Axis 1 - Part Design - Planetary Gearbox

A project log for Yet Another Robotic Arm

Entirely 3d-printed robotic arm created from scratch.

josh-coleJosh Cole 06/17/2020 at 02:280 Comments

Axis 1 - Planetary Gearbox - 3:1 Ratio

In order to control the center of gravity/mass, I wanted the first level to be a planetary gearbox. In this way, I'm starting out with the weight being exactly centered along the shaft of the motor.

Spoiler alert: I forsake this dream of perfectly balanced construction a bit later.

The sun and planet gears are 24T and the annular gear is 72T. Their powers combined create a ratio of 3:1. Paired with my NEMA17 400-step stepper, this will give me an accuracy of +/- 0.3 degrees of resolution. Is that good? Ahh I'm not sure. But we can always revisit this part later!

Mounting Strategies

The sun gear attaches to the motor through a standard set screw. So far I have managed to make Axis 1, Axis 2, and Axis 3 consistently use 6mm set screws. So that is the "standardized" size and it applies here.

The planetary gears have a press-fit bearing installed. Apparently the tolerances needed for a successful press fit are just to make the bore exactly the expected OD of the bearing. My printer is generally off by +/- 0.15mm so this produces a slightly smaller hole then expected. As a result, you can use a press to snugly push the bearing in place.

Lastly, I created some spacers for the top and bottom, to keep the bearing in place.

Reflection: If I were to design this again, I would try hard to avoid the spacer design. In fact, I would probably play around with custom shaft design which has built-in spacer. Will likely work much better.

All of this is sandwiched between two standard planetary gear mount structures. The bottom structure has a NEMA17 mounting holes, as well as some captive nuts on the underside which the top structure bolts down into. The result is this lovely and very strong planetary gearbox.