features described with respect to the adapter device 10 of the first embodiment except that the hollow stub 11 is slightly truncated or shorter than was previously the case furthermore the recess or cavity 18 for accommodating the second bearing 36 u2032 is provided in the second part 9 the second part 9 of the adapter device 10 in this second embodiment thus includes a circular cylindrical cavity 18 u2032 forming a second bearing socket designed to receive and engage with an outer ring of the second roller bearing 36 u2032 e g in a press fit or interference fit or via an adhesive for rotationally fixed engagement therewith this preassembly of the second part 9 of the adapter device 10 with the second roller bearing 36 u2032 may occur either before or after the inner ring of the second bearing 36 u2032 is fixed on the shaft 42 of the motor 40 in a preliminary operation in a separate operation the first part 8 of the adapter device 10 is preassembled with the rotor body 22 in a rotor subassembly thereafter the rotor subassembly incorporating the first part 8 of the adapter device 10 is mounted on the shaft 42 in such a way that a collar 91 at a front end of the second part 9 is received in and engages with the bore 16 of the stub 11 for firm connection therewith e g via a press fit or interference fit or via adhesive fastening and the inner ring.