A Summary of Yarn Structure and Measurement

Here is a brief review of yarn structure and measurement methods

A)     Linear density designation of yarn:

1)      Direct system: based on measuring weight/unit length of a yarn

Tex -> weight  in gram of 1000 meters = 10 decitex (SI unit)

Denier -> weight in grams of 9000 meters...

2)      Indirect system: based on the length/unit weight known as count

Cotton count Nec: count = number of hanks all 840 yarns long in 1lb

Count = K/tex -> K is obtained from tables

3)      Folded Yarns: count of the singles yarn preceded by a number giving the number f single yarns tht make up folded yarn. 2/24s worsted count = tow 24s count worsted yarns twisted together

80x2 -> twisting two 80 tex yarns

R 74tex/2 à twisting two yarns together which result to 74 tex yarn

4)      Measuring Linear Density:

a)      sampling:

For lots of 5 or less cases, 10 packages are selected at random and equal from each

For lots of more than 5 cases, 5 cases are selected randomly and two packages are selected from each at random

b)      Effect of moisture content:

Should be taken into consideration while measuring

Three ways: linear density as received (as it is ), at standard testing atmosphere (4h @50C and 20C 65% RH for 24h), and at correct condition (dry 105C).

Dry weight X (100+standard test)/100

5)      Linear Density from a fabric sample:

Ct to a known size, number of threads are removed

a)      Shirley crimp tester: A device for measuring crimp-free length of a piece of yarn removed from fabric

Percentage crimp = (L1+L0)/L0 X100

B)      Twist:

Increase in twist increases the lateral force holding the fibers

Increase in twist increases the angle that the fibers make with yarn axis

There is a value of twist to give maximum value of strength. Beyond that value makes the yarn vary hard.

Twist has some effects:

a)      Handle: increase in twist level make the yarn more compact and gives a harder feel to yarn

b)      Moisture absorption: high twist yarn holds fibers together to restrict water access to yarn interior.

c)       Wearing properties: high level of twist helps to resist abrasion (pulling fiber out of yarn) and pilling (pulling fibers out of fabric).

d)      Aesthetic effects: twist alters appearance like thickness and light-reflection.

e)      e) Faults: variation of twist levers in yarn change the appearance of a fabric.

1)      Level of twist:

The ideal of amount of twist varies with the yarn thickness. The thinner the yarn, the greater is the amount of twist that has to be inserted to give the same effect. The angle of fiber made with yarn axis is the factor determines the effectiveness of the twist.  Tan(theta) =piD/L.

The greater the diameter of yarn, the lager is the angle produced by one turn of twist.

Twist factor K = turns/unit length/sqrt(count)

The numerical value of factor differs with each system. In direct system of tex,

 K=turns per meter x sqrt(coutn)

The twist in a yarn is not usually distributed uniformly along the its length.

The relationship between twist and thickness: Twist x mass per unit length = constant

2)       Measuring twist:

a)      Direction of twist: twist is conveniently denoted as either S or Z

b)      Withdrawal of yarn from package:

Withdrawal of yarn over the end of a package adds twist to a yarn, whereas withdrawal from the side of the package does not.

c)       Twist in yarns, direct counting method:

This is simplest method of twist measurement. Unwind the twist in a yarn until the fibers are parallel to the yarn axis and to count how many turns are required to do this. The twist tester instrument has a two fixed and rotating jaws.

Number of test

1)      Single spun yarns. A minimum of 50 tests should be made. The specimen length must not exceed the average staple length of the yarn

2)      Folded and cabled yarns and single continuous filament yarns. Minimum of 20 tests carried out with specimen length of not less than 250 mm.

d)      Continuous twist tester:

It is designed to reduce the amount of handling needed on consecutive twist tests and to speed up the testing process.

e)      Untwist-twist method:

This method is based on the yarns contract in length as the level of twist is increased. The end of yarn distant from the counter is attached to a pointer which is capable of magnifying its changes n length.

f)       Multiple untwist-twist method:

This method is subject to a variable error, but can overcome this problem by repeating the untwisting and twisting action.

g)      Automatic twist tester:

This tester takes the tedium out of the large number of tests required for determining twist. This depends on untwist-twist type methods for determining twist levels as it cannot detect fiber straightening automatically.

h)      Take-up twist tester:

This is the difference between the twisted and untwisted length of yarn.

i)        Folded Yarns:

Folded or plied yarns have two levels of twist in them. There is the twist in the individual strands making up the ply and there is the twist that holds the individual plies together. If the twist in single strand is required, the yarn can be analyzed by first removing the folding twist and then cutting out individual yarns to measure individual strands.

C)      Yarn evenness

Yarn evenness can be defined as the variation in weight per unit length or as variation in thickness of the yarn.

1)      Visual examination:

Wrapped on a black surface in equally spaced. The black board are examined under good lighting conditions using uniform  non-directional light.

2)      Cut and weigh methods:

Simplest way of measuring variation in mass per unit length of a yarn. It consists cutting consecutive lengths of the yarn and weighting them. Accurate of cutting is required.

Coefficient of variation (CV%) = 1.25 Percentage mean deviation (PMD%)

3)      Uster evenness tester:

This tester measures the thickness variation of a yarn by measuring capacitance. The yarn is passed through two parallel plates of a capacitor whose value is continuously measured electronically.

a)      Diagram: a diagram should be plotted of the actual variation in mass per unit length along the length of the yarn.

b)      CV or U: the percentage CV or U value gives an overall number for yarn irregularity and hence is the most widely used of the measurements that the instrument makes.

c)       Index of irregularity: to produce a completely regular yarn there would need to be exactly the same number of fibers in each cross-section through the yarn. Meaning that the end of one fiber would have to connect with the beginning of the following fiber.

CVlim = 100/sqrt(n)%  where n is number of fibers in the cross-section

Irregularity (I) = CVmeas/CVlim

d)      Addition of irregularities: each machine that produces yarn from fiber adds to the irregularity of the finished yarn.

e)      Imperfections: in addition to measuring the variability of yarn thickness the Uster tester also counts the larger short-term deviations from the mean thickness to comprise thin and thick places and neps.

f)       Spectrogram: an important type of thickness variation is the regular appearance of a thick or thin place at equal intervals along the yarn length. Rule: the height of a peak in the spectrogram should not be more than 50% of the basic spectrogram height at that wavelength.

g)      Theoretical spectrogram: if the CV of a yarn is zero then the spectrogram consists of a straight line. If the yarn has a completely random distribution of staple fibers, the staple length L has an effect on the spectrogram.

h)      Variance length curve: it is produced by calculating the CV for different cut lengths and plotting it against the cut length on log-log paper. Perfect yarn would produce a straight line plot.

4)      Zweigle G580:

This instrument measures yarn evenness by a fundamentally different method from the mass measuring system of the Uster instrument. It uses an optical method of determining the yarn diameter and its variation instead of capacitance measurments. An infra-red transmitter and two identical receivers are arranged in the tester.

D)     Hairiness

Yarn hairiness is in most circumstances an undesirable property, giving rise to problems in fabric production. The number of hair and the length of hair both vary independently the number of hairs of different lengths is distributed according to an exponential law, in which there are far more short hairs than long ones and the number of hairs falls off exponentially as the hair length increases.

1)      Shirley yarn hairiness tester:

This tester consists of a light beam shining on a small diameter photoreceptor opposite to it.

2)      Zweigle G565:

This tester counts the number of hairs at distances from 1 to 25mm from the yarn edge.

3)      Uster tester 3 hairiness meter attachment:

This device is produced as an attachment for the Uster evenness tester and is connected in place of the normal measuring capacitor.

E)      Yarn Bulk

WRONZ Bulkometer test gives an indication of the covering power of a yarn when it is incorporated into finished products such as knitwear of carpets.

1)      Textured filament yarns:

A large amount of continuous filament yarn has its bulk and stretch increased by some form of crimping process so that it may have the same covering power as staple fiber yarn and approximately the same texture. Tests for yarn stretch, which is related to yarn bulk, usually measure the difference in length between the straightened yarn and the contracted yarn.

a)      HATRA crimp rigidity:

In this test a hank of the yarn is wound under tension sufficiently high as to remove the crimp, the number of turns on the hank being governed by the yarn denier.

F)      Friction

A yarn which is being knitted or woven into a fabric or wound onto a package runs around many guides during the process. Each one causes a drag on the yarn due to friction. Frictional properties of yarn cause an increase or decrease in drag and the tension in the yarn. Limiting or static friction and sliding or dynamic friction. Frictional force is governed by two main factors: the nature of the surfaces in contact and the force that holds the surfaces in contact, known is normal force. The law is called Amonton’s laws:

1)      The limiting frictional force FL is proportional to the normal force R between the surfaces at right angles to the plane of contact.  FL = mu(L)R

2)      With all ordinary surfaces the limiting friction is independent of the area of contact for a constant normal force.

3)      When motion occurs the sliding frictional force F is proportional to the normal force R between the surfaces. F=muR

4)      With all ordinary surfaces the sliding friction is independent of the area of conact and also independent of the speed of motion within limits.

These laws hold fairly well for hard materials, but not for textile materials particularly at low values of normal force.

1)      Coil friction:

The friction that a yarn or similar object experiences when running over a curved surface is governed by the angle of contact with the surface and the tension at either side of the contact. The frictional force increases rapidly with the angle of contact owing to an increase in the normal force rather than to the increased area of contact.

2)      Measuring yarn friction:

The more usual way of measuring yarn friction is to run it around a solid rod and measure the T1 and T2 making use of the equation. The problem is that the frictional force does not conform closely to this equation but depends on factors such as the diameter of the rod, the angle of wrap, the input tension and the running speed.

US standard test condition: speed of yarn 100 m/min, either 180 deg or 360 deg wrap angle but not less than 90 deg. Friction surface 12.7 mm diameter chrome-plated steel of 4-6 micrometer roughness.