16 the free front end of hollow cylinder 7 which is arranged coaxially to the opening disk 16 ends at a certain parallel distance to the opening disk 16 wherein this distance can be varied with the aid of the cogwheel mechanism 6 in dependence on the type of fiber air flow to achieve an optimum degree of opening for the fiber material located opposite the opening disk 16 is a circular segment shaped counter opening disk 25 which is fitted into the front of hollow cylinder 7 and which is provided according to fig7 with pointed spikes 26 at least over an angular region u03b2 of approximately 120 u00b0 the counter opening disk 25 furthermore has a circular segment shaped cutout 27 which is defined by a circumferential angle u03b1 of approximately 120 u00b0 the counter opening disk 25 thus closes off more than half of the front of hollow cylinder 7 the resulting reduced discharge opening 28 of hollow cylinder 7 guides the fiber air flow in a compressed form to the opening disk 16 which opens the fiber material in a joint action with the counter opening disk 25 the counter opening disk 25 is rigidly secured to the wall of hollow cylinder 7 by means of a flange 29 the.