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Principle of HarmonicDrive®

C.W. Musser, inventor of the HarmonicDrive®

C.W. Musser, inventor of the HarmonicDrive®

The innovative concept of the HarmonicDrive® and its unique mechanism are the brainchild of an American inventor, C.W. Musser.
An inventing genius, Musser holds close to 1,500 patents in not only his specialty field of mechanical engineering, but also within many areas in physics, chemistry and biology.
Prior to Musser, efforts had been focused on increasing the rigidity of gear mechanisms with the ultimate intention of "conveying driving power and motion in a faster and more accurate manner than before.
Musser's HarmonicDrive® theory however, is based on elastic dynamics and is about making use of the flexibility of metal. His theory has since been celebrated the world over as a revolutionary method for driving force transmission.

Utilizing the flexibility of metals: HarmonicDrive®

Basically, the HarmonicDrive® -developed to take advantage of the elastic dynamics of metal- is typically made up of only three components: a wave generator, a flexspline and a circular spline (Depending on its shape, the HarmonicDrive® is sometimes made up of four components; but even this four-component Harmonic Drive is based on the same principle of motion). One can grasp the Harmonic Drive's® unique mechanisms from the way its teeth mate with one another.

Wave Generator
The Wave Generator is an assembly of a thin-raced ball bearing fitted onto the periphery of an elliptical cam. The inner ring of the bearing is fixed around the cam causing the outer ring of the bearing to conform to the same elliptical shape of the assembly. The Wave Generator is usually attached to the input shaft.
The Flexspline is a thin-walled steel cup with gear teeth machined into the outer surface near the open end of the cup. The bottom of the Flexspline (cup bottom) is called the diaphragm. The diaphragm is usually fitted to the output shaft.
Circular Spline
The Circular Spline is a rigid steel ring with teeth on the inside diameter. The Circular Spline has two teeth more than the Flexspline. The Circular Spline is usually fixed to a casing.

Principles of motion

Animated CG Clip of Principle of Operation

The Flexspline is forced into an elliptical shape by the Wave Generator causing the Flexspline teeth to engage with the tooth profile of the Circular Spline along the major axis of the Wave Generator ellipse, with the teeth completely disengaged across the minor axis of the ellipse.

As the Wave Generator rotates clockwise with the Circular Spline fixed, the Flexspline is subjected to elastic deformation and its tooth engagement position moves turning relative to the Circular Spline.

As the wave generator rotates 180 degrees clockwise, the Flexspline moves counterclockwise by one tooth relative to the Circular Spline.

For every one full rotation clockwise (360 degrees) of the Wave Generator, the Flexspline moves counterclockwise by two teeth relative to the Circular Spline because the Flexspline has two fewer teeth than there are on the Circular spline. In general, this movement is treated as output power.

Characteristics of the HarmonicDrive®

  1. High-speed reduction ratio

    A HarmonicDrive® has high single-stage coaxial reduction ratios of 1/30 to 1/320. This is why the HarmonicDrive® provides high efficiency gearing without using complex mechanisms and structures.

  2. Backlash-free (lost motion)

    Different from the ordinary tooth-to-tooth mating mechanism, the HarmonicDrive® operates with very little backlash (nearly zero), which is an essential characteristic for motion control.

  3. High accuracy

    With the HarmonicDrive®, high positional and rotational accuracy are assured because a good percentage of its teeth are meshed at all times, and are engaged at two zones 180 degrees apart. This means influences of tooth pitch errors and accumulated pitch errors on rotational accuracy are neutralized to assure high positional and rotational accuracy.

  4. Small number of components and ease of assembly

    High reduction ratios can be achieved with only three gear components, and since all three components are co-axially aligned, the HarmonicDrive® can be easily built into component-assembled products, allowing for simple configurations.

  5. Small-sized and lightweight

    While being less than 1/3 the size of conventional gearing mechanisms in terms of capacity and less than 1/2 the weight, the HarmonicDrive® provides the same levels of torque and speed reduction ratios as its conventional counterparts enabling machinery and equipment to be made smaller and lighter.

  6. High torque capacity

    The Flexspline is made of special steel with a higher resistance to fatigue. Different from a typical power transmission device, each tooth is subjected to a negligible amount of force yet provides a high torque capacity because of the way the teeth come into contact with each other and because roughly 30% of the teeth in the Flexspline are engaged at all times.

  7. Superior efficiency

    The mating portion of each tooth is subjected to very little slide motion. Therefore, motion loss due to friction is reduced in spite of high reduction ratios, and the HarmonicDrive® maintains a high level of efficiency which enables driving motors to be made smaller.

  8. Quiet operation

    With the HarmonicDrive®, quiet, minimal vibration operation is possible because the circumferential velocity of its teeth in mesh is low and the teeth provide a well-balanced force.