Q: Is there only one type of Lubrication regime?
There are actually 4 types of lubrication regimes that equipment can experience and each component usually experiences at least 3 within their lifetime!
As per Noria 2017, the four different types of lubrication are; Boundary Lubrication, Mixed Lubrication, Hydrodynamic Lubrication and Elastohydrodynamic Lubrication.
Essentially, the best type of lubrication regime is Hydrodynamic as it provides the most ideal environment for both the lubricant and the surface. However, there are instances where other types of regimes can exist due to a lack of lubricant or contaminants.
Most components experience Boundary Lubrication on start up where there isn’t a proper layer of lubrication between the two surfaces. As such, the asperities of the two surfaces touch and can create wear. When we look at surfaces under a microscope, we can see tiny asperities (which we can liken to sharp or jagged edges) which are prevalent along the surface.
Even though a surface may appear shiny and smooth, when we microscopically examine them, the actual surface has a lot of asperities. Imagine, if two rough surfaces were sliding against each other, like a piece of sand paper against a wall, eventually parts of the wall will be removed due to the asperities of both the sand paper and the wall.
As more oil is gradually introduced to the component, it begins to experience Mixed Lubrication. In this state, the oil film is still not fully formed and is a bit thicker in some places than others. There are some contact areas where the surfaces will still experience boundary lubrication as well as elastohydrodynamic or hydrodynamic lubrication.
During this period, wear occurs due to the areas that are still experiencing boundary lubrication. However, it can be considered a transition phase as the surfaces move from tone type of regime into another as the lubricant film gradually increases.
When the component is full immersed in the lubricant, it usually achieves Hydrodynamic Lubrication which allows for a full film to be formed between the two surfaces and there is no longer any contact with the asperities. This is one of the most ideal forms of lubrication as it greatly reduces the wear between the two surfaces and the oil film safeguards that these can easily slide over each other thus, decreasing the friction between them. In this type of lubrication, the oil wedge is maintained in all operating conditions and guarantees that the asperities of both surfaces do not interact with each other.
On the other hand, with Elastohydrodynamic Lubrication, something quite special occurs with the lubricant causing the component to deform slightly ensuring that the film is maintained between the two surfaces. This happens at the highest contact pressure and ensures that the asperities do not touch whilst maintaining the oil wedge.
This type of lubrication usually occurs when there is a rolling motion between two moving surfaces and the contact zone has a low degree of conformity (Noria. 2017). Essentially, Elastohydrodynamic lubrication occurs when the lubricant allows the contact surface to become elastically deformed while maintaining a healthy lubricant film between the two contact surfaces.
Noria Corporation. 2017. Lubrication Regimes Explained. https://www.machinerylubrication.com/Read/30741/lubrication-regimes