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Twist effect on strength | angle of twist and twist factor



Twist has effect on the following properties:-
1)      Bending behavior.
2)      Resistance to creasing.
3)      Drape ability.
4)      Strength
5)      Resistance to abrasion.
6)      Elasticity.
7)      Impact Strength.
8)      Stress distribution and analysis.
More twist cause hard yarn production. Excessive
twist causes breakage of yarn. With the increasing twist yarn strength also increases up to a certain limit. After that the strength of yarn decreases with the further increasing of twist.

STRENGTH VS TWIST CURVE:
With the increase of twist, stretch on fiber increases. Strength vs twist curve can be
 divided into 3 sections.
1)      SECTION a. It is the low twist region. In this region yarn strength increases with the increase of twist.
2)      SECTION b. It is the medium twist region. In this region the yarn strength increases with some fiber breakages.
3)      SECTION c. It is the high twist region. In this region the strength does not increase with the increase of twist. In this region the fiber breakage is high.

EFFECT OF TWIST ON YARN STRENGTH:
1)      Twist increases the strength of staple yarn up to an optimum limit.
2)      Finer the yarn higher the twist per inch (TPI).
3)      Higher TPI lower the production.
4)      Higher the TPI higher the cost of production.
5)      Higher the TPI, the more the spirality and thus the obliquity is higher.


ANGLE OF TWIST AND TWIST FACTOR
Although the amount of twist in a yarn is usually expressed in turns per inch (t.p.i.), this is useless for comparing yarns of different counts. What matters is not the number of turns but the angle of twist. This is the angle which fibers on the surface of the yarn make with the axis of the yarn, 0 in
 In this diagram, yarns A and B have the same angle of twist, but yarn B is twice as thick and has half as many t.p.i. as yarn A. Their relative degree, or hardness, of twist cannot be compared in terms of their t.p.i., but it can by comparing their angle of twist. Since this is the same, their properties will also be similar.
If t = t.p.i., then 1/t is the length occupied by one complete turn, or spiral, of twist. Now imagine the surface of yarn B to have been peeled off, in figure C. The figure shown in figure B will now form the diagonal of a rectangle of length l / t and width     Ï€ d. So
tan Ó¨  =  Ï€ d  ¸ l / t  =  Ï€ dt.

The constant k is called the twist factor or the twist multiplier.

We can now use the twist factor either to calculate the t.p.i. required to give the desired degree of twist in any counts, or to compare the degree of twist in yarns of different counts. It is to noted that different counting systems will give different twist factors for the same angle of twist, and therefore to be compared must their counts expressed in the same system

EXAMPLE:- Compare the degree of twist in the following yarns:
a)      36s cotton, 27 t.p.i., and b) 16s cotton, 16 t.p.i.

The two yarns have the same degree of folding twist.
Twist factor is not often used in connection with continuous filament yarns, but with the introduction of the tex system it will be necessary to recalculate twist factors for spun yarns whose yarn number is expressed in the tex system.
Table below gives the angles of twist, based on the a specific volume of 1.1, corresponding to the normal range of twist  factors for single cotton yarns. It also shows the corresponding tex twist factors, derived as above.
                        TWIST EFFECT ON STRENGTH Table
              Angle of twist     Cotton twist factor     Tex twist factors
                                                                              t.p.i.      t.p.m.
                18 36                      3.0                           72.9     2,870
                24 10                      4.0                           97.2     3,827
                28 18                      5.0                          121.5    4,784
 


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