containment shell 3 u2033 with a third variant of the containment shell groove 35 u2032 u2033 and the cooling rib 38 u2033 where the cross section of the containment shell groove 35 u2032 u2033 is substantially trapezoidal with an undercut in this case particles are largely held independently of the direction of rotation fig6 shows a containment shell 3 a with pitched containment shell grooves 35 a a containment shell flange 22 a screw holes 27 a and a mount 34 a for an axis component the containment shell grooves 35 a have a cross section the cross sectional area of which changes in the direction of extent at a first end of the containment shell groove 35 a the cross sectional area is smallest and at the other end of the containment shell groove 35 a it is at largest fig7 shows a containment shell 3 b with pitched containment shell grooves 35 b a containment shell flange 22 b screw holes 27 b and a mound 34 b the containment shell grooves 35 a have a cross section the cross sectional area of which changes in the direction of extent at a first end of the containment shell groove 35 a the cross sectional area is at smallest and at the other end of the containment shell groove it is at.