YARN EVENNESS

  

INTRODUCTION:
Non-uniformity in variety of properties exists in yarns. There can be variation twist.,bulk, strength, elongation , fineness etc.

Yarn evenness deals with the variation in yarn fineness. This is the   property, commonly measured as the variation in mass per unit length along the yarn, is a basic  and important one, since it can influence so samy other properties of the yarn and of fabric made  from it. Such variations are inevitable, because they arise from the fundamental nature of textile fibres and from their resulting arrangement.  

The spinner tries to produce a yarn with the highest possible degree of homogeneity. In this connection, the evenness of the yarn mass is of the greatest importance. In order to produce an absolutely  regular yarn, all fibre characteristics would have to be uniformly distributed over the whole thread. However, that is ruled out by the inhomogeneity of the fibre material and by the mechanical constraints.
Accordingly, there are limits to the achievable yarn eveness.

IMPORTANCE OF YARNEVENNESS:
Irregularity can adversely affect many of the properties of textile materials. The most obvious consequence of yarn evenness is the variation of strength along the yarn. If the average mass per unitlength of two yarns is equal, but one yarn is less regular than the other, it is clear that the more even yarn will be the stronger of the two.The uneven one should have more thin regions than the even one as a result of irregularity, since the average linear density is the same. Thus, an irregular yarn will tend to break more easily during spinning, winding, weaving, knitting, or any other process where stress
is applied.

 

A second quality-related effect of uneven yarn is the presence of visible faults on the surface of fabrics. If a large amount of irregularity is present in the yarn, the variation in fineness can easily be detected in the finished cloth. The problem is particularly serious when a fault(i.e a thick or   thin place) appears at precisely regular intervals along the length of the yarn. In such cases, fabric construction geometry ensures that the faults will be located in a pattern that is very clearly
apparent to the eye, and defects such as streaks, stripes, barre, or other visual groupings develop in the cloth. Such defects are usually compounded when the fabric is dyed or finished, as a result of  the twist variation accompanying them.

Twist tends to be higher at thin places in a yarn. Thus , at such locations, the penetration of a dye or  finish is likely to be lowe than at the thick regions of lower twist. In consequence, the thicker yarn region will tend to be deeper in shade than the thinner ones and, if a visual fault appears in a pattern on the fabric, the pattern will tend to be emphasized by the presence of colour or by some variation in  a visible property, such as crease-resistance controlled by a finish.

Other fabric properties, such as abrasion or pill-resistance, soil retention, drape, absorbency,  reflectance, or lustre, may also be directly influenced by yarn evenness. Thus, the effects of   irregularity are widespread throughout all areas of the production and use of textiles, and the topic is an important one in any areas of the industry.

"UNEVENNESS" OR "IRREGULARITY":
The mass per unit length variation due to variation in fibre assembly is generally known as "IRREGULARITY" or "UNEVENNESS". It is true that the diagram can represent a true relfection of the mass or weight per unit length  variation in a fibre assembly. For a complete analysis of the quality, however, the diagram alone is not enough. It is also necessary to have a numerical value which represents the mass variation.  The mathematical statistics offer 2 methods

• the irregularity U% : It is the percentage mass deviation of unit length of material and is caused by
  uneven fibre distribution along the length of the strand.
• the coefficient of variation C.V.%

In handling large quantities of data statistically, the coefficient of variation (C.V.%) is commonly used to define variability and is thus well-suited to the problem of expressing yarn evenness. It is currently probably the most widely accepted way of quantifying irregulariy. It is given by

coefficient variation (C.V.%) = (standard deviation/average) x 100

The irregularity U% is proportional to the intensity of the mass variations around the mean value. the U% is independent of the evaluating time or tested material length with homogeneously   distributed mass variation. the larger deviations from the mean value are much more intensively taken into consideration in the calculation of the coefficient of variation CV(squaring of the term) C.V.% has received more recognition in the modern statistics than the irregularity value U. The coefficient of variation CV can be determined extremely accurately by electronic means, whereas the calculation of the irregularity U is based on an approximation method. It can be considered that if the fibre assembly required to be tested is normally distributed with respect to its mass variation, a conversion possibility is available between the two types of calculation.

C.V.% = 1.25 * U%

INDEX OF IRREGULARITY":
Index of irregularity expresses the ratio between the measured irregularity and the so-called limiting irregularity of an ideal yarn. The manner in which irregularity is assessed can lead to different ways of expressing the index.

In calculating the limit irregularity, the assumption is made that, in the ideal case, fibre distribution in a yarn is completely random and a practical yarn can never improve upon this situation.Thus, the   measured irregularity will be an indication of the extent to which fibre distribution falls short of  complete randomness. If all fibres are uniform in cross-sectional size, it can be shown that the  limiting irregularity expressed in terms of C.V is given by

C.V.(limit) = 100 / sqrt(N)

This expression also assumes a POISSON distribution in the values around "N"(the mean number of fibres  in the cross section)

Let
C.V.lim = the calculated limit irregularity
C.V. = the actual irreglarity
Then,
Index of Irregularity (I) = C.V / C.V.lim

By calculating the limit irregularity and then measuring the actual irregularity, we can judge the spinning performance.

DEVIATION RATE:

Deviation rate describes by what percentage a mass deviation exceeds or falls below a certain limit. The cut length factor in m averages out the shorter, higher deviations

DR (xy) =  (L1+l2..+Ln) x 100 / L tot

DR = Total relative length in (%) of all deviations of the mass signal which surpass the limit  +/- x% over a total test length of L meters, with the cut length of curve being y meters.

 

FORMULA FOR DR PERCENTAGE:

The standard DR used for yarn is 1.5 m cutlength at a +/- 5% limit. The application of DR is similar to that of the CVm values. One has to take in to consideration that  the DR is based on threshhold values and changes more significantly than CV values when higher mass deviations over long stretches of test material arise.

THe deviation rate is calculated by comparing all the deviations of the positive range with the whole test length Ltot.  The same is valid for all deviations  in the negative range. As the zero line corresponds to the median , the Deviation Rate (DR) can reach the maximum of 5 0%.

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