Question

Define lubricants and lubrication. Mention the various mechanisms involved in lubrication of machine. Discuss boundary lubrication.

Answer 1

Any substance placed between two moving or sliding surfaces with a view to reduce the frictional resistance between them is known as lubricant.

Lubricants may be used in solid, liquid or semi solid form. The process of reducing friction between two metallic sliding surfaces by the introduction of lubricants is called as lubrication. 

BOUNDARY or THIN- FILM LUBRICATION.

Boundary lubrication occurs when the film thickness between two surfaces in relative motion is so thin so that the formation hydrodynamic oil film is not possible and result in direct metalmetal contact. Such a situation may arise when.
1. The load is very high.
2. A shaft starts from rest.
3. Low speed resulting in insufficient supply of oil.
4. Viscosity of the oil is too low.
In boundary lubrication the space between the metal surface is lubricated with oil lubricants a thin layer of which is absorbed chemically or physically and avoid direct metal to metal contact. The thin film will have thickness around 10 A⁰ and consists of one or two molecular layers. The coefficient of friction is about 0.05 to 0.15. the load is carried by the layer of the absorbed lubricant or both metal surfaces. The extent with which the lubricant gets adhered to the metal surface depends on oiliness of lubrican .

Mineral oils blended with vegetables and animal oils, graphite molybdenum disulphide etc are useful for boundary lubrication.

Gears, railways track joints, tractors, rollers etc are provided with this type of lubrication.
For boundary lubrication to be effective molecules should have:-
1. Long hydrocarbon chains.
2. Polar groups to promote spreading and orientation over the metallic surfaces at high pressure.
3. Lateral attraction between the chains.
4. Active groups or atoms that can form chemica/linkages with the metals or other sufaces.