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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 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 subsequent overheating.
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.
When precision ball bearings or rolling bearings fail, the results can be costly in terms of machine downtime and 'lost' production. Fortunately, catastrophic failures of bearings are rare. Usually there are distinct symptoms that indicate the type of damage incurred long before the bearings actually fail. It is therefore important for companies to have a regime in place that enables the symptoms of bearing damage to be recognised early in their development. 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.
Operating Behaviour Indicates Damage
Experience shows that damage to, and subsequent failure of, bearings is seldom due to faults in the bearings themselves, but more often due to the treatment they have received or the use to which they have been put. Often, the first sign of damage is indicated by unusual operating behaviour of the bearings. This can take the form of uneven running, reduced working accuracy, unusual running noises or any combination of the three. It is critical for these early indicators to be logged, as information gained in this early period of degradation can be very useful in identifying the root cause of a problem. Often, 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. 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 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.
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 damage, but rather the inspection of 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. The recommended sequence of measures is shown below:
(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 above methodology is a comprehensive one for carrying out damage assessment. However, its usefulness will decline if the level of damage in a bearing is 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 & Cures' guide, view website: www.bardenbearings.co.uk/bearing_failure_guide.html 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.
The Barden Corporation (UK) Ltd in profile
The Barden Corporation (UK) Ltd is a recognised world leader in super-precision bearing systems. For more than 60 years, Barden has built on its unrivalled manufacturing expertise and its ability to design bespoke engineering solutions for a variety of difficult applications. The company offers an extensive range of super-precision bearings for a variety of applications, including aerospace, turbomolecular pumps, medical instruments, machine tool and motorsport. Now part of the Schaeffler Group, Barden UK holds an enviable position alongside sister manufacturing plants around the world.
For further information, view website: www.bardenbearings.co.uk
The Schaeffler Group in profile
The Schaeffler Group with its product brands INA, LuK and FAG is a leading manufacturer of rolling bearings and linear products as well as a renowned supplier to the automotive industry of high-precision products and systems for engines, transmissions and chassis applications. The group of companies stands for exceptional customer focus, innovative ability and the highest possible level of quality. Sales of over 9.5 billion euros were generated at over 180 locations in more than 50 countries in 2010. With around 70,000 employees worldwide, the Schaeffler Group is one of the largest German and European industrial companies in family ownership.
For further information, e-mail: info.uk@schaeffler.com or view website: www.schaeffler.co.uk Refer to page 87
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Users can set-up a "favourites" area to view specific network parameters. The Warning/Fault pane displays all network issues in one convenient format. "One of the most powerful additions to NetAlytix v2 is the area that displays a basic graphical format of the data being monitored," said Michael Frayne, product manager, network interfaces, Molex Incorporated. "We feel that users will benefit from the enhancements. Molex, through its Brad products, has always been at the forefront when introducing simulation and diagnostic solutions."
The Brad eNetMeter DN is a passive device that continuously monitors a DeviceNet network and sends the information over Ethernet to a PLC or PC monitoring system. The information can be read directly by an EtherNet/IP master. Optionally, data can be accessed through a DLL interface, an OPC server or the NetAlytix v2 software.
eNetMeter DN can be used to diagnose current faults on a non-functioning network through the measurement of hundreds of network parameters. This can minimise downtime by pinpointing the node or location of the fault. The data can also be stored to historian software for future analysis. Typical applications can be found in any industrial or commercial setting where DeviceNet networks are used, including automotive, petrochemical, food and beverage, material handling, forest products, mining and metals.
For further information, view website: www.molex.com/link/xrc.html Refer to page 104
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