A harmonic drive (strain wave gear) is a zero-backlash, high-reduction mechanical gear system widely utilized in compact robotic joint actuators. It comprises three concentric components: an elliptical Wave Generator cam, an externally toothed thin flexible cup (Flexspline), and a rigid internal-toothed outer ring (Circular Spline). Because the flexible cup possesses fewer teeth than the outer ring (typically Z_circ - Z_flex = 2), full rotation of the wave generator advances the flexspline in counter-rotation by a fraction of its circumference.