Abstract: Speed capability of a rotary stepping motor of the harmonic drive type is markedly increased by providing a stiffer, i.e. harder to radially deflect, structure of its rotor. Elimination of magnetically permeable bridging material as heretofore employed is made possible also in an alternate construction. Basically, thickness of a flexrotor or flexspline wall is made greater than hitherto relative to its outside diameter to provide a stiffness insuring efficient operation at increased natural or resonant frequency. Higher capability of the thicker-stiffer flexspline to maintain its sinusoidal shape during operation results in less tooth wear and increased efficiency over a longer operating life. When concentric ring bridging is incorporated, its total wall-to-outside-diameter ratio thickness is advantageously substantially that of the novel flexrotor or flexspline itself to obtain higher step rates efficiently.