Indian Journal of Fibre & Textile Research Vol. 27, March 2002, pp. 38-43

Apron slippage in ring frame : Part I - Establishing the phenomenon and its impact on yarn quality

A Dasa & P Yadav

Northern India Textile Research Association, Sector 23, Raj Nagar, Ghaziabad 201 002, India

and

S M Ishtiaque Department of Textile Technology, Indian Institute of Technology, New Delhi 1 10 0 1 6, India

Received 6 September 2000; revised received and accepted 22 February 200]

The phenomenon of apron to apron slippage exists in every spindles but with different extent. Due to the absence of positive means of motion transmission from bottom to top apron, the top apron always moves at a slower speed than bottom apron and this speed difference is higher when there is material in between them. This apron to apron slippage causes uncontrolled movement of fibres in the apron zone, resulting in poor yam quality.

Keywords: Apron to apron slippage, Bottom apron, Cotton yam, Polyester/viscose yam, Tenacity, Top apron

1 Introduction

In double apron drafting arrangements, two sets of aprons (bottom apron and top apron) are used to control the fibre movement in the main drafting zone, i .e. between middle roller and front roller. To guide the fibres, the upper apron must be pressed with controlled force against the lower apron. The bottom apron is driven by the middle bottom roller by frictional contact and guided through bottom apron guide plate as close as possible to the bottom front roller. The top apron is driven by the bottom apron through frictional contact. In the main drafting zone of ring frame, as the number of fibres are relatively small and at the same time the amount of draft is very high, the strict control over the movement of floating fibres is very essential.

As there are no positive transmission of motion from bottom apron to top apron, it may be thought that whether the aprons are really controlling the movement of fibres in the main drafting zone. The primary condition of controlling the movement of fibres is that both the aprons should move at the same speed. However, if both the aprons move at the

"To whom all the correspondence should be addressed. Phone: 4783586; Fax: 0091 -0 120-4783596; E-mail: nitra@nde.vsnl.net.in

same speed, the fibre strand in between them will also have streamline movement and in that case one can expect a controlled movement of fibres in the main draft zone. But, due to the negative transmission of motion from bottom apron to top apron, there are every possibility of apron to apron slippage, i.e. top apron to move at slower speed than bottom apron which can result uncontrolled movement of fibres in between aprons. This paper deals with the establishing phenomenon of apron slippage and its impact on yarn quality.

2 Materials and Methods

2.1 Measurement of Apron to Apron Slippage

The amount of apron to apron slippage was observed with and without roving in the drafting zone. To measure the apron slippage, a mark was put on the top and bottom aprons. Another reference mark was put on the frame itself. Time taken to make five revolutions by the top and bottom aprons was measured with the help of stop watch. The process is repeated for five times to get the average time. Then the length of the aprons was measured. With the values of time and length, the surface speed of apron was calculated. Difference of the surface speeds of bottom and top aprons was used to calculate the apron slippage, as per the following relationship:

DAS et al. : APRON SLIPPAGE IN RING FRAME : PART I 39

Apron to apron slippage (Sa), %= Bottom apron Top apron

surface speed surface speed 0 x l 0

Bottom apron surface speed

2.1.1 Study on Apron Slippage

To establish the phenomenon of apron slippage, a detailed study was carried out in laboratory pilot plant as well as in a most modem 100% EOU. In the pilot plant, the apron slippages were measured in two different ring frames with both short cradle for cotton and long cradle for polyester/viscose (PN). The apron slippages were measured in five different spindles randomly chosen in each case as shown in Table 1 . In the industry also, the

apron slippages were observed in three randomly chosen ring frames as shown in Table 2. The apron slippages were observed for 30 Ne and 40 Ne cotton running with 0.9 and 1 .0 hank rovings respectively and for 30 Ne PN running with 0.9 hank roving.

The following drafting details were used in the present study: Drafting details

Drafting system Top roller pressure, kg Break draft Tensioning arrangement Front zonelback zone setting, mm

100% Cotton PN

Pilot plant

SKF 235 1 4 1 .4

Spring & pulley

43/60 69/73

Mill

LR (P-3- 1 ) 1 4 1 .2

Spring & pulley

44/72 55/72

Table I - Values of apron slippage in laboratory ring frames

Type of material

24 Ne 100% cotton (carded)

24 Ne 100% cotton (carded)

24 Ne polyester/ viscose (65:35)

24 Ne polyester/ viscose (65:35)

Type of material

40 Ne cotton

30 Ne cotton

30 Ne polyester/ viscose (65:35)

Spindle Speed of apron without No. roving, mrnIs

I 2 3 4 5

I

2 3 4 5

1 2 3 4 5

1 2 3 4 5

Bottom apron Top apron

1 2.68 12 .57 12.79 1 2.79 12.77

1 2.99 1 2.76 1 2.67 12.77 12.73

1 5.58 1 5.76 1 5.82 15 .66 15 .80

1 5.28 1 5.46 1 5.85 1 5.62 1 5.44

12.66 12.53 12.77 1 2.73 1 2.5 1

1 2.97 1 2.62 12.62 1 2.64 12.62

15 .49 15 .45 15 .48 1 5.47 15 .65

1 5.08 1 5 . 10 1 5.64 15.29 15 . 19

Apron Speed of apron with roving slippage without mrnls

roving, % Bottom apron Top apron

0. 1 6 0.32 0. 1 6 0.3 1 2.04

0. 1 5 1 . 1 0 0.39 1 .02 0.86

0.58 1 .97 2.15 1 .2 1 0.95

1 . 3 1 2.33 1 .32 2. 1 1 1 .62

1 2.53 1 2.47 1 2.60 1 2.54 12.57 1 2.60 12.70 1 2.30 1 2.49 1 2.63

14.92 15 . 16 15. 1 1 15 .0 1 15 .08

14.63 14.98 15.26 1 5 . 19 1 5.03

1 2. 1 6 1 2.30 1 2.45 1 2.36 12 . 1 2

12.40 12.34 1 1 .90 1 2.00 1 2.51

14 .44 14.70 14.42 14.75 14.88

1 4. 1 5 14.39 1 5.01 14.58 1 4.64

Table 2 - Values of apron slippage in ring frame of a modern 1 00% EOU

Spindle Speed of apron without No. roving, mrnls

1 2 3 1 2 3 1 2 3

Bottom apron

25.43 25.56 25.49 32. 10 32.0 1 3 1 .79 47.99 48.42 48.49

Top apron

24.97 25.02 25. 1 3 3 1 .40 3 1 .27 3 1 .07 46. 8 1 47. 1 5 47.32

Apron slippage without roving, %

1 .80 2. 1 0 1 .4 1 2. 1 8 2.3 1 2.26 2.46 2.62 2.41

Speed of apron with roving, mrnls

Bottom apron

25. 14 25. 1 6 25. 1 4 3 1 .69 3 1 .50 3 1 .4 1 47.92 47.49 47.92

Top apron

24.40 24.29 24.20 30.58 30.3 1 30.23 45.58 45.26 45.90

Apron slippage with

roving, %

2.95 1 .36 1 . 19 1 .43 3.58

1 .59 2.84 3.25 3.92 0.95

3.21 3.05 4.56 1 .72 1 .32

3.29 3.92 1 .64 3.99 2.59

Apron slippage with roving, %

2.94 3.46 3.74 3.50 3.78 3.76 4.88 4.70 4.22

40 INDIAN J. FIBRE TEXT. RES., MARCH 2002

2.2 Study on Effect of Apron Slippage To study the effect of apron slippage on yarn

quality only two spindles - one with short cradle for cotton and another with long cradle for PN were selected. The spindles were so selected that normal apron slippages with material were on the higher side, i.e. 4.5% for cotton and 4.72% for PN to represent slippage level-2 (Table 3). The apron slippages were then increased to level-3, keeping all the other parameters constant, by making the top apron tight by wrapping cellotape very uniformly on the cradle roller. As the top apron becomes tight, the frictional resistance between the inner surface of top apron and top apron cradle increases, which, in turn, resists the motion of the top apron, resulting in higher apron to apron slippage. It is well known that the increase in tension of top apron to a very higher level leads to intermittent movement of apron. Sufficient care has been taken so that the tension of the aprons does not increase to a very higher level to have the uniform movement of the aprons. Only idea was to increase the apron to apron slippage to level-3 (Table 3) with uniform movement of top apron. On the other hand, the apron to apron slippage was reduced to level- l by rubbing 4 mm of both the edges of bottom and top aprons with the help of emery paper to have better frictional contact between aprons (Table 3).

Ten roving bobbins of the hank 1 . 1 were prepared each from 100% cotton and 65:35 polyester/viscose of 5 1 mm staple length. From each roving bobbin, 24 Ne yarns were prepared with three different levels of slippages. Therefore, at each slippage level ten ring bobbins were prepared in the same spindle from ten different roving bobbins. The cotton yarns were spun with 4.0 TM and PN yarns with 3.2 TM and the spindle speed was kept at 10,500 rpm both for cotton and PN yarns.

All the yarns were tested for evenness and imperfections in UT3 at a speed of 400 mlmin. Tensile properties were measured on SDL universal

tensile tester using 50 cm test length and 10 cmlmin extension rate. The mean values of tenacity and elongation were averaged from 100 observations for each sample, i.e. 10 readings from each bobbins. Hairiness index and diameter U% were measured in Keisokki hairiness tester LASERSPOT Model LST at a speed of 25 mlmin for 1 min. The details of the test results for both cotton and PN yarns at different level of slippages are given in Table 3 .

2.3 Fibre Movement in the Apron Zone The fibre strand in the main drafting field consists

of only a few required fibres. There is hardly any friction field, and the fibre guidance provided by rollers alone is inadequate. Therefore, in the main drafting zone double aprons are provided to reduce the uncontrolled movement of floating fibrel-3• Thus, the aprons really have the most important task on the spinning machines; a controlled increase in the fibre speed in the main draft up to the full speed on the delivery roller4, at the same time reducing the fibre mass to the prescribed values.

To perform the above functions properly, the most important condition is that both the aprons should move at the same speed. As the fibre strand is transported by the frictional contact of both the bottom and top aprons, any variation in speed between the aprons results in turbulent movement of fibres. Figs. 1 and 2 show the streamline and turbulent movement of fibres in the apron zone respectively.

Ideally, both the aprons should move at the same speed (Fig. 1 ) which will result in straightening of the fibres in the drafting zone by removal of hooks during drafting5. But in actual practice, as the top apron moves at a slower speed than the bottom apron (Tables 1 and 2) there is turbulent movement of fibres due to the rubbing action generated by the relative movement of aprons, and friction between aprons and fibres, and between the fibres themselves. The backward drag force on the top layer of the fibre band

Table 3 - Effect of apron to apron slippage on yarn quality

Type of yarn Level of slippage

24 Ne cotton Level l Level 2 Level 3

24 Ne polyester/ Level I viscose (65 :35) Level 2

Level 3

Apron slippage

%

3.48 4.50 8.64 2.87 4.72 7.96

U%

14.64 1 5 . 14 1 7 .3 1 1 0.58 1 1 .55 1 2.93

Imperfections/km

Thin Thick Neps places places +200% 50% +50% 76 788 542 1 12 881 625 342 1 658 862

8 1 04 257 14 1 89 375 38 266 543

Tensile properties Hairiness Diameter

Tenacity Elong- index (HI) U% . cN/tex ation

% 1 1 .83 5.86 682 14.2 1 1 .48 5.99 694 1 5.7 1 0.5 1 6.02 689 1 7.2 15 .44 7.56 3 12 9.8 14. 1 9 7.68 307 1 1 .3 1 3.32 7.44 322 1 2.7

DAS el at. : APRON SLIPPAGE IN RING FRAME : PART I 4 1

in the apron zone by the top apron results i n retardation of the fibres i n that zone while, on the other hand, the fibres in contact with bottom apron move relatively at a faster speed and this results in non-streamline movement of fibres in the apron zone (Fig.2).

This apron to apron slippage results in rolling or rubbing of fibres in the drafting zone, thereby showing entanglement of the fibre instead of straightening in the idealised condition. Figs. 3 and 4 show the photographs of the drafted rovings collected from the front roller nip of the same ring frame spindle, drafted from the same PN roving with two different levels of apron slippage.

Figs. 3 and 4 show that as the apron slippage increases the fibre entanglement also increases due to the reason as discussed earlier.

3 Results and Discussion 3.1 Extent of Apron to Apron Slippage

The apron to apron slippage data in laboratory ring frames and that in ring frames of an industry are given in Tables 1 and 2 respectively. It is observed that always there is apron to apron slippage, i.e. top apron always moves at a slower speed than bottom apron even when there is no material in between them. This is due to the fact that the top apron is getting its motion from bottom apron only by frictional contact. There is no positive means of motion transmission. The slippage between aprons, when there is no material between them, depends on many factors, e.g. friction

_ VI vammmvOZZIlIlOV71M TapApua ------:: - -...----::: - +- DirocIioaoflilnllow �- --------== ---- ----

..f'%\\\S� BoIIaaaApua _ Vb

Fig. l - Idealised streamline flow of fibres in the apron zone (Vb = VI, where Vb is surface speed of bottom apron and VI, the surface speed of top apron)

_ Vt

- Vb Fig.2 -Turbulant flow of fibres in the apron zone (Vb > VI, where Vb is surface speed of bottom apron and VI the surface s�d of top apron)

between top and bottom apron, frictional resistance of inner surface of top apron and apron cradle, condition of apron, contact pressure, surface speed of bottom apron, pressure distribution between aprons, etc. The above factors cannot be identical for all the spindles and, therefore, a wide variation in apron to apron slippage in different spindles is expected as is evident from Tables 1 and 2. When fibre strands are there in between aprons, the slippage values are always higher than that in the absence of material in between aprons. This is due to the fact that when fibre strand is present in between the aprons, it hinders the flow of motion further. Apron to apron slippage in a particular ring frame also varies widely from spindle to spindle when

. there is fibre strand in between. Apron to apron slippage with and without material,

in general. is found to be higher in the industry than

-: -- --

. - .. jLl � -"

�'j.�. �-���:.� Fig.3 - Photograph of fibre strand coming out from the front roller nip at negligible apron to apron slip (0.3%)

Fig.4 - Photograph of fibre strand coming out from the front roller nip at very high level of apron to apron slippage ( 1 0.6%)

42 INDIAN J. FIBRE TEXT. RES., MARCH 2002

that in the laboratory (Tables 1 and 2). This is due to the higher surface speed of bottom apron in the industry than that in laboratory ring frame. Table 2 shows that the slippage i s higher in case of 30 Ne PN than that in case of 40 Ne cotton, probably due to the higher bottom apron speed and different fibre properties.

From the Tables 1 and 2, it is also clear that the speed of bottom apron is always lower when the rovings are drafted than that observed without materials, which is due to the drafting force of the break draft zone that acts opposite to the bottom aprons direction of motion and restricts its movement6 •

3.2 Mass Irregularity

Table 3 and Fig. 5 show that the apron slippage has a significant influence on yarn unevenness both for cotton and PN yarns. The higher the apron to apron slippage, the higher is the yarn unevenness. The same trend is observed both for cotton and PN yarns. As the apron slippage increases, keeping all other parameters same, the movement of fibres gets disturbed as discussed earlier. Due to the uncontrolled movement of fibres in the main drafting zone, the distribution of the number of fibres in the cross­section gets disturbed and this results in the increase in unevenness. The diameter U% also follows the similar trend as shown in Table 3 .

3.3 Imperfections

Figs. 6 and 7 show the effect of apron slippage on thick places, thin places and neps of both cotton and PN yarns respectively. A drastic increase in the imperfections is clearly evident when the slippage increases. As already been discussed, when the apron to apron slippage increases there is a turbulent movement of fibre strand in drafting zone, causing rolling of fibres which results in generation of neps and thick places. As the thick places or neps are generated due to the fibre rolling, this also leads to the formation of thin places.

3.4 Tensile Property

The tenacity of both the cotton and PN yarns deteriorates when apron to apron slippage becomes higher (Table 3 and Fig.8). With higher slippage, the movement of fibres gets disturbed and the fibres become entangled, causing improper migration in the yam structure. Also, due to the non-straightness of fibres the ratio of mean fibre extent to original fibre length7.9, which is defined as spinning-in-coefficient

• Level-1 �Level-2 • Level-3

Cotton

Fig.5 - Effect of apron to apron slippage on yam irregularity (U%)

1800 -r------------,

1600

1400

400

200

o

.1.eveI-1 �Level-2 .Level-3

· Thin places Th ick places

Fig.6 - Effect of apron to apron slippage on cotton yam imperfections

DAS et al. : APRON SLIPPAGE IN RING FRAME : PART I 43

600

• level-1

• level-2

• level-3

400

E � c:: 0 'is � 300 & .5 E � 200

Thin Thick Neps places places places Fig.7 - Effect of apron to apron slippage on PN yam imperfections

(1(,,), reduces and this results in reduction in tenacity. No clear trend has been observed in case of breaking elongation.

3.5 Hairiness No clear trend has been observed for hairiness in

both the yams when apron slippage increases (Table 3).

4 Conclusions 4.1 The phenomenon of apron to apron slippage exists in all the spindles, but the extent of slippage varies widely from spindle to spindle and this may be responsible for spindle to spindle variation in yam quality. 4.2 The amount of slippage is always higher when there are fibre strands in between bottom and top aprons than that observed without fibre strands. 4.3 The extent of slippage found to be higher at higher speed of bottom apron, i.e. at higher spinning speed.

Ill_I.' 1S �leYeI-2

.l_�S ,.

1 13 f • c � e 12 �

1 1

PoiyHtwM6cose Fig.8 - Effect of apron to apron slippage on yam tenacity

4.4 As the apron to apron slippage increases the yam quality deteriorates in terms of evenness, imperfections and tenacity. But, no clear trends have been observed in breaking elongation and hairiness.

From the present study, it is clearly evident that apron to apron slippage is existing in all the spindles to varying extent which is detrimental for yam quality. Efforts can be made to either reduce the slippage to a negligible level or eliminate it by positive drive of top apron. The factors responsible for apron slippage will be studied in details in Part-II of this series.

Rererences I Balasubramanian N, Text Res J, 39 ( 1969) 155. 2 Klein W, Short Staple Spinning Series, Vol. 4 (The Textile

Institute, UK), 1987. 3 Bay E & Baier F, Melliand lnt, (4) ( 1998) 162. 4 Karuer H & Bomhauser P, Int Text Bull, 44 (4) ( 1998) 48. 5 Lord P R & Grover G, Text Prog, 23 (4) ( 1993) 19. 6 Singh A K, Effect of speed-frame apron slippage on yam

quality, M.Tech. thesis, Indian Institute of Technology, Delhi, 1999.

7 Ishtiaque S M & Saxena A K, Indian J Fibre Text Res, 23 ( 1998) 14 1 .

8 Kasparek J , Text Month, 8 ( 1974) 52. 9 Ishtiaque S M, Indian J Text Res, 1 1 ( 1986) 208.