Rolling bearings are one of the most critical components in mechanical equipment, and their performance is directly related to the operating efficiency and service life of the equipment. Most students may know how to select and calculate the life of rolling bearings, but they may not have a deep understanding of the internal mechanism of rolling bearings, so when there are some problems with the bearings, they don't see it so thoroughly. For example, why do bearings from different manufacturers have the same nominal data but the actual life is very different? Why is the actual performance of grease with the same viscosity different on bearings? let's take a look today.
First, think about what functions rolling bearings have? In fact, there are two main functions: supporting the shaft and transmitting the load, and reducing the friction between the running parts. The support and transmission of the load rely on the inner and outer rings + rolling elements, and the cage plays an auxiliary role; reducing friction relies on the rolling method + lubricant, and the seal plays an auxiliary role. The two aspects of force and lubrication are the core of rolling bearings, and they are also the starting point for us to analyze bearing problems.
Understanding rolling bearings from the perspective of force
When rolling bearings support and transmit loads, the inner and outer rings act as direct load-bearing bodies and the rolling elements act as intermediate force conductors. Since the contact area between the two curved surfaces of the rolling elements and the raceways is very small, these small contact areas will generate very high stress when the bearing is subjected to external loads. When the stress exceeds the fatigue limit of the material, contact fatigue failures such as cracks, pits and peeling may occur on the surface of the material under the action of cyclic stress.
The contact between the rolling element and the raceway can be described by the Hertz contact theory. But we need to know that it has some premise assumptions, such as: the contact surface is smooth and frictionless; the contact area is small enough compared to the surface of the object; the materials are isotropic; there is only elastic deformation, etc. Corresponding to our bearings, it means: very good lubrication, very good surface accuracy and finish of the rolling element and raceway, very high material purity, and moderate load without plastic deformation. Only when the contact state is close to the perfect Hertz state can the theoretical prediction of life be more accurate. Of course, due to the complexity of the actual application conditions and the existence of lubrication and pollution, this is difficult to achieve. This is why many correction factors need to be considered when calculating the bearing life.
From the perspective of bearing design and manufacturing, the rolling element and raceway structure design is continuously optimized to improve stress distribution and reduce the influence of stress concentration; better quality bearing steel materials and heat treatment methods are selected to improve the contact fatigue strength limit of the material, and more precise processing is used to improve accuracy. These aspects reflect the level of ability of a bearing manufacturer and the quality of its products (theoretical durability).
From the perspective of bearing application, as a user, you need to know how to choose the right bearing according to the load type. For example, from the contact area mentioned above, we know that ball bearings are more suitable for light loads and high speeds due to their small contact area, while roller bearings are more suitable for low speeds and heavy loads due to their large contact area. Another example is to ensure good bearing seat and shaft accuracy and fit, use reasonable installation tools, etc. to reduce deformation and damage, because this will cause uneven stress distribution or stress concentration. Another example is to avoid overloading and eccentric loading during application. As shown in the figure below, excessive axial load causes shouldering (truncation of the elliptical contact area), which will cause abnormal stress concentration. At this time, a large contact angle bearing should be selected.
