Indian Journal of Fibre & Textile Research Vol. 28, March 2003, pp. 55-59

Spirality of weft-knitted fabrics: Part I-Descriptive approach to the effect

A Primentasa School of Textile Industries, The University of Leeds, Leeds LS2 9JT, UK

Received 5 October 2001; revised received and accepted 11 March 2002

The nature, origin and characteristics of the spirality effect have been examined in detail . The distinction between the spirality effect and other fabric distortions contributes towards the verification, by experiment, that the prime reason for spirality is the yam twist liveliness.

Keywords: Knitting, Spirality, Weft knitted fabric, Yam twist liveliness

1 Introduction 1.1 The Nature of Spirality

In a relaxed state of single jersey tubular fabrics knitted from singles yarns, the wales, instead of being at right angles to the courses, show a pronounced inclination towards the left or the righe3, following a spiral path around the axis of the fabric (Fig. 1). This distortion, known as spiralitl effect, and its magnitude, measured by the spirality angle5, are unpredictable when the fabric is still on the knitting machine because of the imposition of strains on it due to the take-down tension6. If the fabric. undergoes a process of wetting, this distortion may be worse as a result of the reappearance of relaxed torsional forces in the yarn that have been set earlier7. This factor must be recognized in producing a commercially acceptable washable product. .

The main reason for spirality is the unbalanced8.12 as well the residual1314 torque in the yarn, shown by its twist liveliness. Hence, the greater the twist liveliness, the greater is the spiralityl. Moreover, the degree of freedom of yarn movement in the fabric structure contributes significantly to the rise of spiralityl5. The more slack the fabric structure, the greater is the spirality. This slackness can be achieved by changing either the tightness factor or the linear density of the yarn.

The direction of spirality in the fabrics knitted from singles short-staple ring-spun yarns is determined by

a Present address: Department of Textile Engineering, Faculty of Applied Technologies, TEl of Piraeus, 250 Thivon & P. RalIi, Athens 122 44, Greece Phone : + 3-210-4515137; Fax : + 3-210-4122977; E-mail : aprim @teipir.gr

the yarn twist direction. Thus, the technical face of a single jersey fabric exhibits spirality in the Z direction if a Z-twisted yarn has been knittedlO (Fig. 2). As the measurement of the angle of spirality is concerned, either a protrc:tctor or a specially designed transparent plastic board, as is illustrated in Fig. 3, can be used. Furthermore, the percentage spirality, that is considered as the sum of the net spirality caused by the yarn torque and the additional spirality caused by all other factors, can be calculated following the two different geometrical approachesl2.16 (Fig. 4). As the level of acceptable spirality angle is concerned, the opinions are divided, indicating as maximum values five degreesl5.17 or seven degrees I I and the percentage spiralityl8 of 8.

1.2 Knitting Machine and Fabric Distortion

In a knitted fabric, the spirality describes the resulting configuration pf the wales as are skewed

Fig. l -Spirality of a knitted fabric

56 INDIAN J. FIBRE TEXT. 'RES,. MARCH 2003

S-spirality Fig.2-Spirality direction of knitted fabrics

D c

A E B

Fig.3-Transparent board with a protractor configuration

A 8 8' A'r----- ----

Fig,4-Method for the calculation of the percentage spirality (by AATCC)

from the vertical 16, whereas the drop effectl2, cptks.crew or course skew (Fig. 5) concerns. the course skewness from the horizontal and is due to the helical disposition of the coursesl9, In weft circular knitting machines, the yarns are knitted in the circumferential direction 10 and the produced fabrics present a drop that depends on the numbt:Iof the used feeders2o. The degree of drop is a .functioh.:oOhe step S.of the helix (Fig. 6) that depends6ilthe "nQi;nber fcous knitted pet revolution Of aie machirie>tne niimber of courses P[ ,c(5!itiinetre 19 and. the machine circumference' The directi5n of the iilcHnatioii. of tbe drop . depends on the direction of either the revolving cam box or the

w

c c z s

Drop (Course Skew)

w

c c

Spirality (Wale Skew)

Fig.5--Comparison between drop and spirality effects

rotating cylinder. It has been suggested2,3 that both the spirality and the drop effect contribute to the total spirality of the fabric. The effect of the number of feeders on the drop phenomenon has been investigated using the model illustrated in Fig. 6.

Assumptions: The distance between the central lines of two adjacent courses (step S of the helix = KL) is 1 mni. In Fig. 6a, one feeder is used while in Fig. 6b, four feeders are used. The diameter D of the tubular fabric is assumed 23 cm. The various curved

PRIMENTAS: SPIRALITY OF WEFf-KNITIED FABRICS: PART I 57

__ --0. ---.-

Q - g Arrangnt or a Course in a

ltD .. - -- - --. ----- ----Kr'--- - .. -. ' - " - " .. - .. S n 0 t e

Opened Form oCtile Fiaure

Knined ,Fabric (One Feeder' (a)

aD 14---0.-- __ ----------------rT-"-"-" _ . . - . . _ .. _ .. - .. 0

Arrangement of a Course in a Knined Fabric (FoUr Feedml

Ii

Opened Form oflhe Fiaure

(b)

Fig.6-Effect of the number of feeders on the fabric drop (Course skew)

(spiral) lines represent a possible ideal position of the central lines of the successive cqurses.

The opening of the fabric by cutting along a wale shows that the length of a course in the first case (one feeder) is smaller than that in the other case (four feeders). Following a simple geometrical analysis, it can be seen that

OL = r-(7t-X

-D-) 2-

+-S

-2

=

(7tX 23)2 + (0.1)2

= 72.2567 cm

OM =

(7tXD)2 +S( = (7tX 23)2 + (0.4)2

= 72.2577 cm

The difference 0.001 cm of the course length, due to the insertion of three more feeders, can be neglected since the nominal width OK of the opened fabric is 72.2566 cm (1txD = 7tX23 cm). Furthermore, the inclination of the course in the fabric can be:

D

A' y' B'

Fig.7-Spirality of a single jersey fabric knitted from a Z-twisted yarn on a multifeed circular machine with a clockwise-rotating cylinder XX' = Position of a course due to the total spirality; AA' = Position of a wale due to the total spirality; BB' = Position of a wale when spirality (drop) due to the number of feeders exists; XD = Position of a course when spirality (drop) due to the number of feeders exists; X'D = Displacement between two consecutive courses knitted by the same feeder; Of= Spirality (drop) angle due to the number of feeders (YOB); ay = Spirality angle due to the yarn twist liveliness (BOA); and a.,. = Total spirality angle (YOA or A'OY')

0.00138% for one feeder or

58 INDIAN I. FIBRE TEXT. RES., MARCH 2003

appearance of spirality on the knitted fabrics is that the main factor responsible for this defect is the yarn twist liveliness. Twist liveliness is a yarn characteristic that describes the active torsional energy present in the yarn. Its magnitude depends primarily on the torque inserted in the yarn by means of twist. Because spirality appears commonly in fabrics produced from singles yarns, it was decided to produce a range of singles yarn samples having different twist factors in order to investigate the effect of the yarn twist and twist liveliness on spirality.

2.1 Yarn Samples Cotton singles ring-spun Z-twisted yarns of

nominal linear densities 29 tex and 39 tex were produced. For both these yarn linear densities, three nominal twist factors (32, 34 and 39 turns.cm-l.tex!7) were chosen. These produced six yarn samples which were wound onto cones. Quantities of the samples were kept in a normal conditioned place of 202 C and 65% RH for a period of 60 days (conditioned samples), whereas. the rest quantities were kept in a place without any standard atmospheric conditions (unconditioned samples). On the end of the conditioning period, the yarn samples tendency to form snarls (the yarn snarliness) was tested on the appropriate testing apparatus PRIANIC22. The means of the readings are presented in Table 1. The yarns were then knitted, producing single jersey fabrics.

2.2 Fabric Samples

A WILDT Model 5 power-operated, circular onefeeder weft knitting machine was used for the production of the knitted fabric samples. Its diameter was 22.9 cm (9 inch) with a gauge E14 (14 needles per inch). The cam box was revolving in the anticlockwise direction (viewed from above) with a speed of 50 revolutions per minute.

The loop length 1 of the produced fabric samples was calculated from the machine parameters to be 4.83 mm, whereas the tightness factor was calculated using the formula K=tex!7 x /"1 and found to be:

KI = 1.115 tex.mm-I (for 29 tex linear density ); and K2 = 1.293 tex!7.mm-1 (for 39 tex linear density).

It is well known that the tighter the knitted fabric, the less it exhibits spirality due to the restriction of the yarn movement in the knitted structure23 Although in the knitting industry. the commonly used tightness factors range between 1.45 and 1.5, for this experiment the above low tightness factors were

Table I-Yam snarliness and fabric spirality

Testing variables and characteristics

A 29 29 29 B 32.4 34.9 3S.6 C 49.3 60.6 7 1 . 1

D( 2 1 .5 24.5 35.5 D2 IS.0 2 1 .0 2S.5 E 16.3 14.3 19.7

A-Nominal yam linear density, tex; B-Yam twist factor, tums.cm-(.tex; C-Yam snarliness, cm;.

39 39 32.3 33.4 47.2 55.7 20.5 24.0 1 7.5 IS.0 14.6 25.0

39 39.5 70.S 33.5 26.5 20.9

D(-Spirality angle of fabric knitted with unconditioned yam, deg; Dx-Spirality angle of fabric knitted with conditioned yam, deg; and E-DifferenceD(-D2, %

36

to 32 " ..! 28 r 24 ... III 20

Unconditioned yam, 29 lex -+- Conditioned yam, 29 tex -0-Unconditioned yam ,39 lex .... Conditioned yarn, 39 lex

16 +------.------.------.----.----- 30 32 34 36 38 40

TwIst factor, tums.cm'.tu

Fig.S-Effect of yam conditioning on knitted fabric spirality angle

chosen to make more distinguish the effect of the yarn twist liveliness on the spirality of the knitted fabrics. For easier assessment of the spirality angle of the produced fabrics, a latch needle was removed from the cylinder. This produced an apparent needle .1ine (along the wale) in the fabric. The spirality angle was measured with a protractor and the mean values of five readings are shown in Table1.

3 Results and Discussion The increase in the twist snarliness that the yarn

samples of both linear densities exhibited, mainly due to the increase in their twist amount, resulted in the increase of the fabric spirality (Table 1). This is in agreement with the statements of many textile technologists that the spirality depends mainly on the yarn twist liveliness which in this experiment has been expressed in terms of the yarn snarliness.

The fabrics produced from the unconditioned and conditioned yarn samples showed differences in the values of spirality angle (Fig. 8). Although the differences in spirality angle seemed to be rather

PRIMENTAS: SPIRALITY OF WEFf-KNITIED FABRICS: PART I 59

small, the percentage differences would appear to be significant (Table 1). In particular, the percentage reduction in spirality angles of those fabrics produced from the conditioned yarn samples of both linear densities (29 tex and 39 tex) indicated that the atmospheric conditions contributed substantially to the relaxation of the yarn twist liveliness.

The above results indicate the necessity to develop methods for the reduction or, if possible, elimination of the yarn twist liveliness. The less twist lively the yarn, the smaller is the spirality distortion in single jersey garments knitted with a normal tightness factor.

References 1 Nutting T S, HATRA Res Bull, 4 (6) ( 1960) 18. 2 Buhler G & Haussler W, Text Prax Int, 36 ( 10) ( 1981) 1092. 3 Araujo M D & Smith G W, Text Res J, 59 (5) ( 1989) 247. 4 Davis W, Edwards C H & Stanbury G R, J Text Inst, 25 (3)

( 1934) T l 22. 5 Stevens J C, African Text, (December/January) (1985) 28. 6 Charnock IL A, Text Inst Ind, 15 (5) (1977) 1 75 .

7 Fletcher H M & Roberts S H, Text Res J , 23 ( 1) ( 1953) 37. 8 Stacey P, HATRA Res Bull, 4 ( 1) ( 1957) 17. 9 Parker R, Knitting Industry Technical Review, 1 (2)

(1981) 25. 10 Buhler G & Haussler W, Knit Tech, 7 (6) (1985) 373. 1 1 Haigh J S, Wool Sci Rev, (64) ( 1987) 8 1 . 12 Oinuma R & Takeda H , J Text Mach Soc Japan (Engl edn),

34 (3) (1988) 74. 13 Leaf G A V & Glaskin A, J Text Inst, 46 (9) ( 1 955) T587. 14 Munden D L, J Text Inst, 50 (7) ( 1959) T448. 15 Nutting T S, HATRA Notes, ( 13) ( 197 1). 16 Bailey D L, Factors affecting the skew of 100% cotton single

jersey fabrics, paper presented at the Air-jet Spinning Conference, Charlotte, North Carolina, 28 May 1992.

17 Smirfitt J A, An Introduction to Weft' Knitting (Merrow, Watford), 1975.

18 West Point Stevens, NC, USA, (1993). Private communication.

19 Complement aux etudes de vriUage des tricots, ITF Maille Bull Scient, 9 (33) ( 1980) 85.

20 Brackenbury T, Knitting Clothing Technology (Blackwell Scientific Pub., Oxford), 1992.

21 Walker E M &Sleath C E, J Text Inst, 41 (7) (1950) P559. 22 Primentas A, Indian J Fibre Text Res, 28 (2003) 23. 23 Banerjee P K & Alaiban T S, Text Res J, 58 (5) ( 1988) 287.