The strain wave gear as an adaptive machine element

Ever since they were invented, Harmonic Drive® strain wave gears have remained of great interest in a variety of industries, thanks to the constant innovation, improvements and modifications that they are still undergoing. Today they represent the first choice for applications that require a high level of positioning and repeat precision. Whether enclosed in a housing, employed as an actuator with motor and encoder, or as components in customer-specified configurations with specific materials or design elements, the flexibility of strain wave gears makes them suitable for a wide range of applications. However, there is one aspect, in particular, that has remained constant for decades, and this is the principle by which the Harmonic Drive® strain wave gear functions.

Strain wave gear - construction and function

The Harmonic Drive® strain wave gear is made up of three basic components. These are first of all the so-called wave generator, an elliptical steel disc that forms the heart of the gear unit, which has a centric hub and a special thin, elliptically deformable ball bearing. The wave generator is also connected to the motor shaft. Secondly, the wave generator consists of the flexspline – a deformable, cylindrical steel bushing with teeth arranged around its outer circumference – and the circular spline. The latter surrounds both the wave generator and the flexspline as a rigid, cylindrical outer ring, with teeth along its inner circumference. What is significant is the number of teeth – the outer toothing of the flexspline has fewer teeth than the inner toothing of the circular spline. The difference in number is frequently two.  Typically, the wave generator is used as the input member and the flexspline as the output element of the mechanism.

The elliptical wave generator is the driven element and initiates the process of the Harmonic Drive® strain wave gear, with preliminary deformation of the flexspline. The latter is enmeshed with the internally toothed circular spline in the opposing areas of the large elliptical axis. The rotation of the wave generator causes the large elliptical axis to shift, along with the enmeshed area of the teeth. The key to the design and function of the Harmonic Drive® strain wave gear is that the flexspline has fewer teeth than the circular spline. With a difference of two teeth, a half-revolution of the wave generator produces a relative motion between the flexspline and the circular spline corresponding to one tooth. After a complete revolution, the difference is already two teeth.

Properties and benefits of the Harmonic Drive® strain wave gear

The central features and advantages of the strain wave gear compared to other, more conventional types of gear unit, are primarily due to the combination of its simple construction and the unique principle of elastic deformation found in steel gear wheels. Harmonic Drive® gears display absolute play-free performance throughout their service life and maintain a high level of torsional rigidity over the whole torque range. With smaller dimensions and a lower weight compared with conventional gear types, they are suitable for use in lighter and more compact applications and systems. Owing to their high reduction ratios and the fact that they consist of three basic components and have a coaxial orientation, strain wave gears can be easily integrated into existing applications.

A central hollow shaft is sometimes used to create the aforementioned properties. It provides space for shafts, cables and other media or components, and lends itself to user-friendly construction. High single-stage reduction ratios, high efficiency levels, and very good values in terms of position and repeat precision are further advantages of the strain wave gear.

The strain wave gear - areas of application

When Walton Musser first conceived the Harmonic Drive® principle in 1955, he primarily envisaged that it would be used in space travel, which is why it was funded above all by NASA. Over the ensuing years, strain wave gears from Harmonic Drive AG have visited the moon and Mars; they have left our solar system and may one day in the future even find their way to hitherto undiscovered planets. To this day, strain wave gears are still in use in space. But their range of applications has expanded considerably, and they now represent an extremely important component of complex drive systems, alongside motors, controllers, measuring instruments and brakes.

Another application field is that of robotics, where strain wave gears primarily function as axle drives in a wide range of robot designs. They are also found in drive units in flight simulators, as well as in the orientation of parabolic antennas, in the automobile industry, and in machine tools.