The Flexspline is distorted into an ellipse by the Wave Generator and it engages the Circular Spline at the major elliptical axis. Teeth are completely disengaged on the minor axis.

With the Circular Spline fixed in place and the Wave Generator turning clockwise, the Flexspline deforms and turns Counterclockwise relative to the Circular spline.

For each complete revolution clockwise of the Wave Generator, the Flexspline moves counterclockwise by two teeth.

SHD harmonic-drive gearheads from HD Systems Inc., Hauppauge, N.Y. (, are compact and use a cross-roller bearing with high moment stiffness. The units are available in six different gearhead sizes, with thickness ranging from 17.5 to 33 mm, positional accuracy between 1 and 1.5 arc-min, and can handle input speeds of 4,000 to 8,500 rpm. The rated torque at 2,000 rpm ranges from 3.7 Nm (33 lb-in.) to 206 Nm (1,823 lb-in.).