关键词: 纤维变速点, 循环迭代法, 仿真, 牵伸, 压力分布, 纤维运动模型

Abstract:

Objective The dispersion of fiber accelerated points during drafting is one of the main causes for yarn unevenness. Most of the previous studies are about accelerated point distributions of all fibers in a sliver, and there is a lack of research on the motion of single fibers. Therefore, based on the force of a single fiber in the drafting zone, a fiber motion model is established in this research, and the theoretical accelerated points of fibers under different pressure distributions are discussed, which may provide theoretical basis for process design in actual production.
Method Ignoring the difference of the pressure distribution in the transverse direction of a sliver, a drafting model, using software MatLab, was established according to the controlling force and guiding force of a single fiber in the drafting zone, and the position where the fiber is accelerated was determined. In the simulation, it is assumed that all fibers are accelerated at the front roller nip, and then the accelerated points of each fiber is calculated repeatedly by the iterative method, until the difference from the result of the last loop is less than a set error value.
Results The maximum absolute values of errors between the calculated results and the verification results in the opposite direction were all much smaller than the error value, which validates the model. The accelerated point of a fiber was related to its length(Fig. 7). The longer the fiber, the closer the fiber accelerated point is to the front roller nip. When the additional pressure was added close to the back roller, the accelerated point of the fiber with a length of less than 27 mm would move slightly backward compared to the case without the additional pressure. For a fiber longer than 27 mm, the accelerated point of the fiber moved rapidly towards the front roller nip as the length increases. As the position for adding additional pressure was moved forward, the accelerated points for shorter fibers also got closer to the front roller. When the additional pressure was added closer to the front roller, the theoretical accelerated points for fibers longer than 14 mm were all 0.5 mm away from the front roller. In the case of no additional pressure, although the average position of the fiber accelerated points was the farthest from the front roller, the coefficient of variation of the overall fiber accelerated points was the smallest at only 0.431%(Fig. 8). The front additional pressure was found beneficial for the fibers closer to the front roller, but due to the large difference in the accelerated points of short fibers and long fibers, the CV value was greater than that without the additional pressure. When fibers of slivers were longer with better uniformity, increasing the front additional pressure was revealed to be more conducive to the fiber accelerated points closer to the front roller and more concentrated. However, at this time the fiber dispersion was required to be higher to avoid defects such as "thick end" or breakage. With additional pressure added at the middle positions, fibers smaller than 27 mm have larger accelerated point changing rate with the increase of fiber length, and the dispersion of all fiber accelerated points was the largest compared with no additional and back additional pressure, although the average accelerated point is closer to the front roller.
Conclusion The position of the additional pressure in the drafting zone affects fiber accelerated points. Compared with increasing the additional pressure, the stable and gentle pressure distribution in the drafting zone is more conducive to the concentration of accelerated points, but the fiber accelerated points are farther from the front roller, which is more unstable in actual production. As the position of the additional pressure moves forward, the accelerated points of shorter fibers also move forward. Therefore, under the front additional pressure, the accelerated points of fibers are more concentrated and closer to the front roller, which is beneficial to reduce the unevenness of the sliver after drafted. The model established in this research could be used to predict sliver evenness after drafting, and to guide the adjustment of drafting parameters and optimization of the drafting mechanism in actual production. Due to the complexity of actual drafting process, this paper does not consider the difference of the pressure distribution in the transverse direction of a sliver and the cohesion between fibers. Therefore, there are certain gaps between the theoretical and the actual production results, and further research is needed in the future to fill the gaps.

Key words: fiber accelerated point, cyclic iterative method, simulation, drafting, pressure distribution, fiber motion model