In general, if a yarn is spun or
twisted to a range of twist factors, it is found that the yarn strength increases
with the twist factor up to a certain value of it, beyond which the strength
begins to decrease, slowly at first, then more rapidly. The general shape of
the twist-strength curve for continuous filament and spun yarns is shown in
figure 2 below:
The filaments in a continuous
filament yarn make their maximum contribution to yarn strength when they are
parallel to the axis of the yarns, that is when the twist is zero. Any finite
twist, only part of the tension along the fiber axis contributes to the
strength of the yarn, and so, as the angle of twist increases, the yarn
strength decreases. This is illustrated in figure 3 below:
Assume each fiber in the yarn
cross-section to be subjected to a tension t acting parallel to the yarn axis.
Then the tension acting along each fiber is t/cos q ) which is progressively greater than t as
the angle of twist increases. So the yarn strength must decrease as the angle
of twist increases.
It is impossible to spin a staple
yarn below a certain twist factor. Above this minimum twist factor, which
varies with the length , fineness and frictional properties of the fiber, the
yarn strength increases, fairly rapidly at first. At low twist factors, the
yarn breaks mainly as a result of fiber slippage, which is reduced as the twist
factor increases. If this were the only factor, we should expect the
twist-strength curve to take a form such as that shown in figure 3 A above,
reaching a steady maximum where the twist is sufficient to prevent fiber
slippage.
However, as the twist factor
increases, the effects of fiber obliquity (i.e. the angle of the fibers to the
axis of the yarn, or of twist) increases, and we get a decrease in yarn
strength due to this effect, corresponding approximately to the curve 2 A in figure above or to curve 3 B in figure.
The twist –strength curve in
figure 2 B is the result of the simultaneous operation of the two effects
represented by curves A and B in figure 3.
The twist factor which gives the
maximum strength in any given staple yarn is sometimes called the optimum twist
factor. The lowest practicable and the optimum twist factor both depend on fiber
characteristics such as length, fineness, rigidity and frictional properties. Fiber
slippage will cease at lower twist factors for longer fibers, and maximum
strength will be attained at a lower twist factor. Similarly, increased fiber
cohesion produced by lower rigidity or increased coefficient of friction will
decrease the optimum twist factor. Finer fibers are less rigid than coarser
ones and tend to require less twist.
It is necessary to point out that
the twist factor which gives maximum yarn strength does not necessarily, or
usually, give maximum cloth strength. Again, a twist factor which gives maximum
yarn strength usually produces a rather hard, compact yarn. For these and other
reasons staple yarns are usually spun to twist factors substantially below the
optimum.
The relationship between twist
and yarn strength in folded yarns is much more complex, since we have to
consider the amount and direction of both single and folding twists. In
general, the strength of folded yarns is not sensitive to variations in the
single and folding twist, and the twists are usually chosen with other
attributes than strength in mind.
EFFECT OF TWIST ON EXTENSIBILITY:
Spiraling of the fibers causes a contraction in the
length when yarns are twisted. Some of this tends to be recovered when the yarn
is subjected to tension, and so yarn extensibility tends to increase with the
twist factor. The effect is not large over the range of twist factors normally
encountered. Yarn strength is increased and extension reduced by an increase in
spinning tension, presumably because this produces a more compact and cohesive
yarn. This effect is not large because spinning tensions are limited to a
fairly narrow range. The extensibility of folded yarns is decreased as doubling
tension increases. It is also affected by the method of doubling. Wet doubling
plasticizes the fibers and produces more compact and less extensible yarns.
EFFECT OF TWIST ON LUSTER:
Yarn luster is at a maximum when
the fibers are parallel to the yarn axis. Therefore increasing the twist in a
single yarn reduces its luster. In folded yarns the luster can be substantially
modified by varying the relative amounts of twist inserted in the spinning and
doubling operations. For maximum luster the fibers should be parallel to the
axis of the folded yarn. This is achieved when the ratio of the doubling to the
single twist (known as the D/S ratio) is 0.70 for a yarn in which the single
and doubling twists are opposite in direction and the two singles are the same
counts. This has some importance in the preparation of two-fold cotton yarns
for mercerizing, where maximum luster is the main objective.
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