Analysis and Treatment of Mechanical Bearing Failures

1 Rolling Bearing Failure Analysis

1.1 Vibration Analysis of Rolling Bearings

The typical way rolling bearings fail is through pure fatigue spalling of the rolling contact. This spalling, with a spalling surface area of about 2mm² and a depth of 0.2mm to 0.3mm, can be detected by monitoring its vibration. Spalling may occur on the inner race surface, outer race, or rolling elements. Among them, the inner race is more likely to crack due to higher contact stress.

Among various diagnostic techniques for rolling bearings, vibration monitoring remains the most important. Overall, time-domain analysis methods are relatively simple and suitable for situations with low noise interference, making them a good method for simple diagnosis; among frequency-domain diagnostic methods, the resonance demodulation method is the most mature and reliable, suitable for precise diagnosis of bearing failures; the time-frequency analysis method is similar to the resonance demodulation method and can accurately depict the time and frequency characteristics of fault signals, making it more advantageous.

1.2 Analysis of Damage Forms of Rolling Bearings and Remedies

(1) Overload. Severe surface spalling and wear indicate early fatigue failure of the rolling bearing due to overload (additionally, tight fits can also cause some degree of fatigue). Overload can also lead to severe wear of the bearing steel ball raceway, large-area spalling, and sometimes accompanied by overheating. The remedy is to reduce the load on the bearing or increase its load capacity.

(2) Overheating. Changes in color of the roller raceway, steel balls, or retainers indicate that the bearing is overheating. Increased temperature reduces the effectiveness of the lubricant, making it difficult for an oil film to form or completely disappear. Excessive temperature can cause the materials of the raceway and steel balls to anneal, reducing hardness. This is mainly caused by poor heat dissipation or insufficient cooling under heavy load and high speed. The solution is to ensure adequate heat dissipation and additional cooling.

(3) Low Load Erosion. The appearance of elliptical wear marks at each axial position of the steel balls indicates failure caused by excessive external vibration or low load erosion when the bearing is not in operation and no lubricant film is generated. The remedy is to isolate the bearing from vibration or add anti-wear additives to the bearing grease.

(4) Installation Issues. Pay attention to the following aspects:

First, pay attention to the force applied during installation. If there are spaced indentations on the raceway, it indicates that the load has exceeded the elastic limit of the material. This is caused by static overload or severe impact (such as hitting the bearing with a hammer during installation). The correct installation method is to apply force only to the ring that needs to be pressed (do not push the outer ring when installing the inner ring on the shaft).

Second, pay attention to the installation direction of angular contact bearings. Angular contact bearings have an elliptical contact area and only bear axial thrust in one direction. When the bearing is assembled in the opposite direction, the steel balls are at the edge of the raceway, causing groove-shaped wear on the loaded surface. Therefore, it is important to pay attention to the correct installation direction.

Third, pay attention to alignment. If the wear marks on the steel balls are skewed and not parallel to the direction of the raceway, it indicates that the bearing is not aligned during installation. If the skew exceeds 16000, it can easily cause an increase in bearing temperature and severe wear. The causes may include a bent shaft, burrs on the shaft or housing, or the locking nut's pressing surface not being perpendicular to the threaded axis. Therefore, it is important to check the radial runout during installation.

Fourth, pay attention to proper fit. The appearance of circumferential wear or discoloration on the contact surfaces of the inner and outer rings of the bearing is caused by a loose fit with the mating parts. The oxides generated from wear are a pure brown abrasive, which can lead to further wear, heating, noise, and radial runout issues. Therefore, it is important to ensure proper fit during assembly.

For example, if there are severe spherical wear tracks at the bottom of the raceway, it indicates that the bearing clearance has decreased due to a tight fit, leading to increased torque and temperature, causing the bearing to fail quickly due to wear and fatigue. In this case, simply restoring the radial clearance and reducing the interference can solve the problem.

(5) Normal Fatigue Failure. Irregular material spalling occurs on any running surface (such as the raceway or steel balls) and gradually expands, causing increased amplitude, which is a normal fatigue failure. If the lifespan of a standard bearing does not meet usage requirements, it is necessary to select a higher-grade bearing or increase the bearing specification to enhance its load capacity.

(6) Improper Lubrication. All rolling bearings require continuous lubrication with high-quality lubricants to maintain their designed performance. The bearing relies on a layer of oil film formed on the rolling elements and raceways to prevent direct metal-to-metal contact. If lubrication is good, it can reduce friction and prevent wear.

In the operating state, the viscosity of the grease or oil is key to ensuring proper lubrication; at the same time, it is crucial that the lubricant remains clean and free of solid or liquid impurities. If the oil's viscosity is too low, it cannot provide sufficient lubrication, leading to rapid wear of the raceways. Initially, the metal surfaces of the raceways and rolling elements come into direct contact and rub against each other, making their surfaces very smooth, which then leads to dry rubbing, causing the raceway surface to be crushed by particles from the rolling elements. Initially, the surface may darken and lose its luster, eventually forming dents and flaky fatigue spalling. The remedy is to select and replace the lubricant according to the bearing's needs.

When contaminant particles pollute the lubricant, even if these particles are smaller than the average thickness of the oil film, their hardness can still cause wear, even penetrating the oil film, leading to localized stress on the bearing surface, significantly shortening its lifespan. Even a water concentration as low as 0.01% in the lubricant can halve the original lifespan of the bearing. If water dissolves in the oil or grease, the bearing's lifespan will decrease as the water concentration increases. The remedy is to replace the contaminated oil or grease; it is advisable to install better filters, increase sealing, and maintain cleanliness during storage and installation.

(7) Corrosion. The appearance of red or brown spots on the raceway, steel balls, cage, and inner and outer ring surfaces indicates that the bearing has suffered corrosion failure due to exposure to corrosive liquids or gases. This can lead to increased vibration, accelerated wear, increased radial clearance, reduced preload, and in extreme cases, fatigue failure. The remedy is to drain the liquid from the bearing or enhance the overall and external sealing of the bearing.

Causes and Solutions for Bearing Failures in Fans

According to incomplete statistics, the failure rate of abnormal vibrations in fans at cement plants can reach as high as 58.6%, as vibrations can cause an imbalance in fan operation. Among them, improper adjustment of the bearing locking sleeve fit can lead to abnormal temperature rise and vibrations in the bearings.

For example, a certain cement plant replaced the fan blades during equipment maintenance. The blades are fixed in place with locking sleeves and bearing seats on both sides. After the re-test, there was a fault of high temperature in the free end bearing and elevated vibration values.

Upon opening the bearing seat cover and manually rotating the fan slowly, it was found that the bearing rollers at a specific position of the shaft were rolling even in the non-load area, indicating that the bearing operating clearance was excessively variable and the installation clearance might be insufficient. Measurements revealed that the internal clearance of the bearing was only 0.04mm, and the shaft eccentricity reached 0.18mm.

Due to the large span between the left and right bearings, it is difficult to avoid shaft bending or errors in the bearing installation angle. Therefore, large fans use self-aligning spherical roller bearings. However, when the internal clearance of the bearing is insufficient, the rolling elements inside the bearing are restricted by the movement space, affecting their self-aligning function, which in turn increases the vibration values. The internal clearance of the bearing decreases with increased fit tightness, failing to form a lubricating oil film. When the operating clearance of the bearing drops to zero due to temperature rise, if the heat generated by the bearing operation exceeds the dissipated heat, the bearing temperature will rise rapidly. At this point, if not stopped immediately, the bearing will eventually burn out. The reason for the abnormal high temperature of the bearing operation in this case is the overly tight fit between the inner ring of the bearing and the shaft.

During the handling, the locking sleeve was removed, and the fit tightness between the shaft and the inner ring was readjusted, with a post-replacement clearance of 0.10mm. After reinstallation, the fan was restarted, and both the bearing vibration values and operating temperature returned to normal.

Too small internal clearance of the bearing or poor design and manufacturing precision of components are the main reasons for the elevated operating temperature of the bearing. To facilitate the installation, disassembly, and maintenance of fan equipment, it is generally designed to use bearings with locking sleeve fits in the conical holes of the bearing seat. However, issues can easily arise due to negligence in the installation process, especially in adjusting the appropriate clearance. If the internal clearance of the bearing is too small, the operating temperature will rise rapidly; if the fit between the conical hole of the inner ring and the locking sleeve is too loose, the bearing may fail and burn out in a short period due to loosening at the fit surface.

Conclusion

In summary, the failures of bearings should be taken seriously in aspects such as design, maintenance lubrication management, and operational use. This can reduce maintenance costs for mechanical equipment and extend the operating rate and service life of the machinery.