A heavy-duty, all-gear transmission using helical gears is disclosed as a CVT or IVT completely devoid of any frictional or hydraulic elements. By longitudinally moving the input member of a pair of rotating helical gears, and by varying the direction and extent of any superimposed, longitudinal motion, the rotation of the output member can be varied. When the longitudinal motion is generated with a reciprocating, harmonic mechanism, the speed of the output gear varies in a sinusoidal fashion. This novel mechanism employs the principle of combining two equal outputs with sine wave profiles of opposite phase in a differential to produce an averaged, linear profile. Altering the amplitude of any longitudinal translations results in variations in the final output.