Harmonic Drives

Harmonic drive 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. Harmonic drives consist of a thin ring that deflects elastically as they roll on the inside of a slightly larger rigid circular ring.

Basic Elements
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 their inherent low torsional stiffness, distinct speed or torque ripple, and low efficiency, the harmonic drives can be problem for certain precision applications like painting application in automotive plants.