Harmonic Drive Gear is a critical component in countless robotic applications. It looks very simple, but operates rather peculiarly. This gear system produces oscillations when running, yet it can provide an extremely high torque-to-weight ratio with precise accuracy, even if it indeed oscillates. So, how does it actually work?
This seemingly strange harmonic drive gear is actually quite ingenious in its operation—a flexible elliptical structure is set inside a circular shape, causing oscillations when rotating, and only about one-third of the teeth of its two main gears are in contact at any given point—but as a result, it is an exceptionally compact and lightweight drive gear that can provide excellent torque with absolute precision without any transmission backlash or play.
Harmonic drive gear was first conceptualized by C. Walton Musser from Pennsylvania in 1957. He was a highly creative inventor with patents ranging from power steering systems, recoilless rifles, and pneumatic lifejacket inflators to many key jet aircraft ejection seat concepts. Harmonic drive gear was first commercialized in the early 1960s. It consists of three key components:
Wave Generator: A slightly elliptical input hub with a ball bearing on its outer edge.
Flexspline: A flexible cup-shaped component with teeth on its outer circumference. When the elliptical wave generator is inserted, it can radially deform but maintain torsional rigidity.
Circular Spline: A completely rigid ring with internal teeth, similar to planetary gears, and has two more teeth than the flexspline.
When the wave generator rotates, about 30% of the flexspline's teeth at either end contact the teeth of the circular spline at any given point, forming an advancing wave. This causes the flexspline to rotate within the circular spline but in the opposite direction of the input.
The cleverness of this part of the system is its zero backlash—there are no gaps between the gears, making it an ideal choice for precise and refined motor control. Its compact and lightweight design is also well-suited for space-limited applications. Additionally, with fewer moving parts, harmonic drive gear is highly reliable and durable.
Today, harmonic drive gears are ubiquitous both on Earth and beyond. Industrial robots use them for tasks such as welding and assembly that require repeatable, precise movements. The Da Vinci surgical robot, designed for minimally invasive surgery, relies on harmonic drive gears in its arms to achieve zero-backlash fine movements, providing maximum precision for minimally invasive operations.
Boston Dynamics' Atlas humanoid robot and Spot quadruped robot use them in multiple parts such as their arms and legs to achieve smooth and controlled movements while remaining powerful.
Even NASA's Mars rovers, Curiosity and Perseverance, use harmonic drive gears in their mechanical arms and wheels. Sending rovers to another planet without the prospect of maintenance or repair attests to the reliability and durability of this design. While Musser did not see his invention reach Mars, he did witness it go into space.
Harmonic drive gears are a common component of the space shuttle's standard-configured remote manipulator system—the "Canadarm"—used for handling payloads and deploying cargo. Nearly 70 years after its inception, this extraordinary invention, with its unique and purposeful "oscillation", continues to be found in various fields. From precise surgical actions to the exploration of distant planets, the genius of harmonic drive gears lies in their simplicity, reliability, and precision.
