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CHAPTER 2

LITERATUREREVIEW

The literature related to mechanism of roller drafting, ideal drafting,

real drafting and theory of formation of drafting wave are discussed in this

chapter.

2.1 DRAFTING

The reduction of the number of fibres in the cross- section logically

leads to a reduction in linear density of the strand. In terms of fineness, the

following relationship (Klein 1987) is obtained,

Draft = Input linear density/ Output linear density

2.2 THE BASIC MECHANISM OF ROLLER DRAFTING

Drafting is the process of elongating a strand of fibres with the

intention of orienting the fibres in the direction of the strand and reducing the

linear density (Lord 1993). In the roller drafting system, the strand is passed

through a series of set of rollers, each successive set rotating at a surface velocity

greater than that of the pervious set. Each set of rollers grips the strand. the top

rollers weighted by dead weights, springs and pneumatic pressure systems to

press the top and bottom rollers together and to prevent slippage between the

fibre and the rollers. If the surface speed of the back rollers is v2 and that of the

front rollers is v1, then in unit time the back rollers take up a length v2 of strand

and in the same time the front rollers deliver a lengthv1. The length of the strand

is therefore increased in the ratio of v1 to v2 (Foster 1958).

Mechanical draft (D) = v1/v2

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Figure 2.1 Draft through a roller drafting

2.2.1 Ideal Drafting

The theory of ideal drafting is explained using Figure 2.2. The front roller

speed is twice that of the back rollers, so that the draft is two. In Figure 2.1, the

leading fibre A has just reached the front nip, and the fibre is therefore moving at

the speed of the front rollers. All the other fibres are either held in the back nip

or belong to the sliver being fed to the back rollers, and are therefore moving at

back roller speed. The diagram2 shows the position, a moment later, when fibre

B has moved one unit, has just been released by the back nip and has reached the

front nip. In the meantime fibre A, which has been moving at twice the speed of

B, has moved two units, and consequently the leading ends of the fibres A and B

are two units apart, instead of one unit as they were in the original sliver. Since

they are now both moving at the speed, they retain this spacing. The drafting

proceeds according to diagrams3 and 4, and the state of affairs after it has

continued for some time is shown in bottom diagram, with the drafted sliver to

the right of the front roller nip. The effect of the drafting is to space the left hand

ends of the fibre twice as far apart as they were in original sliver (Foster 1958).

The fibre moves at the velocity of back roller and continue till it

reaches the front roller nip then it pickup the speed of front roller.

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Figure 2.2 Ideal Drafting

2.2.2 Real Drafting

Real drafting differs from the perfect drafting described in the above

section in four ways (Foster 1958):

1. The fibres in the sliver or roving are not all the same length and are not

perfectly straight and parallel.

2. The roller nips do not always remain in fixed positions.

3. The rollers do not always run at steady speeds.

4. The rollers sometimes slip, so that the fibres under them do not move at

the speed of the rollers.

First is largely a property of the raw material and cause a wave like

irregularity called the drafting wave. When all the fibres are not of the same

length, the shorter ones are released by the back rollers before their front ends

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have reached the front rollers. These fibres tend to come out of the front rollers

in clots and so cause alternate thick and thin places in the drafted material. This

is called the drafting wave. The other three differences are due to mechanical

faults in the machinery.

2.3 IRREULARITIES DUE TO ROLLER DRAFTING

The quality of the spun yarn is characterised in terms of its evenness,

hairiness, tensile strength, elongation at break and faults. The evenness of the

yarn depends on the quality of preparatory material and irregularities introduced

at the ring frame. The irregularities may be introduced due to drafting wave,

roller slip movement and roller speed variations during drafting at the ring

frame.

2.3.1 Formation of Drafting Wave

If the roller setting is less than the length of the longer fibres, then the

fibres would be held at the same time by both back and front rollers which

would cause the front rollers to deliver small undrafted tufts of fibres. This effect

is known as spewing. Some fibres are also broken when the setting is too close.

All cottons contain fibres of different lengths and when the setting is

wide enough to avoid spewing, there are many fibres shorter in length than the

roller setting. The rear ends of these fibres are released by the back rollers before

their front ends have reached the front rollers. Consider one of the shorter fibres,

at the moment of its release by the back rollers. It is moving at the speed of the

back rollers and it’s partly surrounded by fibres which are held by the back

rollers, and which by friction against it tend to keep moving at back roller speed.

It is not gripped by either line of rollers, and is not therefore controlled directly

by them, but is carried forward solely by other fibres. Such a fibre is called a

floating fibre (Foster 1958).

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The floating fibres are, however, also in contact with other fibres

which are held by the front rollers and therefore dragged forward at front rollers,

and therefore tend to be dragged forward at front roller speed. For some of the

floating fibres, the drag exerted by the fast moving fibres exceeds that of slow

moving fibres, and they are pulled forward at front roller speed, and reach the

front rollers earlier than they should. This means that the number of fibres held

by the front rollers becomes greater than it would otherwise have been, so that a

thick place is formed under the front rollers. This succession of thick and thin

places is called the drafting waves.

2.3.2 Drafting Waves in Staple Fibre Yarn

In drafting staple fibres, the effective length of the fibre is an

important factor. Fibres supplied to a drafting system in normal practice vary in

length, fineness, crimp, finish and natural fibres being more variable than man-

made ones. Each of the variables mentioned alters the force that can be

transmitted by a fibre under these circumstances.

Irregular flow of the fibres changes the position of fibres relative to their

expected positions after drafting and creates unwanted variations in linear

density. With natural fibres, length is quite variable and the error is distributed.

The variation produces a ‘hill’ typical of this type of error, as shown in the actual

distribution in Fig. 2.3(a)and it is easily distinguished from a mechanical error

(Fig. 2.3(b)), which shows up as spikes like those at A and B (also there are less

easily distinguished peaks such as shown at C). The first of these types is often

called a ‘drafting wave’ and it is a fibres borne error. Fig. 2.3(a) shows two hills,

which indicates that two different drafting waves were created, one from a

rearward drafting zone and a large wave from a forward one. Since a difference

between the roll setting and effective fibre length is an important factor,

variations in fibre length can produce undesirable results, which show up as

blotchiness or streakiness in the fabrics.

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Not only is control of fibre length an important matter, but so is the

maintenance of correct roller settings. There are thousands of drafting zones in a

normal mill, and the aim is to set all of them to standard values appropriate to

the material being processed. Changing the ratch setting (i.e. the distance

between the nip lines of successive pairs of rolls in a drafting system) throughout

a mill is a major operation. Settings of all the drafting systems must be

maintained within close tolerances to the values standard in the particular mill.

Also the fibre purchasing agent must seek to acquire fibres within a standard

range of fibre length distributions. These are management and maintenance

problems; changes are not made lightly. Once set up, the ratch settings are

usually maintained until the next maintenance period. Thus, it is useful to

constrain the variability in the fibre population by blending and careful stock

control.

Figure2.3 Actual and theoretical variance distributions in a draw frame sliver

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2.4 CONTROL OF FIBRES WITH MECHANICAL RESTRAINT

The fibre movement should be constrained to minimize drafting

waves, the distribution would approach the theoretical value as shown in Fig.

2.3(a). In the example, to the left of the picture there is a significant hill and in

the centre there is a minor one. The large one came from the main draft zone and

the small one from the break draft (back) zone or an earlier process. Obviously

there must be concern about the irregular flow through the front draft zone

where the draft ratio is the largest.

2.4.1 Pressure Bar

Fortunately, there are some design features in modern machines that

help to restrain the unwanted relative fibre movement. These features work by

adding frictional forces, which tend to keep floating fibres at a speed at or near

that of the back rolls. These floating fibres within the draft zone are not gripped

by either nip. In draw frame design it is normal to incorporate a pressure bar or

some other device to restrain these floating fibres.

2.4.2 Apron

In a ring frame or roving frame, aprons are commonly used to fulfil a

similar function (Fig.2.4 (b)). Aprons are pairs of relatively wide flexible bands.

They are pressed together sufficiently to restrain most fibres so that they move at

the apron speed until the leading ends of the fibres are trapped by the delivery

rolls. The linear speed is usually close to the surface speed of the back roll.

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Figure 2.4 Fibre control in drafting system

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2.5 CONTROLLING FORCE AT DOUBLE APRON ZONE

Figure2.5 Floating fibre movement in apron roller drafting

In the figure, f is the fibre gripped by the front roller nip moving at

the velocity of V1 (fast moving front fibre), b the fibre gripped by the back roller

nip moving at the velocity of V2(slow moving back fibre), and s is the floating

short fibre, neither griped by the front nor by the middle roller nip and move

with the velocity of either V1 or V2 based on the frictional resistance between

short fibre & front fibre, and short fibre & back fibre(Subramanian and Peer

Mohamed 2006).

During normal drafting of fibres in an apron zone of a roller drafting

system the force equation, assuming uniform acceleration, is given below:

Pf = Ffb+ Ffa+ Mf (V1-V2)]/∆t

Pf = Ffb+ Ffa+ Mf V1[1-(1/D1)]/∆t ……………… (2.1)

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Where Pf is the pulling force exerted by front roller nip; F fb, the

frictional resistance between fast moving front fibre and slow moving back fibre;

Ffaisthe frictional resistance between fast moving front fibre and slow moving

aprons; Mf is the mass of fast moving front fibre; V1is the velocity of front roller;

V2 is the velocity of middle roller; ∆t is the time for acceleration; and D1 is the

draft in the apron zone.

Lower value of Pf compared to RHS of Eq. (2.1) results in roller slip

and leaves the strand undrafted.

2.5.1 Condition for Drafting Wave to Occur

The short fibres will be accelerated before they reach the front roller

nip and the drafting wave will be created when

Fsf≥ Fsb+ Fsa+ MsV1 [1-(1/D1)]/∆t ...……………..(2.2)

Where Fsf is the frictional resistance between floating short fibre and

fast moving front fibre; Fsb, the frictional resistance between floating short fibre

and slow moving back fibre; Fsa, the frictional resistance between floating short

fibre and aprons, i.e. the controlling force exerted by the aprons over the floating

short fibres and ms, the mass of floating short fibre.

2.5.2 Condition to Avoid Formation of Drafting Waves

The Eq. (2.2) shows that the formation of drafting wave can be

avoided by (i) exerting controlling force over the floating fibres using aprons and

(ii) increasing the inertial force of drafting fibres. From Eq. (2.2), it is further

clear that either Fsa and Fsb have to be increased and/ or Fsf has to be reduced in

order to avoid formation of drafting wave for a given draft and front roller

speed. The controlling force exerted by the aprons on the floating short fibre F sa

can be increased by reducing the gap between the top and bottom aprons, i.e. by

reducing the spacer thickness. This reduction in gap between the aprons will, on

the other hand, increase the value of Ffa manifold in Eq.(2.1) Under extreme

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conditions, the fall outs of the reduction of gap between the aprons are given

below.

If Pf is less than the RHS value of the Eq. (2.1), it will result in roller

slip and undrafting. If Pf is still greater than the RHS value of the Eq. (2.1), it

would cause increased tensioning of the drafting fibres and may lead to fibre

breakage. Moreover, to keep Pf at greater level, the loading on the front roller

has to be increased at the cost of life of cots and other rotating element.

The yarn uniformity increases due to the reduction in gap between the

aprons up to a certain level. Beyond that level, reduction in spacer thickness

leads to deterioration instead of improvement in yarn quality. Hence to get better

control of floating fibres without undrafting, the controlling force Fsa has to be

increased without causing manifold increase of Ffa and it can be achieved by

suitably distributing the controlling force over different regions of main draft

zone according to the requirement.

2.6 SPACERS

The top aprons are forced by spring pressure against the lower aprons.

The intensity of fibre clamping and thus fibre guidance depends upon this

pressure and also upon the distance between the two aprons. The pressing effect

should be considerable, but not too high, otherwise it is impossible to achieve

controlled drawing of fibres out of the clamped strand. The arrangement must

also permit precise adaption of the minimum distance to the fibre volume. In

order to maintain this closely defined minimum distance between the aprons, the

“distance pieces” of variable height are interchangeably inserted between the

nose bar of the lower apron and the cradle edge of the top apron, i.e. at exit

opening M.

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Figure 2.6 Spacer

The distance pieces are known by various names, such as spacers,

distance clips, cradle spacers. The correct distance piece to use can be

determined within a broad range from tables provided by the manufactures, but

fine settings have to be established by experiment.

2.6.1 Pin Spacer

Rieter introduced a one piece pin spacer which consists of two

smooth metallic pins attached on either sides of the existing spacer. The spacer

provides the gap between the bottom and top aprons and the pin deflects the path

of the material between the exit of apron and front roller nip (Rekipedia).

Suessen introduced a pin spacer called Pin NT with design different from earlier

one, in which it is possible to optimize yarn quality values and spinning stability

independently of each other. In contrast to the one-piece Pin Spacer, the Pin NT

profile has been changed to ensure that the fibres are forced to run under the pin

and not by mistake over the same. This improves the operational reliability

drastically. A leather ring pin was also tried in the main draft zone to control the

movement of floating fibres (Yang Xiaohong 2006).

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2.7 CONTROLLING FORCE EXERTED BY PIN

In the existing double apron drafting arrangement, the aprons cannot

be extended up to the nip of the front roller due to design constraint. Hence, the

controlling force is exerted on the fibres up to a distance of 16-18 mm from the

front roller nip. In the case of pin spacer, the rods extended from the spacer

deflect the path of the drafting material and it increases the friction between the

fibres. The phenomenon can be explained using the following force equation

2.3.

Fsf≤ Fsb+ Fsa+ MsV1[1-(1/D1)]/∆t + Fps ...……………..(2.3)

Where Fsf is the frictional resistance between floating short fibre and

fast moving front fibre; Fsb, the frictional resistance between floating short fibre

and slow moving back fibre; Fsa, the frictional resistance between floating short

fibre and aprons, Fps the frictional resistance between floating short fibre and

pin., i.e. the controlling force exerted by the pin over the floating short fibres and

ms, the mass of floating short fibre.

Figure2.7Controlling force exerted by pin

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The RHS of the equation is increased by the introduction of pin

spacer, which would help in avoiding of formation of drafting wave. However,

as discussed earlier in section 2.5, the above increase may cause undrafting of

fibres. Hence, optimization is required such that the formation of drafting wave

is reduced without causing undrafting of fibres.

2.8 NOVEL TECHNIQUE FOR IMPROVING YARN QUALITY

The condensed yarn production in the ring spinning system, the

conventional ring spinning system having SKF PK 225 drafting system with 3/3

drafting arrangements (Fig. 2.8) was modified.

Figure 2.8 Figure 2.9

Fig.2.8- Existing drafting system arrangements in conventional

ring spinning machine and Fig.2.9- Modified drafting system with mingling

chamber arrangement

Mingling chamber with pneumatic pipeline was designed for

individual spindle. The mingling chamber was made out of aluminium bar with

two holes on either side and was fitted with pneumatic pipe line connections.

The mingling chamber ensures the condensation of drafted fibre strand by means

of intertwining the jet of compressed air in anticlockwise direction.

The air pressure of pneumatic pipe line was controlled with the air

regulator.

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A pressure of 4.5 bar was chosen after several trials for trouble free

processing. Figures 2.10 and 2.11 show the pneumatic pipe arrangement with

mingling chamber attached in the front drafting zone (R. Murugan &

C.Vigneswaran 2011).

Figure 2.10 Figure 2.11

Fig.2.10- Pneumatic pipe line arrangement with mingling

chamber and Fig.2.11- Pneumatic pipe with mingling chamber attachments

in drafting system.

2.9 YARN PROPERTIES DEVELOPMENTS WITH DIAGONALLY

SLOTTED ROLLER

In this spinning method, the width of the main twist triangle is the same

as conventional ring spinning system, To prevent the end fibres from escaping

from the main strand body, a diagonally slotted roller (DSR) was designed with

various slot angles to positively lead the end of fibres in two sides of the twist

triangle to the yarn body and reduce the protrusion of the fibres from the body of

the yarn. The key element of the newly developed spinning system is a pair of

diagonally slotted rollers fixed on the conventional ring spinning frame. The

slots in the DSR roller are not in parallel with the strand and runs with an angle

to the strand and runs with an angle to the strand which is called slot angle. The

slots are made of symmetrically made up from two edges toward the roller to

provide a convergent path for the fibres in the strand at the twist triangle. In DSR

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roller, the slots separate each strand into multiple sub strands and positively lead

them into the entangled yarn structure.

In the spinning system using a DSR roller, a drafted strand enters the

nip of a DSR roller which has a lot of small grooves that positively lead the

drafted fibres into the main twist triangle from both sides and prevent them from

escaping from entangled yarn structure body giving a lower hairiness to the yarn.

The angle of slots on the DSR roller is one of the most important parameters

which properly lead the fibre ends into the entangled yarn structure (Ali Akabar

Merati, Aliasghar Alamdar- Yazdi & Alireza Farokhnia 2013).

Figure 2.12- A schematic of DSR roller (a) side view, (b) front

view, (c) top view and (d) three dimensional view [α= angle of slots relative

to direction of roller]

Each DSR roller is positioned just below and parallel to its

corresponding top front draft roller. The DSR rollers are rotated, being in contact

with the bottom front draft rollers. Figure 1 shows a schematic of a DSR roller

and its various views. As Fig 2.12 shows, the surface of the DSR rollers is made

up of series of diagonal slots that are symmetrically made up from two edges

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towards roller centre to provide a convergent path for the fibres in the strand at

the twist triangle. In this figure, α is the angle of slots having the values 15◦, 30

◦and 40◦(Fig. 2.13).

Figure 2.13 - A schematic of slots on the surface of DSR rollers (a)

15◦, (b) 30◦ and (c) 45◦

2.10 FRICTION

Friction is a surface phenomenon which is governed by surface

properties of the material. This entails a comprehensive understanding of a

material’s morphology and its chemical and physical structural properties.

Morphology, i.e. surface architecture, should influence the nature of contact

between two bodies brought against each other. Textile to textile and textile to

other surface friction plays an important role in governing the behaviour of

textiles in processing, wear and use. Whether the material is a fabric with

complex structure or a single fibre, the contact in friction process can be

considered to occur between two fibres or between a fibre and another surface.

The three major modes of contact used during measurements are the

point, line and the area. These correlate with the nature of contacts found in

textiles during processing and use. Point contacts exist between metallic wires

and fibres during carding, between needles and fibres during sewing in apparel

manufacturing, and between the fibres at the crossover points during stretching

and bending of woven and knitted fabrics. It should be noted, however, that

since textiles are soft and deformable, the seemingly point contact actually

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involves a small area of some shape. The line type of contact can be expected to

exist between fibres during drafting in yarn formation, and during stretching and

bending of yarns and yarn based products in processing and use. Just as point

contact involves contact over some area, so does the line contact. Examples of

area contact, i.e. contacts over significant geometrical areas, include those

between two items of clothing worn by an individual, clothing and skin, clothing

and upholstery, clothing and bed linen, stocking and shoe liner, between

footwear and flooring including carpet (Gupta 2008).

2.10.1 The Basic Law of Friction

Friction is the tangential force that opposes relative movement when

one body slides over another. The two basic laws of friction as follows:

The friction force, F, is propositional to the normal force, N. This implies

that the ratio, F/N, called the coefficient of friction and commonly

denoted by the symbol µ, is a constant for a given pair of bodies.

The frictional force is independent of the geometric area of contact.

F α N

µ= F/N

Here, F independent of the geometric area of the contact

2.10.2 Line Contact

In Line contact, the load is distributed over the material for some

distance. Due to that more amount of force acting on the material. So it requires

more amount of pulling force at delivery point.

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Figure2.14 Line contact

2.10.3 Point Contact

For point contact between two filament yarns, the method adopted

was the one based on capstan geometry. The angle of wrap and diameters of the

fibres being small, the contacts are also small and assumed to be a point. The

value of µ is calculated with the well known capstan equation, given below, in

which θ is the angle of wrap:

T2/T1=eµθ

Figure 2.15 Point contact

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2.11 VARIOUS TYPES OF FRICTION

Figure gives schematics of the principle involved in the measurement

of friction by various techniques. Its examination will indicate that the methods

can be classified and discussed in three different groups:

1. The fundamental study methods based on Amonton’s Law in which the

motion of one body with respect to the other is rectilinear. This group

accounts for the largest number of apparatuses developed and used is for

much of the fundamental studies of the property.

2. The capstan method in which one material, usually a textile, passes over a

length of the curved surface of a cylindrical body, usually a non- textile

material.

3. The twist method, in which two materials, yarn or filaments are twisted

together. Although in this method, the path of each body is helical, the

actual contact is a line parallel to the axis of each material.

Figure 2.16Various types of friction

2.11.1 The Capstan Method

When the textile material is pulled over a cylindrical body, a frictional force

develops between material and cylinder surface due to normal force

generated tensions T0 and T1exerted on the ends.

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Figure 2.17 Capstan method

∑Fx = 0

= T cos (∆θ/2) +µ. ∆N – (T + ∆T).cos (∆θ/2)

∑FY = 0

= ∆N - (T + ∆T) sin (∆θ/2) - T sin (∆θ/2)

For small ∆θ, sin (∆θ/2) = (∆θ/2)

cos (∆θ/2)= 1

Substituting:

∑Fx→ ∆T= µ. ∆N

∑FY → ∆N=T. ∆θ

Substituting:

∆N→∆T= µ.T.∆θ

∫T 1

T 2dtt = µ∫

0

θ

dθ

T2/T1 = eµθ