17 journals in a support 17 a and is thus rotatable in the direction of the arrows c d in addition to adjusting the clearance b as noted earlier the guide element 17 also varies the clearance c between the guide element 17 and the facing portion of the inner wall of the duct 16 by setting the strength of the air stream b 1 b 2 for doffing the fiber material through the fiber removal opening 15 it is feasible to improve apart from the change of degree in the fiber doffing the waste quality at the waste discharge opening 10 situated upstream of the fiber removal opening 15 it is a particular advantage of this arrangement that by virtue of adjusting the air stream the vacuum between the cover designated at 14 c in fig1 and at 14 f in fig2 and the respectively last roll 5 or 4 thus has an effect on the intensity of the separation of foreign bodies through the opening 10 in this manner the ratio between the desired separation of foreign bodies and the undesired separation of good fibers is optimized it is in particular possible to keep the loss of good fibers at a minimum during the foreign body separation the presence of vacuum downstream of the respective separating knife 11 and 23 is advantageous to ensure that a separation of good fibers through the waste removal opening 10 is kept at a minimum such a vacuum is generated by the air stream passing through the fiber removal opening 15 with the air doffed fiber.