What is lubricant?

Lubricant can be expressed as the substance that is applied among surfaces to control the wear and tear and improve performance. Lubricants play an essential part in the safety of machinery components like gears, shaft, linkage, cam, clutches, and many others. The quantities which affect the amount of lubricant are temperature, surrounding conditions, speed of the components, and many more.

Some of the essential functions of lubricants are listed below:

  • To prevent premature fatigue by making a thin layer on the surface.
  • Lubricants protect materials from corrosion or rust.
  • Lubricants are helpful in restricting overheating and deterioration by radiation.
  • It reduces the chances of friction between moving parts.
  • It decreases the possibility of expansion of metals.
  • The maintenance cost of components reduces because of applying lubricants.
  • In an automotive engine, the power loss is decreased by applying lubricants.
  • Unsmooth relative motion is avoided.
  • Lubricants also function as a coolant in heat transfer.

Classification of lubricants

The lubricants can be classified into three categories which are listed below:

  • Liquid lubricants
  • Semi-solid lubricants
  • Solid lubricants

Liquid lubricants

The liquid lubricants are further categorized into three parts,

  • Vegetable and animal oils
  • Mineral oils
  • Blended oils

Vegetable and animal oils

The vegetable and animal oils are obtained from the cottonseed oil or castor oil and crude fat, respectively. These types of lubricant oils are helpful in protecting the metal surfaces because of their effectiveness under elevated temperatures and large loads. However, vegetable and animal oils do have a lot of disadvantages which are listed below:

  • These types of oils are very expensive.
  • They get oxidized easily to provide gummy products.
  • They hydrolyze more often due to contact with water or moisture.

Mineral oils

The minerals oils (petroleum oils) are popularly used compared to other liquid lubricants because they are inexpensive and easily available in huge quantities. These types of oils are obtained from unrefined oil through vacuum distillation. In mineral oils, oleic acid or stearic acid are mixed in order to raise the oiliness of the mineral oil. Some of the characteristics of good lubricants oils are as follows:

  • Non corrosive properties
  • High resistance to heat and oxidation
  • Appropriate viscosity for better performance
  • Freezing point is low
  • Boiling point is high

Blended oils

Blended oils are a mixture of different types of oils having essential properties to improve the lubricant's performance. The extension of oils like vegetable oil, castor oil, palmitic oil, and other oils having large molecular weight composition improves the functioning. The blending oils are better than vegetable and minerals oil due to their service satisfaction, oxidation stability, and other features.

Semi-solid lubricants

The semi-solid lubricants are produced by blending lubricants oil with grease or other thick oily substance. In lubricant oil, petroleum, synthetic hydrocarbon, or other types of oil can be used. Grease is considered in semi-solid lubricants because it can support heavy loads at the slow speed of components of the machine. Also, the internal resistance of grease is much greater as compared to other oils. Some examples of semi-solid lubricants are vaseline, waxes, and various mixtures of oil and fats.

Solid lubricants

The solid lubricants are applied in exceptional cases where the lubricating oils or grease does not provide better prevention. These types of lubricants are utilized in particular conditions where the temperature is higher, and the load is heavier. Some of the solid lubricants are graphite, zinc oxide, and others. They can also function as additives to different types of oils to raise the lubricants' load capacity.

The following diagram represents an unlubricated rotor.

An illustration represents the rotor and hub of a machine. In this condition, there is no lubricant provided to the rotor.
Unlubricated rotor

The following diagram represents mobile liquid lubricant.

An illustration represents the rotor and hub of a machine. In this condition, mobile liquid lubricant is provided to the rotor.
Mobile liquid lubricant

The following diagram represents bonded liquid lubricant.

An illustration represents the rotor and hub of a machine. In this condition, bonded liquid lubricant is provided to the rotor.
Bonded liquid lubricant

Properties of lubricants

The following are five properties observed in lubricants.

  • Viscosity
  • Flash point and fire point
  • Cloud point and pour point
  • Aniline point
  • Corrosion stability


Viscosity is expressed as the property that resists the flow of certain liquids. In lubricants, viscosity plays an essential part in determining the functional features of the lubricants. When the lubricant oil has low viscosity, the liquid film can not be maintained among the sliding surfaces. Also, the viscosity depends on the temperature of the particular fluid, which means that lubricating oils will be thinner with rise in temperature. The change in viscosity is estimated by an arbitrary scale named viscosity index.

Flash point and fire point

Flash point is described as the point at which the lubricant oil releases sufficient vapors that help in the ignition at the lowest temperature if a small flame is kept near the oil. On the other hand, fire point is the lowest temperature at which the vapor of lubricant oil flashes consistently for a short duration of time (approximately 5 seconds). The quantities which affect the flash point and fire point are pressure, oxidants, configuration of lubricants, and ignition sources.

Cloud point and pour point

The cloud point is expressed as the point at which the lubricant oil is cooled and it displays cloudy appearance, whereas the pour point represents the temperature at which the lubricant oil ceases to pour. These points symbolize the adequacy of any particular lubricant oil in cold circumstances. Additives are also added to different lubricant oils for maintaining the higher cloud point and pour point. The quantities which affect the cloud point and pour point are temperature, salinity, and specific gravity.

Aniline point

An aniline point of a certain lubricant oil expresses its smallest equilibrium temperature. When the aniline point is higher, then the amount of paraffinic hydrocarbons increase, and aromatic hydrocarbons reduce. An aniline point is evaluated by combining identical volumes of the lubricant oil and aniline in a single test tube.

Corrosion stability

Corrosion stability is one of the essential properties of an oil, which is evaluated by using a corrosion experiment. In this, a piece of copper is kept in lubricant oil at a certain temperature. After some time, the strip is taken out to estimate the effects of corrosion. In order to reduce the effect of corrosion in lubricating oil, specific inhibitors such as phosphorus, arsenic, lead, and chromium are included.

Common Mistakes

  • Students make mistakes while understanding the mixing of lubricant oils with other things to improve their performance. However, mixing mineral-based lubricants with synthetic ones generate major failure concerns.
  • One of the misconceptions about lubricants is that they don't extensively utilize in industry and applying lubricant to a component is quite simple in industry.

Context and Applications

The topic lubricants are very much significant in several professional exams and courses for undergraduate, graduate, postgraduate. For example:

  • Bachelors of Technology in Mechanical Engineering
  • Bachelors of Technology in Civil Engineering
  • Masters of Technology in Mechanical Engineering
  • Doctor of Philosophy in Mechanical Engineering
  • Bachelors of Science in Physics
  • Masters of Science in Physics
  • Hydraulic fluid and properties of fluid
  • Lubrication and mechanism of lubrication
  • Application of lubrication methods
  • Oil lubrication method, drip lubrication method and their applications
  • Lubrication fluid
  • Water-Based lubricants and Industrial lubricants
  • Viscous fluids
  • Oil-based, silicone-based, and petroleum-based lubricants
  • Lubricity and solvent
  • Lubricate
  • Tribology

Practice Problems

Q1. Which of the following oils is utilized as blending agent?

  1. Minerals oils
  2. Petroleum oils
  3. Both (a) and (b)
  4. None of these

Correct answer: (d)

Explanation: Mineral oil and petroleum oil can not be utilized as blending agents. The blending agent is those oil that helps in the mixing of oils.

Q2. Identify the correct feature of animal and vegetable oils?

  1. Optimum oiliness
  2. Poor oiliness
  3. Good oiliness
  4. None of these

Correct answer: (c)

Explanation: The viscosity of animal oil and vegetable oil is more, which results in good oiliness. The fats in these types of oils are more than other oil.

Q3. Which of the following lubricants are used for heavy cutting?

  1. Diesel
  2. Gasoline
  3. Coconut oils
  4. Cutting oils

Correct answer: (d)

Explanation: For heavy cutting, cutting oils are suitable because they can operate under high temperatures and help in the cutting operation.

Q4. The effective lubricants oil emulsions are mostly used in?

  1. Gears
  2. Heavy cutting
  3. None of these
  4. Both (a) and (b)

Correct answer: (c)

Explanation: The term oil emulsion is the more effective lubricant for the light cutting operation. The emmulsive oil has less lubricating effects in comparison to cutting oils etc.

Q5. Which is the appropriate lubricant used for delicate devices?

  1. Refined oil
  2. Light cutting oil
  3. Minerals oil
  4. Thin vegetable oil

Correct answer: (d)

Explanation: Thin vegetable oils are the most suitable lubricants for delicate devices, light machines, watches, guns, etc., because they provide very little resistance against the motion of these devices.

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