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Harmonic Drive

3D model description

This is a 3D printable version of a harmonic drive. Harmonic drives are mechanical gear systems that have some very nice properties such as no backlash, high gear ratios, and being able to hold their position without power :

Harmonic drives have an elliptically deformable inner gear (the flex spline) that is meshed into a ridged outer gear (the circular spline) using an elliptical horn ( the wave generator) that attaches to a motor. As the wave generator moves it deforms the flex spline and changes its contact points with the circular spline. Every full rotation of the wave generator moves the flex spline by only by the difference in the number of teeth in the flex spline and the circular spline. This design has 32 teeth in the flex spline and 34 teeth in the circular spline. See more details:

Check out the video of the drive in action:

3D printing settings

Harmonic drives are used on a lot of the high end humanoid robots such as Asimo. Unfortunately they are normally really expensive (thousands of dollars), so I'm hoping to make this technology accessible to the hobby market. This design costs around $3 in parts and plastic (+ $11 for the servo). Once the design has been refined a bit I want to use them in bipedal robots such as FOBO :

The main technical addition to the basic harmonic drive design is the stabilizer which attaches to both the flex spline and the circular spline to constrain the output force to the radial axis. This reduces sheering forces on the flex spline that would otherwise destroy it quickly.

I went through around 25 iterations for the design for the flex spline and the current version has stood up to about 3 hours of use without breaking. Some tweaking of the printer settings might make the flex spline more durable. I need your help to make this design better so that everyone can have access to this great technology.

The main parts needed for the drive are:

2 radial ball bearings
1 standard sized continuous servo
48 bbs
1 radial potentiometer

A full parts spread sheet that includes pricing and links to vendor web sites is available here:[hashtag gid]=0
(note that some of the parts must be purchased in greater volume than necessary for a single build)

Printing the parts should take about 2.5 hours depending on your printer setting. The tolerances are really important for this build, so some tweaking of the settings might be necessary to get a happily meshing harmonic drive (be patient and edon’t despair, it is definitely possible).

Modify the servo to be continuous if it isn’t already. A good tutorial on how to do this can be found here:

Assembly of the drive should take about around 20 minutes. Step by step animated instructions can be found here:

Testing the drive can be done with any microcontroller that can control a servo. I used an Arduino Uno to control the Harmonic Drive. Follow the Servo/Knob example that comes with the Arduino development environment (for both the wiring and software).

  • 3D model format: STL and ZIP





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