The Most Common Causes of Bearing Failure and the Importance of Bearing Lubrication0 pages

THE MOST COMMON CAUSES OF BEARING FAILURE AND THE IMPORTANCE OF
BEARING LUBRICATION
RKB TECHNICAL REVIEW - FEBRUARY 2010
1
The Most Common Causes of Bearing Failure
and the Importance of Bearing Lubrication
Ciprian Radu
RKB Bearing Industries - Advanced Software Engineering Unit
Abstract: Bearing failures have great impact on industry and economy. The aim of the present work is to study and spot the major causes that limit bearing efficiency, thus leading to bearing failure. This study shows that the main failure cause is the inappropriate lubrication of the bearing rolling elements (approximately 80% of the cases), followed by inadequate bearing selection (10%), improper mounting (5%), indirect failure (4%), and material defects and manufacturing errors (less than 1%). Key words: Bearing failure, Contamination, Grease, Oil, Lubricant, Lubrication
1. Lubrication of rolling bearings
The rolling bearing is a machinery component that plays a very important role, since it dominates the machine performance. If one of the bearings fails, not only the machine, but also the assembly line stops and the deriving costs may be extremely high. For this reason, every bearing manufacturer should make every effort to ensure the highest quality for each bearing and the most careful use and maintenance on behalf of the user [10].
Lubricants are used between contact surfaces to keep the parts in continuous motion. The main purpose of rolling bearing lubrication is to avoid or reduce the metal-to-metal friction between the rolling and sliding contact surfaces. This is not the only function of rolling bearing lubrication. The supplementary functions are: heat dissipation from the bearing, removal of solid wear particles and contaminants from the rolling contact surfaces, corrosion protection, increase of the sealing effect of the bearing seals [4].
Lubrication is crucial for bearing life. In heavy duty applications, such as rolling mill machines, furnaces, ovens or high temperature fans, rolling bearings may be exposed to higher-than-normal temperatures. For these applications, appropriate selection of the lubricant and lubrication method is very important [13]. In industrial applications there are two types of lubricants suitable for high temperature use: grease and oil. In special cases, bearings are lubricated with solid dry lubricants [4].
Grease lubricant is used for 90% of all rolling bearings. The amount of grease to be used will depend on many factors relating to the dimensional and geometrical parameters of the housing, space limitations, bearing working speed and type of grease used. As a general rule, rolling bearings and their housings should be filled from 30 to 60% of their total capacity. If working speed and temperature rise, then a reduced amount of grease should be used. If the amount of grease is excessive, temperature rises, which may cause the grease to soften and local leakage may appear. With the passage of time grease loses its properties and fresh grease must be resupplied at proper intervals. The relubricating interval depends on the bearing type, dimensional parameters, bearing working speed and temperature [10].
In industrial applications, oil lubrication is used if adjacent machine components are supplied with oil as well, or if generated heat must be dissipated by the lubricant. The heat dissipation is necessary if high rotating speeds and/or high loads are involved or if the bearing is exposed to high temperatures [4]. In this case, the relubricating interval depends on bearing operational conditions (temperature, speed, load etc.), oil quantity and type of oil used. If an oil bath lubrication method is used when operating temperatures are around 50 „aC, then the relubricating interval should be one year. For operating conditions of 70 to 100 „aC, the oil should be replaced every three months. Anyway, it is important that lubrication system and deterioration be verified regularly to determine oil replacement [10].
Another type of bearing lubrication is solid or dry lubrication. This method is used in industrial applications, where rolling bearings are subjected to heavy loads, slow relative movements and high working temperature. Solid lubrication is very effective and yields relatively long running times. The most commonly used solid lubricants are graphite and molybdenum disulphide. These solid lubricants are applied to the raceway surfaces in the form of powder loose, sliding lacquer or paste. Graphite and molybdenum disulphide lubricants can be used for high operating temperatures of up to 450 °C [4].