The Harmonic Drive® Gear belongs to the group of strain wave gears. The Flexspline is slightly smaller in diameter than the Circular Spline resulting in it having two fewer teeth on its outer circumference. It is held in an elliptical shape by the Wave Generator and its teeth engage with the teeth of the Circular Spline across the major axis of the ellipse. As soon as the Wave Generator starts to rotate clockwise, the zone of the tooth engagement travels with the major elliptical axis. When the Wave Generator has turned through 180 degrees clockwise, the Flexspline has regressed by one tooth relative to the Circular Spline. Each turn of the Wave Generator moves the Flexspline two teeth anti-clockwise relative to the Circular Spline.