on the secondary shaft 20 whereas the second fork 29 is arranged on the main shaft 7 and that a single lifting eccentric 30 is provided on each said shaft whereas each fork of each support element 26 27 is engaged with its own block 31 the figures show moreover that the support elements 26 and 27 have grooves 32 which are substantially vertical and first and second lugs 33 and 34 respectively which protrude from the first support element 26 and the second support element 27 and which can be fixed to a first joining element 35 and a second joining element 36 respectively via further lugs the joining elements 35 and 36 support the main teeth 16 and the differential teeth 17 respectively as can also be seen in fig 2 it is also pointed out that the joining elements 35 and 36 are fixed to the ears 33 and 34 by means of screws which are inserted in holes provided in the said lugs and in eyelets provided in the said joining elements moreover further holes 33a and 34a are provided in the said lugs which holes are arranged at the vertices of equilateral triangles and allow screws to be inserted which depending on their degree of projection determine the position of the joining elements 35 and 36 and consequently the position of the teeth 16 and.