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Types of Lubrication

Wedging Film Action Illustration

UBRICATION CAN GENERALLY be defined as the reduction of friction by the use of a fluid lubricant. Although the concept is simple, in practice this procedure becomes much more complex. In fact, 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 fluid immediately adjacent to each surface travels at the same speed and direction of each surface. If you were to travel through the width of the film, differential speed graduates and direction is reversed.

Boundary Condition Illustration

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. Hydrodynamic lubrication often occurs in components such as cylinders, gears, and plain bearings.

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 Illustration

Boundary Lubrication (sometimes referred to as thin film lubrication) is a condition in which the lubricant film become 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 through chemical compounds rather than the properties of the lubricating fluid. Boundary lubrication often occurs during the startup and shutdown of equipment, or when loading becomes excessive. This condition can commonly be observed in certian types of gear sets that need to withstand sliding pressures and shock loading, such as hypoid gears found in automotive differentials.

Mixed Film Lubrication is a combination of both hydrodynamic and boundary lubrication. In such a situation, only occasional asperity contact occurs. This condition can be the result of lubricant breakdown or increased load placed upon the lubricant.

Elasatohydrodynamic Lubrication Illustration

Elasatohydrodynamic Lubrication (EHD or EHL) 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 increases the effectiveness of the lubricant.

A good understanding of these different types of lubrication can assist greatly in the selection of the proper lubrication for a specific application and the prevention of equipment failure.

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