0. Introduction

　　For a vertical mill, the grinding roller is the heart of the mill, and the grinding roller bearing is the heart of the mill. The normal operation of the grinding roller bearing is the key to the reliable operation of the vertical mill. Grinding roller bearings operate in complex environments such as high temperature, impact, heavy load, and low speed. There are also more severe working conditions such as high dust outside the grinding roller. How to correctly use grinding roller bearings and extend their service life in these working conditions has always been a concern for users and designers. Therefore, it is particularly important to analyze the failure modes of bearings during use and how to avoid them.

　　1. Failure modes and causes of grinding roller bearings

　　1.1 Contact fatigue failure

　　The working surface of the bearing is subjected to alternating stress, resulting in failure. It mainly occurs on the working surface of bearings, often accompanied by fatigue cracks. This failure mode first arises from the large alternating shear stress below the contact surface, and then extends to the surface to form different peeling shapes. Its formation process and consequences are: pitting - pitting peeling - shallow peeling - deep peeling.

　　1.2 Wear and tear failure

　　Bearing wear failure refers to the failure caused by the continuous wear of the working surface metal due to the relative sliding friction between surfaces. Wear failure can usually be divided into abrasive wear and adhesive wear according to the form of wear. Abrasive wear refers to the wear caused by the intrusion of foreign hard particles, hard foreign objects, or metal surface debris between the working surfaces of bearings, and the relative sliding of the contact surfaces. Adhesive wear refers to uneven force distribution on the friction surface due to micro protrusions or foreign objects. When lubrication conditions deteriorate severely, local friction generates heat, which can easily cause local deformation and micro welding of the friction surface. In severe cases, the surface metal may melt locally, and the force on the contact surface will tear the local friction welding point from the substrate, resulting in increased plastic deformation.

　　When wear failure occurs, there is continuous wear on the surface of the bearing, which gradually damages the bearing parts and further causes loss of bearing dimensional accuracy and other related problems. Its formation process and consequences are: wear - shape change - larger fitting clearance and changes in working surface morphology - lubricant contamination - complete loss of lubrication function - bearing loss of rotational accuracy and even inability to operate normally.

　　1.3 Fracture Failure

　　This type of failure is mainly caused by two major factors: overload and defects. When the load exceeds the strength limit, overload fracture will occur. Defects such as microcracks, shrinkage cavities, bubbles, large foreign objects, overheated tissue, and local burns in bearing parts can also cause fracture at the defect site during impact overload or severe vibration.

　　1.4 Failure due to changes in bearing clearance

　　This type of failure is often caused by the combined action of internal and external factors. When this failure occurs, the first step is to change the fit clearance, which in turn causes a decrease in bearing accuracy, resulting in the phenomenon of "biting". External factors mainly refer to excessive interference, improper installation, expansion caused by temperature rise, instantaneous overload, etc; The internal factors mainly include residual austenite and residual stress in an unstable state.

　　2. Factors affecting the service life of grinding roller bearings

　　The factors that affect the service life of grinding roller bearings mainly include the condition of the bearings themselves, the use of the bearings, the operation of the mill, the storage of the bearings, and so on.

　　2.1 Bearing itself condition

　　The material, heat treatment process, strength and wear resistance, surface structure, internal geometric structure, etc. of bearings all affect their service life. For users and designers, it is important to understand the working conditions and load status of bearings, and provide relevant parameters and working conditions when selecting bearings. Specific issues with bearings can be considered by the bearing manufacturer.

　　2.2 Bearing usage aspects

　　The use of bearings is an aspect that users can subjectively control and is also a key factor affecting bearing life. From installation to specific operation and maintenance, various factors that affect bearing life are permeated in every link.

　　Installation aspect: The installation quality of grinding roller bearings is an important factor affecting their service life, and many premature failures or damages of grinding roller bearings are related to improper assembly. Users should strictly follow the instructions when replacing the grinding roller bearings. Installing them too loosely or too tightly will cause premature damage to the grinding roller bearings.

　　In the specific installation process, the first thing to avoid is hammering. The installation of bearings requires heating, and attention should be paid to the heating position and temperature control during heating. It is required that the oil be heated together with the bearings in a cold state, and the temperature should be controlled at 100-110 ℃; After installing the bearings, they should be cleaned with kerosene; After the assembly of the entire grinding roller, a pressure test was conducted with a test pressure of 0.05MPa and held for 30 minutes.

　　In terms of usage, it mainly includes sealing, lubrication, and monitoring.

　　Sealing also has a significant impact on the working life of bearings. The function of sealing is to prevent lubricating oil from leaking out of the bearing and to prevent external dust from entering the bearing. In a high dust working environment, if a large number of tiny hard particles enter the interior of the vertical grinding roller bearing, it will deteriorate the working environment of the bearing, reduce the lubrication effect, and even form indentations on the raceway surface, causing pitting corrosion and even peeling of the raceway surface, reducing the working life of the bearing. Therefore, choosing highly reliable sealing components is crucial for improving the working life of bearings.

　　In high temperature, impact, heavy load, and low-speed application environments, it is difficult to form good lubrication conditions for bearings, and the harsh dust environment and impact further deteriorate the lubrication condition. The actual application results show that most of the damage to grinding roller bearings is the result of improper lubrication of the bearings.

　　In terms of monitoring and protection, the sealing pressure and bearing temperature should be monitored in real time, and the control logic and interlocking should not be removed arbitrarily. If abnormal sealing pressure and bearing temperature are found, the grinding should be stopped in a timely manner to prevent damage to the grinding roller bearings.

　　2.3 Grinding Machine Operation Status

　　The vibration of the grinder can affect the service life of the bearings, so reducing vibration, especially horizontal vibration, during the operation of the grinder can provide reliable guarantee for the good operation of the grinding roller bearings. The loading method of MLS vertical mill is centralized loading of the pressure frame. As the grinding disc rotates, a horizontal tangential additional force is generated at the pressure frame. Therefore, for large vertical mills, a buffer device is added at this location, and the effective operation of the buffer device is the key to reducing horizontal vibration.

　　2.4 Storage of bearings

　　Incorrect storage of bearings is also a factor that affects their lifespan. If the bearings are not placed horizontally, it will affect their clearance accuracy; If the packaging is not tight, it will cause corrosion on the working surface of the bearing when exposed to humid air, resulting in small pits and stress concentration points during operation, which will cause failure and affect the service life.

　　3. Conclusion

　　As the core component of the grinding roller, avoiding the failure of the grinding roller bearing and ensuring its service life are the key to the effective operation of the grinding machine. From the selection, installation, use, maintenance and other aspects of grinding roller bearings, there are hidden factors that affect the bearing life. Users must do a good job in each aspect to ensure the normal operation of grinding roller bearings and even the entire production line.