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Rolling element bearings are highly reliable components, and the vast majority of bearings will outlive the equipment on which they are installed. However, while bearings account for a relatively small percentage of all equipment breakdowns, they do fail occasionally. And when they fail, it is usually a critical event, resulting in costly repair and downtime.
When bearings fail, only a small number of failures are caused by material fatigue. Most failures are caused by a condition that usually can be prevented. Typically, the causes of bearing failure can be traced to:
Harsh operating conditions
Faulty storage, handling and installation
This article focuses on bearing failures associated with lubrication problems.
Bearings fail for many reasons, but improper lubrication is at the top of the list, according to a variety of studies:
Improper lubrication: 40 to 50 percent
Improper mounting: 25 to 30 percent
Other causes: approximately 20 percent
Reaching the natural fatigue limit: less than 10 percent
Anti-friction bearings must be lubricated to prevent metal-to-metal contact between the rolling elements, raceways and retainers. In addition, lubrication protects the bearing against corrosion and wear, helps dissipate heat, helps seal out solid and liquid contamination, and reduces bearing noise. A properly lubricated bearing has the best chance of reaching its maximum service life.
As with bearings themselves, there are numerous causes for lubricant failure, including:
Insufficient lubricant quantity or viscosity
Deterioration due to prolonged service without replenishment
Contamination with foreign matter
Use of grease when conditions dictate the use of static or circulating oil
Incorrect grease base for a particular application
Rolling element bearings depend on the continuous presence of a very thin – millionths of an inch – elastohydrodynamic film of lubricant between rolling elements and raceways, and between the cage, rings and rolling elements. Lubricant-related failures can be avoided by selecting a grease or oil that generates a sufficient film to keep bearing elements separated. A good lubricant also provides good boundary lubrication.
Lubricant failures can be detected by the presence of discolored (blue/brown) raceways and rolling elements. Excessive wear on rolling elements, rings and cages follows, resulting in overheating and subsequent catastrophic failure.
In addition, if a bearing has insufficient lubrication, or if the lubricant has lost its lubricating properties, an oil film with sufficient load-carrying capacity cannot form. The result is metal-to-metal contact between rolling elements and raceways, leading to adhesive wear.
Adhesive wear modes include scoring, galling, seizing and scuffing. These failures result when the lubricant film is too thin to prevent the welding together of microscopic projections at the sliding interfaces between two mating parts. After the projections weld together, sliding forces tear the metal from one surface, creating minute cavities on one surface and projections on the other. These defects lead to even more damage. Although adhesive wear starts on a microscopic level, it progresses steadily once it starts.
Lubrication obviously plays a vital role in the performance and life of a bearing. Without lubrication, bearings can be expected to fail early and possibly cause other equipment to fail. The three main concerns with bearing lubrication are:
Specifying the right amount – Rolling element bearings operate at their optimum temperature when the minimum amount of lubricant is used. The quantity of lubricant required also depends on the other functions it must perform, such as cooling and sealing.
Specifying the right type – Rolling element bearings can be lubricated with grease or oil. In special cases, solid lubricants can be used.
Keeping the lubricant clean.
The choice of lubricant depends on conditions such as operating temperature, rotating speed, loads and the environment. Generally, oil is the best bearing lubricant, but it is not always practical because of design considerations.
Grease lubrication should be used when the bearing operates under normal speeds and temperatures. Grease has several advantages over oil, including simpler and less expensive application procedures, better adhesion, and improved protection against moisture and contaminants.
Grease selection varies with the application. Factors to consider include hardness (consistency), stability (ability to retain consistency) and water resistance (emulsification). However, grease is oil suspended in a base or carrier, and when these bases are exposed to moisture or heat, they can turn into soap or carbon ash. Therefore, it may be necessary to use synthetic additives to prevent deterioration of the base.
Overfilling may cause a rapid rise in temperature, particularly at high speeds because the rolling elements have to push the grease out of the way. This leads to churning in the grease, which produces heat. Adding more grease only worsens the problem, creating the risk of blowing out a seal.
Bearings operating at slow speeds, and those requiring corrosion protection, can have their housing completely full of grease. The length of time that a grease-lubricated bearing will operate satisfactorily without relubrication depends on bearing size, type, speed, operating temperature and the grease used.
Oil is the preferred lubricant when speed or operating conditions preclude the use of grease or where heat must be transferred from the bearing. Oil is often used to meet the operating requirements of other components such as seals and gears.
Oil bath systems are suitable for low shaft speeds. To avoid frequent oil changes due to high operating temperatures, an oil circulation system can be used. At high shaft speeds, oil must penetrate the interior of the bearing to remove excess heat. An oil injection system is an effective method to ensure that oil gets to where it is needed. The speed of oil being injected must be high enough to ensure that sufficient oil penetrates the air vortex created during bearing rotation.
The frequency at which oil needs to be changed depends on the operating conditions and the oil quality. For oil bath systems, oil should be changed more often if its temperature exceeds 120 degrees Fahrenheit, or if the machine operates in an environment containing abrasives or contaminants. For circulating oil systems, oil change intervals are determined by checking oil quality to determine the presence of abrasive particles, oil oxidation and additive breakdown.
Oil viscosity is just as important as oil quantity to ensure adequate lubrication. Required viscosity depends on operating temperature. Inadequate lubricant viscosity appears as a highly glazed or glossy surface. As damage progresses, the surface appears frosty and eventually spalls. This type of spalling is fine-grained compared to the more coarsely grained pattern produced by fatigue failure.
In the frosty stage, the fine slivers of metal pulled from the raceway create a “nap” that can sometimes be felt. The frosted area will feel smooth in one direction but have a distinct roughness in the other. As metal is “pulled” from the surface, pits appear and frosting advances to pulling.
Smearing is a form of surface damage caused when two surfaces slide and the lubricant cannot prevent adhesion of the surfaces. Minute pieces of one surface are torn away and rewelded to either surface.
Skid smearing occurs when rolling elements slide as they pass from the unloaded zone to the loaded zone. Too stiff a lubricant also causes this type of damage, which is most likely to happen in large bearings.
Smearing can occur on the roller surfaces and in the raceways of spherical and cylindrical roller bearings. It is caused by roller rotation being retarded in the unloading zone, where the rings do not drive the rollers. Consequently, their rotational speed is lower than when they are in the loaded zone. The rollers, therefore, are subjected to rapid acceleration, and the resultant sliding is so severe that it may produce smearing.
Grooving also results from inadequate lubrication. The areas subject to sliding friction, such as locating flanges and the ends of rollers in a roller bearing, are the first parts affected.
Where speeds are high, inertial forces become important, and the best lubrication is required. Inertial forces acting on the rolling elements at high speed, with sudden starting and stopping, can result in high forces between rolling elements and the cage. The forces skew the cage, and repeated force application eventfully causes the cage to crack and break.
Inadequate flow in a circulating oil system can cause failures of tapered roller bearings. The area between the guide flange and the large end of the roller is subject to sliding motion and is more difficult to lubricate than areas under rolling motion. Therefore, some of the rollers can weld themselves to the guide flange.
Even under the best operating conditions, bearing seals can leak. Typically, seal leaks can be traced to three basic causes:
When a seal leaks, the most common cause is a worn or scored shaft, most often the result of abrasion caused by contamination. The shaft should be replaced or repaired to prevent seal damage.
If the seal’s main function is to exclude foreign matter, the seal lip should face toward the dirt instead of the bearing. If the lubricant is under pressure from heat buildup or other sources, the lip should face the pressure.
A leaking seal is not always an indicator of seal failure. In pillow block bearings, for example, excess grease is often purged through the seals during operation. This indicates that the seal is working properly.