Abstract: An apparatus for advancing a sliver and sensing thickness variations thereof in a fiber processing machine includes a tongue-and-groove roll pair composed of a tongue roll and a groove roll. The groove roll is radially fixedly supported and has a circumferentially extending groove including a groove bottom. The tongue roll projects into the groove and defines, with the groove roll, a nip through which the sliver passes for being compressed and advanced by the tongue-and-groove roll pair. The apparatus further has a sensing device including a biased, movably supported sensor element projecting into the groove of the groove roll and cooperating with the groove bottom upstream of the nip as viewed in a direction of sliver advance for pressing the sliver against the groove bottom and for undergoing excursions in response to thickness variations of the sliver passing between the sensor element and the groove bottom.

Abstract: A method of operating a draw frame including a plurality of serially arranged roll pairs each defining a nip through which a fiber sliver passes from roll pair to roll pair. Each roll pair is formed of an upper roll and a lower roll. A pressing arm carries the upper rolls, and pressing devices are accommodated in the pressing arm for pressing each upper roll against a respective lower roll. The method comprises the following steps: during the working state of the draw frame, passing the sliver consecutively through the roll pair nips while the pressing devices press the upper rolls against respective lower rolls, and during an interruption of the working state the upper output roll is relieved of pressing forces to an extent that at the most only an insubstantial pressure is exerted by the upper output roll on the sliver situated in the nip between the upper output roll and the lower output roll.