disk in accord with the invention are presented fig4 a shows an embodiment having a groove 30 in a cover layer 143 which nearly penetrates through to the base dotted line by the cohesion of the cover layer at its inner circumferential surface a very stable embodiment is created since the two part cover layer extending outwardly at its fastening point is supported on both sides in fig4 b a support disk 14 is presented which has two circumferential grooves 30 by this means three support surfaces are created by the cover layer for the rotor shaft fig4 c depicts an arrangement of three grooves 30 in the support disk cover layer 143 as this cross section shows the outer two grooves are formed less deep in the thickness of the cover layer than is the center third groove the center groove 30 is in this case similar to the groove depicted in fig4 a as it is formed to a depth in the cover layer 143 approaching the body surface 5 in fig4 d cleaning grooves 3 in the cover layer 143 are slanted in reference to the central axis of the body 5 by this device an axial thrust is generated on any contaminating deposit on the rotor shaft 21 whereby such contamination is gradually abraded and the rotor shaft 21 in this.