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Lubricant Failure = Bearing Failure - 2012-07-16
Rolling part bearings are highly reliable parts, and also the overwhelming majority of bearings can outlive the equipment on that they're put in. However, whereas bearings account for a comparatively little share of all equipment breakdowns, they are doing fail sometimes. And once they fail, it's sometimes a crucial event, leading to pricey repair and downtime.
When bearings fail, solely atiny low variety of failures are caused by material fatigue. Most failures are caused by a condition that sometimes are often prevented. Typically, the causes of bearing failure are often traced to:
Harsh operating conditions
Faulty storage, handling and installation
This article focuses on bearing failures related to lubrication issues.
Avoiding Lubricant Failures Bearings fail for several reasons, however improper lubrication is at the highest of the list, in keeping with a spread of studies:
Improper lubrication: forty to fifty %
Improper mounting: twenty five to thirty %
alternative causes: approximately twenty %
Reaching the natural fatigue limit: but ten %
Anti-friction bearings should be lubricated to stop metal-to-metal contact between the rolling parts, raceways and retainers. additionally, 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 most effective probability of reaching its most service life.
As with bearings themselves, there are varied causes for lubricant failure, including:
Insufficient lubricant amount or viscosity
Deterioration attributable to prolonged service while not replenishment
Contamination with foreign matter
Use of grease when conditions dictate the utilization of static or circulating oil
Incorrect grease base for a specific application
Rolling part bearings depend upon the continual presence of a really skinny ¨C millionths of an in. ¨C elastohydrodynamic film of lubricant between rolling parts and raceways, and between the cage, rings and rolling parts. Lubricant-related failures are often avoided by choosing a grease or oil that generates a sufficient film to stay bearing parts separated. sensible|an honest|a decent} lubricant conjointly provides good boundary lubrication.
Lubricant failures are often detected by the presence of discolored (blue/brown) raceways and rolling parts. Excessive wear on rolling parts, rings and cages follows, leading to overheating and subsequent catastrophic failure.
In addition, if an impression has insufficient lubrication, or if the lubricant has lost its lubricating properties, an oil film with sufficient load-carrying capability cannot kind. The result's metal-to-metal contact between rolling parts and raceways, resulting in adhesive wear.
Adhesive wear modes embody scoring, galling, seizing and scuffing. These failures result when the lubricant film is just too skinny to stop the welding along of microscopic projections at the sliding interfaces between 2 mating elements. once the projections weld along, sliding forces tear the metal from one surface, making minute cavities on one surface and projections on the opposite. These defects result in even a lot of harm. though adhesive wear starts on a microscopic level, it progresses steadily once it starts.
Lubricant choice Lubrication clearly plays an important role within the performance and lifetime of an impression. while not lubrication, bearings are often expected to fail early and presumably cause alternative equipment to fail. The 3 main considerations with bearing lubrication are:
Specifying the proper quantity ¨C Rolling part bearings operate at their optimum temperature when the minimum quantity of lubricant is employed. the amount of lubricant needed conjointly depends on the opposite functions it should perform, like cooling and sealing.
Specifying the proper kind ¨C Rolling part bearings are often lubricated with grease or oil. In special cases, solid lubricants are often used.
Keeping the lubricant clean.
The choice of lubricant depends on conditions like operating temperature, rotating speed, masses and also the setting. Generally, oil is that the best bearing lubricant, however it's not invariably sensible as a result of style concerns.
Grease lubrication ought to be used when the bearing operates below traditional speeds and temperatures. Grease has many blessings over oil, as well as less complicated and fewer expensive application procedures, higher adhesion, and improved protection against moisture and contaminants.
Grease choice varies with the appliance. Factors to contemplate embody hardness (consistency), stability (ability to retain consistency) and water resistance (emulsification). However, grease is oil suspended during a base or carrier, and when these bases are exposed to moisture or heat, they will grow to be soap or carbon ash. Therefore, it should be necessary to use artificial additives to stop deterioration of the bottom.
Overfilling might cause a speedy rise in temperature, notably at high speeds as a result of the rolling parts need to push the grease out of the manner. This results in churning within the grease, that produces heat. Adding a lot of grease solely worsens the matter, making the danger of blowing out a seal.
Bearings operating at slow speeds, and people requiring corrosion protection, will have their housing fully packed with grease. The length of your time that a grease-lubricated bearing can operate satisfactorily while not relubrication depends on bearing size, type, speed, operating temperature and also the grease used.
Oil is that the most well-liked lubricant when speed or operating conditions preclude the utilization of grease or where heat should be transferred from the bearing. Oil is commonly used to satisfy the operating needs of alternative parts like seals and gears.
Oil bathtub systems are appropriate for low shaft speeds. To avoid frequent oil changes attributable to high operating temperatures, an oil circulation system are often used. At high shaft speeds, oil should penetrate the inside of the bearing to get rid of excess heat. An oil injection system is an efficient methodology to confirm that oil gets to where it's required. The speed of oil being injected should be high enough to confirm that sufficient oil penetrates the air vortex created throughout bearing rotation.
The frequency at that oil has to be modified depends on the operating conditions and also the oil quality. For oil bathtub systems, oil ought to be modified a lot of usually if its temperature exceeds a hundred and twenty degrees Fahrenheit, or if the machine operates in an setting containing abrasives or contaminants. For circulating oil systems, oil modification intervals are determined by checking oil quality to work out the presence of abrasive particles, oil oxidation and additive breakdown.
Lubricant Viscosity Oil viscosity is simply as vital as oil amount to confirm adequate lubrication. needed viscosity depends on operating temperature. Inadequate lubricant viscosity seems as a highly glazed or shiny surface. As harm progresses, the surface seems frosty and eventually spalls. this sort of spalling is fine-grained compared to the a lot of coarsely grained pattern made by fatigue failure.
In the frosty stage, the fine slivers of metal pulled from the raceway produce a ¡°nap¡± that may typically be felt. The frosted space can feel swish in one direction however have a definite roughness within the alternative. As metal is ¡°pulled¡± from the surface, pits seem and frosting advances to pulling.
Smearing may be a type of surface harm caused when 2 surfaces slide and also the lubricant cannot forestall adhesion of the surfaces. Minute items of 1 surface are torn away and rewelded to either surface.
Skid smearing happens when rolling parts slide as they pass from the unloaded zone to the loaded zone. Too stiff a lubricant conjointly causes this sort of injury, that is possibly to happen in massive bearings.
Smearing will occur on the roller surfaces and within the raceways of spherical and cylindrical roller bearings. it's caused by roller rotation being retarded within the unloading zone, where the rings don't drive the rollers. Consequently, their rotational speed is below once they are within the loaded zone. The rollers, therefore, are subjected to speedy acceleration, and also the resultant sliding is thus severe that it should turn out smearing.
Grooving conjointly results from inadequate lubrication. The areas subject to sliding friction, like locating flanges and also the ends of rollers during a roller bearing, are the primary elements affected.
Where speeds are high, inertial forces become vital, and also the best lubrication is needed. Inertial forces functioning on the rolling parts at high speed, with sudden beginning and stopping, may end up in high forces between rolling parts and also the cage. The forces skew the cage, and repeated force application eventfully causes the cage to crack and break.
Inadequate flow during a circulating oil system will cause failures of tapered roller bearings. the world between the guide flange and also the massive finish of the roller is subject to sliding motion and is tougher to lubricate than areas below rolling motion. Therefore, a number of the rollers will weld themselves to the guide flange.