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Various factors affecting bearing friction factor

Due to the effects of pollution, chemical heat treatment, electroplating and smoothing agents, a very thin film is formed on the surface of the metal, such as oxide film, sulfide film, phosphate film, chloride film, indium film, cadmium film, aluminum film Etc., so that the surface layer has different properties from the substrate. If the outer surface film is within a certain thickness, the actual contact area is still spread on the base material instead of the outer surface film, and at the same time, the shear strength of the outer surface film can be lower than that of the base material; on the other hand, it is not prone to attack due to the existence of the outer surface film Adhesion, so the friction and friction factor can be reduced accordingly.

The thickness of the outer film also has a great influence on the friction factor. If the outer surface film is too thin, the film is easily crushed and presents direct contact with the matrix material; if the outer surface film is too thick, on the one hand, the actual contact area increases due to the softness of the film, and on the other hand, the micro peaks on the two pairs of surfaces The furrow effect on the outer membrane is also more prominent. It can be seen that the outer surface film has a better thickness that is worth seeking.

The friction factor of the metal friction pair varies with the nature of the mating material. Generally speaking, the same metal or metal friction pairs with greater mutual solubility are prone to adhesion, and the friction factor is relatively large; on the contrary, the friction factor is relatively small. Materials of different structures have different friction characteristics. For example, because graphite has a stable layered structure and the separation force between layers is small, it is easy to slide, so the friction factor is small; another example is that the friction pair of diamond pairing is not prone to adhesion due to high hardness and small practical contact area, and its friction factor is also Smaller.

The influence of the temperature of the surrounding medium on the friction factor is mainly caused by the changes in the properties of the surface material. The experiment of Bowden et al. showed that the friction coefficient of many metals (such as molybdenum, tungsten, and qin, etc.) and their compounds is in the surrounding medium. It shows a small value when the temperature is 700~800℃. This phenomenon appears because the initial temperature rise causes the shear strength to drop, and the further temperature rise causes the yield point to drop sharply, causing the practical contact area to increase a lot. However, during high polymer friction pair or pressure processing, the friction factor will show a maximum value with the change of temperature.

It can be seen from the above that the influence of temperature on the friction factor is changeable, and the relationship between temperature and friction factor becomes very complicated due to the influence of detailed working conditions, material characteristics, oxide film changes and other factors.

Relative movement speed. Under normal conditions, the sliding speed will cause surface heating and temperature rise, thereby changing the properties of the surface, so the friction factor will change accordingly. When the relative sliding speed of the friction pair exceeds 50m/s, it will touch the surface. Generate a lot of frictional heat. Due to the short continuous contact time of the contact point, a large amount of frictional heat generated in an instant is too late to diffuse into the interior of the substrate, so the frictional heat is concentrated on the surface layer, making the surface temperature higher and presenting a condensed layer. The condensed molten metal plays a smooth role and makes friction The factor decreases with the increase of speed. For example, when the sliding speed of copper is 135m/s, the friction factor is 0.055; when it is 350m/s, it drops to 0.035. However, the friction factor of some materials (such as graphite) is almost unaffected by the sliding speed. The reason is that the mechanical properties of such materials can be maintained in a wide temperature range.

Regarding border friction, when the speed is lower than 0.0035m/s, that is, in the low speed range from static friction to dynamic friction, as the speed increases, the friction factor of the adsorption film gradually decreases and tends to a fixed value, which reflects the friction factor of the film It also gradually increases and tends to be a fixed value.

Under normal conditions, the friction factor of the metal friction pair decreases with the increase of the load, and then tends to be stable. This phenomenon can be explained by the adhesion theory. When the load is very small, the surfaces of the two pairs are in elastic contact. At this time, the actual contact area is proportional to the 2/3 power of the load. According to the adhesion theory, the friction force is proportional to the actual contact area, so the friction factor is 1 of the load. /3 power is inversely proportional; when the load is large, the two pairs are in an elastoplastic contact state, and the actual contact area is proportional to the 2/3 to 1 power of the load, so the friction factor decreases slowly as the load increases. Tend to be stable; when the load is so large that the surfaces of the two pairs are in plastic contact, the friction factor has nothing to do with the load.

The size of the static friction factor is also related to the duration of static contact between the two pairs of surfaces under load. Under normal conditions, the longer the static contact duration, the greater the static friction factor. This is because the load causes plastic deformation in the contact area. With the extension of the static contact time, the actual contact area will increase, and the micro peaks will be embedded in each other. Deepen and cause it.

In the case of plastic contact, since the surface roughness has little effect on the actual contact area, the friction factor can be almost not affected by the surface roughness. Regarding the dry friction pair with elastic or elastoplastic contact, when the surface roughness value is very small, the mechanical action is smaller, and the molecular force is larger; and vice versa. It can be seen that the friction factor will have a minimum value with the change of the surface roughness.

The effects of the above various factors on the friction factor are not isolated, but interact with each other.


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