With bearings being crucial components, failure can be costly or sometimes catastrophic, so identifying the root cause of failure is essential to prevent it happening again. Brian Williams, quality director at The Barden Corporation, comments
When a bearing fails, it is often removed and replaced without sufficient levels of analysis into the cause, meaning similar failures could occur again in the future.
Examination often reveals the true cause of failure, but this is complicated by the fact that one failure mode may initiate another. For example, corrosion in a ball raceway leaves rust – an abrasive – which can cause wear, resulting in loss of preload or an increase in radial clearance. The wear debris can, in a grease-lubricated bearing, impede lubrication, resulting in lubrication failure and overheating.
As a result, a systematic procedure for securing and inspecting bearings once they become damaged is needed before the failure becomes catastrophic. Regular monitoring and inspection is therefore needed.
When precision ball bearings or rolling bearings fail, the results can be costly in terms of machine downtime and ‘lost’ production. As there are often symptoms that indicate the type of damage incurred long before failure occurs, a regime is needed to enable the symptoms of bearing damage to be recognised early. Once this has been achieved there must also be a system in place that preserves the condition of the bearings when they are removed from the machine in their damaged state. This is critical in assisting the bearing manufacturer to analyse the causes of failure and to avoid similar issues in the future.
Damage to, and failure of, bearings is often due to their treatment or use. The first sign of damage is often indicated by unusual operating behaviour such uneven running, reduced working accuracy and/or unusual noises. Logging these indicators early on can be very useful in identifying the root cause of a problem.
Effective bearing monitoring is the key to problem detection, but in many 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, more formalised monitoring is required – including monitoring lubricant cleanliness, measuring bearing temperature and vibration analysis.
Bearing damage can generally be classified into two groups. Localised damage is usually restricted to specific locations on the bearing and 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.
The second group is widespread damage, and is often the result of an insufficient supply of clean lubricant. Failures of this type can be detected by monitoring the lubricant supply. Oil flow can be monitored for pressure, flow and cleanliness. A magnetic plug gives a crude indication of lubricant condition, whilst a spectral analysis can be used to provide a more precise check.
Using thermocouples to monitor temperature is a 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.
Removing and inspecting
When a bearing has to be removed from a machine due to damage, the cause must be established to avoid future failures. This involves inspection of the bearings, the mating parts, lubrication and sealing, as well as the operating and environmental conditions.
A systematic procedure for removal should be followed for securing and inspecting the bearing:
(1) Determine operating data
(2) Evaluate records and charts from any bearing monitoring devices
(3) Extract lubricant samples
(4) Check bearing environment for external influences and other damage
(5) Assess the bearing in its mounted condition
(6) Mark the mounting position
(7) Dismount the bearing
(8) Mark bearings and parts
(9) Check bearing seats
(10) Assess complete bearing
The earlier a bearing can be dismounted, the more effective the assessment process will be.