is selected in such a way that its shore hardness is less than 96 shore hardness a this relatively soft plastic allows thickness s of moving part 4 to lie around 2 5 mm by the constructive formation of hub ring 2 supporting ring 3 and also by the choice of material good dissipation of the generated heat from supporting ring 3 to hub ring 2 is achieved and indeed in the middle region as well as in the edge regions 0015 fig2 shows likewise enlarged the outer region of hub ring 2 on the outer circumference ring grooves 16 and 17 are inserted which as in fig1 are arranged symmetrically to the radial center plane 18 of supporting disk 1 but at an angle u03b1 to center plane 18 in addition ring grooves 16 and 17 are arranged at a lateral distance a to the side surfaces 9 of hub ring 2 thereby the metallic material of hub ring 2 is led directly up to the edge regions of supporting ring 3 the side surfaces 12 of supporting ring 3 are aligned with the side surfaces 9 of hub ring 2 the chosen design results in good support of supporting ring 3 by hub ring 2 and good heat dissipation from the edge regions as well as improved discharge of electrostatic load on the supporting ring and the rotor shaft and thus a better performance with respect to contamination of the rotor shaft the ring grooves 16 and 17 can be machined very accurately by plunge cutting supporting ring 3 has to be positioned on hub ring 2 by an injection molding procedure 0018 fig3 shows a further.