Lubrication’s principal function is to reduce friction between contacting materials and to limit wear and tear. Lubricants can be either solid or liquid, such as grease or oil. The primary difference between grease and oil is that grease contains a thickening agent. Lubrication is a subfield of Tribology that studies relative motion between contacting surfaces. This blog will take you through the three types of lubrication regimens.
What is a lubrication regime?
Lubrication regimes define the type of lubrication formed or provided under specific operating circumstances based on the amount of contact between the two surfaces.
3 Types of Lubrication Regimes
The Stribeck Curve is a key concept in Tribology. The Stribeck Curve presents the Hersey Number, a dimensionless lubrication parameter.
There are 3 types of lubrication regimes that the Stribeck curve can identify.
1) Boundary lubrication
2) Mixed lubrication
3) Hydrodynamic lubrication
3 Types Of Lubrication Regimes
Boundary Lubrication
The key characteristic of the lubrication regime is the high coefficient of friction of boundary lubrication. There is contact between asperities in this lubricating regime, which sustains the load.
When there is a large contact load or very low entrainment speeds, the lubricant’s hydrodynamic forces are insufficient to maintain the load, resulting in asperity contact. Boundary lubrication can also occur in systems that use a thick lubrication coating.
When the mechanism is turned off, the surfaces become motionless. The lubricant is slowly driven out of contact during this process. High amounts of friction and wear will be observed at system startup.
Efforts have been undertaken to reduce the friction and wear observed in the boundary lubrication regime. The primary methods for accomplishing this are to add additives to the lubricant and surface coatings.
The use of surface coatings helps to reduce wear. This is accomplished by employing layers with high anti-wear performance and a low shear strength interface for the contacting surfaces.
Mixed Lubrication
The coefficient of friction decreases significantly when we enter the second lubrication phase. The primary driver in this is either an enhanced lubricating film layer, a reduction in surface roughness, a reduction in load, or a higher entrainment speed.
As the name implies, there is an element of both boundary and hydrodynamic lubrication working in this regime. As a result, the load is sustained by the contact surface and the lubricant.
However, in boundary lubrication, the amount of contact with sharp edges is lowered, which is the key contributing cause to a decrease in the coefficient of friction.
Hydrodynamic Lubrication
A reasonably thick lubricating coating separates the two surfaces under hydrodynamic lubrication conditions. The load is supported by the hydrodynamic pressure created by the film.
The two surfaces separated by the lubricant must have exceptionally high geometric conformity for hydrodynamic lubrication to occur. This implies that the size of the two surfaces must be almost equal and quite close.
A spinning shaft and a simple journal bearing are excellent examples of this. To establish hydrodynamic pressure between the two surfaces, the space between them must converge and form a wedge. In practice, however, the angle and division between the two surfaces are usually quite modest.
The average film thickness will be one-thousandth of the shaft diameter. Furthermore, the gap between the maximum and lowest film thickness might be as much as a factor of 5.
Once a system has been initiated and is operational, the lubrication regime begins. Because of the speeds and stresses, a wedge of oil might be present between the two surfaces, reducing the possibility of contact between the two. But is there still conflict? Friction persists inside the system, but it is detected in the lubricant.
The lubricants employed in this lubricating regime must be viscous enough to sustain the hydrodynamic effect even when operating circumstances vary, i.e., high loads and low speeds.
A lubricant with an excessively high viscosity may cause excessive drag, resulting in higher temperatures. An overly viscous lubricant will reduce and introduce asperity contact.
Conclusion-
The three types of lubrication regimes on the Stribeck curve have been discussed. The lubricating system’s operating regime is determined by various elements such as layer thickness, surface roughness, load, etc. At low layer thicknesses (and low Hersey number), we get boundary lubrication, which exhibits asperity contact properties, resulting in significant friction and wear rates.
