If a fiber strand (Fig. 25(A)) is held firmly at two spaced points by clamps K1 and K2 and is twisted somewhere between them, this strand always takes up the same number of turns of twist before and after the twist element (T). However, these turns have opposing directions of twist, which are represented in the example in Fig. 25 (A) as Z-twist on the right and S-twist on the left. If the clamps are replaced by rotating cylinders (Z1 and Z2 in Fig. 25 (B)) and the yarn is allowed to pass through the cylinders while twist is being imparted, the result is governed by the false-twist law and is different from the case of the stationary yarns, as previously assumed. A moving yarn entering the section (b) already has turns of twist imparted in section (a). In the example illustrated (B), there are turns of Z twist.
As the twist element is generating turns of S twist in the lefthand section, this simply means that each turn of the Z twist imparted in the first section (a) is canceled by a turn of S twist imparted in the second section (b). The fiber strand thus never has any twist between the twisting element and the delivery cylinder. In a false-twist assembly, turns of twist are present only between the feed cylinders and the twisting element. This principle is exploited, for instance, in false-twist texturing.
Fig. 25 – The false-twist principle
