Basic Modes of Lubrication

In lubrication, two contacting surfaces which have motion relative to each other are seperated by a high pressure film of lubricant.
Based on the method of creating high pressure film of lubricant between the contacting surfaces, the lubrication can be classified into following four groups :
  1. Hydrodynamic Lubrication
  2. Hydrostatic Lubrication
  3. Elastohydrodynamic Lubrication 
  4. Solid-film Lubrication

1.Hydrodynamic Lubrication :

Hydrodynamic 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 :
  1. The shape of the zone between the contacting surfaces.
  2. 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.
The hydrodynamic lubrication is also known as thick film lubrication or full film lubrication.

2. Hydrostatic Lubrication :

Hydrostatic Lubrication |
Hydrostatic Lubrication
In hydrostatic lubrication, the load supporting high pressure fluid film is created by an external source like pump.
The lubrication which is pressurized externally is supplied between the two surfaces.
So, in hydrostatic lubrication the motion of one surface relative to other is not required.

3.Elastohydrodynamic Lubrication :

This type of lubrication occurs when a lubricant is introduced between the surfaces which in rolling contact, such as 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.

4.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 lubricant, such as molybdenum disulphide or graphite are used.


Sujit Kadus

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