Page 289 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of cotton fibers: importance, research, and limitations 263

1.2
1 Comfort index Strength index
0.8
Quality index 0.6

0.4
0.2 Strength-comfort profile

0
0 1 2 3 4 5 6
Twist multiplier (TM)
Figure 7.17 Twistequality index relationship of ring-spun yarn.

that are only held at one end by the nip of the front roller while the other end is free, (2)
n 2 ¼ fibers that are only held at one end by the twisting point while the other end is
free, and (3) n 3 ¼ fibers that are held by both the nip of the front roller and the twisting
point (i.e., fibers firmly held by the triangle). The third category of fibers are those that
exhibit mean fiber length longer than the height of the spinning triangle. Any fiber
entering the spinning triangle that does not belong to any of the above categories
will typically be too short (fiber fragment) and, if not held by other fibers, will likely
result in fly generation. This description implies that the spinning triangle may be
considered as a battlefield in which two forces are acting against each other; the
spinning tension T y and the fiber strength T f . In a situation where a perfect dynamic
balance is achieved, all fibers in the spinning triangle will equally share the support
of the spinning tension. This situation is likely to occur when all fibers in the triangle
belong to the n 3 group. If the majority of fibers in the triangle belong to the other two
groups, a smaller number of fibers will be supporting the load exerted by the spinning
tension. This situation may result in a spinning failure or end breakage at the spinning
triangle.
In practice, the true impact of the ring spinning dynamic tensile test described above
is reflected on the tensile behavior of the ring-spun yarn produced. This point is best
illustrated by the familiar strengthetwist curve of ring-spun yarn. In general, a yarn
would exhibit a maximum strength at the so-called optimum twist level (determined
by the spinning tension and the number of turns or cycles per minute). This point is
illustrated in Fig. 7.17. Ideally, the ring-spun yarn should be produced at the least
optimum twist level since high twist levels in ring spinning will result in reducing
production rate. In addition, high twist will also result in high yarn stiffness and
high fiber compactness. These two characteristics determine the handle and the
comfort of apparel fabrics. A stiff yarn will produce a stiff fabric, and a highly
compacted yarn (high bulk density) will be stiff and may exhibit low insulation
capability due to low percentage of air pockets in comparison with the fiber volume
in the yarn. Therefore, the twist level in the spun yarn should be large enough to
provide maximum yarn strength, yet as small as possible to provide yarn flexibility