Yarn evenness_AyBee Marwat

Abubakkar Marwat (05-NTU-05)

Yarn Evenness

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Table of contents:
Introduction
Types of irregularity
Causes of irregularity
Denoting unevenness or irregularity
Types of variations
Importance of yarn evenness
Measuring and assessing evenness
1-Visual examination
2- Gravimetric method
3- Capacitive method
3.1- Uster evenness tester
4- Mechanical method of roving/sliver evenness measurement
5- Optical method (Zweigle G580)
6- Pneumatic method
7- Acoustic method
7.1- Impulse acoustic method
Index of irregularity

Textile Testing-II (TS-333)

1
1
2
3
4
4
5
5
6
7
7
13
15
15
16
16
18


Yarn Evenness

 Introduction:

2/19

:

Non-uniformity in variety of properties exists in yarns. There can be variation in
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 many
other properties of the yarn and 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 strand (yarn). 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 evenness.
 Types of Irregularity:

:

1) Weight per unit length:
Variation in weight per unit length is the basic irregularity in yarn. All other
irregularities are dependent on it. This is because weight per unit length is
proportional to fibre number i.e.; number of fibres crossing a section of yarn.
Variations in fibre number are the factor influenced by drafting. So any improvement
in drafting or spinning will first reflect in improvement in variability of weight per
unit length.
2) Diameter:
Variability in diameter is important because of its profound influence on
appearance of yarn. Variations in diameter are more easily perceived by eye. Latest
models of evenness testers have therefore a module for determining diameter
variability. Diameter variability is however caused by weight variability. As twist has
tendency to run into thin place, variability in weight gets exaggerated in diameter
variability.
3) Twist:
Twist variation is important because of its influence on performance of yarn and
fabric dye ability and defects. Soft ends are a major cause of breaks in weaving
preparatory and loom shed. They arise from twist variations. Soft twisted yarns take
more dye and so uneven dyeing is caused by high twist variation. Weft bars and
bands are also caused by low twisted yarns. Twist variations come from slack spindle
tapes, jammed spindles. A certain amount of variation is also present along the chase
of cop.
4) Strength:
Importance of strength variation is easy to appreciate. Yarn breaks at the weakest
element and so yarns with high strength variability will result in high breakages in


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further processes. Strength variability is partly dependent upon count variability and
partly upon spinning conditions and mechanical defects.
5) Hairiness:
High variation in hairiness leads to streaky warp way appearance and weft bars in
fabric. More light will be reflected from portions of weft where hairiness is more and
this leads weft bands. High hairiness disturbs warp shed movement in weaving and
results in breaks, stitches and floats. Among other factors, worn out rings and
travelers, vibrating spindles, excessive ballooning and variation in humidity in
spinning room cause variations in hairiness from bobbin to bobbin
6) Colour:
Variations in colour of yarn cause batch to batch variation in fabric colour, which
leads to rejects. This is particularly critical in cloth marketed to garment units.
Variations in colour of yarn and fabric are caused by variations in colour of cottons
used in mixing. Larger lot sizes made from a large number of bales help to mitigate
this problem. Checking of cotton and mixing for colour will also minimize large
variations in colour. HVI testing equipments have therefore a module for checking
colour.
 Causes of irregularity:

:

1) Irregularity caused by raw material:
The natural fibres have variable verities. They have no true fixed length, fineness,
shape of cross-section, maturity, crimp, etc., which have effect on yarn properties
specially evenness. These variations are due to different rates of cell development due
to changes in environmental conditions (nutrients, soil, and weather).
In man made fibres, variations in mass/unit length occurs due to changes in
polymer viscosity, roughness of spinneret orifice, variation in extrusion pressure and
rate, filament take-up speed, presence of delustrant or additives, which can modify
the particular shape and fibre surface geometry.
2) Irregularity caused by fibre arrangement:
Textile fibres are not rigid. Their manipulation during conversion into yarn is an
immensely complex combination of mechanical movement which usually requires
some degree of compromise. The desirable results of relocating large number of fibres
at high speed and arranging in well ordered form tend to be difficult. Fibres
assembled into the form of a twisted strand constitute a yarn.
Fibres are not precisely laid end to end, and gaps are present between them. As a
result of yarns twist, fibres arrange in spiral form in a series of folds, kinks, and
doublings.
3) Effect of fibre behavior:
Fibres shape directly affects yarn regularity. The fibres section, arrangement of
fibre section and space between the fibres will vary from yarn section to section.
Hence the mass of each section will differ.
A thin place in yarn will have lower mass and less strength. In thin regions, yarn
twist tends to be higher since resistance to deformation is lower.


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4) Inherent shortcoming of machinery:
In many engineering processes the units from which the final product is assembled
are positively controlled by hand or machine and positioned with only a few
thousandths of an inch tolerance. In spinning it is surprising how often the individual
fibres are only negatively controlled-at times they are carried forward by air currents
or jostled along by surrounding fibres, or they are held in position by friction and
twist.
Fibre manipulation by rollers, aprons, gills, and other machine parts is hampered
by fibre variation, and the machines can only be set to give the best results within the
limitations imposed by the material.
The drafting wave is one example of irregularity due to the inability of a drafting
system to control each fibre. Where roller drafting is used, the distance from one nip
to the other is greater than the length of the shorter fibres. These short fibres „float‟ in
the drafting zone and move forward in an irregular but cyclical manner which results
in the drafted strand having thick and thin places. The wavelength of this type of
irregularity is about 2-5 times the mean fibre length but it is not necessarily constant
for a particular strand. In addition to a varying wavelength, the amplitude of the
drafting wave is also variable.
5) Mechanically defective machinery:
Since machines even in good condition produce irregular yarns, it is reasonable to
assume that defective machinery will increase the amount of irregularity. The
implementation of an efficient maintenance system is essential if the level of
irregularity is to be kept within bounds. Machines drift out of adjustment, bearings
become worn, components get damaged, and lubrication systems clog and dirt works
its way into the mechanism. Faulty rollers (top roller eccentricity) and gear wheels
usually produce periodic variation.
 Denoting "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 reflection 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:
1. 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.
2. 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 irregularity. It is given by


Yarn Evenness


Coefficient of variation (C.V.%) =

S .D
avg .value

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100 where S.D = Standard deviation

 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 C.V. %.
 C.V.% has received more recognition in the modern statistics than the irregularity
value U%.
 The coefficient of variation C.V.% 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 calculations
C.V.% = 1.25 x U%
 Types of variations: short, medium and long term:

:

Variations in the fibrous strand are classified according to their wavelength with
respect the fibre length used to form that particular strand.
1) Short term variation:
These variations are of the wave length 1-10 times of fibre length. Amplitude of
these variations is greater than long term variations. These result due to faulty
processing at the last machine. Such variations if excessive produce a fabric of
objectionable appearance.
2) Medium term variations:
These variations are of the wave length 10-100 times the fibre length. Such
variations do not cause a pattern as it hides into the adjacent warp yarn. In weft it will
appear as a thick line again hidden by adjacent weft. However excessive variations
give the cloth a streaky appearance.
3) Long term variations:
These variations are of the wave length 100-1000 times the fibre length. Such
variation cause periodic faults known as diamond bars or block bars in the woven
fabric along the weft direction. A weft yarn to cause a diamond bar pattern must have
a long term periodic variation of wave length less than twice the pick length.
 Importance of yarn evenness:

:


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 unit length of two yarns is equal, but one yarn is less


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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 lower 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 luster, 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.
 Measuring and Assessing Evenness:

:

1) Visual examination:
Yarn to be examined is wrapped onto a matt black
surface in equally spaced turns. The black boards are
then examined under good lightening conditions using
uniform non-directional light. A.S.T.M. has a series of
Cotton Yarn Appearance Standards which are
photographs of different counts with the appearance
classified in four grades. The test yarn is then wound
on a blackboard approximately 9.5 x 5.5 inches with the
correct spacing and compared directly with the
corresponding standard.
Motorized wrapping machines are available: the
yarn is made to traverse steadily along the board as it is
rotated, thus giving a more even spacing. It is
preferable to use tapered boards for wrapping the yarn
if periodic faults are likely to be present. This is because
the yarn may have a repeating fault of a similar spacing


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to that of one wrap of yarn. By chance it may be hidden behind the board on every
turn with a parallel-sided board where as with a tapered board it will at some point
appear on the face.
2) Gravimetric Method (Cut and weigh method):
This is the simplest way of measuring in mass per unit length of a yarn. The
method consists of cutting consecutive lengths of the yarn and weighing them. For the
method to succeed, however, an accurate way of cutting the yarn to exactly the same
length is required. This is because a small error in measuring the length will cause an
equal error in the measured weight in addition to any errors in weighing operations.
One way of achieving accurate cutting to length is to wrap the yarn in around a
grooved rod which has a circumference of exactly 2.5 cm and then to run a razor blade
along the groove, leaving the yarn in equal 2.5 cm lengths. The lengths so produced
can then be weighed on a suitable sensitive balance.
If the mass of each consecutive length of yarn is plotted on a graph as in Fig., a line
showing the mean value can then be drawn on the plot. The scatter of the points about
this line will then give a visual indication of the unevenness of the yarn. The further,
on average, that the individual points are from the line, the more uneven is the yarn.
There are two ways of expressing this unevenness:
1. The average value for all the deviations from the mean is calculated and then
expressed as a percentage of the overall mean (%age mean deviation, PMD). This
is termed U% by the Uster.
2. The standard deviation is calculated by squaring the deviations from the mean
and this is then expressed as a percentage of the overall mean (CV %).
The two values are related by the following relation:
CV % = 1.25 PMD
 This method is slow and laborious.
Example:
Suppose a yarn is cut into short consecutive lengths (2.5 cm), and then each length
weighed:
S.No

Weight
“X”

1
2
3
4
5
6
7
8
9
10

20
18
19
17
22
21
18
20
19
21

X

19.5

Deviation from
Mean (X- X )
0.5
1.5
0.5
2.5
2.5
1.5
1.5
0.5
0.5
1.5

 ( x  x)
n

= 13

(X- X )2
0.25
2.25
0.25
6.25
6.25
2.25
2.25
0.25
0.25
2.25

25
20

x  19.5mg

15
Series1
10
5
0
1

22.5
9 = 22.5


2

3

4

5

6

7

8

9 10

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 ( x  x)
Mean Deviation =

= 13/10 = 1.3

n

mean.deviation

Percentage Mean Deviation (P.M.D) =

 ( x  x)
Standard deviation S.D =
S .D

CV% = Avg .value

100

1.58

avg .value

2

n 1

=

1.3

x 100 = 19.5

 100

= 6.66 %

22.5
9 = 1.58

 100

= 19.5
= 8.10 %
Irregularity in the yarn is 6.66 %.
3) Capacitive method:
The measuring device of an electronic capacitance tester is a parallel plate
capacitor. Under certain conditions, the effect of introducing a non-conducting
material such as a sliver or yarn into the space between the plates is to change the
capacity of the capacitor, the change being proportional to the weight of material
present. If, therefore, the material is drawn through the capacitor continuously, the
changes in the capacity will follow the variation in the weight per unit length of the
strand, the unit length being the length of the capacitor. If is necessary to detect the
changes in capacity and to translate them electronically into meter readings which
indicate the coefficient of variation. At the same time a trace of the variation should be
made on a pen recorder if required.
Some of the design problems may be described:
1. Changes in capacity are linearly related to the weight of material present, the
material thickness should not exceed 40% of the distance between the capacitor
plates. Hence interchangeable capacitors are necessary to test a range of
materials from sliver to fine yarns.
2. The length of the capacitor should be as short as possible so that the variations
in weight are measured over short lengths.
3. Shape of the cross-section of the tested strand affects the change in capacity. So
it is necessary that the strand maintains its shape during its passage through
the capacitor.
4. The moisture in the material affects the magnitude of the change in capacity, a
higher moisture content giving a greater change in capacity.
3.1) Uster evenness tester:
Schematic diagram of this capacitance based evenness tester is shown. Two
oscillators A & B have equal frequencies when there is no material in the measuring
capacitor C. When the two frequencies are superimposed the difference in frequency
is zero. The presence of material (yarn) in the capacitor causes its capacity to change
and so alter the frequency of the oscillator A. There will then be a difference between
the two frequencies which varies according to the material (strand thickness) between
the capacitor plates. Suitable circuits D translate these frequency differences into
signals which (1) are indicated on the meter M, (2) drive the pen of the recorder, and


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(3) are fed into the integrator which indicates the average irregularity either as PMD
or CV% according to the model
used.
The actual tester is illustrated which
shows the following external features:
i) the „comb‟ of eight measuring
capacitors of different sizes
ii) the creel and guides to control
the material
iii) the traverse rollers which can
control the material speed over a
range from 2-100 yd/min
iv) the control switches
v) the meter on the main unit which
indicates the momentary variations
in the material
vi) the integrator which indicates the P.M.D. or C.V.
vii) the high-speed pen recorder whose chart speed can be varied between 1 & 40
in./min.
3.1.1) Mass evenness, measuring principal:
i) The sensor for measuring
the evenness of slivers,
rovings or yarns is a
capacitive measuring sensor:
ii) A high-frequency electric
field is generated in the
sensor slot between a pair of
capacitor plates. If the mass
between the capacitor plates
changes, the electrical signal
is altered and the output
signal of the sensor changes accordingly.
iii) The result is an electrical signal variation proportional to the mass variation of the
test material passing through.
iv) This analog signal is then converted into a digital signal, stored and processed
directly by the USTER® TESTER 5 computer.
v) The capacitive measuring principle is very reliable and has good signal stability.
With this measuring principle not only yarns, but also rovings and slivers can be
tested.
The Uster is adjusted and balanced after a preliminary warming-up period. Several points are
noticeable:



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3.1.2) Material speed:
In yarns, drafting wave variations have a wavelength of about 2-2.5 times the
mean fibre length. So the testing speed must be chosen so that frequency of these
fluctuations lies within the capacity of the recording pen to follow them. For example,
with a yarn speed of 50 yd/min and a drafting wavelength of about 3” the pen must
be capable of responding accurately to 10 fluctuations/sec. The recorder of Uster can
cope up with a yarn speed of 100 yd/min.
3.1.3) Chart speed and chart contraction:
The yarn speed & chart speed ratio, i.e. the chart contraction, is chosen to suit the
type of variation being examined.
 For short term variation a ratio of 8-20 is recommended.
 For medium term variation a ratio of 40-160 is recommended.
 For long term variation a ratio of 200-1000 is recommended.
3.1.4) Choice of measuring capacitor:
In order to achieve a change in capacity which is linearly related to the amount of
material between the capacitor plates the thickness of the material relative to the size
of the capacitor must not exceed certain limits. In the Uster instruction manual a Table
states which capacitor in the „comb‟ must be used for the different hanks and counts.
3.1.5) The imperfection indicator:
The introduction of the imperfection indicator in place of the „Hy-Lo‟ offers the
quality control department greater opportunities for detailed analysis of faults in
yarns. Signals from the main units are fed to the imperfection indicator which
simultaneously measures neps, thick and thin places. A count of all three types of
fault is made and the results shown of separate counters. It is possible to adjust the
sensitivity of the system so that a chosen size of fault is counted and smaller faults
ignored. In 1964, a new set of „Uster‟ yarn standards were introduced, the yarn
parameters considered being evenness, neps, thick places and thin places.
Yarns spun from staple fibres contain "IMPERFECTIONS”. They are also referred
to as frequently occurring yarn faults.
Neps-a fault length of 1mm having cross-section 200 % of the average value



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Thick place-a fault length of
approximately the fibre
staple length having a
cross-section of 50 % more
than the average value
Fig:USTER® TESTER 5

allows the following sensitivity
thresholds for thick places:
+35%/+50%/+70%/+100%.
Every time the selected limit is
exceeded, a thick place is
counted.

Thin place-a fault length of approximately the fibre staple length, having a crosssection approx. 50 % less than the average value

The reasons for these different types of faults are due to raw material or improper
preparation process. A reliable analysis of these imperfections will provide some
reference to the quality of the raw material used.
Imperfection indicator record imperfections at different sensitivity levels:



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Thin place
-30% : yarn cross section is only 70% of yarn mean value
Thick place
+35% : the cross section at thick place is 135% of yarn mean value
+100%: the cross section at thick place is 200% of yarn mean value
Neps
400%: the cross section at the nep is 500% of the yarn mean value
Thick places and thin places which overstep the minimum actuating sensitivity of
+35% and -30% , respectively, correspond to their length to approximately the mean
fibre length. Medium length or long thick and thin places are to be considered as
mean value variations and are not counted by the instrument.
The standard sensitive levels are as follows
Thin place : -50%
Thick place : +50%
Neps : 200% ( 280% for open-end yarns)


Neps can be divided, fundamentally, into two categories:
 -raw material neps
 -processing neps

The raw material neps in cotton yarn are primarily the result of vegetable matter
and immature fibres in the raw material. The influence of the raw material with wool
and synthetic fibres in terms of nep production is negligible.
Processing neps are produced at ginning and also in cotton, woolen and worsted
carding. Their fabrication is influenced by the type of card clothing, the setting of the
card flats, workers and strippers, and by the production speeds used.
3.1.6) Uster yarn standards:
Advances in spinning technology and improvements in quality control systems,
have demanded standards on the quality of the yarns rather than the characteristics of
the intermediate products. Yarns from all over the world were sampled and tested,
the results analyzed and the standards prepared. The representation of the standards
is in graphical form. Charts for combed and carded cotton, blends and worsted-spun
yarns are available, but for the purpose of explanation we shall consider the charts for
combed cotton yarns.



Yarn Evenness


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Yarn Evenness

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In figure 1, the yarn count is on the logarithmic horizontal scale and the C.V.
percentage on the vertical scale. Suppose the yarn considered is a 40s (15 tex).
Locating the 40 position on the count scale we travel vertically until meet the 50% line;
reading across to the vertical scale we read off the value C.V. 16.2%. This means that
of all the 40s combed yarns spun, 50% of them will have a C.V. of 16.2 or worse, and
50% of them better (lower) than 16.2%. The spinning technologist can therefore
compare his own product with the products of other spinners. If our own 40s combed
yarn has a C.V. of 13.2% we would be amongst the spinners of good quality yarns as
the intercept on the 5% line corresponds to a C.V. of 13.5%. In other words, only one
spinner in twenty spins 40s combed are evenly as ourselves.
Standard charts for thick and thin places, and neps, are similarly constructed, the
example shown in fig.2 being the nep count chart. For our 40s combed cotton yarn the
number of neps per 1000 m, for the 50% line, is 54. (On the carded yarn chart the
corresponding value for 40s is about 600)
3.1.7) Extreme imperfections:
With the selector switch set to „Extreme Imperfections‟ the thick place, which are more
than double the average cross-section, can be counted. For worsted spinners two
limits have been fixed, +33% and 100% and the two counters register the number of
times that these limits have been exceeded.
4) Mechanical method of sliver/roving evenness measurement:
Principle of mechanical method of evenness testing is compression of fibrous
strand. Wool Industries Research Association has developed W.I.R.A sliver, roving
levelness tester. It can provide a continuous
record of the test performed.
Main features of the tester:
 Positively driven roller R1, with a
rectangular groove
 Negatively driven top roller which has
flange type shape and can be positioned
into the groove of roller R1
 R2 is mounted on an arm (lever) which
is pivoted at P
 Recording pen and graph paper
Test procedure:
In order to check different types of materials several pair of rollers is available. For
example, rollers with groove width of 1/32”, 1/16”, 1/8” and 1/4 “.






Suitable pair of rollers are mounted at the equipment
Magnification lever system is adjusted to get adequate pen movement
Pen movement can be adjusted from 20 times to 400 times of the top roller up &
down movement to get properly magnified trace
Pen to and fro movement is adjusted to get a trace between
trace paper


1

1
2 inches on the




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Material speed to chart speed can be varied from the gear box for material to
chart speed ratio of 2” to 100” of material speed inches/min

It is possible to derive the co-efficient of variation by measuring the height of the chart
at large number of points, and carrying out the statistical calculation.
Assessment of result:
The irregularity trace obtained from the mechanical tester (WIRA) is recorded on
chart paper having 30 divisions, each 1/10 inches wide. Redraw trace will be centered
at line 15 following procedure is adopted to determine standard deviation and CV%

of mass.
Step1 = choose centre line at 15
Step2 = mark two lines 16, 18 above centre line and two lines at 12, 14 below centre
line
Step3 = mark distance a,b,c on a plain paper and measure sum up distance (a+b+c) for
line 18, 16, 14, & 12.
Step4 = record sum up lengths in table
form
Step5 = in this trace 70 inches sliver
had been checked to obtain 7 inches
trace calculate % length of trace on
each line for line 18:
0.88/70 x 100 = 12.6
Subtract 12.6 form 100 for length =
100-12.6 = 87.4
Step6 = prepare a sheet of probability
paper
by
marking
from
12,13,14,15,16,17,18 on vertical side.
Mark horizontal scale (in %age) for the worked out length % plotted.
Step7 = draw the best straight line through the plotted points
Step8 = from straight line read ordinate against values of horizontal line of 15.9%, 5%,
and 84.1%. These ordinates would be 11.95%, 14.72%, 17.47% for this mentioned test.
Step9 = work out standard deviation
S.D = max. Ordinate value – min. ordinate value/2


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17.47  11.95
2
=
= 2.76
Step10 = work out co-efficient of variation of mass
CV% = S.D/mean chart height x 100
2.76

= 46.72 = 5.9 %
5) Optical method (Zweigle G580):
This instrument uses optical method of determining the yarn diameter and its
variation. In the instrument an infra-red transmitter and two identical receivers are
arranged as shown in Fig. the yarn passes at speed through one of the beams,
blocking a portion of the light to the
measuring receiver. The intensity of this
beam is compared with that measured by
Reference
the reference receiver and from the
receiver
difference in intensities a measure of yarn
diameter is obtained.
The optical method measures the
variations in diameter of a yarn and not in
its mass. For a constant level of twist in
Receiver
the yarn the mass of a given length is
related to its diameter by the equation:
Absolute
Mass = CD2 where C = constant, Infra-red
yarn dia.
light
D = diameter of yarn
Yarn

However, in practice the twist level
throughout a yarn is not constant. Therefore the imperfections recorded by this
instrument differ in nature from those recorded by instruments that measure mass
variation.
Advantages: it is very accurate like human eye. Because of the way yarn evenness is
measured, this method is not affected by moisture content or fibre blend variations in
the yarn.
Disadvantage: twist through the yarn is not same thereby changing the diameter and
hence results are not very accurate.
6) Pneumatic Method:
In this method the yarn to be tested is passed through a narrow tube, into which a
stream of air is being forced. The air flow rate is then measured, usually by pressure
change or some associated phenomenon. In theory, the air-flow in an empty tube is
impeded, as yarn is inserted, by an amount related in some way to the mass of
material present in the yarn. Clearly, careful calibration is a necessary procedure in
using equipment of this type and the relationship may not necessarily be a linear one.
In addition, the effects of such factors as temperature and atmospheric pressure, as
well as relative humidity, will have to be controlled more carefully than usual.



Yarn Evenness

17/19

6.1) Procedure:
Two air streams are directed towards each other, and a bundle of filaments is
guided to meet the resulting jet at right angles. The filaments are directed towards the
stream of higher intensity, which thus causes pressure fluctuations in both, and these
changes are measured in the stream of lower intensity to generate a signal
proportional to the mass of yarn interfering with air-flow.
The method is still relatively untried and unproved.
7) Acoustic Method:
Sound waves are used to measure the evenness of yarn. Yarn is moved through a
sound field between a sound generator and a pick up device. Time taken for sound
waves to move across the gap is measured electronically. Transit time of sound is
dependent upon the weight of yarn in the gap.
7.1) Impulse Acoustic Method:
A new method was elaborated to determine the inner (transversal) unevenness of
multifilament chemical yarns and its defectiveness in the yarn‟s cross-section by
means of the impulse-acoustic spectrograms, both separately and in combination with
an analysis of the full stretching diagram. This method was applied for different
chemical yarns. There are no other methods of obtaining such information, including
all the standard testing methods currently used.
7.1.1) Principles of the new impulse
acoustic method of yarn control:
 The impulse-acoustic emissionrecording method is based on
stretching
the
untwisted
multifilament structures (parallel
fibres or bundles of filaments) and
accumulating the filaments‟ deformation energy to rupture step by step
the structural elements, i.e. the
separate filaments. The deformation
energy of every stretched filament
comes free at rupture, and gives
returning impulses owing to the
contraction process. Therefore, at the
moment of rupture, the energy of
contraction is transformed into to the
energy of acoustic oscillation. These
impulses,
their
intensity,
arrangement etc. are recorded, and produce the acoustic spectrum, which is the
picture of deformation and the multi-element material destruction (of the
multifilament yarn) at the time of its deformation. It therefore represents the picture of
distribution of the yarn rupture.
The impulse-acoustic spectrum is shown in the coordinates of deformation/impulse
intensity.


Yarn Evenness

18/19

 The impulse acoustic spectrum consists of a series of separate impulses located at
the axis of elongation or the time of deformation. Great
differences exist among the acoustic spectra of various
yarns. The spectra differ in the impulse intensities and
their positions in relation to the elongation. These data
give an excellent reflection of individual filaments‟
breaking arrangements.
 Higher unevenness and defectiveness of yarn
correspond to a broader width of the spectrum. The
intensity and position of the first impulse, or a small
group of impulses, give information about more
defective filaments. It is necessary to stress that the
information concerned with the first impulse is
perceived only by the impulseacoustic
method.
This
information cannot be perceived
in a standard stretching diagram
such as that commonly carried
out by dynamometric tests (loaddeformation curve).
 At present, together with the
impulse-acoustic
tests,
a
traditional
load-deformation
diagram is also recorded, as it
was established that the best
information and interpretation of
the test results can be obtained by combination of the impulseacoustic spectrum with the full deformation diagram (up to the
point of the yarn rupture). The acoustic spectrum and the full
stress-strain curve both permit every impulse coordinate to be
found.
7.1.2) Equipment for tests:
The testing device for the impulse-acoustic method consists
of a low noise and vibration level electronic dynamometer with
an additional acoustic system. A scheme of the testing device
is shown in Fig. 3. The FPZ-10/1 Tiratest testing machine
(dynamometer) from Thűringisches Prűfmaschinenwerk GmbH
(Germany) was applied for our research. The measuring system
includes standard deformation and load sensors and an additional acoustic measuring chain composed of a sensor,
amplifier, and amplitude detector. The acoustic piezo-sensor is
placed on the dynamometer clamp, as shown in Fig. 4.
Every filament break while stretching the yarn gives an
acoustic returning impulse which is registered by the
measuring system. The form of the acoustic signal is shown in

Figure.4. Clamp; scheme of sensor
position; 1 - sensor, 2 – clamp, 3 –
rod, 4 – connecting cable, 5 – yarn.

Figure 5. Form of acoustic
impulse corresponding to single
filament rupture; U – amplitude of
signal, t – time.


Yarn Evenness

19/19

Figure 5. All signals from the three sensors are sent to an analogue-digital
transformer, and next to a computer for registration, processing and indication. The
resulting information includes the impulse-acoustic spectrum combined with the full
stretching diagram stored in the computer memory and visible on the monitor.
According to the constant amplification factor, the height of each amplitude is
proportional to the energy impulse returning from the filament ends. In this way, all
the impulses of filament breaks as well as the full stretching diagram are reordered
(Fig.6).It should be stressed that an appropriate sensitivity and minimal response time
of the electronic system recording every impulse are necessary to separate each
acoustic impulse while testing yarns consisting of a high number of filaments.
Standard and special software were applied to analyze the test results.
7.1.3) Sample preparation & testing procedure:
The yarn samples are initially untwisted to zero twist; in this manner the bundle of
parallel filaments is prepared. To fix the yarns, its ends are glued between two pieces
of paper (with polyvinyl acetate glue, or another with a short drying time). The
samples are conditioned at standard conditions (relative air humidity of 65 ± 2% and
temperature of 20 ± 2 °С). The velocity of the clamp movement guarantees obtaining a
time to sample break of 20 ± 2 sec. If another procedure was necessary it would be
used.
The clamping length is within the range of 30 mm to 100 mm. This length should
be correlated with the friction forces between filaments. If the friction between
filaments is high, a short length is necessary. However, the best length for standard
tests is 100 mm. The number of tests for one kind of sample should be 5 (or 10 in the
case of filaments of high unevenness).
Index of Irregularity:

:

Index of irregularity (I) is the
measure used to find out the extent to
which actual irregularity deviates from
that due to random. Index of
Irregularity=CVa/CVr where CVa is
actual measured irregularity. A higher
value means that there is more scope
for improving the processes. Table
below shows how irregularity due to
random arrangement and Index of
irregularity vary with count.
Count
20 card
30 card
40 combed
60 combed
100 combed

CVr
7.7
9.26
10.5
12.5
15.2

CVa
17
17.5
14
15.5
18

I
2.20
1.88
1.33
1.24
1.18

It will be seen that while Irregularity
due to random arrangement increases
Index of irregularity reduces with
increase in count. CVr reduces from
yarn to roving and further from roving
to yarn because of the increase in
number of fibres in cross-section. Index
of irregularity on the other hand
increases from yarn to roving and then
from roving to yarn. Typical results are
given in Table below:
Material
Yarn 20s
Roving 1Ne
Drawing sliver
Yarn 100s
Roving 100s (2.8) Ne
Drawing sliver(100s)
0.19Ne


CVr
7.7
1.7
0.6
15.2
2.54
0.66

CVa
17
6
4
18
5
2.5

I
2.20
3.52
6.68
1.18
1.96
3.79

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