Modern developments in rotor spinning to improve economics, productivity and electrical energy saving over ring spinning

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Retd. Jt. Director (BTRA) and Consultant

Substantial improvements in rotor spinning have increased productivity by 8 -10 times of ring spinning apart from labour saving, automation and computerisation making it attractive even in medium to fine counts. Manufacturing cost benefits from rotor spinning increase with increase in labour wages and reduction in capital costs. Increased number of rotors per machine, 4 piecing carriages, automatic transportation of drawing cans to rotor machine, and transport of rotor packages to packing with palletising reduce labour and independent operation of two sides, and individual rotor drive, improve flexibility. Rotor diameter has to be reduced at high rotor speeds to keep down power consumption. Increased twin disk diameter, magnetic bearing, automatic evacuation of filter waste and cooling system for invertor and motor reduce power consumption.
Rotor Spinning is one of the commercially well established and viable technologies offering high productivity and unique properties compared to ring spinning. More than 8 million rotors are currenly working and about 35 – 40 % yarns are made out of rotor spinning. Denims, sports wear, upholstry, furnishings, fancy fabrics and even knitted goods are made from rotor yarns Marked improvements in machine features with high level of automation and computerisation make rotor spinning more attractive even in medium to fine counts. Proprtion of these machines have gone up by 32 – 37 %. Rotor speeds have been increased by 40 – 50 % and power consumption has been reduced by 15 – 20 %. Some of the earlier drawbacks have also been overcome. Against this background, merits and Limitations of modern rotor spinning in relation to ring spinning arebriefly summarised below

As a result of the advantages mentioned earlier, cost of manufacture in rotor spinning is much lower than ring spinning and the difference is more marked in coarser counts. The economics of rotor spinning improves with lower capital cost as interest on loan for capital will reduce. If indigenous manufacturers make automatic 2nd and 3rd generation rotor machines, capital costs will come down and rotor spinning economics will improve and break even point will move towards finer count Estimates of cost of manufacture in rotor and ring spinning by Rieter5 are given in Table 3
Table 3
Comparison of cost of manufacture in rotor and ring spinning, CHF/Kg, (US. $/Kg)
Type of Spinning High wage country Low wage country 8s 34s 8s 34s Ring 1.21 (1.31) 2.51 (2.71) 0.65 (0.69) 1.52 (1.64) Rotor 0.59 (1.64) 1.52 (1.64) 0.39 (0.42) 1.25 (1.34) ( Rotor )/Ring×100 48.7 60 60 82
Manufacturing cost benefits from rotor spinning are more in coarse counts like 8s than in 34s and are also of higher order in high wage countries. Weidner Bohnenberger6 give cost comparison, inclusive of of raw material, in rotor and ring spining for 20 tex (300 Ne) and 30 tex (20s Ne) yarns as given in Table 4.
Table 4
Cost of manufacture in rotor and ring spinning (EUR/Kg and US $/Kg in bracket)
Count Rotor Ring 20 tex (30s) 0.68 (0.91) 1.1 (1.47) 30 tex (20s) 0.5 (0.67) 0.85 (1.14)
The costs given in Table 4 are much lower than that given by Rieter. Break even point at which rotor spinning can produce yarn at lower manufacturing cost used to be 24s – 30s earlier in India but has moved up to 40s – 44s in recent times becauses of higher rotor speeds and higher level of automation offered by the 3nd generation machines.
Fig 2 below compares the cost of ring spinning over rotor spinning over the years (based on ITMF Report).
The extent of reduction in cost of rotor spinning over ring spinning has increased significantly over the years. Fig 2 : Comparison of cost of rotor spinning against ring spinning over the years (ITMF Report)
Table 5, based on a survey by ITMF7, gives the various elements contributing to cost of manufacture in rotor spinning in different countries.
Table 5 : Breakup of Cost in US $/kg of yarn in rotor spinning in different countries, 20s Cd (2006)
Item of cost India China Turkey USA Waste 0.07 0.12 o.09 0.08 Labour 0.01 0.01 0.05 0.11 Energy 0.16 0.13 0.14 0.08 Auxiliary Material 0.08 0.08 0.08 0.08 Capital 0.18 0.18 0.17 0.27 Production cost excluding raw material 0.50 0.52 0.53 0.62 Raw Material 1.02 1.76 1.36 1.13 Total cost 1.52 2.28 1.89 1.75
Power is the major production cost in rotor spinning next only to capital cost in all the countries as seen from Table 5. Labour forms a very low part in production costs particularly in automatic machines.
Machine improvements
Improvements in feed plate design, opening roller housing and opening roller teeth have been discussed in an earlier paper by Balasubramanian8
  • Balasubramanian9 has discussed improvements in design and coating to rotor and navel in another paper.
  • SC-R spinbox improves the spinning stability because of the higher false twist arising from the sharp angle between the axle of the navel and yarn direction.
  • Wax block sizes are subtantially increased thereby reducing the frequency of changing the blocks.
  • Number of piecing carriages has been increased to 4 thereby reducing the waiting time for mending of a broken end. While one of the carriages is taken for maintenance the other carriages are made to attend to postions covered by it.
  • Digital piecing has improved the quality of piecing reducing the difference in strength and appearance between piecing and normal yarn. Online moitoring of quality of piecing is also available in latest models.
  • Digiwinding helps to produce packages with straight face and increases weight of package.
  • Labour cost is further reduced in latest machines by increasing the number of rotors per machine up to 540 and number of piecing carriages up to 4. Automatic transportation of drawing cans to rotor machines and continuous removal and transport of rotor packages to package room and palletising and automatic can exchange systems found in modern set ups will further reduce labour costs.
  • By increasing the gauge between spinning positions larger cans upto 18 inches can be used. This together with bigger packages upto 6 kg, reduces number of can and packages changes per hour and contributes to space utilisation. Material handling costs and waste disposal costs are also brought down by the above measures
  • Two sides of the rotor machine can be independently operated to enable different counts and twist factors used on the two sides. Individual feed drive enables the same machine to be used for spinning fancy yarn. In some of the latest models upto 5 lots can be spun at the same time on a single machine.
  • Autocoro 8 has independent drive and piecing for each rotor position. This eliminates waiting time for piecing and gives higher efficiency, productivity and flexibility compared to machines with central drive.
  • Online quality monitoring for yarn quality, faults and distrubed rotors help to ensure unifrom defect free yarn and reduce customer complaints
  • Thick and places of preset thickness and foreign fibre in the yarn will be cleared.
  • Identical piecings are ensured by improved digital piecer.
    Power consumption
    ITMF survey also gives power consumption in rotor spinning in various countries, which is given in Table 6
    Table 6
    Comparison of power consumption per kg in rotor and ring spinning, 20s Ne
    Country Power cost $/KWh Ring yarn 20s cbd Rotor Yarn 20s Cd Pwer consumption KWh/kg Pwer cost/kg Pwer consumption KWh/kg Pwer cost/kg India 0.095 3.368 0.32 1.684 0.16 China 0.080 3.375 0.27 1.625 0.13 Turkey 0.140 2.000 0.28 1.667 0.14 USA 0.045 3.334 0.15 1.778 0.08
    Power consumption is nearly double in 20s combed ring yarn compared to 20s carded rotor yarn in India, China and USA. Only in Turkey the difference in power consumption is lower. Rieter10 reports power consumption of 0.18 CHF/Kg in rotor spinning, 0.25 CHF/Kg in ring spinning and 0.20 in Air jet spinning in 30s. Thus power consumtion is lowest in rotor spinning amongst the 3 spinning systems. Krause and Soliman11 have reported that in counts corser than 60 tex (10s) rotor spinning consumes less power than ring spinning. However in counts finer than 30 tex (20s), rotor spinning consumes more power wich is contradiction with Rieter’s results.. Kaplan and Koc12 suggest a method for estimating power consumption in different sections of rotor spinning by estimating the time for which the machine runs to make a known quantity of yarn. From this basis they determined the share of pwer consumption in different departments in 20s carded rotor yarn, which is given in Fig 3
    Fig 3 : Share of power consumption in different departments in Rotor spinning, 20s Cd
    75 % pwer is consumed in rotor spinning, 16 % in carding in 20s as per Kaplan and Koc. Rieter10 has given share of power consumtion in different departments in Indian mill for 30s viscose knitting yarn which is given in Fig 4 .
    Fig 4 : Share of power consumption in different departments, 30s Viscose (Rieter)
    Share of rotor is much higher at 85 % while that of carding is much lower at 7 % compared to Kaplan and Koc. Power consumption in rotor spinning inclusive of air conditioning works out as 2.95 KWh/Kg in 20s as per Kaplan and Koc. Further while machines share of power is 73.4 %, the share of compressors is 3.5 %, Lighting 3.6 % and air conditioning is 19.7 %.
    Measures to reduce power consumption
    In rotor spinning, power is consumed mostly by opening action by opening roller, rotor, suction system and winding. Rotor Speed and Diameter
    Fig 5 : Effect of rotor speed on Power consumption with different rotor diameters
    Increase of power consumtion with increase of rotor speed with different rotor diameters is shown in Fig 5. Power consumtion increases more rapidly with rotor speed at 56 mm diam than at 30 mm diameter. As a result, power consumtion at 130000 rotor speed with 30 mm rotor diameter is nearly same as that at 62000 rotor speed with 56 mm rotor diameter. So rotor diameter has to be reduced with increase in rotor speed to keep down power consumtion.
  • Rotor shape and weight have been optimised by computer aided design to reduce air friction in corobox SE 12 by Schlafhorst. This reduces frictional resistance and brings about significant reduction in power.
  • Type of machine
    Self pumping rotors which do not use a separate suction system, like BD series of machines consume 50 – 60 % lower power compared to automatic machines with outside suction sytem. But speeds are much lower with former machines. Rotor bearing
  • Aero bearing, which provides air cushion and axial support to rotor in place of steel ball, is shown in Fig 6 . The air film is produced by compressed air of 6 bar which is continuously monitored. This reduces friction, power consumption and wear and tear.
    Fig 6 : Air bearing
  • Magnetic rotor positioning system by Schlafhorst contributes to power saving as it dispenses energy consuming steel balls, staggered twin discs or air nozzles. The end of rotor shaft is fixed in position by two permanent magnets without contact of rotor shaft (Fig 7 ).This is claimed to be more energy efficient than air bearing as it avoids the energy required to produce compressed air.
    Fig 7 :Magnetic Bearing
  • Thrust bearing by Suessen is claimed to save power.
  • Opening roller<
    Apart from rotor speed, opening roller speed also affects power consumption with higher speed leading to more power13.
  • Suction system
    Suction system for producing vacuum also consumes significant power. Vacuum level decreases with machine running time, due to deposition of waste on filter and so higher rating motor or higher setting is used to minimise fall in suction. But Autocoro 312 and later models offer an electronically controlled vacuum system with the help of a frequency invertor drive to suction fan to maintain constant suction throughout spinning. This even permits in some cases reduction in suction levels without affecting performance. In Reiter R 40 model, vacuum level in rotor is kept constant by autmotic actuation of filter cleaning in the event of drop in suction,monitored by an electronic sensor, to reduce power consumption. Further, a bigger volume main fan is fitted to improve its efficiency.
  • Drive to rotor
    Diameter of twin discs driving rotor has been increased to 78 mm and width of tangential belt reduced to 20 mm by Suessen in their new spinbox to reduce power consumption. Pressure on tangential belt is reduced in R 40 model during normal working to reduce power consumption. The pressure is increased at the time of start up after end break to increase acceleration. This is facilitated by the ability of robot to increase pressure for increasing the speeding up of rotor after piecing. Oerlikon Schlafhorst claims reduction in manufacturing costs by use of plastic bushings.
  • Piecing by Robot
    Coromat by Schlafhorst employs a shutter that allows suction air only at the time when it is needed like yarn search, suction for yarn piecing and doffing of package. Controlled use of suction air in this manner reduces requiement of vacuum requirement thereby saving power.
  • Cooling system for invertor
    By designing better cooling systems for invertor and motor, power savings have been obtained in Rieter R40.
  • Air conditioning
    Instead of air conditioning the entire spinning room, Schlafhorst has developed a system of providing direct air conditioning to the required areas like sliver feed to spin box. This helps to reduce air conditioning costs substantially.
  • Online quality monitoring
    Schlafhorst uses energy saving LEDs and optimised electrically controlled sensors to reduce power in yarn quality monitoring system.<
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    2. K.N.Seshan,P.Chellamani, K.P.R.Pillay, S.K.Khurana, R.M.Mittal, M.C.Sood, N.Balasubramanian and A.N.Desai, Rotor spinning comparisons –Spinning, Weaving, Processing and Overview, A joint project paper presented at 26th Joint Technological Conference, 1985, RP 1
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    11. H.W. Krausa and H.A. Soliman, Energy consumption of rotor type open end spinning machines as compared to ring spinning frame, International Textile Bulletin, 1982, 3rd quarter, p 285.
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