What is lubrication? Explain the mechanism of fluid film lubrication.
Any substance placed between two moving or sliding surfaces with a view to reduce the frictional resistance between them is known as lubricant. The lubricant when interposed between the moving parts makes the surface slippery and eliminates cohesion. 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 Ao 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.