In recent years, more and more attention has been paid to the failure of bearing friction and wear caused by solid impurities. A large number of experimental results show that solid impurities (including those inappropriately treated solid additives) existing in the lubricating medium will not only affect the vibration and noise performance of the bearing, but also aggravate the wear of the bearing and cause the bearing to have an early stage. failure and destruction.
Although some experiments have proved that the influence mechanism of solid abrasive particles on bearing surface damage is mainly related to the change of surface contact performance, and the relationship between the long-term existence of particles is not obvious, but before the 1980s, the mechanism of the relationship between solid abrasive particles and bearing wear was studied. All focus on the study of mechanical wear. Little is known about the performance changes of bearing metal surfaces caused by solid abrasive particles, especially some tribochemical effects. In fact, because the particle size of some abrasive particles is in the order of several microns, their chemical activity and surface energy are large, so under a certain temperature and pressure, this effect is very large, and it is in the entire friction and wear process. The role cannot be underestimated.
In order to further study the failure mechanism and behavior of sealed bearings in the presence of solid abrasive particles, we studied the performance of grease lubricated bearings under low load and high temperature and low temperature conditions by comparing the bench test of sealed bearings with various abrasive particles. failure process. Based on the analysis of the parameters such as the leakage rate of grease, the loss of antioxidants, the total acid value and the wear rate before and after the sealed bearing test, combined with the research and comparison of infrared, SEM and surface roughness, it is found that the solid grinding under high temperature The tribochemical effect produced by the particles is quite significant, and it is confirmed that the life and failure process of grease-lubricated sealed bearings at high temperature mainly depend on the lubrication state of the bearings.
1. Test conditions and procedures
The life test of the sealed bearing is carried out using the bench test machine in the laboratory. The temperature of the sealing box of the testing machine is 125C, the test bearing model is 6200, the spindle speed is 3()00r/min, the radial and axial loads are 9.SN and 29.4N respectively, and the amount of grease added is 0.359. The grease used in the test is a seal Bearings commonly used mineral oil based grease lAiainaZ “.
The particle size and distribution of solid impurities are shown in Table 1. The amount of solid impurities added in the test is 2% of the total amount of grease, and the test results are compared with the results of pure grease without solid impurities under the same test conditions.
Figure 1 shows the experimental analysis procedure. Bearings after different running time tests are taken out one by one, and then anatomical analysis is carried out according to the steps shown in Figure 1, until the end of the life of the sealed bearing
A large number of analysis results show that solid impurity particles have a great influence on the wear and lubrication life of sealed bearings. Table 2 is the comparison result of the influence of four kinds of impurities on the life of sealed bearings. Among them, iron and standard dust can reduce the average life of sealed bearings by about 2, while 2% copper and molybdenum disulfide particles can increase the life of sealed bearings by about 2%. 6.7% and 44%.
Figures 2 to 5 show the change of various deterioration parameters with time during the failure process of the sealed bearing grease at high temperature, which reveals the nature of the failure process of the sealed bearing and the change of the lubrication life. It can be clearly seen from Figure 3 that after the bearing runs for 20 hours at high temperature, the antioxidant of its grease has basically disappeared, and then the grease oxidation intensifies, and the wear and leakage of the bearing increase until the final lubrication state is destroyed, resulting in Seizure and seal bearing failure.
Obviously, although the four kinds of solid impurities have different effects on the chemical deterioration, physical deterioration and wear deterioration of bearing grease at high temperature, their effects are all significant.
2. The role of iron
The 2% iron particles lead to occlusal failure of the sealed bearing at about 250h. The change in the grease leakage rate in Figure 5 shows that iron has no significant effect on the grease leakage of the sealed bearing before occlusion. The results in Figure 3 show that the presence of iron makes the antioxidant additives in the lubricant disappear 10 h after the test, and then the acid value increases rapidly. Infrared analysis also clearly proves that the absorption peak (1700-‘-1720-’em) of the oxidation product increases significantly around 20h. It can be seen that iron has a significant effect on the chemical deterioration of lubricants.
In addition, from the comparative analysis of the results in Figure 4, the content of iron in the lubricant increases rapidly after 70h, the wear intensifies, and the surface roughness of the bearing also increases accordingly (see Table 3), R. The value reaches .012 lmm, which is nearly 5 times higher than the original value, which indicates that the presence of iron at high temperature can not only accelerate the chemical deterioration of the sealed bearing lubricant, but also seriously damage the bearing surface and aggravate its wear deterioration.
Further studies have shown that the mechanism of particulate iron reducing the lubrication life of sealed bearings is that it acts as a catalyst to accelerate the oxidation of the base oil and the decomposition of additives in the lubricant at high temperatures, and at the same time, accompanied by abrasive wear, the result of early seizure failure of the bearing. .
3. The role of dust
Figure 5 shows that the dust dominated by non-metallic oxides has little effect on the grease leakage rate (physical deterioration) of the sealed bearing, and the effect on its chemical deterioration is not as significant as that of iron (Figure 2, Figure 3). Although the triple occlusion was found to fail after 340 hours of test operation, infrared analysis showed that the oxidation products appeared in large quantities after 30 hours, and the total acid value of the lipid also increased rapidly at this time.
Table 3 shows that after the bearing runs for 154h, the vane value can reach 0.093 mm, which is close to the level when it is engaged. The changing trend of iron content (Fig. 4) shows that its dust has a greater influence on the deterioration of the wear resistance of sealed bearings. This shows that in the early stage of the bearing test, the main impact of non-metallic dust on the bearing is abrasive wear. In order to reflect the chemical deterioration of the lubricant, the final bearing fails under the combined action of abrasive wear and further chemical deterioration caused by dust.
4. The role of molybdenum disulfide
Aluminum disulfide, as an extreme pressure exciter, has strong resistance to rabbit load. The results in Figure 3 show that the antioxidants disappeared completely after 170h, which indicated that it also had the effect of inhibiting the decomposition of a-phenyl zeamin at low load and high temperature. In addition, it also inhibited the oxidation process of oil after the antioxidant disappeared (Fig. 2). Infrared analysis showed that only after 60h of test at high temperature, the sealed bearing grease with aluminum disulfide showed obvious oxidation products. Therefore, molybdenum disulfide can obviously inhibit the chemical deterioration of the bearing and prolong its lubricating life.
From the analysis of the results in Figure 5, the effect of aluminum disulfide on the grease leakage rate of the sealed bearing is not very obvious, but it only increases slowly after 2 hours of training. for the sake of. The results of changes in iron content in Figure 4 show that until the sealed bearing fails completely, the wear amount remains at a very low level. Combined with the result that the surface roughness value of the bearing is still small after failure (where = 0.042 mm), it can fully explain that under low load and high temperature The aluminum disulfide particles have the effect of significantly improving the anti-wear properties of the bearing surface.
It was confirmed by SEM observation that the bearing samples with molybdenum disulfide were still slightly worn after the sealed bearing lubricant failed completely. This is the result of the formation of a solid lubricating film on the bearing surface by aluminum disulfide. At present, there are two hypotheses for the slowing down of the chemical deterioration of the sealed bearing grease caused by aluminum disulfide. One is that the formation of the surface film reduces the catalytic activity of the bearing metal surface. Another view is that the molybdenum disulfide particles inhibit the decomposition rate of the antioxidant. The above analysis shows that although it is still unclear what mechanism is the main effect of molybdenum disulfide on the chemical deterioration process in sealed bearings, it is certain that both effects exist.
5. The role of copper
From Figure 2 to Figure 5 and Table 3, it can be seen that the effect of copper on the chemical deterioration, physical deterioration and wear deterioration of the sealed bearing grease at high temperature is similar to that of molybdenum disulfide, and it can also significantly improve the bearing resistance. Grinding performance and increase its lubricating life.
The change of iron content in Fig. 4 and the change of total acid value in Fig. 2 show that under high temperature conditions, copper particles can reduce bearing wear and prolong the lubrication life. Reduced catalytic activity of iron-based surfaces against chemical degradation of lubricants. Although there is a theory that the improved bearing life is due to copper’s ability to retard the decomposition of antioxidant additives.
3 shows that the antioxidant additives disappeared after 20h of the test, and the wear of the sealed bearing and the chemical deterioration of the lubricant were still very little after 70h. The infrared analysis also confirmed that the oxidation products appeared after 700h. This shows that the chemical solid film or adsorption layer and its function exist.
In order to further confirm the existence of the chemical adsorption layer and the influence of temperature, we lowered the test temperature and studied the influence of copper particles on the wear and chemical degradation of bearings and greases at a low temperature that is not easy to form a chemical adsorption layer. The results show that the situation is completely opposite to the high temperature state at 10 ℃ receding degree. Since the chemical adsorption layer cannot be formed at low temperature, copper particles exist in the bearing as a kind of mechanical wear particles, and its abrasive wear is strengthened, which seriously aggravates the wear deterioration and reduces the lubrication life.
It can be seen that the main reason why copper particles improve the bearing anti-wear performance and increase the lubricating life at high temperature is the combined effect of copper inhibiting the decomposition of antioxidants and forming a chemical adsorption layer on the bearing surface to reduce the catalysis of the iron-based surface.
(l) The impact of solid impurities on the life of sealed bearings and the failure of file base grease is significant, and the degree varies according to the nature of the impurity particles. At a high temperature of 125 ℃, copper and platinum disulfide can improve the lubrication life of bearings and reduce the failure rate of grease. Iron and dust aggravate grease failure and reduce bearing life.
(2) The influence of solid impurities on bearing life performance is mainly reflected in the chemical deterioration and wearability deterioration of grease, but the effect on physical deterioration is not obvious. The wear failure behavior at high temperature is mainly controlled by chemical degradation, while at lower temperature it is dominated by abrasive wear.
- Although the mechanism by which copper and platinum disulfide particles can improve the lubrication life of sealed bearings is not completely clear, it is confirmed that these two substances can inhibit the decomposition of diphenyl zeamin at high temperature, and A solid film or chemical adsorption layer may form on the bearing raceway surface.
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