I. Introduction
Sliding bearing, which works under sliding friction. The sliding bearing works stably, reliably and noiseless. Under the condition of liquid lubrication, the sliding surface is separated by the lubricating oil without direct contact, which can also greatly reduce the friction loss and surface wear. The oil film also has a certain vibration absorption capacity, but the starting friction resistance is large.
Sliding bearing is one of the most common mechanical parts in machinery and equipment, and is the key friction pair in large rotating machinery such as fans and motors. From the wear form of sliding bearing, there are mainly slight wear, serious wear and scratch.
Through the analysis of a large number of failed sliding bearings, the main causes of abnormal wear are as follows:
1. Poor lubrication. Due to design error or improper maintenance, the oil supply of the sliding bearing is insufficient during operation, resulting in the rupture of the hydrodynamic oil film, resulting in abrasion, and finally biting; Or because of external oil cut-off of bearing after running-in, such as blocked oil suction screen, defective oil pump, blocked or leaking oil circuit, and failure of pressure reducing valve spring, the lubricating oil circulation is interrupted, and finally the failure is caused.
2. Incorrect assembly or centerline offset. It will cause serious wear and high temperature at one end or part of the bearing, or the wear will be biased to one side due to uneven load.
3. Overload. The overheating caused by this causes metal fatigue, which causes the metal surface of the bearing to break and flake.
4. The entry of dust or external impurities. Large and hard dust particles entering the bearing clearance will scratch the softer bearing surface, or the particles will sink on the softer bearing surface and strain the journal.
5. Corrosion. Due to the corrosion of the bearing surface caused by the acidic substances in the lubricating oil, a surface with fine pits and a large area of bearing material deterioration are formed. In addition, the formation and collapse of oil hollow bubbles will also lead to extremely high local pressure, namely the so-called "cavitation" effect, resulting in local fatigue and pitting on the bearing surface.
II. Fault diagnosis
A factory has an exhaust fan whose main parameters are: the exhaust volume of the main exhaust fan is 21000m3/min, the air pressure is 2000mmH2O, the motor power is 9300kW, and the sliding bearing is made of lead-based babbitt alloy (containing 5%~11% tin) bushing, which has excellent inlaying resistance, sintering resistance and corrosion resistance.
In the process of use, oil samples are often analyzed. At the beginning of February, it was found that the direct reading ferrography analysis data showed a significant upward trend compared with the previous analysis data, especially the large wear particle reading DL increased significantly (Table 1).
The content of lead and tin in spectral analysis is high. In addition to a small amount of normal sliding abrasive particles, there are many non-ferrous metal particles deposited at the full length of the ferrograph, with the size of 10-20 µ m and rough surface, which are clumps with granular surface, with dark color, mixed with blue and purple small spots, and are an effective light absorber. They have no bright and shiny luster under polarized light. They are randomly arranged on the ferrograph and arranged along the direction of the magnetic line of force, Therefore, it can be ruled out as a black oxide (FeO). These nonferrous metal abrasive particles are from babbitt alloy on the sliding bearing, indicating that the bearing bush has abnormal wear, which is caused by poor lubrication, so a warning is given. At that time, the oil analysis and vibration analysis were carried out simultaneously, and the vibration analysis results showed that the vibration values of various points were within the normal range, so the point inspection party did not take any measures. One month later, the oil was analyzed again, and the oil samples from five sample points including the system oil tank were collected for comprehensive analysis. The results showed that the oil samples at all points contained a lot of babbitt abrasives, with obvious melting phenomenon on the surface, and some even became spherical. The wear of the bearing bush had a trend of further development, and the conclusion was still a warning, and the spot inspection party was recommended to stop for maintenance, Check the wear of bearing shells and other parts. After half a month, the spot-checker inspected the bearing pads of the main exhaust fan, and found that the sliding bearing pads of the four sample points had different degrees of scratches, obvious scratches on the surface of babbitt alloy, and some had cracks and peeling. After replacing the bearing bush and making proper adjustment, it works normally.
If it is allowed to develop naturally, it is likely to bite and eventually lead to the motor burning accident. This is a typical case where the oil monitoring technology can predict the hidden trouble earlier than the vibration analysis technology. At that time, the fault of the main exhaust fan was still in the initial stage, and it was not enough to cause large vibration value changes. The vibration value measured on the site was not analyzed by frequency spectrum.
III. Conclusion
Sliding bearings are widely used in metallurgical machinery. It is theoretically feasible to use ferrography analysis to monitor their working conditions. 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 babbitt alloy is a non-ferromagnetic material, while ferrography technology is mainly sensitive to ferromagnetic materials, so it is more difficult, The accurate prediction of faults through oil monitoring technology this time is the result of many years of experience accumulation and has important practical significance.
More about E92 Bushing:
E92 is deriving from E90; the difference between E90 & E92 is Indentations on working surface, which substituted by throughholes. Theses holes will allow greater capacity to collect lubricant, which build up a lubrication film at the start of movement and reduce the frication. It is suitable for high load, lower speed application like construction, transport, and agriculture machinery.
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