Insufficient lubrication is a significant cause of bearing failure, accounting for over a third of all such incidents. Many engineers face this issue during the use of bearings.
This article provides knowledge on the diagnosis and analysis of bearing problems related to inadequate lubrication.
Mechanism of Bearing Lubrication Deficiency
Lubrication primarily serves to reduce friction in bearings. Conventional lubricants operate by forming an oil film between the two surfaces in relative motion, isolating the frictional pair and thus minimizing friction. In the case of rolling bearings, this means separating the rolling elements and the raceways.
When the lubricating film completely isolates the rolling elements and the raceway, this is called hydrodynamic lubrication, an optimal lubrication state.
Poor lubrication essentially means that the lubricating film fails to separate the rolling elements and the raceway, resulting in direct metal-to-metal contact during bearing rotation, thus accelerating bearing failure.
For detailed lubrication mechanisms, one can refer to a series of bearing application technology books by the author. This article merely provides a broad description.
Symptoms of Bearing Lubrication Deficiency
As discussed, the essence of poor bearing lubrication is that the oil film does not fully separate the rolling elements and the raceway, implying direct contact between the rolling body and the raceway, and direct metal-to-metal friction within the bearing during rotation.
Under good lubrication, there is no direct metal friction in the bearing, overall friction is low, and consequently, heat generated from friction is minimal.
In the case of poor bearing lubrication, the lubricating film is damaged, leading to direct metal contact, increased friction, and consequently, more heat generation. This results in high bearing temperatures, commonly referred to as bearing overheating.
Other bearing failures can also lead to increased bearing temperatures and overheating. How do these situations differ from the temperature changes caused by pure lubrication deficiency?
Here, pure lubrication deficiency refers to inherent poor lubrication, essentially due to inferior qualities of the lubricating medium, primarily issues with lubricant viscosity. When pure lubrication deficiency begins to occur, there are no other issues with the bearing, hence only abnormal friction between the rolling elements and the raceway surfaces.
This anomaly does not cause severe damage to the bearing surface in the short term; it merely intensifies rolling friction and increases temperature. At this point, an increase in bearing temperature can be observed.
However, as all the components of the bearing are in good condition, no anomalies appear on the characteristic frequencies of the vibrations of all the bearing components. The onsite observation is that while the bearing temperature increases, other state information such as vibrations remains normal.
Failure Analysis of Bearing Due to Poor Lubrication
Poor lubrication results in direct metal-to-metal contact between the bearing rollers and the raceway. As a result, surface-originating fatigue or surface degradation occurs on the interfaces of the bearing rollers and the raceway, as illustrated below:
The surface roughness of the raceway in the figure has changed, initiating surface deterioration of the bearing. If it is purely due to poor lubrication (excluding issues such as contamination), the metal surface may evolve into a state of surface fatigue. Initially, it presents as an uneven mirror-like surface, which eventually results in surface spalling.
Addressing Poor Bearing Lubrication
There are many factors that cause poor bearing lubrication, such as contamination. The poor lubrication discussed in this article refers to purely poor lubrication. The effects of factors such as contamination will be introduced in subsequent articles.
As mentioned earlier, pure bearing lubrication inadequacy mainly occurs when bearing viscosity and other factors cannot meet the requirements for forming a lubricating film. At this point, factors affecting lubrication viscosity must be adjusted based on the circumstances.
When the equipment has just completed the manufacturing stage, poor lubrication may be related to the choice of lubricants. Check the selection of lubricants and the filling process when poor lubrication occurs. Ensure that the choice of bearing lubrication in the equipment is correct and that the oil volume is accurate.
In the case of poor lubrication during equipment operation, where the equipment was in good condition initially and suddenly exhibits signs of poor lubrication, the factors affecting lubrication during this process need to be examined.
For example, whether the temperature fluctuates; whether there is supplementary lubrication; whether contamination has occurred; and whether different types of grease have been mixed.
If signs of improper lubrication appear in the early stage, timely correction of the lubrication issue can prevent premature bearing failure. At this point, the bearing may not have been damaged. Once the lubrication is corrected, all the fault indications can be readily resolved.
