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End Breaks in the Spinning and Weaving of Weavable Singles Yarns Part 2: End Breaks in Weaving JAMES LAPPAGE1 Canesis Network Limited2, Private Bag 4749, Christchurch, New Zealand ABSTRACT The eight weavable singles yarns studied in Part 1 of this paper were all woven plain weave and the end breaks identified by cause and recorded. End breaks were found to be due to failed splices, abrasion failure and thin places. The incidence of thin-place breaks was found to increase with increasing irregularity (CV%) of the yarn, and as the linear density of the yarn decreased. It is primarily the thin-place end breakage rate which determines the yarn count limit for acceptable weavability, and this is shown to depend on the evenness of the yarn. It is shown how to predict the thin-place end breakage rate from the yarn, fabric and loom parameters.

Part 1 [3] of this paper discussed how the end breakage rate for weavable singles yarns in spinning depends upon the irregularity of the yarn, and how that rate can be predicted from knowledge of the yarn irregularity, the spinning parameters and the parameters of the spinning machine. It was suggested that weavable singles yarns should be spun with a high spindle speed to stress the yarn and remove very weak places which might otherwise survive the spinning operation, and that the spindle speed should not be reduced to reduce the end breakage rate. Spinning is then used as a first point of clearing the yarn, to remove very weak places, which are usually the very thin places, and which are too short to be detected in clearing, but which may be a source of breaks in later stages of processing, particularly in weaving, where breaks are more costly than in spinning. This part of the paper examines the weaving performance of the eight experimental yarns discussed in Part 1 and shows how the thin place end breakage rate in weaving is related to the irregularity of the yarn. It is shown how to predict the thin place end breakage rate from a knowledge of the yarn, cloth and loom parameters.

Causes of End Failure in Weaving Warp ends fail in the loom for any one of the following reasons: (a) abrasion failure, (b) development of excessive hairiness,

1 To whom correspondence should be addressed: tel: ⫹64 (03) 325 2421; fax: ⫹64 (03) 325 2717; e-mail: [email protected] 2 Formerly the Wool Research Organisation of New Zealand.

Textile Res. J. 75(6), 512–517 (2005) DOI: 10.1177/0040517505054189

(c) thin-place break, (d) failed knot, splice or other yarn join, (e) projectile cut, and (f) gross yarn faults. Brorens et al [2] studied the mechanisms of yarn failure in a test which simulates the main stresses applied to warp ends during weaving, namely abrasion and cyclically applied tension, and found that abrasive failure (so called) occurs when yarn incrementally drafts apart under the cyclic stresses imposed. Blanchonette [1] and Weinsdorfer and Wimalaweera [7] have made similar studies on a loom, weaving without weft. They tested the yarn strength before and after “weaving”, and found that cotton/polyester and wool worsted yarns lost about 10 and 20%, respectively, of strength, whereas cotton yarn lost little. This type of failure is promoted by loom parts, particularly the reed, causing twist to be pushed back-andforth along the warp ends, and subjecting the ends to tension peaks during shedding and beat-up. All unsized staple warp yarns suffer a degree of incremental drafting, but in a good weaving yarn this is insufficient for ends to fail during passage through the loom and into the fabric. The resistance of yarn to abrasive failure depends upon the twist and geometry of fiber paths in the yarn. In ordinary singles worsted yarns, the fibers lie in highly parallel helical paths which are more prone to drafting. In woollen yarns there is a high degree of inter-fiber entanglement, which resists incremental drafting. Worsted yarns are also more likely to become excessively hairy because fibers can be pulled out of the yarn structure, or even migrate out (root end first) due to the differential frictional effect occasioned by the fiber © 2005 Sage Publications

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scales. Protruding hairs interfere with adjacent ends, becoming wrapped around them as the ends rotate under redistributing twist, causing the fibers to be further pulled out of the yarn during shedding. This is the primary cause of yarn failure and/or matting of the warp sheet when the weaving of ordinary singles yarn is attempted. Hairiness may cause adjacent ends to ‘stitch’ together, inhibiting complete shedding and presenting ends in the path of the shuttle, rapier or projectile, which may then be severed. Weavable singles yarns such as SoloSpun are highly resistant to abrasive failure and the development of hairiness, and are thus highly weavable in that sense. End breaks due to failed knots, splices or other yarn joins do not depend upon the abrasion properties of the yarn, but only upon the quality of the join. Gross yarn faults (e.g., a large slub) should not exist in well-made yarns which have been cleared. Both these types of end failure are largely under the control of the spinner, and depend primarily on the expertise of the mill. The remaining source of warp end failure is thin place breaks. All yarns spun from staple fibers are irregular, to some extent, and manifest cyclic thick and thin places which occur randomly along the yarn with an overlaid regular variation with a cycle length about four times the mean fiber length (hauteur). This is about the wavelength of the drafting wave peak seen in the spectrograph of an Uster Yarn Irregularity Test. The magnitude of the thick and thin places varies considerably along the yarn, and this is evident in the trace of the yarn profile delivered by the evenness tester. When a very thin place occurs, it may fail under the balloon tension in spinning and a spinning break occurs. When such a very thin place survives spinning in a weavable singles yarn [3], it may fail in weaving, when the peak tensions exceed the local strength of the yarn at the thin place. The magnitude and frequency of very thin places are the factors which determine the spinning limit for ordinary yarns, and the limit in weaving for a weavable singles yarn. Very thin places which survive spinning are substantially very short, usually no longer than the mean

fiber length, and are too short to be detected by current yarn clearers, so they inevitably survive into the warp. The magnitude and frequency of very thin places depend upon the irregularity of the yarn, which in turn depends upon the number of fibers in the yarn crosssection, the variability of fiber diameter and the expertise and equipment of the mill. The best mills produce singles worsted yarns having an Irregularity Index of about 1.1. Weavable singles yarns of this quality, spun from Merino wools in the range 17–24 ␮m mean fiber diameter, have been found by experience to weave with an acceptably low level of thin place warp failures under commonly used conditions of weaving, as long as there are at least 65 fibers, on average, in the yarn cross-section. This arbitrary limit of 65 fibers depends upon the yarn irregularity, mean fiber diameter and conditions of weaving, and can be expected to vary from 65 if the yarn falls outside the range of mean fiber diameters 17–24 ␮m.

Weaving Performance in Plain Weave The eight yarns discussed in Part 1 of this paper, which were spun to achieve a range of evenness values, were all woven on a Sultzer–Rutti rapier loom, square set plain weave at 88% of maximum set, at a speed of 325 ppm, and all warp lengths were 30 m. The nominally 37 tex yarns were drawn with 48 ends/inch in a reed having 16 dents/inch, and the nominally 31 tex yarns were drawn with 54 ends/inch in a reed having 18 dents/inch. The warp width in the reed was 66 inches. Weaving was carefully observed and all warp end breaks were recorded and the cause identified. The end breakage rates are listed by cause in Table I as breaks per 103 ends per 105 picks (108 interlacings). The yarns were all spun from 23.4 ␮m wool (CV ⫽ 23.1%), so the nominally 37 tex yarns had an average of 62 fibers in the cross-section, and the nominally 31 tex yarns had 52 fibers. These were all below the recommended 65 fibers and some thin place end breaks were expected in every warp. In practice, some thin place breaks were recorded in every warp except that for yarn

TABLE I. Weaving performance, plain weave fabrics (end breakage rate per 108 interlacings). Yarn no.

1

2

3

4

5

6

7

8

Evenness CV% Yarn tex (nom) Knots Thin places Abrasion Splice failure Other Total

15.25 37 0 0 0 3.02 0 3.02

15.24 37 1.84 2.45 0 1.84 0 6.13

16.54 31 3.20 26.66 0 3.20 1.07 34.13

16.93 31 1.71 15.41 12.84 3.43 0 33.39

16.08 31 4.04 15.34 0 5.65 0 25.03

16.05 31 1.71 7.71 1.71 3.43 0 14.56

14.99 37 0 1.04 3.12 3.12 0 7.28

14.61 37 0.98 1.95 0 1.95 0 4.88

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514 no. 1. Some splice failures were recorded in every warp, some failed knots in most warps and abrasion failures were recorded in three warps (invariably thick places).

Measurement of Peak End Tensions and Warp Wear The peak tension, P, experienced during the picking cycle can be measured with appropriate instrumentation. Blanchonette [1] measured peak tensions in a 25 tex weavable singles worsted wool warp woven plain weave on a Somet flexible rapier loom running at 370 ppm, and found tension peaks around 0.75 N (74 g) in a single warp end using a DEFAT unit (Zweigle G 590) to measure the tension variation and peak values, with the tension probe located between the back rest and the droppers. He found that the tension varied by as much as 25% in adjacent warp ends, and that single end tensions at various points across the loom varied between the middle and sides of the warp from about 0.8 N (78 g) to about 0.23 N (23 g). The measurement of 0.75 N was made approximately 40 cm from the right-hand side of the loom. Weinsdorfer et al. [5] made similar measurements on a polyester/cotton warp having 15 744 ends and found peak tensions about 75 cN (74 g). Robinson and Gee [6] studied the weaving performance of 71 worsted warps, woven 2/2 twill on a Sulzer VSK weaving machine and measured peak warp tensions in groups of 200 ends at five points across the loom. They found peak end tensions at beat-up averaged 1.33 N (130 g) and 1.16 N (114 g) for “high” and “low” warp tensions, respectively. The highest tension peaks observed by Blanchonette were at beat-up of the weft, with smaller peaks associated with shedding. The highest tensions, however, are likely to occur closer to the fell of the cloth than Blanchonette’s point of measurement, since there may be some dissipation of tension at the healds and the droppers, but it is impractical to measure tensions in the region of the shed and/or the reed. As a result of this difficulty and the doubt about tension measurements, other means of estimating peak warp tensions were sought.

Calculation of the Linear Density of Thin Place at Break The maximum linear density, t tex, of a very thin place which will break under the peak end tension can be calculated if the peak tension and the strength of the thin place at the point of break are known. The strength of the thin place can be calculated if the tenacity at thin places

is known; this is defined here as the absolute tenacity, A. An end break will occur when t ⫽ P/A

.

(1)

The absolute tenacity is not the tenacity found by the usual calculation from mean yarn strength and mean linear density, but is the tenacity at the point of break, based upon the strength of a thin place and the linear density at the point of break. This has been discussed elsewhere [4], where it is assumed that all breaks in single-end strength tests in weavable singles yarns occur at the thinnest place in the test specimen. The mean linear density, t⬘, of the thinnest places (the points of break in the strength test) can be calculated from the mean linear density, m, of the yarn and the mean amplitude of the thin places: t⬘ ⫽ m共1 ⫺ √2CV兲

,

(2)

where CV is the coefficient of irregularity of the linear density of the yarn. The absolute tenacity of the yarn is then A ⫽ mean breaking load/t⬘

.

(3)

The frequency of occurrence of a thin place as fine, or finer than t can be calculated statistically from the irregularity of the yarn. It has been shown [5] that linear density is distributed along the yarn closely to the normal distribution. Accepting this, the probability of finding a thin place of linear density t is given by pr共t兲 ⫽ pr共Z ⬍ 关m ⫺ t兴/␴兲

,

(4)

where ␴ is the standard deviation of linear density, m, of the yarn. A thin place will just break when Z ⫽ 共m ⫺ t兲/ ␴ or

t ⫽ m ⫺ Z␴ .

(5)

(The probability of t is determined from tables of the normal probability function Z; because an occurrence of linear density as fine as t is a rare event, tables reading to 10 decimal places are required.) The average length, L, of warp yarn between places as fine or finer than t is given by L ⫽ ␭ /pr(t)

,

(6)

where ␭ is the mean length of the drafting wave. The end breakage rate is given by end breaks/106 m of warp yarn ⫽ 106 /L

.

(7)

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Analysis of Thin Place End Breaks in the Weaving Trials Without a precise measure of the peak tensions experienced by warp ends, the thin place end breakage rate cannot be predicted with confidence. However, knowing the breakage rates in the weaving trials, it is possible to estimate what the peak tensions were for each of the eight warps woven. The variations in end tensions across the loom observed by Blanchonette [1] and by Weinsdorfer et al. [8] are a further source of difficulty but, for simplicity, it is assumed that the ends all experience substantially the same peak tension. The measured physical properties of the yarns are given in Table II and the statistical parameters and the calculated peak end tensions are given in Table III. In Table II, the average linear density t⬘ of all thin places in the yarn was calculated using equation (2) and the absolute tenacity by equation (3). In Table III the probability of t is found by equation (4), Z is found from tables of the normal probability function and t is calculated by equation (5). It is also assumed that the yarn strength reduces by 20% due to wear during weaving, as found by Blanchonette [1]. For the calculations made in Table III, it is necessary to take into account the end breaks in spinning, since these are part of the total of thin places in the yarn which

would break in weaving if they had survived spinning. For simplicity, the end breakage rates are all expressed as breaks/106 m in Table III. Weaving breaks generally increased exponentially with yarn irregularity, similarly to the spinning breaks, but with some anomalies for yarns 3, 4 and 5. The average peak tension for the nominally 37 tex yarns was 96.5 g, for the nominally 31 tex yarns it was 86.0 g, and the overall average was 91.25 g. These estimates are intermediate between the measurements of Robinson and Gee (114 –130 g) [6] and Blanchonette (74 g) [1]. A major difference between Blanchonette’s experiment, the work of Robinson and Gee, and the present work is the linear density of the yarns used. Robinson and Gee did not give details of all of the yarns woven, but quoted 50 all-wool yarns with an average linear density of 45 tex. They also wove 31 yarns comprised of wool/polyester and polyester/viscose blends, which had more than twice the tenacity of the all-wool yarns, and may account for the highest of the peak tension measurements; for this reason, the lower average peak tension of 114 g is likely to be more relevant to the all-wool yarns. The average peak tensions and yarn linear densities are compared in Table IV and are plotted in Figure 1. The relationship between peak tension and yarn linear density is assumed to be linear over the range of yarn

TABLE II. Physical properties of the experimental yarns. Yarn no. Linear density (tex) Mean breaking load (g) Evenness CV% Standard deviation, ␴ t⬘ (tex) Absolute tenacity (g/tex)

1

2

39.2 304 15.25 5.978 30.7 9.90

3

38.0 291 15.24 5.791 29.8 9.77

4

31.5 219 16.54 5.210 24.1 9.09

5

30.9 217 16.93 5.231 23.5 9.23

31.5 212 16.08 5.065 24.3 8.72

6

7

8

32.0 199 16.05 5.136 24.7 8.06

36.5 258 14.99 5.471 28.8 8.96

36.6 249 14.61 5.347 29.0 8.59

6

7

8

TABLE III. Statistical parameters and estimated peak tensions. Yarn no.

1

Total ends in warp 3184 Picks/cm in weave 19 6 Thin place breaks 10 /m of warp yarn: spinning 6.1 weaving 0 total breaks 6.1 L (m) 163 934 pr(t) (⫻106) 1.732 Z 4.372 ␴ 5.987 t (tex) 13.02 Absolute tenacity after weaving (g/tex) 7.92 Estimated peak tension (g) 90.8

2 3184 19 9.8 46.5 56.3 17 762 0.160 3.861 5.791 15.64 7.82 108.8

3

4

5

3582 21

3582 21

3582 21

3582 21

3184 19

3184 19

45.4 560.7 606.1 1650 0.017 3.238 5.210 14.63

50.8 323.4 374.2 2672 0.011 3.371 5.231 13.27

10.3 322.1 332.4 3008 0.944 3.411 5.065 14.22

15.7 161.9 177.6 5631 0.504 3.571 5.136 13.66

2.1 19.7 21.8 45 872 6.191 4.088 5.471 14.13

3.4 37.0 40.4 24 752 0.115 3.941 5.347 15.53

7.27 93.4

7.38 85.0

6.98 87.8

6.45 77.6

7.17 90.3

6.87 96.0

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TEXTILE RESEARCH JOURNAL From equation (8) P ⫽ 2.674 m

TABLE IV. Yarn linear density and mean peak tension. Source

Linear density (tex)

Peak tension (g)

Blanchonette Lappage Lappage Lappage Lappage Lappage Lappage Lappage Lappage Robinson and Gee

25.0 39.2 38.0 31.5 30.9 31.5 32.0 36.5 36.6 45.0*

74.0 90.8 108.8 93.4 85.0 87.8 77.6 90.3 96.0 122.0*

and

␴ ⫽ m CV

.

Substituting in equation (9)

Z ⫽ 共m ⫺ 2.674 m/A兲/m CV

or

Z ⫽ 共1 ⫺ 2.674/A兲/CV

.

The probability of t [pr(t)] is then found from tables of the probability function Z. The average length of yarn between places as fine or finer than t is

* Averaged values.

L ⫽ ␭ /pr共t兲, and the thin place end breakage rate is calculated by equation (7): Breaks/106 m ⫽ 106 /L

.

This can readily be translated into the commonly used end breakage rate based upon the number of ends and picks woven.

Conclusions

FIGURE 1. Relationship: peak end tension versus yarn linear density.

counts studied. If the origin is accepted as a valid point, the relationship, by linear regression, is: Peak tension ⫽ 2.5833 tex ⫹ 3.0874 共r ⫽ 0.9730兲.

(8)

This approximates to: Peak tension ⫽ 2.674 tex

.

Prediction of the Thin Place End Breakage Rate If a warp end will break under peak tension when a thin place appears of linear density t or finer, then the probability of a break is given by pr共break) ⫽ pr共t兲 ⫽ pr共Z ⬍ ⫺ 关t ⫺ m兴/␴兲 or when

t ⫽ m ⫺ Z␴

now

t ⫽ P/A 共equation 共1兲兲

therefore

P/A ⫽ m ⫺ Z␴

or

Z⫽(m⫺P/A)/␴.

(9)

End breaks occur in the warp due to several causes, most of which depend upon, and are under the control of, the expertise of the spinning and winding departments. These include failed knots, splices or other yarn joins. In weavable singles yarns, end breaks can occur at thin places in the yarn if it is attempted to weave yarns which are below the recommended count limit, which is, for merino wool, represented by an average of 65 fibers in the yarn cross-section, or if a limiting yarn is particularly irregular. The end breakage rate was found to increase exponentially as the irregularity of the yarn (CV%) increased. It appears that it may be possible to predict statistically the likely end breakage rate for thin places from knowledge of the peak tensions encountered by warp ends during the picking cycle, the strength (in the loom), the irregularity and the linear density of the yarn. This prediction may be useful as a guide in determining the weaving limit for weavable singles yarn made from a given lot of fiber. Peak end tensions in the loom have been measured by other authors [1, 6] and some estimations have been made in the present work. These various results show a good, linear correlation between peak tension and yarn linear density. The primary advantage offered by weavable singles yarn is a major saving in the cost of production. A further potential advantage is the possibility to spin and weave yarns which are much finer than two-fold yarns can be

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made, and the production of very lightweight fabrics not previously possible in 100% wool. The major factor limiting how fine a yarn can be woven is the irregularity of that yarn.

3.

4.

ACKNOWLEDEGMENTS 5.

This work was carried out under a project funded partly by the Australian Wool Research and Promotion Organisation, and partly by the New Zealand Government Foundation for Research, Science and Technology.

6.

7.

Literature Cited 1. Blanchonette, I., Tension Measurements in Weaving of Singles Worsted Wool Yarns, Textile Res. J. 66, 323–328 (1996). 2. Brorens, P. H., Lappage, J., Bedford, J., and Ranford, S. L.,

8.

Studies on the Abrasion Resistance of Weaving Yarns, J. Textile Inst. 81, 126 –134 (1990). Lappage, J., End Breaks in the Spinning and Weaving of Weavable Singles Yarns, Part 1: End Breaks in Spinning, Textile Res. J. 75, 507–511 (2005). Lappage, J., The Tenacity of Worsted Weaving Yarns, submitted to J. Textile Inst. Lappage, J., The Distribution of Linear Density from Pointto-Point Along Worsted Spun Yarns, submitted to JTATM. Robinson, G. A., and Gee, E., Yarn Requirements for Efficient Weaving of Worsted Fabrics, in Proc. 7th International Wool Textiles Research Conference 1985, Tokyo, Vol. 3, pp. 1–10. Weinsdorfer, H., and Wimalaweera, W., Investigation on Abrasion and Roughening of Warp Yarns During Weaving, Textil Praxis Int. 42, 611– 64 (1987). Weinsdorfer, H., Wolfrum, J., and Stark, U., The Distribution of the Warp End Tension over the Warp Width and How it is Influenced by the Weaving Machine Setting, Melliand Textilber. 72, 903–907 (1991).