addition to the pin 13 also an elastic member made in the form of a helical spring 17 the line forming the helical spring 17 has appendages 17 a and 17 b the appendage 17 a is engaged in the appropriately shaped cavity constituted by the hole 15 whilst the appendage 17 b has a bent end 17 c that is engaged in a corresponding hole not illustrated made on the bottom face of the corresponding eccentric oscillating element 11 with this arrangement when the member 9 is assembled the helical spring 17 loads the eccentric oscillating element 11 in a resting position from which the element itself can be made to oscillate torsionally deforming the helical spring 17 as may be noted in fig1 provided on a part of the perimetral development of each of the elements 11 is a knurling 11 z for the purposes that will be clarified hereinafter the eccentric oscillating elements 11 are mounted see in particular fig4 a 4 b and 5 in the respective seats 7 in such a way as to project through the respective slit 5 with at least one portion of the respective knurling 11 z it is possible to act on this knurled part of the edge of each eccentric oscillating element 11 to bring about an oscillation of the eccentric oscillating element.