by spring 7 when the blocking shaft is turned clockwise fig2 cam member 6 is positioned under the pawl 5 so that the latter does not act this rotational direction of the blocking shaft corresponds to the roller blind axle during the winding up operation when the blocking shaft 1 moves in the opposite rotational direction the blocking device is also effective as long as the rotation does not exceed a certain speed this corresponds to the normal unrolling of the roller blind thus under normal conditions cam member 6 blocks the action of pawl 5 to engage ratchet wheel 4 as seen in fig2 now if the drive means fail either during raising or lowering of the roller blind or also in the rest position the roller blind will unroll very quickly because of its weight and thereby cause rapid rotation of the blocking shaft 1 counter clockwise the inertia member 9 thereby lags behind the blocking shaft 1 and also lags behind intermediate member 21 attached to blocking shaft 1 as a result of this lag projection 10 of inertia member 9 disposed next to the free end of the control pawl 11 causes the pawl to move out a distance pivoting on pin 25 so that its tooth shaped part 28 engages the internal gear teeth 12 since control pawl 11 is connected to shaft 1 and must rotate with it the engagement of internal gear teeth 12 causes cam member 6 to turn counter clockwise with blocking shaft 1 by this rotation of less than one fourth turn cam 6 moves from its position preventing pawl 5.