16 so that the radial movement of the lever 16 leads to a linear expulsion of the spring end 18 over the circumference of the shaft body 11 until it engages at the stop 24 attached to the housing in the exemplary embodiment illustrated in fig4 a through 4 c the secondary tensioning apparatus is arranged in the form of the spring 17 between the profiled head 14 and the shaft body 11 so that in this embodiment there is no shaft casing 12 as can be seen in detail from fig4 a the spring 17 is arranged between the end face of the shaft body 11 and the profiled head 14 whereby for adjusting the normal operation the shaft body 11 and the profiled head 14 are joined via the associated shearing pin 21 a shaft 41 carrying a gearwheel 42 is movably mounted in the guide slots 40 that are arranged mutually opposing on the profiled head 14 and on the end of the shaft body 11 and that are disposed radially outward in mutual opposition in the neutral position which can be seen in fig4 b the gearwheel 42 is situated inside the contour of the shaft body 11 so that the shaft body 11 can rotate in both directions of rotation without the spring 17 being tensed once the profile head 14 has been blocked by the blocking member not shown and by the shearing rod 21 having been sheared off the relative rotation between the shaft body 11 which continues to rotate while the force limiting device is effective and the fixed profiled head 14 causes the gearwheel 42 to move radially outward until it engages in the.