Q. What are the functions of a lubricant? Describe the various types of lubrication.
Discuss the types of mechanism of lubrication.
Ans. Function of lubricants:
- The foremost important function of a lubricant is reducing friction.
- It reduces wear and tear and surface deformation.
- It acts as a coolant and carriers away the destructive heat produced due to friction.
- It acts as a seal around piston ring. ICE and cylinder wall, thereby preventing the leakage of gases under high pressure from the cylinder are also scaling of bearing against the entry of drift.
- It reduces expansion of metals by friction heat.
- It reduces unsmooth relative motion of the moving sliding parts.
- It reduces maintenance and running cost of the machine.
- It absorbs shocks between bearing and other engine parts thereby reducing noise.
- It also carriers away contaminated products. Later, they can be removed from the system by oil draining or by oil purification system.
Types of lubrication:
1. Thin layer mechanism or boundary lubrication: Following are some conditions under which a continuous lubricant film cannot persist and direct contact of metallic surfaces is possible:
(a) When a shaft starts from its rest state.
(b) When a shaft is moving with slow speed or under high loads.
(c) When viscosity of oils is too low.
For such conditions, a thin layer of lubricant is used which is absorbed on the metal surface by physical and chemical forces on both metallic surface. The long hydrocarbon chain is oriented outwards in almost a perpendicular direction in the monolayer of oil. Multilayer absorption then occurs over the monolayer. The coefficient of friction is such a case is usually 0.05 to 0.15. Thin film lubrication depends on the metallic surface where it is used, the load and the oiliness of the lubricants. For a good boundary lubricant, its molecules should have the following features:
- Long hydrocarbon chains.
- Lateral attraction between the chains.
- Polar groups to promote wetting and spreading over the metallic surface at high pressure.
- Active atoms or group which can form chemical bonds with the metals or other surfaces.
- High viscosity index.
- Heat and oxidation resistance.
- Good oiliness.
Following are the lubricants used for the boundary lubrication:
Solid lubricants, greases and mineral oils: These are thermally stable but possess poor oiliness which is improved by adding small amount of fatty acids or fatty oils.
Vegetable and animal oil and their soaps: They possess oiliness but poor stability because they decompose at high temperature. They are also used as additives to improve oiliness of mineral oils.
Graphite and molybdenum disulphide either alone or as stable suspensions: They can bear compression as well as high temperature and posses' low internal friction.
Fluid film or thick film or hydrodynamic lubrication: A thick film of lubricant is interposed between the two surfaces so that direct surface to surface contact and welding: junctions do not occur. This consequently reduces friction and prevents interlocking and wear also. The thick film is of lubricant films the valleys and covers the asperities. So that there is some internal resistance to the moving surfaces. But there is some internal resistance to the movement of sliding or rolling parts due to the internal resistance between the particles of the lubricant moving over each other. In general friction depends upon viscosity, thickness of the lubricant, the relative area and velocity of moving or sliding surfaces. For example, hydrodynamic friction occurs, in the case of a shaft running at a fair speed as well as in well lubricated bearing with not too high load. This type of lubrication is suitable for delicate instruments, light machines, like watches, clocks, guns, sewing machines, scientific instruments etc. Hydrodynamic lubrication is satisfactorily carried out by hydrocarbon oils. These are generally blended with selected long chain polymers in order to keep the viscosity of oil constant in all seasons of the year as viscosity changes with temperature.
Extreme pressure lubricants: When the moving or sliding surfaces are under very high pressure and speed, the lubricants may decompose or vaporise. They fail to stick to the surfaces because high local temperature is produced between surfaces due to friction heat which causes following harmful effect:
1. Seizers and deformation take place.
2. Formation of welded junction and metal tearing.
To overcome, above extreme pressure difficulties, special additive called" extreme pressure additives" are added to the mineral oil, so that lubricant sustain the extreme pressure and temperature and continue to maintain lubricant characteristics. Extreme pressure additives are the compounds having active groups such as
(a) Chlorine (chlorinated ester)
(b) Sulphur (sulphurized oil) and
(c) Phosphorous (tricresyl phosphate).
At high temperature, they react with metal giving surface layers like metallic chlorides, sulphides and phosphates. These metallic compounds have high melting points (ranges from 650-12000C) and serve as good lubricants under pressure and pressure and temperature conditions.