the circumferential direction but not as high as the webs in the radial direction i e their radially inwardly facing broad side coincides with the internal envelope 26 but their radially outwardly facing side does not reach the outer envelope 24 while the two spacer lugs 34 of each pair in one and the same section of cheese centre are identically arranged the arrangement alternates from one section of the cheese centre to the next so that the sign of the shift in position in the circumferential direction alternates the ring connecting webs 18 and the spacer lugs 34 are arranged to produce a periodicity in each section of the cheese centre so that identical pairs of webs and the pairs of lugs are uniformly distributed over the circumference of the centre and the arrangements recur in every second section whereas the number of webs and the number of lugs which are the same in each section of the cheese centre and are inevitably equal to one another within a section may be either even or odd the number of sections should be even so that the total torsion of the compressed cheese centre between the two end rings 12 and 14 is theoretically zero in one end ring 12 the web connecting section 20 whose axial dimension is smaller than that of the stiffening rings 16 is.