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Explain significance and properties of following:

Viscosity and Viscosity index

Acid value of oil

Cloud and pour point

Flash and fire point

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Viscosity: It is the property of lubricant due to which it offers resistance to its own flow.

Viscosity index: It is defined as rate of change of viscosity with respect to temperature.

Significance:

  • It is the most important property which determines the performance of lubricating oils under operating conditions.

  • A lubricating oil should have sufficient viscosity to enable it in position.

  • On machine part moving at slow speeds under high pressures, a heavy oil should be used as it better resists being squeezed out from between the rubbing parts light oils can be used for lower pressures and high speeds.

  • It is not possible to maintain a liquid oil film between two moving or sliding surfaces if the viscosity is too low and hence excessive wear will occur.

Acid value of oil: It is defined as the number of milligrams of KOH required to neutralize free fatty acids present in one gm of oil.

Significance:

  • It indicates the suitability of the lubricating oil.

  • Higher the acid value, more the corrosion of the machine surfaces, more wear and tear, more significance cost for machine.

Cloud and pour point: It can be defined as that temperature at which a cloud or haze of wax crystal appears at the test when the oil is cooled under prescribed condition.

Significance:

  • The cloud point is an index of the lowest temperature of its utility for certain applications.

Pour point is defined as the lowest temperature at which it will ceases to flow or pour when cooled and tested under prescribed conditions.

Significance

  • It is an index of lowest temperature limit for utility as lubricating oil.

  • It also indicates dissolved wax concentration of lubricating oil.

Flash point and fire point-

Flash point can be defined as the temperature at which the oil gives out vapours that ignite for a moment when a small flame is brought near it. Fire point can be defined as the temperature at which the oil gives out enough vapours which burns continuously at least 5 seconds when a small flame is brought near it.

Significance

  • A lubricant should have a flash point which is reasonably above its working temperature. This ensures safety against fire hazards during the storage, transport and use.

  • A good lubricating oil should not volatize under working condition and even if volatizes, the vapour formed should not catch fire under the working temperature conditions.

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