the pin roll 3 has a circumferential speed of approximately 10 21 m sec whereas the sawtooth roll 4 has a circumferential speed of approximately 15 25 min sec the roll 4 is followed by further clothed rolls 5 and 6 which rotate in the direction c and d respectively and which have increasing circumferential speeds as viewed in the fiber working direction e the rolls 3 6 which have a diameter of approximately 150 300 mm are disposed in a closed housing the pin roll 3 cooperates with a separating opening 7 through which waste is discharged and whose size may be adjusted for adapting it to the degree of dirt contained in the cotton the separating opening 7 is bordered by a mote knife 12 as viewed in the rotary direction a of the roll 3 a further separating opening 8 and mote knife 13 adjoin the roll periphery likewise the rolls 4 5 and 6 cooperate with respective separating openings 9 10 11 bordered by respective mote knives 14 15 and 16 the separating openings 7 11 are in pneumatic communication with a respective suction hood 17 21 also referring to fig1 b the suction hoods 17 18 19 20 and 21 are adjoined by respective suction conduits 22 23 24 25 and 26 which in turn are coupled to a common suction channel 27 the rigid suction conduits 22 26 and the suction channel 27 are formed as a one piece structure made of sheet metal or plastic material the length of the suction ducts 22 26 is different dependent on their distance between the respective suction hoods 17 21 on the one hand and the suction channel 27 on the other hand.