Analysis of yarn tension generated during circular weft knitting in case of positive storage feeding

Abstract

Knitting mechanics is highly affected by yarn tension at various points in the process. Yarn tension plays a crucial role by influencing the knittability as well as the quality of the fabric. Besides these industry practitioners always have to be aware about higher production rate as well as machine performance. Therefore analysis of yarn tension, particularly Yarn Input Tension (YIT) [Yarn feeding tension to the loop formation zone], with respect to course length (CL), machine speed (i.e. rev./min. of needle bed) and machine performance are some very important issues to be studied. To investigate these issues on a modern circular knitting machine (CKM) some theoretical and experimental works were carried out. To relate YIT with CL a model in the form of an equation has been developed based on the mechanical consideration of yarn during dynamic circular weft knitting process that runs with positive storage feed (PSF) system. The predicted CL through this model has been compared with that found from actual fabric by a recognized apparatus, i.e. HATRA Course Length Tester. The t-test was carried out over the obtained results for statistical analysis purpose. It was observed that for cotton and spun polyester knitted fabric, as used in the experimental part, the model worked very effectively through precision prediction by showing very low average mean difference in predicted CL from that measured from the actual fabric. Moreover from the experimental works it was also observed that actual CL remains almost unaffected by machine setting (i.e. stitch cam position) and yarn fineness. To find out the effect of machine speed on YIT, plain jersey fabric samples were knitted on an industrial CKM at different loop sinking depths through adjustment of cam settings. Linear regression analyses representing the relationship between the variables were evaluated. YIT and online yarn length/machine rev. were found to vary, though not so significant, positively with machine speed resulting a r-square value of more than 0.9. To assess the performance of a modern CKM (i.e. the particular industrial machine used for the research study) through monitoring of running yarn tension, a total of 16 different production runs- each for a machine running period of 30 seconds [equivalent to the time required for more than 05 revolutions of the needle bed or machine at 10.5 rev. /min.] were analyzed. Highest tension-peak value for each second was identified through MLT Wesco Yarn Tension and Rate Meter and associated PC software. Run charts were built-up with these selected tension values by statistical software, i.e. Minitab and the p-values were checked to identify special cause variations. It was found that most of the production runs showed no non-random pattern in the tension values based on an alpha value (significance level) of 0.05, representing absence of special cause variations and thus disclosing quite satisfactory machine performance. Finally a noble approach has been shown to bring an improvement in the Quality Adjustment Pulley (QAP) belt cleaning system of a CKM. A compressed-air based lint removal device for the QAP belt has been developed. It has been found that the device acts as a convenient tool for lint removal from the belt. Moreover, the device is more cost-effective and shows better cleaning performance than the traditional brush-shelf type cleaning apparatus that are available in some CKM.