DRAWFRAME

OPERATIONS OF DRAWFRAME

1. Through doubling the slivers are made even 2. Doubling results in homogenization(blending)

3. Through draft fibres get parallelised

4. Hooks created in the card are straightened

5. Through the suction, intensive dust removal is achieved

6. Autoleveller maintains absolute sliver fineness

Quality of the drawframe sliver determines the yarn quality.

Drawing is the final process of quality improvement in the spinning mill.

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.

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.

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.

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 tothe 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.

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.

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.

Drafting arrangement is the heart of the drawframe. The drafting arrangement should be

1. Simple

2. Stable design with smooth running of rollers

3. Able to run at higher speeds and produce high quality product

4. Flexible i.e suitable to process different materials , fibre lengths and sliver hanks

5. Able to have good fibre control

6. Easy to adjust

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

1. Draft ratio

2. Roller settings

3. Material characteristics

4. Pressure exerted by the top roller

5. Hardness of top roller

6. Fluting of the bottom rollers

7. Distribution of draft between the various drafting stages

Drafting is affected by the following raw material factors

1. No of fibres in the cross section

2. Fibre fineness

3. Degree of parellelisation of the fibres

4. Compactness of the fibre strand

5. Fibre cohesion which depends on

(i) Surface structure

(ii) Crimp

(iii) Lubrication

(iv) Compression of the strand

(v) Fibre length

(vi) Twist in the fibre

(vii) 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 Floating fibres are subject to two sets of forces acting in opposite directions. The more number of fibres which are moving slowly because of the contact with the back rollers restrain the floating fibres from accelerating.

The long fibres in contact with the front rollers tend to accelerate the floating fibres to the higher speed.

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.

At some balance point, a fibre accelerates suddenly from low to high speed.

This balance point is compounded by the laws of friction, static friction being higher than dynamic friction.

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.

Thus there may be an avalanche effect which results in drafting wave.

PROCESS PARAMETERS IN DRAW FRAME

IntroductionDrawframe 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 :

1. The total draft

2. No of drawframe passages

3. Break draft

4. No of doublings

5. Grams/meter of sliver fed to the drawframe

6. Fibre length

7. Fibre fineness

8. Delivery speed

9. Type of drafting

10. Type of autoleveller

11. Autoleveller settings

The total draft depends upon

1. Material processed

2. Short fibre content

3. Fibre length

Following are some facts derived from trials

1. Wider back roller setting will result in lower yarn strength

2. Wider back roller setting will affect yarn evenness

3. Wider back roller setting will increase imperfections

4. Higher back top roller loading will reduce yarn strength

5. Higher back top roller loading will reduce end breakage rate

6. Wider front roller setting will improve yarn strength

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.

Most of the improvement in fibre parallelization and reduction in hooks takes place at first drawframe passage than at second passage.

Better fibre parallelisation generally results in more uniform yarns and a lower end breakage rate in spinning.

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.

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.

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. With variable fibre-length distribution(with more short fibre content), the drafting irregularity will be high.

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.

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.

For combed material, if single passage is used, it is better to employ draft of 7.5 to 8.

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.

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.

For synthetic fibres (44 mm to 51 mm), 8 of a total draft can be employed both in breaker and finisher passage.

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.

Especially for synthetic fibres with very high drafting resistance, it is better to feed less than 38 grams per meter to the drawframe.

Break draft setting for 3/3, or 4/3, drafting system is as follows

1. For cotton, longest fibre + (8 to 12 mm)

2. For synthetic fibre, fibre length + (20 to 30% of fibre length)

Break draft for cotton processing is normally 1.16 to 1.26. For synthetics, it is around 1.42 to 1.6.

To meet the present quality requirements, finisher drawframe should be an autoleveller drawframe.

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.

It is advisable to buff the rubber cots once in 30 days (minimum) to maintain consistent yarn quality.

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.

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.

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.

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).

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.

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.

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.

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 Most of the modern autolevellers are open loop autolevellers. This system is effective on short, medium and to some extent long tem variations.

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.

Intensity of levelling and timing of correction are two important parameters in autolevellers.

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.

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)

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.

Higher the speed, higher the correction length

Whenever the back roller setting, guide rails setting, delivery speed, break draft etc are changed, the timing of correction should also be changed.

U% of sliver will be high, if timing of correction is set wrongly

If intensity of levelling selected is wrong, then 1 meter C.V % of sliver will be high.

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. ThenA% = {(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

Life of servo motor and servo amplifier will be good, if

1. it is used for carded material

2. feed variation is less

3. motor is checked for carbon brush damages, bearing damages etc periodically

4. 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: 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 densityLet 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 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 systemBecause 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. 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 ConsideredMechanical 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 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 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 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 AutolevellerAll 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 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 inwarping 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 goodProduction calculated will be more accurate in autoleveller drawframe compared to non autoleveller drawframeVariation 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.