Yarn Twist | Relationship Between Yarn Count and Twist | Principles of Twist Measuring Methods

Yarn Twist:In the manufacture of staple fibre yarns, twist is inserted into the fine strand of fibres to hold the fibres together and impart the desired properties to the twisted yarns. Without twist, the fine strand of fibres would be very weak and of little practical use. A change in the level of twist also changes many yarn properties, such as strength and softness.

Definition:Twist may be defined as the spiral disposition of the components of a thread which is usually the result of relative rotation of the two ends. Twist is generally expressed as the number of turns per unit length of yarn, e.g. turns per inch (tpi), turns per metre (tpm), etc.

What exactly does twist to a yarn?1. The twist in a yarn binds the fibres together and helps to keep them in the respective

positions. It thus gives coherence to yarn.2. Twist gives sufficient strength to the yarn.3. Twist is also used to bring about novel effects that are prominently visible when the yarn

is converted to fabric. This is achieved primarily by having a combination of yarns with different twist levels and twist directions in the fabric.

Nature of Twist:

Types of Twist:There are two types of twist: real twist and false twist.

Real twist: To insert a real twist into a length of yarn, one end of the yarn should be rotated relative to the other end, as indicated in figure (a).

Spun yarns usually have real twist, which holds the fibres together in the yarn.

False twist: When inserting false twist into a length of yarn, both ends of the yarn are clamped, usually by rollers, and twist is inserted with a false twister between the clamping points, as indicated in figure (b).

If the yarn is not traversing along its axis, the twist will be in opposite directions above and below the false twister. If the false twister is removed, the opposite twists will cancel out one another, leaving no real twist in the length of yarn. If the yarn is traversing along its axis, then the section of the yarn moving away from the false twister would have no net twist, as indicated in figure (b).

False twisting is a very important phenomenon, which has considerable practical implications in yarn technology.

Figure: Real twisting and false twisting

Twist Direction:A twist can be either in Z direction or S direction as indicated in the following figure, depending on the orientation of the surface fibre in relation to yarn axis.

Fig. : Twist directionIt is worth noting that twist direction affects fabric properties. For example, following Figure shows two identical twill-weave fabrics with the warp yarn of different twist direction. Fabric A will be more lustrous than fabric B, because light reflected by fibres in the warp and weft is in the same direction. Fabric A will be softer while fabric B firmer, because in Fabric B, the surface fibres on the warp and weft in the region of contact are aligned in the same direction and they may ‘get stuck’ inside each other and reduce the mobility of the intersection. Whereas for fabric A, the surface fibres on the warp and weft in the region of contact are crossed over, and they can

move about easily. The freedom of movement at the yarn intersections is the key for fabric softness.

Fig. : Effect of twist direction on fabric properties

Self-locking Effect:Because of twist in a yarn, the fibres on yarn surface take a roughly helical configuration around the yarn. When the yarn is under tension, these surface fibres are also under tension. However, because of the helical configuration, part of the tension is diverted radially, which creates a radial pressure. This is illustrated in the following figure.

The radial pressure tends to pack the fibres together, increasing the normal force between them, and so increasing their frictional resistance to slipping past each other. The more tension is applied to the yarn, the more it locks together, hence 'self-locking'. An analogy is, when you wind a string around your arm, as you pull the string along the arm and away from each other, the string bites deeper into the flesh.

Without twist, there won’t be any self-locking effect to prevent fibre slippage. Consequently the yarn would have no strength.  

Relationship Between Yarn Count and Twist:

From figure, we get,

 (Where ɵ= twist angle, d= yarn diameter and L= yarn length)

Also from figure, the height (pitch) of one turn of twist is L. Since the twist level is normally specified as the number of turns per metre, the twist level in one metre of the yarn would be:

We also know from experience that yarn diameter is also very hard to measure, because textile yarns by their very nature are soft and squashy. On the other hand, yarn count is normally used as we have discussed in the first topic of this module. But we can relate yarn diameter to yarn count using the expression below:

Thus, K is a factor relating twist level to yarn count. The derivation shows that if two yarns have the same twist factor, they will have the same surface twist angle, regardless of count. Since surface twist angle is the main factor determining yarn character, then twist factor can be used to define the character of a yarn.

It is worth noting though there are minor errors associated with the use of twist factor for the following reasons:

The cubic density may be different for different yarns. It is assumed in the above calculation that this will not change for yarns of the same surface twist angle.

Different fibres with different frictional and other properties will create different yarn character.

Nevertheless, the relationship we have just derived between twist, twist factor and yarn count is one of the most important in the study of yarn technology. This relationship is expressed in different ways for different yarn count systems.

For the tex system:

Please note the unit for twist is also different in the above expressions of twist factor. In addition, twist factor is also known as twist multiplier, twist alpha, or twist coefficient.

Angle of Twist: The yarn twist angle is the angle between a tangent to the helix formed by a fibre on the yarn surface and the yarn axis. If the twist multiplier of a cotton yarn is known, the twist angle can be easily calculated.

Factors Affecting Twist:The twist introduced in the yarn during spinning depends upon a number of factors, such as follows:

1. The count of yarn to be spun2. The quality of cotton used3. The use to which the yarn is put- is the yarn meant to be used as warp yarn or weft yarn,

knitting yarn or any other yarn?4. The fineness of the fibre being spun5. The softness of the fabric into which the yarn is to be converted

The Distribution of Twist in Staple Spun Yarns:If someone twists your head, it is your neck that suffers most. That is because the neck is a ‘thin’ place and offers little resistance to being twisted. By analogy, if a yarn of varying thickness is twisted, it is usually the thin spot in the yarn that gets twisted the most. Invariably, yarns spun from staple fibres (eg. wool, cotton) are not perfectly uniform, and there are thick and thin spots along the yarn length. This variation in yarn thickness will lead to variation in the twist level along the yarn length, because twist tends to accumulate in the thin place.

The fact that twist tends to accumulate in the thin spot along the yarn has several important implications:

1. It exacerbates the variation in yarn linear densityWhile variation in yarn linear density is the fundamental cause of twist variation, concentration of twist in the thin places will make those places even thinner, exacerbating the problem of yarn unevenness.

2. It improves the evenness of a fibre assembly during “drafting against twist”In the drafting stage of woollen ring spinning, the woollen slubbing is drafted while twist is inserted into the slubbing (drafting against twist) to control fibres during drafting. Because twist tends to accumulate in the thin spots, the fibres in thin regions in the slubbing are more difficult to draft than those in the thick places, which have less twist. As a result, the thick places are drafted more than the thin places, thus improving the evenness of the drafted material. This is depicted in following figure.

Figure: 'Drafting against twist' improves evenness3. It has implication for twist measurementsBecause the twist level varies along the yarn length, the twist measured at a short length of yarn may not reflect the true average twist of the yarn. Standard test procedures should be followed to measure the yarn twist accurately.

The relationship between twist and yarn count may be expressed by the following formula:

Where, p is usually greater than 1 but less than 2 for most yarns.

Twist Contraction:When a bundle of parallel fibres is twisted, the distance between the two ends of a fibre will decrease, particularly for fibres near the surface of the twisted bundle. As a result, the overall length of the twisted bundle is shorter than its length before twist insertion. The reduction in length due to twist insertion is known as twist contraction.

The following formula is used to calculate the amount of twist contraction:

Where,Lo = original length before twistingLf = final length after twisting

It should be noted that because of twist contraction and the associated change in length, the count of a yarn will change slightly when twist in the yarn is changed. Twist contraction increases yarn count (tex), because the weight of the yarn is distributed over a shorter length. The following formula can be used

Where,No = count (tex) before twistingNf = count (tex) after twistingC = %contraction

Measurement of Twist:Twist measurement is a routine test for yarns. Because of the variation in twist along yarn length, care should be taken in measuring the twist of staple spun yarns. Some basic principles are discussed here.

Sampling Rules:The following rules should be observed when measuring yarn twist:

1. Tests should not be limited to a short length of the yarn package.2. Beware of "operator bias" - tendency to select either thicker or thinner regions. Taking

samples at fixed intervals along the yarn length will reduce the bias.3. Discard first few metres from package. Being a free end, it could have lost twist.4. Remove yarn from side of package, not over end. Removing yarn over end will change

the twist level in the yarn.5. Tension in Yarn during test e.g. For single worsted yarns: 5 + 1 mN/tex

Principles of Twist Measuring Methods:The two common methods used in twist measurement are straightened fibre method and untwist/retwist method.

(1) Straightened Fibre Method:This method involves counting of the number of turns required to untwist the yarns until the surface fibres appear to be straight and parallel to yarn axis. This method is mainly used for ply and continuous filament yarns.

(2) Untwist / Retwist Method:This is the common method used for staple fibre yarns. It is based on twist contraction (hence also known as twist contraction method).

For this method, it is assumed that the contraction in length, due to insertion of twist, is the same for both direction of twist (S and Z). Suppose we want to measure the twist level in a yarn with Z twist, the yarn is first untwisted (by a twist tester), and a counter on the twist tester will record the number of turns. During untwisting, the yarn would increase in length from its original length L to a new length L’. If the operation is continued, the yarn would have its twist completely removed first and then twisted up again in S direction. As the yarn gets twisted, its length will decrease (twist contraction) from L’ towards its original length L. When its original length is reached, the total number of turns received by the yarn, as recorded by the counter on the twist tester, would be equal to twice the twist in the original yarn (with a length of L).

Automatic twist testers are now available, such as the Zweigle automatic twist tester.

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Mechanism of Twist Insertion to the Strand/Yarn

Twist: Twist is the number of turns about its axis per unit of length of a yarn or other textile strand. Twist is expressed as turns per inch (tpi), turns per meter (tpm), or turns per centimeter (tpcm). It is a very essential process in the production of staple yarn, twine, cord and ropes. Twist is inserted to the staple yarn to hold the constituent fibres together, thus giving enough strength to the yarn, and also producing a continuous length of yarn. The mechanism of twist insertion to the strand during ring spinning has been studied. The twisting of the strand occurs not only due to the rotation of twisting elements, but also due to the winding of yarn on the package. When the yarn is wound on a stationary cop by gripping and winding the yarn by hand, for every coil of yarn wind one turn of twist to the yarn is inserted. Now we will discuss about way of twist insertion to the yarn. 

Twist direction

Twist Insertion to the Yarn When the Spindle is Stationary:  We assume that the spindle is stationary and the traveller rotates in the ring frame. Each revolution of the traveller winds one coil of yarn onto the cop. This is similar to gripping and winding the yarn on a cop by hand. The yarn will rotate 3600 per coil wind while winding the yarn onto a stationary cop by hand; hence the winding causes yarn twisting.

Length of yarn wound per revolution of traveller = πd Turns/cm due to winding = 1/πd

Where d – Winding on diameter of cop or bobbin in cm.

If the yarn is unwound in parallel from the cop, the yarn will retain all the twists present in the yarn, whereas if the yarn is over-end unwound, unwinding a coil removes one turn of twist. The unwinding causes twisting. So, the twists inserted into the yarn during winding are removed during over-end unwinding. The over-end withdrawal may be from any side of the cop. If the traveller rotates in a clockwise direction to wind the yarn onto the cop, each coil of wind inserts one turn of ‘Z’ twist to the yarn. When the same is over-end unwound, every unwinding coil inserts one turn of twist in an ‘S’ direction, and so the resultant yarn will not have any twist.

Twist Insertion into the Yarn when the Traveller is Stationary:  We assume that the traveller is fixed on a stationary ring and that the spindle is rotating at a constant speed. Every revolution of spindle winds one coil of yarn onto the cop. Here winding does not cause twisting, and hence the yarn in the cop will not have any twist. But if the yarn is over-end unwound, every unwinding of a coil of yarn inserts one turn of twist into the yarn.

Turns/cm due to over-end unwinding = 1/πd

The direction of twist insertion during over end unwinding depends on direction of yarn winding. If the spindle rotates in an anticlockwise direction to wind the yarn onto the cop, during over-end unwinding a ‘Z’ twist will be inserted into the yarn. But if the same yarn is unwound in parallel, the yarn will not receive any twist.

Twist Insertion onto the Yarn when both Spindle and Traveller rotate in Opposite Direction: It may be wondered why it should be necessary to rotate the traveller and spindle in the opposite direction, and also how to rotate the traveller in the opposite direction. This is only to enable the reader to clearly understand the mechanism of twisting. When both the spindle and traveller rotate in the opposite direction, each revolution of the spindle and traveller winds one coil each. The length of yarn wound per min and twist/cm can be calculated.

Length of yarn wound per min = π d (NS+NT) Twist/cm due to winding = - NT/ π d (NS+NT) where NS – spindle speed in rpm, NT – traveller speed in rpm.

If the spindle and traveller rotate in clockwise and anticlockwise directions respectively, the direction of twist insertion due to winding would be ‘S’. But during over-end unwinding, the direction of twist insertion would be ‘Z’. + and - signs are used to represent the Z and S twist directions respectively.

Twist/cm due to over-end unwinding = (NT/ π d (NS+NT)) + (NS/ π d (NS+NT)) Twist/cm in the yarn after over-end withdrawal = (NS/ π d (NS+NT)

Twist Insertion onto the Yarn when the Spindle leads the Traveller: In ring spinning, both the spindle and traveller rotate in the same direction. However, the spindle rotates at a higher speed than the traveller. If both rotate at the same speed, only the twisting of yarn takes place without winding. Due to the difference in their rotational speeds, the winding of the yarn takes place on the cop.

Length of yarn wound on the cop per min = πd (NS –NT)

Due to rotation, both spindle and traveller insert twists onto the yarn. If both the spindle and traveller rotate in a clockwise direction, a ‘Z’ twist is inserted to the yarn.

Turns/cm in the yarn = NT/πd (NS –NT) The winding rate should be equal to the delivery rate. Length of yarn delivered (cm/min) = πd (NS –NT)

Here winding takes place in similar conditions to when the traveller is stationary and the spindle is rotating; hence winding does not insert any twist onto the yarn. On the other hand, during over-end unwinding one turn of twist is inserted for every unwound [[*]] of coil.

Turns/cm for unwinding = 1/πd Total twist present in the yarn after over-end unwound = NT/πd(NS –NT) + 1/πd =

NS/πd(NS-NT)

Since yarn from the ring cop is normally over-end withdrawn during the winding process, the spindle speed is taken for calculating the turns/cm in the yarn instead of using traveller speed. However, turns/cm in the roving is calculated by taking the flyer speed into account. This is due to the parallel withdrawal of roving during spinning.

Twist Insertion onto the Strand when Flyer leads Bobbin: Due to the difference in the speeds of the flyer and the bobbin, the winding of roving takes place on the bobbin.

Twist/cm due to twisting = NB / πd(NF-NB) Twist/cm due to winding = (NF-NB)/ πd(NF-NB) Twist/cm in the roving = NF / πd(NF-NB) where NF - flyer speed in rpm, NB - bobbin speed in rpm.

If the roving is unwound in parallel, the roving will have the same amount of twist as in the bobbin, but if it is over-end withdrawn, it will lose a certain amount of twist during unwinding.

Turns/cm due to over-end withdrawal = - (NF-NB)/ πd(NF-NB) Turns/cm in the roving after over-end withdrawal = NB/πd (NF-NB) 

Read more: http://textilelearner.blogspot.com/2012/05/mechanism-of-twist-insertion-to.html#ixzz43ZtkWZjF

Calculation of Twist, Twist Constant of the Ring Frame

Name of the experiment: Calculation of twist, twist constant of the ring frame.

Objects: 1. To find out twist per inch of the ring frame. 2. To find out twist constant of the ring frame.

Specification:

Front roller diameter =  1" Tin cylinder diameter =  10" Whrave diameter = 1.125" Twist change pinion = 48T

Gearing diagram:

Figure: gearing diagram for calculating twist and twist constant of ring frame.

Calculation:

Result: 1. TPI= 21 2. Twist constant= 1008

Conclusion: Ring frame is the final and very important machine for build the yarn onto bobbin in a form suitable for storage, transportation and processing. It is used to twist the drafted strand to form yarn of required count and strength. In this practical we calculate twist, twist constant of the ring frame. By this practical we come to know about the gearing diagram of ring frame. Special thanks to our teacher and his assistance for helping us.

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Twist Constant of the Speed Frame / Simplex M/C / Roving Frame Machine | Calculation of Twist, Twist Constant of the Speed Frame Machine

Name of the experiment: Calculation of twist, twist constant of the speed frame machine.

Introduction: Twist is the spiral turns given to a yarn to increase the strength of the yarn. But in speed frame machine vary small amount of twist is given to the roving to make it able to wound onto a bobbin. For a fibrous material twist is measured by the parameter twist per inch (TPI), twist per centimeter or twist per meter (TPM). For the cotton sample twist is measured by TPI.

In speed frame machines twist per unit is varied with the variation of raw material and its different parameters. This variation of twist is inserted by changing a wheel that is connected with the main driving shaft named twist change pinion (TCP). And the multiply of TCP and TPI, present in a machine is called twist constant. This value is applicable for any required twist with corresponding TCP. So we can find out the required TCP to get a given TPI. The generalize formulae is as below: 

Specification: Front roller carrier wheel :80T(A) Twist constant change pinion carrier:30T(B) Twist constant change pinion:30T(C) Twist change pinion:28T(D) Sprocket wheel:34T(E) Sprocket pinion:36T(F) Spindle carrier wheel:40T(G)

Spindle wheel:22T(H)

GEARING DIAGRAM:

Fig: Gearing diagram of speed frame

Calculation:

 Result: Twist per inch TPI → 1.56 Required TCP → 33

Conclusion: Speed frame is the first machine which enables the winding of the fibrous material on to a package. From this machine the fibre gets a circular shape which is very advantageous to be used in ring spinning. So the importance of this machine is very much. In this experiment we indicate different gearing diagram of the twist inserting portion; specify it and calculate twist and twist constant. We found a satisfactory result. So the experiment is a successful one.

Read more: http://textilelearner.blogspot.com/2012/02/twist-constant-of-speed-frame-simplex.html#ixzz43ZuF5xU7

Yarn Twist | Twist direction | Types of Twist | Twist Principle

Yarn Twist: Twist is the spiral arrangement of the fibres around the axis of the yarn. The twist binds the fibres together and also contributes to the strength of the yarn. The amount of twist inserted in a yarn defines the appearance and the strength of the yarn. The number of twists is referred to as turns per inch. 

Yarn twistThere are different definitions given to twist of the yarn. Some of the definitions given are as follows:

Yarn twist is defined as the spiral deposition of the components of a twist is the measure of the spiral turns given to a yarn in order to hold the constituent fibres or threads together – Skinkle.

When a strand is twisted the component fibres tend to take on a spiral formation, the geometric perfection of which depends on their original formation – Morton.

Twist may be defined as the rotation about the yarn axis of any line drawn on the yarn which was originally, before twisting parallel to the yarn axis – Wool Res. Vol. 3.

Twist may also be defined as thread which is usually the result of relative rotation of the two ends.

Twist direction:  The direction of the twist at each stage of manufacture is indicated by the use of letters S or Z in accordance with the following convention:

A single yarn has S twist if, when it is held in the vertical position, the fibres inclined to the axis of the yarn conform in the direction of the slope to the central portion of the letter S. Similarly the yarn has Z twist if the fibres inclined to the axis of yarn conform in the direction of slope to the central portion of the letter Z.

The Amount of Twist: In B.S. 946: 1952 it is stated that the amount of twist in a thread at each stage of manufacture is denoted by a figure giving the number of turns of twist per unit length at that stage. It affects the characteristics and properties of a yarn including appearance, behaviour and durability.

The amount of twist is an important factor in finished consumer goods. It determines the appearance as well as the durability and serviceability of a fabric. Fine yarns require more twist than coarser yarns. Warp yarns, which are used for the length wise threads in a woven fabric, are given more twist than filling yarn which is used for cross wise threads. 

The amount of twist also depends upon the type of the fabric to be woven:

1. Yarns intended for soft surfaced fabric are given slack twist. They are called as soft twisted yarns.

2. Yarns intended for smooth surfaced fabrics are given optimum twists. Such twisted yarns contribute strength, smoothness and elasticity.

3. Yarns intended for crepe fabrics are given maximum amount of twists.

Types of Twist S-twist Z-twist

Twist PrincipleOne end will be fixed and another end will be turned this is the twist principle.

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Yarn Twist | Twisting Process of Yarn | Mechanism of Twist Insertion to the Strand

Yarn Twist: Twisting is a very essential process in the production of staple yarn, twine, cord and ropes. Twist is inserted to the staple yarn to hold the constituent fibres together, thus giving enough strength to the yarn, and also producing a continuous length of yarn. The twist in the yarn has a two-fold

effect; firstly the twist increases cohesion between the fibres by increasing the lateral pressure in the yarn, thus giving enough strength to the yarn. Secondly, twist increases the helical angle of fibres and prevents the ability to aooly the maximum fibre strength to the yarn. Due to the above effects, as the twist increases, the yarn strength increases up to a certain level, beyond which the increase in twist actually decreases the strength of staple yarn. The continuous filament yarn also requires a small amount of twist in order to avoid the fraying of filaments and to increase abrasion resistance.

Yarn Twist However, twisting the continuous filament yarn reduces the strength of the yarn . Yarn is often ply-twisted in a direction opposite to a single yarn twist to improve evenness, strength, elongation, bulkiness, lustre and abrasion resistance, and to reduce twist liveliness, hairiness and variation in strength .

The twisting of fibres strands are carried out on a roving frame, ring frame, rotor spinning and DREF spinning machines etc. This twisted strand has to be wound on the delivery package in a certain form for easy withdrawal of these strands in the next process. Since the open end of the yarn is rotated in the rotor and DREF spinning systems, the delivery package has to be rotated axially to wind the yarn. The twisting and winding operations are separated in the open-end spinning . However, this is not possible on a roving frame or a ring frame.

There should be two rotating elements (the spindle and traveller or flyer and bobbin) in order to twist and wind the strand on the package. The winding rate should be equal to the delivery rate from the drafting device. As the winding on the diameter of the package varies continuously throughout the process, the difference in speed between the two elements also has to be varied continuously. Since the delivery rate is constant, the product of winding on diameter and the speed difference between the two rotating elements should be kept constant. On a roving frame, this is achieved by adjusting the bobbin speed continuously and keeping the flyer speed constant, whereas in ring spinning, only the spindle is rotated at a constant rate and the traveller is dragged around the ring by the yarn. Due to the frictional force between the ring and traveller, the required speed difference between the spindle and traveller is automatically adjusted. In both the

ring and roving frame of the short-staple spinning system, the bobbin lead is used. For calculating twist in the roving, the flyer speed is taken into account, whereas in ring spinning, the spindle speed is considered .

Twist/cm in the roving = flyer speed in rpm)/ delivery rate in cm/min Twist/cm in the yarn = spindle speed in rpm/ delivery rate in cm/min

The reasons for the above, and the mechanism of twisting strands on a roving frame and ring frame are not explained in textbooks or literature. In the present paper, the mechanism of twisting strands on a roving frame and ring frame is explained.

Mechanism of twist insertion to the strand Twist insertion to the yarn when the spindle is stationary. We assume that the spindle is stationary and the traveller rotates in the ring frame. Each revolution of the traveller winds one coil of yarn onto the cop. This is similar to gripping and winding the yarn on a cop by hand. The yarn will rotate 3600 per coil wind while winding the yarn onto a stationary cop by hand; hence the winding causes yarn twisting.

Length of yarn wound per revolution of traveller = πd

Turns/cm due to winding = 1/πd

Where, d – Winding on diameter of cop or bobbin in cm.

If the yarn is unwound in parallel from the cop, the yarn will retain all the twists present in the yarn, whereas if the yarn is over-end unwound, unwinding a coil removes one turn of twist. The unwinding causes twisting. So, the twists inserted into the yarn during winding are removed during over-end unwinding. The over-end withdrawal may be from any side of the cop. If the traveller rotates in a clockwise direction to wind the yarn onto the cop, each coil of wind inserts one turn of ‘Z’ twist to the yarn. When the same is over-end unwound, every unwinding coil inserts one turn of twist in an ‘S’ direction, and so the resultant yarn will not have any twist.

Twist insertion into the yarn when the traveller is stationary. We assume that the traveller is fixed on a stationary ring and that the spindle is rotating at a constant speed. Every revolution of spindle winds one coil of yarn onto the cop. Here winding does not cause twisting, and hence the yarn in the cop will not have any twist. But if the yarn is over-end unwound, every unwinding of a coil of yarn inserts one turn of twist into the yarn.

Turns/cm due to over-end unwinding = 1/πd

The direction of twist insertion during over end unwinding depends on direction of yarn winding. If the spindle rotates in an anticlockwise direction to wind the yarn onto the cop, during over-end unwinding a ‘Z’ twist will be inserted into the yarn. But if the same yarn is unwound in parallel, the yarn will not receive any twist.

Twist insertion onto the yarn when both spindle and traveller rotate in opposite direction. It may

be wondered why it should be necessary to rotate the traveller and spindle in the opposite direction, and also how to rotate the traveller in the opposite direction. This is only to enable the reader to clearly understand the mechanism of twisting. When both the spindle and traveller rotate in the opposite direction, each revolution of the spindle and traveller winds one coil each. The length of yarn wound per min and twist/cm can be calculated.

Length of yarn wound per min = π d (NS+NT) Twist/cm due to winding = - NT/ π d (NS+NT) 

Where

NS – spindle speed in rpm, NT – traveller speed in rpm.

If the spindle and traveller rotate in clockwise and anticlockwise directions respectively, the direction of twist insertion due to winding would be ‘S’. But during over-end unwinding, the direction of twist insertion would be ‘Z’. + and - signs are used to represent the Z and S twist directions respectively.

Twist/cm due to over-end unwinding = (NT/ π d (NS+NT)) + (NS/ π d (NS+NT)) Twist/cm in the yarn after over-end withdrawal = (NS/ π d (NS+NT)

Twist insertion onto the yarn when the spindle leads the traveller. In ring spinning, both the spindle and traveller rotate in the same direction. However, the spindle rotates at a higher speed than the traveller. If both rotate at the same speed, only the twisting of yarn takes place without winding. Due to the difference in their rotational speeds, the winding of the yarn takes place on the cop.

Length of yarn wound on the cop per min = πd (NS –NT)

Due to rotation, both spindle and traveller insert twists onto the yarn. If both the spindle and traveller rotate in a clockwise direction, a ‘Z’ twist is inserted to the yarn.

Turns/cm in the yarn = NT/πd (NS –NT)

The winding rate should be equal to the delivery rate.

Length of yarn delivered (cm/min) = πd (NS –NT)

Here winding takes place in similar conditions to when the traveller is stationary and the spindle is rotating; hence winding does not insert any twist onto the yarn. On the other hand, during over-end unwinding one turn of twist is inserted for every unwound of coil.

Turns/cm for unwinding = 1/πd

Total twist present in the yarn after over-end unwound = NT/πd(NS –NT) + 1/πd = NS/πd(NS-

NT)

Since yarn from the ring cop is normally over-end withdrawn during the winding process, the spindle speed is taken for calculating the turns/cm in the yarn instead of using traveller speed. However, turns/cm in the roving is calculated by taking the flyer speed into account. This is due to the parallel withdrawal of roving during spinning.

Twist insertion onto the strand when flyer leads bobbin. Due to the difference in the speeds of the flyer and the bobbin, the winding of roving takes place on the bobbin.

Twist/cm due to twisting = NB / πd(NF-NB) Twist/cm due to winding = (NF-NB)/ πd(NF-NB) Twist/cm in the roving = NF / πd(NF-NB) 

Where,

NF - flyer speed in rpm, NB - bobbin speed in rpm.

If the roving is unwound in parallel, the roving will have the same amount of twist as in the bobbin, but if it is over-end withdrawn, it will lose a certain amount of twist during unwinding.

Turns/cm due to over-end withdrawal = - (NF-NB)/ πd(NF-NB) Turns/cm in the roving after over-end withdrawal = NB/πd (NF-NB)

Summary and conclusion: Yarn will rotate 3600 per coil wound while winding yarn onto a stationary cop by hand.

When it is over-end unwound from the cop, all twists present in the yarn are removed. Hence both winding and over-end unwinding cause twisting, but in opposite directions. 

If the yarn is wound onto the cop by feeding the yarn perpendicular to the cop and rotating it, winding the yarn will not cause any twisting. But if the yarn is over-end withdrawn, the yarn will receive one turn of twist per coil unwound. 

If the flyer leads the bobbin in the roving frame, twisting of the roving takes place due to both twisting and winding. 

Since the yarn from the cop is over-end withdrawn during winding, the spindle speed is taken for calculating the twist in the yarn, whereas the flyer speed is taken for calculating the twist in the roving, due to parallel unwinding of the roving during spinning. 

The over-end unwinding of yarn helps in getting extra twist to the yarn, and the parallel unwinding of roving will not introduce any extra twist to the roving. If the roving is over-end withdrawn during spinning, every coil unwound will insert one turn of twist onto the roving. Hence the break draft and the setting of the back roller have to be increased to

facilitate the breakage of the twist present in the roving. Otherwise, undrafting of the strand will occur during drafting. Hence the roving is normally unwound in parallel from the bobbin during ring spinning. 

Read more: http://textilelearner.blogspot.com/2011/07/yarn-twist-twisting-process-of-yarn_1007.html#ixzz43Zugyuip