According to the ideal Hertz contact state mentioned above, the contact fatigue of the bearing usually occurs at the position of the maximum shear stress on the subsurface. According to actual research, bearing fatigue failure often starts from the contact surface rather than from cracks formed under the surface. This is because the working state of the bearing is not so perfect due to the influence of lubrication conditions.
On the one hand, the rolling element of the bearing is not in a pure rolling state. Due to the different linear speeds at different positions, the influence of load and elastic deformation, the existence of start and stop inertia and acceleration, and the changes in load and speed, the rolling element will slide. Compared with rolling, sliding will increase the shear stress on the contact surface. When the lubricant is not selected properly or the lubricant is contaminated, the lubricating oil film is insufficient or the anti-wear performance is poor, which will cause the contact surface stress to exceed the subsurface stress under high friction conditions (as shown in the figure below), thereby causing surface failure.
On the other hand, when the lubrication is poor or the bearing is running at low speed so that the oil film is not established, and the extreme pressure and anti-wear properties of the lubricant are poor, metal-to-metal contact will occur between the rolling element and the raceway, as shown in the figure below at the microscopic level. The uneven parts of the surface shear against each other, the fatigue caused by stress, and the stress concentration of the microscopic wear marks and grooves will cause cracks to appear and expand, and then the material will peel off. Over time, the contact surface will become more and more rough and pitting will appear. Therefore, it is very important to choose the right type of lubricant to establish a stable lubrication state.
Assuming that there is no problem with the selection of lubricants, if the lubricant is contaminated, such as mixed with dust, iron filings, etc., it will cause scratches and abrasive wear on the contact surface; if it is mixed with water, chemicals or corrosive gases, it will cause the lubricant to deteriorate and reduce its lubrication performance. Therefore, it is also very important to keep the lubrication system clean and sealed, and the lubricant needs to be replaced and replenished regularly according to regulations.
Lubrication is not as clear and easy to calculate accurately as bearing force, and it is also the most easily overlooked part. Most of the time, the actual life of the bearing does not meet expectations and is closely related to poor lubrication conditions. We know that grease is mainly composed of three parts: base oil + thickener + additives; of which base oil accounts for about 80-95%; thickener accounts for about 2%-20%; additives account for about 0-15%. Different base oil quality, the quality and type of thickeners and additives selected (see the table below), and the optimization and adjustment of the formula composition are the differences in the performance of greases from different manufacturers.
Let's take the Mobil SHC™ series of greases with excellent comprehensive performance on the market as an example to see what characteristics the grease should have when facing complex working conditions and harsh conditions. See the table below for details. You can save it and find the corresponding required grease characteristics when the equipment has specific requirements in the future.
Since Mobil SHC™ series greases are made of synthetic base oil, high-quality lithium-complex thickener and selected additives, they have excellent performance in high and low temperature performance, adhesion performance, structural stability performance and extreme pressure and anti-wear performance mentioned in the table above. However, for different applications, you still need to find a specific grease model based on the viscosity and consistency of the lubricant, as shown in the table below. Many times, you may not be very familiar with grease parameters, nor are you very professional. It may be more convenient to directly consult the technical service team of the grease manufacturer, and they can analyze specific problems in a more targeted manner.
Summary
The selection of bearings is not as simple as calculating the life span. From selection to installation and then to application, every link is crucial. When encountering bearing problems, force and lubrication are always the starting points we need to consider. When you calculate and select, if some information is missing or the harsh conditions of the application are underestimated, it is safer to choose big brand bearings and greases with excellent comprehensive performance such as Mobil SHC™ series.
