Failure causes and improvement measures of bearing inner race raceway
Abstract: Failure bearing inner raceway system were analyzed, and made a series of appropriate treatment, improvements in order to provide some useful reference for relevant personnel.
Key words: bearings; inner ring raceway; roller; protrusion
Bearing is a component that plays a role in fixing and reducing load friction coefficient in the process of mechanical transmission. However, in the practical application of bearing, the inner race of bearing will fail, which seriously affects the normal operation of machinery. Therefore, it is necessary to carefully analyze the failure causes of the inner race of bearing and take effective measures to deal with them.
1. Failure bearing organization analysis
1.1 Macroscopic observation and metallographic analysis
Fig. 1 is a photo of the bearing inner race. Through macroscopic observation, it is found that the surface of the inner race is covered with various peeling spots, and the spots are scattered and disordered. The spalling phenomenon is most serious at about 4/5 of the contact area width from the surface, and the spots at the spalling place have obvious fatigue arc characteristics.
Cut along the line where the raceway peels off, and make the inner race raceway into a metallographic sample. According to the requirements of GB/T 11354-2005 "examination of microstructure of rolling bearing steels", the raceway of bearing inner race is etched with nitric acid alcohol solution with a mass fraction of 4%. After that, it can be found that the microstructure of its matrix is cryptoacicular tempered martensite and carbide particles, and the carbide particles tend to develop into mesh; There are cracks at the bottom of the spalling pit, and the cracks tend to expand to the side matrix. The microstructure of the spalling part of the bearing inner ring is shown in Figure 2.
Fig. 1 photo of bearing inner race
Fig. 2 microstructure of bearing inner ring spalling
1.2 SEM analysis
After scanning electron microscope (SEM) of the peeling pit, it is found that there are fatigue striations at the bottom of the peeling pit. Due to the high hardness of GCr15 steel, the fatigue striations are fine and discontinuous, and there are also scratches at the bottom of the pit due to friction, as shown in Figure 3. The SEM image of spalling pit is shown in Figure 4. It can be seen from Figure 4 that there are obvious contact fatigue spalling pits on the surface of the bearing raceway; In addition to the scratch and fatigue striation at the bottom of the pit, there are dimples at the joint of the surface of the peeling pit, indicating that the fatigue source is on the sub surface of the raceway; The fatigue crack propagates slowly from the sub surface layer to the surface, and finally the spalling pit is formed by tearing.
Fig. 3 scratch marks of peeling pit
Fig. 4 SEM image of spalling pit
1.3 Chemical composition and hardness analysis
Take samples from the inner race of the failed bearing and conduct chemical analysis. The analysis results are as follows: w (C) ≈ 1.05%, w (Si) ≈ 0.4%, w (Mn) ≈ 0.25%, w (Cr) ≈ 1.55% According to the relevant standards of GB/T 8254-2002 bearing steel, the inner race raceway composition meets the requirements of GCr15 steel.
According to GB/T 230-2004 Metallic Rockwell hardness test method, the Rockwell hardness test was carried out on the inner ring raceway surface, and the average hardness value measured was 62 ~ 63 HRC, meeting the process requirements.
1.4 Analysis of outline dimension of failed bearing
Use the profiler to measure the dimensions, protrusions and mutual differences of the inner and outer race raceways and roller profiles that fail in the same batch. The measurement shows that all indicators of the outer race raceway are normal values; The size and protrusion of most inner race raceways are unqualified; The protrusion of bearing rollers is also unqualified, and the protrusion of most rollers is 2 ~ 3 μ m (the process requires a protrusion of 3 ~ 5 μ m); The diameter difference of rollers greatly exceeds the standard value.
2. Failure cause analysis
According to the above macroscopic observation, metallographic analysis and SEM analysis, the inner race raceway spalling belongs to typical contact fatigue failure. There are two main reasons for contact fatigue failure: ① stress concentration caused by the increase of contact stress in inner race raceway. Different degree of protrusion of inner race raceway, small roller protrusion and large difference of roller diameter will increase the contact stress and form stress concentration. In order to improve the load distribution of the roller, the roller is usually processed into a strip shape to reduce the stress level of the roller and prevent contact fatigue damage in advance. However, the inner race raceway size and protrusion mentioned above are unqualified. The protrusion of most rollers is too small, and the diameter difference between them greatly exceeds the standard, which increases the contact stress and leads to stress concentration, then produces fatigue source, and finally develops into contact fatigue failure. The surface spalling of the inner race raceway is the most serious at about 4/5 of the contact area width from the surface, indicating that the contact stress is the largest and the stress concentration is the most serious. ② Improper heat treatment process and poor microstructure of inner ring raceway. In the microstructure of the inner ring raceway, there are carbide particles with network development. The network carbide particles will destroy the uniformity of the structure and make the mechanical properties of the material worse, especially the fatigue properties of the material. Therefore, the increase of contact stress and stress concentration in the ring raceway above are the main factors inducing contact fatigue failure, while the poor metallographic microstructure in the inner ring raceway, especially the network carbide particles, will accelerate the contact fatigue spalling.
3. Improvement measures
Through the analysis of the above failure causes, it can be seen that reducing the contact stress between the inner race raceway and the roller, eliminating the network carbide particles, and further optimizing the metallographic microstructure of the inner race raceway are the fundamental methods to prevent the contact fatigue damage of the inner race raceway.
The surface quality of inner race and roller has a great influence on the contact stress between inner race and roller. At present, in the process of bearing processing, the quality inspection mainly focuses on the appearance, clearance, rotation flexibility, noise, nondestructive testing and residual magnetic measurement. Due to the special bearing products, there are still many problems in these inspection and treatment methods. It is necessary to add new inspection methods and means to ensure the quality of bearing inner and outer race raceways and roller surfaces. Profilometer is a reliable tool to measure the surface quality of bearing, but its measuring efficiency is low, so it can only be used in the detection of precision bearing. Since vibration testing has been more and more widely used and developed, precise and advanced measuring equipment can be used to ensure the contour size, protrusion and surface quality of bearing inner and outer race raceways and rollers, so as to avoid early contact fatigue damage.
The reticular carbide particles in the microstructure of inner ring raceway are mostly secondary carbides. Although increasing the quenching temperature can reduce the reticular structure of carbides, it will greatly increase the risk of quenching cracking. Because the secondary network carbide already exists before quenching, it is a feasible method to eliminate most of the network structure in the microstructure before quenching. The isothermal annealing treatment before the quenching of the bearing inner race raceway can eliminate some network structures, but its effect is very limited. Therefore, the isothermal annealing treatment can be changed to spheroidizing annealing treatment. There are two benefits of spheroidizing annealing: ① spheroidizing annealing itself can improve fatigue properties. ② After spheroidization, the microstructure is globular pearlite and a small amount of flake pearlite, and there is little reticular grain structure; In addition, the quenching heating time can be appropriately extended during subsequent quenching to further melt the secondary network carbide that has not been melted after spheroidizing annealing and completely remove the network structure. After the spheroidizing annealing process, it was found that no net structure was found in the metallographic microstructure after quenching, which fundamentally ensured the microstructure of the inner race raceway.
4. Concluding remarks
To sum up, the bearing plays a key role in mechanical rotation. However, due to the failure of its inner race in practical application, in order to ensure the normal operation of the machine and prevent the failure of the bearing inner race, it is necessary to carefully analyze the failure causes and take corresponding measures to ensure the normal use of the inner race.
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