Basic Modes of Lubrication

Table of Contents

What is Lubrication?

Lubrication is a science of reducing friction and wear between two contacting surfaces, which have motion relative to each other, by inserting or introducing a substance, called lubricant between the contacting surfaces.

The lubricant may be either liquid, gas, or semi-solid.

Objectives of Lubrication :

To carry away frictional heat
To reduction wear
To reduce friction between two contacting surfaces
To carry away worn out particles
To protect surfaces against corrosion
To prevent the entry of foreign particles like dirt and dust into the contact zone.

In lubrication, two contacting surfaces which have motion relative to each other are separated by a high-pressure film of lubricant.

Based on the method of creating a high-pressure film of lubricant between the contacting surfaces, the lubrication can be classified into the following four groups :

Hydrodynamic Lubrication
Hydrostatic Lubrication
Elastohydrodynamic Lubrication
Solid-film Lubrication

Hydrodynamic Lubrication

As shown in fig., in hydrodynamic lubrication, the two surfaces in relative motion are separated by a relatively thick fluid film to prevent the metal to metal contact.

In this type of lubrication, the load supporting high-pressure fluid film is created due to mainly two reasons :

The shape of the zone between the contacting surfaces.
Relative motion between the contacting surfaces.

The moving surface pulls the lubricant into a wedge-shaped zone at a velocity sufficiently high to create the high-pressure film, necessary to separate the two surfaces against the load.

Hydrodynamic lubrication is also known as thick film lubrication or full film lubrication.

Hydrostatic Lubrication

In hydrostatic lubrication, the load supporting high-pressure fluid film is created by an external source like a pump.
The lubrication which is pressurized externally is supplied between the two surfaces.

So, in hydrostatic lubrication, the motion of one surface relative to another is not required.

Elastohydrodynamic Lubrication

This type of lubrication occurs when a lubricant is introduced between the surfaces which in rolling contacts, such as the mating of gears or rolling elements and races of the rolling contact bearing.

The lubricant is entrapped in the deformation zone between the contacting surfaces.

Solid Film Lubrication

When the bearings are required to operate at very high temperatures, the lubricating oils are not suitable as they may lose their properties at such high temperatures.

In such cases, solid film lubricants, such as molybdenum disulfide or graphite are used.

What is Lubrication?

Objectives of Lubrication :