the drawing the two planetary wheels 6 u2032 u2033 and 6 u2033 u2033 one of which depending on the direction of rotation of the planetary carrier not shown causes the desired tensioning of the internal geared wheel are arranged so as to be approximately diametrically opposed the arrangement is positioned in an annular gap r u2032 u2033 r u2033 u2033 of relatively pronounced wedge shape said annular gap r u2032 u2033 r u2033 u2033 being situated between the internal geared wheel and the sun wheel 11 u2032 4 u2032 with the direction of rotation of the sun wheel 4 u2032 as shown only the planetary wheel 6 u2032 u2033 exerts a tensioning effect on the internal geared wheel if the direction of rotation were reversed this effect would only be exerted by the planetary wheel 6 u2033 u2033 within the associated planetary carrier not shown the planetary wheels 6 u2032 u2033 and 6 u2033 u2033 which tension the internal geared wheel are guided so as to be slidable in circumferential direction of the internal geared wheel 11 u2032 within an elongated hole 13 which is indicated by a dashed line the elongated holes 13 are arranged such that in their initial end stop position i e before any tension or any increase in the tension of the internal geared wheel has taken place by displacement of one of these two planetary wheels the associated planetary wheels 6 u2032 u2033 and 6 u2033 u2033 when seen in the same circumferential direction rest against opposite ends of these elongated holes 13 each of these planetary wheels 6 u2032 u2033 6 u2033 u2033 is tensioned by a spring 14 in the direction of this initial position.