113 (#167) ############################################ WOOLLEN YARN SPINNING be given, but in this case the operation will partake more of the twisting of condensed slivers rather than the spinning of a woollen thread. Considerable parallel- izing in spiral form of the fibres is necessary to produce a typical woollen thread. * * * The twist inserted has a twofold object- 1. To support the thread during drafting; and 2. To make a sound, strongly-spun thread. The drafting-twist inserted is probably dependent upon the longest and shortest fibres to be spun, as well as upon the average fibre-length of the blend. Unfortu- - nately no method has been devised for ascertaining this twist other than the "trial and error," or empirical, method. The problem is not merely one of the absolute turns per inch to be given, but of a gradually increasing twist to compensate for the decrease in thickness of slivers as the movement of the carriage outwards atten- uates them. This will be better understood by looking at the matter from the "angle of twist” point of view. Thus, the first question to be decided is Is it desirable to decide upon and maintain one definite angle of twist throughout the spindle-drafting operation; or should the angle be varied, and if varied what should the varia- tion be ? Adopting the view that for each woollen blend there is a definite twist angle with which the utmost support to the fibres—without interference with the sliding of fibre on fibre—is given, then the two questions to be decided are- 1. What decides the twist angle for any particular blend ? 2. How shall this twist angle be maintained through- out the spindle-drafting (movement of the carriage) ? I 113
154 154 Textile Machine Part