The ESS device prevents the rebounding (re-jumping) of yarn ends in case of yarn breakages
on warp knitting machines processing elastane yarns.
The demand for super light as well as opaque and mouldable underwear fabrics, produced in finer machine gauges, is continuing to rise. The market share of fine-count elastane yarns in the 11 - 14 dtex range is also continuing to rise.
An easily accessible tubular guide, which is driven by the yarn, is currently used as standard for feeding the elastane yarn to the knitting point. This system guarantees a uniform yarn feed. A prerequisite for this is that the yarn tension between the warp beam and the guide causes the yarns to adhere at the tube. However, as the yarn count decreases, the maximum yarn tension that can be achieved also decreases. As a guideline, this should be roughly 1 gram per 10 dtex of yarn. For example, an elastane yarn of 44 dtex can be loaded with four grams, and a yarn of 11 dtex with just one gram.
In order to achieve these low yarn tension levels at fine counts during processing, a modified tubular guide belonging to the new ESS is actively controlled via its own EBA drive. The rubber-coated surface, the precision grinding of the tubular guide, and an additional pressure roller guarantee the non-slip transfer of the peripheral speed to the yarns (Fig. 1). The control functions are also carried out by the EBA computer, which is extended to three positions on the two-bar machines.
This system enables the resulting yarn tension between the sectional beam and the tubular guide and between the tubular guide and the knitting point to be adjusted and the number of friction points, which stress the yarn, can also be minimized. In addition to the yarn being fed in more gently, the yarn tension sequence, which usually increases continuously from the sectional beam to the guide, can be controlled. The variation in the amount of yarn, caused by stretching between the warp beam and the knitting point, can be equalized by making the relevant adjustments to the yarn feed.
Another problem can occur when handling these fine-count yarns. They are supplied as 'clear" types, and are almost invisible, rather like glass yarns. If a yarn breaks and one or more elastane yarns spring back to the sectional beam, they are difficult to locate at high yarn densities or if the light is poor. It is often difficult to find the yarns and even more difficult to feed them into the yarn comb again in the correct order. If this cannot be done successfully, and the yarns become crossed over each other between the sectional beam and the yarn comb, more yarns could be broken as the beam continues its movement.
This problem can be solved by installing two combs of the same gauge, as well as the system consisting of the pressure roll.
Yarn tension on the ESS device
The yarn tension behind and in front of the ESS device should almost be the same (during machine operation); this means: the same number of yarns has to be placed over the measuring system. In case of an elastane yarn run--in value of 500 mm/R, it is possible to use an approximate value of 900 -- 950 mm/R, in order to ensure an almost constant stretching between the warp beam and the ESS device as well as between the ESS device and the guides.
When using filament yarns, a yarn run-in value of approx. 1100 mm/R would be required, this means the elastane yarn has to be elongated (stretched) to this actual length. A part of the elongation comes from the ESS device, the other part is the result of the yarn tension between the guides and the ESS device.
The exact feeding value for the ESS drive axis has to be determined on the machine,
because the feeding value depends on the yarn material to be processed and on the
relevant machine type.