a new technique for feeding yarn in a flat bed knitting

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Engineers, Part E: Journal of ProcessProceedings of the Institution of Mechanical

http://pie.sagepub.com/content/209/2/151The online version of this article can be found at:

DOI: 10.1243/PIME_PROC_1995_209_242_02

1511995 209:Proceedings of the Institution of MechanicalEngineers, Part E: Journal of Process Mechanical Engineering

P K Choy, J Atkinson and T DiasA New Technique for Feeding Yarn in a Flat Bed Knitting Machine

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Technical Note

A new technique for feeding yarn in a flat bed knittingmachineP K Choy, BEngDepartments of Textiles and Mechanical Engineering, University of Manchester Institute of Science and Technology, ManchesterJ Atkinson, BSc, MSc, PhD, CEng, MIMechEDepartment of Mechanical Engineering, University of Manchester Institute of Science and Technology, ManchesterT Dias Dip1 Ing, Dr IngDepartment of Textiles, University of Manchester Institute of Science and Technology, ManchesterThe quality of knitted fabric panels is of considerable importance to the knitwear industries. The latest development inflat bed knittingtechnology improves the eficie ncy of the production b y knitting full y fashioned fab ric panels that a re to exact shape and size. Thisgreatly reduces the cutting waste and length of time fo r the making-up process, but garment manufacturers ar e still facin g the dijlicultyo j retaining dimensional stability of the knitted panels. One of the signijicant factors influencing this undesired property is input yarntension variation. The analysis of this problem is described and a new method of yarn feeding is also sugg ested. T his method utilizesstora ge yarn feed to eliminate the undesired effect of yarn feed speed variation and has been applied as a prototype. The results show apromising and satisfactory improvement in the fab ric qu ality.Ke y words: flat bed knitt ing machine, fabric panels, input yarn tension, yarn feeding

NOTATIONfabric structure constantslength of knitted fabric panelstitch lengthnumber of knitted rowsnumber of needlestime taken to knit one courseyarn input tensiontension before capstantension after capstanyarn feeder traverse velocityrun-in yarn velocityresultant yarn velocitywidth of knitted fabric panel

0 angle of wrapmean coefficient of friction between yarn andthe knitting element

1 INTRODUCTIONGarment panels can be knitted to the required shape,known as fully shaped panels , using present technology,but flat bed knitting users still face the problem of con-trolling the dimensions of individual panels. In fact, thedimensions of individual knitted panels can vary frombetween 5 and 10 per cent of their nominal length orwidth 1). Therefore, fully fashioned garment panels areknitted at the present time with an extra allowance, thatis with a 5-10 per cent waste factor.Knitted fabric structures are elastic by nature, whichis mainly due to the geometrical binding of yarns in theform of stitches. The dimensions of a knitted panel arethe result of the effect of the shape and the size of theindividual stitches as suggested by Munden (2). ThusThe M S was received on 2 June 1994 and was accepted for publication on28 February 1995.E01294 Q IMechE 1995

the fabric dimensions are dimcult to predict and tocontrol. The causes of the variation in dimension ofknitted panels can be listed as follows:(a) dimensional variations due to change of st i tchb) dimensional variations due to change of st i tch1.1 Stitch shape variationsIn flat bed knitting the fabric is subjected to tensionfrom the fabric take-down rollers or holding-downsinkers during the stitch formation process, thus strain-ing and distorting the stitches. The shape of stitchdepends on the relaxation conditions such as dry relax-ation, steaming, washing, centrifuging and tumbledrying.1.2 Stitch length variationsThe research of Munden 2) shows that, in relaxedknitted structures, the number of rows of stitchescourses) and the number of columns of stitches wales)in a unit length are inversely proportional to the stitchlength. Therefore the dimensions of a knitted fabricpanel can be calculated as follows:Length of a knitted fabric panel :

s h a p e ;length.

Ikp = c N~ st

wkp c2 Nn 1st

(1)

2)Equations (1) and (2) indicate that the dimensions ofknitted fabric panels can be made reproducible by knit-ting the stitches of equal length. The relaxed dimensionsare determined by the length of yarn knitted into eachstitch and are unaffected by other knitting variables (3).One of the significant parameters affecting the stitchlength is the yarn tension. According to Buhler e t a l . (4),

Width of a knitted fabric panel:

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152 P K CHOY, ATKINSON AND T DIASaverage yarn tension values up to 150 cN have to beexpected during stitch formation. Buhler et al. alsostated that short-term yarn tension peaks of 600-1OOOcN are not unknown. Yarn will be elongated when it isunder tension and the stitch size becomes smaller whenit is removed from the machine. The phenomenon ofrobbing back described by Lau and Knapton (5) andMunden (2) is another significant factor influencing thestitch length, resulting in a smaller stitch. This unde-sired reduction in stitch length is proportional to theyarn tension.1.3 Yam tensionThe model suggested by Probst (6) describes the yarntension built up along the yarn feed path. For a run-inyarn tension of 3.0 cN, the yarn tension at the take-back spring can be calculated by applying Eulerscapstan equationThe take-back spring has to compensate for 4.5 cN inthe yarn feeder, position 2 in Fig. 1, and for 6.7 cN a tthe yarn guides, positions 3 and 4.Under these condi-tions, the tension at the yarn return spring is approx-imately 14.0 N. Thus the cymbal yarn tensioner shouldbe set to a value tha t is slightly higher tha n 14.0cN inorder to take back the yarn and maintain it straight.However, the setting of the cymbal yarn tensioner to15.0 cN w ould result in the inpu t ya rn tension buildingup to a value as high as 48.0 cN during knitting.Yarn tension varies during the knitting of alternatecourses and the variation is caused by the pulling ofunequal lengths of yarn from the yarn package. Thetension depends on the direction of the yarn feedermovement, as shown in Fig. 1. When the yarn feedertravels away from the yarn feed side of the mach ine, theknitted length of yarn V,c) plus an additional lengthcorresponding to the distance travelled by the yarnfeeder V , will be drawn off from the yarn package. O nthe return travel, the length drawn from the yarnpackage will be equal to the length of the knitted yarnV , minus the distance travelled by the yarn feederV , . This alternate motion results in a yarn feed veloc-i ty variation as shown in Fig. 2, and can be expressedmathematically as

4)According to work suggested by Rieder I), the yarntension is directly proportional to the yarn feed velocity

T ~ ; , e 3)

v, = v v,Take-back

Yam feeder traverse

Fig. 1 Yarn feed path of flat bed knitting machinePart E : Journal of Process Mechanical Engineering

traverse traverseaway from towardsyam feed yam feedside sideFig. 2 Resultant yarn feed velocity

because of frictional forces induced by the yarn guidesand the tensioners along the yarn path, as shown in Fig.1. These frictional forces exert a n influence on the stitchformation a nd limit the maximum knitting speed.Most of the existing yarn feeding methods availablein industry apply yarn feeding techniques at either, orboth, sides of the machine. However, even if the yarntension can be controlled accurately in position 4 thatis at the side of the machine) shown in Fig. 1, theproblem of the yarn tension variation, at the needles,arising from the reciprocating yam feeder traverse hasnot been solved. Thus, these variations have to be com-pensated for unless control can be exercised just beforethe yarn is laid on to the needles in position 3. In fact,Rieder 1) found evidence of an exponential increase ofthe yarn tension with increasing carriage velocity duringknitting.2 A N E W A P P R O A C H TO FEE ING YARN 7)

The yarn velocity variation can be minimized by elimi-nating the yarn feeder velocity component V , in equa-tion 4). This would result in a situation similar to thatin circular knitting. In high-speed circular knittingmachines the yarn tension is maintained at a low con-stan t level of 2-3 cN w ith positive yarn feeding, and thisapproach has become very successful in the control offabric quality.The objective may be achieved by integrating a yarnaccum ulator storage) feeding device directly on to theyarn feeder. Such an arrangement would no t only elimi-nate yarn tension variation but would make it possibleto lay yarn into the knitting needles at very low tension.Such a device can store a certain length of yarn in acompact form and allow the residual stress within thestored yarn to be relieved. By this means, the arrange-ment can compensate for the undesired yarn tensionarising from the variation in yarn feed velocity. The dif-ficulty in storing yarn on t o the feeder is that the lengthof stored yarn will be limited by the dimensions of thefeeder and the yarn has to be stored in a compact form

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A NE W T E CHNI QUE FOR FE E DI NG YARN I N FL AT BE D KNI T T ING MACHI NE 153

CompressedA air

\- ake-backaccumulator

Fig. 3 A schematic diagram of the accumulator feedingdevice

so that a reasonable length is continuously availableduring the stitch formation process.The present design aim is to store yarn in the form ofloosely packed layers. Figure 3 shows the concept ofyarn storage in compact form without entanglement.The accumulator consists of a cavity into which yarn isblown by mean of compressed air. The yarn is stored aslayers due to the air flow inside the cavity. A schematicdiagram of the device is shown in Fig. 3. In addition, afriction yarn feed roller pulls yarn from the yarnpackage to make it available at the side of the machine.This arrangement increases the efficiency of the yarnfeeding. Figure 4 shows an accumulator installed on toa flat bed electronic knitting machine. The size of accu-mulator is similar to the ordinary yarn feeder andallows several feeders to operate on a single knittingprocess. In the case of knitting with several accumulatorfeeders, the compressed air has to be supplied from thecarriage with appropriate adaptors and regulated to thecorrect pressure for different yarn skin frictional proper-ties. The device has been used for more than 20 shortknitting trials (for example 30 minutes) on a modernelectronic machine. During these tests no breakdown

Fig. 4 The new yam feeder instal led on to a knittingmachin e second from the left is the accumula to r y amfeeder)Q IMechE 1995

occurred. However, system breakdown can happenwhen the take-back tension is not enough due to incor-rect adjustment of air pressure on the take-back accu-mulator shown in Fig. 3.3 RESULTS

Figure 5a shows the yarn tension variation during stitchformation when the machine knits a complete course.The tension is measured directly at the yarn feederbefore the yarn is laid on to the needles. The tensionbuilds up to 49 CN and decreases to an average value of20 cN with the conventional yarn feed method withoutusing the accumulator. However, this is reduced toabout 1.5 cN with the yarn accumulator installed.According to Munden s work (2), the stitch length isdetermined by the input tension rather than by anyother knitting parameter. Therefore any change in thestitch length of the knitted fabric is the result of thechange of yarn input tension. The stitch length, which isinversely proportional to the yarn feed speed, clearly

onventional feedeed with accumulator

0 0.1 0.2 0.3 0.4 0 3 0 6IS-

0.4.2

- Conventionalfeed+ Feed with accumulator01 I L I I I , I I

0 0 1 0.2 0 . 3 0 4 0 5 0.6 0.7 0 8 0 9 I 1 1 1.2mlsc-

Fig. 5 a) The effect of accumulator feed method on yarn(b) Th e effect of knitting speed stitch lengthtension

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I54 P K CHOY, A T K I N S O N AND T DIASincreases when knitting with the aid of an accumulator,as shown in Fig. 5b. These results demonstrate the dif-ference in stitch length between knitting with andwithout the new yarn feeding method. There is about a20 per cent increase in stitch length when using theaccumulator feed method.

4 CONCLUSIONSThe yarn tension variation problem on the flat bed weftknitting machine has been described. The dimensionalstability of the knitted fabric panel is influenced by thesize and shape of the stitches. The stitch size is directlyinfluenced by the input yarn tension. In most of theexisting techniques, the yarn tension has been con-trolled before the yarn enters the yarn feeder. Thereforethese feeding systems still cannot solve the fundamentalproblem because the tension induced by the yarn feedertraverse velocity is not compensated for. The yarnfeeding method proposed here does not suffer from thisdrawback since it allows the yarn tension control totake place near to the knitting needles. Experimentswith this new yarn feed method have produced fabrics

that exhibit a more relaxed structure and a larger stitchsize in comparison with that obtained from the conven-tional method. This is an encouraging result as itplainly shows that the yarn tension has been appropri-ately reduced to give a decided improvement in fabricquality.

REFERENCES1 Rider, 0 Schwankungen in den Maschenwarendimcnsionen undMach nahmen zur Minimierung. Wirkerei- u. Strickerei-Technik,2 Munden, D. L. Geometry and dimensional properties of plainknitted fabrics. J . T e x t . Inst. , July 1959,50 7).3 Munden, D. L. A study of the mechanism of loop formation on weftknitting machinery. T e x t . Res. J . , 1966,33, 1072-1080.4 Biihler, G. Rider, 0 and Haussler, W. Verbesserung der Pro-duk tion in der Strickerei durch Vermeidung der Fa serflugprobleme.AIF -Nr 5295, AbschluDbericht, September 1985.5 Lau T. W. Y. and Knnpton, J. J. F. Design and dynamics of non-linear cams for use in high speed weft knitting machine. J T e x t .

Ins t . , June 1978,69 6), 169-185.6 Probst, H. New yarn feed systems lor Aat kn itting mach ines. I n t .T e x t . Bull. Fabric Forming, 1989,3, 50-62.7 UMIST Br. Pat. Application 9313225.6.

1990,40 8), 842-848.

Part E: Journal of Process Mechanical Engineering Q IMechE 1995


a new technique for feeding yarn in a flat bed knitting

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