the surfaces of the channels 13 and 14 with little or no clearance it can be terminated by curved edges 17 and 18 at the front and at the back substantially parallel to the strips 12 and which fit with no or practically no clearance against stopping faces 19 and 20 of the channels 13 and 14 in the axial direction which are near edges of the platforms 1 and 2 if the platforms 1 and 2 move mutually in the axial direction they shear the core 11 which however has the ability to withstand and counter these undesired movements which could separate the platforms 1 and 2 the curved edges of the core 11 at the ends as well as on the sides are also designed to hold the plate 25 supporting the parts 15 and 16 of the easily eroded layer and to prevent it from descending into the channels 13 and 14 it is noted that it is suitable for the strips 12 to extend in front of furrows 21 separating peaks or wipers 22 of the rotor rubbing on the layer parts 15 and 16 and which shape them to complete the leak tightness between a disk 23 of the rotor carrying these wipers 22 and the platforms 1 and 2 the strips 12 can project beyond the layer parts 15 and 16 to.