Robot actuator with flexible helical planetary gear

This design is turning out to be pretty effective for getting a 38:1 ratio out of a small 35mm stepper motor to drive a joint in a robot arm. It’s Jeff Kerr’s design, LoboCNC on Thingiverse, but I’ve tweaked it and rebuilt it in Fusion 360.

Here’s a video of the flexing that occurs inside the gear box. The net effect is that you get way less friction on the tight gears, but you get tightness which gives almost no backlash.


That is pretty darn cool!

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And now with a hall sensor for an endstop so the robot arm can:

  • return each actuator to exact position on power up by using a homing technique
  • enable 360 degree rotation since the hall sensor is not mechanical
  • be very small so it can be added without messing up the overall actuator design
  • easily interface into the ESP32 without a lot of components (achieved zero beyond the hall sensor)


Hi John,

That flexible helical planetary gear is about the coolest thing I’ve seen. Any chance you will make the model available?

Well, I’ve done a heavily modified design located here that has the slip ring integrated, the hall sensor, and a wider shell for my custom PCB, but my design uses LoboCNC’s as its base:


Oh I see. I think I will attempt to modify LoboCNC’s design myself. Thanks for posting your design, I am a big fan of everything you are doing.

I found that I had to rebuild his design from scratch in Fusion 360 since the files he shared would crash Fusion. What design program are you using?

Well I have access to F360, Solidworks, and Inventor. I’ll try to open up his files on each of them tonight.