This application claims the priority of German application 198 37 181.0, filed in Germany on Aug. 17, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an arrangement for condensing a drafted fiber strand in a condensing zone arranged downstream of a front roller pair of a drafting unit, which condensing zone comprises a stationary sliding surface having a suction slit extending essentially in transport direction, also comprising a perforated transport belt which transports the fiber strand over the sliding surface, to which transport belt a nipping roller, limiting the condensing zone on its exit side, is arranged.
An arrangement of this type is prior art in the U.S. Pat. No. 5,600,872. A driven delivery roller pair is arranged downstream of the front roller pair of the drafting unit, the transport belt being wrapped around the top roller of the delivery roller pair and being driven by same. A condensing zone is located between the front roller pair of the drafting unit and the delivery roller pair, which condensing zone comprises a suctioned sliding surface, over which the transport belt is guided. The transport belt has centrical perforations in the transport direction, through which perforations a suction air stream passes. The size of the perforation of the transport belt corresponds to the desired degree of condensing of the fiber strand. When the drafted, twist-free fiber strand is condensed, the outwardly projecting edge fibers are rolled in around the core strand, so that the fiber strand becomes smoother and less hairy. When the twist is subsequently imparted, a yarn results which is smoother and more tear resistant.
In the known arrangement it is disadvantageous that the size of the perforation determines the condensing effect and has to be adapted to the desired fineness of the yarn to be spun. It is furthermore disadvantageous that the suction slit, over which the transport belt slides, cannot reach to the nipping point limiting the condensing zone on its exit side. This means that when the yarn reaches the delivery roller pair a part of the condensing effect is lost.
In the German published patent 41 39 067 C2 it is disclosed that the fiber strand is guided in the condensing zone by means of a non-perforated apron pair, of which one apron each wraps around a roller of the front roller pair of the drafting unit. This apron pair guides the fiber strand into a delivery roller pair arranged downstream, of which the top roller takes the form of a suction roller. The lower apron of the condensing zone also wraps around the bottom delivery roller, while the suction roller in the form of a top roller of the delivery roller pair is not wrapped by an upper apron. In this embodiment, the suction slit is located in the inside of the suction roller, so that really the actual condensing effect first occurs in this area.
It is an object of the present invention to provide a transport belt which is to a great extent independent of the thread unit to be spun, whereby the drive of the transport belt is such that the suction slit extends to the twist block.
This object has been achieved in accordance with the present invention in that the transport belt is driven by a roller of the front roller pair and is pressed onto the sliding surface by the nipping roller.
A drive of this type, in contrast to the prior art described above, permits the extension of the suction slit to the nipping point, so that the condensing effect is not lost before the spinning twist is imparted. The suction air stream becomes then particularly homogenous when the transport belt is designed as a very finely perforated woven belt, for example, made from polyamide filaments.
The transport belt accordingly wraps around a driven bottom roller of the front roller pair, whereby the condensing zone is completely draft-free. The transport belt can drive hereby in turn the nipping roller, which then requires no further drive.