In all the types of the machine, the surfaces of the moving or sliding part rub against each other , due to which there exist resistance which is known as friction.

                 Any substance that is used between two moving surfaces to reduce the frictional resistance between them is known as lubricant; and Phenomena is known as Lubrication.

The good lubricants posses following characters:

  • high Bp
  • low freezing point
  • high viscosity index
  • stable to heat at working condition
  • corrosion preventive
  • high resistance to oxidation etc.

Function of lubricants

  • It acts as coolant and reduces the loss of energy in the form of heat.
  • It reduces the expansion of metal by local frictional heat.
  • It increases the efficiency of machine.
  • It reduces tearing, wearing and surface deformation.
  • It sometimes acts as seal.
  • It reduces the maintenance cost and increase durability of machine.
  • It avoids secure of moving surfaces.

Types of Lubricants

Lubricating oil

It reduces the friction and wear between the two surfaces by providing a continuous fluid film between them they are further classified as ;

(i) animal or vegetation oil

  • The oils of vegetable origin such as mustard oil, olive oil, castor oil, and that of animal origin such as mutton oil, whale oil are the most commonly used lubricants.
  • They posses good oiliness due to which the surfaces of the machine parts stick together even under high temperature and heavy land.
  • costly and easily oxidized and hydrolyzed in contact with the air ; hence less used nowadays.

(ii) Mineral oil or Petroleum oil

  • They are obtained by the frictional distillation of petroleum crude oil; which contain lots of impurities so that it should be well refined before use
  • Most widely used because they are cheap, easily available and stable.
  • They viscosity of these type of oil depends upon the length of hydrocarbon which varies from 12-50 carbon atoms.
  • Lesser oiliness than vegetable oil hence the oiliness can be improved by mixing it with vegetable or animal oil.

(iii) Blended or compounded oil

  • These are special types of oil prepared by adding small quantities of some specific additives to improve the desired lubricating properties of the oil.
  • The commonly employed additives are as follows;
  • vegetable and animal oil
  • aromatic and amino compounds =to control oxidation
  • organic phosphorus compound=to prevent corrosion
  • glycerol= to reduce foam formation
  • organic sulfur , chlorine and phosphorus= to resist extreme pressure

(iv)Synthetic lubricants

  • These are the special types of oil which are not found naturally and are designed for specific task or job.
  • They are used for most drastic and severe condition, such as in aircraft turbines.
  • It posses the following distinguishing characteristics:
  • not inflammable
  • high flash point, high thermal stability.
  • chemical stability.
  • high viscosity index etc.


  • polymerized hydrocarbons (polyethylene , polypropylene)
  • Polyglycals
  • silicones
  • fluorocarbon

Semi solid lubricating oils or greases

  • Greases are the semisolid lubricants prepared by the saponification of facts with alkali and adding hot lubricating oil with continuous agitation.
  • They have higher shear or frictional resistance than oils so supports much heavier loads at lower speed.
  • The main function of the soap is thickening lubricants so that the grease sticks firmly to the metal.
  • According to the type of alkali used during saponification different types of soap are formed which determines the properties of the greases as temperature resistance, oxidation resistance etc.

Solid lubricants

  • Solid lubricants are the materials which is solid in nature however it is able to reduce the friction between two surfaces sliding each other without needing liquid media.
  • These after lubrication at temperature higher than the situations where liquid lubricants are used.
  • Can operate upto 350 ˚c.

Solid lubricants are used in the following conditionsGraphite


  • Most widely used solid lubricant.
  • The structure of graphite is responsible for lubricating effect.
  • The carbon atoms in graphite are arranged in regular hexagon in flat parallel layers and each atom is linked by covalent bond to three other atoms but its distance from fourth one is more than double .This fourth valency bond is not fixed and moves about and hence there is no strong bonding between different layers hence the sheet of carbon in graphite can slide over another.
  • It is very soapy to touch , non-inflammable , not oxidized in air below 375˚c.
  • Used either in powdered form or as suspension.
  • Graphite dispersed in water is called AQUADAG which is useful where lubricating free oil is needed.

Molybdenum disulphide

  • This compound has sandwich like structure where the layer of mo atoms lies between the two layers of sulphur atoms.
  • The weak force of attraction between the mo and sulphur layer is responsible for its lubricating effect.


Paint is an organic coating and may be defined as the mechanical dispersion of mixture of one or more pigments io a liquid consisting of drying oil , highly volatile solvent, thinner and film forming material.

  • When the paints are applied on the surface of material then the paint film dry up the process of evaporation followed by the oxidation and polymerization of drying oils.

Characteristics of the paint

  • high adhesion capacity to the material over which it is intended to be used .
  • easily applicable with brush or spray.
  • spread easily over the protected surface
  • high covering power.
  • not get creak or drying.
  • protect the surface from corrosion.
  • Form a tough, uniform adherent and impervious film.
  • The colour should be glossy and quite stable to the effect of atmosphere and other agencies.

Types of paints


  • varnish is the homogenous colloidal solution of resin (natural or synthetic) in drying oil or volatile solvent or both
  • dries by evaporation  and polymerization of its constituents and forms hard, transparent and glossy film.

Uses of varnish

  • commonly used for protective and decorative purpose.
  • give brightness coat to painted surface.
  • improving appearance of wood surface.
  • protects the metal from corrosion.


  • In a simple definition “An enamel is a pigmented varnish”. It is the dispersion mixture of pigment in varnish.
  • It forms hard and glossy films on drying.


  • It is the colloidal dispersion solution of resins of cellulose derivate (cellulose nitrate or cellulose ester etc) and plasticizer in volatile solvent.
  • dries up by evaporation.
  • forms hard, transparent and water proof film.

Uses :

  • for painting of furnitures.
  • giving finishing coats to automobiles.
  • coating cotton fibres and preparing artificial leathers.

Emulsion paints

  • It is the dispersion mixture of rubber like resin in water and film forming vehicles.
  • contains additives such as extenders, stabilizers, pigments, driers, antifoaming agent and preservatives.
  • when it is applied on the surface water evaporates and resinous particles unite to form uniform, homogenous and clear film.


  • coating porous surface because of the low penetration of water solutions.
  • easy to apply because it is odourless or less odour and non-inflammable.
  • can be recoated many times but it is not possible with ordinary paints.

Application of paints

  • to coat metallic surface in order to protect it from corrosion(paints)
  • to make the furniture attractive in appearance (synthetic paints).
  • to protect as well as decorate the buildings (cement paint).
  • finishing of oven, furnashes , aircraft combustion chambers (heat resistant paints).
  • to give brightness and make uniform protective coating over the surfaces (varnishes).
  • reduce the foul smelling in ships due to marine worms and fungus attack (antifouling paint).

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