Articles by Dr. N.Balasubramanian

Importance of count variability in yarn hardly needs any emphasis. Higher count variability invariably leads to higher strength variability. The weak patches in the yarn lead to frequent end break in further processing, which often reaches annoying levels leading to rejection of bobbins and cones. In latest autoconers, which have settings for rejecting bobbins with count of yarn exceeding beyond certain limits of the nominal, processing of yarns with even slightly high count variations becomes extremely difficult. Winding efficiency reaches unacceptably low levels with such yarns. Higher count variability especially of medium to long length range results in moire like appearance in fabric and increases warp way streaks and weft bars. Ring cuts and soiled ring packages is another problem with higher count CV. To overcome this, wider clearance is kept between ring diameter and full package leading to lower doff weights. With higher count variability, percentage of bobbins exceeding tolerance limits of nominal count increases, leading to sales rejections and market complaints. In shuttleless looms, problem of weft tear is encountered during weaving when count of weft changes abruptly beyond certain limits at the time of pirn change. High count variations in weft are also a cause of warp way fabric creases in processed fabrics like dyed poplins1. Dependence on Length Variability of count depends upon the length of the yarn used for estimating count. Though 120 yd. or lea is normally used for estimating yarn count, sometimes half leas are used especially in coarse polyester blend counts to keep strength measurement within the capacity of strength tester. In very fine counts like 120s, two leas are weighed together to estimate count to achieve better accuracy in weighment. It is well known that CV of count decreases with increase in length but the rate of reduction decreases with increase in length. CV of half lea will be1.414 (i.e.; �� 2) of full lea, if there is no serial correlation between adjoining leas. But there is usually a positive serial correlation because of long term variations. So CV of half lea will be 1.2 to 1.3 times the CV of full lea. The same rule holds good in the case of slivers and rovings. CV of 5yd wrapping of roving is much lower than 1.732 (i.e.; ��3) times CV of ��15 yd. wrapping in inter. The former is usually around 1.4 to 1.5 times that of latter. Tracing the Source of Count Variation Location of source of count variability will be greatly facilitated if wrappings and estimate of CV from the same are based on corresponding wrapping lengths of material at different stages. Thus wrappings and CV of wrappings may be based on 5yd instead of the traditional 15 yd length at Inter. 5 yd length at Inter after drafting will be closer to 120 yd length in yarn than 15 yd length and CV estimates based on the former will be more helpful to show if ringframe is contributing to additional variation. At drawframes, CV of wrappings based on 0.5 yd length will be more useful from the same consideration. Estimation of CV of such lengths can be obtained from modern Evenness testers like Uster tester 3. Norms for CV It is beneficial to set norms for CV not only at Ringframes but also at different stages of processing. Two sets of Norms, one with and another without autoleveller at Drawframe are given in Table 1

Table 1: Norms for CV at different stages

Material	Wrapping Length Yd	Without Autoleveller	With Autoleveller
Yarn	120	2.5 - 3	1.5 - 2
Roving	15	1.5 - 1.8	0.9 - 1.2
Roving	5	2.2 - 2.7	1.3 - 1.8
Drawframe	5	0.7 - 0.8	0.4 - 0.5
Drawframe	1.0 - 1.2	0.5 - 0.7
Drawframe	1	1.5 - 1.6	0.9 - 1.1
Drawframe	0.5	1.9 - 2.1	1.3 - 1.5

Contribution from various processes Ring frame Contribution to count variation from ringframe comes from Variation in mechanical draft between frames Slippage of top roller Stretch of material in creel Variation in Mechanical Draft Variations in mechanical draft come from the use of different change pinions on frames of the same make and drafting system. Some common causes for this defective practice are Lack of sufficient stock of change pinions Using change pinions differing by one tooth on the two sides of a frame, ostensibly to achieve an average count close to nominal. This should be discouraged as it increases variability in count between frame sides. Frames of different makes and drafting systems are used for spinning the same count but the same mechanical draft is not kept on them, Slippage of strand under top rollers arises because of inadequate weighting or improper grip. Count variation is therefore reduced upon conversion of older version of top arms to later versions with higher pressures2, 3. Higher frequency for cot buffing, higher starting diameter for cot up to 30 cm are therefore helpful to reduce count variation4, 5. Disturbances to weighting also comes from worn out springs, leakage of air and improper seating of plunger on rib in pneumatic drafting, leading to count variation. One of the reasons for stretch of strand in the creel is low roving twist. The level of creel breaks can assess this. Misalignment of creel roving bobbin in relation to the creel roving guide is another contributory factor to stretch. Improper location of creel guide rod in relation to the bobbin can also cause stretch. If located too high or too low, stretch takes place when roving unwinds from top most or bottom most portion of roving bobbin. Group Control of Count Group control of count should invariably be preferred to individual frame control as a basis for changing the change pinion except under special circumstances. For this purpose, average count from wrappings taken from all ringframes working on a count should be determined and if the count exceeds the preset limits even after a repeat wrapping, pinion change should be done on all frames. The only exception for this is when frames or drafting systems differ widely in age. Under such conditions, slippage of strand occurs under drafting rollers with older frames and drafting systems leading to a lower actual draft at the same mechanical draft5. In one study2 the mechanical draft had to be reduced from 36.5 to 32.6 to get the same yarn count with the same back material when drafting system was changed from SKF PK211E to SKF PK225. Speedframe Most of the problems enumerated above like variations in mechanical draft between frames and slippage of strand under top roller are sources of count variation at speedframe. In addition, the variation in stretch from start to full position is an added source of variability. Stretch variation during build can be estimated accurately, only if a correct method of estimating wrapping is followed. Traditionally roving bobbin is placed on the top of self-weighted top roller driven by wrapping block, with a spindle inside while preparing wrapping. This method has the drawback of slippage of roving between the self- weighted top roller and wrapping drum. The amount of slippage increases from full bobbin to empty bobbin, often to the extent of 5 - 6 %, leading to errors in estimation of wrapping variation from start to full. A better and more accurate method, free from such error, is to suspend the bobbin from a bobbin holder located at the back of wrap block. The roving drawn from the bobbin is passed through the nip of self-weighted top roller and wrapping drum. Actual studies in the mill showed that wrapping estimated by this method is 5 - 10 % coarser than the one obtained by the traditional method. Draw Frame Major contribution to yarn count variation comes from drawframe. Primarily there are two sources of variability 1. Medium term variations in the sliver and 2. Long term variations in the average hank between frames and between shifts. Medium Term Variations Variations in the length region between 0.2 to 0.7 m, depending on the yarn count and hank organisation influence directly count CV as it corresponds to length of lea divided by draft between drawframe and ring frame. Contribution to this variability comes from irregularities in breaker sliver, which do not get reduced by doubling in finisher. Long Term Variations Long term variability in the hank of drawing sliver arises from variation in average weight of lap weight in lap feed systems and weight of feed sheet to card in chute feed systems, that occur from shift to shift. The piano feed regulating motion in scutcher and pressure switch control in chute feed to card operate on the basis of volumetric control and the degree of openness of the material therefore affects the extent of control. In piano feed belt shift would occur when the sheet becomes bulkier (because of better openness) even though weight per unit length remains the same. Likewise, the pressure developed in the chute would be higher for the same weight with well-opened material. Thus it is necessary to standardise the evenness roller to inclined lattice setting in scutcher and speed of beaters and pressure switch setting in the chute, based on factors like type of cotton, whether the feed is directly from bales or from a sandwich mixing from pre-opened material. This will minimise variations caused by openness of material. The variation in average hank of card sliver from shift to shift have less chance of getting reduced from doubling at draw frame, as slivers made in one shift do not get doubled with those in the next shift. But the variability due to this can be kept down by increasing storage capacity of laps and card sliver cans so that material made at longer intervals of time get doubled. But most of the mills do not have such storage space. Moreover material stored for long lengths run the risk of contamination with dust and fly. Autoleveller Modern draw frames are equipped with autoleveller and sliver monitor to reduce both type of variability mentioned above. The autoleveller brings down the variability in sliver lengths beyond 0.3 - 0.5 m. Further, the rate of reduction in CV of wrapping with increase in wrapping length is steeper with autolevelled product. This will be clear from Fig.1, where the variance length curves of Finisher drawing slivers made on the same drawframe with and without autoleveller are compared. The amount reduction in CV with autoleveller increases as wrapping length increases from 0.5 to 3 m. Variance length curve of Breaker drawing sliver (without autoleveller) fed to this draw frame is also shown plotted in the same Fig. The figure shows that doubling in the draw frame brings down the CV with length 0.5 m and about and the order of reduction increases with length. So the variance length curves become steeper from Breaker to Finisher. The short term irregularity in sliver is however not reduced by doubling or autolevelling. Norms for CV of wrapping of drawing sliver at various lengths with and without Autoleveller are given in Table 1. Functioning of autoleveller gets affected by zero setting, obstruction to free movement of scanning roller, improper functioning of servo motors, drives, solenoids and related parts. Leveling action of autoleveller has therefore to be checked at regular intervals. A method usually followed in the mills is to feed 9 ends and 7 ends of sliver and check the wrapping of outgoing sliver. The extent of deviation of hank from nominal under such circumstances is taken as leveling sensitivity of autoleveller. In normal practice, however, such wide variations in input hank seldom occur and the effectiveness of autoleveller in leveling lower order of variations would be of greater interest. The best method of judging this is by comparing CV of 0.5, 1 and 3 m lengths of material with and without autoleveller. Sliver Monitor Sliver monitor consists of a sensor fitted to the calendar rollers of draw frame to check the hank of the outgoing sliver If the actual hank deviates from the nominal by more than the preset limits the drawframe will be stopped. The limits of deviations from nominal hank at which drawframe will be stopped can be varied. It is normally advisable to set it at +/- 2 %. In some makes, sliver monitor can also be set to stop drawframe if CV of wrapping goes beyond certain limits. If the drawframe stoppage due count exceeding preset limits is high then the hank of breaker sliver should be checked to find if it is deviates considerably from normal. If this is not the case, then the functioning of autoleveller has to be checked. Thus a step by step approach should be followed. Mechanical Condition of Draw Frame Poor mechanical condition of draw frame, coupled with inadequate weighting for top rollers have sometimes been traced as a source of count variation. When older versions of Whitin drawframes were replaced by later versions of Laxmi Rieter drawframes, among other noticeable benefits found by the mills, was a significant reduction in count CV of yarn6 as shown in Table 2.

Table 2: Effect of mechanical condition of Drawframe on Count and Strength CV of yarn (30s)

Drawframe Type	Age	Mechanical Condition	CV of Count%		CV of Strength%
			Mixing1	Mixing2	MIxing1	Mixing2
Whitin J5	Old	Unsatisfactory	4.9	3.8	11.4	8.2
Laxmi Rieter DO2S	New	Good	4.4	3.0	8.1	4.7

Card Sliver Though card sliver is characterised by high variability of wrapping, its contribution to yarn count CV is not that significant because doublings in drawframe reduce the variations substantially. Autolevelling at card is therefore not found very beneficial in bringing down count CV of yarn. This will be clear from Table 3 where card slivers made with and without autoleveller were processed up to ringframe and variability at different stages assessed.

Table 3: Effect of Autolevelling at Card on Yarn Count CV % (20s)

	U % of Card sliver	CV % of 1 yd of Card sliver	CV % of 1yd of Finisher Drawing sliver	Count CV % of Yarn
With Autolevelling	3.8	1.2	1.3	2.6
Without Autolevelling	4.1	7.0	1.6	3.2

Though autolevelling at card brings down CV of 1 yd wrapping of card sliver from 7 to 1.2, CV of 1 yd of finisher drawing sliver comes down only marginally from 1.6 to 1.3 and count CV of yarn drops marginally from 3.2 to 2.6. U % of card sliver also does not have much effect on count CV of yarn. This will be clear from a study where cards giving low U % and wrapping CV % and those giving high U % and wrapping CV % were separately channelised up to yarn. The results (Table 4) show that even with large differences in U % and wrapping CV % of card sliver, there is no significant difference in yarn count CV %.

Table 4: Effect of card sliver irregularity on Yarn count CV % (20s)

Property	Cards with low sliver variability	Cards with high sliver variability
Card sliver U %	4.3	7.2
CV of 6 yd wrapping of card sliver	6.1	7.3
CV of Yarn count %	2.8	2.9

Comber Sliver High comber sliver U % arising from piecing wave can affect yarn count CV, particularly if single post comber drawframe passage is used. This will be clear from the following study where combers giving low and high U % were channelised up to ring frame separately and checked for yarn count CV. The results given in Table 5 show that yarn count and strength variations are higher with yarns made from combers with higher variability.

Table 5: Effect of comber sliver U % on variability of count and strength of Yarn (40s)

U % of comber sliver	CV of yarn count %	CV of yarn lea strength %
3.7	1.75	4.12
6.9	2.78	7.96

Medium Term Variations in Yarn Count variation is traditionally used to indicate variability in 120 yd lengths. Variability in shorter lengths are also equally important though they do not form part of quality monitoring activity in most of the mills. Variability of weights in lengths in the region of 0.5 to 10 m affect the appearance of fabric, lead to moire like defects and contribute to weft bars and warp way streaks. U % of drawing sliver has a direct influence on the variations in these lengths. This will be clear from the results of a study in a mill given in Table 6.

Table 6. Contribution of drawing sliver U % on medium term variations in yarn (20s)

Draw Frame settings, mm Fr/Back	U % of Drawing sliver	CSP	U %	CV of 1 m %	CV of 3 m %	CV of 10 m %	CV of 120 yd %
49 - 56	6	2044	15.6	8.0	6.5	4.6	2.5
44 - 53	4.6	2116	13.8	6.2	5.2	4.0	2.6

Drawing sliver U % was brought down substantially in the mills by optimising roller settings. When this material was channelised up to ring frame, medium term variations in the yarn as indicated by CV of 1 to 10 m lengths came down markedly though CV of 120 yd length showed little improvement. The fabric appearance also improved markedly. This shows the importance of monitoring CV of yarn count in length regions 1 to 10 m. This is conveniently obtained in later model Uster Evenness testers like UT3. References: 1. Warpway Creases in fabrics N.Balasubramanian and C.Chatterjee, Indian Textile J., 1996 Oct, p66 2. Benefits from modernisation of ring frames from second generation Top arms M.Balakrishnan and N.Balasubramanian, BTRA Scan 1980 Sept., XI, p4 3 Yarn quality improvements from second generation Top arms at Ring Frame G.Janakiraman and N.Balasubramanian, BTRA Scan, 1987 Dec, XVIII, p 9 4.Cot buffing at Speed Frame - A useful measure to reduce count variation S.K.Sett, G.V.Aras and N.Balasubramanian, BTRA Scan, 1984 March, XV, p 4 5.Contribution of Ring Frame drafting condition to yarn count variability in fine counts N.Balasubramanian and G.Janakiraman, Indian J. of Fibre & Textile Research, 1990, 15, p 198 6. Influence of mixing characteristics and Draw Frame condition on yarn quality G.V. Aras and N.Balasubramanian, BTRA Scan, 1982 June, XIII, p 10