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):-
(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.
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