Wobbly yet precise high-torque harmonic drive gear

Harmonic drives are a key part of many robotic applications. Deceptively simple, yet quite strange to look at, this gear system is extremely precise, with an impressive torque-to-weight ratio, even if it’s shaky. So how does it work?

The seemingly bizarre harmonic drive gear is actually somewhat genius in how it works. The flexible oval-in-a-circle setup creates a rocking motion as it turns and only about a third of the two main gear teeth are in contact. However, the result is a very compact and lightweight drive gear that can deliver exceptional torque with absolute precision and zero drive lash or play.

The device was first conceived in 1957 by C. Walton Musser of Pennsylvania, a remarkable and inventive inventor who held patents on everything from power steering systems, recoilless rifles, and pneumatic life jacket inflators to many of the key ideas behind jet aircraft ejection seats. I did it. Harmonic drives were first commercialized in the early 1960s. It consists of three main components:

harmonic drive

• Wave Generator: A slightly oval-shaped input hub with a ball bearing race around the outer edge.
• Flexspline: A flexible cup-shaped component with teeth on the outer circumference. It deforms radially but maintains torsional rigidity when inserting a rectangular wave generator.
• Circular Spline: Similar to a planetary ring gear, it is a completely solid ring with internal teeth and two more teeth than a flexspline.

As the wave generator rotates, approximately 30% of the flexspline teeth at both ends are in contact with the teeth of the circular spline at any time, forming a traveling wave pattern. This causes the flexspline to rotate within the circular spline, but in the opposite direction of the input. Check it out:

One of the advantages of this system is that there is no backlash. There is no play between gears, making it ideal for precise and fine motor control. The compact and lightweight design is also suitable for applications where space is limited. With virtually no moving parts, harmonic drives are highly reliable and durable.

Harmonic drives are everywhere today. turn off planet. Industrial robots such as Fanuc and Universal Robots use them for tasks such as welding and assembly where repeatable, accurate movements are important.

Designed for minimally invasive surgery, the da Vinci surgical robot uses harmonic drives in its arms to provide backlash-free, delicate micro-movements, providing ultimate precision while performing minimally invasive surgery on humans.

harmonic drive

Boston Dynamics’ Atlas humanoid and Spot Quadruped robots use this, especially in the arms and legs, to achieve strong yet smooth and controlled movements.

NASA’s Mars rovers Curiosity and Perseverance also feature Harmonic Drives on their robotic arms and wheels. Sending a probe to another planet without any hope of maintenance or repair downtime is a testament to the reliability and durability of this design.

Musser never saw his invention reach Mars, but he at least saw it make it into space. The harmonic drive was a common component of the Space Shuttle’s standard telemanipulator system (aka “Canadarm”), used to handle payloads and deploy cargo on shuttles such as Columbia and Challenger.

Nearly 70 years after its creation, this amazing invention with its unique and purposeful “wobble” continues to be discovered in a variety of fields. From precise surgical movements to exploration of distant planets, the genius of Harmonic Drive lies in its simplicity, reliability and accuracy.

Source: Harmonic Drive