Proper lubrication for rolling bearings is essential for reliable operation. Given that some leading bearing companies have stated that incorrect lubrication can account for more than 30 percent of bearing failures, lubrication is a key influence that can make or break bearing service and life.
The lubricant provides a separating film between the bearing rolling elements, raceways and cages to prevent metal-to-metal contact. By controlling surface contact, the lubricant is able to minimize the effect of surface contact, namely undesired friction that otherwise would generate excessive heat, metal fatigue and wear. The lubricant must also prevent corrosion and contamination damage.
Grease has become a common lubricant choice for rolling element bearings. The practical benefits are apparent: grease is easy to apply, can be retained within a bearing’s housing and offers protective sealing capabilities.
Correct lubrication is critical to bearing performance. There are a range of options depending on the application listed below.
- Mineral or synthetic based lubricants are the most commonly used and are designed for general and high speed use. Finely filtered versions are used for low noise applications. There are versions that are water resistant, offer low or high temperature capabilities.
- Silicon lubricants have wide temperature ranges and change viscosity less with temperature. They also have good water-resistance and are safe to use with most plastics. They are not suitable for high loads and speeds.
- Per fluorinated lubricants or PFPE lubricants are non-flammable, oxygen compatible and highly resistant to many chemicals. They do not react with plastics or elastomers. Many have low vapour pressure and are suitable for vacuum or clean-room applications. Some can also withstand temperatures of up to 300°C.
- Dry lubricants are often specified where standard lubricants may cause contamination such as vacuum environments. Dry coatings such as molybdenum disulphide or tungsten disulphide are often burnished on to the balls and raceways of bearings to give smooth operation and higher running speeds than unlubricated bearings. These coatings are also resistant to water and dilute acids.
- Dampening greases are widely used in automotive parts to prevents rattles and squeaks. They are also used to give a “quality” feel to switches, slides, threads and gears. They can be used in slow rotating bearings in, for example, potentiometers for the same reason.
- Food grade lubricants are required for the food and beverage industries to conform to strict hygiene regulations. HI approved lubricants are required for bearings were there may be incidental contact with food and H2 approved greases are used where there is no contact. These greases are also designed to be highly resistant to being washed out by cleaning processes.
Bearing Lubrication Methods
Grease is normally applied with special lubricating equipment with a head that deposits the grease between the balls forcing it into, and around, the ball (or roller) raceway interface. Upon rotation the grease is distributed within the bearing. Miniature and instrument series bearings are typically lubricated in a clean room environment.
The amount of grease is typically specified as a percentage, such as 30% fill. The percentage represents the actual grease volume compared to the free internal space within the bearing. In other words, if the internal space in the bearing is completely filled with grease with no voids, it would be 100% full. Manufacturers of bearings have different amounts that they would consider standard. This typically ranges from 20% to 40%. In small or miniature bearings grease fill amounts can be as little as 10%.
Oil is applied by the manufacturer with special equipment as well. The amount is not normally specified. In torque sensitive applications excess oil can be removed via centrifuging. This is more common with miniature or instrument type bearings.
Grease Plating (MGGP) – The MGGP process starts with carefully mixing the candidate grease with a volatile solvent. The consistency of the grease is thinned considerably. This mixture can be controlled to achieve different coating thicknesses. The mixture is then injected into the bearing coating all of the internal surfaces. A dipping process is also used in some cases. The solvent is then baked off at low temperature. This method is often used in torque sensitive applications and lubricant migration associated with oil lubrication is an undesirable.