Mechanism of Lubrication

Mechanism of Lubrication:

(i)                Fluid-film or thick-film or Hydrodynamic Lubrication

(ii)             Boundary Lubrication or thin-film lubrication

(iii)           Extreme Pressure Lubrication

All parts are described below one by one.

(i)                Fluid-film or thick-film or Hydrodynamic Lubrication:

                                                                                                                                     In this the moving surface are separated from each other by a thick-film of fluid(at least 1000Aº thick), So  that the direct contact of two surfaces of metals or  junket may be minimized. As a result wearing and tearing of metals get minimized due to less friction.   The lubrication film fins the irregularities of the moving surfaces and forms a thick layer in-between them, So that there are no direct contact between the metal surfaces. Hydrodynamic friction occurs in case of a shaft running at a fair speed as well as in well-lubricated bearing with not very high load. In a journal bearing a film of the lubricating oil covers the irregularities of shafts as well as the bearing surfaces and the metal surface do not come into direct contact with each other. Thus, the resistance to movement is only due to the internal resistance of the lubricant.

        Such type of lubrication is useful in delicate and light machines like watches, clocks, guns, sewing machines and scientific equipments.
 

Boundary Lubrication:

Boundary lubrication is necessary when fluid film lubrication fails to maintain the lubrication. This happens due to the following reasons:

(i)                 The Load is very high.

(ii)               The viscosity of oil is very low

(iii)             The speed is very slow.

At this stage the lubrication is maintained by boundary film lubrication (thickness less than 1000 Aº). It may be possible that some of the (peaks And valleys) asperities so wearing and tearing take place. Hence it is necessary that a thin layer of oil be absorbed by physical or chemical forces on some metal surfaces, which would avoid the direct contact of metals. The property by virtue of oil sticks to the surface of machine parts even at high temperature and heavy loads is called oiliness. For boundary lubrication the lubricant molecules should have:

(i)                 Long chain hydrocarbon.

(ii)               Resistance to heat and oxidation.

(iii)             Low pour point and good oiliness

(iv)             High viscosity index.

On the basis of above characteristics the following types of lubricants are useful:

(i)                 Mineral oils, which are thermally stable.

(ii)               Solid lubricants graphite or molybdenum disulphide as emulsion in oil.

(iii)             Vegetable and animal oils having greater oiliness than mineral oils.

Extreme pressure Lubrication:

When the moving surfaces are working under very high pressure and high temperature the ordinary liquid lubricants either vapourize or decompose in such case extreme pressure lubrication is done. For this special additives (Extreme pressure additives) are used along with the liquid lubricants chlorinated esters, sulphurized oils are example of extreme pressure additives. These additives combine with the metallic surfaces at high temperature to form metallic chlorides, sulphides or phosphides in the form of durable film. These film can withstand very high loads and high temperature due to their high melting points.