Mechanical concepts and indicators of friction and cohesion
1. Friction refers to the tangential resistance of two objects in contact with each other when they move against each other under the action of normal pressure (load). If the load is equal to zero, then the friction force is also equal to zero. The relationship between friction force F friction and normal pressure N is often expressed as F friction = eta × N, where eta is the friction coefficient. 2. Cohesion refers to the tangential resistance of fiber materials in relative movement when the normal force of objects in contact with each other is equal to zero. There are many factors that affect fiber cohesion, mainly the geometric shape of the fiber (surface structure, fiber length, fineness, curl), arrangement and contact state, fiber elasticity and surface oil, etc. In addition, temperature and humidity also have a significant impact. Generally, those with more curls or twists, slender and softer fibers will have greater cohesion. The cohesion performance indicators of several fibers are shown in Table 1-6. There are scales on the surface of wool fibers, and the open ends of the scales point toward the hair tips. This makes the tangential resistance coefficient of wool fibers friction when parallel to each other have obvious directionality. The tangential resistance coefficient of friction against the direction of the scales is greater than the tangential resistance coefficient along the direction of the scales, which is called the directional friction effect or friction difference effect of wool. The tangential impedance coefficient of wool in the direction of the scales and in the direction opposite to the scales is related to the initial tension value during testing, wool fiber diameter, etc. The reference data of the dynamic and static tangential impedance coefficients between wool and wool or other fibers along the scale direction and against the scale direction are shown in Table 1-8.
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