The resistance offered by a fix or moving surface to the free movement of another surface which is in close proximity is known as friction. The resistance is developed due to the inter locking of irregularities (ups and downs) of the moving surfaces. Even polished surfaces have irregularities if we observe it through a magnifying lens. This friction causes a lot of wear and tear of moving parts in machineries in actual practice. This also reduces the efficiency of the machine as the large amount of heat produced due to friction effects the machine parts. In order to reduce the frictional effects, chemical substances are introduced between the moving surfaces which are known as lubricants. The important functions of a good lubricant are
1. It prevents a direct contact between the moving surfaces and reduces wear and tear.
2. It reduces loss of energy due to friction.
3. It dissipates the heat produced in the machine thereby reducing the expansion of metals.
4. It optimizes the maintenance cost of machines.
MECHANISM OF LUBRICATION
There are three types of mechanism by which lubrication is done:
(i) Fluid film or Thick-layer or Hydrodynamic lubrication: The sliding / moving surfaces are separated by a thick film of fluid (1000°A), so that the surfaces do not come in to direct contact with each other and welding of joints does not occur. This thick film of lubricant covers all the irregularities on the moving surfaces and hence reduces the damages wear. Thus the resistance of moving / sliding surfaces is reduced to the internal resistances between the particles of the lubricant moving over each other. Therefore the lubricants so chosen should have minimum viscosity under working conditions and at the same time it should remain in place and separate the surfaces. In such a system the friction depends on the viscosity, thickness of the lubricant, the relative velocity and area of the moving / sliding surfaces. Light machines like watches, clocks, guns, sewing machines etc., are provided with this type of mechanism.
Hydrocarbon oils are generally considered as satisfactory lubricants for fluids film lubrication. Petroleum oils are mixed with additives to maintain the viscosity of oil in all seasons.
(ii)Boundary lubrication or Thin film lubrication: This kind of lubricant is employed where a continuous film of lubricant cannot persist and direct contact between sliding surfaces is possible due to some reasons. Typical cases are movement of a shaft from rest, low speed or heavy load and the viscosity of the oil is low. In such conditions a thin layer of the lubricant is absorbed (surface attached) by physical or chemical reaction or both to the metallic surface. The oil film keeps the metallic surface apart. Vegetables and animals oils are usually used for this type of lubrication process as they possess the property of adsorption. For boundary lubrication the lubricant molecules should have long hydrocarbon chains, polar groups to promote spreading over a metallic surface and lateral attraction between chains and active groups which can form linkages with metals. High viscosity-index, resistance to heat and oxidation and pour-point are some of the good qualities of boundary lubricants.
(iii)Extremes pressure lubrication: When the moving surface is under high pressure or temperature, liquid lubricant fails to adhere and may undergo decomposition or vaporization. To prevent this decomposition, special additives are added to minerals oils. These are called extreme pressure additives. These additives form a more durable film on the metal surface capable of withstanding the high pressure and temperature which they are subjected to. Examples of extreme pressure additives are chlorinated ester, sulphurised oils, tricresal phosphate etc. These compounds react with metallic surfaces at high temperature to form metallic chlorides, sulphides or phosphides. These metallic compounds possess high melting point and serve as good lubricants under extreme pressure and temperature. The films are capable of replenishment when they are broken due to rubbing action of moving parts.
Lubricants are classified on the basis of their physical state as
Liquid lubricant: Liquid lubricants reduce the friction and hence reduce wear and tear of the moving part of the machines. They also act as (i) locking medium (ii) sealing agent, and (iii) corrosion preventer. E.g. Animals oils, vegetable oils and petroleum oils. Balanced liquid lubricants have better properties than single lubricants have better properties than single lubricating oil. In order to get enhanced properties, liquid lubricants are mixed with additives.
No single lubricating oil serves as a satisfactory lubricating for many of the modern machines. The properties of lubricant oils (petroleum oils) are improved by adding special additives. The special additives are
(i)Oiliness carriers: e.g. oleic acid, stearic acid, vegetable oil, animal oil.
(ii)Extreme pressure additives: e.g. chlorinated, sulphurised oils.
(iii)Viscosity improvers: e.g. Polusterene, pollsters.
(iv)Pour point depression additives: e.g. chlorinated esters, naphthalene.
(v)Antibiotics: e.g. Phenols, amino compounds.
(vi)Emulsifiers: e.g. Sodium salt of sulphuric acid.
Synthetic lubricants: sometimes blended oils are found unsuitable for machines operating at severe and special conditions. In order to meet the lubricating requirements under such peculiar operating conditions, viscous fluids are prepared from various organic and inorganic substances which are called synthetic acids.
Synthetic lubricants have high thermal stability as well as low freezing point. They are usually inflammable having high flash and fire point. They also have flexible index. Common examples of synthetic lubricants are polyvalence glycols, silicone fluids etc.
Semi-solids-lubricants or greases-Semi-solid lubricants are obtained by dispersion of soap in liquid lubricating oil. The liquid lubricating oil may be petroleum oil or synthetic oil containing additives.
Greases are prepared by saponification of fat with alkalies like NaOH, Ca (OH)2 followed by adding hot lubricating oil with agitation. Grease is a gel and soap acts as a thickening agent. Depending upon the nature of the soap, used, different varieties of greases can be prepared.
1. Calcium based greases or cup greases: These are emulsions of petroleum oil with calcium soap. They are water resistant since calcium soap is insoluble in water. They are cheapest varieties of all the greases and it is stable up to 80°C above which it separates in to oil and soap.
2. Soda based greases: They are petroleum oils thickened by using sodium soaps. They are water soluble as sodium soap is soluble in water. They are stable up to 175°C. Lithium based grease is obtained by mixing petroleum oil with lithium soap. They are usually used to low temperature since they are thermally stable up to 15°C.
Solid lubricant-Commonly used solid lubricants are graphite and molybdenum disulphide.
1. Graphite: Graphite is the most commonly used solid lubricant. It consists of several flat plate like structure and an atom thickness and are held together by weak Vander Waals forces. Carbon atoms are arranged hexagonally in each plate. These plates can slide easily over one another with little friction as they held together by weak forces.
The distance between two typical plates is 3.4°A. Graphite’s are used either in powder form or suspension form. If graphite is dispersed in oil, it is called oil dag and when it is dispersed in water they are called aqua dag. It is used as lubricant in air compressors, lathes, and food stuffs industries, railways track joints, open gears, chains, I.C engines, cast iron bearing etc.
2. Molybdenum disulphide: It has a sandwich like structure in which a layer of molybdenum atoms lies in between two layers of sulphur atoms. All layers are held together by weak Vander waals forces. Like graphite, molybdenum disulphite plates are 3.13°A. It possesses low friction and it is stable up to 400°C. it is used in powder form or it can be mixed with grease and used. It is mainly used in heavy machineries working under heavy load and high temperature.
Properties of lubricating oils
1. Viscosity-Viscosity is the property of a liquid that offers an internal resistance to its own flow. The unit of viscosity is poise or Newton/sec/square meter.
Viscosity can be measured using Redwood viscometer or Laybolt viscometer. Good lubricating oil should have moderate viscosity.
2. Flash and fire point-Flash point is the lowest temperature at which lubricating oil produces sufficient vapors to give flash when the vapors come in contact with a test flame. Fire point is the lowest temperature at which an oil lubricant produces sufficient vapors that is the lowest temperature at which an oil lubricant produces sufficient vapor that burns continuously for at least five seconds. Generally fire point is 5 to 40°C higher than flash point. Flash point is an indication of the vaporization of oil lubricants. A good lubricant of high flash and fire point is essential for machines working at high temperature. Flash and fire points can be determined by Pensky-Martins apparatus.
3. Cloud and pour point-The temperature at which the lubricant oil becomes cloudy or hazy in appearance on cooling is called cloud point. The temperature at which the lubricant oil ceases to flow or pour on cooling is called its pour point. For machines working under low temperature, lubricants with low cloud and pour point are preferred.
4. Oiliness-It is the ability of lubricating oil to stick to the machine parts under heavy load or pressure. Lubricating oil with poor oiliness will be easily squeezed out when machines work under heavy load. Hence good lubricating oil should have good oiliness. Petroleum oil has poor oiliness. Animal and vegetable oils have good oiliness. In order to increase the oiliness of petroleum oils, they should be mixed with animals or vegetables oil or fatty acids like oleic acid, stearic acid etc.
5. Emusification-It is the ability of oil to get intimately mixed with water to form a homogeneous mixture called emulsion. A good lubricant should have emulsification tendency. Since emulsion will collect dirt, dust and other impurities, there will be wear and tear of the machine parts.
6. Neutralisation – Neutralization value or acid value-Neutralization value is the number of milligrams of potassium hydroxide required to neutralize the free acids present in 1 gm of lubricating oil. Presence of free acids in lubricating oil is harmful to the machine parts since it leads to the corrosion. Hence a good lubricating oil should not have free acids, and should have an acid value less than 0.1.
7. Carbon residue-Most of the lubricants consists of long chain carbon atoms. When a machine works under high temperature and pressure, these lubricant molecules undergo decomposition and result in carbon residue. A good lubricant should produce least carbon residue in use. The carbon residue can be estimated by Conradson method.
8. Volatility-A good lubricant possess low volatility which makes the lubricating thermally stable. For machines working at elevated temperature, lubricants with high volatility vaporize easily and effect adversely the lubrication process.
9. Oxidation stability-Lubricants should have good oxidation stability . Oxidation of oils produces gummy or carbon residue which reduces the efficiency of the lubricants. Oxidation can be prevented by adding anti-oxidants with lubricating oils.
Like it on Facebook, +1 on Google, Tweet it or share this article on other bookmarking websites.