Boundary film or thin film lubrication
Operating conditions :
This type of lubrication is applied when a continuous fluid-film cannot be maintained and direct metal to metal contact is possible due to certain reasons
Shaft comes into action from rest (as in the case at the starting and stopping positions ) or has intermittent decreases in speed.
The speed is very; low.
The load is very high
If the viscosity of the oil is very low.
Working temperature has large variations.
In such lubrication, it is necessary that a thin layer of oil be adsorbed on rubbing surfaces by physical or chemical or both forces on the metal surfaces this property which is responsible for maintaining a boundary film of oil by adsorption is known as oiliness.
Oil which has more oiliness than another oil of the same viscosity gives a lower coefficient which has been adsorbed on the metal surfaces forming a thin film of metal soaps which acts as s lubricant. This kind of lubrication is known as a boundary or thin film lubrication.
The coefficient of friction under these conditions ranges from 0.05 to 0.015
The boundary lubrication is effective at high temperature as the fatty acids break down at high temperature.
Vegetables and animals oils have greater oiliness than the mineral oils because of their chemical constitution, particularly their fatty acid contents.
In case of vegetable and animal oils, as the operating temperature is raised, a remarkable increase in friction and wear occurs at a temperature near to the melting point of the lubricant film because at the melting point long molecules get disoriented and the boundary film breaks down.
Under the conditions of very loads and temperature, the solid lubricants such as graphite and molybdenum disulphide, either alone or as stable suspension in oil are also used for boundary lubrication.
Oils used: animal or vegetable oils and their soaps are used for effective boundary lubrication the lubricant molecules should have-
- Long hydrocarbon chains
- Polar groups to increase spreading and orientation under high pressure
- The lateral attraction between hydrocarbon
- Good oiliness
- Resistance to heat and oxidation.
- Low pour point
- High viscosity index etc.
It is used in low speed, high load and pressure conditions as in case of axles, bearings, gears, wire ropes, pulleys, tractor rollers, lathes.