A part of the stored kinetic energy is stored in additional rotating fly wheels which circulate with the vibrator shaft. The fly wheels are arranged so that together they represent essentially no mass. Individual fly wheels have no useful operating function other than that of energy storage. In one embodiment the sum of the masses of the individual fly wheels has a null value with respect to the centrifugal force generated when the shaft is rotated, and/or the sum of the masses of the individual fly wheels corresponds to the distribution of mass of a rotationally symmetrical cylinder part.
