26 and said second bearing sleeve is surrounded by a second bearing bushing the second bearing bushing 39 and second axial bearing ring 35 are arranged so as to substantially face one another a third retaining ring 40 which has been pushed onto the impeller shaft 13 bears at the side facing away from the retaining ring 31 against the second bearing sleeve 38 as can be seen from fig1 and 2 the plate spring pack 41 is arranged between the retaining ring 40 and the shaft section 13 a and exerts a spring force on the clamped assembly composed of impeller 16 an impeller nut 43 which fastens the impeller 16 to the impeller shaft 13 via a disk 42 retaining ring 25 first bearing sleeve 26 retaining ring 31 second bearing sleeve 38 retaining ring 40 and inner rotor 17 in such a way that the clamped assembly is held in abutment in particular by way of the inner rotor 17 with a certain degree of elasticity against an abutment surface 44 which arises owing to the different diameters of the shaft sections 13 a and 13 b wherein the diameter of the shaft section 13 b is greater than the diameter of the shaft section 13 a and the diameter of the shaft section 13 a is greater than the diameter of that part of the impeller shaft 13 which adjoins the shaft section 13 a at the side facing away from.