Describing the principle of yarn formation and yarn structure fibre deposition in rotor, integration into yarn and formation of wrapper fibres, N Balasubramanian provides details of influence of opening roller teeth specifications, speed, transport tube design, on yarn quality and spinning performance based on R&D work over the years.
Among the various spinning systems developed to improve productivity, rotor spinning is one commercially established system for coarse to medium counts. Main problem restricting production rate in ring spinning is that spinning and winding are carried out continuously. This necessitates rotation of heavy package at the rate of twisting. This is obviated in rotor spinning by separating twisting and winding. Fibres are separated from the input material and deposited on a rotor rotating at high-speed. Fibres in the rotor get twisted into already formed rotating yarn tail lying on the rotor. Since a small length of yarn alone has to be rotated to impart twist, much higher production rates are achievable without attendant problems. Thus fibre separation from sliver, deposition on rotor for the formation of yarn represents essential features of rotor spinning.
Methods of fibre separation
Two methods of fibre separation have been used:
- Drafting system.
- Opening roller.
The former system is no more in vogue and most of the rotor spinning machines employ opening roller. Opening roller permits much more intensive fibre individualisation than roller drafting as the latter is restricted by the mechanical draft and inability of drafting rollers to run at high speeds. But roller drafting keeps the fibres straight while opening roller is associated with certain amount of fibre breakages apart from resulting in fibre hooks. Outline of rotor spinning with the essential elements is given in Figure 1.
Feed roller and feed plate
Sliver is fed by a feed roller mounted on a feed plate to the rotating pins of opening roller. Feed roller or feed plate is spring loaded and moves up and down according to thickness of sliver. Suessen have developed an improvement in their spinbox SC 1-M to ensure favourable opening roller action on the fibre beard projecting from feed roller. A fixed fibre support is incorporated in front of feed plate as shown in Figure 2 and is rigidly attached to opening roller housing.
As fixed beard support is fixed, it does not follow minute variations in thickness of sliver to affect the settings between feed plate and opening roller. As a result all spinning positions have the same opening roller action on the fibre beard. Different types of feed plates offered by Schubert and Salzer for their RU 11 rotor machine. While universal feed plate handles fibre of 2.5-inch length, a special plate is offered for 1.5-inch fibres. In the latter opening roller pin starts acting on the fringe at an earlier point to get better opening. Toothed feed roller is offered for long staple fibres of 4 inch and above to facilitate easy withdrawal of fibres.
Action of opening roller is similar to that of licker-in in card but is more intense because of higher order of speeds. Fibres are almost individually removed from the fringe held by feed plate and feed roller. Apart from individualisation, extraneous trash, foreign material and neps are removed by opening roller action. Amount of opening and fibre separation achieved in opening roller region has critical influence on evenness of yarn.
Performance of opening roller is decided by
- Type of teeth.
- Teeth specifications.
- Surface speed of opening roller.
- Condition of teeth.
Type of teeth
Two types of teeth are common: Pin type and Saw tooth type. Pin type has a higher wear resistance, longer life and further it results in fewer fibre breakages. But fibre removal from opening roller to transport tube is much more smoother with saw tooth wire even at high speeds. However, opening roller speed has more critical influence on fibre breakages with saw tooth type teeth. Pin type roller is however more difficult to manufacture and does not offer the wide range in tooth specifications as saw tooth wire. Nibikora et al1 found better yarn quality with pin roller than saw tooth roller with spun silk/cashmere blended yarns.
Important teeth specifications are rake angle a, distance between adjacent teeth t and width of the base of the teeth b and height of wire h (Figure 3). t and b determine the number of teeth per unit area.
Number of teeth per unit area = 1b x t
Rake angle is the angle front edge makes with horizontal (Figure 3). Action of opening roller will increase as rake angle reduces and at the same time fibre breakages will increase. Most usual clothing have a rake angle of 66 to 80° for cotton and 90° and above for synthetics. For coarse cottons and waste with higher trash, opening roller with lower rake angle is preferred. For finer cottons higher rake angle should be used. For man-made fibres, 90° or negative rake should be used.
Simpson and Murray2 found that lower rake angle a leads to more fibre breakages and poor yarn quality with cotton yarns from medium staple (29 mm). Wire with 90° rake angle resulted in inferior yarn quality because of poor fibre individualisation while 60° rake angle gave more fibre breakages. Trash removal was better with lower rake angle. Rake angle 75° gave the best yarn quality. Trash removal was better with lower rake angle. Kirschner and Bay3 found that negative rake wire gives less rotor dust than normal wire with man-made fibres.
Higher height increases intensity of combing action and ensures better individualisation and trash removal and should be preferred for coarse trashy cottons. Lower height should be used gentler action but fibre individualisation will be inferior.
Number wire points per unit length and number of rows of teeth per unit width determine tooth density. For short staple cottons and waste higher tooth density in the range of 12 to 18 teeth per cm2is recommended. For man-made fibres lower tooth density in the range of 5 to 9 teeth per cm2 should be used. Some common opening roller tooth profiles are given in Figure 4.
OK 36 Viscose staple fibre and blends, h = 1.2mm, t = 4 mm, tooth density = 14.06t/cm2, a = 90°OK 37 - Man-made fibres, h = 2 mm, t = 4.7 mm, tooth density = 13.3 t/cm2, a = 99°OK 40 cotton and blends with high cotton content, h = 3.6 mm, t = 2.62 mm, a = 65°OK 61 blends of cotton and man-made fibre, polyester/viscose, 100% man-made fibre, h = 3.6, t = 3.93 mm, a = 75° OS 15 Polyacrylonitrile staple fibre, OS 21 Cotton and Polyester, OB 20 100% cotton at high speeds.
Vigo and Barella4found that coarse Polyester/viscose yarns are less affected by opening roller tooth height and density. But OK 37, which has lower tooth density and higher height, gives a better yarn quality than OK 36 with finer yarns. OK 36 results in more fibre breakages with such material. Vaughn and Cox5 found that opening roller type does not have much effect on yarn quality.
There is an optimum opening roller speed at which best yarn quality is obtained as shown by Rakshit and Balasubramanian6, 7, and other workers. Figures 5, 6 and 7 show how single thread, U% and imperfections vary with increase in opening roller speed in 14s Ne yarn. Strength improves with increase in opening roller speed, reaches a maximum and drops with further increase in speed. But U% and imperfections initially reduce steeply with increase in opening roller speed and later marginally with further increase in speed. With increase in opening roller speed, fibre individualisation improves leading to better yarn quality.
Further removal of trash particles and foreign matter improves with individualisation of fibres leading to fewer imperfections in yarn. However, fibre breakages occur because of opening roller action, which increases with speed. Therefore yarn strength improves initially with speed reaches an optimum and deteriorates beyond a point. However, improvement from individualisation and trash removal at high speed overweighs the effect of fibre breakages resulting in marginal improvement in evenness and imperfections.
Fibre removal from opening roller teeth to transport tube is also hampered at high opening roller speeds. Leading end of fibre forms hook upon removal by opening roller2. Hooking tendency and breakages increase at higher opening roller speed. Optimum speed depends upon fibre and its properties. Optimum roller speed is around 8000 to 9000 rpm for coarse and trashy cottons, 7000 to 8500 for long staple cottons and 6000 - 7000 for man-made fibres.
Shahid et al8 also found similar trends with viscose jute blends. Tenacity and elongation increase initially and unevenness and imperfections reduce with opening roller speed and afterwards strength reduces and imperfections increase. Using Box and Behnken factorial design, Salhotra and Balasubramanian9 found the optimum opening roller speed to be around 7000 rpm. Kong et al10 quantified the amount of opening by a measure called number of points per fibre (ppf).
ppf = Number of opening roller points per unit time. Total number of in feed fibres over the same time.
Studies on 60 tex yarn showed that irregularity and imperfections reduces with increase in ppf up to a critical level beyond which no reduction is found because of increased fibre breakages. Duru and Babaarslan11found strength reduces but unevenness and hairiness reduce with increase in opening roller speed with polyester/waste blends. 7000 - 8000 rpm provides most effective cleaning. Fibre straightness and alignment improves with increase in opening roller speed at constant airflow speed and with increase in airflow rate at constant opening roller speed12.
Barella and Vigo13 conclude that best yarn quality was obtained at opening roller speed above 6000 and TM of 160 for 23.57 tex yarn for processing conditions remote from optimum. Optimum values of opening roller speed and TM were found to be different for yarn regularity and tenacity.
Another important function of opening roller is to remove trash and foreign matter from cotton and molten chips from man-made fibres. As fibres in individual state carried by teeth of opening roller, trash and foreign matter being heavy are ejected into a trash box below. One of the problems encountered in rotor spinning is due to fibres circulating around opening roller. Instead of moving into transport tube some fibres continue to move with opening roller because of inadequate suction at entry point of transport tube. If suction is increased, trash removal under opening roller suffers.
To overcome this problem, Suessen have provided in their compact spinbox a bypass opening with adjustable air intake, after the area of trash extraction (Figure 8). Incoming air at this point meets the teeth of opening roller tangentially with the effect that fibres are lifted out of the interspaces of saw teeth. This helps to improve fibre removal from opening roller to transport tube. At the same time air speed at trash extraction point is kept lower by adjusting the air intake in bypass to achieve efficient trash removal.
Bypass opening at opening roller helps to produce a cleaner yarn with fewer imperfections. Suessen have also developed Speed Bypass for man-made fibres, which are more prone to recirculate. To facilitate easy passage of fibres to the transport tube, the gap between opening roller and housing is increased from bypass region to the junction of opening roller and transport tube. Another method tried to reduce circulating fibres around opening roller was tried out by introducing high-pressure slot ejector at the entrance in a research project by Voidal14. Trash content removed by opening roller depends upon trash in input sliver and varies between 35 - 50%.
Micronaire value of opening roller lint is found to be much lower than that of input material indicating that low micronaire fibres being weaker get broken and removed along with trash. High level of trash in the input sliver leads to rapid wear of opening roller teeth and causes accumulation of fine trash and dust in rotor groove. Normally trash in input sliver should be below 1.2 - 1.4%.
A certain amount of fibre breakages is inevitable and arises because of the action of teeth on material at high speeds. Several methods are used to determine the extent of fibre breakages. One common method is to stop the machine and collect fibres deposited on rotor as ring. By doing this several times, sufficient amount of material can be collected to test fibre length. Mean fibre length of the fibres thus collected is compared with that of input sliver.
% Fibre breakages = l - l1l ×100
Where l = mean length of fibre in input sliver and l1 = mean length of fibre in rotor ring.
Another method is comparing short fibre % in input sliver and that in rotor ring. Effect of opening roller speed on fibre breakages with different cottons is studied 6,7 and the results are shown in Figure 9.
Figure 9 shows that opening roller action is associated with certain amount of fibre breakages at all speeds. Order of fibre breakages is nearly same as opening roller speed is increased from 4000 to 6500 rpm but increases markedly as speed is increased to 9000 rpm. Further, fibre breakages are more with long staple cottons because of longer combing action by opening roller. Breakage of long fibres is further corroborated by the frequency distribution of fibre length of material at different opening roller speeds, shown in Figure 10. While a peak indicating mode is seen around 28 mm in raw cotton, the peak is completely absent at 600 and 9000 rpm speeds. So the loss in strength of yarn at opening roller speed of 9000 rpm is because of higher fibre breakages. But the improvement in evenness and imperfections at 9000 rpm is because the improved fibre individualisation outweighs the effect of higher fibre breakages.
Salhotra and Chattopadhyay15 introduced tracer fibres of known length into the input drawing sliver and measured length of the fibres collected from rotor ring on a scale. This enabled estimation of amount of fibre breakages and also incidence of multiple breaks. % breaks increased from 12.5 to 66.6% as opening roller speed is increased from 4000 to 8000 rpm. Multiple breaks occur at higher speeds. Pattern of breaks is similar with pin and saw tooth roller. A probability model was developed to estimate multiple breaks of fibres. Numbers of breaks as well as multiple breaks increase with opening roller speed and length of input fibre16.
Ishtiaque17 found that too high an opening roller speed causes excessive breakages and results in low spinning in coefficient. Too low an opening roller speed also reduces spinning in coefficient because of inadequate fibre individualisation. Breakages of polyester fibres during opening roller action were classified as those due to direct contact with wire point and those without direct contact. Direct breaks form major part of breaks18. Trilobal fibres and saw tooth roller gave a higher % of direct breaks than circular and pin type roller.
Developments in opening roller
Opening roller consists of a pinned or saw toothed combing device mounted on a core. After long period of working, teeth wear out and need to be replaced by a new one. To reduce replacement cost, several manufacturers, like Burckhardt, Rieter have developed sleeves with combing device, which can be removed and refitted on the core. By this only the sleeve needs to be replaced at the time of wear and the core, which has minimum wear, can be reused. Further sleeve can be fitted other way round so as to change rake angle.
Wear and tear of teeth is more with trashy cottons and polyester fibres with delustrant titanium dioxide. To reduce wear of teeth hardened teeth with diamond and nickel coating have been developed. Nickel polished rollers are also available Rieter offers extra resistant XR coating for polyester fibres. Ekatex offers partial or complete coating of opening roller with electroless nickel dispersions with controlled incorporation of hardening particles.
Fibres picked by opening roller are sucked into transport tube. Velocity profile of the air in the opening roller near transport tube is shown in Figure 11. The velocity increases in the radial direction from the opening roller towards wall of transport tube and as a result, pressure reduction occurs in the radial direction away from opening roller teeth. This helps in removing the fibres from the teeth to the high velocity air stream rushing into transport tube as fibre reaches the junction of the two. Fibre transport tube narrows down from the start to exit as a result of which air velocity increases and straightens the fibres further. Air turbulence is also minimised as a result. Upon reaching the exit of transport tube, fibres are further accelerated while being deposited on rotor.
In order to achieve satisfactory doffing and transportation, velocity of air must be 1.5 - 2 times circumferential velocity of opening roller. Kong and Platfoot19 conclude on the basis of two dimensional simulation of airflow that too high or too low ratio of circumferential velocity of opening roller to airflow velocity is not conducive to good quality of yarn due to recirculation of air at outer cover side or opening roller side.
High speed cinematography showed20 that fibres get detached from opening roller teeth in the region of trash box and get drawn into the air stream flowing into transport tube. Leading ends of fibres form hooks as they are removed by opening roller. Trailing ends bend over the knife-edge at the junction before being drawn into air stream. Transfer tube with narrow rectangular cross-section has got more straightening effect on fibres than one with circular cross-section. Transfer channel design optimised by using mathematical equations by Lawrence and Chen21. Optimum design gives 3-fold improvements in fibre straightening than that in commercial models. Ratio of airflow velocity at inlet of transport tube to the circumferential velocity of opening roller has a critical effect on quality of fine count open-end yarns22.
Nield23 reported that some fibres reverse their direction during their passage because of contact with the wall of transport tube. Simpson and Murray2 and Chattopadhyay and Sarkar24 confirm the reversal. While Simpson and Murray found this to be more prominent at higher opening roller speed, Chattopadhyay and Sarkar report the contrary. Further, reversal is more with shorter fibres. Reduction in reversal at higher speed is more prominent with short-staple cotton. Fibres also form hooks and loops during passage through transport tube. Ursiny25developed a probability model for fibre opening and transport. This enabled determination of optimal transport tube length for specific range of partial drafts, input sliver hank, fibre fineness and length. Suessan has developed a compact spinbox, which has an undivided fibre channel when box is opened for cleaning. This ensures turbulence- free airflow. A self-centering seal further helps reduces leakages.
The velocity of fibres and draft of material at different stages from sliver to yarn in rotor spinning are given below.
D01 = v1v0
D12 = v2v1
D23 = v3v2
D3p = vpv3< 1
Dp4 = v4vp< 1
V0 = Surface velocity of feed rollers.
V1 = Surface speed of opening roller.
D01 = Draft at opening roller and feed roller junction.
V2 = Surface velocity of fibres at the exit end of transport tube.
D12 = Draft in transport tube.
V3 = Surface velocity of rotor.
D23 = Draft at transport tube rotor junction.
Vp = Velocity of yarn at peeling point, Condensing draft at peeling off point Dp4 = vpv3< 1
V4 = Velocity of yarn at naval, Condensing draft at naval D4p = v4vp< 1
It is easy to appreciate drafts D01 and D12 as velocity of fibres increases progressively from feed roller to opening roller and from entry to exit of transport tube. D23 represents the draft as fibre is deposited on the rotor. A partial draft between feed roller and rotor has significant effect on yarn quality. Optimum drafts between feed roller and opening roller and between opening roller and rotor were arrived at for 26 - 40 tex cotton yarns by Barella and Vigo26.
Vp is peel of velocity of the yarn as the fibre ring is peeled off and twist continuously inserted on it. The peeling velocity is much lower in magnitude than rotor velocity resulting in condensation of material. Further condensation takes place at naval due to twist insertion. Thus fibres get compacted to give required linear density. The condensing action during peeling and twist insertion is known as back doubling. The number of back doublings is given by
B = Π D Nv4
As T = Nv4
B = Π D T1000
Where B = Number of back doublings
N = Rotor Speed, rpm
V4 = Yarn production rate, m/min
D = rotor diameter, mm
T = Twist/m
A layer of fibre is deposited on rotor wall, which slides into the groove because of centrifugal force during each revolution, and a build-up of layers takes place till the yarn count is reached. Chen and Slater27 developed a mathematical model to study the movement of fibre from the inner wall to rotor groove. The motion is caused by increased centrifugal action from smaller to larger diameter and finally to the groove.
The number of layers also indicates the back doubling. Back doubling contributes to the high level of evenness in rotor yarns. Irregularities with periodicity less than circumference of rotor get destroyed by back doubling. Higher rotor diameter increases back doubling and improves yarn evenness. However, Krause and Soliman28 found that although back doubling improves short-term regularity, actual magnitude of back doubling does not have much effect on levelling of periodic variations.
The yarn extending into the rotor groove has a tapered shape with maximum width at the point of take off and minimum width at the point when a new ring layer is formed. The tapered tail where integration takes place moves forward ahead of peripheral speed of rotor and the lead is given by (rotor circumference/number of ring layers). After a number of revolutions equal to number of ring layers, yarn take off point comes back to the original position. The yarn tail is pressed against the rotor groove by the centrifugal force and rotates with the rotor. This imparts twist to the fibre ring integrated into yarn.
After sufficient number of fibre rings are laid on rotor groove, rotating action of the yarn tail pressing against the ring layer, generates twist which flows to the peeling point P (Figure 12). Prior to peeling point in the region A, strand is slightly twisted and as the strand gets lifted at peeling point, it gets fully twisted in the region B. The peeling point is also known as twisting zone. Analysis of yarn motion showed that the bent shape of yarn tail blocks twist and as a result twist near peeling point is low. Twist level at peeling point is higher for finer yarn than coarser yarn29. Photographs of yarn confirm that twist in the yarn near entrance to navel is higher than that at peeling point30. Twist level inside rotor was determined by Cormack et al31by applying correction factor to the results from photographic technique. Twist inside the rotor is shown to be higher than mechanical twist because of false twist generation.
Positive and Negative mode
If peeling point moves in the same direction as rotor, positive mode of spinning takes place. But if peeling point moves in opposite direction to rotor, negative mode of spinning takes place which results in poor yarn quality.
Major types of fibre integration reported in literature are discussed below:
Trail end of fibre deposited in rotor groove is peeled off and integrated into yarn as tail end of yarn sweeps past it as shown in Figure 13. There is a reversal in direction of fibre and minimum disturbance of other fibres occurs during integration. Figure 13(a) shows starting of integration and Figure 13(b) shows completion of integration.
With type 2, integration of fibre into yarn takes place as the fibre emerges from transport tube. Leading end is picked by the yarn tail and trailing end is flung into rotor. There is no reversal of fibre and fibre ring in rotor groove gets disturbed as trailing end is drawn through the fibres (Figure 14).
Third type of integration is shown in Figure 15. Leading end of fibre is laid on rotor but trailing end is still in transport tube. The fibre gets picked up at some point in the middle as the yarn tail sweeps past it. The fibre is folded in the form of a leading hook while being integrated.
While the above three are some commonly noticed form of integration, there are other innumerable forms of integration.
Structure of rotor yarns and formation of wrapper fibres are discussed by Kampen et al32in detail. Odd fibres deposited in rotor wall slide into tying zone A (Figure 12) instead of fibre ring in groove. Such fibres are picked by yarn tail (Figure 16), and get wound on the yarn with S twist initially (Stage 2 in Figure 16). After lifting from rotor groove, fibre continues to get twisted in S direction so long as angle between fibre and yarn axis exceeds 90°. A local concentration of wrapper fibre turns occurs at the turning point while the angle is 90° (Stage 3 in Figure 16). When the winding angle decreases subsequently below 90° wrapper fibre reverses direction and gets twisted in Z direction (Stage 4 and 5 in Figure 16). Upon leaving rotor surface the wrapper fibre remains attached to yarn untwisted (Stage 6 in Figure 16). Increasing false twist at navel increases both S and Z wrapper turns.
Longer fibre length and higher friction between fibre and rotor groove increases number of wrapper turns. At the navel, false twist disappears and as a result S twist in wrapper gets over twisted causing waisting and Z twist in wrapper gets untwisted and may get stripped back during passage through navel. The waisting of S twisted wrapper and strip back effect of Z twisted wrapper portions cause defects in fabric. Koc and Lawrence33 have studied the mechanism of wrapper fibres in polyester yarns under different spinning conditions and its effect on yarn properties.
Structure of rotor yarn
Rotor yarn has broadly 3 parts. The core is similar to ring yarn with fibres following helical path arising from flow of twist into the rotor groove. The sheath is less strongly twisted with helix angle of fibres varying along the length. This gives the characteristic matt appearance of yarn. This is because surface fibres slip in relation to rotor to a greater extent than core fibres34. This is supported by the work of Nakajima35. Fibre helix wavelength increases from core to surface and helix angle reduces from core to surface. The 3rd part is wrapper fibres tied round the yarn, which has been discussed in detail earlier. The cross-section of rotor yarn is therefore has dense core and a soft sheath. The packing density variation from core to surface is as shown in Figure 17.
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35. N Nakajima: TX591 Project, School of textiles, NC State University, Raleigh, 1970.
Retired Joint Director
Bombay Textile Research Association (BTRA)