Abstract: Two flyweight shafts 12,22 are coupled by means of gears 18,30 each attached to one flyweight shaft 12,22 and two intermediate gears 54,56 each engaging one of these gears 18,30 and mesh with each other. Each of the intermediate gears 54, 56 is mounted on one of the oscillating cranks 32, 34 pivotably mounted on the associated flyweight shaft 12 and 22, respectively. The oscillating cranks 32 and 34 are held parallel to each other and can be pivoted to effect a relative phase displacement of the flyweight shafts 12, 22. The oscillating cranks 32,34 are provided with counterweights 62,64 and are thereby mass balanced to prevent oscillations from becoming effective on the oscillating cranks 32,34 and thus on the adjusting mechanism 60 during vibratory operation due to intertial forces.