First, the greater the viscosity of the lubricating oil, the easier it is for the oil film to form. We know that the viscosity of lubricating oils decreases with increasing temperature. In the grinding rolls, the overall temperature is higher due to the influence of external hot air. In general, oil mill lubrication is used in the coal mill. The lubricating oil is not circulated and cooled in use, but only through the shaft to dissipate heat, the heat is concentrated in the grinding roller, and the entire grinding roller bearing and the oil reach a heat balance at a higher temperature. In this case, it is necessary to use a higher viscosity lubricating oil to expect the oil to still maintain viscosity at the working temperature. Another option is to use a lubricant with a relatively high viscosity index, such as certain synthetic oils. Thus, the viscosity of the lubricating oil decreases with increasing temperature, and the lubricating oil ensures viscosity at the operating temperature.
Secondly, to form an effective oil film, there must be a certain relative speed and reasonable pressure. In the grinding roller, the bearing is relatively stressed, which also makes the pressure between the two relatively moving contact surfaces of the roller and the raceway relatively large. If this pressure is too large, it is easy to crush the oil mill. Therefore, from the perspective of bearing use, we recommend the use of lubricants with extreme pressure additives to ensure the lubrication of the bearings under pressure.
Look at the speed again. This is actually another major obstacle to the lubrication of the roller bearings. The selection speed of the grinding roller is relatively low, generally about 35 to 55 revolutions per minute. At such speeds, it is more difficult to form an effective oil film. In addition to increasing the viscosity of the lubricating oil at the working temperature and increasing the extreme pressure additive, from the perspective of the bearing, the lubricating effect can be improved by the following means. Figure 4 is an enlarged view of the contact area between the inner roller and the raceway of the bearing.
Figure 4 Lubrication diagram of roller and raceway surface
We can see that the role of the oil film is to separate the two contact faces, and the effect of the separation depends on the thickness of the oil film and the sum of the surface roughness of the two contact faces. From this we introduce a parameter λ to characterize the lubrication effect.
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