which its upper longitudinal edge 33 contacts the partition wall 26 i e the pivot flap 32 is located in the position b one sees that the link 42 retains its vertical position during this movement in this position the opening arm moves in the arrow direction 54 while the opening roll 12 now operates in the counter clock wise direction of arrow 56 if now a foreign article executes a blow on the lower edge 34 of the pivot flap 32 the latter deviates into the angular position shown in broken lines against the bias of the spring means of fig2 a during this movement the switching cam 52 moves into the position shown at 52 39 and actuates a switch which in turn serves to stop the machine and or to generate a warning signal fig4 shows a representation of an embodiment which is similar to the embodiment of fig2 but in which the lever 45 has a different angular position in comparison to the corresponding lever 45 of fig2 the switching drive unit 46 of this embodiment is also not pivotally connected to the shield or housing of the opening arm but rather to the end of a further one arm lever.