Harmonic drive

mechanism it describes is strain wave gearing. The gear works on the elastic deformation of metal and has only three parts — a wave generator, a flexspline and a circular spline — the circular spline carrying two more teeth than the flexspline, so each turn of the wave generator advances the flexspline by two teeth. Because teeth stay fully engaged along the ellipse's major axis, the gearing is described as having zero backlash, with high torque, high reduction ratios, positional accuracy and repeatability in a compact, light package whose size and weight do not change with the ratio.