flush with a front side 12 of rotor 3 the cross shaped arrangement of vanes 4 on the outside of rotor 3 is indicated in fig7 a cover 13 which may be fastened to the stator also has a recess 14 with which locking element 7 i e spherical section 11 is able to engage recess 14 is provided with a ramp shaped base 15 of recess 14 of cover 13 as is apparent in fig9 in a top view recess 14 has a kind of horseshoe shaped or u shaped outer contour ramp chamfer 16 formed by the base merging steplessly with a planar surface 17 of cover 13 a singular representation of locking element 7 is illustrated in fig1 and 11 which is designed in the manner of a spherical section 11 a convex outer contour 19 is present between two front surfaces 18 of an arbitrary preferably planar design which are the sides of a plate the convex outer contour follows a spherical shape a rounding or chamfer 20 being provided in the transition to front surfaces 18 alternatively to the spherical shape i e as a spherical rounding the outer contour may however also have other shapes the outer contour.