Babbitt alloy is one of the commonly used materials for sliding bearings, and due to its unique mechanical properties, it is widely used as a sliding bearing material in many rotating machinery. It is common to encounter situations in daily work where production is halted due to sliding bearing failures, resulting in significant losses. Summarize and accumulate experience, refer to relevant literature knowledge, and briefly discuss the factors and solutions of Babbitt alloy bearing faults.
1. Babbitt alloy loosening
The reason for the loosening of Babbitt alloy is often due to insufficient cleaning of the base metal before casting, material tinning, and insufficient casting temperature. When the Babbitt alloy loosens from the base metal, the bearing accelerates fatigue, and lubricating oil enters the loosening separation surface, causing the bearing to wear out quickly.
Solution: Re solder and pour Babbitt alloy.
2. Abnormal wear of bearings
During the acceleration and running in process, slight running in wear and mating wear of the shaft diameter are normal. But when the bearings have the following faults, abnormal or severe wear will occur.
1) Defects in bearing assembly. Inappropriate bearing clearance, misalignment of bearing shells, poor contact of shaft diameter in bearing shells, and inability of shaft diameter to form a good oil film during operation can cause rotor vibration and bearing shell wear.
Solution: Replace the bearing or scrape it again and mark it properly, then reassemble it to meet the technical requirements.
2) Bearing machining error. Cylindrical bearings are not round, improper size and distribution of oil wedges in multi oil wedge bearings, excessive or insufficient bearing clearance, excessive deflection of thrust disc end face of thrust bearings, and uneven thickness of bearing pads can all cause serious wear.
Solution: Use process axis inspection to repair the irregular shape of the bearing tile.
3) Rotor vibration. Due to rotor imbalance, misalignment, oil film oscillation, fluid radicalization and other faults, high amplitude is generated, causing severe wear, burns, and fraying of the bearing shell.
Solution: Eliminate the factors causing vibration and replace worn bearings.
4) Oil supply system issue. Insufficient or interrupted fuel supply can cause severe friction, burns, and shaft gripping.
Solution: Solve the oil supply system problem by cleaning or replacing the oil, repairing or increasing the cooler to lower the oil temperature.
3. Bearing fatigue
The following reasons can cause bearing fatigue:
1) Overloading of the bearing causes the oil film in the bearing area to rupture, resulting in stress concentration in local areas, local contact cracks, and fatigue failure after propagation.
2) Loose bearing shells, excessive or insufficient bearing clearance, and vibration generate alternating loads on the bearings, especially with large amplitude fluctuations. The shear stress on the bearing surface can easily cause fatigue cracks in the bearing shells. The location where cracks occur is generally at the peak of a steep pressure gradient, where multiple cracks converge, causing extensive cracking and loosening of the tile.
3) The thicker the Babbitt alloy, the more sensitive it is to fatigue and prone to fatigue failure.
4) The strength of Babbitt alloy decreases with increasing temperature, and fatigue cracks occur during operation at high temperatures, accelerating bearing fatigue wear damage.
Solution: The bearing pressure ratio should be controlled within an appropriate range. The bearing clearance should be controlled within the design requirements. Babbitt alloy tiles with a thickness controlled between 1-1.5mm and good fatigue resistance. Control the temperature of the bearing shell.
4. Bearing corrosion
Bearing corrosion is mainly caused by chemical reactions of lubricants. If the lubricant is selected improperly or loses its lubrication performance due to aging, and cavitation and high temperature occur during operation, surface corrosion may still occur. Corrosion damage and wear damage have some similarities, but from the surface of the bearing shell, corrosion is caused by an oxide layer that changes color locally or completely due to corrosion. Under a metallographic microscope, corrosion deposits can be observed in the chemical corrosion pits. Corrosion does not occur in the oil film bearing area like wear, it can be found in any part.
5. Bearing cavitation
Bearing cavitation is the generation of tiny bubbles in the low-pressure area of the bearing (where the pressure is lower than the saturated vapor pressure of the oil). These bubbles are carried to the high-pressure zone and crushed, instantly forming a pressure shock wave that impacts the surface of the bearing, causing fatigue cracks or metal layer peeling on the surface metal quickly. When the bearing is working, the amplitude and speed of the shaft diameter vortex are large, and the hydraulic pressure difference in the clearance is large, which is prone to cavitation. High speed bearings are prone to cavitation due to strong eddies or interruptions in oil flow at the junctions of oil holes, oil grooves, and bearing cross-sections. The viscosity of lubricating oil decreases or the presence of water or air in the oil can easily cause cavitation.
Methods to mitigate cavitation: reduce oil disturbance, increase oil viscosity, increase fuel supply pressure, and other measures.
6. Loose bearing housing
The main reason is that the pressure between the bearing cover and the bearing seat is not tight, and there is a gap between the bearing sleeve and the bearing cover. When the rotor is working, the bearing shell becomes loose, which affects the stability of the bearing oil film. This type of vibration caused by gaps has nonlinear characteristics, and there may be subharmonic components of 1/i times the rotational speed frequency and superharmonic components of i times the rotational speed frequency (where i is a positive integer) in the vibration frequency. To avoid bearing looseness, an interference fit of 1-4mm should be maintained between the bearing covers during bearing assembly.
7. Inappropriate bearing clearance
The bearing clearance is too small, and due to excessive shear friction loss caused by oil flow in the clearance, the bearing heats up. The clearance is too small, the oil volume decreases, and there is no time to take away the heat generated by friction. The clearance is too large, even a small excitation force (such as unbalanced force) can cause significant bearing vibration, and the vibration is large when the critical speed is exceeded. Excessive bearing clearance in high-speed light load rotors can alter the dynamic characteristics of the bearings and cause unstable rotor operation. The large clearance between bearings is similar to a loosening problem, and there will be many harmonic components of rotational frequency in the frequency spectrum of shaft vibration. The fit clearance of bearings should be controlled within the design value or recommended value range. For bearings with high installation requirements, it is best to use a dedicated measuring rod to measure the bearing clearance. Check the contact and tightness between the measuring rod and the tile block to ensure that each tile block is in good contact and the clearance meets the requirements. High speed five pad bearings are generally not allowed to be scraped. Sometimes, in order to reduce rotor vibration, the bearing clearance should be controlled at the lower limit of the allowable value. It is best to have an embedded thermometer to detect the temperature of the bearing shell. Generally, it should not exceed 12000C, otherwise the Babbitt alloy will melt.
8. The bearing temperature is too high
In large rotating machinery, bearing temperature or return oil temperature is a regular detection item. The reason for the high temperature is:
1) The bearing clearance is too small
2) The bearing load is too high
3) Oil cooler malfunction, resulting in an increase in inlet oil temperature
4) The shape or assembly of the bearing does not meet the requirements
If the oil temperature is found to be too high during machine operation, it must be closely monitored to identify the cause and promptly dealt with.
Conclusion
In short, sliding bearings must strictly meet the standards from assembly to maintenance, ensuring good lubrication and temperature monitoring, in order to ensure the good operation of the equipment.