make an embodiment in which only the lower part 7 is made in the form of a screen 11 the upper part 8 need not be made in the form of a screen but could be a cover with the previously mentioned surface depressions and raised surface areas fig7 shows another possibility for means 6 for dirt separation in combination with a presentation device 9 and controls 10 in a guiding device 2 a screen 11 is held by the holder h1 h2 below the screen 11 is a suction device 3 in hinges s1 s2 which are located outside the running path of the fiber material on the guiding device 2 a screen 12 is mounted pivotably by means of bolts one side of the swiveling motion is delimited by a stop a0 which corresponds to an open position ol the screen continues to be connected to the shaft of a servo motor 13 in extension of a bolt the servomotor 13 is controlled by controls 10 the servomotor allows for right as well as left direction operation in case of a position guided servomotor i e with bearing control the stop a0 can be omitted since the servo motor would then be able to position and hold the screen 12 exactly when fiber material is laid on the screen 11 the servomotor 13 can be steered in the opposite running direction so that the screen 12 is moved with the other bolts up to stop a1 a2 the screen rods of the screen 12 reach into the gaps between.