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Twist Insertion

Factors affecting twist insertion :-

the insertion of twist in rotor spinning is influenced by design of two machine components, namely , the doffing tube or withdrawl tube & the rotor.

 Effect of doffing tube on twist insertion :-.

  • Under dynamic equilibrium, the twist propagates from the navel of the doffing tube into the first 8-10mm. Of fibres collected in the rotor groove.
  • This length of 8-10mm. Of the fibres as the peripheral twist extent & is important for obtaining both a good spinning performance & acceptable yarn properties.
  •  It has been reported that the degree of peripheral twist present during spinning is controlled by the much reported false twist effect of doffing tube.
  • The sudden drop in twist after the navel is an indication of the false twist effect.
  •  If the false twist is suppressed, as in spinning with rotating doffing tube, the peripheral twist extent is small, & a considerably  higher twist factor is needed to spin without a high end breakage rate.
  •  In controlling the insertion of twist in the yarn , the doffing tube also influences the yarn structure. Results show that , as more false twist is inserted , then the more the wrapper fibre are present on the yarn surface.
  •  At the navel of the doffing tube, the yarn twist back to the true twist value, which causes part of the wrapper fibres to become untwisted in producing the final surface structure.
  • It is ,however, the degree of twist in the core of the yarn that gives the yarn strength.
  • Since the core fibres are those in the rotor groove the yarn strength shows good correlation with the peripheral twist extent.
There are two possible reasons for this.
  •  Firstly , results are given which shows that the twist in the core is not always the same as that calculated from the rotor speed & the yarn delivery rate; It varies directly with the peripheral twist.
  •  Secondly, the core of the yarn does not consider entirely of helically wound fibres , but it shows a twist difference across its diameter that is inversely proportional to the peripheral twist. Because of the relatively high tensions present within the rotor & high rotational & draw-off speeds of the yarn, wear readily occurs on the doffing tube navel. This of course will influence the false twist & ultimately the yarn properties. In designing the doffing tube, two factors therefore to be considered : control of the false twist effect & the reduction of wear.

Control of false twist effect :-.

The false twist depends on the frictional behaviour between the yarn tail & the navel of the doffing tube & is influenced  by various factors, which include the type & count of the yarn, the draw off speed & the texture of the contact surface, as well as the atmospheric conditions.

 A very important factor, however , is the ratio of the yarn draw off tension f2, measured downstream of the doffing tube & to the calculated tension f1 in the yarn tail between the navel & the rotor wall.

The tension f2 must be greater than f1 if spinning is to take place. However, for stable conditions , the ratio should be within the range of 1.2-2 .

For given yarn count , f1 is dependent upon the rotor speed & the rotor diameter & is not significantly changed by changes in the doffing tube design. The tension f2, however, will be affected by the doffing tube profile & frictional properties.

Any such changes in the doffing tube design, in combination with f1, will control the false twist effect.

 A compromise must therefore be reached in trying to improve the false twist effect in order not to exceed a value of 2 for the ratio f2/f1.

Taking these things into consideration various navel designs are made. Click here to watch them.

Reduction in wear :-.
  • The draw off speed through the doffing tube is usually between 60 & 150 m/min. While the rotational speed  of the yarn tail in contact with the navel is up to 80000 rpm.
  • On average, depending upon the diameter of the navel, the rubbing speed of the yarn across the navel up to 1500 m/min.
  • This can lead to severe wear, quickly destroying the texture & making slight but significant modification to the profile of the navel.
  • The rate of wear depends upon the fibre being spun,& is reported that synthetic fibres that are strongly pigmented, e.g., spun dyed polyester fibre , give the fastest wear rate.
  • Doffing tubes are usually made from mild steel, which is easily machined but has not a sufficiently high resistance to this rapid wear action.
  •  By coating the surface of the mild steel doffing tube with a sintered aluminum oxide, the life of  component is more than doubled.
  • The high friction & rubbing of the yarn on the navel can also produce high temperature spots resulting in localized melting of fibres on the yarn surface.
  •  Increasing the wear resistance of the component will not prevent localized melting.
Effect of the rotor parameters on twist insertion :-.
  • It is the compactness of the fibres in the rotor groove that both aids twist insertion, to give a good peripheral twist extent & produces an improved yarn strength.
  •  Results have shown that the degree of fibre 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 fibre compactness in the rotor groove , several patented device have fitted mechanical means, which exert a controlled force on the fibre in the groove.

 



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