Lubrication
• When two metallic parts make a contact and move relative
each other, due to surface irregularities frictional heat is
generated.
• This results in a loss of power transmitted between them.
• Frictional losses can be minimised by maintaining a layer of
unguent known as lubricant between the two moving
surfaces.
• The lubricant prevents the two contact surfaces to be
separated by a small distance.
• A film of lubricant is said to be present between the contact
surfaces.
Dept. of Mech & Mfg. Engg. 1
Purpose of lubrication:
• To reduce the friction between the surfaces of machine
parts
• To carry away the heat generated due to friction & to
cool the parts
• To clean the parts by washing away the deposition of
carbon & metal particles caused by wear
• To seal the space between the piston & cylinder & to
prevent the leakage of working fluid
• To cushion the parts against vibration & impact
Dept. of Mech & Mfg. Engg. 2
Properties of a lubricant
1. Viscosity:
• It is the property of lubricant by virtue of which it offers
resistance to shear.
• If the viscosity is too low, then a liquid film cant be maintained
between the 2 moving surfaces.
• If the viscosity is too high, then it will offer great resistance to
the moving surfaces of the parts.
• Viscosity decreases with increase in temperature
• So viscosity of a good lubricant should not change with varying
operating temperature
Dept. of Mech & Mfg. Engg. 3
2. Flash & fire points:
Flash point: It is the lowest temperature at which the fumes
of oil will not catch fire when the flame is brought in contact
with it.
Fire point: It is the lowest temperature at which the oil
fumes catches fire & will continue to burn when the flame is
brought in contact with it.
So a good lubricant should posses a flash point
temperature higher than the temperature at which it is
used.
Dept. of Mech & Mfg. Engg. 4
3. Oiliness:
Oiliness: It is the ability of the lubricating oil to adhere to the
rubbing surfaces.
When a thin film of oil is subjected to high pressure, the oil
film will be squeezed out of the lubricated surfaces.
So a good lubricant should adhere to the surfaces and
maintain an oil film between the rubbing surfaces.
Dept. of Mech & Mfg. Engg. 5
4. Cloud & pour points:
Cloud point: It is the temperature at which the wax & other
substances in the oil separate out from the oil, when the
lubricant oil is cooled.
Pour or freezing point: It is the lowest temperature at which
the oil stops to flow when cooled.
These two points will indicate the suitability of lubricants
for use in cold conditions.
Dept. of Mech & Mfg. Engg. 6
5. Carbon residue:
• Lubricant oils contain high % of carbon in combined form.
• At higher temperature, they decompose depositing a
certain amount of carbon. The deposition of carbon
deposit is highly objectionable.
A good lubricating oil should deposit a least amount of
carbon while in use at higher temperatures.
Dept. of Mech & Mfg. Engg. 7
6. Volatility:
When working temperatures are high, some oils vaporize
leaving behind a residual oil having different lubricating
properties.
A good lubricating oil should have low volatility.
Dept. of Mech & Mfg. Engg. 8
Types of lubricants
Type of Examples Application
lubricant
Solid Wax, graphite, Used where oil
lubricants soap, graphite film cant be
with grease maintained due to
high pressure
Liquid Mineral oils, Ordinary
lubricants vegetable oils, machinery, steam
animal oils & I C engines
Semi solid Grease Used where low
lubricants speed & heavy
pressure exist
Dept. of Mech & Mfg. Engg. 9
Commonly used lubrication systems in I.C
engines
1. Splash lubrication:
The connecting rod is
dipped into the oil of the
crank case & at the time
of rotation the oil is
splashed due to
centrifugal force & it
reaches different parts,
requiring it.
Dept. of Mech & Mfg. Engg. 10
2. Pressure/Force lubrication:
Here a pump is used to supply
lubricating oil.
Strainer: To prevent the
admission of impurities that may
pass into the oil system.
Oil filter: It acts like a fine filter.
Relieve valve: To maintain the
oil pressure constant in the
system.
Oil cooler: To cool the oil
before entering into the
distribution line.
Dept. of Mech & Mfg. Engg. 11
