Abstract: Four shafts fitted with eccentric weights are arranged as four parallel lines with one pair placed immediately above the other pair and the ends of each are fitted with a notched driving roller. A double-notched belt driven by a motor passes alternatively under and above the rollers for the first pair of shafts, around a return, and above and under the rollers for the second pair of shafts. Idler rollers are associated with each shaft to ensure that the belt moves properly. By means of a slide, the motor and the return can be moved laterally in relation to the shaft lines, thereby permitting the continuous adjustment of the relative position of the eccentric weights for each pair of shafts. Therefore, the level of the resulting force of vibration is adjustable to between a maximum value and a zero value.