The Twist Contraction Method

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.

Twist Measurement By Microscope

A microscope having a rotating stage what its periphery graduated in degrees can be used to measure the twist angle and the yarn diameter and enable the turn per inch to be derived.After micrometer eyepiece is employed and it is first calibrated against a stage micrometer.With the stage set a zero degrees the yarn,mounted in a suitable holder,is brought into focus and the moving hairline of the eyepiece brought into line with the yarn axis.The stage is now rotated until the hair line is tangential to the helix formed by the twisted fibers on the yarn surface.The angle through which the stage has been rotated is then recorded. The yarn diameter is measured in terms of eyepiece divisions and then converted to thousands of an inch.At this point the yarn diameter d is known and also the twist angleThe length of yarn occupied by one turn of twist is therefore given by

A number of observation will,of course,be taken since both the twist angle and the yarn diameter will very from point to point along the yarn.Twist determination by means of some from of twist counter is precise only for plied or cabled yarns.For single yarns an accurate determination is almost out of the question by this means.Certainly no adequate idea of twist contraction in this latter case is possible.

Twist Measurement Of Plied Yarns

For a variety of technical reasons many textile structures are designed either completely or partly with the component yarns in doubled or cables form.Generally speaking,the inference is that the desired properties of the structure could not be obtained, or at least to the required degree if only single yarns were used.Luster,regularity ,strength extensibility,elasticity,liveliness,and docility are the kind of yarns properties which may be 'built' into a plied yarn by the right selection of the single yarns and the technique of construction.It is necessary,therefore,to able to analyze plied yarns to determine such details as doubling twist,single twist,take-up,resultant count,count of component yarns,and so on.

A number of papers on the properties of double yarns and the effects varying structural details are be found in the literature.The former is concerned with cotton and staple rayon yarns,and the latter with the mechanical properties of twisted continuous-filament yarns.In the other note which follow we shall deal only with the measurement of twist and the changes in length due to twist.The simple types of yarn will be mentioned but the basic methods can be extended to cover more complicated plied yarn structures.

'Take-up' Twist Testers

'Take-up' is defined as the difference in length between the twisted and untwisted thread,expressed as a percentage of the untwisted length.It is positive or negative according to wether the doubling twist causes contraction or extension in the folder yarns.Special twist testers are available which have device on them to accommodate the change in the specimen length as the test proceeds,and so they are commonly known as 'Take-up' tester.

Crimp Rigitidy Test

The ability of stretch and bulk yarns to collapse longitudinally is importance because the degree of crimp rigidity influence the properties of the fabrics product from this yarns.Crimp rigidity is a measure of the ability of a textured yarn to recover from stretch,and is related to the bulking potential of the yarn.In the tester developed by the Hosiery and Allied Trades Research Association, a load equivalent to 0.1g per denier is suspended from a skein of yarn which is then impressed in water at room temperature.After two min its Length, 'L' is measured.The load is then reduced to 0.002g per denier and after a further 2 min the reduced length,L2,is measured.The crimp rigidity is give by formula:

The apparatus for the test,supplied by Shirly Development Ltd is shown .The gate tensioner is adjusted so that the crimp in the yarn is just removed when the skein is being wound.A range of weight is required to accommodated the various denier encountered.Thus if the nominal denier were 20 then a skein of 50 strands would be wound and the heavy weight used would 100 g to give the required 0,1g per denier initial loading.The appropriate light weight would be 2g.For a 200 strand skein is used, the heavy weight 400g and the light weight 8g.A table gives the recommended skein strands and weight value.

The time taken to carry out a test is about five min.For firms who need to carry out large numbers of test .

Yarn Hairiness

One of the important difference between continuous filament yarns and yarns spun from staple fibers that the latter are 'hairy' that is fiber ends and loops of fibers stand out from the main body of the yarn.It is not intended to discuss this yarn property in the present volume but the interested reader may refer to a recent paper by barella, who has himself investigated the hairiness of yarn.In his conclusion Barella states that: 'The non-existence of a method sufficient satisfactory from both a scientific and a practical point of view for the easy measurement of hairiness in routine industrial process has been noted.The scientific method are slow and the quicker ones are open to criticism in that they are not soundly based.It would therefore be desirable to devise a new method that is superior to the existing ones.Furthermore, according the nature of the fabrics and the characteristics they must possess,the yarn hairiness can either be a desirable or an undesirable,property, so that its measurement and control are important.

Textured Yarn

In recent years there have been important development in the production of special
yarns based on thermoplastic continuous filament yarns.These yarn may be classified as follow:
(1) BULK YARN:- These are multifilament yarns which have been processed to produce an increased mass or bulk per unit lenght.
(2) STRETCH YARN:- As the term implies,these yarn have the ability to be stretched to high elongations; further, they have the power to recover rapidly from elongation,hence their use in stretch-to-fit apparel.
(3) MODIFIED STRETCH YARN:- These are basically stretch yarns which have had the characteristic modified to suit particular end use.
It will be appreciated that new types of yarn may require new testing technique, for research and development of the yarns and foe the routine quality control checks on the yarn supplied to the fabric producer.Several test methods have been reported but at the moment the testing situation is little fluid.The test described below has been in use for several years but even so the method has its critics.