Many variables influence the coefficient of friction between two rubbing surfaces, the most important being the shape and nature of the surfaces, their smoothness, the clearance between the journal and bearing, as well as the viscosity, film-forming tendency, oiliness, friction speed, bearing pressure, lubricant supply method, and temperature of the oil.
The purpose of this article is to present the most accurate information on lubrication mechanisms in order to predict their exact effects under any given set of conditions.
The following are the mechanisms of lubrication:
- Thick-Film lubrication (called hydrodynamic or Fluid-Film lubrication)
- Thin Film lubrication (Boundary lubrication)
- Extreme Pressure lubrication
There is a thick film of fluid between moving/sliding surfaces (at least 1000 A° thick) which prevents direct contact between moving surfaces and the welding of junctions between them.
Lubricant films provide lubrication between moving/sliding surfaces by covering/filling the irregularities, preventing direct contact between them. Friction is therefore reduced as a result.
Under operating conditions, the selected lubricant should have the lowest viscosity possible (to reduce resistance between lubricant particles), while remaining in place and separating the surfaces at the same time.
Thick-film lubricants commonly use hydrocarbon oils (low-molecular-weight hydrocarbons containing 12 to 50 carbon atoms) because of their properties.
A long-chain polymer is blended with ordinary hydrocarbon lubricants to maintain the viscosity of the oil throughout the year.
Thin Film lubrication
Whenever a continuous film of lubricant cannot persist, this type of lubrication is preferred. A material that can be adsorbed by either physical or chemical forces can lubricate the clearance space between moving or sliding surfaces.
As long as the peaks on the surface are high enough, the adsorbed film protects the metal surfaces from each other.
These types of lubricants are available from vegetable and animal oils, which can be physically adsorbent or chemically react with the metal surface to form a thin film of metallic soap.
The disadvantage of these oils is that they will break down at high temperatures, despite their good oiliness.
Mineral oils, on the other hand, have high thermal stability, and by adding vegetable/animal oils to mineral oils, the oiliness of the minerals is also increased. Thin-film lubrication can also be performed with graphite and molybdenum disulfide.
Extreme Pressure lubrication
The third one under the category of the mechanism of lubrication is extreme pressure lubrication. Under conditions where moving/sliding surfaces experience very high pressures and speeds, lubricants may not adhere to these surfaces and may even decompose.
Mineral oils are enhanced with special additives to meet these extreme pressure conditions. Additives designed to withstand extreme pressure are called extreme pressure additives. In addition to forming durable films on metal surfaces, these additives can withstand very high loads and temperatures as well.
The additives that play an important role are organic compounds that have active radicals or groups, such as chlorine (in chlorinated esters), sulfur (in sulfurized oils), or phosphorus (in tricresyl phosphate). When these compounds are present at high temperatures and react with metallic surfaces, they can produce metal sulfides, metal phosphides, or metallic chlorides.