1. Introduction
Sliding bearings are bearings that operate under sliding friction. Sliding bearings work smoothly, reliably, and without noise. Under liquid lubrication conditions, the sliding surface is separated by lubricating oil without direct contact, which can greatly reduce friction loss and surface wear. The oil film also has a certain vibration absorption ability, but the starting friction resistance is relatively high.
Sliding bearings are one of the common mechanical parts in machine equipment, and are key friction pairs in large rotating machinery such as fans and motors. From the perspective of the wear forms of sliding bearings, there are mainly slight wear, severe wear, and scratches.
Through analysis of a large number of failed sliding bearings, the main causes of abnormal wear are as follows:
1.1 Poor lubrication. Due to design errors or improper maintenance, the sliding bearing experiences insufficient oil supply during operation, resulting in the rupture of the hydrodynamic oil film and causing abrasions, ultimately leading to Z biting and death; Or due to external oil interruption of the bearing after running in, such as blockage of the oil suction net, defects in the oil pump, blockage or leakage of the oil circuit, and failure of the pressure reducing valve spring, the lubricating oil circulation may be interrupted, resulting in failure.
1.2 Incorrect assembly or centerline offset. It can cause severe wear and high temperature on one end or part of the bearing, or cause wear to shift to one side due to uneven loading.
1.3 Overloading. The overheating caused by this leads to metal fatigue, causing the metal surface of the bearing to break and peel off.
1.4 Entry of dust or external impurities. Large and hard dust particles entering the bearing clearance can scratch the softer bearing surface, or the particles may sink into the softer bearing surface and strain the journal.
1.5 Corrosion. Due to the acidic substances in the lubricating oil, the bearing surface corrodes, forming a surface with fine pits and a large area of bearing material deterioration. In addition, the formation and collapse of oil bubbles can also lead to extremely high local pressure, known as "cavitation", causing local fatigue and pitting on the bearing surface.
2. Fault diagnosis
A certain factory has an exhaust fan with the following main parameters: main exhaust fan displacement of 21000m3/min, air pressure of 2000mmH2O, motor power of 9300kW, sliding bearings made of lead based Babbitt alloy (containing 5% to 11% tin) lining, which has excellent resistance to inlaying, sintering, and corrosion.
In the process of use, oil sample analysis is often conducted on it. In early February, it was found that the direct reading iron spectrum analysis data showed a significant upward trend compared to previous analysis data, especially the large abrasive grain reading DL, which increased significantly (Table 1).
The content of lead and tin elements is relatively high in spectral analysis. Except for a small amount of normal sliding abrasive particles, there are a large number of non-ferrous metal particles deposited along the entire length of the iron spectrum plate, with a size of 10-20 µ m and a rough surface. They are clusters with granular surfaces, with dark colors mixed with blue and purple small spot colors. They are an effective absorber and do not have bright flickering luster under polarized light. They are randomly arranged along the magnetic field lines on the iron spectrum plate, thus ruling out the possibility of them being black oxides (FeO). These non-ferrous metal abrasive particles come from Babbitt alloy on sliding bearings, indicating abnormal wear of the bearing shells, which is caused by poor lubrication. Therefore, a warning is issued regarding this. Since the oil analysis and vibration analysis were conducted simultaneously at that time, and the vibration analysis results showed that the vibration values of each point were within the normal range, the inspection party did not take any measures. One month later, another oil analysis was conducted on it, collecting oil samples from a total of five points including the system oil tank for comprehensive analysis. The results showed that the oil samples at each point contained a large amount of Babbitt alloy abrasive, with obvious melting phenomenon on the surface, and some even became spherical. The wear of the bearing shell showed a further trend of development, and the conclusion was still a warning. It was recommended that the inspection party stop the machine for maintenance and check the wear condition of the bearing shell and other components. Half a month later, the inspection team conducted a dismantling inspection on the bearing shells of the main exhaust fan and found that the sliding bearing shells of the four sample points had varying degrees of abrasion. The Babbitt alloy surface had obvious scratches, and some even showed cracking and peeling. After replacing the bearing shell and making appropriate adjustments, it works normally.
If left to develop naturally, there is a high possibility of biting and ultimately leading to motor burnout accidents. This is a typical case where oil monitoring technology can predict potential faults earlier than vibration analysis technology. At that time, the failure of the main exhaust fan was still in its early stages and did not cause significant changes in vibration values. Spectral analysis was not conducted on the measured vibration values on site.
3. Conclusion
Sliding bearings are widely used in metallurgical machinery, and the use of ferrography analysis for condition monitoring is theoretically feasible. It can predict the hidden dangers of faults earlier than vibration analysis technology. However, in the specific practical application process, due to its relatively small wear and the fact that Babbitt alloy is a non ferromagnetic substance, and ferrography technology is mainly sensitive to ferromagnetic substances, its difficulty is relatively high. The accurate prediction of faults through oil monitoring technology this time is the result of years of experience accumulation and has important practical significance.
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