in the radial direction a radially outer area 19 accommodating outer rings a of bearing device 14 a b or being rotatably fixedly coupled therewith and an area located radially farther inward being reduced in the axial direction by the width of bearing device 14 a b as cooling area 20 cooling bore holes 21 which are distributed at regular intervals around rotation axis 17 and which extend in the axial direction parallel to rotation axis 17 are introduced into cooling area 20 cooling bore holes 21 are situated in such a way that they overlap with bearing devices 14 a b in the axial projection the free diameter of cooling bore hole 21 is as wide as the width of bearing devices 14 a b in the radial direction in the illustrated longitudinal section slots are provided between cooling area 20 of rotor shaft 7 and bearing devices 14 a b and in the overall view annular disks are provided as clearances 22 a b which are open in the direction of hollow shaft chamber 18 a flow related connection between hollow shaft chamber 18 and cooling bore holes 21 is thus established by clearances 22 a b so that a gear oil is able to enter.