Yarn feeders can be divided into "positive" or "negative" types depending on the possibility of controlling the yarn feeding speed and uniformity.
A yarn feeder is the negative type when the needle takes the yarn directly from the package during the stitch formation step, and the feeding tension of the yarn cannot be controlled.
This feeding technique can generate differences in the yarn length used for stitch formation. This is due to the variable tension of the yarn since a new spool has a certain diameter which gradually reduces as more yarn is unwound and fed into the machine. In addition, the spool can be too hard or too soft.
Circular knitting machine manufacturers have eliminated this problem by implementing two distinct solutions:
1. The motorised yarn accumulator levels off the yarn tension since when rotating, it accumulates a certain quantity of yarn on a constant-diameter pulley and then stops. The yarn wound on the accumulator is then conveyed to the thread guide always maintaining the same tension. The machine takes up the yarn, gradually emptying the accumulator, which is then restarted automatically to replenish its yarn reserve.
This solution is particularly indicated when the same type of feeding technique cannot to be applied to all the feed systems due to the structure of the knit stitches. Therefore, yarn accumulators are mainly used on machines for the manufacturing of fabrics of pre-set length, or also of continuous cloths with Jacquard patterns.
2. Positive feed systems control the tensions of the yarn fed by means of a drive wheel or a drive belt system.
The drive wheel systems, which in the past were much more widespread than today, consist of two conical toothed wheels. The thread passes between the two wheels and the quantity of yarn can be adjusted by approaching or withdrawing the wheels. This positive system grants a smooth feeding of the yarn on all the feed systems.
Today, the belt system has by far become the most common positive feeding system. The belt makes the spool rotate, and the number of rotating spools corresponds to the number of feed systems.
By adjusting the belt RPM, the quantity of thread can be increased or reduced. This system grants an accurate control of the yarn tension (picture 80).
The Thread Guide
The tread guide is the fundamental element of a yarn feeder.
On circular knitting machines each thread guide corresponds to a feed system. The thread guide is a steel or ceramic plate with a hole for the thread. The thread guide is positioned near the hook of the needle and, besides feeding the yarn, it opens and protects the latches
The thread guides of double-bed machines feature two holes: one is used for conveying the yarn to the needle on the cylinder while the other hole only serves for feeding the dial needles when these are working.
Some machines have more thread guides for the same feed system, e.g. the circular knitting machines for continuous fabrics or continuous striped jersey or those equipped with Jacquard selection systems. The whole set of thread guides mounted on these particular machine models is called stripe pattern motion.
Stripe pattern motions usually includes from four to six threading-in options and a yarn retaining/cutting device. A gripper is positioned between one thread guide and the next to keep the threading-in position while changing the colour on the stripe pattern motion. The machine's head controls the gripper which holds the thread while the scissors cut the thread as soon as it stops.
The yarn remains threaded-in the thread guide, held by the gripper. The thread is released from the gripper and fed to the needles only when the thread guide is activated again. Thanks to a centralised programming system, the different thread guides are only operated when necessary depending on the colour or yarn change.
Special thread guides with double threading-in are used for generating special patterns, for example plating.
Together with the thread guides operating in the stitch formation position, special additional thread guides are employed for feeding the weft yarns.
Picture 82 - A thread guide of a double-bed knitting machine
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