A new mechanism for the regulation of feeding tension of elastic filaments has been developed, in cooperation with a reputable manufacturer of textile machinery, to obtain “core spun” yarns. This device reduces, of a very significant form, the coefficient of variation of the elongation of the yarn and, consequently, the fabrics obtained with this yarns are more regular appearance. This mechanism is fundamental when the new spools of T-400, with diameter and weight superiors to the standard size, are processed.