Rotor spinning

Overview of development:-

The idea of producing yarn by the rotor –spinning techniques is far from new. Patent applications this method was filed before the 2nd World War. The first usable design was put forward by Meimber at Spinnbau Company in Bermen in 1950. The first machine of this kind was shown in 1955 at Brussels exhibition.

The first machine suitable for industrial use was presented in 1967 with the name BD200 in ITMA exhibition. The current share of rotor-spun yarn is around 20% of total staple-fiber- yarn production & it is increasing steadily. The speed of rotor machine is about 100000 rpm now days previously which was 30000 rpm. Rotor spinning is more economical than Ring spinning for the counts up to NM70 (Ne40).

The invention relates to a spinning rotor for an open end rotor spinning machine, particularly a spinning rotor comprising a rotor shaft, a rotor cup having an opening, an inner chamber, a rotor groove, a conically widening slide wall extending from the opening to the rotor groove and a rotor base arranged opposing the opening and designed with a bore, through which the rotor shaft extends at least partially, wherein the rotor shaft is connected by means of a connection element to the rotor cup and the rotor shaft and rotor cup comprise a common rotational axis.

In conjunction with open end rotor spinning machines, a large number of the most varied spinning rotors are known from the patent literature and generally consist of a rotor shaft for mounting the spinning rotor and a rotor cup for producing a thread. Spinning rotors of this type in modern open end spinning machines reach rotational speeds of far above 100,000 min −1. Rotational speeds that are as high as this in total place special demands with regard to imbalance, mounting and stability of spinning rotors of this type. As spinning rotors of this type are heavily stressed, for example as a result of mechanical vibrations, the highest demands are also made of the fastening between the rotor shaft and rotor cup.

Spinning rotors are described, for example, in German Patent Publications DE-OS 28 12 297 or DE 199 10 77 A1, in which the rotor cups are connected to the rotor shaft, in each case via a hub, into which a bore is let. The connection is implemented as a press fit here and is non-releasable.

Furthermore spinning rotors are known from German Patent Publications DE 40 20 518 A1 or DE 103 02 178 A1, in which the rotor cups only have one central bore in the region of the rotor base, in which the rotor shaft is inserted. The rotor shaft is, in this case, equipped with a bearing collar, on which the rotor cup is fixed by a weld connection. A weld connection for fixing a rotor cup on the rotor shaft is also described in German Patent Publication DE 3519 536 A1. In this known device, the rotor cup has an extra thick base. The rotor shaft is fixed to this rotor cup base by means of friction welding. The aforementioned connections between the rotor cup and rotor shaft in total have the disadvantage that either the connection is relatively heavy, which has a very disadvantageous effect on the acceleration capacity of the spinning rotor, or a change in structure occurs in the components in the course of the attachment of the two rotor parts and this is not unproblematic because of the high rotational speeds of such spinning rotors.

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A further economic aspect, of interest in some fields, arises from the possibility of spinning mill-waste fibers with the aid of rotor –spinning m/c. This m/c is an excellent recycling m/c. Further more this machine is the first final spinning m/c that can be fully automated. This fact has contributed substantially to the improvement in economics of rotor spinning.

Tasks of the rotor-spinning machine:-

The basic tasks of the rotor spinning machine are

Opening (& attenuating) almost to individual fibers (fiber separation). Cleaning. Homogenizing through back doubling. Combining i.e. forming a coherent linear strand from individual fibers. Ordering (the fibers in the strand must have an orientation as far as possible in the

longitudinal direction). Improving evenness through back-doubling. Imparting strength by twisting Winding.

R_40 –Rotor Spinning m/c

Principle of operation:-

The principle is illustrated as

Principle of Rotor Spinning:-

The general principle of rotor spinning is shown in Figure. The input fiber strand is a drawn sliver. A sliver may have more than 20,000 fibers in its cross-section. This means that a yarn of 100 fibers per cross-section will require a total draft of 200. This amount of draft is substantially higher than

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that of ring spinning. Drafting in rotor spinning is accomplished using a comber roll (mechanical draft) which opens the input sliver followed by an air stream (air draft). These two operations produce an amount of draft that is high enough to reduce the 20,000 fibers entering the comber roll down to few fibers (5-10 fibers). In order to produce a yarn of about 100 fibers per cross-section, the groups of few fibers emerging from the air duct are deposited on the internal wall of the rotor and a fiber ring is formed inside the rotor. The total draft in rotor spinning is, therefore a combination of true draft from the feed roll to the rotor (in the order of thousands) and a condensation to accumulate the fiber groups into a fiber ring inside the rotor. The total draft ratio is the ratio between the delivery or the take-up speed and the feed roll speed. This should approximately amount to the ratio between the number of fibers in the sliver cross-section and the number of fibers in the yarn cross-section.

Consolidation in rotor spinning is achieved by mechanical twisting. The torque generating the twist in the yarn is applied by the rotation of the rotor with respect to the point of the yarn contacting the rotor navel. The amount of twist (turns per inch) is determined by the ratio between the rotor speed (rpm) and the take up speed (inch/min). Every turn of the rotor produces a turn of twist, and a removal of a length of yarn of 1/tpi inches.

The winding operation in rotor spinning is completely separate from the drafting and the twisting operations. The only condition here is that the yarn is taken up at a constant rate. This separation between winding and twisting allows the formation of larger yarn packages than those in ring spinning.

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Sequence of operation:-

The feed stock in form of either card sliver or draw frame sliver from first or second passage drawing. The sliver runs from a can beneath the spinning unit into the feed trumpet. A feed roller grips the sliver & pushes it over the feed trough into the region of the opening roller. A spring ensures firm clamping of the sliver by urging the trough towards feed roller. In the event of an end-break, the feed unit is stopped either by stopping the feed roller rotation or by pivoting the in feed trumpet, in each case sliver feed stops automatically. The signal pulse causing this effect is generated by a yarn-sensing arm.

In the in conventional spinning processes, the fiber strand at in feed is maintained as a coherent structure & is merely attenuated during spinning. In rotor spinning, the fiber strand is opened to individual fibers. This task is performed mainly by the opening roller. This small roller which is clothed with needles or saw teeth, combs through the fiber beard projecting from the nip between the feed roller & the tough it transports the plucked fibers to the feed tube. An air flow is needed for further transport of the fibers to the rotor. This is generated by central fan that draws air by suction through leads from each rotor box. To facilitate generation of this under pressure, the rotor box must be hermetically sealed as far as possible. The suction stream in the feed tube lifts the fibers off the surface of the opening roller & leads them to the rotor. In the course of this movement, both the air & the fibers are accelerated because of the convergent form of the feed tubes. This represents a second draft following the nip trough/ opening roller & giving further separation of the fibers. Moreover partial straightening of the fibers is achieved in this air flow. A third draft arises upon arrival of the fibers on the wall of the rotor because the peripheral speed of the rotor is several times as the speed of the fiber. This is a very important feature because it contributes significantly to good orientation of the fibers. The last straightening of the fibers occurs as the fiber slides down the rotor wall into the groove under the influence of the enormous centrifugal forces work with in the rotor.

Side view of the rotor spinning m/c

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On average, from one to five fibers emerge simultaneously from the feed tube. After sliding down the rotor wall they come to rest in a longitudinally oriented form in the rotor groove. Because the rotor is turning steadily under the stationary feed tube continual deposition of the fibers in the groove is achieved. In this way a continuous fiber ring is built up in the groove. This operation is referred to as back doubling. If nothing further were done the rotor would be chocked in no time. However since the whole purpose is to form these fibers into a new yarn, the free end of a yarn is allowed to extend from the rotational axis to the rotor periphery. Centrifugal force acting at this point presses the yarn end firmly against the wall of the collection groove, exactly as in case of the fibers in the ring. The yarn end therefore adheres to the rotor wall. Accordingly as the rotor turns it carries the yarn along & the latter rotates around the rotation hub (navel) like one arm of the crank.

Each revolution of the rotor inserts in true twist in the yarn. When the yarn has reached its maximum twist level as determined by the prevailing force conditions, the yarn end begins to run about its own axis i.e. it rolls in the rotor groove. Now, open yarn end is resting in the binding-in region on a strand of more or less parallel fibers, rolling of the yarn end to grasp the fibers from the ring & twist them in & so on. A yarn is thus spun continuously. It is simply necessary to pull this yarn out of the rotor by means of withdrawal rolls & wound on a cross wound package.

Speed interrelationship:-

Normal & maximum revolutions & speeds are

Rpm of opening roller :5000 -10000 rpm Rpm of rotor up to 100000 rpm Delivery speed: up to 200m/min.

  Fiber/Machine Interaction in Rotor Spinning:-

As indicated above, the sliver of some 20,000 fibers per cross-section is drafted using a combination of mechanical and air draft. Obviously, a fiber strand that has been carefully prepared by carding and drawing to straighten the fibers will find it unpleasant to be treated

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by a toothed opening roll as it enters the system. This major fiber entanglement after a long journey of straightening and parallelization provides the first test to the fibers in the rotor spinning system. The comber roll drafts fibers by detaching a fiber beard presented by a feed roll and passing them into the rotor at much higher speed than the advance of the beard. Fortunately, cotton fibers are flexible and tough enough to withstand the comber roll action. Normally, wire-wound clothing is recommended for cotton and cotton blends where pinned combing rolls are suitable for fragile fibers such as acrylic and rayon.

The extent of the opening action imposed by the combing roll will depend on the extent of fiber length. As the fiber length increases, the force acting on the fiber beard increases significantly. This can result in fiber damage. Thus, a moderate fiber length is required for rotor spinning. In fact, long fibers such as pima cotton or Egyptian cotton may suffer waste of fiber fragments if used on the rotor spinning system. Comber noils produced from combing these long staple fibers are more suitable.

As the opened fibers flow around with the combing roll, friction between the fibers and the comber roll metal chamber results in a fiber velocity lower than the surface speed of the combing roll. Those fibers are normally in a disoriented shape. In this regard, fiber attributes such as fiber resilience, fiber/metal friction, crimp, stickiness, and surface finish are of keen importance. The tendency to increase the combing roll speed makes these fiber properties even more critical. This increase is often associated with high yarn hairiness and yarn imperfections.

Although the primary role of the combing roll is to open the fibers, it can also act as a cleaning unit by separating trash particles from cotton. Obviously, this additional function can easily overstress the combing roll making it wear rapidly. It is important, therefore, that the input sliver exhibit a great level of cleanliness. A maximum trash level of 0.1% is typically recommended by the machine maker. In addition, fine trash and dust content can accumulate in the rotor groove leading to yarn defects and end breakage.

Fibers coming out of the comb roller are airborne through an air duct. This zone of draft is of a special significance because of its impact on fiber orientation. Since laminar airflow is hardly a reality, fibers are likely to suffer turbulence as they flow through the air duct adding more disorientation. This factor partially contributes to the weakness of rotor-spun yarn. Long fibers are more vulnerable to air stream disturbance than medium or short fibers. In order to minimize fiber disorientation, the airflow in the duct should have a velocity exceeding that of the surface speed of the opening roll. Investigators suggested speed ratios ranging from 1.5 to 4. To obtain such a fast airflow, the inside of the rotor is run at a vacuum which may be achieved by designing the rotor with radial holes to allow the rotor to generate its own vacuum (self-pumping effect). Alternatively, an external pump can be used as in most modern machines.

Another approach to minimize fiber disorientation in the air duct is by designing it in a tapered shape toward the rotor to allow acceleration of the fibers as they approach the rotor inside surface. This action may also straighten the leading fiber hooks coming out of the opening roll. Fibers emerging from the air duct come into contact with the rotor inside surface, which is typically faster than the fibers. This also assists in straightening the fibers disoriented in the previous zones.

The mass flow per unit time of fibers in the rotor spinning system, particularly from the air duct to the take-up zone provides an interesting insight into the contribution of fibers to the quality of rotor-spun yarn. The product of the fiber mass and fiber velocity can determine this

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quantity. For a stable process, this mass flow must exhibit a continuity that can be determined by the following simple mass-flow equation:

This above equation indicates that the ratio between the number of fibers in the yarn cross-section (nfy) and the number of fibers in the air duct (nfd) is governed by the ratio of the fiber velocity in the air duct (Vfd) and the yarn velocity (Vfy). The equation summarizes an important phenomenon that is unique to rotor spinning; the doubling effect. As indicated earlier, the effect of the air draft is to reduce the fiber strand down to few fibers (2-10 fibers). These fibers are then landed into the inside surface of the rotor as it takes many layers of fiber to make up sufficient number of fibers per yarn cross-section. As successive layers of fibers are laid into the inside surface of the rotor, a doubling action occurs. This action tends to even out short-term irregularities in the yarn. This doubling action contributes largely to the low irregularity of rotor spun yarn. One should not overlook the fact the elimination of the roving process also contributes greatly to the Low Mass irregularity of rotor yarns.

The number of doubling in rotor spinning can be estimated by the ratio nfy/nfd. Thus, an increase in the number of fibers in the yarn and/or a reduction in the number of fibers in the air duct can enhance the uniformity of the yarn. This point partially reveals the critical importance of fiber fineness in rotor spinning. Machine manufacturer commonly states that rotor spinning requires very fine fibers. It is our opinion that this statement should be qualified by a specific value of fiber fineness. Obviously, if the fiber is coarse, less number of fibers will be allowed in the yarn cross-section (for a given yarn count) and the effect of this on yarn strength and irregularity are well known. On the other hand, if the fibers are too fine, the risk of extremely high flexibility (as with micro denier synthetic fibers) and/or low maturity (as with cotton fibers) may arise. In this situation, the benefits of manufacturing fine fibers may be offset by the high tendency of fibers to entangle and disarrange. In a previous study [7], we found that polyester fibers of 0.7 denier provided higher yarn irregularity than those of 0.9 denier. In the same study, we found no significant improvement in the strength of the yarn made from 0.7 denier over that made from 0.9 denier fibers.

If one observes a rotor-spun yarn under a microscope, one will easily notice that along the yarn axis there are many fibers that are not completely tied into the yarn. Those fibers have a free end that wraps itself around the yarn periphery and causes constriction of the yarn. This is an inevitable defect that is peculiar to rotor-spun yarns. It is commonly called "fiber belts" or "wrapper fibers". According to Hunter [8], those fibers are introduced to the yarn in the rotor as a result of fibers that are trapped from the wrong direction, i.e. from the section the yarn has just left, or by fibers that are fed from fibers fed directly onto the yarn-forming point and by the yarn between the doffing tube and rotor groove coming into contact with the airborne fibers.

In connection with the influence of fiber attributes on yarn quality, we should point out that wrapper fibers are largely useless. They fail to contribute to the strength of the yarn and they

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provide no improvement to any quality aspect. In fact, they should be treated as waste fibers that happen to stick to the yarn body. The inevitability of wrapper fibers, however, has led many machine manufacturers to claim that they may have some merit including improvement of yarn abrasion resistance.

Although wrapper fibers are a result of a technology deficiency, their presence is greatly enhanced by some levels of fiber attributes. For instance, long fibers tend to form wrappers that are so tight that the belt looks more like a thin place. Short fibers, on the other hand, form slack and loose belts. The number of wrapper fibers is often estimated by the ratio between the staple fiber length and the rotor circumference (FL/p d). Other fiber attributes that may contribute to wrapper fibers include fiber stiffness and fiber fineness; stiffer and coarser fibers tend to become wrapper surface fibers.

One important feature that separates ring spinning from rotor spinning is the tighter fiber control in the former due to the higher spinning tension. In rotor spinning, fibers are not firmly gripped at any point of their flow; a differential tension such as that discussed in ring spinning does not exist. Accordingly, no significant fiber migration (to enhance yarn strength) is expected in rotor spinning. This point reflects the importance of fiber/rotor groove friction, and fiber-to-fiber friction. The lack of significant tension also results in some fibers that are only partially twisted leading to inferior yarn strength. These deficiencies can only compensate for by high fiber strength and optimum fiber fineness.

The different aspects of fiber/machine interaction discussed above result in a structure that consists of three layers: a core that is truly twisted (similar to ring-spun yarn), an outer layer that is partially twisted, and fiber wrappers. The true twist in rotor-spun yarns results in a natural curling tendency, similar to ring-spun yarns. However, this torque is partially balanced by a torque caused by the wrapping effect of the wrapper fibers, particularly those that take an anti-clockwise direction. The more such anti-clockwise banding fibers there are, the lower will be the curling tendency in the rotor yarn. Yarns with low curling tendency also display low yarn extensibility by virtue of their "liveliness". These features reveal two important points:

a. In rotor spun yarn, the true amount of twist is difficult to measure b. The actual twist in rotor spun yarn is typically less than the nominal twist as set by

the ratio between the rotor speed and the take-up speed.

With regard to the first point, the inverse connection between the number of fiber wrappers and the curling tendency is normally used to obtain an indirect measure of rotor yarn structure by measuring its curling tendency or the residual twist (difference between the measured yarn twist and the nominal twist). A typical value of residual twist may range from 10% to 40%.

With regard to the second point, rotor-spun yarn always requires higher levels of twist than comparable ring-spun yarns. This means that the yarn will be usually stiffer and will produce a fabric of poor hand. For this reason, some knitters prefer ring spinning over rotor spinning in the medium count range (20’s to 30’s) substituting economical benefits by quality demands. Proper fiber selection can play a major role in producing a flexible rotor-spun yarn. In general, it is well known that fine, long, and flexible fibers provide less resistance to twisting than coarse, short, and stiff fibers. Through optimization of this combination, a rotor spun yarn of acceptable flexibility level can be produced.

Can Rotor Spinning Produce Yarns of Fine Count?

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As indicated earlier, rotor spinning has superior economical advantage over ring spinning in the coarse to medium counts. In recent years, there have been many attempts to push rotor spinning further into the area of fine counts When we speak of fine counts, we generally mean yarns of maximum 40’s cotton count. In order to produce fine yarn counts on rotor spinning, two main factors must be addressed: (a) machine-related factors, and (b) material-related factors.

Examining the spinning tension of rotor spinning may summarize the machine-related factor. This is the tension on the yarn, Ty, delivered from the rotor expressed by the following equation:

Where v is the rotor rotational speed in radians/sec, r is the rotor radius, and m is the coefficient of friction between the yarn and the navel surface in contact with the yarn.

The above equation indicates that the spinning tension is highly sensitive to the rotational speed of the rotor and the radius. The product vr is a primary design criterion in rotor spinning; recent trends are to increase rotor speed and reduce rotor diameter so that a balance in spinning tension is always maintained.

In light of the fact that the value of the product vr has virtually reached its technological limit, a reduction in yarn Tex will result in a reduction in the spinning tension (which is already low compared to ring spinning). The importance of spinning tension as a controlling factor of the fiber flow was indicated earlier. A reduction in yarn Tex will also result in a smaller area of yarn/navel contact. This will reduce the coefficient of friction, m, leading to a further reduction in spinning tension. More importantly, less area for friction heat imposed by the high rotational speed to dissipate. This last point is critical in spinning synthetic fibers or cotton/synthetic blends.  In relation to the material-related factor, the earlier discussion of fiber/machine interaction pointed out the problem of fiber disorientation during spinning, and its impact on yarn strength. This factor, in addition to the loss of fibers through wrapping, makes it difficult to improve the rotor-spinning limit. Furthermore, the need for lower twist level to improve yarn flexibility and fabric hand makes matters additionally complex.

Fine counts are associated with high quality yarn (defect free and certainly trash free). This means that the quality of the fibers must be upgraded to produce fine counts. The sliver fed to the machine should be prepared carefully so that it exhibits the lowest irregularity possible, and the lowest trash level possible. In case of light sliver, inter-fiber cohesion is critical. These criteria indicate that fiber properties such as trash content, short fiber content and inter-fiber friction are extremely important, not only for producing acceptable quality levels, but also for minimizing end breakage during spinning. In recent years, low level combing (8% comber noil extraction) has been used to upgrade cotton fibers used in producing fine rotor yarns. Combing upgrades the cotton quality by removing neps and short fibers, and by providing better fiber orientation in the fiber strand. The added-cost by combing is justified by lower end sown during spinning, and slight reduction in twist. Bischofberger [9] reported that with optimum noil removal in combing (8-14%), and under similar spinning conditions, the yarn count can be increased from 30's to 36's from the same raw material at a constant rate of ends down for both counts of 150 ends down/1000 rotor hours.

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Irrespective of raw material, yarn strength was found to increase by about 10% with combing and strength uniformity was improved.

Combed rotor-spun yarns yield better filling insertion rates during weaving because of lower rates of filling stops.

Combed rotor yarns result in better knitting efficiency because of the low fly deposition and the smoothness of yarns. The uniformity and handle of single jersey knitted fabrics were significantly improved as a result of using combed rotor yarns.

In light of the above discussion, one can develop a list of fiber properties in rotor spinning according to the order of their importance. We suggest the following list:

Comparison between the Best & the Worst Levels of Coarse Yarn Quality

Uster Statistics, 100% Cotton Carded-Rotor-Spun, Count Range, Ne = 4’s-10’s]

Yarn

Property

95%

Statistics

5%

Statistics

Percent

Difference

(% Improvement)

Yarn Strength, YS (cN/tex)

Yarn Elongation, YE (%)

Yarn Hairiness (H)

9.5

6.5-5.8

16-10

15-14.5

11.0-9.0

5.5-4.5

58%

62%

60%

Count Irregularity C.V%(Ne)*

Uster

6.0-4.0 0.8 84%

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Irregularity C.V%m

Strength Irregularity C.V%(YS)

Elongation Irregularity C.V% (YE)

IPI/KM

13-15

8.5-9.5

8.0-9.0

75-500

10-11.5

4.0-5.5

3.0-4.0

3-200

25%

45%

58%

80

Comparison between 100% Cotton & 50/50 Polyester/Cotton Yarn Quality [5%-Uster Statistics, Carded-Rotor-Spun, Count Range, Ne = 10’s-40’s]

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Raw material used-

Short staple spinning m/c (up to 60 mm fiber length) require

Cotton (CO) Cotton waste ( secondary m/t recycled m/t) Cotton noil Blends of two or more of these materials. Polyester fibers (PES). Polyacrylonitrile fiber ( PAC) Poly amide fiber (PA) Viscose (CA) Blends of man-made fibers ( mostly PES/ CV & PAC/CV) Blends of cotton & man made fibers ( mostly CO/ PES & CO/CV)

Raw material requirement:-

Fiber length:-

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Yarn

Property

100%Cotton

50/50Polyester/Cotton

Yarn Strength, YS (cN/tex)

Yarn Elongation, YE (%)

Yarn Hairiness (H)

14.5-13.5

9.0-6.5

4.5-3.5

17.5-14.0

14-8.5

5.5-3.5

Count Irregularity C.V%(Ne)*

Uster Irregularity C.V%m

Strength Irregularity C.V%(YS)

Elongation Irregularity C.V% (YE)

Total Imperfections/km (IPI)

0.8

11.5-15

5.5-8

4.0-6.0

200-350

0.9-0.51

11-17

5-10

5.5-8.5

85-400

Following m/t can be processed according to Reiter Company

Cotton:-

Waste <7/8 inches ( for yarns up to 15 tex count) Short-staple cotton < 1 inch ( for yarns up to 30 tex count ) Medium staple cotton < 1 1/8 inches (for yarn up to 17 tex count )

Man made fibers:-

Staple length up to 60 mm for count = 12 tex yarns

Fiber fineness:-

Finer fibers preferred in rotor spinning usually in the range of

Cotton 2.8 to 4.5 micronaire. Man- made fibers 1, 1.2 to 1.7 dtex.

Coarse fibers lead to deterioration in spinning conditions; this necessitates the use of higher twist co-efficient.

Fiber strength:-

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Due to poorer exploitation of the fiber substance, fibers of the greatest possible strength as shown in diagram.

Dirt & dust:-

The rotor-spinning machine reacts very sensitively to the trash content of cotton. Coarse particles such as husk particles stay caught in the rotor groove. They can prevent yarn formation at this point, & this in turn can lead to an end down or to fiber agglomeration at the particle. This gives a thick place at the agglomeration point & immediately a thin place after this. More trash content also lead to more NEP generation. Small particles also lead to deterioration in quality.

Clean raw m/t is therefore a precondition for spinning of yarn on the rotor-spinning m/c. in accordance with recommendations from Reiter, the following residual trash content should not be exceeded in the feed sliver:

Up to Ne 6 : 0.3% Up to Ne20 : 0.2% Up to Ne 30 : 0.15% Up to Ne 50 : 0.10%

Other foreign matter:-

Quartz & mineral dust present in cotton causes wear & tear in m/c Foreign fibers lead to ends down. Honey dew makes fiber to stick to m/c parts & cotton free of honey-due should be

used. Spin finish should be taken off before feeding to m/c. it acts same as honey due. Remnants of the yarn lead to thick places in the yarn, so they should not be used.

Preparation of the raw material:

The processing stages in rotor spinning, not only are characteristics of the raw material important, the manner in which this m/t is prepared is also significant. The most optimized process line given by Reiter is shown in diagram. The machine selection should be made according to the raw m/t. Card sliver can be directly fed to the m/c or one, two or three passages can be given before being feed to the rotor. Reiter proposes two passages of draw frame two get best results in term of strength, evenness & handle. Third passage is not even used for cotton- synthetic fibers because back doubling the rotor leads to high degree of fiber/ fiber transverse doubling.

Reiter machine sequence for rotor spinning.

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Twist insertion in yarn in rotor spinning process:-

Real twist is applied by the motion of the rotor acting on the yarn arm that passes from the rotor groove to the yarn with drawl point inside rotor. Each revolution of the rotor causes about one turn of twist to be inserted in the yarn & 1/T inches (T= twist per inch) of yarn are removed. There can be movement of fibers with respect to the metal of the rotor during twisting. This is because of the fibers are not firmly held by any discrete nip point. Twist usually runs back along the rotor groove & some fibers are laid onto an already twisted core of fibers. This affects the yarn structure. The center of the end of the yarn withdrawal tube is fitted with a non- rotating navel through which the departing yarn flows as shown in the figure. Sometimes a separate plate is introduced to prevent the premature capture of the incoming fiber by the outgoing yarn. This makes a less desirable transport system because of the complexity of the passage way, but in separating the incoming fibers & outgoing yarn fulfills a useful function.

The yarn entering the navel rolls on the inside surface. This rolling action produces a false twist in the section of yarn inside the rotor. The false twist is in addition to any real twist created by the rotor. Twist is trapped in the running yarn b/w the point of twist application & the nearest upstream yarn trap. In the present case, the point of twist application is at the navel & the twist trap is on the collecting surface of the rotor. The flare radius of the navel affects the false twist as shown in the table. Spinning performance & yarn character depends on the twist in yarn inside the rotor rather than the apparent twist in the delivered yarn. Navel is grooved to increase the false twist. Also grooved navel tend to make the yarn weaker, bulky, neppy& hairy particularly at higher rotor speeds. The groove causes the yarn to bounce off the surface of the navel for very brief periods of time. Yarn tensions measured inside the rotor are very close to the theoretical figure given by the formula w2Rr

2 n but there are pulses due to the yarn riding over the grooves, if near there are not too many of them.

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False twist also affects by how close the front surface of the navel is set towards the flat inner surface of the rotor. Local shear in the air is produced by the rotor wall moving close to the stationary navel. This shear can produce a small amount of false twist in the yarn. Enlarged potion of the yarn can interact with this space if the gap is set too narrowly. There will be difference in the co-efficient of friction of the navel surface & the yarn, also navel wears. It has become common to use ceramic navels because of their longer life. As the navel varies it also cause the yarn character to vary. It is also important to make all the yarn from same kind of navels to eliminate the barre’ effect in the fabric.

The range of usable twist multipliers is much affected by these considerations; a typical set of twist multipliers is shown in the figure. Generally the twists are higher than the ring yarn & the combination of higher twist & the most disorganized yarn surface create a rougher handle. At one time this was of major concern, but fabric finishing techniques have improved & the market has adjusted for the difference the lower costs outweighs the tactile disadvantages. As mentioned earlier, end-break rates, amongst other things, a function of rotor diameter & speeds. Where as the larger rotors used in 1980s gave minimum end-breakage rates/lb at about 70000 rpm, the newest small rotors at about 28 mm with rpm of 100000 gave more end breakage rate. The geometry of the twist is shown in the figure below

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The minimum twist level achievable is of interest because low twist yarn will have a good handle & m/c productivity will also increase. Generally the minimum twist diminishes with rotor speeds of 70000rpm & then level off, under some circumstances it raises at higher speeds. The lowest value of twist is a function of the radius at the base of rotor groove & the type of navel in use. The navel might be grooved or non-grooved; they might be steel or ceramic. Generally, the higher the rotor speed the less is the need of grooved navels. The combination of the m/t is recommended by the manufacturer which is followed by trials.

Effect of the rotor parameters on twist insertion:-.

It is the compactness of the fibers in the rotor groove that both aids twist insertion, to give a good peripheral twist extent & produces improved yarn strength.

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Results have shown that the degree of fiber compactness in the rotor groove will depend on the rotor speed, the rotor diameter & the tightness of the groove angle.

It is now well known that stronger yarns are obtained with large diameter rotors & a 30 degrees v-grooved rotor, provided that the rotor speed does not produce a spinning tension greater than the yarn strength.

In order to increase the degree of fiber compactness in the rotor groove, several patented device have fitted mechanical means, which exert a controlled force on the fiber in the groove.

Winding:-

The yarn is withdrawn & wound on a cross wound package by keeping the surface speeds constant i.e. decreasing the the rpm of winding cylinder with in crease in dia due to yarn take up.

The rotor:-

The rotor is the main spinning element of the rotor-spinning m/c. Yarn quality ,character working performance of yarn prudctivity, & costs etc. all depend chiefly on the rotor. The most important parameters of the rotor that exert influence are

The rotor form The groove The rotor diameter Rotational speed along with The rotor bearing Co-efficient of friction b/w the fiber & the rotor wall. The air-flow conditions inside the rotor Liability to fouling

Rotors are replaceable element in the m/c.

Rotor-Bearing Dynamic System Simulation:-

Generally speaking, rotor dynamics is a branch of structure dynamics with a strong focus on "beam or shaft model" adding an important supporting component, "bearing". Since rotating machines and turbo machinery are widely used mechanical equipments, rotor dynamics has been singled out from the structure dynamics field to become a special technological subject.

Basically, a rotor structure contains a shaft, several key components, such as compressors, turbines and couplings, and of course, the most important supporting elements, bearings. Dynamic or vibration behaviors concerning with a rotor-bearing structure include: (1) critical speed, (2) synchronous (rotor speed is the same as vibration frequency) response, (3) asynchronous response (rotor speed is different from vibration frequency), (4) stability and (5) reliability aside from material fatigue.

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The following chart depicts the integrated Rotor-Bearing Dynamic system simulation procedure:

Automation of the rotor spinning machine :-

The extent of realization:-

Unlike the ring spinning m/c , the rotor spinning machine is easy to automate. Accordingly , with the exception of can transport , can changing & sliver –piecing , all the operations on the rotor spinning m/c have already been automated. i.e.

Piecing ends down Package change Yarn length measuremnet. Monitoring the quality individual yarn. Production-monitoring.

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Rotor- cleaning

Degree of auto mation in yarn piecers:-

Up to 4 automatic yarn piecers travel continually around the m/c or back & forth on the respective sides, while runnung on the rails mounted above the m/c. Two or more robots are rather expnesive , but use of extra devices bring a major improvements in the efficiency , especially in the spinning of coarse yarns. The doffer replacing the complete packages by empty tubes is some time incorporated in the same robot used as piecer. But it can also be designed as aseparate unit. Three or four Robots as designed by Rieter Company are shown in the figure below.

Several manufacturers donot require a specially prepared empty tube; in other; a winding device is provided at the end of the m/c to wind the length of piecing thread onto each tube.

The package removed by the spinning positions is transfeered by the doffer robot to one or two conveyor belts running along the packages to one end of the m/c. The belt or belts convey the packages to one end of the m/c where they can be placed in cartoons or carriges by hand or by means of a robot .

Package handling:-

Transport of packages to the end of the m/c is is effected by the conveyer belt. The packages running to the m/c end are ready for removeal, & at present this step is usually carried out manually, with the packages removed from the belt being laid inb previously prepared cartons or on pallets.

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Length mesurements:-

Special measurement devices provided on each spinning positions register the exact length wound on the package & stops the spinning position when a predetermined yarn length is reached.

Monitoring of quality at the spinning position:-

Each spinning position is fitted with a measuring head manufactured by Zellweger Uster mostly. These heads monitor thick , thin places & neps & periodic faults.

Logging of operating data:-

Such equipment mainly provide the following data

Machine number Yarn fineness Material Twist Delivery speed Production Efficiency of the m/c Efficiency of the individual spinning m/c Down times Number of piecing per package Number of doffing operations Number of disturbances Ends- down rates Defective spinning postions

Technical & technological data:-

Number of spinning positions per m/c up to 220 Count range 12- 125 Tex (5 – 50 Ne) Draft 25- 400 Speed of rotation of opening roller 6000- 11000 rpm Rotation speed of rotor up tp 120000 rpm Rotor diameter 32 -65 mm Delivery speed ( m/ min) up to 200 Package mass up to 5 kg Angle of taper 2 o - 4o 20’ Winding angle 29o – 45o

Comparison of rotor spun yarn with the ring spun yarn :-

Breaking strength lower than ring spun yarn

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CV% of strength better than ring spun yarn Elongation at break higher than ring spun yarn Mass irregularity ( over short lrngths) better than ring spun yarn Imperfection index lower than ring spun yarn Volume greater than ring spun yarn Abrasion resistance higher than ring spun yarn Stiffness higher than ring spun yarn Handle harder Power consumption less than ring spun yarn

Possible yarn counts Ne 3 – 60 Ring Ne 6 – 200

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Energy consumption with productivity lower as compared to ring m/c.

Aesthetic properties Surface rougher than ring yarn Hariness lower than ring yarn Lusture on the dull side

More capital costs & more maintanace cost as compared to ring machine.

References

Manual of textile technology by W.klien Hand book of yarn production by Peter .R.L ord www. Reiter.com Spun yarn technology by Eric Oxtoby.

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