Properties of Lubrication System | Function of Lubricant

Desirable Properties of Lubrication System | Function of Lubricant |

Define 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 gas, liquid, semi-liquid or solid.

Definition of Lubricant.
Lubricant is a substance inserted or introduced between the two contacting surfaces having relative motion, so as to reduce friction and wear.
Any fluid having some amount of viscosity can be regarded as a lubricant.

The following are the function of lubricant:

Function of lubricant

The lubricant performs the following function:

  1. To reduce the friction between the contacting surface.
  2. To reduce the wear.
  3. To carry away the frictional heat.
  4. To protect the surface against corrosion.
  5. To prevent the entry of foreign particles, like dirt and dust, into the contact zone.

The list of physical and chemical properties of lubricants:

1. Viscosity

2. Viscosity index 

3. Oiliness

4. Specific gravity

5. Specific heat 

6. Flashpoint 

7. Fire point

8. Pourpoint

9. Foaming

10. Oxidation stability

11. Acidity

12. Alkalinity

13. Demulsibility

1. Viscosity:

Viscosity is the property of fluid by virtue of which offers a resistance to the movement of one layer of fluid over an adjacent layer.

It is the most important physical property of a lubricant. If the viscosity of the lubricant is extremely low, it can not form a layer of fluid film between the two sliding surfaces. 

If the viscosity of the lubricant is high, the frictional resistance and heat generated increase.

(i) Absolute viscosity or Dynamic viscosity:

The absolute viscosity or dynamic viscosity can be defined as the shear stress in the fluid per unit velocity gradient.

(ii) Kinematic viscosity:

The kinematic viscosity is defined as the ratio of the absolute or dynamic viscosity to the mass density of a fluid.

2. Viscosity index:

The variation in viscosity of the lubricating oil with temperature is, though undesirable, it is inevitable. The viscosity index is used to indicate the variation of viscosity with temperature.

3. Oiliness:

Oiliness is the ability of the lubricant to cling to the metal surface. There is no absolute measure of oiliness. It is a measure of the lubricating qualities under boundary lubrication conditions.

4. Specific gravity or relative density: 

Specific gravity or relative density is the ratio of a density of lubricant to the density of water at the same temperature and pressure.  Most mineral oils have relative densities in the range of 0.85 to 0.95. 

5. Specific heat:

Specific heat is the amount of heat required to raise the temperature of one unit mass of lubricant through 1 Degree Celsius. 

6. Flashpoint:

Flashpoint is the lowest temperature at which the lubricating oil gives sufficient vapour to support a momentary flash without actually setting fire to the lubricating oil when a flame is brought within 6 mm from the surface of the oil. 

7. Fire point:

The fire point is the temperature at which the lubricating oil gives off sufficient vapour to burn it continuously when ignited.

In order to avoid fire, the lubricating oil should have a high fire point.

8. Pourpoint:

The pour point is the temperature at which the lubricating oil will cease to flow when cooled.

The lubricating oil should have as low a pour point as possible. The pour point imposes the lower limit on the lubricating oil working temperature and determines the suitability of lubricating oil for low-temperature applications.

9. Foaming:

Excessive foaming of lubricating oil due to churning of the oil in the presence of air may lead to inadequate lubrication and other problems.

10. oxidation stability:

Oxidation of the lubricant is the chemical reaction of oxygen in the air with the hydrocarbon in lubricant. At atmospheric temperature,  the oxidation rate of Mineral oil is fairly low. 

However, it is greatly accelerated at temperatures above 90 degrees Celsius. The insoluble oxidation products may clog oil holes, oil pipelines, filters, and other elements of the lubricating system.

11. Acidity:

The acidity of the lubricating oil is expressed in terms of the neutralization number.

Neutralization number is defined as the mass of Potassium Hydroxide in a milligram required to neutralize one gram of lubricating oil.

12. Alkalinity:

The alkalinity of the lubricating oil is expressed in terms of the base number. The base number is defined as the mass of Potassium hydroxide in milligrams which is equivalent to the alkali present in one gram of lubricating oil.

13. Demulsibility:

Demulsibility of the lubricating oil is its ability to separate from water. It is expressed in terms of demulsibility number.


Demulsibility number is the time in seconds required for a known volume of oil to separate from an equal volume of condensed steam. 

The lubricating oil with good demulsibility will have a low Demulsibility number.

Desirable properties of lubricant:

The following properties are desirable in lubricant used for bearing

  1. It should have an optimum viscosity for the given application.

  2. It should have a high viscosity index

  3. It  should have good oiliness property

  4. It  should have a high specific heat

  5. It should have a high thermal conductivity

  6. It should have a high flash point

  7. It  should have a high fire point

  8. It  should have a low pour point

  9. It  should have  anti-foaming property

  10. It  should have high oxidation stability

  11. It should not be acidic.

  12. It should have good demulsibility i.e It should have a low demulsibility number.

  13. It should be chemically stable with bearing material and atmosphere over the range of temperatures encountered in the operation.  

   15. It should be commercially available at a reasonable cost.

Also Read: What is Hydraulic Braking System

Properties of Lubrication System | Function of Lubricant Properties of Lubrication System | Function of Lubricant Reviewed by Harish on August 05, 2021 Rating: 5

1 comment:

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