What is Lubricant

What is Lubricant


In a machine, the surfaces of moving or sliding parts rub against each other, and this resistance is known as friction, it causes a lot of wear and tear to the surfaces of moving parts, any substance that is introduced between two moving/sliding parts is known as lubricant, the main purpose of a lubricant is reducing wear, friction, force transfer, transporting foreign particles and heating or cooling the surfaces.


  1. REDUCED FRICTION: lubricant provides a thin film that separates moving parts in machines, by adding lubricant to two contacted surfaces, reduces friction, wear and heating on the parts surface.
  2. COOLING: certainly, friction causes heating when metals rub against each other, and lubricants are used to reduce heat, metal expansion and material destruction.
  3. ANTI RUST PROTECTION: lubricants protect metal parts from rust by forming a film that protects the surface of the part from attack by oxygen, moisture or corrosive gases in the air.
  4. SEALING EFFECT: the lubricant enters the gap between the cylinder liner, piston and piston rings in internal combustion engines, it prevents leakage of gases from the engine cylinder by creating a hydraulic film to maintain the internal pressure and protect the system.
  5. CLEANING EFFECT: lubrication keeps the engine clean by removing carbonized substances or dust inside the engine, therefore lubricant itself cleans out foreign substances like soap.
  6. LOADING FORCE BALANCE: lubricant protects systems against increased load by forming an oil film to disperse load and forces applied to frictional surfaces.


Considering the nature of motion between moving or sliding surfaces, there are different types of mechanisms by which the lubrication is done.

1. THICK FILM LUBRICATION/HYDRODYNAMIC LUBRICATION: the moving surfaces are separated by the pressure of a continuous unbroken film or layer of lubrication, In this type of lubrication the load is taken completely by the oil film, hydrodynamic lubrication depends on the relative speed between the surfaces, oil viscosity, load, and clearance between the moving or sliding surfaces.

2. THIN FILM LUBRICATION/BOUNDARY LUBRICATION: this type of lubrication is preferred where a continuous film lubricant is not possible to establish a full fluid condition, in such cases, the clearance space between the moving/sliding surfaces is lubricated by such a material which can get adsorbed on both the metallic surfaces, this adsorbed film helps to keep the metal surfaces away from each other.

3. EXTREME PRESSURE LUBRICATION: when the moving or sliding surfaces are under very high pressure and speed, a high temperature is attained, under such condition, liquid lubricant fails to stick to the moving parts and may decompose and even vaporize, to meet this extreme pressure condition, special additives are added to the minerals oils, these are called “extreme pressure lubrication”.


Lubricants are classified based on their physical state, as follows.

  • Liquid lubricants: Animal fat oil, Vegetable oil, Mineral oil
  • Semi-solid lubricants: Grease
  • Solid lubricants: Graphite, Mica, Talc powder


Lubricating oils also known as liquid lubricants and further classified into three categories.

  • Animal and Vegetables Oils
  • Mineral Oils
  • Blended Oils


It is a low molecular weight liquid based on natural hydrocarbon with approximately 12 to 50 carbon atoms, which is abundant and stable under service conditions, mineral oils are not expensive to produce and are widely used in applications appropriate to their properties, but the oiliness of mineral oil is lower, so adding higher molecular weight compounds increases the oiliness of mineral oil.

No single oil has all the properties required for a good lubricant, blended oils are additives added oils, so adding the right additives is essential to make it work well, additives are used to improve various properties such as viscosity index, oxidation stability, oiliness and pour point, blended oil to superior to mineral oils.


Grease is a semisolid liquid that consists of a liquid lubricant mixed with a thickener, lubricating oil is the principal component and it can be either petroleum oil or a synthetic hydrocarbon of low to high viscosity, the oil does the lubricating while the thickener holds the oil and provides a resistance to flow, additives are added to grease to improve load-carrying ability, oxidation resistance, and corrosion control, but grease cannot effectively dissipate heat from the bearings, so grease work at relatively lower temp.



A solid lubricant is a material used as powder or thin film which reduces friction and wear of contacting surfaces in relative motion and provides protection from damage, they can provide improved lubrication at higher temperatures, other terms commonly used for solid lubrication include dry lubrication, the most common solid lubricants are graphite, molybdenum disulphide, tungsten disulphide and zinc oxide, they can withstand temperature up to 650° c and can be applied in continuously operating situations.


High boiling point
Low freezing point
High kinematic viscosity index
High resistance to oxidation and heat
Corrosion resistance
Stability to decomposition at the operating temperatures


Viscosity is the property of a fluid that determines its resistance to flow, it is the most important property of any lubricating oil, because it is the main determinant of the operating characteristics of the lubricant, higher viscosity lubricants are thick and don’t flow, while lower viscosity lubricants have a closer consistency to water and do flow, viscosity of liquids decreases with increasing temperature and, consequently, means if the viscosity of the oil is too low, a liquid oil film cannot be maintained between two moving/sliding surfaces, and if the viscosity of the oil is too high excessive friction will result, viscosity of good lubricating oil should not change much with change in temperature.

The viscosity of a lubricant is influenced by temperature, mostly used to characterize the viscosity temperature behavior of lubricating oils (viscosity index V. I), if the viscosity of lubricating oil falls rapidly as the temperature is raised, it has a low viscosity index, and if the viscosity of lubricating oil is slightly affected on raising the temperature, its viscosity index is high, the viscosity index of a lubricant is determined by measuring the kinematic viscosity at 40°c and 100°c, synthetic fluids have high viscosity index.

Flash point is the lowest temperature at which the lubricant oil gives off enough vapors that ignite for a moment produced by continuous heating of lubricants, in most cases, the fire points are 5° c to 40° c higher than the flash points, a good lubricant should have flash point at least above the temperature at which it is to be used.

Pour point is the lowest temperature at which the lubricant oil become semi-solid or cloudy and ceases to flow, pour points indicates the suitability of lubricants used in cold condition, lubricant oil used in a machine working at low temperatures should possess low pour point otherwise, solidification of lubricant oil will cause jamming or decrease work efficiency of machine, the pour point should be especially considered when selecting engine oil to be used in winter.

Oxidation stability is a reaction that occurs when oxygen is combined with lubricating oil, variables such as high temperatures, water and acids accelerate the rate of oxidation, also the rate of oxidation is dependent on the quality and type of base oil as well as the additive package used, this property is particularly important to identify the life and storage period of engine oil.

Emulsifiability is important property of lubricants, it is the ability of the oil to separate from water, the water enters a hydraulic system can mix the oil and water collecte the dust and dirt, which can affect the machines and increase wear, corrosion and fluid oxidation, good lubricating oil is to have lower demulsification.

Total acid number test measures the level of acidity in a lubricant, it is useful in monitoring acid buildup in oils due to depletion of antioxidants, oil oxidation causes acidic byproducts to form, high acid levels can indicate excessive oil oxidation and can lead to corrosion of the internal machine components, it is an important quality measurement of lubricating oil formulations quality control.

Total base number (TBN) refers to the quantity of acid, expressed in terms of the number of milligrams of potassium hydroxide per gram of oil sample (mg KOH/g),TBN mostly ranges from 6–8 mg KOH/g in modern lubricants, more higher TBN is more effective in suspending wear-causing contaminants and reducing the corrosive effects of acids over an extensive period of time, when the TAN is higher than the TBN the lubricant requires replacement.

The thickener is a material that in combination with the base oil and additives will produce solid to semifluid structure, thickener is important elements in deciding the properties of the grease, the primary type of thickener is metallic soap, these soaps include lithium, aluminum, organophilic clay, polyurea, fumed silica, fluoropolymers and others, lately, complex thickener-type greases are gaining popularity, they are being selected because of their high dropping points and excellent load-carrying abilities, thickeners are generally used only for the special cases, such as operation in high temperature.

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