Classification Of Lubricants

Classification Of Lubricants:

According to the state of lubricant there are classified into the following classes:

(i)                 Liquid lubricants or Lubricating oils

(ii)               Semi-solid Lubricants, and

(iii)             Solid Lubricants

These are describing below one by one.

(i)                 Liquid Lubricants:

                                The main function of lubricating oils is to reduce friction or wear between two moving metallic surfaces by providing a continuous film in between them. Liquid lubricants are further classified as:-

(1)   Animal Oils and Vegetable Oils

(2)   Minerals Oils(Petroleum oil)

(3)   Blended Oils(Compounded Oil), and

(4)   Emulsions

(1)      Animal Oils and Vegetable Oils:

                          Animal and vegetable oils are very important having good oiliness and obtained from the animal and vegetable kingdom contain glycerides of higher fatty acids. Before developing the petroleum industries these were widely used. For example

Animal oils- whale oil, seal oil etc.

Vegetable oils- palm oil, mustard oil, cotton-seed oil, etc.

(2)   Mineral Oils (Petroleum Oil):

                                                           These are obtained by fractional distillation of petroleum. Although they have poor oiliness but they are widely used as lubricants because they are cheap stable and available in abundance.

(3)   Blended Oils (Compounded Oil):

No single oil serves as the most satisfactory lubricant for many of the modern machines because poor oiliness and viscosity. So some specific additives are incorporates into the oils to improve their properties.

(4)   Emulsions:

                                     An emulsion is a two phase system of two immiscible liquids in which one liquid as the dispersion medium and the other as dispersed phase. Emulsions are important lubricant which are used in several machines like boring, milling etc. Emulsions are of two types:

(a)    Oil in Water Type:   Oil in water type emulsions are prepared by mixing the oil with sufficient amount of water in the presence of water soluble emulsifying agent. They are used generally in diesel motor piston, boring machines, milling machines etc.

(b)   Water in Oil Type:    Water in Oil type emulsions are prepared by mixing the water in sufficient amount of oil in the presence of water-insoluble emulsifying agent.

(ii)               Semi-Solid Lubricants:

                                                     These are neither solid nor liquid. They exist in the gel form like paste or grease. They consist of soap dispersed throughout a liquid lubricating oil may contain specific additives for specific purposes. These type of lubricants have higher frictional resistance than oils and therefore can support much heavier loads at lower speeds.

For example, greases, Vaseline, creams etc.

(iii)             Solid Lubricants:

                                              Lubricants which exist in solid form are called solid lubricants. These are used in heavy machinery which operate under very heavy load and low speed. The solid lubricants are used either dry powder or, suspended form in oil, grease or water so that they stick firmly to the metal surfaces. For example, graphite, molybdenum disulphide, soap stone, wax, mica etc.

A brief description  of graphite and molybdenum disulphide as a solid lubricant is given below.

(a)   Graphite:  Graphite is widely used as solid lubricants due to the following reasons:-

(i)                 It is soapy in touch.

(ii)               It is non-inflammable.

(iii)             It is not oxidized in air upto 375ºC.

(iv)             It is used in the form of powdered.

(v)               It has unique layer structure Like in fig.(a). Each carbon atom is sp2 hybridysed  and covalently attached to three neighboring carbon atoms lying in the same plane. The bond length of C-C in ring is 1.42  . These rings constitute a number of layers of atoms. The layers are held together by weak vander waals forces and are separated by a large distance of 3.4Aº. Due to weak vander waals force it is soft and smooth. Hence it is useful as lubricant.

(vi)             When graphite is dispersed in oil it is called ‘Oildag’ and  When it is dispersed in water it is called ‘aquadag’.

                           Fig (a). Layer Structure of graphite
 

(b)   Molybdenum Disulphide( MoS2):-

Molybdenum disulphide is used as solid lubricants due to the following reasons.

(i)                 It possesses very low coefficient of friction.

(ii)               It is stable in air upto  400ºC.

(iii)             It has a higher specific gravity than graphide.

(iv)             It is used as either in powdered form or in additives.

(v)               It has Sand-witch like structure as in fig(b). The layer of molybdenum atoms lie between two layer of sulphur atoms. The two layers of sulphur atoms are separated by a distance of 6.26Aº and the distance between molybdenum layer and sulphur layer is 3.13Aº. The layers are held together by very weak vander waals forces. Due to vander waals forces, it is soft and smooth in nature.

                                     Fig(b). Layer Structure of Molybdenum.