A lubricant reduces friction of the metal surfaces. It keeps metal surfaces clean by carrying away possible deposits and providing a seal to keep out dirt. A lubricant carries away the heat generated in bearings and gears, etc. preventing overheating, seizure and possible breakdown.
Desirable Properties of Lubricating oils are:
Viscosity is the resistance of a fluid to motion. It is an internal friction. The load carrying capacity of the bearing depends on the viscosity of the lube oil, speed of rotation of the journal, and the load on the bearing.
1.Viscosity of the lubricant decreases with increase in temperature.
2.Heat generated in the bearing is proportional to viscosity.
3.The temperature rise in the bearing is proportional to the heat generated and inversely proportional to the heat flow from the bearing.
4.Rate of lubricant flow depends on viscosity. Hence viscosity is indirectly related to the temperature rise in the bearing.
5.Correct bearing clearance ensures the required flow of lubricating oil through the system in maintaining the bearing and the shaft at right temperature and lube oil at right viscosity.