spring 63 one then obtains the position according to fig4 in which the wedge 3 sits on a side edge 66 of the recess 60 and does not fully extend into the groove 10 the lifting unit can during this movement continue to work practically in the illustrated position however there exists an undesired instability factor when shortly before the end of the 180 u00b0 swing the shifting wedge 3 which partially engages the groove 10 reaches the opposite locking block 6 a destruction of a part does not occur since this locking block also gives way against the force of its spring 63 with respect to this reference is made to later discussions relating to fig7 during the brief standstill of the drive shaft 1 after its 180 u00b0 rotation the outer end 34 of the wedge 3 under the action of the spring 63 is again caught in the recess 60 and is centered so that during the next control operation it is positioned in front of the groove 10 and can move into the groove without problems it is important that the center axis 69 of the recess during the displacement of the locking block 6 is inclined only insignificantly toward the radius 10 of the stationary drive shaft a modification of the support of the locking block on the connecting rod 14 can be taken from fig5 to 7 a locking block 7 which again has a groove shaped recess 70 is provided with a center pin 71 which extends into a rectilinear slot.