Revision: Lubricants Applied Chemistry 1 BE Civil Engineering Semester 1 (FE First Year) University of Mumbai

Any substance placed between two moving or sliding surfaces with a view to reduce the frictional resistance between them is known as lubricant.

Lubricants may be used in solid, liquid or semi solid form. The process of reducing friction between two metallic sliding surfaces by the introduction of lubricants is called as lubrication.

HYDRODYNAMIC OR FLUID-FILM OR THICK -FILM LUBRICATION.

In this type of lubrication, the lubricant is forming a thick film having about 7 000 A0 thickness between the moving surfaces so that the direct surface to surface contact and welding of junction rarely occurs. The coefficient of friction is very low i.e., 0.007 to 0.03 under hydrodynamic lubrication.

When oil is introduced between the moving surfaces, some of the oil molecules are held up tightly at the surface due to adsorption. The remaining oil molecules are loosely arranged away from metal surfaces. Frictional resistance is only due to the internal resistance between the particles of lubricants moving over each other. Hence lubricant chosen should have the maximum viscosity.

Hydrodynamic lubrication occurs when the surfaces are rigid and retain the shape during operations. For hydrodynamic lubrication to occur ,two essential conditions are to be satisfied: (1) liquid must be viscous and, {2) the shape of the surface should be such that a wedge shaped film should be formed. Journal bearing consist of a shaft or journal that rotates freely in a supporting metal sleeve or shell with lubricating oil in the interface between them. During normal operations the shaft rotates at sufficient speed to force the oil between the conforming curved surfaces of the shaft and the shell thus creating the oil wedge and hydrodynamic film. This film allows these bearing to support extremely heavy loads. Watches, clock, sewing machine, fans, guns etc. requires hydrodynamic lubrication.

POUR POINT.
After cooling oil, the temperature at which it ceases to flow is called its pour point. Good lubricant should have low POUR POINT.

OILINESS.
Oiliness of a lubricant is the measure of its capacity to stick on to the surface of machine parts under conditions of pressure or load. Mineral oils have very poor oiliness whereas vegetable oils posses good oiliness.

FLASH POINT.
Flash point is defined as the lowest temperature at which the lubricants gives off enough vapours to cause a momentary flash when a standard test flame is brought near it Good lubricant should have high FLASH POINT.

Flash point:- Flash point is defined as the lowest temperature at which the lubricant gives
off enough vapours to cause a momentary flash when a standard test flame is brought near it.

Fire point:- Fire point is the lowest temperature at which the oil vapours catch fires for at least 5 seconds, on being lighted by a test flame.

Fire point is usually 5-40ºC above the flash point. A good lubricant should have flash point at least above the working temperature . This ensures safety against the risk hazards. Thus flash point acts as a guide for the safe storage, transportation and use in machine.
Good lubricant oil should have hiah flash and fire point.