 Good morning to one and all. Today we are going to learn about mechanism of lubrication. So at the end of this session students will be able to explain the different types of lubrications. So the content of today's lecture that is three types of lubricant systems we are going to learn that is first one fluid film or thick film or hydro dynamic lubrication. Second boundary lubrication or thin film lubrication and third one is extreme pressure lubrication. So first one fluid film or thick film or hydro dynamic lubrication. In this the moving sliding surfaces are separated from each other by a thick film of fluid so at least 1000 angstrom unit thick so that the directive surface-to-surface contact and welding of junctions rarely occurs. The lubricant film covers or fills the irregularities of the sliding or moving surfaces and forms a thick layer in between them so that there is no direct contact between the material surfaces. This is the diagram with which we can explain the mechanism of this particular lubrication. So these two are the moving surfaces this is lubricant which is introduced in between these two moving surfaces and this is a load or the pressure which is carried by the surfaces. This consequently reduces where the resistance to movement of sliding or moving parts is only due to the internal resistance between the particles of the lubricant moving each other. Therefore the lubricant chosen should have the minimum viscosity under working conditions and at the same time it should remain in place and separate the surfaces. In such a system friction depends on the viscosity, thickness of the lubricant and the relative velocity and area of the moving or sliding surfaces. The coefficient of friction in such cases is low as 0.001 to 0.03. Hydrodynamic friction occurs in the case of shaft running at the fair speed as well as in well lubricated bearing with not too high load. In general bearings a film of the lubricating oil covers the irregularities of the shaft as well as the bearing surfaces and the metal surfaces do not come into direct contact with each other. So this is diagram with which we can explain this particular meaning. This is a central shaft bearings these are available and in between them the lubricant is introduced and this lubricant is so thick that there is no contact between such as shaft as well as the bearings. Thus the resistance to movement is only due to the internal resistance of the lubricant. Delicate instruments etc are lubricated with this type of lubrication. Hydrocarbon oils are considered to be satisfactory lubricants for fluid film lubrication in order to maintain viscosity of the oil in all seasons of the year. Ordinary hydrocarbon lubricants are blended with selected long chain polymers. Moreover hydrocarbon petroleum fractions generally contain small quantities of unsaturated hydrocarbons which gets oxidized under operating conditions forming a gummy products. So antioxidants like amino phenols are added in general bearings. Second lubrication system is boundary lubrication or thin film lubrication. This type of lubrication is run when a continuous film of lubricant cannot persist and direct metal to metal contact is possible due to certain reasons. This happens when these are the reasons that is first one a shaft starts moving from the rest or the second the speed is very low. Third the load is very high and the fourth one is viscosity of the oil is too low. Under such a conditions the clearance space between the moving or sliding surfaces is lubricated with an oil lubricant. A thin layer of which is adsorbed by physical or chemical forces on both the metallic surfaces. These adsorbed layers avoid direct metal to metal contact. The load is carried by the layers of the adsorbed lubricant on the both the metal surfaces. The coefficient of friction is 0.05 to 0.15. This is the diagram with which we can explain this mechanism. These are two surfaces in between them a thin film of the lubricant is introduced and this lubricant will carry the load. The oil film keeps the distance apart between of the moving surfaces of the order of the height of the asperities. Vegetable and animal oils and their soaps possesses property of adsorption. They are either physically adsorbed to the metal surfaces or react chemically at the metal surfaces forming a thin film of a metallic soap which acts as a lubricant. The load is carried by the layers of adsorbed lubricant. Although the fatty oils possesses a greater adhesion property called oiliness than mineral oils yet they tend to break down at a high temperature. In order to improve the oiliness of mineral oils, small amount of fatty oils or fatty acids are added. Graphite or molybdenum disulfide either alone or as a stable suspension in oil are also used for boundary lubrication. These materials form a film on the metal surfaces which possesses low internal friction and can bear compression as well as high temperature. So, for boundary lubrication the lubricant molecules should have long chain hydrocarbon, polar groups, internal attraction between the chains, active groups or atoms which can form chemical linkages with the metals or other surfaces. Also high viscosity index, resistance to heat and oxidation, good oiliness and low pore point are some of the good qualities of the boundary lubricants. So, here pause the video and answer this question. Question is in case of a boundary lubrication the coefficient of friction is options a 5 to 15, b 0.5 to 0.15, c 0.05 to 0.15 and d 0.001 to 0.03. Welcome back the answer for this particular question that is option c is correct that is 0.05 to 0.15. Third mechanism that is extreme pressure lubrication. When the moving or sliding surfaces are under very high pressure and speed a high local temperature is attained. Under such a conditions liquid lubricants fails to stick and may decompose and even vaporize. To meet these extreme pressure conditions spatial additives form on material surfaces more durable films capable of withstanding very high loads and high temperature. Important additives are organic compounds having active radicals or groups such as chlorine as in chlorinated estrus, sulphur as in sulphurized oils and phosphorus as a tricracial phosphate. These compounds react with metallic surfaces in prevailing high temperature form metallic chlorides and iron sulphate that is which melts respectively at 650 degrees Celsius to 1100 degrees Celsius and serve as a good lubricant under extreme pressure and extreme temperature. If by chance the low shear strength film are broken by the rubbing action of moving parts they are immediately replenished. So for this particular lecture I have used a textbook of engineering chemistry by Jane and Jane as a reference. Thank you.