Lubrication serves the primary purpose of reducing friction between contacting materials and limiting wear and tear. Lubricants can take the form of either solid or liquid, grease or oil respectively. Note: The only difference between grease and oil is that grease has a thickening agent. The study of lubrication has its own field within Tribology, which is the study of relative motion between contacting surfaces. In this post, we will cover the 3 types of lubrication regimes
What is a Lubrication Regime?
Lubrication regimes describe what type lubrication is created or presented under certain operating conditions, dependent on the level of contact between the two surfaces.
The Stribeck Curve and The 3 Types of Lubrication Regimes
A fundamental concept within Tribology is the Stribeck Curve (see below). The Stribeck Curve introduces a dimensionless lubrication parameter called the Hersey Number. The equation for the Hersey Number is below:
Hersey Number = (η.N)/P
- η = dynamic viscosity of the fluid
- N = Entrainment speed (the speed the lubricant is drawn into contact. It can be calculated by obtaining the mean speed of the two surfaces with respect to the contact)
- P = Normal load per length of contact
Therefore, the Stribeck Curve shows the relationship between speed and the coefficient of friction, for a given dynamic viscosity and load. Additionally, the x-axis of the curve is commonly labelled λ which is the ratio of lubricating film thickness and surface roughness.
The Stribeck curves axis are labelled as follwed: x-axis: Hersey number or λ (ratio of lubricating film thickness and surface roughness) and y-axis: Coefficient of friction. There are 3 types of lubrication regimes which can be identified by the Stribeck curve.
- 1) Boundary lubrication
- 2) Mixed lubrication
- 3) Hydrodynamic lubrication
3 Types Of Lubrication Regimes
Boundary Lubrication
We can see from the Stribeck curve that boundary lubrication (1) has a high coefficient friction, which is the main characteristic of the lubrication regime. In this lubrication regime, there is contact between surface asperities which supports the load.
Boundary lubrication occurs when there is a high contact load or very low entrainment speeds, which results in insufficient hydrodynamic forces in the lubricant to support the load, resulting in asperity contact. Boundary lubrication can also occur in systems that generally operate with a thick lubrication film.
It occurs when the system is shut down, and the surfaces become stationary. During this process, the lubricant is forced slowly out of the contact. Upon start-up of the system, high levels of friction and wear will be witnessed.
As a result of the excessive friction and wear seen in the boundary lubrication regime, efforts have been made to minimise this. The main strategies for achieving this is by adding additives to the lubricant and surface coatings.
Introducing surface coatings helps minimise wear. This is achieved by using coatings that have excellent anti-wear performance whilst also providing a low shear strength interface for the contacting surfaces.
An example of lubricant additives used is extreme pressure (EP) additives. These additives will react with the contacting surfaces under extreme operating conditions in the contact zone, producing a low shear strength compound allowing a lubricant to be located precisely where required.
Mixed Lubrication
As we move into the second lubrication regime, we can see a significant drop in the coefficient of friction. The key driver in this is either an increased layer of lubricating film, reduction in surface roughness, reduction in load or a greater entrainment speed. An increase in lubrication film thickness can result in the surface asperities being submerged in some areas by the lubricant.
As the name suggests, it is believed to be operating in this regime there is an element of both boundary and hydrodynamic lubrication. As a result, the load is supported by a combination of both asperity contact and the lubricant.
However, the level of asperity contact seen in boundary lubrication is reduced, which is the major contributing factor to a reduction in the coefficient of friction.
Hydrodynamic Lubrication
As we move into regime 3, we enter hydrodynamic lubrication. Hydrodynamic lubrication conditions consist of the two surfaces being separated by a relatively thick lubricating film. The hydrodynamic pressure generated by the film is what supports the load.
For hydrodynamic lubrication to occur, the two surfaces separated by the lubricant must have an extremely high geometric conformity. This means that the dimensions of the two surfaces must be almost identical and a very close match.
A great example of this is a rotating shaft and a plain journal bearing. For the lubricant to produce a hydrodynamic pressure between the two surfaces, the gap between them must converge and create a wedge. However, the angle and partition between the two surfaces tend to be extremely small in real cases.
Looking back at the previous example of a shaft and a journal bearing, the mean film thickness will be in the order of one-thousandths of the shaft diameter. In addition to this, the difference between the maximum and minimum film thickness could be up to a factor of 5.
The lubrication regime occurs once a system has started and is up and running. The speeds and loads allow a wedge of oil to be present between the two surfaces, minimising the risk of asperity contact. But there is still friction? Friction remains within the system however the friction is found in the lubricant.
The lubricants used for this lubricating regime must have sufficient viscosity to maintain the hydrodynamic effect even with a change in operating conditions, i.e. high load, low speeds.
A lubricant with a viscosity too high will result in excessive drag, therefore, creating elevated temperatures. A lubricant too viscous will result in the λ reducing and introducing asperity contact.
Elastohydrodynamic lubrication
What if the two surfaces and contact they create isn’t conformal? The pressures seen locally at the contact will be a lot greater than those witnessed during hydrodynamic lubrication. This is when elastohydrodynamic lubrication occurs.
What is a non-conformal contact? As discussed above, conformal contact is when the two contact surfaces have an extremely similar geometric profile e.g. a shaft and plain bearing. Therefore, a non-conformal contact is when the two surfaces are distinctly dissimilar geometrically.
Examples of non-conformal contacts are point contact between gear teeth and a ball in a bearing race. For the image below (meshing gear teeth), we can see that the profile of the teeth does not match at the point of contact, giving a non-conformal contact.
In these examples, the point of contact is extremely small, and the pressures seen at these contacts can be as high as 3-4 GPa.
There are two types of EHL, soft EHL and hard EHL. Soft EHL involves the lubrication of soft contacting bodies i.e. rubber.
Soft EHL involves pressures much lower than those recently discussed above, and only the elastic deformation of the contacting bodies needs to be considered. This helps differentiate it from hard EHL, which occurs when the materials have a high Youngs Modulus.
The lubricant film can be as small as tenths of micrometres in hard EHL, which is extremely thin and would lead to you thinking that asperity contact is possible with a lubricant film this thin.
However, how a lubricant performs under EHL conditions is still not fully understood, and in practice, even with a thin lubricating film of tenths of micrometres contact between asperities can be avoided.
The extremely high pressures seen in hard EHL increase the viscosity of the lubricant at the point of contact. A large increase in the viscosity within the contact zone leads to the lubricant behaving much more like a solid rather than a liquid. At pressures of 500 MPa, the viscosity of the lubricant can increase 20,000 times compared to its viscosity at atmospheric pressure.
Summary
We have covered the 3 types of lubrication regimes of the Stribeck curve and looked into the lubrication regime EHL. What regime the lubricating system operates depends on a range of factors such as the film thickness, surface roughness, load etc. At low film thicknesses (and low Hersey number), we have boundary lubrication, which has characteristics of asperity contact leading to high friction and wear rates.
As we increase the lubricant film, we move into mixed lubrication. In this regime, the load is supported by both asperity contact and hydrodynamic pressures present in the lubricant film. The last regime, hydrodynamic lubrication, relies on only the hydrodynamic pressures in the lubricant supporting the load. However, we still see some friction as a result of the friction present within the lubricant itself.
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