together and pull them apart half a dozen times while prototyping, and being able to fasten bits securely by hand is a huge time saver.

I used a few dollops of furniture wax, which seemed to make it run smoother:

IMG_2328.JPG

The torque is large but not ridiculous, and I can make it stall by hand if I really try, but overall it’s still remarkable for a single stage gearbox. (Also, not many moving devices have sliding wood-on-wood surfaces, so if you made this out literally almost any other material you’d likely have better results. )

Files up here for anyone that wants to make their own:

https://www.thingiverse.com/thing:3576090

 

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