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Types of Lubricants - What are the Basic Types of Lubricants?

What are the four types of lubricants? Lets look at a few definitions.
To lubricate means nothing more than to make a surface smooth and slippery. But it can also refer to the act of apply a lubricant.
A lubricant is any substance that reduces friction by creating a slippery film between two surfaces. Lubricants permit one surface to move easily over the other surface.
Although the reduction of friction via the use of a fluid appears to be a simple concept, it is actually far more complex. Lubrication by this method can be divided into four basic types or regimes.

Hydrodynamic or Full Fluid Film Lubrication

Although the reduction of friction via the use of a fluid appears to be a simple concept, it is actually far more complex. Lubrication by this method can be divided into four basic types or regimes.When a fluid lubricant is present between two rolling and/or sliding surfaces, a thicker pressurized film can be generated by the movement of the surfaces (velocities). The non-compressible nature of this film separates the surfaces resulting in no metal to metal contact.
The condition in which surfaces are completely separated by a continuous film of lubricating fluid is commonly referred to as Hydrodynamic or Full Fluid Film Lubrication. In this regime, the lubricant immediately adjacent to each surface travels at the same speed and direction of each surface. As you travel through the width of the film, differential speed graduates and direction is reversed.

Hydrodynamic film thickness can be formed by wedging the lubricant through a convergent gap with the tangential surface velocities. Known as wedging film action, this is similar to a car tire hydroplaning on a wet road surface.Although hydrodynamic lubrication is the ideal situation, in many instances it cannot be maintained. Factors which affect hydrodynamic lubrication include Lubricant Viscosity, Rotation Speed or RPM, oil supply pressure and Component Loading. An increase in speed or viscosity increases oil film thickness. An increase in load decreases oil film thickness.

Boundary Lubrication

Boundary Lubrication (sometimes referred to as thin film lubrication) is a condition in which the lubricant film becomes too thin to provide total separation. This may be due to excessive loading, speeds or a change in the fluid’s characteristics. In such a situation, contact between surface asperities (peaks and valleys) occurs.Friction reduction and wear protection is then provided via chemical compounds rather than properties of the lubricating fluid. Boundary lubrication often occurs during the start up and shut down of equipment or when loading becomes excessive.

Mixed Film Lubrication is a combination of both hydrodynamic and boundary lubrication. In such a situation only occasional asperity contact occurs.

Elastohydrodynamic Lubrication

Elastohydrodynamic Lubrication(EHD or EHL)is the last of the types of lubricants discussed here and occurs as pressure or load increases to a level where the viscosity of the lubricant provides a higher shear strength than the metal surface it supports. This regime can occur in roller bearings or gears as the lubricant is carried into the convergent zone approaching a contact area or the intersection of two asperities. As a result the metal surfaces deform elastically in preference to the highly pressurized lubricant, which increases the contact area and thus increasing the effectiveness of the lubricant.