Indian Journal of Fibre & Textile Research Vol. 25, June 2000, pp. 97- 1 07

Some preliminary investigations into the mechanics of 1 x 1 rib loop formation on a dial and cylinder machine

Sad han Chandra Ray' & P K Banerjee

Department of Textile Technology, Indian Institute of Technology, New Delhi 1 1 0 0 1 6, India

Received /6 April 1999; accepted 18 May 1999

An attempt has been made to find out the intluence of the variables affecting loop length as well as to gain an insight into the mechanics of loop formation on a dial and cylinder machine with I xl rib gating. For the purpose, the movement of yarn across the knitting elements during loop formation has been studied with the help of a travelling microscope and the loop length has been measured in fabrics produced under different conditions of knitting. The forward movement of yarn inside knitting zone general ly continues up to the cylinder knitting point. The backward movement of yarn occurs in atleast two phases-the first phase occurs just after cylinder knitting point and the second phase occurs around the dial knitting point. It has also been observed that the loop length in general increases with the increase in cylinder cam setting, dial cam setting and dial height and decreases with the increase in input tension. A wide range of loop length can be produced on a dial and cylinder machine only by changing the timing of knitting. General ly, the lower loop length results at higher delayed timing.

Keywords: Cast-off loop, Cylinder needle, Dial needle, Knitting. Loop length

1 Introduction The properties of the knitted fabrics are mainly

governed by two parameters, namely length of yam in a loop and shape of the loop. Although the shape of loop is finalized upon relaxation treatment of the fabric, the length of loop is mostly decided on the machine during loop formation. It is obvious that if the variables governing the length of loop as well as the mutual interaction betw�en the variables inside knitting zone ( KZ) are known, it would be possible to produce a knitted fabric with a pre-determined loop length.

Although the scientific studies in the field of weft knitting are being carried out over the past seventy years, the mechanics of loop formation has become a subject matter of research only since early 60's. Moreover, such experimental and theoretical studies have been mostly restricted to single-jersey loop formation on circular bed machine only.

Experimental studies have , been conducted on circular single jersey machines to find out the effect of yam-yarn and yarn-metal friction, yarn input tension, cam setting, cam shape, fabric take down load and yam properties on loop length l - I I and on yam tension profile inside the knitting zone5. 1 2- 1 9. It

'Present address : Institute of Jute Technology, 35 8allygunge Circular Road, Calcutta 700 0 1 9, India

has been observed that the length of yarn consumed per stitch is governed by the frictional properties of the yam as well as the input tension whereby the nature of interaction between loop length and input tension depends on the shape of the stitch cam and range of input tension. Loop length is also found to be related to the take do+.n load whereas machine speed has hardly any significant effect. However, the information on systematic studies pertaining to the mechanics of loop formation on double-jersey machine is not available in accessible international journals, whereas an understanding of the same is a prerequisite for (a) optimizing the knitting process, and (b) redesigning machine components. The present study has, therefore, been planned for gaining an insight into the knitting action on a dial and cylinder (double-jersey) machine with I x l rib gating as I x l rib is the simplest of all double-jersey structures.

A I x 1 rib machine is equipped with two sets of needles, namely cylinder needles (CN) and dial needles (DN) arranged on cylinder bed (XY plane) and dial bed (XZ plane) respectively (Fig. I ) which are mutually perpendicular to each other. Between the beds there is vertical as well as horizontal gap; the vertical gap is known as dial height (VG) and it is adjustable. As illustrated in Fig. I , the extension of CN and DN intersect at a point (S), termed in this study as point of intersection . The l ine joining the

98 INDIAN 1. FIBRE TEXT. RES. , JUNE 2000

Fig. I-Side view of knitting zone (needles at knitting points under synchronised t iming)

points of intersection of all the needles would henceforth be termed as reference l ine. On account of the movement of cylinder needle and dial needle a10ng two mutually perpendicular planes, a complete rib loop is formed in multiple planes and is made up of one cylinder loop, one dial loop and two links (Fig. 2). In the course of knitting, the neighbouring needles of both sets may form loop simultaneously by drawing yam from the supply package and reach their respective knitting points-cylinder knitting point (CKP) and dial knitting point (DKP)-at the same time, designated as synchronised (SYN) timing, or the dial needles may form loop and reach the knitting point with a phase lag to the neighbouring cylinder needles, designated as delayed (NO) timing. The extent of phase difference between the knitting points of cylinder needle and dial needle in terms of number of needle spacings is adjustable on machine and hence needs to be considered as an important factor in the study of mechanics of loop formation on a double­jersey machine.

In order to simulate the knitting process with the help of a computer, Le. to model the loop formation process, it is essential to observe the action of the knitting elements and the movement of yam across needles inside KZ which run at a circumferential speed of about I mls and take the yam from the supply package at the rate of 3 .0-4.0 mls under normal conditions of knitting. Understandably, this i s not possible without taking recourse to expensive aids such as video photography, etc .. However, for the present purpose, the machine was turned very slowly by hand and the instantaneous positions of the needles and yam were followed b)' means of a travelling

Fig. 2-Components of rib loop unit

microscope. Although, such a method would result in low yam input tension, the facets of knitting process

d 'd ' d fC d l 1 .20 un er conSl eratlOn 0 not get a lecte . A comprehensive l ist of factors which are likely to

alter loop length on a double-jersey machine has been proposed by Little2 1 . It has been demonstrated 1 .20.2 1 that the changes in dial height, cam settings and take­down tension result in significant changes in loop length . However, Knapton22 opined that the adjustment of dial height may be the most misused control on a double-jersey machine. According to Marvin & Arauj02\ the disposition of the fabric on a double-jersey machine is such that most of the fabric weight and take-down load are carried by the dial needles and the loops on these needles are stressed more than the loops on the cylinder needles. Moreover, a rearrangement of the angular relation between CN and DN could alter the distribution of stress on the fabric at the point of loop formation and fabric passage.

2 Materials and Methods

2.1 Experimental Approach To gather the general ized information on the nature

of yam movement across knitting elements inside KZ

and the effect of some input variables on loop length,

RA Y & BANERJEE: MECHANICS OF I x l RIB LOOP FORMATION 99

experiments were carried out on two - course gauge dial and cylinder machines (Table 1 ) equipped with t x t rib gating under different timings of knitting. Observation of yam movement inside KZ was done by turning the machine very slowly by hand and recording the instantaneous positions of the knitting elements and yam segment by means of a travelling microscope. Yam segment was marked with indelible ink at the initial yam-needle contact point for its identification inside KZ. As the presence of cast-off loop and bed verges cause segmentation of the loop arm on a dial and cylinder machine, the points of such segmentation were incorporated in the calculation of yam displacement across needles with the help of computer. For the sake of comparison, the yam movement across needles on a single-jersey machine was also studied. Yam displacements were calculated from the co-ordinates of the marked point and of the needles, which were subsequently converted to yarn speed across needles.

To identify the roles played by some input variables in determin ing loop length, the fabric samples were made by using variable yam input tension, cylinder cam setting, dial cam setting and take-down load under different timings of knitting. The unroved loop length was subsequently measured for all the samples. For establishing the occurrence of robbing back of yam inside KZ, the length of yarn contained in a loop at knitting points on the machine was marked for a few combinations of input variables. The marked loop length was measured in unroved state by a travelling microscope. The unroved yarns were subjected to decrimping load (obtained on Instron) during length measurement . While conducting experiments, the cam settings and input

Table I-Machine particulars

Particular Machine A Machine B Machine C (Mellor (Raj interlock (Single Bromley machine) jersey interlock mlc) mlc)

Gauge 1 6 20 1 2

Diameter, in. 30 1 6 3.5

No. of feeders 1 2 8

Dial height, mm 1 .6 1 .6

Horizontal gap 1 .8 1 .5

between beds, mm

Cylinder 40"/0", 2.5*a/ 46"/0", O*a/ 45"/45"

stitch cam O , O*a 9 , 7*a

Dial stitch 40"/32", .94*a/ 39"/ 39", 2.4*a/-

cam 0 , \ .5*a O , 2.4*a

tension were monitored with the help of a travell ing microscope and a digital yam tensiometer respectively.

2.2 Stitch Cam Characteristics The method of characterizing the particulars of a

stitch cam has been demonstrated in Fig. 3 where 'a' represents the distance between two neighbouring CN and DN, while a, � & Y represent angles and C1 & C2 are some constants. The characteristic values of stitch cams are listed in Table I and the values of cam settings maintained during observation of yam movement inside KZ are shown in Table 2.

2.3 Measurement of Yarn and Needle Movements Travelling microscope was employed for observing

the yarn and needle movements. This microscope permitted the fol lowing of yam and needle movements in vertical plane only, i .e. along X and Y directions. But following of the movement of a yam element in the rib knitting machine would involve monitoring its coordinates in space, i.e. of X, Y and Z values . Accordingly, to monitor the Z-coordinate, a

�CI x a + C 2 x o 1 R e p r& se n t o t ion : r;(f t9 . C , x a / y_ C � x a

Fig. 3-Characterization of stitch cam

Table 2-Cam settings and depth of needles at knitting points

Type of Cam setting, mm Depth of needle below machine Cylinder Dial reference/ sinker line

A

B

C

1 .6

2.2

4.5

1 .5

1 . 5

Cylinder Dial needle needle

3.5

4. 1

4.5

3.6

3.3

100 INDIAN J. FIBRE TEXT. RES., JUNE 2000

mirror, as shown in Fig. 4, was positioned just above the KZ at 45° angle to the horizon. The movement of the yarn along YZ plane could then be projected and observed through the microscope on XY plane.

2.4 Observation of Yarn and Neeate Movements under Quasi-static Condition

By turning the machine manually, one CN was made to just catch the feed yarn. In that position, a mark was applied on the yarn lying across the preceding DN with indelible ink. The coordinates of the marked point and of relevant CN and DN were recorded. The machine was then turned through a small angle so as to effect a short displacement of CN. The new positions of the marked point and relevant needles were recorded. This procedure was continued till the eN under observation reached the running position after passing through the knitting points. This method of observation of yarn and needle movements was fol lowed on all the machines under study. However, the observation was restricted to the XY­plane only in case of single-jersey machine.

The length of yarn entering the KZ was also observed intermittently fol lowing the same method.

2.5 Measurement of Take-down Load For wider fabrics (applicable to the machines under

study), the take-down load 'is usual ly generated by applying some strain on the fabric being formed on machine (i.e. higher rate of take up compared to the rate of fabric formation). So, the amount of strain on the fabric near take-up roller can be expected to be related to take-down load. But as the fabric shrinks and its dimension changes after its formation inside KZ, the measurement of strain on the fabric, very far away from KZ, would not provide the right information. However, if the dimension of the fabric just after its formation on the machine is measure}! first and then the relaxed fabric is brought back to measured dimension by applying some known load,

-i{----_- A

� -;>f---t----_- A'

Fig. 4--Method of projection of horizontal displacement into vertical displacement

then the actual load acting on the wale l ines under consideration can be determined.

For the present study, it was decided to mark the fabric at a distance of 5 cm below the fel l of fabric with the help of a template (flexible metal strip). The length of the template chosen was a compromise between

(a) desirability of measuring as large a distance as possible to achieve minImum error of measurement,

(b) desirability of marking as close to fabric fel l as possible to approach the true value of load acting on the wale l ines just beyond the cast-off point, and

(c) controlled manoeuverability of template and marker pen in the somewhat inaccessible region of fabric formation zone.

This 5 cm length of fabric would subsequently contract length-wise as well as width-wise when the fabric is allowed to relax. The fabric width can be restored to the value prevalent during its formation by stretching till the number of wales per unit length equals twice the value of the machine gauge. After adjusting the fabric width in this manner, the segment of fabric lying within the marked l ines is stretched to the original length (5 cm) on machine. The force required to stretch length-wise is then divided by the number of wale lines for working out the load per wale l ine on machine.

3 Results and Discussion

3.1 Yarn Displacement and Yarn Speed The calculated values of displacement of the

marked point on yarn across DN and CN under different timings are plotted in Figs 5-7. It is observed from the Figs 5 and 6 that the values of displacement are higher in the case of SYN timing than those under 3 ND timing for the same amount of circumferential displacement of CN on machine A. Similarly, the values of yarn displacement are higher in the case of 2 ND timing than 4 ND timing on machine B . This trend is maintained even at the running position of the needles when the displacement is stable. Therefore, it may be inferred that the yarn displacement. decreases with the increase in delay and consequently one could expect lower loop length at delayed timing.

It is also observed from the displacement curves (Figs 5 and 6) that from the point of initial contact (CP) between yarn and needle, the values of yarn

RA Y & BANERlEE: MECHANICS OF I xl RIB LOOP FORMATION 1 01

� 5 ,----------------------------------,

.9 8. 10 1 � '0

r .-a A(,.TOU DN (SYN) Acrota ON (3 NO)

• -- I- Aero •• eN (SYH) 0---0 Aeros, eN (� ND)

CKP CKP2 CKP4 CKP6 CKPB

Displacement of eN along X- axis( l div.=08mm)

Fig. 5-Displacement of the marked point under SYN and 3 NO timings on machine A .

12 r----------------------

§ 1 0 .5 8. 8 ] � 6 � '0

------- AcrolS ON (2 ND) Across ON (4 NO)

-t- - - + Acron eN (Z ND) 0--0 Acron eN (4 ND) O ��-L·�I�I �I�ILLI �I_LI �I�I_LI �I�I �I_LI �I�ILLI JI_LI �I�I J

GP CKP CKP2 CKP4 CKP6 CKP8

Displacement of eN along X- axis( t div �O·6ITJrn) Fig. 6-0isplacemenl of the marked point under 2 NO and 4 NO timings on machine B

displacement across DN and CN gradually increase up to CKP under any timing. After CKP, there may be further increase or decrease or even cyclic decrease and increase in displacemept depending upon the timing of knitting. But whether the timing is synchronised or delayed, there is always some decrease in the displacement of the marked point across DN and CN as the needle under study crosses the knitting points before the displacement value becomes stable. However, the amount of decrease in the displacement of the marked point and the point of such occurrence vary with the timing of knitting.

On machine A, in the case of SYN timing, the displacement of the marked point across DN increases up to a distance (CKP2) of 2*a beyond CKP and then decreases; however, the maximum displacement of the marked point across CN occurs at CKP. The percentage differences between the maximum value and the stable value of displacement of the marked point across DN and CN are 1 8.9 1 and 23.63

.S 8.

1 5.----------------------------------, ...- Acro$! Yf!ree

+- - + ACTO" needle

1l 1 0

i .s '0

O �-LLL-L���������L-���� I 2 3 4 5 6 7 B 9 10 I I 12 13 14 15 ·16 17 CP KP

Displacement 0.1 needle along X - axis( l div.=06mm)

Fig. 7-0isplacement of the marked point across verge and needle on machine C

respectively. In the case of 3 ND timing, the displacement of the marked point across DN and CN increases continuously up to CKP. Then there is a very slow decrease in displacement for quite some time followed by cyclic rise and fall in displacement before attaining a stable value at a distance (CKP8) of about 8*a beyond CKP. The percentage differences between the maximum value and the stable value of displacement of the marked point across DN and CN are 1 2.63 and 20.22 respectively .

On machine B, the displacement of the marked point acro�s both DN and CN gradually increases up to CKP. In the case of 4 ND timing, the displacement of the marked point after CKP across both DN and CN decreases gradually but very slowly before attaining a stable value at a distance of about 8*a beyond CKP. However, in the case of 2 ND timing, the displacement across CN beyond CKP continues to increase. The displacement across DN decreases marginally after CKP but increases again subsequently. Both of these displacements across CN and DN become maximum at a distance (CKP4) of 4*a from the CKP, i .e. when the neighbouring DN reaches DKP. Afterwards, the displacements decrease and attain stable values. The percentage drops in the displacement of the marked point from the maximum to the stable value across DN and CN are 4. 16 and 1 2. 1 3 only, i .e. very low in the case of 2 ND timing. The corresponding values are 17 .58 and 14.48 respectively in the case of 4 ND timing.

In case of single-Jersey machine also (Fig. 7), the displacement of the marked point across both verge and needle increases continuously from the point of initial contact between yam and needle. However, in this case, the reduction in displacement value starts even before the knitting point and continues steadily

1 02 INDIAN 1. FIBRE TEXT. RES. , JUNE 2000

beyond knitting point until a stable value is attained. Moreover, the percentage decrease in the displacement of the marked point across verge and needle due to yam flow back are 27.27 and 25 .6 1 and hence comparatively higher than those occurring in double-jersey machines.

The values of speed of the marked point on the yam across ON and CN are shown in Figs 8- 1 2. It is observed from these figures that the speed of the marked point across ON and CN at different stages of loop formation is higher in the case of SYN timing up to CKP. The values gradually decrease as the delay increases. Higher speed of yam across ON and CN indicates that more length of yam is flowing across them. This would suggest a possibility of larger loop length for lower delay.

It is also observed that the values of yam speed across ON and CN are maximum at the moment the

0.6 r----------------------,

- 0. 2

__ Yarn entering ______ Acrogs DN + --+ Acr09!1 eN

•

2 3 5 6 7 8 9 10 1 1 12 13 14 15 1 6 Displacement of Ct'I along X- axis( 1 div. =08mm)

Fig. 8-Speed of the marked point across DN and CN and rate of yarn entering KZ under SYN timing on machine A (D = machine diam.; N = machine rpm)

O .5 �------

004 +-- + I\('ro�!t eN

-O. 1 CUpL..J.�-'---'-.LCK-PL..J.----'--'---'-.L......JL..J.C-KPJ....4-'---'

-.L......J�----'-

-'CL.,K':'-.-'P B Displacement of CN 'alon,g X - axis( 1 rliv. =O-Bmml

Fig. 9-Speed of the marked point across DN and CN under 3 ND timing on machine A (D = machine diam.; N = machine rpm)

CN catches the feed yam. Afterwards, these values gradually decrease. This is due to the fact that when a particular CN (CN1 ) catches the feed yam and draws it , the neighbouring CNs (CNo, etc .) ahead of CN"

0.40 Acr099 ON

0.30

+--1- Acron eN

� 0.20 .:::: i 0. 1 0 .5 1 0 - - - - - - - - - - - -

-0. 1 0 + CP CKP4

Displacemenl of CN along X- axis( l div. =Oumm )

Fig. I O-Speed of the marked point across DN and CN under 2 ND timing on machine B (D = machine diam; N = machine rpm)

0.6 .----------------------,

0.5 � 04 � 0.3

u .S '0 <U � D. l

� Yarn enterlDl ______ Across DN

+ - - + Ar:rn!l' eN

Displacemellt of CN along X-axis( l div.; O{)mm) Fig. I I -Speed of the marked point across DN and CN and rate of yarn entering KZ under 4 ND timing on machine B (D = machine diam.; N = machine rpm)

(J .n ACrOSq vcric ----- + - - + AcrOSIL nt'edl� 0.-1

� + 0.2 + " !j " 1. '"

-0.2

-004 LJ...--1_L.-.1.---1--L.--1_'----'--......... O-,---____'_--'_-'----'----'--' I 2 3 4 � 6 7 8 9 10 I I 12 13 14 15 16

CP KP

Displacement of needle aloIlg X - axis ( l di v.=O-6mm)

Fig. 1 2-Speed of the marked point across verge and needle on machine C (D = machine diam.; N = machine rpm)

RAY & BANERJEE: MECHANICS OF l x l RIB LOOP FORMATION 1 03

which are yet to cross CKP, also draw yam from the package across CN1 • So, the total requirement of yam inside KZ passes at this instant across CN I . The moment CNo crosses CKP a�d/or does not draw yam any further from the package, the yam drawn from the package by CN1 is used by this needle only. By this time or with a time gap, the following CN (CN2) enters the KZ and draws yam from the package. However, the amount of yam drawn by CN2 does not pass across CN1 • Moreover, the yam flows back to CN1 from CNo which has crossed CKP.

The speed of the marked point on the yam across ON and CN approaches zero near CKP and then fluctuates within a small range of positive and negative values. The negative values of yam speed across ON and/or CN mean that the yam is flowing back towards the needle, i .e . robbing back is taking place. In the case of single-jersey machine also, the speed of the marked point follows the same trend. However, the speed becomes negative, i .e. robbing back takes place before the knitting point.

The measured values of the rate of yam entering the KZ are plotted in Figs 8 and 1 1 for two cases, namely SYN timing on machine A and 4 NO timing on machine B. It is found that in both the cases the rate of yam entering the KZ is higher than the rate of yam flowing across ON or CN. This is easily understood when one considers that the rate of yam entering the KZ represents the cumulative consumption rate of all needles enclosed by KZ at any instant. Moreover, the rate of yam entering the KZ

does not remain constant during the loop formation process. There is cyclic rise and fall in the rate of the yam entering the KZ and the distance between the two peaks is equal to the distance between the two neighbouring needles in the same bed. Hence, a constant rate of yam feeding may result in input yam tension variation as the requirement of yam by the needles varies within the knitting cycle.

3.2 Effect of Input Variables on Loop Length The experimental values of unroved and marked

loop length under different conditions of knitting are shown in Tables 3 - 9 .

Timing of knitting plays an important role 'in

governing the loop length. The unroved and marked loop lengths and their dependency on timing are shown in Table 3 . It is observed that the values of unroved as well as marked loop lengths vary with timing. The maximum loop length in the case of machine A is obtained under SYN timing, and in the case of machine B the same is obtained under 2 NO timing. The values of loop length gradually decrease with the increase in delay on both the machines . For simi lar combinations of input variables, there is 1 5 .88% difference in loop length between SYN and 3 NO timings on machine A and 1 2.22% difference in loop length in between 2 NO and 4 NO timings on machine B . So, the adjustment of dial bed with respect to cylinder bed makes it possible to produce a wide range of loop lengths, keeping the cam settings and other input variables unchanged.

The values of marked loop length differ from those of unroved loop length irrespective of the timing of knitting. Moreover, the values of marked loop length recorded at three different points namely CKP, OKP and LFP (loop forming point), are also different under the same timing. In general, the marked loop length at LFP, i .e. at running position of the needle, is minimum out of the three and is almost equal to unroved loop length . The marked loop length at OKP is generally higher than that at CKP. However, this difference decreases with the increase in delay and under 4 NO timing on machine B, the marked loop length at OKP becomes lesser than that at CKP. The reason for the marked loop lengths at CKP and OKP being, in general, greater than the unroved loop lengths is robbing back or flow back of yam across needles inside KZ. Tht! slight difference between the marked loop length at LFP and the unroved loop

Table 3-Unroved and marked loop length under different timings of knitting

Type of Input Cam setting, mm Timing Loop length, mm machine tension Cylinder Dial Unroved Marked at

cN CKP DKP LFP A 5 1 .6 1 .3 SYN 1 1 .08 1 2.05 1 3 .96 1 1 .22 A 5 1 .6 1 .3 3 ND 9.32 9.96 9.94 9.34 B 5 2.2 1 . 5 2 ND 9.65 9.93 1 1 .02 9.72 B 5 2.2 1 .5 3 ND 8.89 9.63 1 0.3 1 8.90 B 5 2.2 1 .5 4 ND 8.47 9.40 8.99 8.55

CKP - Cylinder knitting point; DKP - Dial knitting point; and LFP - Loop forming point

104 INDIAN 1. FIBRE TEXT. RES. , JUNE 2000

Type of machine

A

A

B

Table 4-Effect of input tension on loop length

Timing Cam setting, mm Input Unroved

SYN

3 ND

4 ND

Cylinder Dial tension loop length

1 .6 1 .3

1 .6 1 .3

2.2 1 .5

cN 3 5

mm 1 1 . 1 2 1 1 .08

9 1 1 .00 1 2 1 6 20 3 5 9 1 2 1 6 20 3 5 9 1 2 1 6 20

1 0.72 1 0.45 1 0.20 9.68 9.32 8.64 8.5 1 8.30 8.08 8.46 8.47 8.35 8.25 8 . 1 0 7.98

Table 5-Effect of take dO'Yn load on loop length

Timing Input Cam setting, mm Take down Unroved

SYN

3 ND

2 ND

4 ND

tension Cylinder Dial load per loop length cN

1 .6

5

1 .6

5

2.2 5

2.2 5

1 . 3

1 .3

1 .5

1 .5

wale, cN 8.6 1 1 .0 1 5 .9 1 7 .6 1 9 .5 7.6 1 2. 1 1 3 .5 1 6.9 27. 1 7.6 1 8. 1 26.4 9.5 1 8.5 23.3

mm 1 1 . 1 6 1 1 . 1 3 1 1 .30 1 1 .36 1 1 .37 9. 1 7 9.33 9.39 9.45 9.65 9.66 9.65 9.69 8.44 8.47 8.59

length could be due partly to the error of measurement and partly to the relaxation of unravelled yam.

The effect of input tension on loop length under three different timings is shown in Table 4. It IS observed that under any timing, the unroved loop length decreases .with gradual increase In input tension. The unroved loop length decreases by 8 .27% and 16.52% under SYN and 3 ND timings on machine A and by 5 .67% under 4 ND timing on machine B respectively for a rise in input tension from 3 cN to 20 cN.

The effect of take-down load on loop length is shown in Table 5. It is observed that there is gradual increase in unroved loop length with the increase in take-down load but the change is very slow.

The values �( loop length using varying cam settings (cylinder and dial) are listed in Tables 6 and 7 .

Table 6-Effect of cylinder cam setting on loop length

Type of Timing Input Cam setting, mm Unroved machine tension Dial Cylinder loop length

eN

A SYN 5

B 2 ND 5

B 3 ND 5

1 .3

1 .5

1 .5

1 .0 1 .3 1 .6 2.0 2.2 2.5 2.0 2.2 2.5

mm 1 0.20 1 0.68 1 1 .08 9.45 9.65 9.90 8.57 8 .89 9.40

Table 7-Effect of dial cam setting on loop length

Type of Timing Input Cam setting, mm machine tension Cylinder Dial

A SYN

B 2 ND

cN

5

5

1 .6

2.2

1 .0 1 . 3 1 .5 1 .0 1 .3 1 .5

Unroved loop length

mm 1 0.69 1 1 .08 1 1 .34 9.34 9.56 9.65

% of total loop length at

Table 8-Proportions of cylinder loop, dial loop and links

Timing

CKP DKP

Cyl . loop 40. 1 0 34.35

SYN Dial loop

1 7.00 30.63

Running position 38.68 3 1 .90 Cylinder cam setting = 1 .6 mm, Dial cam setting = 1 .3 mm, and Input tension = 5cN

Link 42.90 35.02 29.42

Cyl. loop 4 1 .24 37. 1 8 34.27

3 ND Dial loop

32.09 34.45

Link 58.76 30.72 26.28

RA Y & BANERJEE: MECHANICS OF I x l RIB LOOP FORMATION 1 05

Table 9-Effect of cast-off loop on loop length

Type of Timing Cam setting. mm Unroved loop length. mm machine Cylinder Dial In presence of In absence

cast-off loops of cast-off loops

A

B

SYN

2 ND

1 .6

2.2

1 .3

1 .5

1 1 .08

9.65

1 1 .62

1 0.05

It is observed that the higher cam settings always produce larger loops. The unroved loop length increases by 8 .62% in case of SYN timing on machine A for increase in cylinder cam setting from 1 .0 mm to 1 .6 mm and by 4.76% & 9.68% under 2 ND and 3 ND timings on machine B respectively for increase in cylinder cam setting from 2.0 mm to 2.5 mm.

Loop length also increases with the increase in dial cam setting. Unroved loop length increases by 6.08% due to increase in dial cam setting from 1 .0 mm to 1 .5 mm under SYN timing on machine A and by only 3 .32% under 2 ND timing on machine B for the same range of dial cam settings. So, the effect of cam settings on loop length is less prominent in the case of delayed timing than under SYN timing. Moreover, the change in cylinder cam setting appears to have greater effect on loop length as compared to the change in dial cam setting.

The proportions of cylinder loop, dial loop and links obtained under SYN and 3 ND timings using similar input variables are shown in Table 8. It is observed that in the case of 3 ND timing, no dial loop is formed even at CKP and the loop under formation is made up of cylinder loop and links. However, the same loop is made up of cylinder loop, dial loop and links at DKP and as the proportion of cylinder loop and links decreases at DKP, the dial loop is formed by sharing or robbing of yam from those components. Although CKP and DKP occur simultaneously in the case of SYN timing, the relative position of rib loop with respect to CKP and DKP is different. This is reflected by the recorded data on the proportion of loops under SYN timing at CKP. In fact, a rib loop is not completely formed at CKP. It is also observed from Table 8 that the proportion of cylinder loop and dial loop is higher than the proportion of links at running position and therefore at LFP under both the timings. Moreover, the proportion of yam in the l inks keeps on diminishing as the needles pass through knitting points and travel to LFP. During this period, the proportion of dial loop keeps on increasing. The

cause of this phenomenon was investgated and it was found that the cast-off loops slip towards the centre of the two beds from their respective bed verge levels after needles pass through the knitting points.

The loop lengths produced in presence and absence of cast-off loops are given in Table 9. The loop length produced in absence of cast-off loops is much greater than the loop length produced under normal conditions of knitting. So, the cast-off loop has effect on loop length and the mechanics of loop formation in the absence of cast-off loops would be different to that in presence of cast-off loops and in this regard, the mechanics of loop formation on double-jersey machine has similarity with that of single-jersey machine5.

3.3 Yarn Movement and IxI Rib Loop Formation After catching the feed yam, as. the CN (CN1) goes

downward, the marked point on the yam moves forward and its distance from CN I and DN2 (DN on which yam was m

"arked) increases till CN1 reaches

CKP (Fig. 2). Beyond CKP, the nature as weIl as the degree of movement depend on the timing of knitting. In the case of synchronised timing, where both the knitting points occur simultaneously, the forward movement of the marked point stops at CKP. However, the distance of the marked point from DN increases further beyond CKP. But in the case of delayed timing, the marked point may move either backward or forward or there may be no movement of the marked point during the period when the neigh­bouring DN (DN2) approaches DKP. In fact, the nature of movement of the marked point inside KZ

varies with the extent of delay.

Whether the timing is synchronised or delayed, the backward movement of the marked point takes place before it gets a stable position on the loop. This backward movement was never observed before CKP on double-jersey machines. Moreover, the flow back of yam takes place across either CN or DN or both. The relative magnitude of flow across CKP and DKP as well as the type of needle across which the yam flow back takes place would depend on the timing of knitting. In the case of 4 ND timing on machine B, the extent of flow back occurring around CKP is higher than that occurring around DKP (Fig. t t ) . However, the situation is reversed in the case of 2 ND timing on the same machine (Fig. t 0). It has also been observed that the extent of flow back on the two double-jersey machines is comparatively lower than that at single­jersey machine. The probable source of such

1 06 INDIAN J. FIBRE TEX)'. RES. , JUNE 2000

difference could be the profile of the ascending side of cylinder stitch cam. Flat-bottomed or low-angled ascent is very common on double-jersey machine which restricts the yam flow back from needles beyond CKP . .

It was also observed that the configuration of the loop arms changes during the process of loop formation and a complete loop can be assumed to be . made up of a number of segments. The lengths of the two arms (trailing and leading) held by a needle are also not always equal during loop formation. Moreover, the configuration as well as the length of the loops made by the two neighbouring CN and ON are also not same during loop formation, although one arm of the two loops is common.

The needles come in contact with the feed yam slightly above the reference line and start forming new loop. However, casting off of the old loops (cylinder loop & dial loop) takes place on the respective bed verge level. So, a part of the new loop is formed by a needle while the old loop is still hung around that needle. The cast-off loops change the configuration of new loop arms by segmenting them across the contact points. The old loops on the needles are highly strained while passing over the latch of the hook during casting off and as a result the cloth fell position between the two beds gets shifted at KZ.

Sometimes, the cast-off loops get shifted from the respective bed verge levels. The cylinder cast-off loop generally shifts up above the link in between new cylinder loop and dial loop and pulls the link downward. The dial cast-off loop sometimes becomes tensionless and moves with the new dial loop. As the needles approach the running position in their respective cam tracks, the cast-off loops come closer as an effect of take-down load and move downward.

It was also observed that a complete rib loop is formed in multiple planes and the loop arms as well as their segments are inclined at different angles to the vertical plane (XY). The angle of inclination of the loop arms/segments mainly depend on the position of the bed verges and the dimensions of the needles making the loop.

It is expected that ON would deflect the yam from the vertical plane only when its hook catches the yam. However, it was found in the case of delayed timing that the yam, although not gripped by hook, moves nonetheless slowly inwards (Z-direction) due to the frictional contact with ON. Moreover, this inward movement of the yam over the stem of ON is not continuous. Stick and slip phenomenon takes place till

the hook of ON catches the yam. This aspect was investigated and it was found that the yam tension plays a minor role whereas the coefficient of friction between yam and needle plays a major role in the stick and slip phenomenon.

4 Conclusions 4.1 The forward movement of yam inside KZ

generally continues up to CKP. With delayed timings, the backward movement of yam occurs in atleast two phases. The first phase occurs just after CKP and the second phase occurs around OKP.

4.2 The values of displacement as well as speed of yam segment are generally higher in the case of synchronised timing and tend to decrease with the increase in delay. Moreover, the speed of yam across the needles varies inside KZ.

4.3 The loop arms connecting two neighbouring CN and ON are divided into segments at the contact points with cast-off loop and bed verges and these segments make different angles to the vertical plane.

4.4 The speed of yam entering the KZ is not uniform. This speed variation may lead to tension variation in the case of positive feeding.

4.5 A wide range of loop length can be produced on a dial and cyl inder machine (double-jersey) only by changing the timing of knitting. Generally, the lower loop length results at higher delayed timing.

4.6 The significant difference between the marked loop length at knitting points and the unroved loop length indicates the occurrence of robbing back.

4.7 Loop length gradually decreases with the increase in input tension, irrespective of the timing of knitting.

4.8 Loop length increases with the increase in cam settings. However, the effect is less prominent under delayed timing compared to SYN timing. Moreover, the change in cylinder cam setting appears to have more effect on loop length than the change in dial cam setting.

4.9 The proportion of cylinder loop, dial loop and links at CKP, OKP and running position indicate that the finalization of loop arm configuration and length of the loop arms take place beyond the knitting points.

4.10 Take-down load has marginal effect on loop length.

References I Aisaka N, Kawakami T & Shindo T, J Text Mach Soc Jpn,

IS ( 1 969) 228. 2 Banerjee P K & Alaiban T S, Text Res J, 58 ( 1 988) 422. 3 Black D H & Munden D L, J Text illst. 6 1 ( 1 970) 325.

RAY & BANERJEE: MECHANICS OF I x l RIB LOOP FORMATION 1 07

4 Ghosh S & Banerjee P K, Text Res J, 60 ( 1 990) 203. 5 Henshaw D E, Text Res J 38 ( 1 968) 592. 6 Knapton J J F & Munden D L, Text Res J, 36 ( 1 966) 1 072. 7 Knapton J J F & Munden D L, Text Res J, 36 ( 1 966) 1 08 1 . 8 Munden D L, J Text Inst, 5 1 ( 1 960) 200. 9 Munden D L, J Text Inst, 5 1 ( 1 960) 7 1 2.

1 0 Nutting T S , J Text Inst, 5 1 ( l 960) TI90. I I Oinuma R, J Text Mach Soc Jpil, 32 ( 1 986) 36. 1 2 Araujo M De, Rocha A M, Neves M & Lima M, Melliand

Textilber, 1 6 ( 1 987) 406 (EI 79). 1 3 Peat D & Spicer E R, HATRA Research Report, 26 ( 1 973). 14 Pietikaeinen L, Melliand Texti/ber. 1 0 ( 1 98 1 ) 603.

1 5 Pietikaeinen L & Yalkama L, Melliand Tcxtilber. 1 2 ( 1 983) 1 87.

1 6 Proctor J, J Text Inst. 38 ( 1 947) 406. 1 7 Wray G R & Burns N D , J Text illS!, 67 ( 1 976) 1 1 3. 1 8 Wray G R & Burns N D. J Text Inst. 67 ( 1 976) 1 1 9. 1 9 . Wray G R & Burns N D, J Text illS!, 67 ( 1 976) 1 23. 20 Gray P & Hurt F N, HATRA Research Report. 49 ( 1 978). 2 1 Little T J , Text Res J, 4 8 ( 1 978) 36 1 . 22 Knapton J J F, Text Inst Ind. 1 0 ( 1 972) 39. 23 Marvin A W & Araujo M D De. Cattail ill a competitive

world (The Textile Institute, Manchester & The Textile Association, India), 1 979, 2 1 3 .