the rail 6 the small edges 7a 8a of the plate 7 and 8 facing towards the frame 2 are situated near the vertical trajectory of the suction cup at a distance from that trajectory such that the suction cup is able to pass close to the two plate 7 and 8 without being caught on them the two plates 7 and 8 are fixedly connected to each other and are supported in a manner to allow pivoting together about an axis 8b extending along the long exterior edge of the plate 8 which is closest to the rail 6 the pivoting operation is effected by a suitable device such as an air cylinder or a screw jack which is not shown in the drawing but is arranged as the cylinder in fig1 a detector 9 which is preferably a photoelectric detector is placed slightly below the point where the edges 7a and 8a intersect in order to detect the presence of a flat sheet as the sheet passes in front of the detector a suction pipe 10 is disposed to extend horizontally above the detector 9 between the detector and the intersection formed by the edges of the plate 7 and 8 the suction pipe 10 is formed as shown in fig6 by a tubular element which is closed at its axial.