Farm Power And Machinery (1+1)

Properties of lubricating oil

• The different properties of the lubricating oil :

Viscosity
It is the property of a lubricant by virtue of which it offers resistance to flow. The oil should be viscous enough to maintain a fluid film between the two mating surfaces. The viscosity is measured by an instrument called viscometer and expressed in terms of viscosity number. The unit of viscosity in SI system is Newton-second per square meter (N-s/m2) or Pascal seconds (Pas). Viscosity is inversely proportional to temperature i.e., as the temperature increases the viscosity decreases and vice versa.

Flash point and Fire point
When the oil is heated, vapors are released. Flash point is the lowest temperature at which oil is to be heated until sufficient inflammable vapour come off which produces a momentary flash when brought to flame. When the vapours are released continuously and the flame persists for longer period, then that temperature is called fire point. A good lubricant has its flash and fire point above the temperature at which engine work, so that fire hazards are reduced.

Cloud point and Pour point
When lubricating oil is cooled, the temperature at which wax and other substance in the oil crystallize and separate out from oil is called Cloud point. The lowest temperature at which the oil ceases to flow when cooled is called Cloud point. Cloud point and Pour point indicates the suitability of lubricant for use in cold conditions. A good lubricant required to give service at low temperature should possess low pour point and cloud point.

Oiliness and Wettability
The ability of the lubricating oil to adhere to the surface is known as oiliness which depends upon on its wettability and surface tension. A good lubricant should have oiliness enough to adhere to the surface even at very high pressure.

Volatility
A good lubricant should have low volatility at working temperature.

Carbon residue
Lubricating oil has higher percentage of carbon in the combined form. A good lubricant should not deposit carbon when used at high temperature.