that region formed on the inwardly facing surface of the inwardly extending portion is an internal ring gear 12 concentric with respect to the longitudinal center line of the hollow cylindrical roller body 10 provided in the region of the ring gear 12 is a circumferential groove which accommodates a seal illustrated in the form of an o ring 28 which provides a seal relative to the adjacent outside circumferential surface of the stator housing 85 or the mounting flange 93 secured thereto disposed within the bore which passes through the stator housing 85 at the left in fig4 of the mounting flange 87 which supports the rotor shaft 81 is the step down transmission 31 which is in the form of a planetary transmission with three transmission stages which are disposed in succession in the axial direction of the assembly the planetary transmission is of a conventional design configuration from the point of view of the function thereof so that the following brief description is sufficient to arrive at an adequate understanding thereof for the purposes of the present invention in the first stage the planetary transmission includes a planet wheel carrier 32 on which three planet wheels 33 are mounted by means of.