Download - Card
OBJECTS OF BLOWROOM
(1) Opening:
As we receive bales in compact form, (i.e. for reduction of cost during
transportation from ginning to spinning mills.) blow has to reduce tuft size
gradually. In blow room reduction in tuft size is about 0.1 mg.
(2) Cleaning:
It removes all type of impurities. Impurities involves – plant material,
foreign matters, seeds, seed coats, stem, leaf, soil, sand particles, packing
cloths, metal pieces (nails, strips). B.R. removes 40-70% impurities. As far as
B.R. cleaning is concern, we can say that, improved cleaning is achieved at the
cost of high fibre loss. Normally fibre represents 40-60% of BR waste.
Cf =
(3) Dust Removals:
Dust particles are very fine in size & are completely enclosed in the
flocks. Generally dust particles are held back during suction. During suction 64%
of dust is removed & more intensive as smaller the tufts.
(4) BLENDING:
Blending is essential for yarn production. A blending is possible at every
stage (i.e. transverse blending). For good blending at initial stage simultaneous
extraction of fibres from bales is essential.
(5) Even feed of material to CARD:
Old Blow Room feeds lap, which is produced by scutcher. So scutcher
should ensure very uniform weight. As far as Blow Room & carding is concerned
we can say that both are performing same functions at different levels or
intensity.
BR: Macro level opening and cleaning.
Card: Micro level opening and cleaning.
Carding is the most vital machine in staple yarn manufacturing. It
strongly influences process performance & yarn quality. Though concept was
invented in 1770 & has first commercial appearance in 1850. Since then concept
of carding has not undergone much of a change.
CARDING
Objects :-
(1) Individualization of fibre tufts to almost single fibre stage.(Fibre to fibre
Separation)
(2) Cleaning: Cleaning of fibre stock for production of fault free yarn.
Cleaning includes removal of…
(i) Seed coats/ large impurities.
(ii) Neps and immature fibres
(iii) Micro dust
(3) Mixing of fibres to average out variations in fibre characteristics – to
produce a yarn with uniform characteristics.
(4) Formation of random way of oriented fibres called sliver – to produce
an assembly which can be easily manipulated into yarn.
Basic requirements of Carding Process:-
In carding machine two basic actions are taking place between two wire
covered surfaces, these actions are
(i) Carding action
(ii) Stripping action
Carding action:
The separation of fibres from tuft held in between two wire surfaces is
called carding action. To happen this carding action there are following
conditions,
(1) There should be two very closely placed (0.3 mm or 20-30 times dia. of
fibre) wire covered surfaces.
(2) The interacting surfaces should be placed in such a way that wire points
on it should face each other, i.e. point to point arrangement
(3) Interacting surfaces should move either in opposite direction or in same
direction. If they move in same direction, the surface with fibres on it
must move at a higher velocity than other surface.
Above figure shows a carding action & forces acting on fibre tuft during carding…
α = Inclination of first rack with base
f = force acting on fibre tuft
K & P = force is resolved into two components
K= known as carding component, tries to press the fibres towards
the point of other surface.
P = as it acts towards the base of clothing shows fibre retention
capability of clothing.
K = f sin α ………. (i)
P = f sin β ………. (ii)
The fibre can only moves downwards along the wire point provided, P overcomes
the frictional resistance between wire & fibres i.e. µK. Where µ is coefficient of
friction between wire points & fibre. When fibre is slipping towards tip of wire
point. This is also an important condition for carding; otherwise the fibre will have
a tendency to roll between two clothed surfaces. i.e. when α =90
(Holding of fibre by either of surface will not happen.)
Hence for effective carding,
P > µ.K ………………… (iii)
From (i) & (ii) we can write,
F cos α > µ f sin α
Cos α/ sin α > µ
Cot α > µ ……………….(vi)
From above equation it is clear that as angle of wire indication increases µ goes
on reducing. The intensity of carding action can be manipulated for different fibre
by selecting suitable inclination i.e. α .
Stripping action:-
The process of fibre transfer from one surface to another surface is called
stripping. The necessary conditions are…..
(1) Two wire covered surfaces facing each other.
(2) The distance between them should be around 0.3mm or less. (Nearly 20
to 30 times the dia. of fibre)
(3) Inclination of both surfaces in such a way that point on one surface
should face back of wire from other surface. i.e. point to back
arrangement.
(4) Interacting surfaces should move either in same direction or opposite. If
they are moving in same direction, linear velocity of surface on which
fibre is to be transferred; should be higher than surface with fibre.
Force acting on fibre component shown in figure, P' & S are two resolved
components of fibre tension “f”. P' is acting perpendicular to back flank & S is
acting along back flank.
P' presses the fibre against the tooth where as stripping component S
tends to push fibre off the tooth. β is the angle of inclination of back flank. Then
P' & S' are
P' = f sin β
S' = f cos β
Stripping will happen only when…..
S > µ . P'
f cos β > µ. f sin β
cot β > µ
cot (α - ө) > µ
Ex.
Sripping action – between licker-in & cylinder.
Carding action – between cylinder and flats.
Sripping action – between cylinder & doffer.*
(* Wire points are facing each other i.e. point to point action, but wire point
density on doffer is more than cylinder and cylinder & doffer are set very close (5
thou). Also transfer is facilitated by release of air current.)
Carding machines which are available in the market, we can divide into
two broad groups as fallows…..
Roller and cleaner card
Revolving flat card
Roller & cleaner card used for fibres like wool, jute, flax & cotton waste.
Revolving card used for fibres like cotton and synthetic.
Revolving flats can be studied broadly under following heads…
(1) Passage of material through machine.
(2) Taker in region:
(i) Feed unit
(ii) Opening cum cleaning unit.
(3) Carding region.
(4) Condensing region.
(5) Coiling region.
Feed to Card:-
The product of card is sliver; requires uniform along its length. Sliver
quality has direct influence on yarn quality especially carded yarn. To make a
sliver uniform, it is important to ensure that feed to card is free from irregularity.
Since there exist a direct relation between sliver and yarn quality. The form in
which material is feed to the card is either lap or tufts. Based on these forms we
have two feeding systems…
1. Lap feed system.
2. Chute feed system.
Lap feed system:-
Advantages:-
1. Production of even lap in scutcher is relatively easy.
2. The system is flexible, i.e. different material can be processed on different
card.
Functions: -
1. To feed lap uniformly to the licker-in.
2. To tear apart lap into minute tufts without neither plucking nor causing any
damage to fibres.
3. To eliminate trash particles, short fibres etc.
Limitations:-
1. During lap formation compression of opened tufts takes place to form a
lap. Due to again compression the purpose of opening is defeated.
2. During joining of new lap with exhausting lap, some overlapping is
required which is a source of fault generation & cause for extra waste
also.
3. During transport and storage excess handling (additional workers),
deposition of dust & fibres on lap etc.
4. Additional storage pace required.
Chute feed:
To overcome the limitations of lap feed, a new feeding system was
developed. In chute feed material is pneumatically conveyed to card and fed in
opened tuft form. Thus process of gradual opening is continued and load licker-in
action and fibre reduces.
Basic elements of chute feed system:
Material conveying system:
To carry material from blow room to a group of card with the help of air.
Air pressure controlling system:
For smooth flow of material through material conveying system, air
pressure is required to maintain at constant pressure.
Feed mechanisms:
To feed material at uniform wt/length and width in the chute.
Delivery system:
To feed the material to the card.
A sensing mechanism at chute:
It keeps a certain amount of material as reserve and also to control the
feed of material to chute.
Limitations:
1. Not flexible: since the same material supplied by the blow room feeds a group
of card, simultaneously.
2. Change of mixing results in rework able waste created by running out.
3. Difficult to ensure even feed – photoelectric system cannot sense opened and
compact tuft.
4. Higher nep generation.
Basic concept:
From blow room tufts are transported pneumatically to the vertical
reserve boxes / chute attached to several cards. The chutes are filled to
predetermined height by tufts. The packing of tufts should be as uniform as
possible. The material is taken out from the bottom and fed to licker-in. To obtain
even feeding, the tufts should also be distributed evenly across the full width of
the chute. Since tufts are conveyed pneumatically first chute will fill first followed
by rest. Hence control is necessary to ensure uniform chute.
There are two basic types of continuous feed…
(1) Single piece chute
(2) Double piece chute
However the installation can be open or closed distribution type. In
open type feed duct ends after the last card. Where as in closed type the duct
goes beyond the last card and joins the distribution again.
Single piece chute system (Aero feed system):-
Various elements of aero feed are shown in fig. a condenser sucks the
material from blow room and delivers it to flock feeder by way of feeling trunk.
Flock feeder opens the tuft to desired size. A kirshner type beater is used to open
the tufts. In opening zone fresh material, i.e. blow room receipt and returned
material are mixed. The fan (2) blows these tufts into horizontally closed circuit
loop (4) situated above the card. A separating head (5) incorporated in the duct,
divert part of tufts from the air current into the vertical feed chutes above the
card. The feed chute (6) ensures a uniform supply of material over the full
working width of card. Flock meter (3) controls the flow of material.
Flock Feeder:
It consists of three elements…..
1. Beater
2. The filling & return trunks
3. Condenser:
The material is fed to kirshner beater by way of two-ridged roll &
two feed roll. A light barrier (4) regulates supply of material from blow room. A
return trunk consists of two chambers; one is feeding material to beater &
another is mixing material into fresh delivered material, a light barrier in return
trunk monitors the amount of return material.
Fan:
A air current generated by fan is centrifugal type. It delivers the air at the
rate of 1 to 1.3 cubic ‘m’/sec.
Separating head & Card feed chute:
A separating head is located above card feed chute. It has
adjustable nose & glass windows on both sides. A adjustable nose facilitates
change in cross section of distribution duct. The change in cross section of
duct retards the flow of material & under influence of
gravity assisted by part of air current being diverted downwards. The stream
of huts is deflected by the nose of accelerated again. After the last card,
surplus goes into return trunk.
The card feed is ‘U’ shaped metal sheet. The open end of trunk
is provided with glass wall. The material from the chute is
drawn off by a pair of feed rolls & delivered to card feed roller. A separation of
air & tubes is achieved by adjusting nose in separating head. A raising a nose
produces more separations & vice versa.
The feed wt/mt depends on excess static pressure in the
installation. The drop in pressure is linear i.e. pressure goes on decreasing
from first to last separator by 2 mm head of water. The wt/mt is adjusted by
changing distance between glass & rare wall of chute. The wt/mt is ranges
from 600 to700 grams.
Double piece chute:-
Basic features of chute feed:
1. Upper &lower chute separated by a feed roller & beater.
2. A pair of feed rollers is positioned at the end of the lower chute.
3. Have air escape holes & a pressure sensor fitted to control a preset
compacted volume of tuft in the chute.
Working principle:
The upper chute receives tufts from distribution ducting & the
transporting air is exhausted through the air escape holes. The feed roller and
beater remove the material at a slower rate, enabling, incoming tufts to build up
in this top chute. Increase in pressure is sensed by sensor & when desired
pressure is achieved feed is stopped. The opened tufts are dropped into the
lower chute. These tufts are fed to card by pair of feed roller, which feeds slower,
then intake. When pressure is built-up into lower chute, feeding to beater is
stopped.
Advantages & disadvantages of chute feed:
1. Continuous feed to card.
2. More uniform material feed across width & length.
3. Very fine opening of tuft can be achieved & thus reduces the excess
load on card.
4. Biggest disadvantage of chute feed is a very high generation of neps in
chute while cotton is being transferred from blow room to card.
History and developments:-
Carding has always been considered a very important process as far as
spinning good quality yarn. Carding is becoming more and more important with
introduction of modern spinning systems. To achieve a desired quality people
even today reducing carding production. A carding production can be increased
without affecting quality if card is well maintained. A development listed below
shows that card production rate is gone up considerable high from near 4kg/hr
production to 100kg/hr in a span of 45 year.
Development during last 45 years.
Years Process parameters production
1955 Lap feed 4kg/hr.
1965 Metallic wires. 22kg/hr.
1975 Cylinder speed 32kg/hr.
1985 Development of carding elements 42kg/hr.
1995 Better pre-carding 60kg/hr.
2000 Cylinder speed 600rpm 100kg/hr.
From above data it can be concluded that a significant development in carding
took place during last 20 years.
Feeding device:-
Feed plate and feed roller:
Function:-
1) Lap is held firmly, uniform firmness across width to avoid plucking.
2) Lap is presented to licker-in for gentle opening avoid fibre damage.
Feed roller is loaded with lever and weight lever and spring, pneumatic.
F1=friction force acting between lap sheet and feed roller.
F2=friction force acting between lap sheet and feed table.
µR=coeff. of friction between F.R.&lap.
µF= coeff. of friction between feed table and lap.
P= normal force acting.
F1=µR P & F2=µF P
For uniform feeding
F1 > F2
µR P > µF P
µR > µF
to ensure this condition feed roller is fluted & feed plate is highly polished.
Though this simple analysis show there is no influence of normal pressure, infect
if influence on µR. µR increases with increase in pressure apart from feeding lap
feed roller has to avoid fibre slippages during plucking by taker-in. The total
pressure applied=220kg. For better grip in case of heavy feed, wire covered feed
rolls also used. (DK 780 card)
The licker-in can perform its task most effectively if:-
1) Lap is held firmly with uniform firmness across its full width in order to avoid
plucking by licker-in .
2) Lap should be presented in such a way that opening action should be gentle to
avoid fibre damage.
A conventional feed device consists of feed roller & feed plate. The profile the
front portion of the feed plate decides the intensity of opening & some extent
possibility of plucking. The feed plate have curved surface followed by horizontal
plateau which then bevels off steeply towards licker-in. The dimensions of the
horizontal plateau and the guide surface has a strong influence on the quality and
of waste. A sharp nose gives good retention of the fibres and intensive opening.
On the other hand rounded nose results in poor retention and bad opening.
A length of guide surface is too short; they can escape action of licker-in. They
are scraped off mote knives and are lost in waste receiver. Long surface presses
the fibre into licker-in surfaces. This gives better take-up fibres at the cost of
reducing cleaning efficiency.
Geometry of feed plate:-
The profile of speed plate should be so designed, that the teeth of licker in
progressively penetrate the thick mat of fibres and comb it thoroughly over a large
length without damaging them.
Let a—length of horizontal plateau.
b—length of guiding surface
β—angle of inclination of guiding surface
to avoid fibre breakage , the distance between nip A and line of entry of taker-in
teeth into top layer of lap is equal to ¼ of staple length of fibre ,considering 0.5
coeff. of straightening .it means
ABCD=l/4 l=staple length .
a+b =lm lm =model length .
Now geometrically
ABCD= a+2πd/360 x (90°-β) +CD
d= thickness
a+2πd/360 x (90°-β)+CD=l/4
β=360/2πd x (a+πd/2+CD-l/4)
If CD≈0
β = 360/2πd x (a+πd/2-l/4)
for longer fibres we have to increase distance CD. (point of action).
Top & bottom layer point of action is at different levels.
New Design:-
With conventional design material is presented against the
direction of rotation of licker-in, which takes sharp turn over feed plate nose &
causes harsh action. Rieter has developed a feed system that enables
presentation of lap in direction of rotation of licker-in. Here feed roller is located
below feed plate which is present against it by spring pressure. Owing to
opposite direction of rotation (F.R & licker-in) at interacting point(b/a) lap moves
downward in direction of rotating of licker-in.
Taker-in:-
This is a cast roller with dia-9”(250mm). A saw tooth clothing is
applied to it. Beneath the rollers is grid elements/carding segments, above it is a
protective ceasing of sheet metals. Licker-in has to perform following tasks
To tear apart the lap into minute tufts without damaging fibres
To lead tufts over dirt eliminating (grid bars) parts under the roller.
To transfer the fibres to the cylinder surface.
Operation of the licker-in: -
A licker-in wire points passes through thick mass of fibres presented by
feed device at high speed (1000rpm,13m/s).This causes a draft of 1000 between
feed roller & licker-in giving 600,000 pts/sec. As a result the lap fringe is torn
away into minute pieces & carried forward by taker-in teeth.
Due to intensive treatment by taker-in we can observe that 50% of
fibres transferred on cylinder in the form of flocks/tufts &less than 50% in the
form of individual fibres .The intensity of opening by taker-in is governed by.
1. Feed material parameters:-
Lap liner density i.e thicker-in.
Degree of openers of tufts in lap.
Degree of orientation of fibres in lap.
2. Taker-in parameters
wire point density &angle of inclination(Acute angle)
3. Processing parameters
Distance between taker –in & feed plate
Rotational velocity
Martial throughput rate.
High performance cards requires alternative assemblies in order to be able to
deal with high material through-put with conventional assembly waste extraction
and opening intensity is likely to suffer due to reduction in degree of combing and
generation of air vortices near mote knives.
Degree of combing:-
It is defined as number of takerin teeth acting per fibre in lap.
C= no. of wire points /fibre
=No. of wire points passing through material per minute
No. of fibre feed per minute
= Z. n.Nf. l
V.Nx.107
Z: No. of wire points on lickerin.
n: rpm of lickerin.
Nx: linear density of lap Ktex.
V: surface speed of feed roller.
Wt. Of single fibre = Nf.l.10-6 gm. ------(1)
Nf: linear density of fibre.
l: fibre length.
Quantity of material feed per minute = V.Nx gm. -------(2)
No of fibres feed = V .Nx x 107
Nf .l
Opening intensity (f)
f=1/c= V.Nx.10 7 n.Z.Nf.l
Normally at lickerin C=0.3
Cylinder C=10-15
Air vortex:-
As licker-in revolves with very high speed it generates air current
around it, between feed plate and first mote knife and between first and second
mote knife. The generation of vortices are due to placement mote knife
perpendicular to the streamline of licker-in.
When plate is placed at right angle to motion of liquid as shown in fig ,a
boundary layers reaching to edges finds difficult to take sharp turn .due to strong
deceleration behind plate vertices are formed. This leads to drop in pressure
causing re-deposition of trash affecting degree of cleaning.
The degree of cleaning therefore can be improved by:-
Weakening strength of vortices
Preventing generation of vortices
Increasing intensity of opening.
Opening intensity /combing degree can be improved by:-
Enhancing speed
Having more wire points on taker-in
having more no. of combing position.
(A) Weakening of vortices (fig-b)
Mote knifes have been reduced to one
(B) Prevention of vortices generation: -( fig-a)
A large space between feed plate & vertical baffle will reduce
formation of vortex. The trash particles, teased out from lap by licker-in
teeth, get into trash box in a direct flight path. However this leads to more
lint loss.
(C)Enhancement of opening intensity: -
(1) High speed: -
In most of modern cards taker-in speed has been enhanced from
a level of 450-600 rpm to a speed of 800-1300 rpm. An excessive high
speed might result in high waste, fibre damage loss of good spinnable
fibres also, and taker-in loading due to incomplete fibre transfers.
(2) More no. of wire points:-
Very little can be done in this respect because too much increase
in wire point density will reduce gap between two wire point rows
leading to fibre chocking this results in reduced opening efficiency.
(3) More no. of combing position:-
There are two way of doing this.
(i) Increase no of rollers.
(ii) Addition of combing surface.
Increase no of rollers:-
More no of rollers will enhance the opening of tufts thoroughly
before transferring to cylinder. The degree of combing for no. of taker-in working
in series is
C=n1.z1.l.Nf + n2.z2.l.Nf + - - - - - -
V.Nx.107
= l Nf x [ n1z1+n2z2+- - - - - ]
V Nx 107
When more than one licker in is used speed of the licker in is kept higher
progressively. The multiple lickrein will following benifits
1. Improved dirt and dust elimination.
2. Improved untangling of neps.
3. The possibility of speed increases and hence productions increase.
4. Preservation of clothing, and hence longer life of the clothing, especially
flats.
5. Better yarn quality.
Additional carding segments: -
(1)Addition of combing surfaces:- (under taker-in)
Generally combing segments are kept below the taker-in after the mote
knife as shown in fig. In Rieter card, for example, the flocks are first guided over
mote knife then over carding surface then again over mote knife and again over
carding plate, before they get transferred to cylinder. A special clothing need to
mounted on the combing surface so that it does not get chocked with fibres.
(2) Between taker-in and the flats &
(3) Between flats and doffer:-
Taker-in delivers fibres still in the form of flocks, if not lumps to main
cylinder. These flocks are compact and relatively poorly distributed over licker-in.
Since there is no opening action between taker-in and cylinder they are passed
as such to cylinder. These tufts have a negative influence on quality of carding
especially for high production card. This shortcoming can be practically
eliminated by inclusion of carding segment. Since they ensure further opening,
thinning out and primarily spreading out and improved distribution of flocks over
total surface.
Between taker-in and the flats Between flats and doffer
1st few flats with takes parts in carding action gets loaded too quickly to a
large extent and hence impairs carding action .The carding segments positioned
at front opens flocks and distributes uniformly over cylinder surface.
The carding segment following flats (Between flats and doffer) also
has a positive influence on yarn quality in terms of improving neps, thick, thin
place, U% and yarn strength and fibre transfer too.
In short we can say that carding segments brings following advantages.
Improved dust & dirt elimination
Improved untangling of neps
The possibility of speed increase and hence production increase
Preservation of clothing and hence longer life of clothing especially flats
Better yarn quality
Fiber transfer taker in to cylinder:-
Transfer of fiber from taker in to cylinder takes place through “stripping
action”. The most important parameter that strongly influence is the ratio of
surface speed between the cylinder and taker in and angle of inclination of taker
in teeth. The surface speed ratio 1:2.The angle of inclination of taker in teeth
should be such that it should facilitate transfer. The condition for this is Cot α >
µ.
The teeth extended in same direction.
V1>V2
Point to back action.
R—tension on fiber tuft
S1—normal component
S2—stripping component.
S1=R.sin α, S2=Rcos α
For stripping S2 > µ S1
S2 / S1 > µ
R. cos α / R. sin α >µ
Cot α>µ ------------------<A>
Angel of inclination—front rate +5° to +10° to vertical for cotton fibres.
For synthetic -10° to -15°
Condition<A> helps to keep the fiber on surface of taker in which facilitate easy
transfer.
Carding section
It consists of cylinder a chain of flats front & back plates & an undercasing.
Carding action carries out the separation of fibre tufts. The action is so intensive
in nature that tufts are individualised to almost single fibre stage. A fibre
separation also facilitates removal of neps, dirt, dust &other foreign impurities.
The cylinder: -
It is most important part of carding m/c . It is manufactured from cast iron but
now sometimes made of steel. The cylinder is machined internally at both ends
to accommodate the cast iron spider with spokes & hubs. It is supported by a
main shaft which rest on roller bearing on pedestal bracket bolted to the main
frame side. Most cylinder have a diameter 1280-1300mm & rotates at speed
between 250-600rpm.The concentricity of cylinder must be maintained within
extremely narrow limits. This is because of distance between cylinder & doffer is
0.1mm.(very very close).
The casing of cylinder: -
Beneath the cylinder is a grid made of sheet provided with slots. The grid
removes impurities & maintain a constant airflow condition. As most of trash is
removed in taker-in zone &left with very micro dust, the cleaning by grid is very
small due to this now a days grid is replaced with sheet. This sheet avoids
formation of small vortices. It also gives better fibre orientation on the surface of
cylinder &reduces no. of neps at high cylinder speed.
The cylinders at the front have front plate between flats& stationary flats.
By adjusting distance between cylinder & front plate flat waste can be influenced.
(i.e. level & quality of flat waste). A narrow spacing gives little waste & wide
spacing produces more stripping.
This setting is not suitable for use as a means adjusting flat waste
sometime along with short fibre it may eliminate long fibre, which may lead to
fibre loss. So one optimum setting is done it may not be altered without excellent
reason.
Flats: -
Carding action is result of interaction of two wire covered surfaces on fibre.
Together with cylinder flats from main carding zone.
Here flats are desired to performs like:
Opening of flock to individual state
Elimination of remaining impurities
Eliminations of short fibres
Untangling of neps, & dust removal
High degree of longitudinal orientations of fibres
In order to perform above function continuous carding surfaces is needed
.If we go for a stationary carding surface continuous operation of carding will be
hindered as we need frequent cleaning of surface .(as we know that with high
speed of cylinder throws fibres &impurities towards top carding surface .Top
carding surface get loaded with fibre & impurities).
To overcome above difficulties top surfaces is made up of individual
clothing strips. The strips are linked to each other by chain & driven slowly. This
facilitates cleaning of loaded flats as they are guided towards cleaning
devices .To make carding surfaces 40-46 flats are needed which are placed at
operative position. To make a chain continuous in total 100 -120 flats are
required.
Construction of flats:-
Flats are cast iron bar having inverted “T” shaped cross section. Lower
surface is machined flats and contain wire points, stretching to a breadth of 22
mm. As both ends of flats rest over flexible bend, length of flats is slightly longer
than working width of the cylinder. It is very important to maintain constant
distance between flats and cylinder across all width of the cylinder. The bending
rigidity of flat should as high as possible. A “T” shaped cross section enhance
bending rigidity and reduce sagging of flats at the middle. The flats connected
together in the form of chain are made to slide over flexible bends. It is important
make sure that they slide smoothly without rocking. As flexible bend follows
curved path like cylinder surface and flats are of round flats it will rest as a single
point which will lead rocking movement of the flats. To avoid flat surface to made
concave so it rest on both points.
The flats are so positioned over flexible bend that the distance between working
face of the flat and cylinder is not constant but decrease gradually in material
flow direction i.e. flats are tilted slightly and this arrangement is known as heel
and toe arrangement. This facilitates gradual opening of fibres.
Theory of carding action
Classical theory
Carding requires
Two oppositely inclined wire points covered surface
They should move with high relative speed provided that sliding
component of tension cutting on fibre is strong enough to move the fibre
down to the wire towards its base.
Classical theory does not take in to account the centrifugal force and presence
of air current.
Strang’s theory
The theory is based on Prandtl’s boundary layer theory. According to the
boundary layer theory the series of concentric layer of the air of infinitesimal
(very small) thickness surround the cylinder. These layers also rotate along with
the cylinder with different velocity. The velocity of the layer is constant with the
cylinder surface will be equal to the cylinder as we move from cylinder towards
flat velocity decreases and near the flat it is equal to zero.
Because air has such different velocities within surface between flats and
cylinder when tuft is introduce into this boundary layers it is subjected to shearing
action of air. The shearing force is calculated as
F=RVA/d
R= coeff. of viscosity of air at a given temp.
V= velocity of air current
A= projected area of fibre tuft in the direction perpendicular to air stream
d= depth of air boundary layer
The force increases as d become closer. It is the shear force that separates the
tuft into individual fibre For fibre transfer from licker-in to the cylinder boundary
layer of the of licker-in and cylinder are responsible.
Kaufman’s Theory
Tuft held on the cylinder surface approaches the cylinder flat zone at very
high speed. Tufts are introduced into narrow gap between cylinder and flats,
which is generally very smaller than the size of the tuft. Due to this compression
of the tuft occurs. Since flats are almost stationary as compared to fast moving
cylinder. Compression force intensity will be more at the flat and the cylinder get
disturbed over layer surface due to its higher surface speed. According to
kaufman compression force acts on 6 time layers surface of cylinder compared to
flat. The penetration of teeth into tuft is immediately followed by shearing force on
tuft due to great difference in speed between flat and cylinder. As a result tuft is
pulled apart into pieces. This process is repeated till the tuft size is reduced
considerably. Generally opening and separation process is complete by 6-7 flat
and seperated fibres evenly load cylinder.
Other force on fibre tufts:-
(1) Centrifugal force:-
F=ma a=linear acceleration
=mrω2 ω=angular velocity
=w/g x r. (2πn/60)2
Where, m=mass of tuft = w/g = Wt. (gm)
Gravitational accln
r= Radius of cylinder (m)
ω= angular speed of cylinder (d/s)
n=rpm
If r=0.645 mtr n=400rpm
F=W (0.645) (2.π.400) 2 = 115W
9.81 x 602
Here F is 115 times the Wt. of tuft. The acts through the center of gravity of tuft
held by wire points and try to lift the tuft.