Mechanism of lubrication (Types of lubrication)

Mechanism of lubrication(Types of lubrication)

Fluid film (or) Thick film (or) Hydrodynamic lubrication
Condition - Under low load and high speed
Under the condition of low load and high speed a thick fluid film of lubricant is maintained between the two solid surfaces. The thickness of fluid film is atleast 1000A. Since the thick fluid film separates the two solid surfaces there is no direct contact between solid surfaces this reduces wear and tear. The co-efficient of friction in such is as low as 0.001 to 0.03

Example
Consider the rotation of a shaft with respect to a stationary bearing.
When a lubricant is added to the system, It occupies the annular space between the shaft and the bearing and forms a hydrodynamic wedge so long as the shaft rotates, the hydrodynamic wedge will remain and prevent contact between the two solid surfaces. When the load becomes very high, the lubricant will be sequeezed out of the wedge and friction will occur.

Boundary lubrication (or) Thin film lubrication
Condition: under high load and slow speed
Under the conditions of high load and slow speed, a continuous fluid film cannot be maintained between the moving surfaces. Under such conditions, the thickness of the fluid film should be less than 1000A. Such a thin film, consists of 2 or 3 molecules thick. To form a thin film the lubricant has to be adsorbed on the metal surface by physical or chemical forces. In some cases, the lubricant will react chemically with the metal surface forming a thin film of metal soap, which will act as a lubricant. This thin film is known as boundary film. The co-efficient of friction in such cases is around 0.05 to 1.15.
The effectiveness of boundary lubrication depends on the oiliness of the lubricant. Oiliness is the ability of a lubricant to stick on to the surface. Vegetable oils and their fatty acids have more oiliness. (eg) Oleic acid (C17H33COOH), stearic acid (C17H35COOH) etc. The polar carbonyl group (-COOH ) of these oils reacts with the metal surface to form a continuous thin film of lubricant. Hydrocarbon chain of the fatty acid gets oriented outwards in a perpendicular direction.

Extreme pressure lubrication
Condition: Under high load and high pressure
Under the conditions of high load ( high pressure) and high speed, more heat is generated between the moving surfaces. As a result of this, the liquid lubricant fails to stick and undergoes decomposition or evaporation. Under these conditions, for effective lubrication, special additives known as extreme pressure additives are used along with the lubricants.
Import extreme pressure additives are organic compounds having active radicals or groups such as chlorine (e.g Sulphurized oils) etc. These compounds react with metallic surfaces to form metallic chlorides, sulphides etc. These metallic compounds possess high melting points and serve as good under extreme pressure conditions.