One of the effects of putting into a strand of fibre or filament is to cause the strand to contract in length.Suppose a yarn is twisted Z way and has a length, h.Let the twist be completely removed to produce an untwisted strand of length h+c, c being the contraction due to twist.If the strand is now twisted S way with a number due to turns equal to those removed,we would expect that the strand will again contract to the original length, h .Several twist testers are based upon this concept.The yarn is first gripped in the left hand clamp which is mounted on pivot and carries a pointer.After led through the rotating jaw,The yarn is pulled through until the pointer lies opposite a zero line on a small quadarnt scale; the jaw is then closed.At this stage the specific is under a small tension and has a nominal length of 10 in.As the twist is remove,the yarn extends and the pointer assumes a vertical position,so removing the tension.Eventually all the twist is taken out but the jaw is kept rotating in the same direction until sufficient twist has been inserted to bring the pointer back to the zero mark again.The total number of turns registered on the revelation counter is divided by 20, that is twice the number of inch in the test length.The result of this division is the turns per inch in the specimen.A moments reflection will show that twice the number of inch is used as a divisor since only half the revolution of the jaw actually remove the twist.
The ease of operation of this type of twist is obvious.No great operator skills is required,the end point is clear,good lighting is not essential, and smaller number of tests than the standard method recommends may be used as.
Attention must be drawn to possible source error when using this type of twist.It was mentioned earlier that one would expect that the number of turns required to cause the untwisted yarn to contract to its original length is equal to the number of turns originally in the yarn.Unfortunately this is not always true.It has been found that for some yarns the turns per inch determined by the twist contraction method are underestimated.while for other yarns they are overestimated when the values are compared with those obtained by the standard straightened fibre method.
