Rotor Spinning - II
The rotor
Figure 1. Rotors used in Rotor Spinning
The rotors used in rotor spinning are shown in Fig.1. The important rotor parameters which have significant effects on the spinning process and
yarn quality are:
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fiber feed-in conditions (feed-in height relative to the rotor groove, feed-in direction, fiber feed-in speed relative to peripheral rotor speed).
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rotor groove diameter
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rotor groove shape (aperture angle, groove radius and depth)
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rotor wall and rotor groove roughness
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rotor wall inclination and surface quality
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rotor speed
The most important of these are rotor diameter and speed, the design of the rotor groove and the rotor wall. These are discussed below.
Rotor speed and diameter
Rotor speed depends, in part, on rotor diameter. Rotor speeds typically lie in the 120-210 m/sec range, but mostly between 150-190 m/sec, with a
tendency to be higher with a smaller rotor diameter. Currently the smallest rotor diameter used industrially is 28mm with rotor speed up to
150,000rpm, though some machines can reach 160,000 rpm. Technically the rotor diameter is not dependent on yarn count or vice versa. However,
a larger rotor is required for coarser yarns because they are frequently produced from relatively inferior cotton, which results in increased trash
deposit in the rotor. A larger rotor diameter reduces the frequency of cleaning required to remove trash. The numbers of fibers deposited over the
yarn peel-off point at each rotor revolution also increases as the rotor diameter decreases, which results in a higher number of overlapping fibers at
the yarn peel-off point in the rotor groove. This overlapping fiber winding generates additional torque around the yarn axis, which counteracts twist
migration in the rotor groove zone.
An increase in rotor speed results in improved spinning stability up to a specific maximum speed, depending on such factors as rotor diameter.
However, too high a rotor speed results in poor spinning stability due to the large centrifugal forces on the fibers in the rotor groove which increase
spinning tension and result in yarn breaks. Too low a rotor speed also results in poor spinning stability. If yarn tension is too low, the yarn twist
cannot adequately propagate into the rotor groove.
Depending on the speed, a reduction in rotor diameter in the range from 56 mm to 28 mm results in higher productivity and lower energy
consumption, but more irregularities in yarn structure (e.g. more belly band places) and reduced spinning stability with more yarn breaks.
Both rotor speed and rotor diameter affect spinning tension. Since yarn tension may not exceed a certain value because of the risk of increasing yarn
breakages, rotor diameter must be reduced if faster speeds are required. With a smaller rotor and correspondingly increasing rotor speed, fiber
contact pressure due to centrifugal force in the rotor groove increases. This makes twisting more difficult. In addition, embedded trash particles can
only be removed with greater difficulty.
The rotor groove
The configuration of the rotor groove has a pronounced effect on spinning stability, rotor groove dust loading and yarn quality. The most important
parameters in this connection are groove radius, groove angle and groove surface roughness. The more open (depending on yarn diameter) and
smoother the rotor groove, the better the yarn twist penetration, resulting in good spinning stability and bulkier yarns. Rotors with deep, narrow and
rough grooves offer advantages in terms of yarn quality.
There are three different types of rotor grooves available: G-groove, S-groove and U-groove. Fig. 2 shows the shapes of different types of rotor
groove.
G-groove
S- groove
U-groove
Fig. 2: Different types of rotor grooves.
A G-groove rotor with a narrow groove is suitable for finer count yarns providing the fibres are clean since this design of groove difficult to clean
and has a tendency to produce moiré effects in the yarn. This type of groove is recommended for knitted yarns. The S-groove rotor has a sharp edge
and is suitable for dirty cotton and coarser counts with less tendency to produce a moiré effect. The U-groove rotor has a wider groove suitable for
coarser counts and produces a higher strength yarn than the S-groove.
The rotor wall
The roughness of the rotor wall is determined by the rotor coating. A rough surface gives higher yarn quality. On the other hand, too much
roughness with a small wall inclination angle may affect spinning stability. Rotor wall unevenness amplifies this effect. Suitable rotor geometry,
precision production and the right fiber feed-in (closer to the rotor groove in the case of a small wall inclination angle) are important requirements.
The take-off nozzle
As has been noted, to start spinning a seed yarn is introduced into the yarn tube until it contacts the collecting surface and become trapped in the
strand of fibres. Yarn withdrawal then commences with the fibre layer peeled from the collecting surface. Continuity is maintained by the
continuous stream of fibres arriving on the collecting surface to replace the fibres removed as the withdrawal point moves steadily around the
collecting surface usually in the same direction as the rotor rotation. This is termed normal withdrawal. The yarn follows a smooth curve and yarns
of good appearance are produced. Occasionally there is a spontaneous change of motion of the yarn withdrawal point and it moves around the rotor
in the opposite direction. This may be described as reverse withdrawal. In this case, because of air drag, the yarns form an ‘S’ curve and yarn
appearance is adversely affected.
Theoretically one turn of twist is inserted for each revolution of the gyrating yarn end. It is important to note that the direction of twist insertion is
determined by the direction of rotation of the rotor, and that it remains unchanged whether normal or reverse withdrawal takes place, and that the
amount of twist inserted is virtually constant throughout the yarn package.
The yarn is withdrawn from the rotor through the take-off nozzle which protrudes into the rotor. The position of the take-off nozzle relatively to the
rotor has an effect on the quality of spinning. With a standard setting, the front edge of the take-off nozzle and the rotor groove should lie in one
plane. Deeper take-off nozzle projection improves yarn quality to some extent but impairs spinning stability at the same time.
In order to achieve good spinning stability, it is important for the highest possible degree of yarn twist to be available at the yarn peel-off point in
the rotor groove. This ensures a high yarn twisting torque and, as a result, the greatest possible yarn twist penetration into the rotor groove zone and,
consequently, reliable twisting-in of the deposited fibres. The effect of the yarn take-off nozzle on twist diffusion in to the actual spinning zone is
very significant. In the rotor spinning process, the take-off nozzle plays about the same role as lappet guide in the ring spinning. It affects the degree
of twist present in the rotor and therefore the spinning conditions in the rotor groove. Additionally, it exerts strong frictional forces on the yarn at the
time of false twist release.
The materials used in take-off nozzles are generally ceramic and steel. Different configurations and their characteristics are:
- Smooth take-off nozzle:
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Produces good yarn values and little hairiness.
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Should always be used if spinning stability allows it.
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Recommended especially for high twist yarns
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Well suited for delicate fibers
- Fluted take-off nozzle
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Better spinning stability
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Poorer yarn values with higher yarn hairiness.
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Best suited for low twist yarns with a high trash content.
- Spiral take-off nozzle:
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Improved yarn regularity
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Reduced spinning stability
- Swirl take-off nozzle:
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Increased yarn hairiness.
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Best-suited for knitted and terry fabrics.
Modern rotor spinning machines
The main differences between different rotor machine designs are:
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the method of opening and feeding
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the types of rotors and bearings used
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whether they are designed for particular types of fibre and ranges of yarn count
Other features include the following:
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single-sided or double-sided machines
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pneumatic transfer of trash to a common container at the end of the machine with duplicated waste filter units which permit cleaning without
interruption of production.
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automatic control systems which stop the feed sliver when an end breaks. This leads to a significant reduction in the amount of spinning
waste, an important economic advantage compared with ring spinning.
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automatic yarn length counters which actuate signal lamps to indicate the need for doffing, with an automatic synchronized stop if too long
an over-run is permitted by the operative.
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the use of air suction during doffing to hold spun yarn so that, on completion, the yarn forms the tail at the end of the package tube. In this
way doffing can take as little as 30 seconds
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sensors providing a yarn monitoring system which detects foreign fibres or other impurities in the yarn
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laser-guided positioning systems
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yarn waxing as part of the spinning process
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automatic rotor cleaning with brushes at predetermined intervals
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a robot system which increases the contact pressure of the rotor belt briefly at an individual spinning position in order to accelerate the rotor.
This ensures reliable and rapid acceleration during piecing. The machine can therefore be operated with significantly lower overall contact
pressure along the whole of its remaining length. Lower overall belt contact pressure means reduced belt wear and reduced energy
consumption.
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a package transfer arrangement with two winding heads provided for each rotor so that the full package can be removed while the next
package is being formed. This method may be most advantageous on long staple spinning machine for thick carpet yarns.
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automatic conveyance of full packages to the next stage
Rotor spinning performance:
Yarn breakage
In addition to yarn quality, running performance plays a major role in evaluation of a spinning process. Running behavior is often expressed in
terms of the number of yarn breaks per unit mass of yarn. Spinning yarn breaks, and in fact all yarn breaks in general, are rare events. In rotor
spinning, it is important to distinguish between different types of yarn break, which are attributed to quite different causes [10]:
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spinning yarn breaks
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tension yarn breaks
Spinning yarn breaks occur in the yarn peel-off zone in the rotor when continuous fiber spin-in is interrupted. An increasing number of fibers
remain at the yarn peel-off point in the groove and are no longer spun into the yarn end which becomes thinner, ultimately breaking. In the case of
spinning yarn breaks, the main factor is sliver quality (e.g. impurities in the sliver) and uncontrolled fiber accumulations in the event of inadequate
cleaning, especially in the trash extraction zone.
Tension yarn breaks take place in the already spun yarn, normally between the take-off nozzle and take-up rollers, leaving a short, broken yarn end
in the rotor groove. The cause of tension yarn breaks is always excessive spinning tension, which affects the weakest yarn point i.e. the point
between the take-off nozzle and the take-up roller.
The stability of the rotor spinning process decides whether a problem such as trash particles, foreign fibers, dust etc. will result in a yarn break or
not. Spinning stability in rotor spinning is largely influenced by the following four factors:
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The numbers of fibers in the yarn cross section.
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Fiber length relative to rotor circumference
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The extent and degree to which yarn twist can penetrate into the rotor groove (twist-in zone length ).
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The speed and reliability with which the fibers move from the impact point on the rotor wall to the rotor groove
For better spinning stability, all fibers at the yarn peel-off point in the rotor groove must be continuously spun into the rotating yarn elbow.
Adequately high yarn torque, i.e. an adequately high degree of yarn twist in the yarn end, is necessary to this end.
Structure and properties of rotor-spun yarns
Rotor spun yarns are well known for their unique three-part structure:
- wrapper or belt fibres
- sheath fibres
- core fibers
Figure 3. Structure of Rotor Spun Yarn (a) Schematic drawing (b) Optical Micrograph
Fig.3 shows the surface structure of typical rotor yarn. The core contains densely-packed fibers similar to ring-spun yarns. Sheath fibres are loosely
packed round the yarn core at a low angle to the yarn axis. The wrapper or belt fibres are wrapped around the outside of the yarn at a very large
inclination to the yarn axis. It has been reported that fibre migration in rotor yarn is relatively local: fibers in each layer are only tied to the fibers of
adjacent layers. Rotor spinning generates lots of hooks and looped fibers even if a well parallelized sliver or roving is fed into the rotor. The typical
distribution of fibers shapes in rotor-spun yarn the yarn is: 39% folded or buckled fibres in the core, 31% straight fibres in the core, 15% leading
hooks and 15% trailing hooks in the outer layers.
Given its structure, fibers in rotor yarn are less packed than ring yarn. Rotor yarns are known to be 5-10% bulkier than ring yarn. Across the crosssection, the packing is not uniform. The packing is maximum at a point approximately one third to one quarter of yarn radius from the central axis.
This has been attributed to greater buckling if fibres in the core. As a result, packing of rotor yarn is concentrated nearer the yarn axis and less
towards the outer surface of the yarn in comparison to ring yarn.
Rotor spun yarn is less strong than comparable ring spun yarn. This is because of the straight, parallel arrangements of fibers and denser packing of
fibers in ring spun yarn which contrast with the higher numbers of disoriented folded fibers in rotor spun yarn, lower levels of fibre migration, less
packing and the presence of non-load bearing wrappers and belt fibres.
Rotor spun yarns are generally more extensible than ring spun yarns. The higher breaking extension of rotor yarn is due to presence of a lot of
hooked, looped and disoriented fibers in the structure. However, the dense, more tangled structure of fibres in the core offers very little freedom of
movement of fibres in rotor yarns. Rotor yarns are therefore less flexible than ring yarns which have a more uniform helical arrangement of fibres.
Due to its unique structure, rotor yarn shows higher abrasion resistance than ring spun yarn. The loosely-wrapped sheath fibers can easily yield to an
abrasive surface, and, given its greater bulk, the yarn can flatten, giving further abrasion resistance. Rotor yarns also have fewer irregularities and
imperfections compared to carded ring-spun yarns. This has been attributed to the mechanism of yarn formation, i.e. back doubling in the rotor
groove before twist insertion which irons out irregularities.