the interior of the external sleeve 45 until the grooves 67 make contact with the corresponding projections 69 on the rotating sleeve the grooves 67 and the projections 69 form a coupling which enables the operator to rotate the rotating sleeve 39 together with theexternal casing formed by the external sleeve 45 to press the locking pin 29 against the action of the counter spring 30 and thus disconnect the yoke 31 and thus the transmission shaft 1 from the drive shaft 5 once the operator releases the tongue 67 its elastic characteristics return it to the rest position shown in fig7 and 8 i e alignment with the cylindrical external surface of the external sleeve 45 as can be seen particularly from fig1 the rotating sleeve 39 can also be used without the protective casing formed by the external sleeve 45 as shown in the aforementioned fig1 rotating sleeve 17 is precisely the same as sleeve 39 and can be fitted by simply replacing the disc shaped support element 35 and the support element 47 with respectively a disc 18 and an elastic ring 20 which fit into grooves corresponding to the grooves 36 and 51 on the yoke 31 in such a way that the same rotating sleeve and the same yoke can be used both with or without the aforementioned protective casing it is to be understood that the drawings show but one example as a practical demonstration of the invention whose forms and design can be varied without being beyond the scope of.