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Module 01 Introduction to Tribology | Prof. Vinayaka G.



Tribology is defined as the ‘science and technology of interacting surfaces in relative motion and of
related subjects and practices’. The word Tribology was derived was from a Greek word ‘Tribos’
meaning ‘rubbing’, thus tribology literally means ‘science of rubbing’. It’s a science that deals with
friction, lubrication and wear in all contacting pairs. The Figure 1.1 shows the interactions between
friction, wear and lubrication and the common portion of all the three is the study of tribology. It
generally deals with the technology of lubrication, friction control and wear prevention of the surfaces
having relative motion under load. The British Lubricating Engineering working group defines
tribology as “the science and technology of interactive surfaces in relative motion and the practices
related thereto.”

It’s clear that two main constituents of tribology: friction and wear when controlled and reduced
automatically increases the service life of machine elements. The awareness of the subject, Tribological
problems and their solutions give rises to significant savings. Tribology is the art of applying
operational analysis and other techniques to problems of great economic significance viz. reliability,
maintenance and wear of technical equipment ranging from household appliances to aircraft. Surface
interactions in a Tribological interface are highly complex and requires a detailed understanding of
various disciplines including physics, chemistry, applied mathematics, solid mechanics, fluid
mechanics, heat transfer, thermodynamics, machine design and material science.

Wear Friction


Figure 1.1 Tribology-Interaction between wear, lubrication and friction

Department of Mechanical Engineering, VVIET, Mysuru 1

Module 01 Introduction to Tribology | Prof. Vinayaka G.P


 September 1964: Conference on Lubrication in Iron and Steel Works in Cardiff (UK).
Realization of considerable losses due to lack of knowledge related friction and wear of machine
 After this realization UK Minister of State for science formed a committee to investigate the
education, research and the need of industry related to lubrication.
 Committee after discussions concluded that only lubrication could not provide complete
solution to deal with friction and wear of machine components.
 An interdisciplinary approach embracing solid and fluid mechanics, chemistry, and material
science is essential. Since there was no word for such new concept, a new name “Tribology”
was coined in 1966.
 Major breakthrough in Tribological science came in 1981 with development of “Scanning
tunneling microscope” (STM) and systematic theory based on “Contact mechanics”. Such
developments provided tools to predict and estimate the behavior of a single asperity contact.
 Subsequent development of Atomic Force Microscope(AFM) in 1985 allowed measurement
(surface topography, friction force) of all engineering surfaces. Atomic Force Microscope can
be used for studies of adhesion, scratching, wear, lubrication, surface temperatures and
measurements of elastic/plastic mechanical properties.


Tribology is crucial to modern machinery which uses sliding and rolling surfaces. Examples of
productive friction are brakes, clutches, driving wheels on trains and automobiles, bolts, and
nuts. Examples of productive wear are writing with a pencil, machining, polishing, and shaving.
Examples of unproductive friction and wear are internal combustion and aircraft engines, gears,
cams, bearings, and seals. According to some estimates, losses resulting from ignorance of
tribology amount in the United States to about 4% of its gross national product (or about $200
billion dollars per year in 1966), and approximately one-third of the world’s energy resources in
present use appear as friction in one form or another. Thus, the importance of friction reduction
and wear control cannot be overemphasized for economic reasons and long-term reliability. The
purpose of research in tribology is understandably the minimization and elimination of losses
resulting from friction and wear at all levels of technology where the rubbing of surfaces is
involved. Research in tribology leads to greater plant efficiency, better performance, fewer
breakdowns, and significant savings.

Department of Mechanical Engineering, VVIET, Mysuru 2

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

A lubricant is a substance, usually organic, introduced to reduce friction between surfaces in
relative contact. The ability of a lubricant to reduce friction is known as lubricity. Lubrication is
the science employed to reduce friction and wear of one or both the surfaces in close proximity
and moving relative to each other by interposing a substance called lubricant.


Though lubrication was basically conceived with the purpose of reducing friction, lubrication
also performs several functions such as:

1. Reduce sliding and rolling frictions for prolonging the life of parts and components in a
machine to prevent wear.
2. Protect bearings and other components from rust and corrosion.
3. To work as a coolant thereby absorbing heat from the components.
4. Act as a cleansing agent by carrying away the chips and other foreign matters.
5. Act as a sealant by preventing the escaping of gas or the working fluid
6. Reduce vibration, noise and shock between the gear teeth and other components.


Tribological element is analyzed on the basis of the modes of lubrication or the regime of the
lubricant. The three regimes lubrication are as discussed with reference to the Stribeck Curve as
shown. This plot for a hypothetical fluid-lubricated bearing system presents the coefficient of
friction as a function of the product of absolute viscosity (η) and rotational speed in revolutions
per unit second (N) divided by the load per unit projected bearing area (P). The curve is obtained
from an actual test of friction conducted on journal bearing and hence the Elastohydrodynamic
lubrication cannot be represented as
journal bearing do not operate in this
regime. Normally there are four
regimes of lubrication namely:

1. Boundary Lubrication
2. Mixed-Film Lubrication
3. Fluid-Film Lubrication
4. Elastohydrodynamic Lubrication

Department of Mechanical Engineering, VVIET, Mysuru 3

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

Boundary Film Lubrication

This is the thin film lubrication where the organic film is physically adsorbed or chemisorbed.
The film of lubrication occasionally breaks by the asperities of the two surfaces and often metal
to metal contact takes place. This often exists during the starting and stopping or when heavy
load rotates at very low speeds or at high temperature when the lubricant tends to squeeze out.
The co-efficient of friction in boundary lubrication varies from 0.08-0.2.

Mixed Film Lubrication

When under conditions of fluid film lubrication, the viscosity of the lubricant or the relative
velocity between the surfaces decreases or if the load increases, the lubricant thins out and the
interacting surfaces in relative motion come closer and if slight asperities comes in contact with
the surface the regime of lubrication is mixed film lubrication. The load is partly carried by the
lubricant film and partly by the asperities that are in contact. The transition between the
hydrodynamic/Elastohydrodynamic and boundary lubrication regimes is a gray area known as a
mixed lubrication in which two lubrication mechanisms may be functioning. There may be more
frequent solid contacts, but at least a portion of the bearing surface remains supported by a partial
hydrodynamic film

Fluid Film Lubrication or Full Film Lubrication

in this regime the lubricant film is sufficiently thick to prevent the opposing surfaces from
coming into contact and provides low friction, high resistance to wear, good damping
characteristics, better cooling and minimum chance of fatigue failure of surfaces. The viscosity
of the lubricant plays a very important role in this type of lubrication mechanism. Fluid film
lubrication can further be divided as:

i. Hydrostatic Lubrication
ii. Hydrodynamic Lubrication

Hydrostatic Lubrication: in hydrostatic lubrication, high pressure oil is supplied over the whole
range of load and speed from external pump source to separate the two surfaces (journal and
bearing) by thick oil film. The friction is nearly zero and co-efficient of friction is nearly 0.0001
as the load fully floats and the friction depends mainly on the viscosity of the oil. However, this
method is not largely employed by industries as it involves complicated oil pumping system and
leak prevention and collection devices.

Department of Mechanical Engineering, VVIET, Mysuru 4

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

Hydrodynamic Lubrication: in hydrodynamic lubrication relative velocity between the two

surfaces is used to generate liquid pressure between two surfaces. The load is transmitted on the
self-renewing film of the lubricant. They are generally preferred as their cost is less and are
design of the system is less complicated, however their load carrying capacity is less and cannot
be used as air bearings.

Elasto-Hydrodynamic Lubrication (EHD): Elastohydrodynamic (EHD) lubrication (EHL) is

a subset of Hydrodynamic (HD) lubrication in which the elastic deformation of the contacting
solids plays a significant role in the HD lubrication process. This regime of lubrication can be
considered as an extension of the hydrodynamic lubrication. In case of EHD lubrication because
of the heavily loaded components the generated pressure of the lubricant oil is so much that it
causes small elastic deformation of the components resulting in the formation of small pockets
in which the lubricant is trapped and spreads along with the continued deformation. This occurs
mainly during the metal working processes such as rolling/drawing between the die and the roll.

Figure1.3 Relation of Film thickness with Lubricant regimes

Department of Mechanical Engineering, VVIET, Mysuru 5

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

In heavily loaded contacts, high pressures can lead to both changes in the viscosity of the
lubricant and elastic deformation of the bodies in contact, with consequent changes in the
geometry of the bodies bounding the lubricant film. In EHL, adhesive wear occurs during start–
stop operations and corrosive wear of the bearing surfaces can also occur as a result of interaction
with the lubricant.


The lubricant chosen must primarily replace the dry friction between the
moving pairs and also eliminate the thermal deformation or stresses
induced. Load and speed of the moving parts are the two major factors
which affect selection of lubricants as shown in the figure. Other criterion
for selection of the lubricants is mainly done on the basis of:
i. Component design and life
ii. Type of operation (Continuous/Periodic/ Intermittent) and machine
iii. Longest drain interval
iv. Operating parameters such as power consumption, number of cycles, type of tools etc.
v. Easy availability and Low cost
vi. Effect on environmental conditions
Selection stages of Lubricants: Considerations at the following three levels of a machine
development is vital for choosing a lubricant.

i. Design Level
Based on design parameters particularly the pressure and speed a designer can suggest the
appropriate lubricant. If the machine is a conventional type, then for a given viscosity different
grades of lubricant oils might be used based on the input from the manufacturers guide book.
ii. Manufacturers Level
Based on the designer’s recommendations the manufacturer can review the additive package or
develop a new package conducting physio-chemical and field tests. The lubricant type is then
chosen as per the standards set by the organizations like ISO/SAE/AGMA/ASTM.
iii. Selection at User Level
The end user can choose the lubricant based on the catalogues of the machine and the lubricants
specified for different operations. The user adopts some stringent analysis such as spectroscopic
analysis of the lubricant oil to predict the possible state of replenishment of the lubricant.

Department of Mechanical Engineering, VVIET, Mysuru 6

Module 01 Introduction to Tribology | Prof. Vinayaka G.P


Lubricants can be broadly classified under four categories namely:

A. Lubricating oils
B. Greases (Semi-Solid Lubricants)
C. Solid Lubricants
D. Gaseous Lubricants

The lubricating oils used are mostly the mineral oils produced by petroleum refining. They are
composed of hydrocarbons and other aromatic compounds. However, some amount of fatty oils
and fatty acids like vegetable oils, fish oil etc. are used to increase the oiliness of the mineral
oils. Synthetic lubricant oils are produced by synthesizing different constituents of hydrocarbons
so that the products derived can be operated at temperature ranging from -70℃ to +300℃. Based
on the applications lubricating oils are further classified into:

i. Automotive Lubricating oils

ii. Industrial Lubricating oils
iii. Metal working oils
iv. Industrial specialty oils
v. Marine Lubricating oils
 Automotive lubricating oils occupy a unique position as they have to cater to the needs
of varieties of engines, loads, temperatures and environments. The types of automotive
lubricating oils include crankcase oils, transmission oils, gear oils, preservatives, brake
oils etc. All the above mentioned oils are standardized by SAE, API and BIS
specifications for maintaining quality and uniformity in properties.
 Industrial lubricating oils are designed to cater to the needs of machineries, operate at
high temperature and loads. The oils used must reduce friction, prevent wear, rusting and
resist chemical or physical deterioration. The characteristics of the industrial lubricating
oils should be: High chemical stability, maximum oil film strength, resistance to
formation of deposits and emulsions. The commonly used industrial lubricating oils
include Turbine oils, hydraulic oils, refrigeration compression oils, spindle oils,
industrial gear oils etc. The industrial lubricating oils are graded by IS0-VG, AGMA and
ASLE specifications.

Department of Mechanical Engineering, VVIET, Mysuru 7

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

 Metal working oils serve the dual purpose of lubrication of the cutting zones and also
cooling of the tool and the workpeice. Cutting fluids, hot and cold rolling oils, wire
drawing and deep drawing oils are included in this category. The characteristics of the
metal working oils must be: controlled friction, reduce wear of tool/die/roll surface,
compatibility with both the workpeice and the tool, spread and protect newly generated
virgin surface of the workpeice material, adaptability to work under varied working
conditions such as pressure, temperature and sliding velocities, immediately cool the tool
and workpeice material, longer shelf life, thermal stability, must act as the flushing agent
by carrying away the chips without forming any residues.
 Industrial specialty oils are used for a dedicated function of heat transfer, heat treatment
and other finishing operations in order to improve the quality and serviceability of the
product. Quenching oils, rubber process oils, rust preventives etc. come in this category.
 Marine lubricating oils are modified versions of automotive oils to make it compatible
with working under sea water conditions.

Derived from the Greek word ‘Crassus’ meaning fat as initially animal fats were used as grease.
Grease form the semi-solid class of the lubricants and were developed for applications where oil
would run-out of the lubricating zones. It’s a semi fluid product of dispersion of thickening
agents in liquid lubricants. Some amount of fatty acids and other ingredients are added for
imparting special properties. The components of grease include:

i. Fluid about 80-90% by volume which can be selected from mineral oils, ply glycols or
ii. Fatty materials about 3-15% of the total usually animal fatty acids or oils.
iii. Thickeners in the form of base or alkali about 1-4% of the total. These are mainly the
metallic soaps of calcium, aluminum, sodium barium and or lithium components. Also
polyurea and clay are used as inorganic thickeners.
iv. Additives and modifiers: graphite, MoS2, Zinc oxides are used to increase the tackiness, low
temperature performance, water resistance, oxidation resistance and anti-wear properties.

Greases have some advantage over lubricating oils such as they can be used where sealing is not
foolproof, for very low or very high altitude applications where chances of access to dirt, dust
and water exist.

Department of Mechanical Engineering, VVIET, Mysuru 8

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

The disadvantages of greases are they cannot be employed to work as a coolant and cleansing
agent as their co-efficient of friction is higher and they are not suitable for high speed bearings
and they are not suitable for very high temperature applications. Grease is classified from the
type of thickeners used in their manufacturing as:

1. Soap Grease
2. Non-Soap Grease
i. Calcium Soap Grease in a typical manufacturing process of calcium soap grease, mineral oil,
fatty acid, calcium hydroxide (hydrated lime) and water are mixed and cooked to bring to
saponification. The process is complete after adjustment of water content. The water resistance
is good on the other hand, and thus the grease performs well in applications exposed to water.
it can be used up to about 100°C
ii. Lithium Soap Grease Lithium soap grease is the most widely used multi-purpose grease,
from general industrial uses to automobiles, bearings and home electric products. It consists
of mineral or synthetic oil and lithium stearate or lithium salt, a hardened fatty acid derived
from castor oil. Usable in a wide range of temperature, and has excellent water resistance
and mechanical stability.

iii. Aluminum Complex Soap Grease Aluminum complex grease is made from a complex soap
which is formed by reaction of aromatic carboxylic acid and stearic acid on aluminum hydroxide.
Aluminum complex grease is characterized by a very fine fiber structure, high dropping point
(200°C or above), excellent heat and water resistance and mechanical stability.

i. Urea Grease Typical urea grease formulation uses organic compound containing more than
two urea groups (-NH-CO-NH-) as the thickening agent. Because of its excellent heat and
water resistance, urea grease is an optimum choice for continuous casting lines and iron mills,
and actually is the most widely used non-soap grease.
ii. Bentonite Grease This grease thickened by organic bentonite is often called "grease without
dropping point" or "grease without melting point," because it does not lose the grease structure
even at extremely high temperature. The grease can offer other advantages like good shear
stability, but its application is limited because of rather poor rust prevention, hardening
tendency when exposed to high temperature condition (200°C or above) for a long time, and
poor ability to keep oil film on the bearing race surface during high speed rotation.

Department of Mechanical Engineering, VVIET, Mysuru 9

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

Advantages of Greases:
a. Remains at application point & adhere to surface.
b. Less-frequency of application.
c. Good for inclined/vertical shafts.
d. Seal out contaminants & less expensive seals needed.
e. Water resistant & reduces oil vapor problems.
f. Provide lubrication cushion in extreme conditions such as shock loading, reversing
operations, low speeds & high loads.
g. Reduces noise and vibration.


The limitations of lubricating oils and greases such as physical and chemical degradation due to
high operating temperature, low volatility of the lubricating oils and undesirability for prolonged
storage like in missile components, requirement for regular re-lubrication led to the development
of solid lubricants. Graphite and Molybdenum di-sulphide (MoS2) are the best known lamellar
solids which work as excellent lubricants. Graphite is not advised to be used under moisture
conditions and MoS2 works well in vacuum and in dry air and gives less friction at lower
temperature than graphite. Primary limitations, low tensile strength and lack of ductility of bulk
carbon make it good powder form solid lubricant. Their high thermal and oxidation stabilities at
temperatures of 500 to 6000℃ enable use of this solid lubricant at high temperatures and high
sliding speeds. Solid lubricants are used in industries as additives in lubricating oils and greases,
as dry powder, as composite blocks, as resin bonded films or dispersions in other fluids. Polymer
based lubricants such as PTFE, polyacetal, polyethylene, nylon and polyamide are commonly
used as they offer the advantages of absorbing vibrations, ease of manufacturing, and better
conformability. The major drawback in usage of polymer based lubricants is they cannot be
employed at higher temperatures.


Gas (i.e., Air, Nitrogen, and Helium) lubrication is used for ultra-thin film thickness(separation)
between tribo-pairs. Gas lubricated bearings have numerous advantages over liquid and solid
lubricated bearings for a wide range of applications. A gas bearing is virtually frictionless, silent,
clean and vibration free. Gas bearings can be used for extremely large surface velocities. A gas
bearing can eliminate the risk of contaminating a process with lubricant.

Department of Mechanical Engineering, VVIET, Mysuru 10

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

Advantages and Disadvantages of a Solid Lubricant:


Following are the desirable properties of a lubricant:


Viscosity is the measure of resistance to flow of a fluid. This resistance is mainly due to the
internal friction and is a molecular phenomenon. It is the measure of physical ability of the fluid
to maintain lubrication under specified conditions of operating speed, temperature and pressure.
Viscosity of the fluid determines or affects the following: friction loss, heat generation, fluid film
thickness, mechanical efficiency, wear, load carrying capacity etc.

Consider two parallel plates separated by a distance ‘h’ meters and let the space between the
plates be filled by a fluid as shown in the Figure. Of the two plates, the top plate is considered to
be movable and the bottom plate is assumed to be fixed. The topmost layer of the fluid is in
contact with the moving plate and assumes its velocity ‘u’ ms-1. If ‘A’ be the area of the plate in
m2, and if a Force of magnitude ‘F’ Newton (N) is applied for the plate to move, the plate shears
across the top most layer of the fluid and the shear stress ‘τ’ in Nm-2 is given by:

𝐹 𝑑𝑢
𝜏= =
𝐴 𝑑𝑦

Department of Mechanical Engineering, VVIET, Mysuru 11

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

According to Newton’s law of Viscosity, this shear stress is directly proportional to the velocity
gradient or the rate of shear strain.

𝑑𝑢 𝑑𝑢
𝜏 ∝ = 𝜇.
𝑑𝑦 𝑑𝑦

The constant of proportionality μ is called the absolute viscosity. From the above equation the
absolute viscosity is given as:


and can be defined as the shear stress required to produce a unit rate of shear strain.

The newton law of viscosity can be stated as “the shear stress on a fluid element layer is directly
proportional to the rate of shear strain.” The constant of proportionality is called the co-efficient
of viscosity. Fluids that obey Newton’s law of viscosity are called Newtonian fluids and those
which do not obey are called Non-Newtonian fluids.

The SI unit of viscosity is Pas. The unit of Viscosity in CGS is called Poise. The ratio of the
absolute viscosity to the density of the fluid is called kinematic viscosity. Mathematically it is
given by,


The SI unit of kinematic viscosity is m2s-1. In CGS system it is Stokes.

The temperature and pressure have an accountable effect on viscosity and thereby on the
functioning of the lubricant.

Department of Mechanical Engineering, VVIET, Mysuru 12

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

2. Viscosity Index

Viscosity index (V.I) is an empirical number that indicates the effect of temperature change on
the viscosity of the lubricant. For high V.I the expected change in viscosity with change in
temperature is very small whereas for lubricants with low V.I the change in viscosity with respect
to temperature is very large. Oils with V.I between 25-50 are called low viscosity index oils and
V.I between 50-80 are called medium viscosity index oils and oils having V.I more than 80 are
called high viscosity index oils. Most of the lubricants used in industrial applications are high
viscosity index oils as temperature change is almost inevitable.

3.Boiling and Freezing point

A good lubricant must possess a high boiling point and a very low freezing point so that the
range of temperature within which it can be employed for working is very high.

4.Flash and Fire Point

The flash point of an oil is the temperature at which sufficient concentration of vapour is released
from the oil at its surface to be able to ignite when an open flame is applied. At the flash point,
a lower temperature, a substance will ignite briefly, but vapor might not be produced at a rate to
sustain the fire. The fire point of a fuel is the lowest temperature at which the vapour of that fuel
will continue to burn for at least 5 seconds after ignition by an open flame. Flash point and fire
point are important for petroleum products for safety reasons. Low flash point oils are generally
more volatile oils and if used for applications involving higher temperature operations higher
rate of oil consumption shall occur.
5.Pour Point
It indicates the lowest temperature at which the oil will just begin to flow when cooled under
prescribed conditions.
6.Penetration value
It is normally applicable for measuring the softness or the consistency of grease which depends
upon the yield value and viscosity at low rate of shearing. This empirical index defines the degree
to which a grease resists deformation under application of force. The test is carried out by a
penetrometer and is referred to as cone penetration of lubricating greases. The depth of
penetration is measured in tenth of a millimeter and the result is quoted as worked penetration
value of the grease.

Department of Mechanical Engineering, VVIET, Mysuru 13

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

7.Neutralization number

This means is to assess the acidic impurities in the lubricating oil and is a very important property
for corrosion reduction. The neutralization number is normally expressed as the number of
milligrams of potassium hydroxide (KOH) required to neutralize one gram of oil. The acidity in
lubricating oils is generally not allowed to develop in excess of 1.5mg KOH/gm for oils like
spindle oils, turbine oils and hydraulic oils.

8.Demulsification Value

It is the resistance to formation of emulsions. It is the time required for water separation to occur
from an oil and water mixture that has been prepared under specified conditions.

9.Oxidation resistance

The lubricant must not form any oxides with its interacting environment at any operating
temperatures. The lubricant must be resistant to oxidation.

10. Chemically Inert

The lubricant must not be chemically active by forming any new chemical compounds when
made to come across any additives or coolants. The lubricant must be chemically inert.


The lubricant used must be non-toxic as its disposal must not be hazardous to the environment.

12.Thermal Stability

The lubricant must be thermally stable at any range of working temperature. It should not
degrade at any higher temperature.

1.4.6 Variation of Viscosity with Temperature

The viscosity of the lubricant is mainly due to intermolecular forces. With increasing
temperature, the molecules move farther apart and the intermolecular force decreases resulting
in the decrease of viscosity. Thus, viscosity of a lubricating oil decreases with increase in
temperature. In case of gaseous lubricants, the momentum transfer is the dominant contribution
to its viscosity. With increase in temperature of gas the velocity of molecules increases resulting
in increasing of momentum transfer and consequently the absolute viscosity. The variation of
viscosity with temperature is given by a generalized equation:

Department of Mechanical Engineering, VVIET, Mysuru 14

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

ln 𝜇 = 𝐴 +

‘A’ and ‘B’ are constants and T is the

absolute temperature. To study the effect of
temperature, change on viscosity a parameter
called Viscosity Index is used. To find the V.I
of an oil, its temperature-viscosity
relationship must be compared with two
standard oils. The V.I is calculated from the

𝑉. 𝐼 = 𝑋 100

Where, U is the viscosity at 100⁰F of the oil whose viscosity index is to be calculated. Lis the
viscosity of the oil at 100⁰F at O viscosity index having the same viscosity at 210⁰F as the oil
whose V.I is to be calculated and H is the viscosity at 100⁰F of 100V.I.

1.4.7 Variation of Viscosity with Pressure

The viscosity of the fluid rises with increase in pressure though the phenomenon is notable at
pressures of several hundreds of atmospheres. In case of gaseous lubricants, with increase in the
pressure the molecules are forced to come closer thereby increasing the intermolecular forces,
this increases the viscosity. The following exponential law represents the trend of variation of
viscosity with pressure:

𝜇𝑝 = 𝜇0 𝑎 𝑝

Where, μp: absolute viscosity at pressure ‘p’ MPa. μo: absolute viscosity at the atmospheric
pressure and ‘a’ is constant varying between 1.01-1.04 for pressures up to 40MPa.

1.5 Grades of Viscosity

Commonly used viscosity grades are:

I. ISO (International Standards Organization) viscosity grades

II. SAE (Society of Automotive Engineers) viscosity grades
III. AGMA (American Gear Manufacturers Association) viscosity grades

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Module 01 Introduction to Tribology | Prof. Vinayaka G.P


Features of ISO Grades of
viscosity are:

i. Consists of 18 viscosity
grades in the range of 2cst -
1500cst at 40℃
summarized in the table

ii. Viscosity grade indicates

the mid-point kinematic
viscosity in cst at 40℃.

iii. The classification is

silent about any aspect on viscosity temperature or other behavioral characteristics, quality, type
of hydrocarbon and application of lubricant.

iv. the graduation is based on the principle that the mid-point kinematic viscosity of each grade
is nearly 50% greater than the preceding one.


SAE viscosity grades are categorized into SAE Engine oil and SAE Transmission lubricants.
The classification is based on viscosity values at 210⁰F. the suffix ‘W’ indicates winter grades,
i.e. these grades have good cold start-up characteristics. These characteristics mean lubricant
retaining fluidity at low temperatures and are easily pumped.

Engine Oil Transmission Oil

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Module 01 Introduction to Tribology | Prof. Vinayaka G.P


AGMA grades standardize gear oils based on additives used. These are rust and oxidation
inhibited gear oil, extreme pressure gear lubricant and compound oil. The classification is as
shown below:


The accurate method for determining viscosity is based on Hagen-Poiseulle law for laminar flow
of capillaries. The law can be written as:

𝜋. 𝑅 4 𝑑𝑝
𝑄= .
8𝜇 𝑑𝑥

Where, Qis the flow rate, R is the radius of the capillary tube, and is the pressure gradient
and is equal to:

𝑑𝑝 𝜌𝑔𝐻
𝑑𝑥 𝐿

Where ‘H’ is the vertical distance and ‘L’ is the length of the tube. Substituting for the pressure
gradient and the flow rate as V/t, the equation for Hagen-Poiseulle law can be rewritten as:

𝑉 𝜋. 𝑅 4 𝜌𝑔 𝐻
𝑡 8. 𝜇𝐿

On substituting for 𝜂 = where η is the kinematic viscosity, as most of the viscosity measuring
devices (Viscometers) measure the kinematic viscosity, the expression for η is given by:

𝜋𝑅 4 𝑔𝐻𝑡

Department of Mechanical Engineering, VVIET, Mysuru 17

Module 01 Introduction to Tribology | Prof. Vinayaka G.P

A viscometer is an instrument used to measure the viscosity of a fluid. For liquids with viscosities
which vary with flow conditions, an instrument called a rheometer is used. Viscometers only
measure under one flow condition. In general, either the fluid remains stationary and an object
moves through it, or the object is stationary and the fluid moves through it. The drag caused by
relative motion of the fluid and the surface is the measure of viscosity. The flow conditions must
have a sufficiently small value of Reynold’s number for the flow to be laminar.


Rotational viscometers use the idea that the
torque required to turn an object in a fluid is a
function of the viscosity of that fluid. They
measure the torque required to rotate a disk or bob
in a fluid at a known speed. This is perhaps the
only viscometer that measure the absolute
viscosity of the oil.

It is in the form of two concentric cylinders of

which one rotates in the oil whose viscosity is to
be measured. The absolute viscosity is measured by measuring the frictional drag due to viscous
shear on the inner cylinder at a particular velocity. This viscometer is suitable for measuring the
viscosities of oils having higher viscosity.


These viscometers are also known as glass capillary
viscometers or Ostwald viscometer. IT consists of a U-shaped
glass tube held vertically in a controlled temperature bath. In
one arm of the U is a vertical section of precise narrow bore
(the capillary). Above there is a bulb, with it is another bulb
lower down on the other arm. In use, liquid is drawn into the
upper bulb by suction, then allowed to flow down through the
capillary into the lower bulb. Two marks (one above and one
below the upper bulb) indicate a known volume. The time taken for the level of the liquid to pass
between these marks is proportional to the kinematic viscosity. A conversion factor is provided
for determining the kinematic viscosity by measuring the time taken.

Department of Mechanical Engineering, VVIET, Mysuru 18

Module 01 Introduction to Tribology | Prof. Vinayaka G.P


The main disadvantage of the capillary tube

viscometer is the errors that arise due to the
variation in the head loss and other parameters.
The Saybolt viscometer is an efflux type
viscometer that consists of a vertical cylindrical
chamber filled with liquid whose viscosity is to be
measured and is surrounded by a constant
temperature bath and a capillary tube ((length
12mm and diameter 1.75mm) attached vertically at the bottom of the chamber. For measurement
of viscosity, the stopper at the bottom of the tube is removed and time for 60ml of liquid to flow
is noted which is named as Saybolt seconds. The kinematic viscosity is then determined by using
the equation below:

𝜗 = 0.002𝑡 −

Where, ‘υ’ is in Stokes and ‘t’ is in seconds.

Department of Mechanical Engineering, VVIET, Mysuru 19