THEORIES OF LUBRICANT

Fluid film lubrication or thick film lubrication or hydrodynamic lubrication .
When a fluid lubricant is present between two rolling and/or sliding surfaces, a thicker pressurized film can be generated by the movement of the surfaces (velocities). The non-compressible nature of this film separates the surfaces resulting in no metal to metal contact.

• The condition in which surfaces are completely separated by a continuous film of lubricating fluid is commonly referred to as Hydrodynamic or Fluid Film Lubrication .Thickness of the film should be less a1000 angestrom.
• 1.The coefficient of friction is very low.
• 2.The lubricant should have minimum viscosity under working condition .
• 3.Hydrocarbon oils are effective lubricant .
• 4.useful in watches,clocks,sewing machines,working under low load and fair speed of parts.

THIN FILM or BOUNDARY LUBRICATION

Boundary Lubrication (sometimes referred to as thin film lubrication) is a condition in which the lubricant film becomes too thin to provide total separation. This may be due to excessive loading, speeds or a change in the fluid’s characteristics. In such a situation, contact between surface asperities (peaks and valleys) occurs. when a continous film of lubricant cannot persist and direct metal to metal is possible due to certain reasons This happen when a shaft starts moving from rest ,or the speed is low , the load is very high ,vicosity of the oil is too low.

Vegetable and animal oils.,stearic acid ,Oleic acid .

Extreme pressure lubrication :
The contact between the metal surfaces increases and more heat is generated due to increased friction .This results in either decompotion or evaporation of liquid lubricant which renders it ineffective .so extreme pressure additives are used along with lubricant .
Additives are chlorine and sulphur in the form of chlorinated waxes and sulphuried fats .
when the moving /sliding surface are under very high pressure and speed ,a high local temperature is attained and under such condition