Developments In Drafting At Ring Frame
Among the various components of ring frame, drafting has the maximum influence on yarn quality and ring performance. Drafting at ring frame considerably influences not only evenness and appearance of fabric, rejections due to yarn faults. Therefore conversion of drafting is given a high priority in the efforts to upgrade a ring frame and payback from such investments is attractive.
Irregularity in drafting
Irregularity added in drafting is mainly caused by:
1. Inadequate control over the movement of short and floating fibers.
2. Slippage of strand and fibers under the drafting rollers.
3. Variations in speed of drafting rollers.
4. Mechanical faults
Development in drafting has to be focused mainly on reducing the contribution from these factors.
Casablanca drafting
Casablanca A500 drafting represents the first development to improve the control over floating fibers. Prior to that, a self-weighted light middle top roller was used to control short fibers while allowing long fibers to slip underneath. Even now some woolen ring frames employing this principle are found in operation in the mills. In Casablanca drafting a pair of endless aprons, driven by middle top and bottom roller and held together by a tensor, is used to control the floating fibers in the front zone. The back zone is merely a break draft zone with low drafts. The back top roller is self weighted while middle and front top rollers are weighted by a spring, encased in a cage, through a saddle.
The main drawbacks of this system are:
1. Being self-weighted, the back top roller is subjected to slip page.
2. Because of bigger diameter if back top roller, the back zone setting is 55 mm, which is too wide for short staple cottons.
3. The pressures on the front and middle top rollers, which are already low, are further affected by ageing of spring over a period of time.
4. Top rollers are guided by cap bar nebs at the end. Misalignment of roller often occurs because of grooving of cap bar nebs and disturbance in cap bar settings.
5. Plain bearings are used for top rollers, which need frequent lubrication. The lubrication used to attract fluff.
Even so, the quality of yarn obtained on A500 drafting can be improved by optimizing the apron spacing, break draft and roving twist. The pressure on top rollers could be increased by insertion of addition washers inside the spring cap and by minimizing disturbances in auto lever setting. The intermittent slippage of count variability in the yarn as shown by the following study.
Intermediate roving and corresponding yarns spun from them were collected from 10 spindles of ring frames with A500 from two mills and count variability in both materials was determined. A wrapping length of 7 yards was used for roving as this corresponds with 120 yards of yarn after drafting. The results of these studies, shown in Table 1, indicate a significant increase in count variability within as well as between bobbins is increased after drafting. This means the top roller slippage varies from time and time within a spindle because of variations in drafting force in break draft zone.
Top arm drafting
Top arm drafting represents a major breakthrough in improving the quality and performance of drafting. Most of the problems encountered with Casablanca drafting are overcome in top arm drafting by adopting pendulum system of central arbor guidance.Center suspension of top rollers ensures self-alignment of top roller to a position axially parallel to bottom roller. Further, an apron guide bar or nose bar is used to guide the bottom apron. The nose bar provides firm guidance to the bottom apron and the strand over it and further brings down the distance between apron release point and the front roller nip. As a result, the fiber control is improved.
The advantages of top arm are:
1. Self-alignment of top roller in relation to bottom roller results in better grip over fibers.
2. Heavier weighting by the use of better grade springs reduces slippage. The spring pressure is also nearly constant over a long period of working.
3. Higher drafts are achievable because of better control over fibers.
4. Cleaning of draft zone and removal of roller lapping is facilitated because of easy access to parts as top arm together with top rollers can be lifted up.
About 1-1.5% unit’s better U% and 15-20 % reduction imperfections are obtained by conversion to top arm drafting. Further CV of count of yarn is reduced significantly as shown in Table 2. The improvement arises because of reduction in slippage of back top rollers because of heavier weighting.
In SRK drafting, coiled springs are used and in Suessen WST drafting, chrome vanadium hardened steel plate springs are used foe weighting. In Reiter and Laxmi Rieter drafting, top rollers are pneumatically loaded by compressed air. Top rollers are individually weighted in SKF while in the first generation WST UT3 drafting, the pressure from plate spring is transmitted to top rollers by means of a weigh-bridge. In Laxmi Reiter drafting also a weigh-bridge is used to load top rollers. Individual weighting is more advantageous compared to distribution of weights by weigh bridge as any defects or diameter variations in any top roller do not affect weighting on other top roller.
Second generation top arm
Though several developments have taken place in top arm drafting over the years, broadly they can be divided as first and second generation top arms. The major improvements in second-generation top arm are:
· Higher pressure on top roller.
· Provision for varying the pressure on the top roller as per the requirement.
· Modified top arm support bar.
· High stability cradle.
Top roller pressures, found in first and second generation top arms are given in Table 3.
The roller settings in top arm drafting for cotton, 38-mm staple fiber and blends are given in Table 4 and figure 1. Front top roller offset forward by 2-3 mm while middle top roller is offset backward by 2 mm. Offsetting of front top roller forward is done to ensure better flow of twist to front roller nip while offsetting of middle top roller backward is done to ensure firm guidance of bottom apron on nose bar. Normally back bottom roller setting is kept as 50 mm for cotton and around 55 mm for polyester/cotton blends. However the minimum bottom roller setting in the latest P3-1 arm of Reiter is 60 mm.
Top arm support bar
Top arm support bar with a cross section of ‘D’ shape instead of circular, found in first generation, is used in UT600 to minimize disturbances in height of top arm. The top arm clamp bracket has a profile identical to the support bar and is securely fitted on it at the flat portion. This obviates slippage of top arm clamp around the support bar. In SKF PK225, top arm support bar has two groves, in one of which arm is securely locked by a screw and in another setting the height of the arm is done by another screw and in another setting the height of the arm is done by another screw. This minimizes disturbance to height setting because of slippage of top arm around support bar.
High stability cradle
High stability cradle is another improvement in UT 600 top arm. The weighting pressure from retainer spring acts directly on the cradle and from there the middle top roller. This eliminates the yarn defects caused by lifting of front edge of cradle due to defective cradle spring; a defect sometimes encountered in other drafting systems.
Modified cradle for short staple cottons
The top arm drafting systems currently available do not permit front roller settings below 43 mm. While processing short staple cottons of 23 mm and below staple lengths, this setting is too wide and increases the irregularities. A modified cradle was developed with reduced cradle length. This is in conjunction with a shorter wide nose bar enabled the front bottom roller setting to be reduced to 40 mm. About 30% reduction in thick and thin places and 0.8 units lower U% in yarn were obtained with this modification for short staple cottons. Alternately higher ring frame drafts could be achieved with the same level of imperfections in yarn.
Pressure setting
Setting of pressure to the desired level is done by means of height gauge as shown in figure 2. To minimize subjective errors in height setting, "pointer height gauge " is used in Suessen drafting. Pressure checking with the help of a top roller pressure-checking device, which has a dial gauge in it, is the best way to minimize variations in pressure. In SKF PK 225, there is provision for adjusting the pressure in front top roller to three different levels by means of a cam that is adjusted by a wrench.
Improvements from second generation top arm
Conversion to second generation generally improves U%, imperfections and Classimat faults in the yarn. The order of improvement varies with the mixing and age of top arm and is generally more prominent with polyester blends. Further winding breaks due to slubs and weak places, comes down. The results of studies carried in this regard in the mills are given in Table 5.
Table 5 shows that improvements in evenness and imperfections are seen in cotton as well as polyester blend counts with second generation top arms. In fine cotton counts and polyester viscose blend counts, CV of count of yarn also shows a significant reduction with second generation top arm. This is because of the heavier pressure on the back top roller. This hypothesis is further confirmed by the fact that a lower mechanical draft has to be kept at ring frame with PK225 drafting compared to PK211 E drafting to get the same count of yarn from the same back material. The mechanical draft in 58 s has to be reduced from 36.5 to 32.6 with PK225.
Table 6 shows that Classimat faults are also reduced with PK 225 drafting because of heavier pressure on top rollers and improved control over fibers. It is interesting to note that the reduction is obtained even in short length faults with PK 225. This means belief, short length faults are also influenced by drafting.
Age of top arm
A significant deterioration in yarn quality is also found to take place with wear and tear of parts and ageing of springs even in top arms. While part of the deterioration may be due to wear and tear of cots and aprons, remaining part is due to wear and tear of springs, saddles and other weighting elements of top arms. Top arms which are about 4 years old are found to give 1 unit higher U% and about 20% higher long thin and thick faults compared to new top arms of the same type.
Latest top arms
P3-1 top arm represents the latest top arm of Rieter and Laxmi Rieter used in g5/series of ring frames. The bottom and top rollers have bigger diameter and front roller top roller has a higher offset of 4mm. Cradle length is longer and as result the distance of uncontrolled fiber movement is reduced. To minimize torsion vibration of back rollers, the drafting system has a separate drive operated by its own motor in Rieter G30 ring frame. The 1008 spindle machine is divided into two modules, each of which is driven by its own motor for the same purpose.
In HP drafting, which is the latest version of suessen drafting, the weighting is effected by a spring plate, which distributes 75% of load to the middle top roller and balance 25% on front top roller. An anti friction clip of synthetic material fitted on cradle brings top apron nip close to front roller nip. Pressure release systems are provided to bring down pressure to 10-20% of the full load when frame is stopped for long duration of time.
Measuring top arm pressure
Sun Tarp Guage is an unique, compact & portable instrument for measuring Top Arm Roller Pressure in ring frames The instrument measures the spring force is spring loaded drafting system & pneumatic pressure in case of pneumatic drafting system used in ring & speed frames. The spring force or pneumatic pressure is however expressed in terms of kgf.
In order to achieve optimum productivity & yarn quality from ring & speed frames, the top rollers are loaded at specified levels and thus uniformity between top arms on the same frame is maintained. In the case of continuous working, however, variations occur from the set loads, which affect the quality of drafted material. Frequent methodical check up of top roller loads, is therefore, necessary and the sun tarp guage enables the same to be done easily & directly on front, middle & back rollers in all types of drafting systems used in ring
Individual weighting and weigh bridge
In SKF and UT 600 top arms, the three lines of top rollers are individually weighted. But in Rieter and Laxmi Rieter drafting, pressures on the top rollers are distributed through a weigh-bridge. Fulcrum position has to be varied depending upon roller setting and type of cradles. One disadvantage of this system is that any defect in one of the top rollers will affect weighting on the other top rollers.
Improved weighting arm
Constant load on the top rollers is very important to yarn quality. Taking into account the fact that also spindle of one machine should produce yarn of identical quality, constant load and small load deviations are significant quality characteristics of any new weighting arm.
Opening and closing of the weighting arm PK 1601 requires handling forces that can be uncomfortable for operators. During longer periods of machine standstill, the load of all weighting ends have to be reduced in order to avoid marks in the soft rubber cots of the top rollers which could otherwise impair the quality of the yarn. The existing weighting arm offers three load steps for every top roller. The setting has to be done manually and this takes a lot of time.
The underlying concept for the new weighting arm PK 6000 as shown in the figures 2 and 3 was to use pneumatic pressure instead of metal springs to generate the load which can be adjusted infinitely. A pneumatic ring tube (1) in the machine supplies each weighting arm with compressed air (2). When the machine is operating and the weighting arm is closed, a pneumatic valve (3) is open so that the compressed air can enter the pneumatic spring (4). The pneumatic energy is transformed into mechanical forces on the top roller (5) by weighting elements with the pressure plates (6). When the weighting arm has to be opened e.g. when the yarn is broken, the operator has to push back the handle (7), which is connected, to the push rod (8). The pneumatic valve (3) is pushed down, air inlet is blocked and the compressed air can flow out of the rubber spring. The helical spring (9) moves the weighting arm upwards automatically. This locking device has the advantage that the arm can be opened and closed without the load, which results in low handling and low frictional forces in the swivel joints. The design of the new drafting system id completed by a new top apron cradle (10) made from composite materials with individual tensioning of the each apron.
The pneumatic concept has brought a number of advantages including improved product quality, quick load setting and partial load reduction, considerable technological flexibility, easy and ergonomic handling, easy mounting and modern styling. The pneumatic principles guarantee constant load on all top rollers at every spindle position. The load remains constant even when the diameter of the top rollers decreases as a consequence of grinding the rubber cots of the top rollers. The surface structure of the new top apron cradle reduces the friction during apron operation and helps to preserve the aprons. Individual aprons tensioning makes the system less sensitive to tolerances in the apron length, improving the fiber guidance and consequently the yarn quality. There is no longer any need to remove and dismantle the top apron cradle for service work.
The load setting and the partial load reduction, necessary during long standstill can be done easily. The infinite pressure setting allows the machine to be adapted to the yarn requirements. Mounting and setting of weighting arms on the machine is much easier due to improve accessibility of the fastening and setting elements. Finally the compact design has a modern appearance and smooth surfaces mean less cleaning.
Advantages of Pneumatically Loaded Weighting Arms
1. Adaptation of the weighting arms to the design of all modern ring frames and roving frames is possible.
2. Pneumatic loading and partial load release is centralized.
3. The new arms have a smooth surface because of clear and compact design which minimizes fly accumulation on machine parts. The new weighting arms are without lever.
4. Easy setting of the drafting system and easy handling of the arms is possible.
5. There is no possibility of overloading, constant load on all elements of the drafting system is maintained.
6. Load pressure system is independent of the draft zone setting.
7. Differences in top roller diameters arising from grinding of cots do not produce differences in loading.
8. A wide range of staple length can be processed on ring frames and roving frames fitted with new arms.
9. The new loading system is user friendly, completely safe and results in comprehensive improvement of the quality of yarns which are produced.
Top rollers
Originally top roller were of detachable type being made of two components, viz.;
1. Bearing unit
2. Top roller boss
The boss can be removed from the bearing unit to facilitate greasing. The boss is retained on the bearing by means of a snap ring. With such a design play was found to develop between the units after years of usage leading to irregularities. To overcome this non-detachable type of top roller were developed. The inner wall of the top roller shell forms the outer race of the bearing in this design as shown in the figure 3. Hence the inner wall has to be precision ground, thereby ensuring accurate movement of top roller even after years of usage. Double row of bearing is used to minimize the play further. To minimize bending of top roller, a saddle collar is introduced instead of recessed top roller in the middle.
Cots
Hardness, diameter and width are important characteristics of cots influencing irregularities, softer cots on front roller, with a shore hardness of 65o, brings down the U and thick and thin places in yarn significantly in cotton counts. The improvement arises from the improved grip over the strand because of the extended contact at the nip. As a result, slippage of fibers under the nip is reduced. The reduction in imperfections varies with the mixing and ranges from 20-30%. Softer cots are however more susceptible to wear and tear and roller lapping and are therefore not preferred for polyester blends. Bigger cots diameter up to 30-mm diameter brings down slubs, crackers ad other Classimat faults particularly with polyester blends. Roller lapping is also reduced with bigger diameter. Lower cots width up to 25 mm improves pressure over the nip and so contributes to better drafting.
Step nose bar
Nose bar with a step or depression in the height for a portion of the width at the front side improves the control over short fibers, as shown in the figure 4. The improvement arises from the curved path the strand takes while passing over the nose bar. Extent of improvement varies with the amount of depth, and there is an optimum depth at which improvements are maximized.
The optimum depth value varies with the mixing and is between 1.2-2.4. About 20-30 reduction in thick and thin places in the yarn is found with step nose bar. Conversion of plain bar to step nose bar can be done by milling the front portion to a desired height and polishing and plating it. This calls for much less investment than a new step nose bar. By introducing soft cots and step nose bars, about 30-50% reduction in imperfections is obtained as shown in Table 7.
Offset drafting
Offset drafting by INA represents another development for improving yarn quality. The back bottom roller is raised by a height of 13.5 mm and back top roller is offset backwards by 24 mm so that the nip point is taken back by 15 mm around the circumference of bottom roller as shown in figure 5. The incoming roving therefore wraps round the circumference of bottom roller over a considerable length before it leaves it.
Further, back zone bottom roller setting is reduced from 51 to 44 mm. As a result of these actions the strand takes a V-shaped path as twist in roving is broken down. The strand width is reduced leading to improved inter fiber friction in the main drafting zone. For a 1.25 Ne roving, the strand width is reduced from 2.5 to 1.45 mm. Detailed studies show that thick and thin places and U% of yarn are significantly reduced in cotton counts with offset drafting. The improvements are more prominent in carded counts from short staple cottons and at high ring frame drafts. Mill experience also confirms the improvements. It is unfortunate therefore that there is no manufacturer offering offset drafting at present.
Fibre guide apron in high draft drafting system
Aprons have the most important task of fibre guidance in drafting systems. They provide a controlled increase in the fibre speed in the main draft up to the full speed on the delivery roller, at the same time reducing the fibre mass to the prescribed value. The most important requirements for quality aprons are: running characteristics, durability and dimension maintenance .
The aprons are usually made of calf leather (usually used for bottom aprons) and synthetic rubber. The behavior of aprons depends upon the material as in the case of leather, the friction coefficient is lower. Natural leather aprons are expensive and, contrary to rubber aprons which are produced as a tube, always have a join.
The modern rubber aprons have a three layer construction:
1. The inner layer assures a trouble-free, jerk-free sliding over the tension bar.
2. Above this a tough cord thread is wound in a spiral form. This center layer affords the necessary dimensional stability.
3. The finely polished and chemically treated outer layer is responsible for drafting result and the regular fibre control.
These three layers are connected to become one unit by vulcanization. Only aprons cut exactly parallel guarantee perfectly straight running and do not deviate.
Bottom apron
The bottom apron is driven by the middle cylinder and guided through the bottom guide plate to as close as possible to the lower delivery cylinder. As the apron must be taut a tension device is usually used but for space reasons this cannot be set up where the apron is not taut therefore great attention must be paid to the tension device.
Top apron
The top apron is driven by bottom apron and must for its part drive the cradle roller. For technical reasons, a short apron extruded to exactly match the inner diameter can be used. The tension of the apron is attained by tightening over the elasticity of the plastic cradle or by springy elements.
Spacer plate
The size of the differing fibre masses, and the yarn fineness is adjusted by a spacer plate. With this clip the nipping point within the two aprons can be shortened or lengthened in relation to one another, without straining the long fibres too much. Narrow space plates results in better values in the yarn thanks to better fibre guiding too close to the delivery roller. A too narrow adjustment often leads to loss of breaking strength and to poorer functioning. When a closer spacer is used in conjunction with modified nose bar, reduction in the imperfections can be maximized.
Ring can spinning
Ring can is a sliver to yarn spinning machine by Suessen, suitable for counts up to 30 - 40. Total drafts achievable in this system are around 180-200. Positively driven aprons guide the silver from the can in the creel to the back pair of rollers as to minimise stretch. The machine is equipped with Suessen HP drafting. Break drafts are higher up to 3 with lower back zone setting. Higher loads are kept on second and third top roller. Three drawing passages are given in carded mixings to ensure favorable presentation of major hooks to ring frame.
Though the machine occupies more space, additional space requirement is offset by the omission of speed frame. As roving frame is omitted the stretch introduced at this stage is avoided. However, contradictory results are reported in regard to quality of yarn in ring can as compared to conventional spinning.
Further, mill experience indicates the need for some more improvements in ring can especially in the sliver transporting system and drafting system.
Compact spinning
In the traditional ring spinning, fibers in the selvedge of strand emerging from front roller nip do not get fully integrated into the yarn because of the restriction to twist flow by the spinning triangle. These fibers show up partly as protruding hairs or as wild fibers. The spinning triangle is because of higher width of the strand as compared to final yarn diameter.
Further the fibers are tensioned to varying extent depending upon their position in the spinning triangle. This results in non-simultaneity in the occurrence of breaks in fibers, as the yarn is broken. As a result full realization of fiber strength is not achieved in the yarn. The hairiness is a source of weft bars and warp way streaks in the fabric. Long protruding hairs from the yarn contribute to multiple breaks in weaving and fabric faults like stitches and floats.
In compact spinning, spinning triangle is eliminated by incorporating a condensing zone after main drafting zone, thereby overcoming the drawbacks of conventional spinning. This will be clear from a comparison of mode of yarn formation in two systems as shown in figure 6. The condensing zone has a revolving perforated apron with suction underneath. The fibers collected on the perforated track of the apron and get condensed. A low-tension draft is kept in the condensing zone to assist in the condensation. Compact spinning systems are offered by Rieter, Suessen and ITV-Zinser. Slight variations in the design of condensing zone are found in the three systems, but the final effect is to get the strand reduced substantially in width.
In Rieter Comforspin, a perforated drum replaces front bottom roller of drafting system. A second top roller also presses on the drum. There is a suction system generating vacuum udder the drum. Condensing of strand takes place between the two top rollers under the perforated drum non rotating insert inside the drum with a specially shaped slots helps in achieving condensation. In the Elite system of Suessen, condensing zone consist of a profile tube with a perforated lattice apron running over it. A delivery top roller press on the apron drives the apron. A suction system under the profile tube causes condensation of the strand.
In ITV-Zinser system, illustrated in figure 7, condensing zone consists of a revolving perforated apron. The size of perforation in the apron is varied as per the count of the yarn to get the desired condensation.
Compact spinning offers the following benefits: -
1. 15-20% reduction in hairiness of yarn, the reduction being more pronounced in long length hairs.
2. 10-15% improvement in yarn tenacity. Further, the strength of the weakest element is considerably improved leading to fewer end breaks in subsequent stages.
3. Twist can be reduced by 10% while maintaining same yarn strength.
4. Significantly better evenness of diameter and hairiness.
5. Significantly better abrasion and resistance because of absence of long protruding hairs. As a result end breaks and droppings in loom shed and linting in knitting are reduced.
6. Size percentage can be reduced by 30-50%
7. Singeing can be omitted
8. Lower twist can be employed in doubling.
9. Improved appearance and lustre of fabric with a softer handle, reduced pilling and better dye uptake.
Automation and Integration
Rieter G 33 ring spinning frame incorporates a number of automatic devices. First is the SERVOgrip, which has been described as a revolutionary doffing method. With this device doffing can be performed without the objectionable underwinding which may give rise to yarn faults and end-breaks. Second is the FLEXIdraft which is a multi-motor drafting mechanism which eliminates manual change of gears for regulation of draft and twist whenever count changes are required. Changes in the main draft yarn twist and cop build-up are performed by manipulation of controls at the machine control panel.
Conclusions
We have briefly discussed some of the developments in drafting at a ring frame and the ways in which it can affect the nature of yarns. As the ring frame is the last stage in spinning and since high drafts are applied here, drafting system at this stage has considerable influence on the evenness and imperfection level in the yarn. The modern drafting systems have mainly two important features, with which one can most effectively react to spinning requirements such as, the problem free use of high total drafts and the possible use of sufficiently high load pressures on the ring frame.
Submitted by
Jayakesh B.S.(99369)
Nitiz Kumar Sood (99378)
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