Overview

A 3D printed strain wave gear, using a HTD 300-5M-20 and 608 bearings. The belt has 60 teeth, for 30 revolutions of stepper the outer gear rotates once, resulting in a -30:1 gear ratio.

Bill of materials

Design

Using the parametric design I printed exactly the STL output. The parametric model was designed and mainly checked against an HTD 300-5M-20 that I used here. I added space for nuts on the wave generator and set screws. To assemble the wave generator 3D-printed pins for the bearings with a spot for snap rings.

	HTD Belt	Tooth spacing / depth
	8M	8mm / 3.38mm
	5M	5mm / 2.06mm
	3M	3mm / 1.17mm
		The picture shows the belt teeth against a millimeter tape. So far is seems that 3M belts require the gear to be too precise and don't deal with forces, 5M seems like a good mix for prototypes.

Using a 20mm wide belt, gear parts should be less than 10mm each when the sides are assembled there isn't friction between the two halves.

Control

I used a Arduino MKR1000 using the Timer5 library, controlled by the Blynk app. The sketch controlled a DRV8825 stepper module. I can make details available upon request. Here is the configuration of the Blynk app:

Measurements

Torque

I can feel there is more torque on the gear output. What is an effective way to measure torque? I have 5M and 8M and belts of different diameters.

Current

I would want the stepper speed and current to the controlled/captured at the same time. A current sensing resistor supplying the input of an op-amp, conditioned, then captured by the ADC, should work but is there an easier/better way?

Resolution

A laser diode mounted on the gear output pointed at a wall would allow measurements to find the effective repeatable resolution. Then finding out what force results in what loss of resolution or repeatability.