Harmonic Drive LLC | 42 Dunham Ridge, Beverly, MA 01915 | 800-921-3332
Harmonic Drive and Harmonic Planetary are registered trademarks of Harmonic Drive LLC.

Harmonic Drive® strain wave gears are in a class of their own when it comes to motion control and mechanical power transmission. With operating principles and construction that maximize output torque and minimize size and weight, Harmonic Drive® gears offer advantages such as high reduction ratios in a single stage, zero backlash, and high precision that cannot be equaled by conventional gear trains. Click here to see our full product lineup.


Strain Wave Gear Principle

Harmonic Drive® strain wave gearing utilizes a unique operating principle which is based upon the elastic mechanics of metals. The greatest benefits are the zero-backlash characteristics and the weight and space savings compared to other gears because our gear mechanism consists of only three basic parts. They are the: Wave Generator, Flexspline and Circular Spline.

This simple three element construction combined with the unique operating principle allows extremely high reduction ratio in a very compact and lightweight package. Neither the size nor weight of the gear vary with the reduction ratio. The high performance attributes of this gearing technology including zero backlash, high torque, compact size, excellent positional accuracy and repeatability are all a direct result of the unique operating principle.

The Components

Wave Generator


The Wave Generator is comprised of a specially designed thin raced ball bearing that is fitted onto an elliptical hub. This serves as a high efficiency torque converter and is used as the input of the gear and is connected to the motor shaft.



The Flexspline is a thin cylindrical cup made from alloy steel with external teeth on the open end of the cup. The Flexspline is radially compliant but torsionally is very stiff. When the Wave Generator in inserted into the Flexspline the gear takes on its elliptical shape. The Flexspline is used as the output and is connected to the output flange.

Circular Spline


The Circular Spline is a rigid ring with internal teeth. When the gear is assembled it engages the teeth of the Flexspline across the major axis of the Wave Generator ellipse. The Circular Spline has two more teeth than the Flexspline and is fixed to the gear housing.


How it Works



Below is an exaggerated two-dimensional diagram demonstrating how the three gear elements engage and rotate.

The Flexspline is slightly smaller in diameter and has two fewer teeth than the Circular Spline. The elliptical shape of the Wave Generator causes the teeth of the Flexspline to engage the Circular Spline at two opposite regions across the major axis of the ellipse. For every 180 degree clockwise rotation of the Wave Generator the Flexspline teeth are advanced counterclockwise by one tooth in relation to the Circular Spline. Each complete clockwise rotation of the Wave Generator results in the Flexspline moving counter-clockwise by two teeth from its original position relative to the Circular Spline. Because the gear teeth are always fully engaged in a region along the major axis, Harmonic Drive® gearheads have Zero Backlash.

In the cup-type Harmonic Drive component sets used in the CSF-GH and CSG-GH gearheads, the bottom of the Flexspline (output side) is constrained to a circle while the top of the Flexspline is elliptically shaped. This causes the Flexspline gear teeth to flare outwards slightly. We call this the coning angle and it serves to preload the gear teeth of the Flexspline and Circular Spline. This preload ensures zero-backlash performance for the life of the gear. Since there is no true backlash in the gear, we measure the torsional windup of the gear at +/- 4% of rated torque and all Harmonic Drive gearheads have less than 1 arc-min lost motion. The preload on the gear assures it will remain stable for the life of the gear.

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Superior Gear Performance Using an S Tooth Design

Harmonic Drive developed a unique gear tooth profile that optimizes the tooth engagement. This tooth profile (the “S tooth”) enables up to 30% of the total number of teeth to be engaged simultaneously. Additionally the large tooth root radius increases the tooth strength compared with an involute tooth. This technological innovation results in high torque, high torsional stiffness, long life and smooth rotation.

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Please view the short video to get a better understanding of How Harmonic Gearing Works.

Contact us if you have any questions or would like more information. We love to talk about our technology, and welcome your call or email.


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