lego micromotor reduction

This is a recreation of the internal gear reduction of the 9v lego micromotor, the final gear reduction is approx. 545:1 .  I doubt this model can be used for anything, it cant transmit a lot of torque and has some backlash….it is still a very cool reduction.

This recreation is not identical to the real lego 9v micromotor, it has a 48:1 first stage reduction, this model has a 49:1 first stage reduction (miscalculation by me), the second stage should be identical. The real lego micromotor has a reduction of 720:1.

It took me a week to design this, with quite some attempts and plastic, i am still not sure if the calculated gear ratio is correct.

I dont think this type of reduction has been used anywhere else, i also dont really know what to call it, probably something like "2-stage rolling gear reducer" or "2-stage difference/delta rolling gear reducer". Update juli-2026: i found out that this reduction is quite similar to a "ferguson paradox drive".

gitlab link with code ad the math behind this gear ratio: https://gitlab.com/COPERNICON/lego-micromotor-gearbox

about this model:

• this model is mainly designed for FDM 3D printing

• sadly cant transmit a lot of torque due to tolerances/play

• aggressive chamfers added to all gears for easier FDM 3D printing

• fits the lego technic building system

3d printing notable settings:

• standard 0.4mm nozzle

• 0.2mm layer height

• 3 wall parameters

• 10% infill

designed in freecad 1.0.2 on linux, i used cura 5.9 for slicing

UPDATE aug-2026: uploaded ALL freecard project files, same copyright applies