How it works: The Harmonic Drive gear, Precise, High-torque, With a Wobbling Motion

The Harmonic Drive

The Harmonic Drive is a fundamental component in countless robotic applications. Although simple and intriguing to watch in action, this gear system combines exceptional precision with an impressive torque-to-weight ratio, even with its characteristic wobbling motion. But how exactly does it work?

This seemingly unusual mechanism operates ingeniously: its flexible-ellipse-within-a-circle configuration creates a wobbling motion as it turns, engaging about one-third of the teeth of its two main gears at any given moment. The result is a compact, lightweight gear capable of delivering high torque with extreme precision and zero backlash or play between the teeth.

Origins and Components of the Harmonic Drive

The Harmonic Drive was created in 1957 by C. Walton Musser, an inventor from Pennsylvania with patents in diverse fields, including power steering systems, recoilless rifles, and life jacket inflators, as well as various key ideas for jet aircraft ejection seats. It was first commercialized in the early 1960s and consists of three main components:

• Wave generator: A slightly elliptical input hub with a ball bearing race around its outer edge.

• Flexspline: A flexible, cup-shaped component with teeth along the outer perimeter. It deforms radially but retains torsional rigidity when the oval wave generator is inserted.

• Circular spline: A completely rigid ring with internal teeth, similar to a planetary ring gear, with two more teeth than the flexspline.

When the wave generator rotates, about 30% of the flexspline’s teeth come into contact with the circular spline’s teeth, creating an advancing wave pattern. This movement causes the flexspline to rotate within the circular spline but in the opposite direction of the wave generator.

The Advantage of No Backlash

The main advantage of this system is the absence of backlash between the gears, enabling precise, fine motor control. Its compact and lightweight design makes it ideal for applications with limited space. Additionally, with few moving parts, the Harmonic Drive is extremely reliable and durable

Today, the Harmonic Drive is widely used in many fields, even beyond Earth. Industrial robots from companies like Fanuc and Universal Robots use these gears for tasks such as welding and assembly, where precise, repeatable movements are essential.

The da Vinci surgical robot, designed for minimally invasive procedures, uses Harmonic Drives in its arms, allowing micro-delicate, backlash-free movements crucial for surgical precision.

Robots like Atlas and Spot from Boston Dynamics use the Harmonic Drive in their arms and legs for controlled, smooth movements while maintaining strength and stability.

Even Mars rovers like Curiosity and Perseverance rely on the Harmonic Drive in their arms and wheels. The presence of these gears in interplanetary missions, where maintenance is impossible, proves the reliability and durability of the design.

Although Musser did not live to see his invention reach Mars, he did witness its use in space. The Harmonic Drive was a standard component of the Remote Manipulator System, also known as “Canadarm,” used on space shuttles like Columbia and Challenger to handle and deploy payloads.

Almost 70 years after its invention, this remarkable mechanism, with its distinct yet purposeful wobble, continues to be found in a wide variety of fields. From surgical precision to the exploration of distant planets, the Harmonic Drive stands out for its simplicity, reliability, and precision.

Read the original article on: New Atlas

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