3D-printed gearboxes are always an interesting design challenge, especially if you want to make them compact. [ZeroBacklash] created a little strain wave gearbox (harmonic drive) for when you want to trade speed for torque on NEMA 17 stepper motors.
Strain wave gears work by deforming a stationary flexible spline into an ellipse so the teeth engage the internal teeth of the output spline. Add a couple of extra teeth on the output side, and you get a high-reduction gearbox with fewer parts and reduced volume than equivalent spur gearing. Keeping the flexible spline stationery is achieved by making half of it engage with a stationary spline with the same number of teeth.
In this case, there are 60 teeth on the input side and 62 on the output, giving a gear ratio of 30:1. The flexible spline is deformed using a set of bearing balls and an elliptical plug on the shaft of the motor. It makes for a compact design that matches the frontal size of the stepper motor and is only about 27 mm long. [ZeroBacklash] has not released any design files, but the idea should be simple to replicate.
We’ve featured a couple of 3D printed harmonic drives of different sizes, but they usually use a pair of ball bearings as the wave generators, which doesn’t lend itself well to smaller designs.