This is a mechanical speed changing device, invented in 1950s, that
operates on a different principle form, and has capabilities beyond the
scope of, conventional speed changers. They consist of a thin ring that
deflects elastically as they roll on the inside of a slightly larger
rigid circular ring. The basic elements of harmonic drive are circular spline, flexspine, and wave generator, all assembled in a normal configuration. As the wave generator rotates, it imparts a continuous motion to the flexspline. This causes meshing of the external teeth of flexspine with internal teeth of the circular spline. The meshing moves in a rolling fashion. It allows for full tooth disengagement at the two point along the minor axis of the wave generator. Flexspline has two teeth less than circular spline, so each complete revolution of the wave generator causes a two-tooth displacement of the flexspline in relation to the circular spline. This displacement is in the opposite direction. This way harmonic drive works as a speed reducer.
Characteristics
A harmonic drive uses an egg-shaped metal cam to create speed reduction. This means that eccentric motion is inherent in the design and translates to a lack of smoothness or ripple in the velocity and torque profile. They have less stiffness due to flexspline. In order to achieve a near-zero backlash, this drive preloads the bearing and flexspline thereby reducing the operating life. Because of preloading they provide low efficiencies. They are very compact and lightweight drives offering low backlash and high reduction ratio capabilities. Due to its inherent low torsional stiffness, distinct speed or torque ripple, and low efficiency, the drive can be problem for certain precision applications like painting application in automotive plants.