the ties 358 360 on the other hand and vice versa these two axes both being parallel to the pivoting axis o described above furthermore each of the two ties 358 360 is articulated respectively on the upper and lower connectors 328 and 330 at its upper and lower ends each of these joints can consist of a simple hinge 370 comprising a pivoting hub 372 engaged through a double lug integral with the connector in question and an orifice at the end of the tie 358 or 360 the passage orifices at the ends of the ties 358 360 are made as oblong holes and preferably have rounded bearing surfaces in order to allow the ties 358 360 also to pivot in the plane of fig1 the hinges 372 can also be replaced by more sophisticated joints for example knuckles to provide an arrangement according to fig9 the ties 358 and 360 consequently maintain a constant and predetermined distance between the connectors 328 and 330 while by means of the ring 352 they carry the central element 326 in a floating manner between the connectors 328 and 330 fig1 through 17 illustrate various possibilities for relative movements and pivoting between the connectors 328 and 30 and how these movements are compensated by corresponding positioning of the central element 326 the relative movements and pivoting between the connectors 328 and 330 which are illustrated in fig1 through 14 subject to stress only those joints in the region of the hinges 372 between the ties 358 and 360 and the connectors whereas the joints in the region of the ring 352 are.