Twist Is a Double-Edged Sword: Why “A Little More Twist” Does Not Necessarily Mean Stronger

For staple-fiber yarns, twist is a necessary means of imparting strength. An untwisted fiber bundle has almost no practical strength because there is insufficient friction between fibers, and many fibers simply slip when stress is applied. Twisting causes fibers to entangle with one another and generates radial pressure, which increases friction, reduces fiber slippage, and raises yarn strength.

The benefit of twist, however, is not unlimited. As the twist factor continues to increase, the inclination angle of fibers in the yarn becomes larger, and the effective contribution of the axial component of fiber force decreases. At the same time, the pre-tension borne by fibers due to their helical configuration increases, and localized stress concentration may appear before the fibers have fully utilized their own breaking strength. The combined result of favorable and unfavorable factors is that yarn strength reaches a maximum at a certain twist factor and then declines beyond that point. This turning point is the critical twist factor.

The existence of a critical twist factor means that twist design is essentially a search for a balance between binding efficiency and the efficiency of fiber axial contribution. The critical value itself is not a universal constant; it is affected by fiber length, fineness, friction properties, and yarn structure. The larger the angle between the fiber and the yarn axis, the smaller the projection of fiber strength onto the axial direction. The higher the friction coefficient between fibers, the lower the twist required to achieve effective binding. Therefore, discussing an “optimal twist” without reference to specific fiber conditions and yarn specifications is technically invalid.

More importantly, the influence of twist is not limited to strength. Within the normal twist factor range, breaking elongation increases with twist, while luster and hand feel change in the opposite direction: a higher twist factor means a larger fiber inclination angle, lower luster, and a harder hand feel. Twist selection therefore simultaneously affects strength, elongation, and appearance, making it a process decision under multiple constraints.