Page 288 - Handbook of Properties of Textile and Technical Fibres

262 Handbook of Properties of Textile and Technical Fibres

transformed into heat energy. The frictional heat may be limited by the so-called
thermal load capacity of the ringetraveler system (Elmogahzy and Chewning, 2001;
Lord, 2003; Stalder, 1991); a traveler burns when the ringetraveler system reaches
its limit of thermal load capacity.
The forces applied on ringetraveler system described above are ultimately
transmitted to the fibers exiting the drafting system of ring spinning via the so-
called spinning tension (see Fig. 7.16). This is displayed in the so-called “spinning
triangle,” which is a unique and fascinating zone in which fibers are subjected to
continuous dynamic tensile test immediately before they are being twisted into yarn.
Spinning tension is defined as the tensile force applied on the yarn at the onset of
twisting: that is the yarn tension at the point where the fibers in the spinning triangle
are being twisted. The critical importance of this parameter lies in the fact that it con-
tributes largely to both the quality of ring-spun yarn and the spinning performance.
Spinning tension results in a closed packing of fibers during twisting, which enhances
the yarn strength. Variation in spinning tension directly results in variation in yarn
strength. Excessive tension or tension peaks may result in an end breakage during
spinning. In fact, more than 80% of end breakage during ring spinning is believed
to result from tension peaks at the spinning triangle. The relationship between traveler
speed and spinning tension is expressed as by the following equation (Elmogahzy and
m ring=traveler
Chewning, 2001; Lord, 2003): T y ¼ m t V t 2 d ring , where a is the angle
sin a
between yarn from traveler to bobbin and a straight horizontal line from traveler to
spindle axis. This equation clearly indicates that the increase in traveler speed will
result in an increase in the centrifugal force, and consequently, an increase in the
spinning tension. In practice, an increase in spinning tension above a certain critical
limit will immediately result in fiber end breakage.
After the above brief introduction on the spinning tension during ring spinning, a
logical question should be how cotton fibers react to this dynamic tension, which
can happen at a frequency ranging from 10,000 to 25,000 cycles per minute. Well,
it all happens in the spinning triangle where different fibers are subjected to different
levels of tension depending on their position in the triangle. Fibers in the center of the
triangle are usually slack, and those in the outer layers are under maximum tension.
When fibers are released from the nip of the front roller, those exhibiting high tension
tend to move toward the center displacing the initially central fibers to the outer layers.
This phenomenon is called “fiber migration,” and its main effect is to enhance fiber
cross-linking, and consequently to improve yarn strength. The dimensions of the
spinning triangle (width and height) largely contribute to how fibers react to the
dynamic tensile forces. Indeed, the width of the triangle can be simulated by the width
of a fiber bundle in a tensile test nipped by the front roller, and the height represents the
length from the nip of the front roller to the twisting point as shown in Fig. 7.16.
Normally, the width of the triangle is a function of the amount of draft or the total
number of fibers delivered, and the pressure on the front roll. The height of the triangle,
on the other hand, is directly determined by the spinning tension.
Variations in fiber length and fiber displacement in the draft zone typically result in
three different fiber arrangements in the spinning triangle (Fig. 7.16): (1) n 1 ¼ fibers