**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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