element 30 b as viewed in the direction of rotation of the roll 3 b an overpressure prevails between the cover 29 and the roll clothing 3 ii the pressure sensor 24 c provides for a measurement of the vacuum whereas the pressure sensor 24 d provides for a measurement of the overpressure between the air guiding element 22 d and the clothing 3 ii a vacuum prevails whereas between the air guiding element 22 c and the clothing 3 ii an overpressure is present by virtue of the vacuum air enters through the air passage opening 21 1 from the outside into the intermediate space between the cover 29 and the clothing 3 ii whereas by virtue of the overpressure air escapes outward through the air passage opening 21 ii which is drawn away through the suction hood 19 a in this manner the degree of separation of foreign particles such as trash and the like may be varied at the separating knives 31 a 31 b turning to fig6 in a cleaning assembly which is disposed in a closed housing 37 and which may be a cvt 4 model manufactured by tr u00fctzschler gmbh amp co kg monchengladbach germany the fiber material to.