The key to detecting the early signs of a problem is effective bearing monitoring. This can take many forms, but for the vast majority of bearing applications the monitoring supplied by the machine operator is usually sufficient to detect unusual noises at an early stage. In situations where downtime is critical or hazardous, then more formalised monitoring is required. A number of methods are available including monitoring lubricant cleanliness, measuring bearing temperature and vibration analysis.
The type of condition monitoring employed is as much a factor of the experience of previous failures as the production environment in which the bearings are used. Bearing damage can generally be classified into two groups - localised or widespread. Localised damage is usually restricted to specific locations on the bearing. This can take the form of indentations caused by rolling elements, corrosion or fractures. It can be recognised most easily using a combination of vibration and lubricant monitoring. Vibration methods will also reliably detect fatigue damage at any early stage, but are not suitable for detecting lubrication problems.
Widespread damage is often the result of an insufficient supply of clean lubricant. Failures of this type can be detected by monitoring the lubricant suppply. Oil flow can be monitored for pressure, flow and cleanliness. A magnetic plug gives a crude indicaton of lubricant condition, whilst a spectral analysis can be used to provide a more precise check.
Temperature can be monitored using thermocouples and gives a very reliable indicator of impending bearing problems. Normally a system should reach a steady state temperature and will show a sudden rise when there is a lack of lubricant. Typically, with grease, the temperature will rise unevenly over time if there is a general deterioration in the grease condition.
Securing damaged bearings
When a bearing has to be removed from a machine due to damage, the cause must be established to avoid future failures. Inspection of the bearings alone is not normally enough to pinpoint the exact cause of the damage, but rather the inspection of the mating parts, lubrication and sealing, as well as the operating and environmental conditons.
A systematic procedure for removal should be followed to securing and inspecting the bearing. The recommended sequence of measures is shown below:
- Determine operating data
- Evaluate records and charts from any bearing monitoring devices
- Extract lubricant samples
- Check bearing environment for external influences and other damage
- Assess the bearing in its mounted condition
- Mark the mounting position
- Dismount the bearing
- Mark bearings and parts
- Check beating seats
- Assess complete bearing
The above methodology is a comprehensive one for carrying out damage assessment. However, its usefulness will decline if the level of damge in allowed to become excessive. The earlier a bearing can be dismounted, the more effective the assessment process will be.
For your copy of Barden's 'Bearing Failure: Causes and Cures' guide, please click here
. The guide describes the 12 primary causes of bearing failure, illustrated by close-up, colour photographs. Specific remedies are also suggested for each failure type.
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