The fabric take-down and winding motions have been designed to facilitate stitch knock-over and fabric take-down procedures. The take-down and winding functions are kept separated in order to allow a smooth running of the machine and avoid possible fabric distortions.
The take-down motion consists of 2 or 3 rollers placed beneath the cylinder. In the simplest system configuration (i.e. the two-roller) the fabric passes between two rollers that stretch it by rotating in opposite directions. Anyway, the best system is the three-roller take-down motion which pulls the fabric without slipping and without exerting too much pressure that could damage the fabric.
From a mechanical point of view, a take-down system can be either equipped with a swivelling arm or with a lever and spring mechanism. Modern take-down systems are motorised and the latest models also incorporate an electronic control.
Picture 97 - A 2-roller take-down motion
Circular knitting machines pose some problems as regards the winding of the fabric, as the fabric itself is delivered in tubular form and must be spread flat prior to winding. The spreading of the tubular fabric generates some distortions because of the different distances between the various zones of the tubular fabric emerging from the take-down system and the same zones wound on the fabric roll. These differences reflect into uneven winding tensions (the tension is lower in the fabric centre and higher at its edges). To avoid these problems, a metal frame called "spreader" has been incorporated before the fabric winding system. The spreader increases the width of the tubular fabric by giving it an almost circular shape, equalising the distances between the various zones of the fabric and the nip line of the winding system.
A special take-down system has been designed for variable needle-bed machines since these machines do not use the whole needle-bed. This special take-down motion features independent rollers to adjust the tension during the knitting process and differentiate the tension between the central part of the cloth and the edges. On the most recent machines, it is possible to set up to 99 different tension values.
The fabric winding motion is provided with a clutch. In this way, to grant a steady peripheral speed, the angular speed of the winding roller can be gradually reduced as the diameter of the fabric roll increases.
At the ITMA '99 fair in Paris, an Italian manufacturer of circular knitting machines exhibited an innovative winding system mounted on a 30-inch circular knitting machine. In practice, this machine features an "open" base that allows the fabric cutting and opening on only one side prior to winding. Obviously, in order to allow the take-up of the open fabric, the width of the winding roller must be twice the width of a standard one.
The take-up step is carried out on the already opened fabric, and the edges of the fabric are kept tensioned by means of two rollers with worm-screw profiles.
Thanks to this innovative solution, no further rollers squeeze the knit fabric. This avoids the problem of central marks which is particularly serious on elastane fibres.
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