Rieter

Piecing

Index

After the operation of the  circular combs has been completed, the detaching rollers feed back part of the previously formed web. The nippers ( Fig. 3) swing forward and lay the fiber tuft that has just been combed onto the portion of the web projecting from the detaching rollers. When the detaching rollers now rotate again in the web take-off direction, they draw the fiber tuft that is immediately combed through the top comb and out of the fringe. The coherent web at the detaching rollers is thus lengthened by a new web strip. As a result of this operation the newly formed coherent web consists of small fiber tufts laid on top of each other in the same way as roofing tiles. The subsequently formed sliver still contains these periodic irregularities, a distinct source of faults in the operation of rectilinear combers.

The sliver produced in this way has a wave-like structure (Fig. 20) with periodic variations. These variations are visible in the spectrogram as combing cycles in the form of peaks (at about 30 - 75 cm) (Fig. 21). Both the spinning mill and the machine designer must strive to keep this irregularity as low as possible. The designer therefore employs eccentric withdrawal of the web from the web plate (Fig. 53/Fig. 54). The spinning mill can influence this via the machine settings. The fiber tufts drawn off by the detaching rollers can be compared with very flat parallelograms, although normally the leading edge is blunter than the trailing edge. By using correct machine settings it is possible to lay these parallelograms on each other in such a way that any unevenness is partly canceled out. On the other hand, incorrect setting will cause an increase in unevenness. In order to carry out the piecing operation, the detaching rollers must perform a back-and-forth movement (Fig. 50) in which the forward component (V) is larger than the backward component (R), so that effective take-off (T) is achieved. In modern combers backward movement amounts to about 60% of the forward movement.

The back-and-forth movement of the detaching rollers derives from a differential gear. An intermittent rotation (Fig. 51, A) is superimposed upon a constant basic rotation (B) generated by the comb shaft. The intermittent rotation is somewhat faster than the basic rotation. If these rotations are acting in the same direction (A + B), the result is a rapid acceleration of the detaching rollers in the forward direction (detaching operation) (Fig. 51, left). If the superimposed rotations are acting in opposite directions, not only does the intermittent rotation (A) cancel out the whole effect of the basic rotation, but it also causes a reversal of the detaching rollers (C), since the speed of the intermittent rotation is higher than that of the basic rotation.

Fig. 50 – The back-and-forth movement of the detaching rollers

Fig. 51 – The mode of operation of the differential gear of the detaching rollers