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

Properties of lubricants

An essential properties of lubricants principal function is to prevent friction by forming a boundary layer between two surfaces. Heat should be dissipated from surfaces. Carry impurities to the filters, Defends against oxidation and corrosion, and Gear Transmissions.


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

Lubricating oil composition:

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

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)


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.

Additives that are commonly used are:

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

Pour Point of Lubricant

The pour point of a lubricant a properties 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 less wax particles there are.


Flash Point of Lubricant

The flash point a properties of lubricants is the lowest temperature at which a combination of oil, vapour, 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

  • Viscosity: Viscosity a properties 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. Viscosity of the oil decreases with the increase in temperature. Viscosity of the lubricating oil should not go down below a certain specified value at the highest operating temperature of the bearings. If too 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 of an oil to spread and attach itself firmly with the bearing surfaces. Oiliness of the lubricating oil should be high for better lubrication.


  • Flash and fire point: Flash point a properties of lubricants of an oil is the minimum temperature at which it gives off enough vapour so that a momentary flame is obtained when a naked flame is brought near the oil surface. Fire point is the minimum temperature at which an oil continuously burns. 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 more.
  • 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 delergency.


  • 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 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 mass flow of oil and also increase oxidation. Hence the lubricating oil should be free from foaming trouble.


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


In addition a 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 lubrication

1. It reduces friction between moving parts.

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

3. It minimizes power loss due to friction.

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

5. It provides 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.

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 a properties of lubricants protects rubbing surfaces. Zinc dithiophosphate is one such antiwear (and oxidation inhibitor) compound (ZDDP).


Detergents a properties of lubricants have a tendency to neutralise deposits before they develop under high temperature and pressure circumstances, or as a result of utilising a sulphur-rich fuel.


Dispersant a properties of lubricants is a substance that is used to scatter or suspend pollutant deposits. Polyesters and benzylamides, for example, are common dispersants that burn cleanly.

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