If the strand is passed forward (by the delivery movement) between rubbing rollers (N), which are also moving to and fro, then it will be continuously twisted with alternating Z- and S- twist over successive short portions (Fig. 67 and Fig. 68). The counter-torque created in the yarn will, however, eliminate this twist immediately after the yarn leaves the roller nipping line. If – instead of one strand – two fiber strands are passed through while arranged parallel and very close to each other, then the counter-torque can no longer operate solely on one yarn. It must operate on both, and causes twisting of the two threads around each other. A plied thread is created with continually varying twist direction – Z-twist where S-twist is present in both yarns and S-twist where the yarns had originally Z-twist. In most cases, the strength of the self-twist thread made in this way is not quite sufficient because of the untwisted pieces between the twisted portions – it must be additionally twisted subsequently.

In worsted spinning, its sole field of application, self-twist spinning (also known as Repco spinning) has been in use for several years, although not on a very large scale.

Fig. 67 – Self-twist

Fig. 68 – Forming a yarn by means of self-twist