• spinning tension,

• end breakage,

• minimising waste.

8.1.1 Understanding spinning tension in ring spinning

Control of spinning tension on the ring frame is extremely important in controlling yarn characteristics as well as spinning performance. If tension is too high or too low, it will cause higher end breakage leading to poor productivity, higher wastage and ultimately making the process uneconomical. It is important to understand both the theory involved and the practical issues involved in controlling spinning tension.

8.2.1 The effect of spindle speed

Yarn tensions are directly proportional to the square of the traveller or spindle speed. When the spindle speed is increased, the friction work between ring and traveller (and hence the build-up) increases as the 3rd power revolutions-per-minute of the spindle. If the spindle speed is too high then the traveller sustains thermal damage and fails. This speed restriction is felt particularly when spinning high-strength cotton yarns. When spinning 100% cotton, fibre dust from the cotton acts as a lubricant, but not all cottons generate the same amount of lubricating film. Traveller speed is not a limiting factor in cotton combed yarn intended for knitting. Because the yarn TPI is lower, the yarn strand is not strong enough, and so the yarn tension becomes the limiting factor. Yarn strength is not a problem for polyester/ cotton blends and cotton weaving counts, and the limiting factor is traveller speed. For a ring diameter of 40 mm, spindle speeds of up to 19 500 rpm should not be a problem. Rings like Titan (from Braecker), NCN (bergosesia), etc. are able to meet the requirements.

For spindle speeds greater than 20 000 rpm, ORBIT rings or SU-RINGS should be used. Because the area of contact is larger with these rings, with higher speeds and pressure, the heat produced can be easily dissipated. Normal rings and traveller profiles cannot produce good quality yarn if run at speeds greater than 20 000 rpm.

ORBIT rings are very useful in working 100% polyester at high spindle speeds, where high tension generates a lot of heat between the ring and the traveller. Because the yarn strength of polyester is very high, heat dissipation is the only limiting factor. ORBIT rings with higher area of contact will be able to run well at higher spindle speeds.

The maximum spindle speed for a given yarn tension, or the minimum tension for a given spindle speed, can be determined using the following equation³²:

where n is the traveller speed in thousands of revolutions-per-minute, T₀ is in g-wt, N is the count of the yarn in tex, and H is the balloon height in cm.

A single stable balloon cannot form if the tension is less than the value given in the equations. If, at a given spindle speed, the tension is reduced or the balloon height is increased, at a certain point the balloon collapses and forms a neck. The spinning tension decreases as the diameter of the package increases during the build of a bobbin, so balloon collapse is most likely to occur when the yarn is winding onto the full diameter of the package. Collapse is more likely with longer balloons, even though tension increases with balloon height.

The effect of spindle speed on yarn properties has been observed by many researchers. Anbarasan³³ reported that the benefits of advanced speed systems were subsequently extended by the introduction of variable speed systems such as the inverter drive system. This system offers the possibility of selective and continuous speed adjustment for the complete cop build. Optimum spindle speeds can thereby be achieved so that an almost constant spinning tension is maintained throughout the cop build.

8.2.2 The effect of traveller mass

Winding tension increases with traveller mass, and heavier yarns require a greater centripetal force to keep them rotating. Traveller mass is used as a variable to increase tension and generate higher centripetal force for heavier yarns. The traveller mass is usually chosen according to the linear density of the yarn being spun. In fact, all the implications for winding tension apply to tension in the yarn forming zone.

Traveller mass controls winding tension, which in turn produces tension in the lower balloon at the other side of the traveller. Because of yarn/traveller friction, the balloon tension is rather less than half the winding tension. The balloon tension in turn gives rise to the spinning tension between the lappet and the front roller nip; this depends on the winding tension and on the centrifugal force and air drag on the balloon. Within practical limits the coefficient of traveller/ring friction varies inversely with traveller mass, directly with ring diameter, and increases to a maximum at a particular rotational speed.

Heavier travellers are generally used for thicker counts, and the maximum traveller mass consistent with a good spin is chosen, to give an acceptable end breakage rate. As a last resort it may be necessary to reduce the spindle speed in order to control the end breakage rate.

The minimum traveller mass is determined by the need to avoid balloon collapse when winding onto the maximum package diameter and maximum balloon length. The maximum mass is limited by the need to avoid excessive tension, which causes a high end breakage rate. A range of traveller masses can be employed for a given set of conditions, and this range becomes narrower as the spindle speed increases.

The minimum tension required to avoid balloon collapse is proportional to (spindle speed)², and the tension produced by a given traveller is proportional to f(spindle speed)², where f is a coefficient that depends on the ring/traveller coefficient of friction, yarn/traveller coefficient of friction, and the angle at which yarn is wrapped around the traveller. The minimum traveller mass would be independent of spindle speed if f remained constant but in practice, although f is constant at low spindle speeds, it decreases at higher speeds. A slightly greater mass is therefore required at higher spindle speeds.

Minimum traveller mass is proportional to the yarn count and to (balloon height)², but is inversely proportional to ring diameter. If the traveller is too light then the balloon diameter becomes excessive, causing the yarn to beat against the separators. It can also cause traveller ‘flutter’ or ‘chatter’, a vibration with audible impacts between the traveller and the ring, which results in higher peak tensions and increased end breakages as well as traveller and ring wear.

If the traveller mass is greater than the minimum then a firmer package of increased capacity can be produced, and the twist differential across the traveller increased. This may increase yarn strength by parallelising the fibres as they are gathered from the delivery roller nip, and may reduce the end-breakage rate, though a heavier traveller will also increase power consumption.

It is generally assumed that in cotton spinning the tension ratio (mean individual thread breaking load)/(mean spinning tension) must not be less than 14 in order to achieve an acceptable end-breakage rate. Higher tensions with ratios of 4 or 5 are acceptable for polyester/cotton blends, and a ratio of 12 for worsted spinning. Yarn emerging from the front roller nip is only partially twisted and its strength may be only about one-third of the ultimate individual thread breaking load.

The minimum traveller mass for a particular ring frame is that which is just sufficient to prevent balloon collapse during winding on the shoulder of the cop bottom, that is, during winding at the maximum balloon height onto the maximum package diameter. Since both the minimum balloon tension and the traveller-ring frictional force are approximately proportional to the square of the spindle speed, the mass of the lightest traveller that can be used is largely independent of the spindle speed, n. The minimum mass is, however:

where 2R and H are in inches and N is in tex. An approximate value of the minimum traveller weight, M, in grammes is given by the expression

8.2.3 The effect of traveller/ring friction

Rings and travellers are the dominant elements in the ring spinning process. The traveller has, among other duties, the function of regulating the spinning tension. The spinning tension must be high enough to keep the thread balloon stable, but not so high as to cause yarn breakage. Winding tension increases with the frictional coefficient between the ring and the traveller (μ_RT). It should be noted that the spinning tension in the yarn is directly related to winding tension, so any factor that increases winding tension will also increase tension in the yarn during ring spinning.

Reducing the friction coefficient to the lowest possible level is key to the success of ring spinning. At this point perfectly balanced spinning geometry will have been achieved, meaning that the spinning tension is on a constant balanced level. Friction will be reduced if the rings and travellers establish a symbiosis. The ring/traveller system in short staple spinning operates dry, but at high speeds normal friction systems only work with additional lubricants. The fibres protruding from the yarn body between ring and traveller are crushed and form a steadily regenerating lubricating film in a process known as fibre lubrication. The resulting friction coefficient varies according to the fibre, which may be either dry, wax-containing cotton, or synthetic with added softening agents. The coefficient of friction (μ) can vary in extreme cases from 0.08 to 0.12, explaining why different traveller weights must sometimes be applied in otherwise identical spinning conditions. In order to maintain the same friction or spinning tension with different coefficients of friction, different traveller weights must be used. Dry cotton has a high friction coefficient, while manmade fibres have low to medium coefficients of friction, depending upon the manufacturer.

It has been shown by Catling³⁴ that the effective coefficient of friction between the ring and the traveller is a very important factor in the balloon stability equation, and largely determines the yarn tension. It was found that traveller speeds are, in practice, limited by the fact that at high speeds an abrupt and unpredictable increase in the friction coefficient is apt to occur. In some circumstances this may merely increase the winding tension (and thereby the number of thread breakages) but more usually the increase in friction is accompanied by overheating of the traveller and very rapid wear of both ring and traveller.

8.2.4 The effect of ring diameter

Package capacity is approximately proportional to (ring diameter)², so a large diameter ring may be thought desirable, but increasing the ring diameter also increases yarn tension. When thicker counts are being spun, frames with larger rings are fitted because the yarn is strong enough to withstand the increased tension and a larger package size is desirable. However, the ring diameter is restricted by yarn tension and the minimum angle of lead, as well as other factors such as power consumption, spindle rpm, and traveller speed limitations.

This is why we do not see ring spinning machines with very large rings and very small bobbins (tubes). Large ring radii and yarn packages are used for coarser and stronger yarns, but for finer yarns both the ring radius and package size must be smaller. Most worsted yarns are spun using rings between 45 and 75 mm in diameter at spindle rotational speeds between 7000 and 12 000 rpm. Short staple yarns are spun with smaller rings, but at about twice the spindle speeds used for worsted yarns.

8.2.5 The effect of balloon height

The balloon does not behave exactly like a tensioned string in ring spinning, because of its relatively large radius and the influence of traveller and ring/ traveller friction. A stable balloon system can be formed for many combinations of balloon height (H), ring radius (R), package radius (R₁), spindle speed, yarn count and yarn tension. This is because the length of yarn forming the balloon is not fixed even though the axial balloon height is fixed; the yarn length is self-adjusting so that the balloon shape permits the new horizontal force on each element in the yarn to provide the necessary centripetal force on that element.

For a given value of R and h there is a limiting value of P (as defined in 8.2.1) for which a node can be formed. The limiting value of P decreases as the ratio R/H or R₁/H increases; the maximum theoretical value of H/P is π for a ring of infinitely small radius.

To avoid neck formation R/P and H/P must be small and so P must be large. For a given yarn count, this means that either T₀ must be large or ω must be small, both of which are undesirable.

A neck will gradually form in the balloon if unfavourable conditions arise, such as increase in H or a reduction in traveller mass. Balloon collapse occurs when the neck makes significant contact with the package, thereby causing a breakdown in the spinning process. Collapse is most likely to occur when yarn is winding onto the maximum package diameter with the longest balloon length, that is, 58 when H is at a maximum and P is at a minimum.

By restricting the maximum balloon radius, and thereby reducing air drag on the balloon, control rings allow the creation of a longer balloon without a neck, and with a much lower yarn tension than would otherwise be possible. Each section of the total balloon then has a smaller value of H. H/P is thus effectively reduced and balloon collapse avoided even though the overall balloon length has increased. By stabilising tension (T₀), the benefits from the use of control rings permit larger package length, higher spindle speeds and reduced power consumption.

Yarn tension increases as normalised balloon height increases, particularly when the height is greater than 10 cm. When the ratio of yarn-length-in-balloon to balloon height is greater than 1.2, the yarn tension has a ‘lower’ value and the effect of balloon height on the tension is very weak.

The influence of balloon control rings is quite considerable, especially at long cops. A reduction of the yarn balloon is advantageous and may even be a prerequisite for optimum performance. If balloon control rings are mounted at the correct distance, with the yarn balloon restricted as long as possible during one lift of the ring rail, then a marked increase in performance is possible. Balloon control rings are used to contain the yarn-loop, by reducing the yarn tension and decreasing the balloon flutter instability.³⁵ Flutter instability here refers to uncontrolled changes in a ballooning yarn under dynamic forces, including the air drag. Due to the significant variation in balloon length and radius during the bobbin filling process, the optimal location for the balloon control ring is difficult to pinpoint. In order to address this difficulty, this study³⁵ investigates the variation in the radius of a free balloon, and examines the effect of balloon control rings of various diameters at different locations on yarn tension and balloon flutter stability. The results indicate that the maximum radius of a free balloon and its corresponding position depend not only on the yarn length to balloon height ratio, but also on yarn type and count.

A control ring of suitable radius and position can significantly reduce yarn tension and decrease flutter instability of free single loop balloons. The balloon control rings are usually fixed to, and move in sync with, the ring rail. However, the reported results suggest that, theoretically, a balloon control ring that always remains approximately half way between the yarn-guide and the ring rail during spinning could lead to significant reduction in yarn tension.

8.3.1 Theoretical models

The problem of yarn breakage during spinning is serious and difficult to solve due to the complexity of the breakage mechanisms. Yarn breakage phenomena are complicated because the causes are numerous, including the influences of humidity and temperature, but also because the tension and strength of the yarn always vary. In practice, almost all yarn breakages occur at the exit of the front roller, probably because, at that point, the yarn has not yet been twisted and is still weak. Hori³⁶ has used simple mathematical models of yarn tension and strength variation to calculate the probability of breakages. He concluded that:

Evenness of the yarn is thus more important than lower tension on spinning frames in reducing yarn breakages.

De Barr³⁷ also discussed the problem of end breakage in ring spinning in terms of a simple mathematical model. Absolute values of end breakage rate could not be calculated, but the treatment enabled the relative importance of such factors as tension, regularity of yarn, angle of lead, etc., to be assessed. The results, some of which would be applicable to any process in which yarn breakages are involved, show that the most important factor affecting end breakage rates is yarn regularity.

The method of calculating end breakage rates was based upon that described by Hori,³⁶ but the model used was more realistic, and the treatment extended so as to enable the effect of changes in, for example, angle of lead, to be investigated. The variation of T_T/C with angle of lead, α, has been used to calculate the variation of end breakage rate with angle of lead.

From the parameters of the strength and tension distributions, the probability, p, that in any interval, Z_S, the mean tension mT₁, being greater than the mean strength m_S, was calculated by the following equation:

Ghosh et al.³⁸ proposed a statistical model for predicting the end breakage rate based on the following assumptions:

The probability that the tension exceeded the strength of the links just passing from the front roller's nip was found by the relationship

where Z is the standard normal deviation and Φ(Z) can be expressed

8.3.2 Measuring the end breakage rate

Accurate measurement of the end breakage rate can be used in determining operatives’ work allocation, and provide valuable data for quality-control purposes.¹⁵ Unfortunately, end breakage rates are particularly difficult to measure with enough accuracy for the results to be useful. Not only are there systematic variations between apparently similar frames at the same time, and for the same frames at different times, but there are also many chance variations resulting from imperfectly understood and randomly occurring phenomena. Useful results can, however, be obtained if care is taken, and the procedure outlined below will be found adequate where the major systematic variations have been eliminated.

Observation of all end breakages occurring on one side of one ring frame during the build of a complete bobbin may be regarded as a 'unit breakage test'. It is usually convenient for one observer to make several unit tests concurrently by observing all the sides attended by one operative, but it is not practicable to extend this by watching more than one operative at a time. Ends gaited after doffing, and those accidentally gaited by the spinner, should be ignored. Multiple breaks on adjacent spindles often have a single cause and should therefore be noted separately.

To reveal trends with time of day, ambient conditions, part of building cycle, etc., records should be made in consecutive constant time intervals of, say, 15 minutes, and ambient atmospheric conditions should be recorded at not less than hourly intervals. After each complete build-cycle, the results should be summarised for each unit test. Trends relative to ambient conditions or stage of build may be revealed by plotting the mean rate for a number of unit tests during consecutive 15 min intervals.

Before making use of the observations, however, it is desirable to determine the reliability of the results in view of chance variations that inevitably occur. Clearly, the greater the number of unit tests comprising the series of observations, the more reliable the results. However, a quantitative procedure is required for determining the number of unit tests needed to ensure that the average is within a particular range of the true breakage rate for each set of conditions.

Suppose the end breakage rates obtained from h unit tests are b_1, b₂, b₃,…, b_h; then the average rate is

and the mean of the squares is

Then, if a further quantity

is determined, the true breakage rate lies within the range B – 2E and B + 2E, and the number of unit tests required to give a breakage rate reliable to ± q% is given by

8.4.1 The effects of spinning tension and yarn strength

Most end breaks occur close to the front roller nip, where the fibres are being twisted, and so the strength and tension in this region are of the greatest importance. It is important to note that increasing the spinning tension does not necessarily increase the end breakage rate, because the corresponding increase in twist density in the balloon leads to an increase in yarn strength in the critical nip zone which, in some circumstances, may be sufficient to reduce the end breakage rate. The effect is particularly noticeable when spinning tension is increased without any increase in spinning speeds. The balloon diameter is reduced, and thus the angle at which yarn is wrapped around the traveller is increased. As a result, both the twist differential and the tension differential across the traveller are increased, and, over a substantial range, the end breakage rate is reduced by increasing the spinning tension.

At high speeds and with large packages, however, the interaction of these factors gives rise to operating difficulties. To avoid balloon collapse, limiting spinning speed and use of proper traveller weight are extremely important while winding on the largest diameter and with the largest balloon, that is, just on completion of the cop bottom, particularly when spinning is carried out without a balloon control ring.

At this point in the build, when the balloon is at its largest and the air drag (and therefore the balloon tension) is maximised, the angle of wrap of the yarn round the traveller is minimised, so that both the tension and the twist differentials across the traveller are very small. Maximum tension is transmitted to the nip zone at a time when minimum twist potential is available for consolidation of the newly formed yarn, and consequently end breakages are common.

8.4.2 The effect of yarn irregularities

The weakest part of a forming yarn, where most end breakages occur during ring spinning, is at the point of twist insertion in the spinning triangle. Three factors are therefore of importance⁴⁰:

Clearly, where there are a large number of fibres in the cross section of the forming yarn, the yarn will be more able to withstand the spinning tension and tension fluctuations, provided that the mean spinning tension is kept well below the breaking load of the yarn (typically 30% below mean yarn strength). End breakage problems will arise when the number of fibres in the cross section of the fibre ribbon varies significantly and/or the peak value of tension fluctuation is too high. The variation of the number of fibres in the cross section causes thin and thick places in the fibre ribbon. As these pass through the twist insertion point at the apex of the spinning triangle, the thin places are more easily twisted than thick places, so thin parts of the ribbon tend to have more twist than thicker parts. If a very thin part of the ribbon becomes over twisted and weak, the yarn will be susceptible to peak tension fluctuations.

It is a well-established fact that yarn irregularity cannot be eliminated even under the best processing conditions. But there are many conditions that increase the variability of lengths of yarn, and which may lead to excessive spinning breakage rates.

The importance of irregularities in yarn strength in determining the end breakage rate has been noted by Hori,³⁶ and it is clear from findings by De Barr³⁷ that the end breakage rate is more sensitive to variations in irregularity of yarn strength than to any other factor. The effect is so marked that the range between the lowest attainable irregularity and the highest irregularity that can be tolerated is very small.

8.4.3 The effect of twist in the yarn

Most end breaks in ring spinning occur in the material just issuing from the rollers, and the end breakage rate is very dependent upon the twist in the yarn between thread-guide and the roller nip.⁴¹ The twist in this zone is always be greater than the equilibrium twist in the yarn on the package, so that the strength of the yarn here is normally greater than might be expected from the known spindle and delivery speeds. Furthermore, although the twist in the yarn in this zone decreases with the twist factor, this reduction will be less than the decrease in twist in the yarn on the package. The end breakage rate in soft-twisted yarns should not be as much greater than that in hard-twisted yarns as might otherwise have been expected. Indeed, the increased twist in the roller nip – thread-guide zone may well be very important in the spinning of soft-twisted yarns.

A reduction in end breakage rate can be achieved by using a traveller that presents a considerable obstacle to the passage of twist across it, but this reduction may be at the expense of yarn quality.

The angle at which yarn is wrapped around the thread-guide is also a factor determining the amount of twist in the yarn issuing from the roller nip. In the usual arrangement, the thread-guide is positioned substantially on the axis of rotation of the spindle, and the spindle-roller geometry is such that the yarn from the roller nip to the thread-guide makes an angle of about 30° with the spindle-axis. This suggests an average angle of wrap of 30° but, under running conditions, the actual angle of wrap will vary cyclically during each revolution of the balloon from 30° + θ₀ to 30° – θ₀.θ₀ is the angle of inclination to the spindle-axis of a yarn element leaving the thread-guide and entering the balloon.

Under most spinning conditions, the instantaneous angle of wrap becomes negligibly small once per revolution and there is effectively a direct path for twist and tension to be transmitted from the yarn balloon to the nip zone. In a system known as vertical spinning, the roller nip is directly above the thread-guide, so that the angle of wrap of the yarn around the thread-guide does not vary appreciably during each revolution of the balloon. It might be expected that the difference between normal and vertical spinning would greatly influence the end breakage rate, but this does not seem to be the case. It may therefore be inferred that the capstan (or angle-of-wrap effect) of the passage of the yarn over the thread-guide is equally effective as a twist barrier and as a tension barrier.

8.4.4 The effect of temperature and humidity

The temperature and humidity of the atmosphere in the mill are important factors in controlling breakage rate.³⁷ While no published data relating to the effect of temperature is available, common experience suggests that low temperatures should be avoided. Some research into humidity has been carried out at the Shirley Institute,³⁷ where it was found that effects in the card room and spinning room were in one respect dissimilar. In the card room it was found that very low humidity, such as is experienced in frosty weather, led to excessive drafting irregularity, especially at the draw frame, which would give rise to higher spinning breakage rates. In the spinning room the breakage rates per l00 spindles per hour were 18, 29 and 28 for percentage relative humidity values of 20, 45 and 65, indicating that very low humidity may be beneficial in reducing spinning breakage rates.

8.5.1 The causes of fly generation

In conventional spinning, it is impossible to collect all the fibres emerging from the front rollers into the forming yarn. Some fibres escape as they emerge from the front rollers, while others are thrown out later by centrifugal acceleration. Many short fibres are lost as fly during the processing of staple fibres on spinning machines, and a considerable quantity of fibre debris and dust is released. Fly and dust are deposited on machine components or are continuously being whipped up and around by rotating and circulating devices such as spindles, drums and drive wheels. They have therefore always caused significant disturbance to service and maintenance, as well as diminishing quality. This problem has intensified further with high production speeds and high drafts. In ring spinning machines, the most fly and dust are released in the main drafting zone and the spinning triangle (up to 85%), while the balloon and travellers account for most of the remainder.

8.5.2 The factors affecting fly generation

For exercising better control over fly generation it is essential to know about the influencing factors. While drafting zone and the spinning triangle are the major sources of fly generation, relative humidity also plays a major role to control it.

8.5.3 Mechanisms of twist propagation

Although twist is actually inserted into the region between the thread-guide and the traveller, the yarn between the front roller nip and the thread-guide, and that wound onto the package, is also twisted. Twist must therefore pass both the thread-guide and the traveller. There is, however, an important difference in that the twist passes the traveller in the same direction as the yarn, while across the thread-guide the yarn and twist are moving in opposite directions.

The thread-guide and traveller offer frictional resistance to the passage of yarn, and the tension in the yarn on the downstream side of either must be greater than on the upstream side. In a similar way, the thread-guide and traveller offer resistance to the passage of twist, and the torsional stress in the yarn on the downstream side (in the direction of twist propagation) must be less than on the upstream side. The twist (turns per inch, say) in the yarn in the balloon must therefore be greater than that in the yarn between front roller and thread-guide, or in the yarn as it is wound onto the package. Under steady-state conditions, however, the rate at which the twist passes the traveller must be equal to the rate at which twist is inserted in the balloon zone. It follows that the twist in the yarn in the balloon must be greater than calculated.

The distribution of twist between front roller nip and package was studied by DeBarr and Catling.⁴⁴ Grandrelle or mock grandrelle yarns were photographed during spinning using the technique described by Axson,⁴⁵ and the observations confirmed the above predictions. The conclusion of De Barr and Catling does not seem to have been substantiated by direct measurement of twist in the roller nip – thread-guide zone.

Kanai and others⁴⁶ obtained data on twist in the roller nip–thread-guide zone by direct measurement and concluded that the turns per inch of a yarn between the nip point of rollers and the traveller has been shown to exceed that of a yarn on the bobbin. The twist density of a yarn above the snail wire has been found to be 15% greater than in the yarn below it. On the other hand, Wegener and Landwehrkamp⁴⁷ found that there was a reduction of about 22% in the twist at the balloon control ring, and another reduction of about 19% at the thread guide. In worsted spinning, Gessner⁴⁸ found that without a balloon control ring, the twist in the balloon yarn is 123% and 107% below and above the control ring, respectively, and the twist in the roller nip – thread-guide yarn is 78% of the bobbin yarn twist, contradicting the findings of De Barr and Catling. Wegener and Landwehrkamp did not, however, mention whether these results applied to all stages of the doff and to the entire chase cycle. Subramanian and others⁴⁹ used a simple method for measuring the twist density in the yarn between the roller nip and the thread-guide of a ring spinning frame, which was found to be reliable in comparison to results obtained by high speed photography. The method consisted of plucking the yarn passing between the roller nip and thread guide with the fingers and counting the turns.

8.5.4 The factors affecting twist variation

Although twist variation is inherent to the mechanism by which twist is being imparted in ring spinning, proper selection of spindle speed, traveller weight and drafting angle will help to provide effective control over it.

8.6.1 Control of yarn tension

Yarn tension is much lower in rotor spinning as compared to ring spinning, but controlling yarn tension is still an important factor in avoiding end breakage. This is because the yarn formation mechanism is force-restrained rather than position-restrained in rotor spinning.

8.6.2 The factors affecting yarn tension

Although yarn tension is much lower in rotor spinning as compared to ring spinning, when considering very high speed of rotor and occurrence of twist loss it is extremely important to know in detail about the effect of different factors on yarn tension to keep the end breakage rate within control.

1. Tension yarn breaks. These are found in spun yarn, normally between the take-off nozzle and take-up rollers. The yarn ends on the yarn package have a blunt appearance, and short broken yarn ends are in general found in the rotor groove.

2. Spinning yarn breaks. These occur in the yarn peel-off zone in the rotor groove when continuous fibre spin-in is interrupted.

3. Yarn breaks that result from sliver breaks, or breaks due to similar interference factors outside the spinning box, which can be easily controlled by proper maintenance and good practice.

8.7.1 Factors affecting end breakage rate

End breakage in rotor spinning is much lower as compared to ring spinning due to lower yarn tension. Rotor and opening roller speed are the two major factors influencing end breakage in rotor spinning.

8.7.2 The mechanisms of twist loss in rotor spinning

The machine selected twist in rotor spinning does not accurately correspond to twist measured in the yarn. One reason for this is that surface layer fibres in open-end yarn have a different level of twist from those forming the body of the yarn. A stage will be reached where some fibres are not yet fully twisted while others are already twisting in the opposite direction. These simultaneous opposing twisting effects will cause the measured twist to be lower than the theoretical twist.⁶⁸

According to Kleinhansl,⁶⁹ twist loss in the spinning chamber results from torsional stresses in the twisted yarn acting backward past the separation point, and so imparting twist to the strand of fibres in the rotor groove before its full count has been reached. Fibres incorporated into the yarn after that point are not fully twisted and so the total twist is reduced. London and Jordan⁷⁰ concluded that the difference between measured and machine twist is a function of doffing tube design. According to Lord and Grady,⁷¹ the difference between measured and machine twist values, expressed as a twist gradient, is a function of machine design, operating functions and fibre characteristics.

Salhotra⁷² has put forward a hypothesis to explain twist loss through the incidence of sheath fibres. According to Salhotra the transmission of twisting torque cannot be one hundred percent efficient; there is always a possibility of leakage through fibre ends at the yarn forming point. In the extreme case of a single fibre in the yarn core at yarn formation point, one can reasonably expect a loss of twisting torque in the yarn core.

8.7.3 The factors affecting twist loss

Control of twist loss is extremely important considering its influence on yarn quality and spinning performance. Major factors that influence twist loss are amount of twist and opening roller speed.

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