meaning that an outer locking ring 36 will not be necessary finally the embodiment illustrated in fig7 differs from the versions discussed previously in that the tongues 54 on the back of the plates 40 are round in section these extend radially across the plates and their cross section grows progressively smaller forming the shape of a cone towards one end the direction of taper being dependent on whether the plates are designed to be introduced radially into the grooves provided in the disc 10 from the outside or the inside the tongues are attached to the plates themselves by a narrow neck 56 clearly the invention is not limited to the embodiments illustrated and discussed here but can be varied extremely widely within the framework of the underlying idea thus it would be conceivable to provide the supporting disc with grooves running peripherally and of e g dovetail form into which tongues of equivalent design may be introduced each plate may have more than one cuneiform tongue these having a combined effect and running radially and peripherally at some distance from one another as is apparent in fig1 the supporting disc 10 has an annular zone 58 without grooves 22 28 which is of a depth and radial width sufficient to allow the inner ring of plates 19 each with its tongue 35 to be introduced radially into the wedge.