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What factors affect bearing friction?

What factors affect bearing friction?

2022-07-19

1. Material properties

The friction coefficient of metal friction pairs varies with the properties of the paired materials. Generally speaking, the same metal or metal friction pair with greater mutual solubility is prone to adhesion, and its friction factor is larger; on the contrary, the friction factor is smaller. Materials of different structures have different friction properties. For example, graphite has a stable layered structure and small bonding force between layers, so it is easy to slide, so the wholesale bearing friction factor is small; for example, the friction pair of diamond pairing is not easy to stick due to its high hardness and small actual contact area, and its friction factor is also high. smaller.


2. Temperature

The influence of the temperature of the surrounding medium on the friction factor is mainly caused by the change in the properties of the surface material. The experiments of Bowden et al. show that the friction factors of many metals (such as molybdenum, tungsten, tungsten, etc.) and their compounds are significantly different in the surrounding medium. The minimum value occurs when the temperature is 700~800℃. This phenomenon occurs because the initial temperature rise reduces the shear strength, and further temperature rise causes the yield point to drop sharply, causing the actual contact area to increase a lot. However, in the case of polymer friction pairs or pressure processing, the friction coefficient will have 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 specific working conditions, material properties, oxide film changes and other factors.


3. Surface properties

Due to pollution, chemical heat treatment, electroplating and lubricants, etc., a very thin surface film (such as oxide film, sulfide film, phosphide film, chloride film, indium film, cadmium film, aluminum film, etc.) is formed on the metal surface. ), so that the surface layer has different properties from the substrate. If the surface film is within a certain thickness, the actual contact area is still sprinkled on the base material instead of the surface film, and the shear strength of the surface film can be lower than that of the base material; on the other hand, it is not easy to occur due to the existence of the surface film. Adhesion, so the friction force and friction factor can be reduced accordingly.


The surface film thickness also has a great influence on the friction factor of custom bearing. If the surface film is too thin, the film is easily crushed and the direct contact of the substrate material occurs; if the surface film is too thick, on the one hand, the actual contact area increases due to the soft film, and on the other hand, the micro-peaks on the two dual surfaces are The furrowing effect on the surface film is also more prominent. It can be seen that the surface film has an optimum thickness worth seeking.


4. Relative movement speed


In general, the sliding speed will cause heating and temperature rise of the surface layer, thereby changing the properties of the surface layer, so the friction factor will change accordingly.


When the relative sliding speed of the paired surfaces of the friction pair exceeds 50m/s, a large amount of frictional heat is generated on the contact surfaces. Due to the short continuous contact time of the contact point, a large amount of frictional heat generated instantaneously cannot diffuse into the interior of the substrate, so the frictional heat is concentrated in the surface layer, making the surface temperature higher and a molten layer appears. The molten metal plays a lubricating role, making friction The factor decreases as the speed increases. For example, when the sliding speed of copper is 135m/s, its friction factor is 0.055; when it is 350m/s, it is reduced to 0.035. However, the friction factor of some materials (such as graphite) is hardly affected by the sliding speed, because the mechanical properties of such materials can be maintained over a wide temperature range.


For boundary friction, in the low speed range where the speed is lower than 0.0035m/s, that is, the transition from static friction to dynamic friction, as the speed increases, the friction coefficient of the adsorption film gradually decreases and tends to a constant value, and the friction coefficient of the reaction film It also gradually increases and tends to a constant value.


5. Surface roughness


In the case of plastic contact, since the influence of the surface roughness on the actual contact area is small, it can be considered that the friction factor is hardly affected by the surface roughness. For a dry friction pair with elastic or elastoplastic contact, when the surface roughness value is small, the mechanical effect is small, and the molecular force is large; and vice versa. It can be seen that the friction factor will have a minimum value with the change of surface roughness.


6. Load


In general, the friction coefficient 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 two dual surfaces are in elastic contact, and 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 custom designed bearings friction factor is 1 of the load. /3 power is inversely proportional; when the load is large, the two dual surfaces are in an elastic-plastic 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 with the increase of the load. tends to be stable; when the load is so large that the two dual surfaces are in plastic contact, the friction factor is basically independent of the load.


The magnitude of the static friction factor is also related to the duration of static contact between the two dual surfaces under load. In general, the longer the static contact duration, the greater the static friction factor. This is due to the action of the load, which causes plastic deformation at the contact point. With the extension of the static contact time, the actual contact area will increase, and the micro-peaks are embedded in each other. caused by deeper.


The effects of the above factors on the friction factor are not isolated, but interrelated.


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