Bearings are used to provide stable, low-friction movement for equipment. While each bearing will differ according to its
use and specifications, they all face one common enemy: water contamination.
The use of lubrication serves two main purposes:
(i) aid in decreasing friction losses, and (ii) provide a load bearing film. Both of these characteristics rely on the consistency of viscosity within the oil. Viscosity is a measure of the resistance to flow of a liquid, which at a more fundamental level depends on how strongly the molecules attract each other. Thus, any contaminant that might impact the viscosity of any oil should be monitored extremely closely. The contaminating impacts of water in oil have a direct impact on the viscosity and load bearing capacity of an oil. Water causes increased oxidation or breakdown of the carbon bonds within an oil, which ultimately changes the viscosity of an oil.
The viscosity specified by engineers during the design phase is essential to the smooth operating of any machine. Altering the viscosity, by way of water contamination, can decrease bearing life considerably and reduce overall machine reliability.
Altering Oil Viscosity in Bearing Lubrication
The most common methods for lubrication in bearings consist of hydrodynamic and elastohydrodynamic.
Without the protective lubricating film in hydrodynamic lubrication, particles from the wear induced by contact would insert themselves between the fine tolerances and increase the rates of destruction exponentially.
Hydrodynamic lubrication uses the motion of moving machinery parts to force lubricating oil up and around bearing surfaces. With each cycle of movement, more oil is pushed up and over components and results in a thin layer of lubricating oil which is always intact and constantly flows in and out of the system. The coating prevents the surfaces of the bearings from interacting with one another, resulting in abrasion or more severe particle contamination.
In elastohydrodynamic lubrication, the lubricant flows between two contact surfaces creating a thin load bearing film. As flow pressure increases to the maximum operating pressure, the viscosity of the oil becomes the main driving factor in the lubricating films load bearing ability. This film serves the purpose of protecting the softer bearing materials from abrasion. In both these cases, it is quite easy to see how viscosity plays a central role in bearing life and reliability.
Water Contamination and Corrosion
One of the most serious problems that anti-friction bearings face is that of corrosion, or more accurately etching which evolves into more severe spalling and pitting. Etching involves the degradation of a material surface, such as a bearing raceway or inner ring, due to the presence of oxidising elements such as water. The detrimental impacts of water on bearings can clearly be seen in Figure 1.
Etching in its beginning stages can be used as evidence of water in lubrication oils, and bearings might be able to be refurbished and used, such as in Figure 1a. However, once etching has progressed far enough, it can lead to extreme spalling such as in Figure 1b and render the bearing useless.
Impacts of Water Contamination on Bearing Life
Not only does moisture decrease overall lubricant efficiency and increases wear rates in bearings, but it will also impact the bottom-line of companies and show up as increased maintenance and down-time costs.
Whether from condensation due to temperature fluctuations during storage or from exposure to humid environments, the threat of water contamination is always present. Water also renders some additive packages ineffective while it reacts with others to form varnishes, sediment and other harmful compounds.
Figure 3 shows how increasing water content can decrease bearing life significantly. Similarly, decreasing water content has an exponential impact on the life of bearings.
All of this leads to pitting and particle contamination, which results in increased friction, decreased performance and a reduction in bearing life.
Decreasing Water Contamination Should Be A Top Priority
Water contamination can be just as harmful as solid particulate contamination. Water acts as an oxidising agent, changing the physical properties of lubricating oils, specifically viscosity. Changes in viscosity decrease the load bearing capacity of lubricating films and can result in the breaking of these films.
This break in the film will allow for the interaction between surfaces which should not be in contact. Softer bearing materials will be subject to abrasion and fatigue and particles will break off resulting in contamination and increased wear.
This all contributes to higher maintenance costs and more breakdowns. Thus, decreasing water contamination and ingression sources for bearing lubricants should be a top priority for any industrial facility.