Several Problems in the Test of Friction Coefficient

1. Through the process of some enterprise exchanges, it is learned that the measured coefficient is greater than 1, as provided by the data Cpp sample, A s = 1.189, lLd = 0.591, and the value is greater. For this reason, the author has consulted a large amount of data, but did not find any material with a coefficient of friction greater than 1, repeated experiments with different materials did not show this phenomenon.

It is known from relevant data that the friction that occurs in static friction is called static friction. When the cutting force increases gradually but the two objects remain relatively stationary, the static friction force increases as the shear force increases, but the increase of the static friction force can only reach a certain maximum value. When the size of the outward force is greater than this maximum, the two objects will slide relative to each other. This maximum value of static friction is called "maximum static friction force." This limit friction is expressed in f max. The size of the maximum static friction force is proportional to the positive pressure N between the contact surfaces of the two objects, that is: fmax = lLSoN. The maximum static friction force is represented by fmax, and N is the positive pressure. The proportional constant, LSo, is called static friction coefficient and is a unitless value. μS. It is related to the material of the contact surface, the smoothness and roughness, the hygrothermal condition and other factors, irrespective of the size of the contact surface. From the above description of the situation, static friction coefficient lLS. Should not be greater than 1.

2. The friction that occurs in dynamic friction is called "dynamic friction." When the force exerted on the object is greater than the maximum static friction force, the object begins to move. After moving up, if the applied force is reduced, the object stops moving again. This situation shows that after the object moves, there is also a force that prevents the object from moving, that is, there is frictional resistance. The friction generated when this object is moving is called friction. The ratio of the dynamic friction force to the positive pressure is the "dynamic friction coefficient." In the formula, L is the proportional coefficient, and its value is related to the material and surface roughness of the two objects in contact with each other, and the relative speed of the relative sliding between the two objects. In most cases, the relative friction coefficient is relative to the sliding speed. The increase is slightly smaller. From the definition, the coefficient of dynamic friction must be less than the coefficient of static friction, and it will not be greater than one. The fact that the friction coefficient is greater than 1 in the user's actual test should not be a true reflection of the material. The reason should be found in the aspects of operation, whether the surface property of the material is changed artificially.

If the dynamic friction coefficient is larger than the static friction coefficient during use, the dynamic friction coefficient must be smaller than the static friction coefficient from the above analysis. The result should be an abnormality on the back surface of the test material (such as pinholes, foreign objects, etc.), or the test process. In the external force and so on. This situation can be observed in the friction curve, where there is an abnormal mutation.