LUBRICANTS,CLASSIFICATION AND CHARACTERISTICS

Lubricant is substance that reduces friction and wear at the interface of two materials. Or we can say that a lubricant is a substance introduced to reduce frictional force and heat generation between the surfaces in mutual contact, when the surfaces move. The lubricant at interface reduces the adhesive friction by lower the shear strength of interface. 

Based on the shear strength of lubricant or molecular state, lubricants are classified in four categories.

• Gaseous lubricants
• Liquid lubricants
• Semi-solid lubricants
• Solid lubricants

  Gaseous lubricants:-

Gas(i.e, Air, Nitrogen, and Helium) lubrication is used for ultra thin film thickness(separation) between tribo-pairs. Gas lubricated bearings have numerous advantanges over liquid and solid lubricated bearings for a wide range of applications.   A gas bearing is virtually frictionless, silent, and vibrationfree.   Gas bearings can be used for extremely large surface velocities.   A gas bearing can eliminate the risk of contaminating a process with lubricant.  A gas / air bearing can be hydrodynamic or hydrostatic.  In hydrodynamic bearings the gas is introduced into the bearing surfaces by the action of the bearing.  In hydrostatic bearing the gas is introduced under pressure from an external source.Air bearings are also widely used to allow frictionless movement of large heavy items over flat surfaces using air pads. Liquid lubricants:- There are two systems for oil classification. The SAE (Society of Automotive Engineers) viscosity grade and the API (American Petroleum Institute) classification that designates the type of engines for which the oil was designed. The SAE viscosity grade is known as the “W” number when classifying winter oils. In general, the lower the first number, the better the oil performance in extremely cold conditions. Conversely, the higher the second number the better the oil protection at higher temperatures. The API designation is typically “S” designation for gasoline engines and a “C” designation for diesel engines. Most of today’s oils carry an SH,CF or SJ,CF designation signifying that they are suitable for use in all gasoline or diesel automotive applications. Semi-solid lubricants(Grease):- In layman’s language Grease is: A black or yellow sticky mass used in the bearings for lubrication purpose. Lubricating greases consist of lubricating oils, often of quite low viscosity, which have been thickened by means of finely dispersed solids called thickeners. It consist of base oils(75 to 95%), additives(0 to 5%) and minute thickener fibers(5 to 20%). Solid lubricants:- A solid lubricant is basically any solid material which can be placed between two bearing surfaces and which will shear more easily under a given load than the bearing materials themselves. The coefficient of friction in dry lubrication is related to the shearing force and the bearing load.One way to apply solid lubricant is powder coating. We can use powder form of solid lubricant and rub against the tribo surfaces.Examples are Graphite and molybdenum disulfide (MoS2),Graphite etc. A good lubricating oil generally possess the following characteristics: Suitable Viscosity: The viscosity of oil should not change with rise in temperature. Oilness: It ensures the adherence to the bearings and spread over the surface. This property makes oil smooth and very important in boundary lubrication. Strength: The lubricant must have high strength to avoid metal contact and seizure under heavy loads. Chemical Stability: The lubricant should not react with surfaces and any deposit in the cylinder. Pour Point: It should be low to allow the flow of lubricant at low temperature to the oil pump. Flash Point and Fire Point: The lubricating oil should not burn inside the cylinder, otherwise it will leave heavy deposit and poisonous exhaust. Therefore, the flash point and fire point of the lubricating oil must be high. Neutralization: The oil should not have a tendency to form deposits by reacting with air, water, fuel or the products of combustion. Cleaning: The oil should act as cleaning agent inside the engine and should carry any deposits with it. It should also have non-foaming characteristics, low cost and be non-toxic.