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  1. LUBRICANTS  Friction: Resistance observed between two moving or sliding surfaces creating wear and tear.  Lubricant: Any substance introduced between two moving or sliding surfaces with a view to reduce the frictional resistance between them is know as a lubricant.  Lubrication: The process of introducing lubricant between moving/sliding parts is known as lubrication Dr Prasanna Kumar Sharma
  2. Functions of Lubricant o To reduce frictional resistance between surfaces and reduce deformation, wear and tear between moving/sliding surfaces. o To reduce loss of energy in the form of heat (Coolant). o To reduce waste of energy i.e., to increase efficiency of machines. o To reduce irregular expansion of metals. o To reduce welding of the two surfaces. o To reduce or avoid rough relative motions of moving / sliding parts. o To reduce running and maintenance cost of the machine. o To reduce the leakage of gases under high pressure like a seal or Teflon. Dr Prasanna Kumar Sharma
  3. Mechanisms of Lubrication 1. Fluid-film (or) Thick-film (or) Hydrodynamic lubrication 2. Boundary lubrication (or) Thin-film lubrication 3. Extreme pressure lubrication Dr Prasanna Kumar Sharma
  4. 1. Fluid Film/Thick-Film/Hydrodynamic Lubrication (~ 1000 Å) Characteristics: o The surfaces are separated by a thick-film (at least 1000 Å thick) and hence there is no direct surface to surface contact. o No welding of junctions. o Since thick film lubricant covers/fills the irregularities on the both surfaces, there is no direct contact between material surfaces and so the wear is reduced. Dr Prasanna Kumar Sharma
  5. In Fluid Film Lubrication, the Lubricating properties depend on: o Viscosity of lubricant (Lubricant should be of low viscosity). o Thickness of lubricant layer. o Relative velocity and area of moving/sliding surfaces. Dr Prasanna Kumar Sharma
  6. Example: Shaft running (Journal bearings) Lubricant oil covers the irregularities of the shaft as well as the bearing surfaces. Dr Prasanna Kumar Sharma
  7. o Examples where fluid film lubrication is used are : i) Sophisticated instruments ii) Light machines like watches, guns, sewing machines etc. o Examples for fluid-film lubricants are: Hydrocarbon oils are considered to be satisfactory lubricants.  To maintain viscosity throughout lifecycles long chain polymers are used as blenders with normal hydrocarbons oils.  Small amount of unsaturated hydrocarbons present in hydrocarbon oils produced from petroleum fractions, which causes oxidation and produce gummy like products. Hence, anti-oxidant like aminophenol are used in journal bearings with lubricant Dr Prasanna Kumar Sharma
  8. Mechanism of fluid film lubrication Dr Prasanna Kumar Sharma
  9. 2. Boundary Lubrication/Thin-film Lubrication Characteristics of thin film lubrication are: o High viscosity-index. o Resistance to heat and oxidation. o Good Oiliness. o Low pour-point. Dr Prasanna Kumar Sharma
  10. Mechanism of Boundary Lubrication/Thin-film Lubrication This Lubrication takes place due to: o Adsorption of lubricating oils to both surfaces by physical/chemical means. o The adsorbed layers on the both metal surfaces carry the applied load. o Co-efficient of friction, f = 0.05 - 0.15 and distance between surfaces is to be the order of the distance of the asperities. Dr Prasanna Kumar Sharma
  11. For boundary lubrication, the lubricant molecule should have: (i) Long hydrocarbon chain with polar groups. (ii) Polar groups promote spreading and orientation over the metallic surfaces at high pressure. (iii) Lateral attraction between the chains. (iv) Active groups or atoms, which can form chemical linkages metal or other surfaces. Dr Prasanna Kumar Sharma
  12. Examples of Boundary lubrication  Vegetable and animal oils (glycerides of higher fatty acids & their soaps). o These oils either physically adsorbed to metal surfaces or react chemically at the metal surfaces. o Although these oils posses greater adhesion property, yet they tend to breakdown at high temperatures. Hence, fatty acids are added to improve the oiliness.  Graphite and Molybdenum disulphide alone or oil suspension may be used because: o They have Low internal friction o They can bear/withstand compression o They are thermally stable Dr Prasanna Kumar Sharma
  13. Mechanism of Boundary Lubrication Dr Prasanna Kumar Sharma
  14. 3. Extreme-pressure Lubrication  When moving/sliding surfaces are under very high pressure and speed, a high local temperature is attained.  In such conditions, liquid lubricants fail to stick and may decompose and even vaporize.  To avoid this, special additives are added to mineral oils. These are called “extreme-pressure additives”. Mechanism  The “extreme-pressure additives” form on metal surfaces more durable films, capable of withstanding very high loads and high temperatures. Examples:  Organic compounds containing chlorine, sulphur and phosphorus. o These compounds react with metallic surfaces, at prevailing high temperatures, to form metallic chlorides, sulphide or phosphides. o These metallic compounds possess high melting points. Dr Prasanna Kumar Sharma
  15. CLASSIFICATION OF LUBRICANT Based on Physical state, lubricants are classified as: a) Lubricating oils or liquid lubricants b) Semi solid lubricants or greases c) Solid lubricants Dr Prasanna Kumar Sharma
  16. 1. Lubricating oils or liquid lubricants Purpose: o Provide a continuous fluid film. o Provide a cooling between the surfaces. o Act as a sealing agent. o Act as corrosion preventing materials. Properties of liquid lubricants: o Low pressure i.e., high boiling point. o Adequate viscosity for particular service conditions. o Low freezing point. o Heat stability. o Stability to decomposition at the operating terms. o High oxidation resistance. o Non-corrosive properties. Dr Prasanna Kumar Sharma
  17. Types of Liquid Lubricants a) Animal and Vegetable oils: - Usable under very high temperature and heavy load.  Disadvantages of its usages are 1. Costly 2. Undergo oxidation easily in contact with air and forms gummy and acidic products, and get thickened. 3. Tendency to hydrolyze in contact with moist-air or aqueous medium. So, they are used as “blending agents” with other mineral oils. b) Mineral oils or petroleum oils: - They are obtained by distillation of petroleum. o Length of hydrocarbon chain varies between 12 to 50 carbon atoms. o Shorter- chain oils have lower viscosity than the longer- chain hydrocarbons. Dr Prasanna Kumar Sharma
  18.  Liquid lubricants are most widely used lubricants because they are 1. Cheap 2. Available in abundance 3. Quite stable under service conditions. o But they have poor oiliness character compared to animal and vegetable oils. o So, high molecular weight compounds like oleic acid, stearic acid are used to over come this problem. c) Blended oils: o No single oil serves as the most satisfactory lubricant for many of the modern machines. Hence, additives are used to improve the properties. These blended oils give desired lubricating property required for a machinery. Dr Prasanna Kumar Sharma
  19. Additives used are: a) Oiliness- carriers: o Coconut oil, caster oil, and palmitic, stearic and oleic acids. b) Extreme-Pressure additives such as: o Fatty esters or acids which form oxide film with metal surface. o Organic materials containing sulphur. o Organic chlorine compounds. o Organic phosphorous compounds. o Sometimes lead (Pb) compounds could be used as high pressure lubricants. c) Pour-point depressing additives: o Phenol, condensation product of chlorinated wax with naphthalene. Dr Prasanna Kumar Sharma
  20. d) Viscosity index improvers : hexanol e) Thickeners : Polystyrene or polystyers f) Antioxidants or inhibitors : Aromatic phenolic or amino compounds g) Corrosion preventers : Phosphorous or Antimony organic compounds h) Abrasion inhibitors : tricresyl phosphate i) Antifoaming agents : glycols and glycerol j) Emulsifiers : sodium salts of sulphonic acid k) Deposit inhibitors : detergents such as salts of phenol and carboxylic acids Dr Prasanna Kumar Sharma
  21. 2. Semi-Solid Lubricants or Greases o Semi solid consisting of a soap dispersed throughout a liquid lubricating oil. - May be Petroleum oil or synthetic oil with a specific additive. Preparation: Saponification of fat (such as tallow or fatty acid) with alkali (like lime, caustic soda etc.,) ↓ Addition to hot lubricating oil under agitation o To increase the heat resistance of grease, inorganic solid thickening agents ( like finely divided clay, bentonite, colloidal silica, carbon block etc.,) are added. o Have higher shear or frictional resistance than oils and hence support much heavier load at lower speeds. Dr Prasanna Kumar Sharma
  22. Applications of Greases: o When oil cannot remain in place due to high load, low speed, intermittent operation, sudden jerks etc. o Work at high temperature o When external contamination may create problem o When dripping or spurting of oil is undesirable Types of greases: o Calcium based greases or cup-greases o Soda-based greases o Lithium-based greases o Axle- greases → lime with resin and fatty acids o Graphite greases o Soap stone Dr Prasanna Kumar Sharma
  23. 3. SOLID LUBRICANTS Solid lubricants are used when: o Other lubricants can not be used o Contamination undesirable o Too high temperature or load are involved o Combustible lubricants not acceptable Examples of solid lubricants used are: a) Graphite (or) b) Molybdenum disulphide Dr Prasanna Kumar Sharma
  24. Examples of Solid Lubricants 1.Graphite: o Very soapy in touch o Non inflammable o Not oxidized in air below 375°C o Oil + graphite → oildag o Water + Graphite → aquadag - Emulsifying agent (tannin) o Grease + graphite → graphite -greases Dr Prasanna Kumar Sharma
  25. Examples of Solid Lubricants 2. Molybdenum disulphide: o Low coefficient of friction o Stable in air up to 400 ° C  Soapstone, talc or mica are also used as solid lubricants. Dr Prasanna Kumar Sharma
  26. Properties of Lubricants 1. Viscosity:  The property of a liquid or fluid by virtue of which it offers resistance to its own flow . - Viscosity should not be too low or too high. (viscosity is inversely proportional to temperature) 2. Flash - Points and Fire - Points :  Flash Point: The lowest temperature at which the oil lubricant gives off enough vapour that ignites for a moment, when a tiny flame is brought near it.  Fire Point: The lowest temperature at which the vapour of the oil burn continuously for at least five seconds, when a tiny flame is brought near it. Dr Prasanna Kumar Sharma
  27. 3. Oiliness: A measure of its capacity to stick on to the surfaces of machine parts under conditions of heavy pressure or load. o For high pressure - high oiliness oil should be used. o Important for extreme – Pressure lubrication 4. Cloud and Pour points:  When an oil is cooled slowly, the temperature at which it becomes cloudy or hazy in appearance, is called its CLOUD POINT. The temperature at which the oil ceases to flow or pour, is called its POUR POINT. 5. Volatility: o Good lubricant should have low volatility. o It is measured by vaporimeter. Dr Prasanna Kumar Sharma
  28. 6. Emulsification: o The property of oils to get intimately mixed with water forming an emulsion. o Emulsions have a tendency to collect dirt, girt, foreign material etc., causing abrasion and wearing out of the lubricating parts of the machinery. o A good lubricating oil should form an emulsion with water which breaks off quickly. 7. Carbon residue: Normally lubricants consist of high % of carbon containing compounds. o Lubricants decompose due to raise in temp. and deposit carbon creating problems to : a) IC engines and b) Air compressors. o A good lubricant should deposit least amount of the carbon . Dr Prasanna Kumar Sharma
  29. 8. Corrosion stability:  Corrosion Test: o A polished copper strip is placed inside a lubricating oil for a specified time and temperature and then checked for any tarnishing . o To prevent or retard corrosion effect of lubricating oils, additives such as Phosphorous, Arsenic, Antimony, Chromium, Bismuth or Lead are added. 9. Decomposition stability: o Lubricating oils must be stable to decomposition at the operating temperatures by : a. Oxidation: To prevent it anti oxidant or inhibitor are used. b. Hydrolysis: Moisture in oils causes hydrolysis of esters c. Pyrolysis : At high temperature Dr Prasanna Kumar Sharma
  30. 10. Aniline point: (A.P.) o The minimum equilibrium solution temp. for equal volumes of aniline and oil sample. o A good lubricating oil should have higher aniline points (A.P) o Higher A.P means higher % of paraffinic hydrocarbons and hence lower % of aromatic HC. (Aromatic HC dissolves natural rubbers and few synthetic ones) 11. Precipitation Number: o The percentage of asphalt present in oil. o Precipitation Number is used to differentiate the different classes of lubricants. Dr Prasanna Kumar Sharma
  31. 12. Specific Gravity: A.P.I. ( American petroleum Institute) degree A.P.I degree = 141.5/sp. gr. at temp(60°F) -131.5 where 141.5 → modulus of the A.P.I scale. 13. Ash Point: o For used oil it is important to get an idea as to how much abrasion and wear it may cause 14. Saponification number: o Number of milligrams of KOH required to saponify 1g of oil. Dr Prasanna Kumar Sharma
  32. 15. Mechanical stability: o At very high pressures of operation, the stability of a lubricant is judged by four balls extreme pressure lubricant test. 16. Neutralization Number: o Is a scale to determine the amount of acidic or basic constituents of an oil. o Acid Number: Amount of KOH required in milligrams to neutralize the fatty acids in 1g of oil. o Good lubricating oil → acid number value < 0.1 Dr Prasanna Kumar Sharma