LEGO Compatible mini Harmonic Drive

thingiverse

Ever since I learned about the invention and uses of Harmonic Drives(technically known as strain wave gearing) I've been fascinated by them. The input is an elliptical wave generator that pushes the inside walls of a flexible gear, which engages the gears in the case with 2 or 3 more teeth. Similar to a planetary gearset it produces immense gear reduction in a small package. Harmonic drives are used in precision industry, namely robotic arms and things like solar panel extenders in satellites. Their benefits include no backlash(on one made of steel, not this), no back driving, and coaxial output from the motor. https://en.wikipedia.org/wiki/Strain_wave_gearing I've seen several 3D printed harmonic drive projects but they are usually large and heavy with lots of print time and extra hardware like bearings(lots of them). This is the smallest one I've seen and even better it's Lego compatible. I also think the spiral pattern looks pretty cool, I opted for that instead of a longer can to keep size down. The outer ring has 28 teeth, the flexible gear has 26 teeth. The ratio can be calculated by: r = (26-28)/26 = -.077 meaning that for every 100 input rotations it gets 7.7 output rotations in the opposite direction, or about 13:1. I was originally going to have 3D printed pins friction fit the base plate into the Lego XL motor but I printed them in the wrong orientation, so they snapped immediately, so I just jammed some Lego pegs in there and it stays well enough. I've provided Solidworks parts you can tinker with, I'm just too lazy to print more designs to find the right tolerance. Printing is fairly straightforward. I used PLA. Use supports for the flexible gear spiral section, and the screw holes on the base plate. To get it working smoothly I had to do a bit of de-burring and greasing of the wave generator and gears. All you need to assemble are two M3 bolts and whatever Legos you want to use. I've also provided the parts labelled "prototype" for a quick print to see if your printer tolerances and calibration works well with this, the larger encased model uses the same dimensions.