1. surface properties
Due to the effect of pollution, chemical heat treatment, electroplating and lubricant, a thin layer of surface film (such as oxide film, vulcanized film, phosphating film, chlorination film, imprison membrane, cadmium film, aluminum film, etc.) is formed on the metal surface, which makes the surface with different properties from the matrix. If the surface film is in a certain thickness, the actual contact area is still scattered in the matrix material rather than the surface film, and the shear strength of the surface film is lower than the shear strength of the matrix material; on the other hand, the adhesion is not easy to occur because of the presence of the surface film, so the friction force and the friction factor can be reduced.
The thickness of surface film also has a great influence on the friction coefficient. If the surface film is too thin, the membrane is easily crushed and the substrate is exposed to direct contact. If the surface film is too thick, the actual contact area increases because of the soft membrane. On the other hand, the micro peak on the surface of the two pairs is also more prominent on the surface membrane. It can be seen that the surface film has the best thickness to be sought.
2. material properties
The friction coefficient of the metal friction pair varies with the properties of the matched material. Generally speaking, metal friction pairs with the same metal or high solubility are easy to adhere, and the friction factor is large. Materials with different structures have different friction characteristics. For example, graphite has a stable layer structure with small bonding force between layers and easy sliding, so the friction factor is small, and the friction pairs, such as diamond pair, are not easy to bond because of the high hardness and small actual contact area, and the friction factor is also small.
The influence of the ambient temperature on the friction factor is mainly due to the change in the properties of the surface material. The test of Boden and others shows that the friction factor of many metals, such as molybdenum, tungsten, and the compounds, and their compounds, appear the smallest value at the temperature of the surrounding medium at 700~800 C. The emergence of this phenomenon is due to the initial temperature rise to reduce the shear strength, further temperature rise and the sharp decline of the yield point caused a lot of actual contact area increase. However, when the polymer friction pair or pressure is processed, the friction factor 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. The relationship between temperature and friction factor becomes very complicated because of the influence of specific working conditions, material properties and changes of oxide film.
4. relative motion velocity
In general, sliding speed will cause surface heating and temperature rise, thus changing the properties of the surface layer. Therefore, the friction factor will change accordingly.
When the relative sliding speed of the friction pair surface exceeds 50m/s, a large amount of frictional heat is generated on the contact surface. Because the continuous contact time of the contact point is short, the mass of friction heat produced instantaneously is not too late to spread to the interior of the matrix, so the friction heat is concentrated on the surface, which makes the surface temperature higher and the melting layer appear. The molten metal fluid is lubricated and the friction factor decreases with the increase of velocity, such as when the sliding velocity of copper is 135m/s, The friction coefficient is 0.055, while it decreases to 0.035 at 350m/s. However, the friction factor of some materials, such as graphite, is almost unaffected by the sliding velocity, because the mechanical properties of these materials can remain unchanged in a wide range of temperature.
For the boundary friction, in the low velocity range of lower than 0.0035m/s, that is, the friction factor of the membrane gradually decreases with the speed increasing, and the friction factor of the membrane gradually increases and tends to the fixed value.
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