DRAWFRAME
OPERATIONS OF DRAWFRAME
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Through doubling the slivers are made even
Doubling results in homogenization(blending)
Through draft fibres get parallelised
Hooks created in the card are straightened
Through the suction, intensive dust removal is achieved
Autoleveller maintains absolute sliver fineness
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Quality of the drawframe sliver determines the yarn quality.
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Drawing is the final process of quality improvement in the spinning mill.
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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 its linear density. In a roller drafting system, the strand is passed
through a series of sets of rollers, each successive set rotating at a surface velocity greater than that of
the previous set.
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During drafting, the fibres must be moved relative to each other as uniformly as possible by overcoming
the cohesive friction. Uniformity implies in this context that all fibres are controllably rearranged with a
shift relative to each other equal to the degree of draft.
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In drawframe, the rollers are so rotated that their peripheral speed in the through flow direction increases
from roller pair to roller pair, then the drawing part of the fibres, i.e. the draft, takes place. Draft is defined
as the ratio of the delivered length to the feed length or the ratio of the corresponding peripheral speeds.
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Drawing apart of the fibres is effected by fibres being carried along with the roller surfaces. For this to
occur, the fibres must move with the peripheral speed of the rollers. This transfer of the roller speed to
the fibres represents one of the problems of drafting operation. 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.
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Roller drafting adds irregularities in the strand. Lamb states that though an irregularity causing mechanism
does exist in drafting, drafting also actually reduced the strand irregularities by breaking down the fibre
groups.
Drafting is accompanied by doubling on the drawframe, this offsets the added irregularity.
Variance(sliver out) = Variance(sliver in) + Variance(added by m/c)
In Statistics , Variance is the square of standard deviation.
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Two passages of drawing with eight ends creeled each time would produce a single sliver consisting of 64
ribbons of fibre in close contact with each other. In the ultimate product, each ribbon may be only a few
fibres thick, and thus the materials of the input slivers are dispersed by the drawing process. The term
doubling is also used to describe this aspect of drawing.
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Drafting arrangement is the heart of the drawframe. The drafting arrangement should be
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Roller drafting causes irregularities in the drafted strand since there is incomplete control of the motion of
each individual fibre or fibre group. The uniformity of the drafted strand is determined by
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Simple
Stable design with smooth running of rollers
Able to run at higher speeds and produce high quality product
Flexible i.e suitable to process different materials , fibre lengths and sliver hanks
Able to have good fibre control
Easy to adjust
Draft ratio
Roller settings
Material characteristics
Pressure exerted by the top roller
Hardness of top roller
Fluting of the bottom rollers
Distribution of draft between the various drafting stages
Drafting is affected by the following raw material factors
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No of fibres in the cross section
Fibre fineness
Degree of parellelisation of the fibres
Compactness of the fibre strand
Fibre cohesion which depends on
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
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Surface structure
Crimp
Lubrication
Compression of the strand
Fibre length
Twist in the fibre
Distribution of fibre length
3-over-3 roller drafting arrangements with pressure bar is widely used in the modern drawframes. Bigger
front rollers are stable and operated at lower speeds of revolution, this necessitated pressure bar
arrangement for better control of fibres. Some drawframes are with 4-over-3 drafting arrangement, but
strictly speaking it behaves like a 3-over-3 drafting system except for the fact that fourth roller helps to
guide the sliver directly into the delivery trumpet.
DRAFTING WAVE
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Floating fibres are subject to two sets of forces acting in opposite directions.
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The more number of fibres which are moving slowly because of the contact with the back rollers restrain
the floating fibres from accelerating.
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The long fibres in contact with the front rollers tend to accelerate the floating fibres to the higher speed.
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As the floating fibres move away from the back roller, the restraining force by back roller held fibres
reduces, and the front roller influence increases.
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At some balance point, a fibre accelerates suddenly from low to high speed.
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This balance point is compounded by the laws of friction, static friction being higher than dynamic friction.
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When one floating fibre accelerates, the neighbouring shor fibres suddenly feel one more element tending
to accelerate them and one fewer trying to restrain them.
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Thus there may be an avalanche effect which results in drafting wave.
PROCESS PARAMETERS IN DRAW FRAME
Introduction
Drawframe is a very critical machine in the spinning process. It's influence on quality, especially on
evenness is very big. If drawframe is not set properly, it will also result in drop in yarn strength and yarn
elongation at break. The faults in the sliver that come out of drawframe can not be corrected . It will
pass into the yarn.
The factors that affect the yarn quality are :
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The total draft depends upon
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The total draft
No of drawframe passages
Break draft
No of doublings
Grams/meter of sliver fed to the drawframe
Fibre length
Fibre fineness
Delivery speed
Type of drafting
Type of autoleveller
Autoleveller settings
Material processed
Short fibre content
Fibre length
Following are some facts derived from trials
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Wider back roller setting will result in lower yarn strength
Wider back roller setting will affect yarn evenness
Wider back roller setting will increase imperfections
Higher back top roller loading will reduce yarn strength
Higher back top roller loading will reduce end breakage rate
Wider front roller setting will improve yarn strength
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Higher draft in drawframe will reduce sliver uniformity, but will improve fibre parallelisation. Sometimes the
improvement in fibre parallelisation will overcome the detrimental effects of sliver irregularity.
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Most of the improvement in fibre parallelization and reduction in hooks takes place at first drawframe
passage than at second passage.
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Better fibre parallelisation generally results in more uniform yarns and a lower end breakage rate in
spinning.
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Higher the weight of sliver fed to drawframe, lower the yarn strength, yarn evenness, and it leads to higher
imperfections in the yarn and more end breakages in ring spinning.
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Irregularities arise owing to the instability of the acceleration point over time. The aprons and rollers are
used in the drafting zone to keep the fibre at the back roller velocity until the leading end is firmly gripped
by the front roller, but individual fibre control is not achieved.
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Drafting wave is caused primarily not by mechanical defects as such but by the uncontrolled fibre
movement of a periodic type resulting from the defects. As the fibre-accelerating point moves towards the
front rollers, the draft increases( and vice versa), so that a periodic variation in linear density inevitably
results.
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With variable fibre-length distribution(with more short fibre content), the drafting irregularity will be high.
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More the number of doublings , lower the irregularity caused due to random variations. Doublings does
not normally eliminate periodic faults. But it reduces the effects of random pulses. Doubling does not have
any effect on Index of Irregularity also, since both the irregularities are reduced by square root of the
number of doublings.
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Fibre hooks influences the effective fibre length or fibre extent. This will affect the drafting performance.
For carded material normally a draft 7.5 in both breaker and finisher drawframe is recommended. Seven
of a draft can be tried in breaker, since it is a carded material.
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For combed material, if single passage is used, it is better to employ draft of 7.5 to 8.
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If combers with four doublings are used, it is better to use two drawframe passages after combing. This
will reduce long thick places in the yarn.
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In case of two drawframe passage, first drawframe passage will reduce the periodic variation due to
piecing. Therefore the life of servomotor and servo amplifier will be more , if two drawframe passage is
used. Quality of sliver will also be good, because of less and stable feed variation.
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For synthetic fibres (44 mm to 51 mm), 8 of a total draft can be employed both in breaker and finisher
passage.
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The number of doublings depends upon the feeding hank and the total draft employed. Most of the
modern drawframes are capable of drafting the material without any problem, even if the sliver fed is
around 36 to 40 grams per meter.
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Especially for synthetic fibres with very high drafting resistance, it is better to feed less than 38 grams per
meter to the drawframe.
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Break draft setting for 3/3, or 4/3, drafting system is as follows
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For cotton, longest fibre + (8 to 12 mm)
For synthetic fibre, fibre length + (20 to 30% of fibre length)
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Break draft for cotton processing is normally 1.16 to 1.26. For synthetics, it is around 1.42 to 1.6.
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To meet the present quality requirements, finisher drawframe should be an autoleveller drawframe.
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Since the drawframe delivery speed is very high the top roller shore hardness should be around 80
degrees. It should not be less than that.
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It is advisable to buff the rubber cots once in 30 days (minimum) to maintain consistent yarn quality.
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Coiler size should be selected depending upon the material processed. For synthetic fibres, bigger coiler
tubes are used. This will help to avoid coiler choking and kinks in the slivers due to coiling in the can.
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Speed of the coiler will also affect the coiling. Speed of the coiler should be selected properly. In
drawframes like RSB D-30(RIETER) , any coiler speed can be selected through the variator type pulley.
Since, the option is open, there is also more probability for making mistakes. One should take enough
care to set the coiler speed properly.
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Whenever coiler speed is adjusted, the diameter of the coil is also changed. Hence it is necessary to
check the gap between the sliver and can. If it is more than 5 mm, then turn table position (can driving
unit) should be altered so that the gap between coil outer and can inner is around 5 mm.
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Pressure bar depth plays a major role in case of carded mixing and OE mixings. If it is open, U% will be
affected very badly. It should always be combined with front roller setting. If the pressure bar depth is
high., Creel height should be fixed as low as possible (especially for combed material).
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Top roller condition should be checked properly. While processing 100% polyester fibres, fibre scum
should be removed by a wet cloth from the top roller atleast once in a shift.
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Sliver funnel size should be selected properly. Very wide funnel will affect the U%. But very small funnel
will end up in more sliver breaks at the front.
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If the department humidity variation is very big, then corresponding correction to be made for checking the
wrapping of sliver (sliver weight). Otherwise, there will be unwanted changes in the drawframe, which will
affect the count C.V.% of yarn.
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Most of the Autoleveller drawframes are working on the principle of OPEN LOOP control system. Sliver
monitor should be set properly. Whenever there is a problem in sliver weight, this will stop the machine.
Sometimes sliver monitor may malfunction. If it is found malfunctioning , it should be calibrated
immediately.
Autolevelling
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Most of the modern autolevellers are open loop autolevellers. This system is effective on short, medium
and to some extent long tem variations.
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Mechanical draft should be selected properly in autoleveller drawframes. To decide about the mechanical
draft, drawframe should be run with autoleveller switched off. If the sliver weight is correct, then the
mechanical draft selected is correct. Otherwise, the gears should be changed so that the sliver is weight is
as per the requirement without autolevller.
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Intensity of levelling and timing of correction are two important parameters in autolevellers.
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Intensity of levelling indicates the amount of correction. i.e If 12% variation is fed to the drawframe the
draft should vary 12% , so that the sliver weight is constant.
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Timing of correction indicates that if a thick place is sensed at scanning roller, the correction should take
place exactly when this thick place reaches the correction point (levelling point)
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Higher the feed variation, higher the correction length. e.g. if feed variation is 1 %, and if the correction
length is 8 mm, if feed variation is 5% the correction length will be between 10 to 40 mm depending upon
the speed and type of the autoleveller.
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Higher the speed, higher the correction length
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Whenever the back roller setting, guide rails setting, delivery speed, break draft etc are changed, the
timing of correction should also be changed.
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U% of sliver will be high, if timing of correction is set wrongly
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If intensity of levelling selected is wrong, then 1 meter C.V % of sliver will be high.
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Most of the modern autolevellers can correct 25% feed variation. It is a general practice to feed 12%
variation both in plus and minus side to check A%. This is called as Sliver test. The A% should not be
more than 0.75%. A% is calculated as follows;
If no. of sliver fed to drawframe is N, check the output sliver weight with "N", "N+1", "N-1" slivers.
Then
A% = {(gms/mt(N-1) - gms/mt(N)} / gms/mt(N) x 100
A% = {(gms/mt(N+1) - gms/mt(N)} / gms/mt(N)} x 100
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Life of servo motor and servo amplifier will be good, if
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4.
it is used for carded material
feed variation is less
motor is checked for carbon brush damages, bearing damages etc periodically
if the delivery speed is less
DRAW FRAME AUTOLEVELLER
Introduction
Autoleveller is an additional device which is meant for correcting the linear density variations in the
delivered sliver by changing either the main draft or break draft of the drafting system, according to the
feed variation.
There are two types of Autolevelling systems. They are:
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Open loop system
closed loop system
Most of the drawframe autolevellers are open loop auto levellers.
In open loop autolevellers , sensing is done at the feeding end and the correction is done by changing
either a break draft or main draft of the drafting system.
In closed loop system, sensing is at the delivery side and correction is done by changing either a break
draft or main draft of the drafting system.
Most of the earlier card autolevellers are closed loop autolevellers. But the latest cards have sensing at
the feed rollers and as well as at the delivery calendar rollers. We can say , both closed loop and open
loop systems are being used in such autolevellers.
Open loop system is very effective, because the correction length in open loop system is many fold
lower than closed loop system. But in case of closed loop system, it is confirmed that the delivered
sliver is of required linear density. In case of open loop system, since the delivered material is not
checked to know whether the correction has been done or not, Sliver monitor is fixed to confirm that the
delivered sliver has the required linear density
Let us discuss about an autoleveller system which is being used in most successful drawframes like
RSB-951, RSB-D-30 etc.
Major Components
This system is an electronic levelling system. The major components in the system are
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Scanning roller
Signal converter
Levelling CPU
Servo drive (servo motor and servo leveller)
Differential gear box(Planetary gear box)
Function of the scanning roller is to measure the variation in the feed material. All slivers fed to the
machine pass thro a pair of scanning rollers. One of the scanning roller is moveable. These scanning
rollers are loaded either by a spring loading system or a pneumatic loading system. Pneumatic loading
is always better, because the pressure in kgs will be always same(consistent), irrespective of the sliver
feed variation. But in the case of spring loaded, the pressure on scanning rollers may vary depending
upon the feed variation.
The variations in sliver mass of the incoming slivers displace the scanning roller. The distance moved
by the scanning is proportional to the sliver mass fed. This displacement of scanning rollers are
transformed into voltage by a signal converter and is fed to an Electronic Levelling processor. With
analogue system, electronic levelling processor is a servo amplifier, but in the case of digital system, it a
CPU.
It is the Electronic Levelling processor which furnishes the correct target value to the servo drive (servo
motor and servo leveller).
Delivery speed of the machine and electric signal values arrived at by the slivers fed are the two
important signals for the correction.
Servo drive takes the information and is converted in such a way that servomotor RPM and direction is
decided for appropriate correction.
Planetary gearing (Differential gearing) with its controlled output speed drives the middle and back
roller. i.e. Sliver entry of the drafting system
Because the servo motor RPM and direction varies according to the feed variation, and the servo motor
and servo leveller generates a control speed of the planetary gearing, the required change in main draft
is accomplished, compensating for the weight variation of the sliver fed.
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If the slivers fed are too heavy, the entry speed is reduced i.e. draft increased
If the slivers fed are too light, the entry speed is increased i.e. draft reduced
Delivery speed (the front roller speed) remains constant and hence the production remains constant.
Points to be Considered
Mechanical draft or nominal draft should be selected properly. Before switching on the autoleveller,
gears should be selected such that, the wrapping average (linear density of sliver) should be less than
plus or minus 3%.
If the feed variation indicated in the A% display of sliver fed is continuously showing more than -5% or
+5%, then the mechanical draft selected is not correct.
If the mechanical draft selected is correct, then the indication in A% display of sliver fed should be
between -5% red lamp and 0% green lamp or +5% red lamp and 0% green lamp. In other words, green
lamp(0% variation indication) should be on atleast for 80% of the running time.
Atuoleveller is meant for correcting
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Continuous long term variation in the fed sliver
Medium term variation
Seldom occurring abnormal variations in the sliver fed due to deviations in carding and
comber
Short term variations in the sliver fed
Variations like comber piecings
Scanning rollers should be selected properly. In some drawframes like DX7-LT OR DXA7-LT, the
scanning roller is same for all sliver weights and all types of material. But in case of RSB drawframes,
there are different sizes of scanning rollers. It depends on sliver weight fed and the type of material
processed.
Scanning roller pressure is not a constant. It depends on the material being used. It is selected so that
minimum A% is achieved in the sliver. For the same material if the scanning roller pressure is changed,
the linear density of the delivered material will also change. Hence enough care should be taken so that
whenever the pressure is changed, the wrapping should be checked and adjusted.
Following are the two important parameters for Quality Levelling
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Levelling action point ( time of correction)
Levelling intensity
Both feed variation sensing and correction are being done when the machine is running (continuous
process) at two different places (i.e. sensing is at one place and correction is at an other place). Hence
the calculated correction should be done on the corresponding defective material. This is decided by
Levelling action Point. The time required for the defective material to reach the correction point should
be known and correction should be done at the right time.
Levellling action point depends upon
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Break draft
Main draft roller setting
Delivery speed
Levelling Intensity is to decide the amount of draft change required to correct feed variation. The
correlation between mass and volume for different fibres is not same. Therefore the levelling intensity
may be different for different fibres. Levelling intensity is selected based on the following trial.
Wrapping of the delivered sliver should be checked with "n", "n+1", "n-1" sliver at the feeding side. The
sliver weight of the delivered sliver should be same for all the three combinations or should be the
minimum.
This can be checked if the sliver is checked at UT 3(uster)or premier tester 7000 for mass variatons (
U%).
If levelling correction point and levelling intensity is selected properly, then the cul length C.V% of 1
meter will be less than 0.5, if the sliver is tested in UT-3 instrument.
Advantages of Autoleveller
All variations are corrected.
Count C.V.% will be consistent and good, hence the yarn will be suitable for knitting.
Off counts will be very less in the yarn, hence off count cuts will come down drastically in autoconers.
Thin places in the sliver, hence in the yarn will be low.
Ring frame breaks will come down, hence
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Pneumfil waste will be less
Fluff in the department will be less, therefore uster cuts will be less
Fabric quality will be good because of lower number of fluff in the yarn
Labour productivity will be more
Machine productivity will be more
Idle spindles will be less
RKM C.V.% will be low, because of low number of thin places.
Workability in warping and weaving will be good, because of less number of thin places and lower end
breaks in spinning and winding.
Sliver U%, hence yarn U% will be good
Production calculated will be more accurate in autoleveller drawframe compared to non autoleveller
drawframe
Variation in Blend percentage will be very less, if both the components are autolevelled before blending,
hence fabric appearance after dyeing will be excellent.
Cautions Needed
As long as the autolevelling system is set properly and all the components are working properly, the
above said benefits can be achieved. Otherwise, the negative impact will be very big compared to
working without autoleveller. If the autoleveller malfunctions, it is better to run the machine without
autoleveller.