Devices convert non-uniform rotational motion into uniform rotational motion and vice versa. A motion conversion mechanism includes a housing, a common shaft, a symmetrical differential reduction gear, rings for differential power flows, cams having an inner working surface, cams having an outer working surface, and sliders with fingers. The inner working surface cam profile is described by the polar radius as a function of the polar angle and is an equidistant curve distanced outwardly from a first-order derivative of a basic closed curve by the size of the finger's radius. The outer working surface cam profile is described by the polar radius as a function of the polar angle and is an equidistant curve distanced outwardly from a second-order derivative of a basic closed curve by the size of the finger's radius. In a single revolution of the shaft, each ring performs two half revolutions back and forth.