Effect on yarn strength
For short fiber yarns, the direct function of twisting is to obtain strength, but it does not mean that the greater the degree of twisting, the greater the strength of the spinning yarn. The reason is that twisting already has factors that are conducive to improving the strength of the spinning yarn. , and there are factors that are not conducive to the strength of spinning yarn. 1. Favorable factors: (1) The twist coefficient increases, the centripetal pressure of the fiber on the yarn uranium increases, the friction resistance between fibers increases, and the possibility of the spinning yarn breaking due to slippage between fibers decreases. (2) Twisting reduces the uneven strength of the spun yarn in the length direction. When the spinning yarn is under the action of tensile external force, the breakage always occurs at the weak point of the spinning yarn. The strength of the spinning yarn is the external force that the weak link can withstand. As the twist coefficient increases, more twists are allocated to the weak loop, which increases the strength at the weak loop more than other places, thereby increasing the strength of the spinning yarn. 2. Unfavorable factors (1) Twisting makes the fibers in the yarn tilt, which reduces the axial component of the fiber, thereby reducing the strength of the spinning yarn. (2) During the twisting process of the spinning yarn, the fibers are prestressed. When the yarn is stressed, the fiber’s ability to withstand external forces is reduced. The influence of twisting on the strength of spinning yarn is the unity of the above favorable factors and unfavorable factors. When the twist coefficient is small, favorable factors play a dominant role, which is manifested in that the strength of the spinning yarn increases with the increase of the twist coefficient. When the twist coefficient reaches a certain value, unfavorable factors play a dominant role, and the strength of the spinning yarn decreases as the twist coefficient increases, as shown in Figure 2-9. The twist coefficient when the yarn strength reaches a maximum value is called the critical twist coefficient (ak in the figure), and the corresponding twist is called the critical twist. In process design, a twist less than the critical twist coefficient is generally adopted to improve the production efficiency of the spinning machine while ensuring the strength of the spinning yarn.
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