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Plain Bearing Failure
- Inadequate supply of lubricant. This can occur at start up in low temperature conditions if the lubricant viscosity in the reservoir is so high that flow rate is too low. (Viscosity at start up should not exceed 2000 cSt.)
- Use of lubricant of too low viscosity grade for the application
- Lubricant supply is too hot leading to thin lubricant film
- Misalignment leading to edge loading.
- Excessive load causing high temperature and thin lubricant film. (Excessive start-up load, preventing or delaying the generation of a hydrodynamic lubricating film can be a problem with hydrodynamic bearings.)
- Excessive shaft orbiting due to bearing instability (e.g. sub-synchronous whirl), but this can also give rise to fatigue failure.
- Shaft operating load line close to an oil groove causing breakdown of the oil film or oil starvation. The particular case of gear bearings, where the load line is a combination of gravity and gear thrust, should be noted.
- Loss of bearing area by some other failure mechanism, e.g. fatigue, electrical erosion damage.
1.0 Lubrication breakdown
1.6 Lubrication Breakdown: Summary
Characteristics
Melting of white metal, often resolidifying in cooler parts of the bearing (e.g. in the oil grooves, or in the upper half of the bearing, though in the case of minor wipes it will be confined to the trailing edge of the damaged region.
Re-deposited bearing material does not form a metallurgical bond with the underlying material and can often be easily peeled off (particularly if whitemetal).
Circumferential scoring of wiped bearing usually occurs.
Possible Causes
Wiping is caused by a failure to form a hydrodynamic oil film at start up or reduction in lubricant film thickness (sometimes complete breakdown of the lubricating film), resulting in overheating and melting of the bearing material (partial melting in the case of the higher strength lead-bronzes and copper-leads). It should be noted that in high-speed machines wiping can occur without complete film breakdown if the temperature in the oil film rises above the melting point of the bearing material (about 240°C in the case of
whitemetals).
A number of possible causes are listed below. It is unlikely that failure occurs as a consequence of poor bearing manufacturing quality.
Actions
The first check is whether the failure has occurred for the first time or whether there is a history of similar failures on the particular machine, or other similar machines. The operating conditions of the equipment leading up to failure are very important in understanding the cause. For example: has the failure occurred at start-up; was there an increase in vibration levels; has there been a problem with the lubricant pump or was the oil at the correct operating level in a self-contained system; was the filter element blocked?
The profile of the wiping patch may indicate the occurrence of misalignment. Measuring the shell thickness can in some instances indicate the exact position of the shaft when the failure occurred.
Check that the position of wiping corresponds to the predicted minimum film thickness in the bearing (this can be calculated using the techniques listed below). If the actual and predicted positions do not correspond then look for conditions that may result in the applied load being in a different direction to that expected. Note again that changes in gear loading will alter the direction of the resultant load.
Check damage to the mating shaft. Surface scoring and/or thermal cracking can occur; this risk is greater with harder bearing materials (e.g. copper and aluminium alloys).
Useful Analysis Techniques
Calculation of bearing operating conditions (lubricant flow rate, temperature, lubricant film thickness, attitude angle) can provide good supporting evidence. See, for example, ESDU Data Items 84031, 90027 (journal bearings), 82029, 83004 (thrust bearings).