adjusting the position of the members 1 and 3 on the shafts 5 and 7 and by adjusting the center distance between the shafts 5 and 7 it is possible to vary the distance l a support may be provided between the two members 1 and 3 in a direction which is at right angles to the two shafts 5 and 7 the two members 1 and 3 may be mounted on the shafts 5 and 7 so that in addition to the rotation coupling they can be urged in order to axially slide in opposite directions to each other according to arrow f7 fig3 and 8 on the shaft 5 corresponding to the shaft 79 of fig1 to 3 the members 1 and 3 of each working group present outside the shoulders 1a and 3a respective surfaces 15a and 17a which overlap in two zones t1 and t3 shown in fig5 the frontal surfaces 15a and 17a superimpose also on the frontal surfaces inside the shoulders 3a and 1a the interspace between surfaces 15a and 17a corresponds to the one indicated by l as defined above between the minor basis of the frontal end surfaces inside the shoulders 1a and.