Harmonic Drive

This is a harmonic drive. A harmonic drive is a very simple gear reducer that lends itself well to 3D printing.

It uses a rigid outer gear and an inner flex spline. The flex spline has a few less teeth (typically 2 less) than the outer gear. You stretch the flex spline so that it engages on opposite sides (typically) of the outer gear. The stretching mechanism, called the wave generator, rotates. This causes the flex spline to slowly advance along the outer gear in the opposite direction on the spinning wave generator. The ratio is...

**Ratio = ( Flex - Outer ) / Flex**

In my case that is...

**(78 - 80) / 78 = -1/39**

A good harmonic has zero backlash and is very strong. A large percent all of teeth are fully engaged at all times. It will also hold the position with the motor off. That is great on things like robot arms during loss of power. It also puts less load on servo motors.

I used Ninjatek Cheetah filament for the flex spline. It the slightly less fl