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Twist In Yarn

  • Accordingly , in rotor spinning , it is not possible under given conditions to lower the yarn twist coefficient below a certain value {=a min.}  Because otherwise the length of the binding-in zone will be reduced to zero.
  •  The yarn twist momentum will then be negligible & transmission of twist to the fibres in the ring will not be assured. Therefore minimum value of twist coefficient {A min.} Is independent of yarn strength.
  • Twist / metre=rpm of yarn separation point / withdrawl speed {l}.
  • Now since the lead of yarn sparation point over rotor rpm is very small we can safely write.
  • Twist / metre = rotor rpm /l                                                                                         where l is expressed in terms of m/min
     
  • The spun yarn is withdrawn through a passage in the navel t;
  •  The yarn therefore rolls continually on the trumpet shaped mouthpiece of this nozzle.
  • The partial rolling of the yarn gives rise to false twist between the twisting-in point {g} for the fibres & the navel {t}.
  •  The yarn in the spinning section {b} therefore exhibits more turns of twist than the spun yarn.
  • Moreover , the twist level increases continuously from the navel towards the rotor wall.
  • Twist level at separation point is about 20-40% higher than those at navel.
  •  This difference arises from variations in tension along the yarn.
  • Yarn tension is generated by the withdrawl rolls during withdrawl in opposition to the centrifugal force in the rotor.
  • Tension is highest at the withdrawl rolls themselves & declines towards rotor wall.
  • However , the yarn tension & twist level are inversely proportional , that is, if there are sections of low tension in the yarn {b}, these will exhibit more twist whereas sections of high tensions {a} take up less twist.
  •  It is only these additional turns at the separation point, caused by false twist & yarn tension variations, that enable spinning to be done under stable conditions.
  • The false twist effect is dependent upon roughness of the contact surface of the navel & rotor rpm.

 



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