Effect of twist on yarn strength | yarn twist | twisted yarn

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 strength of continuousfilament yarns tends to increase slightly as the twist is increased from zero. The binding effect of twist enables the stronger filaments to support the weaker. This effect is small because there is normally little difference in tensile properties between filaments.

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.