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8 Essential Properties of Lubricants | Purposes of Lubrication Expectations vs Reality

The properties of lubricants

An essential property of a lubricant’s principal function is to prevent friction by forming a boundary layer between two surfaces. Heat should be dissipated from surfaces. Carries impurities to the filters, Defends against oxidation and corrosion, and gears transmissions.

Lubricant oil:

Lubricants are chemicals that are used to reduce the force of friction between moving parts of a machine in contact.

Lubricant oil composition:

Lubricant oil fractions are a complex combination of straight and branched-chain paraffinic, naphthenic aromatic hydrocarbons with boiling temperatures ranging from 302 to 593 degrees Celsius. Some specialty lubricant oils have boiling points ranging from 177 to 815 degrees Celsius. The anticipated use determines the grade of lubricating oil base to utilize.

Classification of Lubricants

  1. Liquid Lubricants (Ex.: Mineral Oil, Petroleum Oil, Vegetable Oil, etc.)
  2. Semi-Solid Lubricants (Ex.: Petroleum Jellies)
  3. Solid Lubricants (Ex.: Graphite, Molybdenum, Disulphide, etc)

Lubricant Additives:

The usage of additives has nearly totally contributed to improvements in lubricating oil. The following are the three primary reasons for adding additives:
1.To keep the lubricant in service as long as possible by preventing chemical change and degradation.
2. To shield the mechanism from dangerous combustion products and lubricating oil that isn’t working properly.
3. To enhance the oil’s current physical qualities while also introducing additional beneficial features.

Lubricant additives that are commonly used are:

  1. Barium
  2. Calcium
  3. Phosphorous
  4. Sulfur
  5. Chlorine
  6. Zinc
  7. Oxidation inhibitor

The Pour Point of Lubricant

The pour point of a lubricant is a property of lubricants is the lowest temperature at which it will continue to flow. The oil thickens and ceases to flow freely below the pour point.

The quantity of wax particles removed during crude processing affects the pour point. The greater the pour point, the more wax particles there are. The lower the pour point, the fewer wax particles there are.


Lubricant Flash Point

The flash point properties of lubricants are the lowest temperature at which a combination of oil, vapor, and air becomes flammable. It is measured by gradually heating an oil-vapor-air combination until it ignites in a typical laboratory container. Lubricating oils have a flash point of 204 °C (400 °F) or above.

The 8 essential properties of lubricants are given below.

  • The viscosity of lubricant oil: Viscosity, a property of lubricants, is the resistance offered by it to the deforming stress, i.e., it is the resistance offered to the flow of the lubricant. The viscosity of the oil decreases with the increase in temperature. The viscosity of the lubricating oil should not go down below a certain specified value at the highest operating temperature of the bearings. If too much thick oil is used, it will lead to power loss, higher operating temperature, and excessive wear and tear of the parts. If the oil is too thin, it cannot lubricate properly and lead to rapid wear of moving parts.


The viscosity of four different oils is shown in the figure below. In thinner, low-viscosity oils, the ball sinks quicker, whereas, in high-viscosity oils, it sinks less.


  • Oiliness: It is the properties of lubricants that allow them to spread and attach themselves firmly to the bearing surfaces. The oiliness of the lubricating oil should be high for better lubrication.


  • Flash and fire point A property of lubricants of an oil is the minimum temperature at which it gives off enough vapor so that a momentary flame is obtained when a naked flame is brought near the oil surface. A fire point is a minimum temperature at which oil continuously burns. The fire point is always greater than the flash point. The flash point of the lubricating oil should be higher than the operating temperature of the bearing.
  • Volatility: When the lubricating oil is exposed to a high temperature for a long time, it may evaporate. This property is known as volatility. The loss of lubricating oil is known as loss of evaporation. The lubricating oil should have low volatility at the operating temperature. If it has high volatility, oil consumption will be higher.
  • Detergency: The lubricating oil should carry away small particles to keep the interior of the engine clean. This property of lubricating oil is known as detergency.


  • Demulsibility (water separation): The lubricating oil should not form an emulsion when brought in contact with water. The property of resisting emulsification is known as demulsibility. The emulsion leads to a collection of dust, dirt, etc., and will increase friction, wear, and oxidation. Hence, it should be avoided.


  • Foaming: It is the condition in which minute air bubbles are held in the oil. This will reduce the mass flow of oil and also increase oxidation. Hence, the lubricating oil should be free from foaming trouble.


  • Corrosiveness: The properties of the lubricant should not attack the engine materials chemically. The oil should prevent corrosion, and it should not contain sulfur.


In addition, lubricating oil should have a high film strength to withstand loads. It should be non-toxic and cheaply available.

Purposes of lubrication (or) functions of a lubricant

1. It reduces friction between moving parts.

2. It reduces the wear and tear of the moving parts.

3. It minimizes power loss due to friction.

4. It provides a cooling effect—During circulation, it carries heat from the hot moving parts and delivers it to the surroundings through the crankcase.

5. It provides a cushioning effect – It serves as a cushion against the shocks of the engine.

6. It provides cleaning action – Impurities such as carbon particles are dissolved during its circulation.

7. It provides a sealing action – It helps the piston rings to provide an effective seal against high-pressure gases in the cylinder from leaking out.

8. It reduces noise.

The barrier of corrosion:

Corrosion inhibitors protect alloy bearings and metal surfaces from chemical assault.

Improvers for anti-wear:

With this film boundary lubrication, an anti-wear improver, the properties of lubricants protect rubbing surfaces. Zinc dithiophosphate is one such antiwear (and oxidation inhibitor) compound (ZDDP).


Detergents the properties of lubricants have a tendency to neutralize deposits before they develop under high temperature and pressure circumstances, or as a result of utilizing a sulfur-rich fuel.


A dispersant is a property of a lubricant, a substance that is used to scatter or suspend pollutant deposits. Polyesters and benzylamine, for example, are common dispersants that burn cleanly.

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