Yarn
Definition and Classification of
Yarn
Yarn: A yarn is an assembly of
fibres that are laid or twisted together to form continuous strand.
Yarns may be made from either staple
fibres or filament fibres. Staple fibres are twisted into yarns, filament
fibres need little or no twist to hold them together in yarn.
Classification of yarns on the basis
of processing:
I. Carded or
Karded yarn.
II. Combed yarn.
III. Semi-combed yarn.
II. Combed yarn.
III. Semi-combed yarn.
Carded or Karded yarn: It is an inferior quality of yarn. In processing this yarn, no comber is used. It contains more short fibres and naps.
Combed yarn: It is a superior quality of yarn. In manufacturing combed yarn comber is used. So this yarn contains almost no naps or short fibres.
Semi
combed yarn: It is a medium quality of yarn. It is manufactured by mixing some
combed slivers. For example, 50% combed sliver means if there are 8 slivers 4
carded and 4 combed slivers are mixed at post comb draw frame to produce this
type of yarn.
What is yarn?
Definition of yarn
Name
given to any textile fibre
when prepared in spinning process for being woven into fabric. Yarn is the most
diverse description and its value varies according to its luster, bleaching,
dyeing properties, its fineness, strength, elasticity, uniformity of diameter,
smoothness, suppleness, and natural colors.
Yarns
can be single, folded or fancy or twisted to right or warp way; twisted to left
or weft away.
Single
yarns or threads are fibrous matter twisted together in spinning.
Cotton
thread is dull, soft, pliable, and of moderate strength and elasticity.
Cotton
waste thread of low-counts is of two varieties.
Linen
thread is of 2 varieties – line and tow – both are stiff, rough and are
elastic.
If
spin wet, they are compact, smooth, and brighter than when dry spun. Bleaching
treatment makes it pure white and becomes lustrous and soft.
Woolen
thread is soft, spongy, hairy, elastic, moderately strong and possesses felting
properties. Bleaches indifferently but dyes easily.
Worsted
thread from combed wool has uniform diameter, luster, and are with a good
chemical resistance.
Silk
is made into net, spun and noils threads. Net-silk gives organize and tram. For
their diameter both are the strongest, most elastic textile threads. Spun silk
is less lustrous and elastic.
Mixed
yarn is obtained by mixing and spinning different fibres.
Folded
threads are made to impart increased strength, elasticity and smoothness and
are glazed or unglazed for special purposes like lace
making, hosiery etc.
Fancy yarn
is made by twisting together threads of different materials of different
counts, colors etc.
Filament yarns | Bullet proof, ballistic yarns | Lusture
What is filament yarn?
Man first learnt the technique of filament
yarns from the silk warm; which extrudes fibroin – a viscous proteinous liquid
through its two spinnerets. No sooner these filaments come in contact with
atmospheric heat than they solidify into yarns.
Manmade yarns can either be (a)
regenerated (Viscose, polynosic, acrylics and acetate) or (b) synthesized from
petroleum byproducts (polyfibres). Fibres forming molecules themselves are like
a yarn – too long as compared to width. These molecules are capable of linking
and forming a long chained polymer.
As chemicals, raw materials and time of
reactions as well as temperature used in manufacturing, vary from lot to lot,
these polyfibres exhibit different dye take-up from one lot to another and
should never be mixed at any stage.
What is Lusture?
Fibre and filaments are most lustrous when
they remain parallel to yarn axis. Twist reduces lusture. More the twist less
the lusture. Polyfibres possesses natural synthetic sheen.
Lusture is a phenomenon of light
reflection. The smoother the surface more the reflection. In twinkle nylon,
light rays are made to enter one yarn face and remerge from the same face. This
principle of total internal reflection becomes possible because of a
triangular, prismatic cross-section of the nylon filament.
Yarn with a mixture of coarse and fine
fibres will be more hairy than all coarse or all fine fibre yarn and diffuse
light instead of reflecting it. Synthetics have a typical glitter which is not
always a desirable attribute.
Bullet proof, ballistic yarns
Fabrics which can withstand and resist the
impact of single or rapid small fire power are made with super strong synthetic
continuous filaments. The most popular among them, KEVLAR – 29, is used for
making vests for army personnel.
Manufactured by Du Pont U.S.A in several
types, Only Kevlar 29 – an Aromatic Polyamide (Aramide) is tough enough to
absorb the force of a bullet and dissipate the heat and impact energy without
cracking.
Properties of bullet proof yarn
-
Tenacity
superbly high – twice and more than polyester and nylon.
-
Average
molecular chain length (M) 20,000.
-
Breaking
strength 20 dynes/tex at 2% strain.
-
Specific
tensile strength 525 lb/in.
-
Sp.
Strength modulus doubles of glass and 20 times more than best steel.
-
Chain
length 108 µ (µ = 10-6).
-
Tenacity
22 g/dtex.
Sometimes, impact of powerful close range bullets of Colt, Mauser or AK-47, do
crack KV fabric. Then the next generation fibre P84 can be used. Its tensile
strength is 33g/dtex against KV’s 22-or its strength is 1.5 times more. It is
an aromatic Poly Cimide. In KV or P84 – molecular chains are highly oriented
along fibre axis as shown in fig. There is no amorphous region. Each chain is
pressed side by side and held together by Van-der-walls, forces. Hydrogen and
co-valent bonds become armor when woven and shrunk to correct specifications,
though, such reflections are beyond the limits of this small feature.
Twist | Twist in yarns
What is Twist? Write about Twist in yarns
Higher the twist more the yarn strength,
till optimum twist is reached. But due to property of ‘fabric assistance’ even
less than optimum twist will produce fabrics
of maximum strength.
It also costs money. The circular threads
in cloth become elliptical in shape due to flattening. High twisted thread
resists flattening, size penetration and their covering power is reached. Spun
threads are invariably irregular and hairy and both these specialties help to
hide minor weaving faults.
A cotton fibre
has about 50 crimps per cm which help a lot in spinning. Voil and crepe threads
are purposely hard twisted for producing an open structure cloth. In voil it is
25-30 TPI and in crepe 40-60 TPI. Crepe threads are twisted to a degree when no
compactness occurs on further twisting. On wetting, threads swell and contract,
producing a low profiled wavy structure. Only hydrophilic fibres swell. Silk
and viscose are the best crepers. Yarn’s irregularity may be short, medium or
long term, depending upon whether it is 10, 100, or 1000 times, the average
fibre length.
Twist in yarns
All staple threads need to be twisted in
direct proportion to the square root of thread count.
It varies according to the type of
spinning machinery used and end use of the thread. Filling or weft threads are
generally given lower twist than warp, as they do not bear shedding and other
tensions or reed friction.
Knitting yarns are softly twisted and a
few yarns used in suitings, in worsted, higher twist is imparted to yarns to
reduce fabric’s fuzziness.
Pot spun threads need more twist than ring or flyer
spun threads. Up to a certain stage, twist beyond which a reduction in strength
takes place. As it costs money and reduces production; yarns are seldom-twisted
full. Due to the property of “Fabric assistance” a cloth woven with lesser
twisted yarns or threads, is as strong as that woven with optimum twisted
threads. But voil and crepe threads are purposely hard twisted. Threads needed
for knitting or weaving women’s wear
are soft twisted.
It is then is termed soft, medium, medium
hard or hard. For crepon or sheersucker effect and for cord effect, threads are
made hard twisted. It can be S, left handed (regular or open hand) or Z right
handed (reverse or cross hand).
A hard twisted thread becomes wiry,
resists liquid penetration and flattens with difficulty. It also acquires a
certain amount of elasticity and a tendency to snarl and resist change. An mf
yarn is as strong without twisted condition as in twisted condition. It
disturbs parallel configuration of monofilaments and their load sharing
capacity. A certain amount of twist helps in weaving of mf yarns. Terms, twist
per meter and twist per inch are used in continuous filaments and staple spun
yarns respectively. A certain amount of lengthwise contraction occurs in yarns
with increase in twist, but the amount is quite small. Yarns for American
Georgette are given up to 3500 TPM. Polyester filament such as 75/34/0 and
150/30/0 are used as filling in shirtings and suitings without twist.
Polyester weaving, Acrylic, Viscose/rayon yarns
Technological points of Polyester weaving, Acrylic, Viscose/rayon yarns by rotor spinning
Polyester weaving yarns
These
yarns are normally for technical fabrics that have high performance
requirements. They are used in tapes, belts and substrates and composites. Care
has to be exercised to obtain the maximum possible yarn strength. This means
that the fibres should not be damaged in the opening area or at the nozzle.
Suggestions are:
1. The rotor diameter has to
be selected according to the count being spun, i.e. 40 mm SB rotor for Ne 8/1
to 16/1 and 35 mm or 31 mm SB rotors for finer yarns.
2. The hardened steel or
boron treated steel rotors, SB, are recommended for 100% polyester.
3. The nozzles should have a
low false twisting action. The flat type nozzle is recommended. The smooth
nozzle SGF is suggested as the least aggressive nozzle. When a slightly higher
level of false twist is needed the grooved nozzle S3KF is sometimes used.
4. The soft twist tube may
be needed to improve the spinning stability; however it may result in a
reduction of yarn strength.
5. The opening roller wire
is frequently difficult to select. The uncoated OS 21 wire does the least
damage to the fibre and is the best surface if it has an acceptable lifetime.
However, the polyester may have a finish that is abrasive and this tends to
wear out the wire in an unacceptable short period of time.
6. Coated opening roller
wire such as OS 21 /6 DN may improve the lifetime, but the fibre may be damaged
by the diamond coating and create dust build up in the rotor groove. To
minimize fibre damage the opening roller speed should be as low as practicable.
Additionally the feed plate should not over control the fibres increasing the
fibre opening forces.
7. In some cases the
needle-opening roller has proved to be the best for certain applications. Check
with the business unit.
Acrylic yarns
Acrylic
yarns are used for a variety of products from knitted apparel to industrial
awnings and substrates. Most of the acrylics are either dyed fibre, package
dyed, or piece dyed as fabric. Acrylics are normally compatible with rotor spinning,
but some of the end products have to be high in lustre, soft and bulky. This is
a challenge in rotor spinning because of the yarn structure and wrapper fibres.
1. For knitting yarns, the
wrapper fibres should be minimized. Larger rotors and open grooves (57 degree
rotor) are preferred. The softest yarns are produced with 56mm, 48mm and 40mm
rotors if the spinning economics are acceptable.
2. Yarns can be spun using
the 35mm and 32mm rotors, but the yarn will be compact and somewhat stiff.
3. The nozzle has to be
selected to meet the end use requirements. Sometimes the grooved nozzle such as
the K4KK is good for producing a hairy yarn under a stable spinning condition,
but unfortunately the yarn may shed badly in the knitting machine. This effect
is partially dependent upon the fibre finish. In any event the fibre should be
produced specifically for rotor spinning.
4. A smooth nozzle is
frequently preferred for acrylics.
5. The soft twist tube may
be necessary to improve the spinning stability and allow low twist yarns to be
produced; however, the hairiness will be slightly increased.
6. The OS 21 opening roller
works well. The diamond-coated wire tends to damage the fibre and cause
shedding on the machine.
7. The yarns that are going
directly to the knitting machine have to be waxed at spinning. Knitting yarns
that are going to be package dyed should not be waxed until after the dyeing
operation.
Viscose/rayon yarns
Viscose
was one of the earliest fibres to be spun on the rotor spinning because it did
not contain trash and it was not temperature sensitive. However care has to be
taken to optimize the machine for high performance spinning.
1. The rotor diameter depends
upon the yarn count and the staple length. Viscose can be spun on rotors of any
diameter, from 28mm to 56mm.
2. The rotor surface should
be hardened steel or boron.
3. Rotor speeds can be
relatively high if the fibre is produced with a finish for high speed rotor
spinning:
48 mm up to 70,000 rpm,
40 mm up to 80,000 rpm,
35 mm up to 95,000 rpm,
31 mm up to 115,000 rpm
30 mm up to 125,000 rpm
4. The “S” rotor groove is
used to produce a strong uniform yarn.
5. The nozzle has to be
selected according to end use requirements. Some applications need the smooth
nozzle while others need the grooved nozzle.
6. The OB20 DN or the OB20/4
DN opening roller wires are commonly used. In plants where various types of
fibres are spun, the OS21/ 6 DN can also be used for viscose.
Denim, Drapery and upholstery yarns
Technological points in Denim, Drapery and upholstery yarns in ring spinning
Denim yarns
Denim
yarns now cover a wide range of applications from traditional cotton “Jeans
fabric” produced from Ne 5.6/1 to ladies fashion shirts produced from Ne 22/1
and finer. Denim was originally defined as woven,
cotton, fabric
produced from an indigo dyed warp and a natural colored filling and most of the
yarn counts fell between Ne 5.6 and 12/1.
The
warp yarns used for the indigo dyeing process and the subsequent
re-beaming operation have to have a low “cling” tendency. This is not the case
for filling yarns
nor for yarns going into the natural warps or beam dyed warps.
Many
spinners choose to produce yarns that can be used for all applications, but the
result may be a compromise of the fabric appearance.
1. The filling yarn must be
uniform in appearance from the inside of the package to the outside. There
should be no rotor loading that changes the yarn hairiness from a clean to
loaded condition. Filling variations are very obvious when used across a dyed
warp and are considered as defects.
2. The warp yarn is not
sensitive to slight yarn variations. In many instances, a “ring like” uneven
appearance is desired.
3. Rotors for coarse and
medium count yarns have been developed with a wide-angle groove to minimize
rotor loading and subsequent yarn variations. The D lll rotor is frequently
used for spinning denim yarn. For cotton that is very abrasive, boron treated
steel with a diamond /nickel coating is recommended.
4. For rope dyeing warp
yarns the following rotors are suggested:
a.
Ne 5, 6/1 to 12/1 - rotor 48 D lll D at 50,000 to 65,000 rpm.
b.
Ne 5.6/1 to 12/1 - rotor 40 D lll BD at 60,000 to 80,000 rpm.
c.
Ne 6/1 to 12/1 – rotor 35 D lll BD at 75,000 to 90,000 rpm.
5. In some cases the D lll
rotor is used for beam dyed warps and filling to simplify the logistics of the
spinning plant.
6. Where a strong, lean yarn
with low hairiness is needed for filling or beam dyeing, the 40 mm S D rotor is
suggested.
7. Denim yarns are normally
spun with grooved ceramic nozzles, depending upon the required yarn characteristics.
The spiral nozzle is not recommended because the coarse yarns are too lively
and have excessive torque. The K3KK, K4KK, K8KK and K4KD are normally used.
When the K4KD nozzle is used, the yarns go into rope dyed warps.
8. Lively, coarse denim
yarns tend to kink and snarl at the next process. To overcome this tendency,
the soft twist tube can be used. However it should be noted that this tube
tends to slightly increase the yarn bulk and hairiness.
9. The opening roller wires
used for denim yarns normally are standard for cotton i.e. OB 20, OB 20 – DN or
OU 25 D. These wires produce a uniform yarn with good evenness.
10.
“Ring Like” denim yarn is a relatively uneven yarn created by irregular fibre
opening. This effect can be produced by using a special opening roller that can
be made available through the technical department.
Drapery and upholstery yarns
Drapery
and upholstery yarns are frequently spun from synthetic fibres, nylon,
polyester, acrylic and viscose in 100% form or as blends with a variety of
fibres to obtain a special texture or optical effect:
Fibres
tend to over 1.5″ long and of relatively coarse denier. The hardened steel 56
mm rotors with 57- degree grooves are frequently used, - (65 mm for special
applications). For 1.5″ fibres the 40 SB rotor is suitable.
1. Yarn counts are in the
range Ne 3/1 to 6/1.
2. Care has to be given to
controlling the feeding of the sliver. If the sliver is too cohesive and will
not open easily, the sliver feed is irregular and spinning is unstable.
Attention has to be given to prevent the sliver slipping between the feed
roller and the feed plate. The fluting of the feed roller should securely grip
the sliver.
3. The opening roller should
have a wear resistant coating or be of the wear resistant pinned ring type.
4. The nozzle has to be
selected depending upon the type of fibres being spun. In some cases the K4K
nozzle is used and in others instances, for 100% polyester, the smooth flat
steel nozzle SGF is used.
5. Many times the spinning
components for these products have to be selected by the business unit
technical department.
Sewing threads are put up on
different types of thread packages called spools, cops or tubes, cones, king
tubes or vicones, containers, cocoons, and prewound bobbins to suit different
types of threads, machines, and sewing needs. Sewing machines require specific
types of thread packages in order for the thread to be presented correctly to
the machine. Thread packages may be color coded by size and type of thread to
assist operators in correct thread section. Thread is sold by length instead of
weight. The following figure shows several types of thread packages –
Spools – Spools contain relatively short
yardage and have thread wound in a parallel position. Spools have a flange
either end that interferes with off winding on industrial machines. Spools are
designed for home sewing use.
Cops – Cops are used primarily on
lockstitch machines where a variety of colors are used and productions runs in
any one color are short. Thread is cross-wound on cops or small tubes to
increase in off winding.
Cones – Cones are symmetric, tapered forms
made of paper or plastic that hold over 5,000 meters of cross-wound thread.
Cones provide good off-winding performance for high-speed machines. Cones are
the most economical packages for sewing threads in situations when thread
consumption is high and production runs are long with limited shade changes.
Vicones or king tubes – Vicones or king tubes may be parallel
tubes or low-angle cones with a flange at the bottom, which is designed to
contain spillage of smooth or continuous filament threads during of winding.
Containers – Containers are designed to handle
lively monofilament threads that may be difficult to control with traditional
packages. Very large spools of thread may have lubricant applied as the thread
is off-wound.
Cocoons – Cocoons are centerless thread
packages designed for insertion in shuttleless of multi-needle quilting
machines and some types of embroidery machine.
Prewound bobbins – These are precision-wound packages
designed to replace metal bobbins in lockstitch machines. Generally, more
thread is available and the length is more consistent on prewound bobbins than
on operator wound bobbins. Downtime is minimized by eliminating time for
winding bobbins. Off winding is also improved because of precision winding.
Prewound bobbins are available in different thread types and sizes for
different models of machines.
What is fancy yarn? Properties and uses of fancy yarns
Definition of fancy yarn | Properties and uses
Definition
“Fancy yarns” are those in which some
deliberate decorative discontinuity or interruption is introduced, of either
color or form, or of both color and form. This discontinuity is incorporated
with the intention of producing an enhanced aesthetic effect. In discussing
fancy effect in fabric, we must also include the metallic yarn and yarns that
have a metallic appearance, and therefore in the discussion of fancy yarn structure
we have included a short section about these yarns, although by strict terms of
the definition, metallic may not be “fancy” since they may not demonstrate any
visible discontinuity of either color or form.
Some Properties of Fancy Yarns
*Fancy yarns
usually have a base or core yarn which is a conventional plain yarn, this yarn
is combined with the effect yarn.
*The effect yarn
can be held in place with a binding yarn.
*They are
intentionally produced to have a distorted or irregular construction.
*There are many
types of fancy, novelty and decorative yarns. They can be produced in many
ways.
¬Color can be applied by printing or dyeing
pattern onto yarn.
¬Sports of colored fibre can be twisted in
with base yarn.
¬Two or more threads of different,
softness, thickness, weight, color or fibre content can be twisted together.
¬Raised textured can be introduced by
controlling the amount and direction of twist.
*Fancy yarns can
be natural or man-made or a combination of both.
Uses of Fancy Yarns
*Fancy yarns are
used in weaving of suiting, shirting, dress material, upholstery, furnishing
fabric & woolen tweeds.
*They are also
used in the knitting industry.
*These are used
for beauty & appeal enhancement.
*By using fancy
yarns the end product gets a boost in its visual appeal, which in turn improves
its sales realization.
*These yarns can
be used in power looms, hand looms, flat knitting and all type of circular
knitting machine.
*These fancy
yarns are synthesized on state of art hollowed fancy machines.
Different structures and formations of fancy yarn
Marl Yarn | Spiral or Corkscrew Yarn | Gimp Yarn | Diamond Yarn | Eccentric Yarn | Boucle Yarn | Loop Yarn | Snarl Yarn | Mock chenille Yarn | Knop Yarn
Structures and formations of fancy
yarns
Fancy
yarns
fall in to different categories, depending upon the basic morphology of yarns.
They are shortly described below;
1. Marl Yarn:
The simplest of fancy effects, a marl yarn is one in which two yarns of the
same count and twist, but of different colors, are folded together to form a
balanced yarn. They are, therefore, essentially plain folded yarns with the
additional characteristics that the yarns folded together are of a different
color or texture. As such, they barely count as “fancy yarns” at all, except in
that they result in a suitable, but noticeable, modification to the appearance
of finished fabric.
2. Spiral or Corkscrew
Yarn: A spiral or corkscrew yarn is a plied yarn that
displays a characteristic smooth spiraling of one component around other.
Following figure shows the basic structure, which is straightforward, and
except in the differing lengths of the two yarns involved, very similar to the
structure of marl yarn. Indeed, just like marl yarn shown also in the following
Figure, it can be produced relatively simply on a double frame or under the
ring spinning system. It is more textural in appearance than marl, and the
finer counts may also appear in some of the laces used in lingerie.
3. Gimp Yarn:
A gimp is a compound yarn consisting of a twisted core with an effect yarn
wrapped around it so as to produce wavy projections on its surface. This
structure is shown in following figure. Since a binder is needed to ensure the
stability of the structure, the yarn is produced in two stages. Two yarns of
widely differing count are plied together, thick around him, and the reverse
bond. Reverse binding removes most of the twist inserted during the first
process. It is this removal of twist that creates the wavy profiles, since it
makes the effect yarns longer than the actual length of the completed yarn.
4. Diamond Yarn:
A diamond yarn is made by folding a thick single yarn or roving with a fine
yarn or filament of contrasting color using S-twist, and cabling it with a
similar fine yarn using Z-twist. Multi-fold or `cable` yarns may be produced by
extending and varying this technique, to produce a wide range of effects.
Clearly, a true diamond yarn would show some compression effect which in the
interests of clarity has been omitted from the following figure.
5. Eccentric Yarn:
An eccentric yarn is an undulating gimp yarn, often produced by binding an
irregular yarn, for example a strip, slub or knop yarn, in the direction
opposite to the initial stage, creating graduated half-circular loops also
along the compound yarn.
6. Boucle Yarn:
The following figure shows the basic structure underlying a boucle yarn. This
is a compound yarn comprising a twisted core with an effect yarn (or roving)
combined with it so as to produce wavy projections on its surface. To simplify
the diagram, the core has been shown as single bar, rather than as two yarns
being intertwined with the effect yarn, as would be the case in reality.
Boucle yarns belong to the gimp yarns and
loop yarns. The effect is achieved by the differential delivery of the effect
component wraps around the core yarns either tightly or loosely according to
the amount of excess delivery and the level of the doubling twist inserted.
These wraps are bound by the effect yarn. The effect is similar to a
gimp.
7. Loop
Yarn: A
loop yarn consists of a core with an effect yarn wraooed around it and overfed
so as to produce almost circular projection on its surface. The following
figure shows the structure of a loop yarn, in this case somewhat simplified by
showing the core as two yarns, is twisted, and partially entraps the effect.
8. Snarl
Yarn: Like
the loop yarn, the snarl yarn is based around a twisted core, although, again
for the sake of simplicity, the core has been shown in following figure as two
parallel bars. A snarl yarn is one which displays 'snarls' or 'twist'
projecting from the core. It is made by similar method to the loop yarn, but
uses as the effect a lively, high twist yarn and somewhat greater degree of
overfeed.
9. Mock
chenille Yarn: A
mock chenille dose not at all resemble a true chenille yarn in its appearance
as a yarn, but when it is woven into a fabric it will give an effect very similar
to that of a chenille. It will, however, seem much harsher in handle because it
does not involve cutting the loop of the effect yarns and so it lacks the
'velvety' fell.
10. Knop
Yarn: A
knop yarn is one that contains prominent bunches of one or more of its
component threads, arranged at regular or irregular intervals along its length,
as shown in very simplified form in the following yarn.
It is usually made by using an apparatus
that has two pairs of rollers, each capable of being operated independently.
This makes it possible to deliver the foundation threads intermittently, while
the knopping threads that create the effect are delivered continuously.
Twist Measurement Techniques
Direct counting method
The essence of the method is to unwind
the twist in a yarn
until the fibres
are parallel to the yarn axis and to count
how many turns are required to do his. A suitable instrument has two jaws at a
set distance apart. One of the jaws is fixed and the other is capable of being
rotated. The rotating jaw has a counter attached to it to number the whole
turns and fractions of a turn. Before starting any tests the samples should
have been conditioned in the standard testing atmosphere.
Testing is started at least one meter
from the open end of the yarn as the open end of the yarn is free to untwist so
that the level of twist may be lower in that region. As the yarn is being
clamped in the instrument it must be kept under a standard tension (0.5 cN/tex)
as the length of the yarn will be altered by too high or too low a tension. The
twist is removed by turning the rotatable clamp until it is possible to insert
a needle between the individual fibres at the non-rotatable clamp end and to
traverse the needle across to the rotatable clamp. The use of a magnifying lens
may be required in order to test fine yarns. The twist direction and the mean
turns per centimeter or per meter are reported.
Continuous twist tester
This apparatus is designed to reduce the
amount of handling needed on consecutive twist tests and to speed up the
testing process. Yarn passes through the rotating jaw end and is wound up on a
rotating drum as it is moved on. Twist is assessed by the same principle as on
other twist testers but after removing the twist it is put back into the yarn
by rotating the counter back to zero. The rotating clamp is opened and its jaws
moved forward to meet the fixed clamp; the jaws are then clamped on the yarn.
The fixed clamp is opened and moving jaws are returned to the starting
position, taking a new length of yarn with them; the drum takes up the slack in
the yarn.
Untwist-twist method
This
method is based on the fact that yarns contract in length as the level of twist
is increased. Therefore if the twist is subsequently removed, the yarn will
increase in length reaching a maximum when all the twist is removed. The method
uses a piece of equipment such as that shown in Fig. 4.9 in which the end of
the yarn distant from the counter is attached to a pointer which is capable of
magnifying its changes in length.
At the start of the test the yarn is
placed under a suitable tension, either by a clip-on weight or by a weighted
arm as shown. The test procedure is to untwist the yarn until all its twist has
been removed and then to continue twisting the yarn in the same direction,
until it returns to its original length. The basis of the method is the
assumption that the amount of twist put in is equal to the twist that has been
removed. However, this is not necessarily the case. For woolen yarns the method
may give results up to 20% below the true value, whereas for worsted yarns the
results may be 15% higher owing to fibre slippage. One source of error in the
method is that at the point of total twist removal the fibres in the yarn are
unsupported so that any tension in the yarn may cause the fibres to slip past
one another, so increasing the length of the yarn. The difference in length if
unnoticed will cause an error in the measurement of turns per unit length.
Another source of error is the fact that with some yarns, when the twist is
removed, the amount of twist to bring it back to the same length is not equal
to the twist taken out.
Because of these problems the method is
not recommended for determining the actual twist of a yarn but only for use as
a production control method. There is a US standard for this method but it
warns that the measured values are only an approximation of the true twist. It
suggests that 16 samples are tested using a gauge length of 250 or 125 mm.
However, the method is easy to use and has less operator variability than the
standard method so that it is often used for measuring the twist in single
yarns.
Multiple untwist-twist method
The straightforward untwist-twist method
is subject to a variable error owing
to the fact that the number of turns to return the yarn to its original length
is not the same as the number of turns to take the twist out. This is mainly
because when the yarn is spun some of the distortion becomes permanently set
into the fibres so that when the twist is removed the yarn is not as straight
as it should be. This is particularly a problem in yarns made from wool fibres,
especially those that have been deliberately treated in order to set the twist.
The multiple untwist-twist method aims
to overcome these problems by repeating the untwisting and twisting action
which causes the error due to this source to be progressively reduced. In the
test, shown diagrammatically in Fig. 4.2, the yarn is untwisted and retwisted
back to its original length as in the normal test and the number of turns A noted. The value A contains an unknown error dl.
Without the counter being zeroed, the direction of turning is reversed and the
yarn untwisted and twisted back to its original length. This ought to bring the
yarn back to its original condition, however owing to the errors the counter
will show a small number of turns instead of zero. This reading is taken to be B and is due to the errors dl and d2. By untwisting and retwisting a third time a further reading C is obtained which contains the
errors rfl, d2 and d3 as shown. Combining the readings
A, B and C gives:
A - 2B H-
C = 4x
Where, x is the number of turns in the length of yarn tested.
The method relies on the errors dl, d2 and d3 becoming progressively smaller so that the remaining error in
the above equation is the difference between d2 and d3 and
can be ignored. It is possible to carry out further untwisting and twisting in
the same manner to reduce the error even further.
Automatic twist tester
An automatic twist tester is produced
(Zweigle D302) which takes the tedium out of the large number of tests required
for determining twist. This necessarily depends on untwist-twist type methods
for determining twist levels as it cannot detect fibre straightening
automatically.
Take-up twist tester
Take-up
is the difference between the twisted and untwisted length of a yarn. Twist
testers are available with a movable non-rotating jaw which is slid away from
the rotating jaw to take up the slack as the twist is removed. This allows the
length difference to be measured.
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