separating surface 24 inside surface 25 and outside surface 26 of these collars are so heavily cross arched that they will adapt to all curvatures of contour 13 and of the outside surface of the rotor shaft with collar 152 of fig5 a core ring 27 of circular cross section is surrounded by a ring jacket 28 of suitable wearing material with collar 153 in fig6 the core ring 271 has a barrel shaped cross section and is enclosed in a sleeve 281 of wearing material for example this jacket may have a greater thickness on the outside than on the other three sides thus the collar of fig7 has a core ring 272 with rectangular cross section which makes possible a particularly simple manufacture by cutting from a cylindrical pipe the jacket 282 is strengthened by the curvature on the outside and inside in cushion like fashion the outer layer may possibly be thicker than the inner layer with collars of fig5 and 7 ring and jacket may be connected in one piece by the jacketing according to fig8 and 9 these parts can be produced separately and then joined which does not preclude from being permanently joined later by vulcanization the core ring 273 of fig8 has a t shaped cross section and the jacket 283 of this collar 154 has a u shaped cross section reaches with the inward projecting flanges of its retaining rims 29 around the flange ends and encloses the stringer so that.