LUBRICANTS
INTRODUCTION AND LUBRICATION
THEORY
INTRODUCTION
Lubricants are defined as The chemical substances which
reduce friction between two sliding/moving metal surfaces and
reduce wear and tear of machines.
The lubricants form layers between two moving surfaces in
contact, thus reducing frictional resistance. This further helps in
reducing destruction of metals.
Friction: It is the force of resistance to the relative motion of
two solid surfaces in contact
Coefficient of friction:
It is ratio of force of friction to
applied force or load.
The process of reducing frictional resistance between moving/
sliding surface by the introducing of lubricants in between them is
called lubrication.
Functions of lubricants :
Lubricants act as coolants as they reduce frictional heat.
Thereby controlling expansion of metals which helps in
maintaining shape, size and dimensions of metallic parts of
machine.
Lubricants act as sealants as they do not allow escape of
gases from engine under high pressure.
1
LUBRICANTS
INTRODUCTION AND LUBRICATION
THEORY
Lubricants prevent the attack of moisture on machine
surface which helps in controlling corrosion.
They act as cleaning agents since they have tendency to
wash off solids produced due to combustion.
Reduces maintainence cost of machine by reduction of wear
and tear of metal surfaces
The basic principle of lubrication is lowering the coefficient of
friction using lubricant.
1)
Theories
of
lubrication
(Mechanism)
During lubrication of a machine, we interpose films of
lubricant between the moving metallic surfaces in order to
reduce frictional resistance, wear & damage.
There are mainly 2 types of mechanism by which lubrication
is done.
LUBRICANTS
I.
INTRODUCTION AND LUBRICATION
Fluid
film
or
thick
THEORY
film
or
hydrodynamic lubrication
In this type of lubrication, thick film of lubricant
(thickness atleast 1000Ao ) is applied between
moving surfaces.
This mechanism for lubrication is useful in machines
where load is low and speed is not very high.
In this lubrication, the lubricant chosen should have
the minimum viscosity under working condition & at
the same time it should remain in place & separate
the surfaces.
In such system, friction is independent of the load,
but
depends on the viscosity, thickness of film &
relative velocity & area of surfaces.
The coefficient of friction in such cases is as low as
0.001 to 0.03.
LUBRICANTS
Light Load
INTRODUCTION AND LUBRICATION
THEORY
Oil molecules
Bearing
which are
lining
tightly held
Journal
up
(Shaft)
(thick)
1000 A0
Rotating
Shaft in
floating
Loosely
(Thin)
arrange Oil
molecules
Running
lubricant.
LUBRICANTS
INTRODUCTION AND LUBRICATION
THEORY
Uses :
a. Sewing machines
b. Clocks
c. Scientific instruments .
Hydrocarbon Oils:
are considered to the satisfactory
lubricants for fluid Film lubrication. In order to maintain suitable
viscosity of the oil in all seasons of a year, ordinary hydrocarbon
lubricants are blended with selected long chain polymers.
Moreover hydrocarbon petroleum fractions, generally contain
small quantities of Unsaturated hydrocarbons, which oxides under
operating conditions forming gummy product. So anti oxidant (like
amino phenol) are used in journal bearing.
LUBRICANTS
INTRODUCTION AND LUBRICATION
THEORY
II. Boundary Lubrication OR Thin film
lubrication:
High Load
Adsorbed thin layer of lubricant
Low Velocity
This type of lubrication is done when a continuous
film of lubricant cannot persist
& direct metal to
metal contact is possible due to certain reasons.
A thin film is applied (which is less than 10A o)
between two moving surfaces
The machine which is operated under high load and
low speed follows this type of boundary lubrication.
This happens when (i) a shaft (steel rod) starts
moving from rest. or (ii) the speed is very low. or (iii)
the load is very high & viscosity of the oil is too low.
Hence to reduce friction in such machines a thin film
of lubricating il is introduced between them.
This film get adsorbed (Surface attached) on metal
surface due to Physical or Chemical or both the
forces & it is retained (keep in place) there.
LUBRICANTS
INTRODUCTION AND LUBRICATION
THEORY
The coefficient of friction in such case is usually 0.05
to 0.15. So that machine surfaces remain protected
from friction.
The lubricant generally applied are the vegetable, animal
oils & their soaps possess a great tendency of adsorption on
surfaces.
The soaps are suitable at moderate temperature. But at
high temp, they get decomposed. Hence these are not
suitable to be used in I.C. Engines. In such cases, Mineral
alls blended with fatty oils or fatty acids are used because
the thermal stability of mineral oils is high.
Other substitutes are solid lubricants such as graphite or
molybdenum disulphides. either alone or their stable
suspension in oil. Which is also suitable.
In short, the lubricant possessing high viscosity index,
resistance to heat & oxidation, goods oiliness, low pour
paint are most suitable for boundary lubrication.
LUBRICANTS
INTRODUCTION AND LUBRICATION
THEORY
Uses :
a. Rail axle bones
b. Tractors
c. Rollers
When the moving/sliding surfaces are under very high
pressure & speed, a high local temperature is attained &
under such conditions liquid lubricants fail to stick & may
decompose & even vaporize.
To meet these extreme pressure conditions, special
additives are added to Mineral Oils. These are called
extreme pressure additives. These additives form on
metal surface more durable films, Capable of with
standing very high loads & high temp.
Important additives are organic compound having active
radicals or groups such as Chlorine (Chlormated esters),
Sulphur (Sulphurised oil) or Phosphorus (Tri-cresyl
phosphate).
LUBRICANTS
INTRODUCTION AND LUBRICATION
THEORY
These compound reads with metallic surfaces at high
temp to form metallic chlorides, sulphide or phosphides.
These compound posses high M.P. (1000oC) + Serve as
good lubricant under extreme pressure & temp conditions.
