Emam Shad Ahmed�Instructor�Department Textile Engineering �Daffodil Polytechnic Institute.
Objectives:
Drawing is the operation by which slivers are blended, doubled and leveled.
In short staple spinning the term is only applied to the process at a draw frame.
In drawing slivers are elongated when passing through a group of pair rollers, each pair is moving faster than previous one.
This permits drawing and elongating of several slivers to make them strong and uniform.
Objectives:
Necessity:
C, card;
GI, drawframe
GIl, drawframe II
H, roving frame
R, ring spinning machine
Fig. Passage of material on Drawframe m/c
Main sections of Drawframe:
Creel Section
Coiler Section
Drafting Section
Sliver Condensing Section
Questions:
Objectives:
The drafting arrangement:
Requirements of drafting arrangement:
drafting operation.
Influences on the draft
Cont…
Elements of drafting arrangement:
Bottom roller:
Bottom rollers are made of steel and are mounted in roller strands or in frames by means of needle or ball bearings.
They are positively driven from main gear transmission.
On order to improve their ability to carry the fibers along, they are formed with flutes of one of the following types.
- Axial flutes
The diameter of the bottom roller lie between 20-90mm, but normally lies between
25-50mm.
The distance between drafting roller is adjustable and depends on fiber length.
As shown in fig.
Top roller:
The top rollers are not positively driven; they are mounted by the
means of ball bearings.
These top rollers are made of steel and they are covered with thick coating of synthetic rubber.
An important matter avoid this coating is its hardness.
Soft rubbers coating can grip the fiber strand more perfectly than that of hard one, but soft coating of rubber wear out more quickly.
Hardness of rubber coating on top rollers in specified in terms of degree shore.
Soft coating -> 60-70° shore Medium coating -> 70-90° shore Hard coating-> Above 90° shore
Top roller pressure:
To clamp the fibers, the top rollers must be forced at high
pressure towards the bottom roller.
This pressure can be generated by:
(The Rieter company)
(The Saco Lowell company)
Questions:
Objectives:
Forms of drafting Arrangement:
In extreme case the break draft and total draft lies between 1.05 –
2.5 and 3.5 – 12 respectively, but usually they are 1.25 – 1.8 and 4-8 respectively.
The degree of reduction in the linear density of the fibre material
is called as the draft.
Attenuation defined as: Attenuation = Draft x [100/100-p] Where, p: elimination or loss of fiber into waste
Draft can be expressed in two forms:
Actual Draft = Linear Density of Input / Linear Density of output
Mechanical Draft = Linear Speed of Output / Linear Speed of Input
If the percentage waste removal during the carding machine is
(W) then the actual and mechanical draft are related to each other as:
Mechanical Draft = Actual Draft (1 – W/100)
The drawing operations are primarily concerned with converting card slivers into drawn slivers in which fibers have been straightened and aligned with a high degree of parallelism.
It is essential that the short-, medium-, and long-term irregularities of drawn slivers are as low as possible, with no periodic variation present.
If the roller are rotated that their peripheral speed in the through flow direction increases from roller pair to roller pair, then the drawing apart of the fiber, i.e. draft.
This is defined as the ratio of the delivery length [LD] to feed length [LF], or the ratio of the peripheral speeds:
V= LD / LF = VD / VF ……………….V= peripheral speed of cylinder C
The drafting arrangement has three drafting zone: Break draft zone [S1] : DB = V2 / V1 Middle draft zone [S2] : DMiddle = V3 / V2 Main draft zone [S3] : DM = V4 / V3
The total draft is always product of the individual drafts and not
sum:
Dtotal = DB x DMiddle x DM = V4 / V1
Fig. Drafting Arrangement
What is drafting force?
Drawing apart of the fibres is effected by fibres being carried along with the roller surfaces.
For this to occur, the fibres must take on the peripheral speed of
the rollers.
This transfer of the roller speed to the fibres represents one of the problems of drafting operations.
The transfer can be effected only by friction, but the fibre strand is fairly thick and only its outer layers have contact with the rollers, and furthermore various, non-constant forces act on the fibres.
Drafting takes place in three operating stages:
Drafting force is heavily dependent upon:
Drafting force(DF) influenced by different parameters during drafting:
- Nip contact
- Type of drafting system
Fig. Draft through a roller drafting .
Fig. Forces acting on a fibre (f) during drafting
Fig. Drafting Force Diagram
Where,
F - Magnitude of drafting force
D - Magnitude of draft
m to n – Highest impact of draft w.r.t. drafting force
Questions:
Objectives:
Fibres arriving for processing exhibit very considerable length variations. In a drafting field, they are therefore found in two conditions:
- Guided (a, b, c)
In guidance of floating fibres is favorably influenced by:
Fig. Guided and floating fibres in
drafting field
The top rollers must be pressed against the bottom rollers with
considerable pressure to ensure that the fibres are transported.
This pressure is not only effective in the vertical direction, but spreads through the fibre stock also in the horizontal direction.
The compression of the fibres, and thus the inter-fibre friction, is transmitted into the drafting field.
The intensity declines, however with increasing distance from the nip line and finally reduces to zero.
The friction field is an extremely important medium of fibre guidance.
It keeps the disturbing effect of
drafting within tolerable bounds.
Fig. The friction field created in the
fibre by applied pressure
Friction field affects by:
A large number of fibres called as 'back beard fibres' are held by the back rollers moving slowly towards front rollers at a velocity VR
Relatively small number of fibres called as 'front beard fibres' held by front rollers are moving faster at a velocity VF .
At a given instant, the back roller releases some of the back beard
fibres.
Since, FF - Tensile force FR - Friction force FD - Drafting force
For the fibre to be just accelerated,
FF ≥ FR
Fig. Frictional Force during Drafting
This force FF is the drafting force for a single fibre. The total
drafting force is,
FD = FF
Once the fibre is accelerated to the front roller velocity, the nature of friction between its leading portion and neighboring front beard fibres will be static one.
While their friction between trailing end and back beard fibres is a dynamic one, slowly reducing due to reduced
area of contact, as the fibres progress forward.
This dynamic friction force at trailing end is less than FR.
Before the fibre is accelerated, fibre
straightening followed by elongation takes place.
This is illustrated in neighboring Fig.
Fig. Stick-slip phenomenon
Where,
n is the number of fibres being accelerated to the front roller velocity at given point of time.
Questions:
Objectives:
Figure shows the general frictional behavior of liquid-lubricated textile yarns relating the frictional coefficient with yarn speed, lubricant viscosity, and pressure between yarn and guide.
The dashed line represent the contribution of the boundary and
hydrodynamic components respectively.
The frictional behavior seen in practice is the solid line, which is the result of the boundary and hydrodynamic contribution to the friction.
The semi-boundary region represents the transition between boundary and hydrodynamic frictions.
It is the region of minimum friction for the system.
Fig. General frictional behavior of yarn
This occurs between draft range of 1 to 2. This drafting range is
called "Critical drafting range,"
For cotton sliver, the critical drafting region lies somewhere between 1.15 and 1.4, and for cotton roving (on the ring spinning
machine) between 1.3 and 1.7.
For synthetic fibres, for which the stick-slip effect is usually more stro
-ngly marked, the range lies somewhat
higher, depending upon delustring, spin finish, etc.
Operating in the critical drafting region can be risky.
Fig. Drafting force diagram for Stick-slip zone
The drafting arrangement has two drafting zone:
Break draft zone [S1] : DB = V2 / V1 Main draft zone [S2] : DM = V3 / V2
The total draft is always product of the individual drafts and not
sum:
Dtotal = DB x DM = V3 / V1
Fig. Drafting Arrangement
The task of the break draft in the first draft field is to straightened and extended the fibres to such degree that the main draft can immediately cause fibre movements.
The main draft must be adapted to the drafting conditions, mainly the fibre mass in the drafting field and the arrangement of the fibres in the strand.
The draft can be increased with increasing fineness of the intermediate product and also with increasing parallelisation of the fibres.
In addition to the reduction in diameter, draft causes:
all of which represent important operations for spinning
Perfect / Ideal Drafting | Real / Actual Drafting |
- All fibers in sliver are straight and having equal length. | - Not straight & equal length. |
- Perfect orientation of fiber along the sliver axis. | - Not perfectly orientated. |
- Distance between consecutive fiber ends is uniform throughout the sliver. | -Not constant. |
- All fiber in sliver are parallel to the sliver axis. | - Not parallel |
- Speed of the fiber = speed of the back roller until gripped by front roller. | - Not |
- Arrangement of the fiber, variation, front end fiber density of the sliver is uniform w.r.t. draft through out the sliver. | - Not |
Questions:
Objectives:
Drafting irregularities are generated in the roller drafting due to the lack of complete control on the fiber movement in the drafting zone.
Some of these irregularities are in form of periodic thickness variations, which are recognised by their amplitude and wave-length.
Classification of drafting irregularity variation:
Sliver Variation | Wavelength Band | Some possible causes |
Ultra short term | 10 cm | Fibre distribution, Mechanical vibration. |
Short term | 10cm to 10m | Drafting waves, Piecing waves. |
Medium term | 10m to 100m | Card, Broken sliver, Chute feed, Scutcher |
Long term | 100m to infinity | Blending, Opening, feeding, Chutes, etc. |
Factors Influencing Drafting Irregularity:
The factors determining zone drafting irregularity are;
The uniformity in the drafted strand is determined by:
One of the tasks of the draw frame is dust removal.
Each roller of the arrangement has an associated cleaning device so that fly and fibers tending to adhere to the rollers can also be carried away.
The air draw away is passed via tube directly into the exhaust ducts of the air-conditioning system, or to filters within the machine.
The accumulation of the fibrous mass on the surface of the rollers causes unevenness in drafting and
sometimes also causes sliver
breakages causing the machine to stop.
Fig. Suction clearer for drafting arrangement
Coiling zone of the draw frame includes following components:
To avoid disintegration of the web, it must be collected together in a converging tube (1), immediately after the delivery roller and guided to the sliver trumpet.
The bore of this sliver trumpet (2) must be adapted precisely to
the sliver volume ( = sliver hank).
The sliver trumpet is therefore replaceable.
The bore ( in mm) should be determined approximately by the
relation.
d = k x √(ktex)
Where,
k varies between 1.6
(for' finer slivers). to 1.9
(coarser slivers).
Fig. Delivery Assembly
1
2
The fibre web leaving the front pair of drafting rollers is passed through a converging tube and is guided to a specially designed condensing funnel called as the trumpet guide as shown below:
The degree of condensing at the trumpet guide is essential for providing a good fibre to fibre cohesion to hold them better in a sliver.
However if too much condensing is done then the drawn sliver develops thick places.
After condensing of fibres at the trumpet guide back into a sliver form, the sliver is passed through a pair of calendar rollers which does a further compressing of the fibre mass and ultimately deposits the drawn sliver into a sliver can.
Fig. Delivery Assembly
1
2
The drawn sliver coming out of the calendar rollers is passed through a coiler tube fixed on a coiler plate.
The coiler gears fixed on the coiler plate help to rotate the coiler tube so that sliver can be laid in the can in form of special coils.
The can rests on the rotating plate, with the rotation of the plate the can also rotates.
The rate of rotation of the can is kept slower than the rate of rotation of the coiler tube which helps in proper deposition of drawn sliver in a spiral arrangement.
Fig. Sliver Coiler
It is necessary to keep the sliver deposition rate slightly higher
than the sliver delivery so that blockage of the sliver in the
tube may be avoided.
However this difference should not be too large where false draft
may arise in the sliver.
Fig. Sliver Coiler
Modern high-performance draw frames are usually fitted with automatic can changers.
These reduce the burden on the personnel, enabling more machines to be allocated to one person reduce the necessity for attendance of the operative at the machine, and also increase efficiency.
They can be classified into:
Single-step changers give higher machine efficiency because full cans are replaced by empty ones at full speed i.e. without stopping the machine.
Multiple step changers give lower machine efficiency because machine must be brought to stop during the change.
Questions:
All diagrams covered in Unit-I & Unit-II have to draw in the
assignment book.
Objectives:
Faisal EI-Sharkawy et al have provided a simplified treatment to obtain the relationship between the drafting Force and the parameters like fibre length, number of fibre ,in the cross section, roller setting and draft.
Assumptions made in the development of the equations are:
1] The drafting force is dependent on the degree of contact between the beards under the front and back roller grips.
2] Fibres in the sliver are straight are parallel to the sliver axis and have the same length.
3] Fibre to fibre contact is constant.
4] Sliver are perfectly regular, with same number of fibre in cross section
subjected to draft D between two pairs of rollers,so that the number fibre
Consider a sliver with N fibres in its cross section on that is being
in the drafted sliver will be N/D.
Under steady-state condition, the (back beard)beard will be gripped by the back roller and another beard will be gripped by the front draft roller.
The taper of the front and back beards in the drafting zone will be
…..(1)…….(from x = eL to x = L)
given by the following equation
Y = (N\D) (x- eL)/L
and
Y' = N(1-x/L)
…..(2)…….(from x = 0 to x = L)
Where, Y= number of fibres in the cross-section of the front beard. Y' = number of fibres in the cross-section of the back beard. D = draft.
x = any position in front of the back roller.
L = fibre length.
Cont….
e = a dimensionless quantity that can take any value between zero and one; it can be considered as a measure of the degree of disconnection between the front and back beards.
The drafting force will be proportional to the number of contact points between the two fibre beards. i.e., proportional to the sum of the areas of the smaller triangle A1 and A2 in fig.
F α A1 + A2
It is easy to show from the above that:
A1 = NLD (1- e)² / 2 (D + 1)²
and
A2 = NL (1 - e)² / 2 (D + 1)²
Hence,
F α NL (1- e)² / (D + 1)
………(3)
Fig. Taper of back and front beard in d/f system
According to Equation (3), when the draft D increases, the
drafting force decreases.
However, for very low drafts, the drafting force increases first to a peak as the draft increases and then decreases as shown in fig.
D0 is the draft corresponding to the maximum force.
Up to this draft, there is no fibre slippage but simple straightening out of fibres, which results in the extension D0 -1 for the sliver.
This region is characterised by increasing contraction of the sliver
cross-section.
This is due to the removal of fibre crimp and alignment of the fibres.
Fig. Relation between D/F force (F)
and Draft (D)
As the fibre crimp is removed, the effective stiffness of each fibre increases sharply, and the sliver resistance to further deformation increases with it.
Meanwhile the Poisson contraction effect increases both the number of fibre to fibre contacts and the pressure at these contact points, thus increasing the sliver resistance to fibre slippage.
In order, allow for this extension, Equation 3 must be corrected so that it now becomes:
F α NL (D0 - e)² / (D - D0 +1) ……………………….(4)
When the sliver cross-section remains unaltered with the change of draft, drafting force is inversely proportional to the draft.
When the values of 1/D are sufficiently low, there is almost
proportionality between the force and the reciprocal of the draft.
For high values of l/D, i.e., as the draft approaches Do, there is a departure from linearity.
Equation (4) also allows a quantitative study to be made on drafting force as a function of the roller setting.
The reciprocal of the drafting force, 1/F, is plotted against the reciprocal of e (which is a quantity proportional to the roller setting) in Fig., where, as can be seen, there is a curvilinear relationship between these two quantities.
Fig. The reciprocal of the draft Vs
reciprocal of roller setting
Fibre crimp affects the geometrical arrangement of the fibre.
The more highly crimped fibre (in terms of crimp frequency and crimp amplitude) exhibit a large drafting force because it has more opportunity to interlock spatially, entangle and mesh with neighboring fibres.
Increased spatial interaction of crimped fibres also accounts for the larger lateral cohesion of the fibres in a crimped sliver compared to the ease of removing decrimped fibre from the surface of the sliver made of decrimped fibre.
Increased fibre crimp is analogous to increased surface roughness
with a result in improved fibre cohesiveness.
With an increasing crimp, friction force increases in the static and low speed regions
Questions:
Objectives:
The number of fibres in the sliver influences the level of the drafting force in two ways,
First, in the additive sense, in that more fibre in a cross-section of sliver means more fibres moving relative to one another and more locations of resistance to this motion; hence, a large drafting force required.
Second, as a result of bulk compressibility of the sliver, the effect of an increase of fibres obtained by vertically sandwiching input sliver is more than additive.
Because the increased sliver thickness means a relatively larger nip constriction and this results in more resistance to drafting caused by more fibre to fibre contact points as well as more frictional restraint at each contact.
The packing density (more compactness) of the sliver entering the drafting zone greatly affects the drafting force, in direct proportion.
Since relative fibre sliding is diminished in the compacted spot, this accordingly becomes a thick spot in the output sliver.
Common source of fluctuating packing density:
The drafting force increases rapidly with increase in compactness of the sliver.
The total drafting force is given by:
FD = { nZ NZ / (nZ + NZ)} FZ
Where,
z = a 1 cm section.
NZ = the number of fiber in section z gripped by the front roller. nZ + NZ = the total number of fibre in section z.
FZ = the force per unit length acting on a single fibre as it is
withdrawn from the sliver of the density of section z.
Sliver density ρ, is the ratio of volume of fibre to the total volume occupied by the sliver .
If fibre to fibre friction is increased as a result of very high top roller pressure, the friction force per fibre increases giving a disproportionately high drafting force.
When the roller setting is decreased, the sliver becomes more compact and drafting force increases accordingly.
Roller lapping tendency increases with:
Higher crimp level coupled with suitable spin finish formulation and percentage add-on will reduce the lapping tendency.
This will result in abnormal increase in fibre to fibre friction, hindering the drafting performance.
For the synthetics and fibres with high crimp level, these draft should kept at lower levels compared to cotton.
Synthetic fibres are crimped with the permanent set. Crimped fibre configuration improves the fibre cohesion
Higher creel and web draft for the synthetics creates localised drafts at weaker places, partially decrimping the fibres, increasing the built up the internal stresses.
With high creel draft for synthetic fibres may try to relieve the stress during drafting resulting in disturbances of neighboring fibres, in this high elastic recovery of the synthetic fibre takes place.
Questions:
depend.
Objectives:
Monitoring systems can be distinguished according to whether they monitor
The first group, devices that detect faults but do not correct them, includes:
It is offered at present only by the Zinser company as MECATROL.
The so-called "toothed roller leveller" consists of a toothed roller pair (1) and a fluted/pressure roller pair (2) forming a small drafting device in front of the actual drafting arrangement.
A fault in an individual sliver can be reduced to about 40-50%.
Fig. MECATROL of Zinser
What is autoleveller?
Need of autoleveller
Object of autoleveller
To maintain consistent hank of sliver.
Principle: Adjust the Draft continuosly, which will depend on thickness of material fed.
Types of autoleveller
Open Loop
Closed loop
Types of autoleveller
▪
▪
Open Loop | Closed Loop |
This system is effective for short, medium and to some extent long term variations | This system is effective for long term variations |
Lack of self monitering | Self monitering |
More popularly used | Rarely used |
Various parts of autoleveller
1
4
3
2
5
Important parameters for quality levelling
Levelling Intensity
Levelling action point
correction point should be known.
The total volume of all slivers is measured at the infeed and adjustment is effected with the appropriate time delay in the main drafting field.
Detection is usually carried out mechanically (rollers with
grooves, bores or steps) or by capacitive sensors.
Fig. Evener draw frames with open-loop control
In this system, the evenness of the delivered sliver is measured, not the infeed sliver as is the case with open loop control.
Nevertheless, the adjustment is still made in the main drafting
field.
Mechanical or pneumatic sensing devices are generally used.
Fig. Evener draw frames with
close-loop control
To avoid the disadvantages of both open- and closed-loop control principles, open-loop and closed-loop devices are combined into an integrated autolevelling system.
Capacitative sensing is generally used in the infeed, and
mechanical or pneumatic sensing in the delivery.
Fig. Evener draw frames with
open-loop control
If there is a sudden deviation from the set volume as the material passes through, a corresponding signal is sent to a regulating device that is to correct the fault.
Owing to the mass inertia of the system, compensation cannot be effected suddenly, but must be carried out by gradual adaptation.
A certain time elapses before the delivered sliver has returned to the set volume.
During this time, faulty sliver is still produced, although the
deviation is being steadily reduced.
The total length that departs from the set value is referred to as the correction length (I).
Fig. Correction Length
In closed-loop systems, the correction length is additionally increased by the dead-time.
In that case, it depends upon the dead time (II) and the correction time (III).
The correction length depends upon the system and the speed of operation, and therefore varies considerably.
Fig. Correction Length for close
loop system
Evening is performed exclusively by adjustment of the draft. 'Theoretically, there are two possibilities for such adjustment,
namely via the break draft and the main draft respectively.
However, the main draft is always used because it is larger, and
therefore finer adjustments are possible.
Also, use of the break draft would run the risk of entering the stick/slip zone.
Draft variation can also be carried out by adjusting the infeed or delivery speed.
Adjustment of the infeed speed is generally used, since then lower masses have to be accelerated and decelerated at lower speeds.
Additionally, the delivery speed, and hence the production rate remains constant.
It is nothing but the process control technique in the draw-frame.
Structure :
Integrated systems, comprehending the complete process, are finding steadily increasing application.
The Zellweger MILL DATA - System is an example. The complete installation (see Fig.) comprises:
decentralized process data systems are subordinated.
Zellweger SLIVERDATA:
This data system continually monitors:
The results are available on call. Production monitoring covers:
Quality monitoring relates to:
If settable tolerance levels are exceeded, alarm lamps light up and the
corresponding machine is stopped.
What is blending and Doubling?
Blending is the combining of different fibres together intimately to achieve a desired product characteristic.
Doubling is the combination of several slivers that are then attenuated by a draft equal in number to the slivers combined, thereby resulting in one sliver of a similar count.
Why for Blending ???
Raw materials used in the spinning mill are always inhomogeneous in their characteristics.
In part, this is inevitable owing to the different cultivation conditions of natural fibers and the different production conditions for manmade fibers.
Partly, it is deliberate in order to influence the end product and the process.
Blending is performed mainly in order to:
Every doubling gives simultaneous blending -including the 6-8 doublings at the drawframe.
This blending intensity is adequate for processing cotton. However, if cotton and synthetics are to be processed together,
operation of the normal drawframe will no longer be optimal.
This machine (see Fig.) has four preliminary drafting arrangements and one downstream drafting arrangement.
Each preliminary drafting arrangement processes a separate set of six slivers.
The webs produced in this way are brought together on a table and fed to the downstream
drafting arrangement. The sliver issuing from this point is coiled in cans.
Fig. 21 – Principle of the blending draw-frame
Three passages are almost always needed with normal draw frames. (blending draw frame and two subsequent draw frames), two passages suffice when a blending draw frame is used (one normal draw frame followed by one blending draw frame).
In addition to this advantage, and the improved intermixing, a further favorable aspect should be mentioned, namely that-each raw material
component can be processed in a drafting arrangement of its own.
The disadvantages are:
are not required).
Advantage
component can be used for better quality of output with less damage of the fibres.
Technical Data
Delivery speeds, meters per minute: up to 800 Production per delivery, kg per hour: up to 300 Deliveries per machine: 1 or 2
Doublings : 4 to 8
Maximum feed weight kg : 55 Delivery hank, ktex: 3 to 7
Waste, in %: 0.5 to 1