Today, bearing manufacturers can reliably predict the operating life of bearings, which makes it easier to specify the right bearing for an application.
Bearings do fail, if for no other reason than components do not operate forever. A number of causes contribute to failure, including material fatigue; operating with a heavier loading than specified; inadequate or unsuitable lubrication; careless handling; poor sealing; and bearing balls or rollers installed too tightly, with insufficient internal bearing clearance.
Each of these contributors leaves known clues on the bearings or the races, so it is fairly easy to determine what happened. Primary types of damage are:
• Mechanical wear
• Indentations and brinelling
• Surface distress
• Electric current damage
Other forms of damage show up as flaking or cracks.
Wear: If everything goes well during the operating life of a bearing, it will reach its predicted end of life through normal wear, or simple material fatigue. You can calculate fatigue–it is a function of the number of bearing revolutions and the magnitude of the load. It shows up in a bearing as cyclical shear stresses that appear immediately below the load-carrying surface. These stresses can cause cracks to the surface of the raceway. When a rolling element moves over the cracks, some material breaks away, which is known as flaking or spalling. Flaking/spalling is a long wear process, so it gives clues as it’s occurring. These clues are increasing noise or vibration in the bearing. Eventually, it will lead to bearing failure, but you have time to change the bearing before it causes machine operation problems.
The most frequent causes of excessive wear are foreign particles that get into the bearing, insufficient or incorrect lubrication, and excessive vibration on non-running bearings.
Foreign particles: Seals are critical to ensure that foreign particles do not get into the bearing. For the most part, seals should not fail unexpectedly unless the wrong seal was installed or installed improperly. However, routine care and maintenance can alert you to any potential problems.
Foreign particles, though, can also get into the bearing through contaminated lubricant or during mounting.
Lubricant prevents metal-to-metal contact, which damages the rolling elements and the raceways. Look for small indentations in the raceways and rolling elements.
Lubricant also keeps temperatures down during bearing operation. Temperature control is critical as high temperatures contribute to bearing seizure where a bearing locks into position and will not move. Another issue with temperature maintenance is the softening of bearing material. Depending on the load, the material will deform if too hot, which results in noise, vibration, and non-smooth movement. Indications of poor lubrication include surfaces with a blue or brown hue, or with a mirror like finish.
Wear caused by vibration: Anytime lubricant is not flowing between the rolling elements and the raceways, a bearing can be damaged. The main cause of such damage when a bearing is not operating is through vibration, either during idle times (normal machine vibrations) or during transportation.
As the bearing experiences vibration, the rolling elements undergo metal-to-metal contact with each other and the raceway, leading to small indentations or depressions in the raceway and small particles breaking away from their surfaces. The resulting damage is known as false brinelling. Damage from balls shows up as sphered cavities, and with rollers it is fluting on the raceways.
Roller bearings are more susceptible to false brinelling than ball bearings because rollers move in one direction so they experience more wear.
Radial preloading can secure a bearing from vibration it may experience during transport. Use of a vibration damping base can also help. If vibration will be an issue on a machine, consider ball bearings instead.
For more information on rotary-bearing lubrication, wear, and failure prevention, go to linearmotiontips.com baby-sister website bearingtips.com. Here are just a few related articles from that resource.
Turning the volume down on ball bearing noise in your motion system
In this interview bearingtips.com speaks with NMB Field Applications Engineer, Rich Picard about bearing vibration, noise …
… and the things to look for when specifying bearings for your motion system.
NKE — Application analysis for lubrication of rolling bearing
About 40% of all premature bearing failures are caused by lubrication problems.
Daniel Stöckl and Klaus Grissenberger, application engineers at NKE Austria GmbH, show you how to prevent lubrication issues before they happen.
Timken’s 14 step procedure for bearing analysis
Timken engineer’s give a step-by-step process for an accurate and complete analysis when investigating any bearing damage or system breakdowns.
Three bearing killers and how to stop them: Part 1
Bearing failure can grind your operations to a halt, resulting in significant lost time and production.
Here are the first three of six common factors that can cause bearings to fail, and what you can do to prevent them.
Three more bearing killers and how to stop them: Part 2
Part two of the two-part bearing-killers article above.
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