Lubricants, Lubrication:

• The lubricants are defined as, the chemical substances which reduce friction between two sliding/moving metal surfaces and thereby reduce wear and tear of machines.
• The lubricant keeps the two surface apasrt, thus the frictional resistance reduces. This helps in reducing the destruction of material.
• Lubrication is a process by which wear gets reduced, with the use of lubricants.

Boundary-film Lubrication mechanism:

• This type of mechanism is useful under certain working conditions of machines, such as:

1. If viscosity of liquid lubricant oil is low.
2. The machine is to be operated at comparatively low speed.
3. During operation of machine, a shaft starts moving from rest at fixed intervals or
4. The machine is operated under high load.

• A continuous thick film of lubricant cannot persist in between two sliding surfaces under such working conditions. Hence, to reduce friction in such machines a thin film of lubricating oil is introduced in the clearing space of the moving surfaces.
• The film gets adsorbed on metal surfaces due to physical or chemical or both the forces and it is retained there. The coefficient of friction falls to the extent of 0.05 to 0.15 and load on the machine is carried by the adsorbed thin film between the sliding surfaces.
• The layer of lubricant is checked, so that the machine surface remain protected from friction. The lubricants applied are the soaps of vegetable or animal oils, as they possess a great tendency of adsorption on surfaces.
• These soapds get attached to surfaces either by physical forces or by chemical forces and form a very thin film which covers all irregularities of metal surfaces, thereby reducing the frictional wear.
• The soaps are suitable at moderate temperature. But at high temperature, they get decomposed. Hence, these are not suitable to be used in internal combustion engines. Mineral oils blended with fatty oils or fatty acids are used, because the thermal stability of mineral oils is high.
• Other substitute are solid lubricants such as graphite or molybdenum di-sulphides, either alone or their stable suspension in oil, which is also very suitable. These suspensions or solids form a thin film on metal surfaces which can bear load as well as hugh temperatures. eg. mineral oils blended with vegetable and/or animal oils, solid lubricants such as graphite or molybdenum di-sulpide.
• The machines where boundary lubrication is commonly applied are gears, rail axle boxes, tractors, rollers etc.