21 March 2013

Twist Mechanism

Twist:
Twist is the number of turns about its axis per unit of length of a yarn or other textile strand. Twist is expressed as turns per inch (tpi), turns per meter (tpm), or turns per centimeter (tpcm). It is a very essential process in the production of staple yarn, twine, cord and ropes. Twist is inserted to the staple yarn to hold the constituent fibres together, thus giving enough strength to the yarn, and also producing a continuous length of yarn. The mechanism of twist insertion to the strand during ring spinning has been studied. The twisting of the strand occurs not only due to the rotation of twisting elements, but also due to the winding of yarn on the package. When the yarn is wound on a stationary cop by gripping and winding the yarn by hand, for every coil of yarn wind one turn of twist to the yarn is inserted. Now we will discuss about way of twist insertion to the yarn. 
Twist direction
Twist Insertion to the Yarn When the Spindle is Stationary:  
We assume that the spindle is stationary and the traveller rotates in the ring frame. Each revolution of the traveller winds one coil of yarn onto the cop. This is similar to gripping and winding the yarn on a cop by hand. The yarn will rotate 3600 per coil wind while winding the yarn onto a stationary cop by hand; hence the winding causes yarn twisting.
  • Length of yarn wound per revolution of traveller = πd
  • Turns/cm due to winding = 1/πd
Where d – Winding on diameter of cop or bobbin in cm.

If the yarn is unwound in parallel from the cop, the yarn will retain all the twists present in the yarn, whereas if the yarn is over-end unwound, unwinding a coil removes one turn of twist. The unwinding causes twisting. So, the twists inserted into the yarn during winding are removed during over-end unwinding. The over-end withdrawal may be from any side of the cop. If the traveller rotates in a clockwise direction to wind the yarn onto the cop, each coil of wind inserts one turn of ‘Z’ twist to the yarn. When the same is over-end unwound, every unwinding coil inserts one turn of twist in an ‘S’ direction, and so the resultant yarn will not have any twist.

Twist Insertion into the Yarn when the Traveller is Stationary:  
We assume that the traveller is fixed on a stationary ring and that the spindle is rotating at a constant speed. Every revolution of spindle winds one coil of yarn onto the cop. Here winding does not cause twisting, and hence the yarn in the cop will not have any twist. But if the yarn is over-end unwound, every unwinding of a coil of yarn inserts one turn of twist into the yarn.

  • Turns/cm due to over-end unwinding = 1/πd
The direction of twist insertion during over end unwinding depends on direction of yarn winding. If the spindle rotates in an anticlockwise direction to wind the yarn onto the cop, during over-end unwinding a ‘Z’ twist will be inserted into the yarn. But if the same yarn is unwound in parallel, the yarn will not receive any twist.

Twist Insertion onto the Yarn when both Spindle and Traveller rotate in Opposite Direction:
It may be wondered why it should be necessary to rotate the traveller and spindle in the opposite direction, and also how to rotate the traveller in the opposite direction. This is only to enable the reader to clearly understand the mechanism of twisting. When both the spindle and traveller rotate in the opposite direction, each revolution of the spindle and traveller winds one coil each. The length of yarn wound per min and twist/cm can be calculated.
  • Length of yarn wound per min = π d (NS+NT)
  • Twist/cm due to winding = - NT/ π d (NS+NT) where
  • NS – spindle speed in rpm,
  • NT – traveller speed in rpm.
If the spindle and traveller rotate in clockwise and anticlockwise directions respectively, the direction of twist insertion due to winding would be ‘S’. But during over-end unwinding, the direction of twist insertion would be ‘Z’. + and - signs are used to represent the Z and S twist directions respectively.
  • Twist/cm due to over-end unwinding = (NT/ π d (NS+NT)) + (NS/ π d (NS+NT))
  • Twist/cm in the yarn after over-end withdrawal = (NS/ π d (NS+NT)
Twist Insertion onto the Yarn when the Spindle leads the Traveller:
In ring spinning, both the spindle and traveller rotate in the same direction. However, the spindle rotates at a higher speed than the traveller. If both rotate at the same speed, only the twisting of yarn takes place without winding. Due to the difference in their rotational speeds, the winding of the yarn takes place on the cop.
  • Length of yarn wound on the cop per min = πd (NS –NT)
Due to rotation, both spindle and traveller insert twists onto the yarn. If both the spindle and traveller rotate in a clockwise direction, a ‘Z’ twist is inserted to the yarn.
  • Turns/cm in the yarn = NT/πd (NS –NT)
  • The winding rate should be equal to the delivery rate.
  • Length of yarn delivered (cm/min) = πd (NS –NT)
Here winding takes place in similar conditions to when the traveller is stationary and the spindle is rotating; hence winding does not insert any twist onto the yarn. On the other hand, during over-end unwinding one turn of twist is inserted for every unwound [[*]] of coil.
  • Turns/cm for unwinding = 1/πd
  • Total twist present in the yarn after over-end unwound = NT/πd(NS –NT) + 1/πd = NS/πd(NS-NT)
Since yarn from the ring cop is normally over-end withdrawn during the winding process, the spindle speed is taken for calculating the turns/cm in the yarn instead of using traveller speed. However, turns/cm in the roving is calculated by taking the flyer speed into account. This is due to the parallel withdrawal of roving during spinning.

Twist Insertion onto the Strand when Flyer leads Bobbin:
Due to the difference in the speeds of the flyer and the bobbin, the winding of roving takes place on the bobbin.
  • Twist/cm due to twisting = NB / πd(NF-NB)
  • Twist/cm due to winding = (NF-NB)/ πd(NF-NB)
  • Twist/cm in the roving = NF / πd(NF-NB) where
  • NF - flyer speed in rpm,
  • NB - bobbin speed in rpm.
If the roving is unwound in parallel, the roving will have the same amount of twist as in the bobbin, but if it is over-end withdrawn, it will lose a certain amount of twist during unwinding.
  • Turns/cm due to over-end withdrawal = - (NF-NB)/ πd(NF-NB)
  • Turns/cm in the roving after over-end withdrawal = NB/πd (NF-NB) 
  http://textilelearner.blogspot.com/2012/05/mechanism-of-twist-insertion-to.html

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