OpenTorque Actuator

A powerful, compliant actuator for legged robotics

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OpenTorque is an open-source implementation of the quasi-direct-drive actuation scheme pioneered by the MIT Biomimetics lab. The basic premise is this: use the largest possible motor with the smallest possible gear reduction. This results in an actuator that is robust, highly backdrivable, and capable of proprioceptive force sensing and open-loop impedance control. These are all ideal attributes for building legged robots.

OpenTorque uses a large-diameter brushless quadcopter motor, a 3D-printed planetary gearbox, and the awesome ODrive motor driver ( for control. 


  • Peak torque (theoretical): 56 Nm
  • Continuous torque (measured): 28 Nm
  • Weight: 1.2 kg
  • Gear ratio: 6:1
  • Cooling: Air-cooled
  • Cost: Approx. $150

This project was inspired by the following:

Gearbox Assembly Helical v10.step

STEP file containing all the parts

step - 4.11 MB - 06/30/2018 at 22:33


View all 12 components

  • Bipedal Robot Concept

    Gabrael Levine07/04/2018 at 04:11 0 comments

    I'm building a bipedal robot inspired by Agility Robotics' Cassie. It uses 10 OpenTorque actuators. 

    These actuators are backdrivable and able to absorb kinetic energy during the walk cycle. As a result of this, there's no need for the complex series-elastic linkages present in Cassie. Instead, each leg consists of a simple parallelogram linkage with an extra joint at the ankle. 

    Carbon fiber tubes are used for the linkages in order to keep inertia as low as possible. The total weight is in the ballpark of 30 pounds -- substantially lower than the 60-70 pounds that Cassie weighs. This is helped by the use of 3d-printed plastic rather than CNC aluminum parts. (All the structural parts will be printed out of NylonX.)

    To control this robot, I'm going to use reinforcement learning. I'll create an OpenAI training environment with a simulation of the robot, then let the controller learn a stable walking gait on its own. This is a lot easier than programming a walking gait by hand, and it's already been done successfully on the Minitaur robot. 

View project log

  • 1
    Print the parts
    • Use 50% infill and a wall thickness of at least 1.2 mm. 
    • Nylon is the recommended material for the sun and planet gears. I used Taulman Alloy 910. 
    • Print the planet gears on rafts to ensure the first layer comes out perfectly flat. Otherwise you can run into issues with the gears meshing. 
    • All the parts are designed to print without supports. 
  • 2
    Install the threaded inserts

    Printed parts:

    • Ring Gear
    • Planet Carrier Front

    Non-printed parts:

    • 21x M3 threaded inserts


    1. Place the inserts in the holes (marked in red) and set them in place with a soldering iron. 
  • 3
    Assemble the planet carrier

    Printed parts:

    • Planet Carrier Front
    • Planet Carrier Back
    • 3x Planet Gears

    Non-printed parts:

    • 3x M3 14mm countersunk screws
    • 3x 5x40mm steel dowel pins
    • 6x F625ZZ bearings


    1. Insert the F625ZZ bearings into the Planet Gears. Use two bearings per gear, one bearing goes on each side of the gear. 
    2. Insert the 3 dowel pins into the 5mm holes on the Planet Carrier Front.
    3. Slide the planet gears onto the dowel pins. (depicted on the left)
    4. Place the Planet Carrier Back on (depicted on the right), and secure it in place with three M3 14mm countersunk screws. 

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Enjoy this project?



nick wrote 11 hours ago point

this looks to have real potential, cant wait to see how it develops!

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