Company emphasises importance of failure root cause analysis

4th November 2011

The identification and analysis of the root cause of bearing failure is critical to preventing similar failures from reoccurring, advises precision bearings manufacturer The Barden Corporation quality director Brian Williams.

Companies should introduce a regime that enables the early recognition of the symptoms of bearing damage, as well as put a systematic procedure in place to secure damaged bearings, he says.

Many bearings that fail are removed and replaced without sufficient levels of analysis into the cause, which may result in similar failures reoccurring, resulting in further damage and costly unscheduled downtime.

“Companies can address these issues by developing a systematic procedure for securing and inspecting bearings once they become damaged. Engineers should not wait until the bearing failure becomes catastrophic, as this makes root cause analysis difficult. Instead, engineers need to perform regular monitoring and inspection of the bearings,” Williams urges.

Examination of the failure mode often reveals the true cause of bearing failure, but this procedure is complicated by the fact that one failure mode may initiate another. For example, corrosion in a ball-raceway bearing produces rust (an abrasive), which may cause wear and result in a loss of preload or an increase in radial clearance.

The wear debris can, in a grease-lubricated bearing, impede lubrication and result in lubri- cation failure and subsequent overheating.

When precision ball bearings or rolling bearings fail, the results can be costly and result in machine downtime and lost production; however, significant bearing failures are rare, and there are usually distinct symptoms that indicate the type of damage incurred before the bearings actually fail.

Operating Behaviour Indicates Damage

Further, Williams reports that damage to, and the subsequent failure of, bearings are seldom the result of faults in the bearings themselves, but more often the result of treatment they have received or the nature of their application.

In most cases, the first sign of damage is unusual operating behaviour of the bearings, which may take the form of uneven running, reduced working accuracy, unusual running noises or any combination of the three.

Williams says that it is critical for these early indicators to be logged, as informa- tion gained in this early period of degradation can be very useful in identifying the root cause of a problem, because, as a bearing becomes more damaged, root cause analysis becomes increasingly difficult.

“The key to detecting the early signs of a problem is effective bearing monitoring by the machine operator, but in situations where downtime is critical or hazardous, more formalised monitoring is required. A number of methods are available, including monitoring lubricant cleanliness, measuring bearing temperature and vibration analysis,” he says.

Williams adds that 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.

Further, bearing damage can be localised, which is usually restricted to specific locations on the bearing, and can take the form of indentations caused by rolling elements, corrosion or fractures. Localised damage can be recognised most easily using a combination of vibration and lubricant monitoring.

Alternatively, widespread damage is often the result of an insufficient supply of clean lubricant, and failures of this type can be detected by monitoring the lubricant supply.

“Temperature can be monitored using thermocouples, which are very reliable indicators 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,” says Williams.

When a bearing has to be removed from a machine as a result of damage, Williams says, 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 an inspection of the mating parts, lubrication and sealing, as well as the operating and environmental conditions.