Influence of Rotor, Navel parameters and Winding tension on yarn quality and performance in Rotor Spinning

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Retd. Jt. Director (BTRA) and Consultant
Abstract Influence of rotor speed, rotor groove, type of navel and winding tension on yarn property and spinning performance are discussed. There is an optimum rotor speed at which minimum twist is lowest and the optimum increases with reduction in rotor diameter. With increase in rotor speed, strength drops marginally, elongation steeply, while neps increase steeply because of closely wound wrappers and stripback effect. Strength reduces and elongation comes down steeply and neps increase with increase in rotor diameter. Spiral groove and plain smooth navel give better yarn quality and grooved navel gives better spinning performance in coarse counts. Rotor spinning has established itself as a commercially viable technology with much higher productivity than ring spinning for coarse and medium counts. However to get the optimum benefits from this technology, the machine parts and process parameters have to be properly chosen taking into account the raw material. In this article mode of twist generation, centrifugal force and torque available for twisting are first discussed. From this the influence of rotor speed and diameter on minimum twist at which spinning is possible is deduced. The effect of rotor speed, rotor diameter, groove design, navel type and winding tension on yarn quality and performance are discussed.
Twist generation
Fig 1 : Twist generation in rotor spinning
Fig 1 shows how twist is generated. Yarn tail is pressed against rotor groove as well as navel by the centrifugal force caused by rotation of rotor. Yarn tail therefore gets twisted as it is withdrawn from rotor. However, friction between yarn and navel does not allow the twist to flow until a sufficient extra twist is built up. The extra twist is the false twist in the yarn tail. Yarn in twisting zone is very weak and requires a higher twist than in fully spun yarn to enable spinning. This is provided by false twist. As the yarn passes the contact point with navel, it unwinds and false twist is removed.
Centrifugal Force
Centrifugal force on the end of yarn tail inside the rotor is given by
t× n2×R2
t = Yarn linear density (tex)
R = Rotor groove radius (m)
n = rotor speed (rpm)
Fig 2 shows variation of centrifugal force with rotor speed for 2 rotor diameters.
Spinning tension F (cN) is given by
F = k1 R2 n2 t
Where k1 is a constant
Peeling tension = k2 Χ Centrifugal force
If yarn strength in cN is given by S, the S/Peeling tension>1 Fig 2: Centrifugal Force vs Rotor Speed
Grosberg1 gives the following empirical equation for spinning tension
F = 0.6 tω2 R2 Where ω = angular velocity of rotor
Grosberg and Mansour2 reported that measured tension values are 28 % higher than that predicted from the above equation because of air drag, frictional force on yarn in contact with base and inner wall of navel. Allowing 20 % increase in tension due to air drag gave the following equation for tension.
F = 0.72 tω2 R2 A close agreement between predicted and actual values is found with this equation. Lotka and Jackowski3 found a good correlation between spinning tension and yarn count. Further there is a good correlation between CV of yarn tension and CV of yarn count and CV of feed sliver count. Tension variations are more pronounced with finer yarns than coarser yarns. Influence of rotor spinning parameters on tension and twist distribution inside rotor are discussed based on numerical simulation by Xu and Tao4.
Torque to form yarn from fibre ring depends on the number of fibres in yarn cross-section, yarn radius, rotor speed and twist factor.
Torque available is given by M = P r α
Where P = yarn tension at the point of yarn formation
r = yarn radius
α = twist factor
Low tension in the yarn reduces torque available for twisting.
P < � t(l/2)Rw2
Where � = coefficient of friction between fibre ring and rotor wall
l = Average fibre length
R = Rotor radius
W = angular velocity of rotor
Minimum Twist
There is a certain minimum twist below which yarn cannot be spun on rotor machine. Minimum twist multiplier depends upon rotor speed as well as rotor diameter as shown in Fig 3. With increase in rotor speed minimum twist reduces, reaches an optimum and then increases.
Fig 3 : Relationship between rotor speed, diameter and minimum twist multiplier
Upon increasing rotor speed, centrifugal force and spinning tension increase and twist has to be increased to improve yarn strength to withstand the tension and minimum twist will be higher. At the same time, if rotor speed is reduced beyond a limit, spinning tension is reduced to such an extent that false twist generated by centrifugal force action against navel is insufficient to cause twist integration at peeling point. So a higher minimum twist may be required to ensure spinning stability. Thus there is an optimum rotor speed for minimum twist level at which yarn can be spun. The optimum is higher with lower rotor diameter as shown in fig 3. Thus though higher rotor speed is achievable with 32 mm diam than 40 mm diam, minimum twist level at which yarn can be spun is higher with former.
Rotor Speed
Rotor speed is an important parameter as it affects productivity of the machine. Several studies have been reported on the effect of rotor speed on yarn properties. Manohar, Rakshit and Balasubramanian5 found that rotor speed has insignificant or marginal effect on yarn strength. But elongation is brought down steeply with increase in rotor speed as shown in Fig 4. The fibres are peeled off and twisted at higher tension at higher rotor speed which makes the yarn compact. Further curliness in fibres are straightened. Wrapper fibres are also increased at higher rotor speed because of higher centrifugal force which increases the pressure of yarn on navel. This increases frictional resistance leading to higher false twist. Kampen6 et al showed that twist difference % at the naval increases with rotor speed as shown in Fig 5. As a result elongation drops with increase in rotor speed.
Fig 4: Effect of rotor speed on yarn elongation Fig 5: Effect of rotor speed on twist difference % Fig 6 : Effect of rotor speed on U% of yarn Fig 7: Effect of rotor speed on imperfections in yarn
Yarn irregularity U% and imperfections increase markedly with increase in rotor speed (Fig 6 and 7). With increase in rotor speed fibre individualization and trash removal by opening roller will be inferior. Neps in particular increase steeply with rotor speed. Fibres have also less time to align themselves in rotor groove and fibre alignment will be poor7. As a result U% and imperfections increase. Steep increase in neps is partly because of close wrapping of wrapper fibres which increases the mass and these places therefore get counted as neps. Examination of nep portions, collected with the help of imperfection selector of Uster tester, under a low power microscope revealed a significant proportion is due to wrapper fibres5. Such neps increase markedly with increase in rotor speed because of highezr false twist and consequently higher incidence of wrapper fibres. Tandem card has not shown any consistent improvement over normal card in rotor spinning. This may be because individualization at opening roller region minimizes the benefits from tandem card. Kampen6 et al found while imperfections increase with rotor speed in cotton, no such effect is found with polyester and polypropylene. This may be because of better tying in of wrapper fibres because of longer fibre length which minimizes strip back effect. Simpson and Patureau8 on the other hand found increase in yarn strength up to 40000 rpm and reduction thereafter with 49 tex yarn and continuous reduction in strength with rotor speed in 25 tex yarn. This is partly because deterioration in fibre parallelization at higher speed7. Good carding and strong and finer fibre enable higher rotor speed. Fibre orientation deteriorates with increase in rotor speed. Difference in strength between combed and carded rotor yarn decreases at higher rotor speed because of wrapper fibre. Simpson and Murray7 found increased deterioration in yarn quality with rotor speed with combed material. Not only yarn regularity and imperfections deteriorate with higher rotor speed but also number of wrapper fibres/unit length and frequency of incidence of wrapper fibres increases9. Rotor speed affects yarn tenacity, elongation and regularity in a linear manner10. Box and Behnken factorial design however showed best yarn quality at lowest rotor speed11.
Rotor diameter
Manohar, Rakshit and Balasubramanian5reported that increase of rotor diameter up to 46 mm did not have much effect on strength with tandem card but increasing it up to 56 mm leads to a significant drop in strength. But with normal card, strength decreases continuously from 46 to 56 mm diameter (Fig 8). Yarn elongation is very sensitive to rotor diameter and drops markedly with increase in diameter (Fig 9). This is because of higher centrifugal force on yarn tail which increases wrappers and makes yarn more compact. Koc9 et al also found lower tenacity and lower elongation with higher rotor diameter. While U% and thick and thin places are not much affected, neps increase markedly with increase in rotor diameter (Fig 10). Increase in irregularity due to higher wraps per unit length of wrapper fibres is compensated by the improvement from back doublings with higher diameter and as a result U% is unaffected. Neps increase with rotor diameter because of higher wraps per unit length and strip backs at navel as shown by Kampen6 et al. Koc9 et al also showed that while numbers of wrapper fibres per unit length decrease, wraps are more tightly wound with more wraps per unit length with higher rotor diameter. Tandem card has resulted in lower nep incidence than normal card with waste mixing because of improved fibre individualization.
Fig 8: Effect of rotor diameter on yarn strength, gm
Fig 9: Effect of rotor diameter on yarn elongation
Fig 10: Effect of rotor diameter on neps
Nawaz12 et al also confirmed that increase in rotor diameter from 33 to 40mm results in significant deterioration in yarn evenness. Simpson and Patureau8 found that fibre orientation deteriorates with increase in rotor speed, rotor diameter and use of grooved navel. Barella10 et al reported that rotor diameter affects tenacity and regularity both linearly and quadratically. Vila13 et al concluded that rotor speed, rotor diameter and preparatory process influence hairiness of rotor yarns. However, Lord14 claims that hooked fibres will reduce with increase in rotor diameter and will therefore improve yarn quality.
Rotor diameter should be sufficiently large to enable formation of fibre ring on the rotor groove. Normally rotor diameter should be at least 1.2 times that of fibre length. Higher rotor diameters should be used as count becomes coarser. Energy consumption and spinning tension increase with rotor diameter and so lower rotor diameters should be used at higher rotor speeds. Speed range for different rotor diameters are broadly given in Table 1
Table 1 Rotor diameter, mm Rotor Speed range, rpm 33 75000 � 130000 36 60000 � 120000 40 55000 � 100000 46 80000 � 95000 56 65000 � 75000
Rotor wall height
By increasing rotor wall height and increasing the distance between peeling point and navel, wild fibres in yarn wrapper decreases and strength is improved15. This enables spinning of finer count.
Rotor Groove
Important dimensional characteristics of rotor and groove are (Fig 11)
1. Angle of rotor wall to vertical, A
2. Design and location of rotor groove, G
Rotor groove angle G is made of two components G1 and G2, angles with respect to horizontal
Fig 11: Rotor Groove and angle of rotor
Rotor wall angle A ranges from 12 � 500. Smaller angle facilitates higher rotor speed.
Groove angle G ranges from 30 � 600. Large angle is used for coarser counts. With narrower groove angle, compaction of yarn is improved leading to higher strength. But dust and trash accumulation will be higher. Higher rotor groove is conducive to self cleaning. Different types of rotor groove offered by Rieter are given in Fig 12.
Fig 12: Different types of groove by Rieter
� U and DS have wide groove angle and are intended for coarse and denim yarns
� TC groove has a wider groove and extended groove angle compared to T but with same groove shape. TC is therefore preferred for very coarse counts and denims and leads to yarns with higher abrasion resistance.
� T and K grooves are narrow with small groove radius and are universally suitable for all counts of cotton and man- made fibres for weaving . G has also a narrow groove but larger groove radius and is suitable for bulky knitting yarns.
� Compared to the G rotor, groove angle and groove radius are larger in GM, but with same groove shape. The latter has therefore merits for use in fine count range.
� U and DS grooves have high groove angle and are ideally suited for coarse, soft and bulky twisted knitting yarns and denims from cottons and blends.
Fig 13: Various types of rotor grooves by Schlafhorst
Various types of rotor grooves Offered by Schlafhorst for different applications are given in Fig 13.
� T and K produce yarns with structure and quality comparable to ring yarns
� S groove is suited for raising yarn
� V groove is suitable for yarns with high displacement resistance of acrylic fibres.
� V shaped groove results in a stronger yarn than U shaped but dust accumulation and wedging of trash will be more. V shaped is suitable for finer counts and U shaped for coarser counts
Nield and Ali16 report increased fibre disorder and inferior quality with increase in friction in transport tube, rotor and naval.
Self Cleaning Rotor
Design of Self cleaning rotor by BD D1 machine is shown in Fig 14. Groove shape in self cleaning type permits cleaning of rotor groove of accumulations by the rotating yarn tail as the air flow is downwards.
Fig 14: Self cleaning Rotor
Murray and Fork17 found lower dust accumulations in rotors with perforations than normal rotors. Linting tendency in knitting is reduced with yarns from perforated rotors.
Rotor Drive
Two types of drive are found
1. Individual drive where each rotor is driven by a motor fitted on its shaft
2. Central drive which is more common, where a single belt running along the machine drives all rotors. The belt is driven by a motor at the end of the machine.
Merits of individual drive are
1. Less noise
2. Reduced energy consumption
3. Higher efficiency, since total stoppage of machine is avoided for maintenance and repair.
However spinning zone becomes hotter, requiring a separate cooling arrangement
Central Drive
There are two types
1 Direct bearing type
2. Indirect bearing type
Direct Bearing type
Rotor is mounted on a ball bearing. Cooling arrangement is provided for bearing
Indirect bearing
Twin disc bearing of Suessen is the most common type of indirect bearing. It consists of a combination of twin discs mounted on bearings to support rotor shaft and an oil bearing ball to absorb the radial thrust. A tangential belt drives the rotor shaft which in turn drives the twin disc. Advantages of this system are
1. Easy change of rotor
2. Large reduction in diameter between twin discs and rotor helps to reduce the speed of rotation of twin discs substantially with a consequent reduction in stress on the bearing
3. Easy heat dissipation because of the large surface area of tangential belt
4. Maintenance of close tolerances in axial positioning of rotor. In later models, diameter of twin discs has been increased to78mm and an interchangeable grease cartridge containing the ball support is incorporated to improve lubrication usage.
Reiter improved upon this by using an aero bearing in place of the ball to provide the axial thrust. Axial thrust of rotor is through an air film which is produced by compressed air. Aero bearing minimizes mechanical friction and wear. Schlafhorst employs a hybrid bearing to reduce wear by use of ceramic pin on rotor shaft.
Self pumping and External suction systems
Suction required to transport the fibre to rotor is provided by the rotation of rotor in self pumping system. However the suction thus generated is low and as a result fibre straightening in transport tube is low and frequent cleaning of rotor is required of accumulated dust and trash. External suction system is therefore invariably used in modern rotor machines. One drawback with this system is that discharge of air takes place between rim of rotor and cover leading to deposition of dust and trash on the rim which slide into rotor groove. This drawback is overcome in self cleaning rotor discussed earlier. Further power consumption will be higher than self pumping type.
Friction between yarn and navel is caused by pressing action due to centrifugal force. Yarn rolls around itself as it passes through the navel to overcome the friction, resulting in false twist in the yarn tail in the rotor. False twist is essential to improve the strength of yarn at peeling zone and enable spinning at normal twist factor and at high speed. Without false twist, minimum twist at which yarn can be spun will be very high. Guo18 et al have developed a mathematical model for twist distribution in the contact zone of navel and for predicting yarn twist level inside rotor. Twist flow past navel reduces with coefficient of friction or wrap angle and contact surface factor. If navel is made to rotate at the same speed as rotor, twist stoppage at navel will be minimum.
Several types of navels are available for desired quality of yarn to suit different end usages and for getting higher productivity.
� Smooth navel made of steel gives the best yarn quality in terms of evenness and imperfections19. Cheng and Cheng20 found higher yarn strength with smooth navel compared to 4 groove navel. Raudbaari and Eskadandamejad 21 found maximum strength lower number of thick places with smooth navel compared to spiral and grooved navels in 50/50 nylon cotton blends.. Navel type affects hairiness but has no effect on abrasion resistance.
� Spiral navel reduces the contact area of the yarn and gives higher strength and smoother and closed surface compared to normal navel. However contradictory results have been reported about the benefits from spiral navel. The benefits seem to vary with type of material. Cheng and Cheng20 found higher strength with spiral compared to normal in 40 tex yarn. Maghassem and Fallalpur22 found best performance with spiral navel without torque stop and close setting between navel and rotor. However Tyagi23 et al found lower tenacity and elongation with spiral nozzle compared to notched nozzle in acrylic/cotton yarns. Nibikora et al found best yarn quality with spiral grooved ceramic navel and pin type opening roller with silk/cashmere blends24. False twist by active and passive rotating torque was investigated by Voidal25 . The tempo of turning displacement influences false twist production.
� Navels with grooves or notches lift the yarn off navel surface for short periods and cause the yarn tail to vibrate at high frequencies. The vibrations facilitate twist movement into rotor groove and improve spinning stability. Vibration reduces roller friction while sliding friction is increased. These navels are therefore preferred with short staple cottons and waste to reduce end breakage rate and achieve higher rotor speed. However, irregularity, imperfections, hairiness and harshness increase with such navels26. The extent of deterioration of quality increases with the number of grooves. Nawaz12 et al reported that finely grooved navel gives less hairiness compared to coarsely grooved navels with built in notches. Roughening up of yarn is more with coarsely grooved navel.
� Closer setting between navel and rotor increases the contact area of yarn on navel and the false twist as a result. Yarn hairiness, neps and lint generation are affected by the setting between navel and rotor27. Setting is adjusted with the help spacers inserted behind navel. Van der Marwa and Veldaman28 found improved yarn quality by reducing the distance between navel and rotor by 2mm. However close setting increases spinning instability29.
� With larger navel diameter, wrap length on the navel surface is increased. This leads to 10% higher strength, 5 % lower elongation, and reduction in end breakages and rotor deposits with short staple cottons30.
� Whirl inserts are also used inside nozzle to open up the wrapper fibres and thereby produce a bulky and hairy yarn with soft handle. One example is the Belcoro navel KS2R4.
� Asymmetrical groove design by EmillBroell (Fig 15) results in less microdust and debris and can help to increase rotor speed by 15%31.
Fig 15: Comparison of asymmetric and normal groove design
� Navels with smaller radius of curvature result in smoother yarns with fewer wrapper fibres31.
� Ceramic navels have a higher precision from groove to groove than steel navels and as a result have a longer service life.
� Erbil32 et al found that number of notches, physical form of notches (concave/convex), structure of navel surface, surface geometry (flat/spiral) have critical influence on hairiness of blend rotor yarns. K4Ks navel (ceramic, 4 notches, spiral) results in maximum hairiness and K6KF (Ceramic, 6 notches, flat) gives minimum hairiness. Najar33 et al replaced navel by an air jet nozzle to give false twist to the yarn. Use of air jet nozzle with Z direction false twist and jet angle of 900 improves strength, hairiness, abrasion resistance of yarn with a slight deterioration in regularity.
Torque stop
Torque stop is a small replaceable twist blocking device by Schlafhorst to improve spinning stability and reduce end breakage rate. It conserves the twist between deflecting point and rotor. Yarn hairiness and imperfections however increase with torque stop.
Twist loss in rotor spinning
Twist loss in rotor spinning takes place because of slippage of yarn tail on rotor surface. Twist loss34 is given by Tl = ((Tm � Ty)/ Tm) Χ 100
Where Tl = Twist loss %
Tm = Machine twist
Ty = Actual twist in yarn measured by untwist twist method.
Manich35 et al found increase in twist loss with linear density, twist multiplier, diameter of navel and with grooved navel. Twist loss increases with navels that produce more false twist. Rotors that produce more friction with yarn reduce twist loss. Palamutcu and Kadoglu36 report lower twist loss with coarser and shorter fibres which may be because of lower incidence of wrapper fibres. Opening roller teeth type has significant effect on twist loss. Higher opening roller speed improves fibre separation, reduces sheath fibres and as a result improves twist efficiency36,37. Salhotra38 also found that twist loss increases with increase in sheath fibres. Further, % sheath fibres increases with fibre length and as a result longer fibres result in more twist loss. Increase of navel diameter increases twist loss.
Winding Tension and Package Build
Tension draft between take up roller and winding drum should be kept between .94 and .9839. Lower tension draft below .94 will result in unsatisfactory winding and tension draft above 0.98 will result in too many end breakages. Strength is improved and elongation reduced as tension draft is increased up to .99. Unevenness and imperfections increases and hairiness reduces as tension draft is increased up to .97.
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