lubrication

1. CHAPTER - 22
LUBRICATION AND LUBRICANTS
Introduction
Discovery of the beneficial effects of lubrication must have followed closely upon making of
the most primitive mechanical contrivances, and it would have been quickly recognized that
the practice of lubrication not only reduced the muscular effort (or force and power required)
of using the contrivance, but also reduced the wear and tear of the working parts. Thus it was
observed that the practice of lubrication increased mechanical efficiency, reduced wear,
reduced excessive heating of the parts and all the three are of vital importance for the
existence of machines to-day.
From history we find that even 500 years back, Assyrians and Egyptians used lubrication for
their cart movements. Since then, although lubrication was in practice, but the development
of the technology was very slow. Industrial revolution gave birth to study and development of
this branch of engineering. Later on, World War II brought sea change in the lubrication
practice for running of developed and efficient war equipment. After this only, the subject
“Lubrication Engineering” came into existence.
Definitions
The activity of application of a suitable material between two rubbing surfaces in relative
motion to reduce friction between them is called lubrication. The material such applied
between the rubbing surfaces is called the lubricant. Thus, any material capable of reducing
friction between two rubbing surfaces in relative motion can be called a lubricant irrespective
of its state (i.e. solid, liquid or gaseous).
From the above statement, it is observed that the basic purpose of lubrication is to reduce
friction between rubbing surfaces. This does not mean that only the presence of a lubricant
serves the purpose of effective lubrication. Effective and purposeful lubrication means
“Application of correct quality of lubricant in proper quantity at right period of time” Here
the important points may be noted: i.e. the application method, correct quality, proper
quantity and the right period.
Purposes
The purpose of lubrication may be divided into two categories i.e. primary purposes and
secondary purposes. The primary purposes for lubrication are:
1. Reduction of mechanical power required to run the system
2. Reduction of wear of various components of the equipment and
3. Reduction in excessive heating and rise of temperature causing loss of properties of
the components.
The secondary purposes may be enlisted as below:
1. Extended useful life of the equipment and its components
2. Reduced unscheduled down time due to break downs
3. Lower production cost
4. Reduced power cost
5. Removal of wear debris
6. Sealing from environment
7. Rust prevention

2. 8. Reduction of thermal stresses
9. Improve material property
10. And many others.
It is not obvious, that the three primary purposes mentioned above have some relations, but
are found to have the same origin. Lubricant interposes a partially or fully, continuous
stratum of fluid between the mutually opposed parts of the machine to which it is applied. It
thus prevents partly or wholly direct contact of solid members with one another. When the
application is ideal, and the layer of fluid is complete and continuous, solid contacts and solid
frictions are completely eliminated together with the possibility of abrasion or seizing.
Therefore, the physical and chemical properties of the lubricant are of much importance from
the subject point of view.
Lubricants are classified based on their physical state as under:
a. Liquid : Lubricating oils,
b. Semi-solids : Greases and
c. Solid lubricants : Powder.
Properties of liquid lubricants
Although there are many physical and chemical properties of lubricant, the following
properties are important and within the scope:
1. Viscosity: It is the measurement of internal resistance to flow of liquids and is the
most important single property of lubricating oils. The viscometer is to an oilman
what the ruler is to a carpenter. Viscosity determines the ability of oil to support a
load on a fluid film, the power consumed in friction and the amount of heat that
will be generated.
2. Flash point: The flash point of an oil is the temperature to which it must be heated
to give off sufficient vapours to form an inflammable mixture with air. At this
point, vapour flash upon application of a lighted burner and then go out for want
of more vapour.
3. Fire point: Fire point is the temperature to which the oil must be heated to burn
continuously after the test burner has been applied to the escaping vapours. As a
general rule, fire point is 300
C to 450
C above the flash point.
4. Neutralization number: It is the number, which identifies the acidity or alkalinity
of oil. It is the weight in milligrams of potassium hydroxide required to neutralize
the acid content of one gram of oil.
5. Pour point: The pour point of an oil is an identification of its ability to move at
low temperatures. The test is important for lubricating oils that are to be used in
cold surroundings; particularly they must flow to suction side of an oil pump.
6. Insoluble rating or precipitation number: The insoluble rating is the amount of
insoluble material in oil.
It has been stated that the viscosity is the most important single property of lubricating oils.
This property changes with pressure and temperature of the fluid. While viscosity increases at
very high pressure, the viscosity reduces with rise in temperature in liquids. For engineers,
the phenomenon of change of viscosity with temperature needs attention.

3. Viscosity index (VI)
The rate of variation of viscosity with temperature is different for different oils. For example,
mineral oils with napthenic base vary more over the same temperature range than those of
parafene base oils. The rate at which the viscosity of an oil changes with temperature is
expressed by an empirical number known as viscosity index (VI).
A relatively small change in viscosity with temperature is indicated by high VI, whereas, a
low VI shows a large change in viscosity with temperature.
A particular oil (Pennsylvanian oils), consisting of mainly parafines is arbitrarily assigned a
VI value of 100 as they exhibit a relatively small change in viscosity with rise in temperature.
Another oil (oil of Gulf-coast origin) consisting mainly napthenes is assigned a VI value of 0
as they exhibit a large change in viscosity with rise in temperature.
In industry, lube oils with high VI are preferred since they have almost the same viscosity
over a range of temperatures.
Determination of VI
VI = × 100
Where,
L = Viscosity at 1000
F of the low viscosity standard oil having VI = 0
(gulf-
oil) and also having the same viscosity as the oil under test at 2100
F.
U = Viscosity of oil under test at 1000
F.
H = Viscosity at 1000
F of the high viscosity standard oil having VI = 100
(Penn. oil)and also having the same viscosity as the oil under test at
2100
F.
Viscosity grades
Commonly used viscosity grades have been standardized as:
1. ISO viscosity grades (ISOVG)
2. SAE viscosity grades for engine oils and transmission oils and
3. AGMA grades (American gear manufacturers association).
ISO grades
It has 18 viscosity grades (i.e. 2,3,5,7,10,15,22,32,46,68,100,150,220,320,460,680,1000 and
1500) in the range of 2 cst to 1500 cst (at 400
C). The viscosity grade indicates the mid-point
viscosity. For a particular grade the kinematic viscosity is supposed to be within 10% of the
mid-point viscosity. However, this classification is silent about the viscosity-temperature
relationship or other characteristics like the quality, type of hydrocarbon their specification
etc.
SAE grades
SAE classification is based on the viscosity at 990
C. Sometimes, suffix ‘W’ is used which
means “winter”. These oils have good cold start-up characteristics i.e. the oil retains its
fluidity at low temperature also. The classifications are as under:

4. SAE Engine oil SAE Transmission oil
Grade Viscosity cp
at -- 0
C
Viscosity cst at
990
C (2100
F)
Grade Viscosity cst at
990
C (2100
F)
0W 3250 at 30 3.8 70W 4.1
5W 3500 at 25 3.8 75W 4.1
10W 3500 at 20 4.1 80W 7.0
15W 3500 at 15 5.6 90 11.0
20W 4500 at 10 5.6 140 13.3 – 24.0
25 6000 at 05 9.3 250 24.0 – 41.0
20 - 5.6 - 9.3
30 - 9.3 – 12.5
40 - 12.5 – 16.3
50 - 16.3 – 21.9
60 - 21.9 – 26.1
AGMA grades
This classification standardizes gear oils on the basis of additives used. There are rust and
oxidation inhibited gear oils, extreme pressure gear lubricants and compound oils.
No. Viscosity range
cst at 400
C
Equivalent
ISO grade
Grades like 7EP, 8EP, 8AEP,
8Comp and 8AComp are also
available for special purposes.
1 41.4 – 50.6 46
2 61.2 – 74.8 68
3 90 – 110 100
4 135 – 165 150
5 198 – 242 220
5EP 280 – 352 320
6 414 – 506 460
6 EP 612 – 748 680
7Comp 900 – 1100 1000
Lubricating oils are classified as under:
1. Animal and vegetable oils,
2. Mineral or petroleum oils,
3. Blended oils and
4 Synthetic oils.
Animal oils are extracted from crude fat and vegetable oils are obtained by crushing the
seeds. Both these oils need further treatment before use as lubricating oil. The treatment
processes involve cooling, filtration, neutralization from free fatty acids, coagulation
treatment for removing suspended impurities etc.
These oils possess good “oiliness” and hence they stick to the surface of machine parts even
under high temperature and heavy loads.
Animal and vegetable oils have very limited use because:
 They are costlier
 Less resistant to oxidation and after oxidation, forms gummy and acidic products
 Get thickened when coming in contact of air.

5. Mineral of petroleum oils are obtained by distillation of petroleum crudes. They are cheap
and stable under service condition and hence most widely used.
Impurities like wax, asphalt etc. are removed from crude oil before use by dewaxing, acid
refining, solvent refining and such other processes. These oils are less resistant to oxidation
and possess lower oiliness. These drawbacks are removed by adding number of additives.
Desirable properties of lubricating oils can be improved by adding small quantities of various
additives. The oils thus obtained are known as blended oils or compounded oils. Some of the
additives can be classified as under:
Lubricant protective additives
Additive type Purpose Examples
Antioxidant Retard oxidative decomposition Aromatic amines, hindered phenols
etc.
Metal deactivator Decrease catalytic effect of
metals on oxidation rate
Amines, sulphides etc.
Antifoamant Prevent persistent foam
formation
Silicon polymers
Surface protective additives
Rust corrosion
inhibitor
Prevent rusting and corrosion
of metal parts
Metal phenolates, sulphonates etc.
Anti-wear agent Reduce wear and prevent
scoring and seizure
Zinc thiophosphates, organic phosphates
Friction modifier Change co-efficient of
friction
High molecular weight organic
phosphorus and phosphoric acid esters
Detergent Keep surfaces deposit free Magnesium phenolates
Dispersant Keep insoluble contaminants
dispersed
Polymeric alkyl thiophosphate
Performance additives
Viscosity
improver
Reduce rate of change of
viscosity with temperature
Polymers of olefins, alkylated styremes
Pour point
depressant
Enable lubricant to flow at
low temperatures
Polymethacylates, alkylated napthene
Seal swell agent Cause swelling of elastomers
by chemical reaction
Aromatic hydrocarbons
Semi-solid lubricant or greases
The semi-solid lubricant obtained by mixing oil with thickening agent (to obtain its
consistency)is known as “grease”. The oil is the principle component and it may be oil of
high or low viscosity. Thickeners are primarily soaps of lithium, sodium, aluminum etc. Non-
soap thickener include carbon black, silicon gel, bentonite etc.
The fibrous structure of the thickener traps the oil and enables the lubricant to cling to
moving parts. However certain properties of grease must be noted as under
 Grease flow only under pressure (unlike oils)
 Water contamination degrades grease very fast
 Grease have high shear and frictional resistance
 It can support heavier loads at lower speeds

6.  Co-efficient of friction is much higher
 It cannot effectively dissipate heat like oils.
Solid lubricants
They are used either in powder form or mixed with oil or water. Low spots on the surface are
filled by these lubricants which then form solid films having low frictional resistance
(µ=0.005 – 0.01). Two most commonly used solid lubricants are:
1. Graphite and
2. Molybdenum disulphide (MoS2).
They have low co-efficient of friction, they are very soapy, non-inflammable and stable in air
even upto 3500
C.
Graphite dispersed in oil is called oil-dogand is used in piston ring-cylinder contact in some
cases. Graphite dispersed in water is called aqua-dog and is useful where a lubricant free
from oil is needed.
Synthetic lubricant
These lubricants exhibit unique combination of properties like high temperature stability,
extended temperature range, long service life in in reactive environment etc. They are not
produced by normal manufacturing or refining process in petroleum industry, rather they are
synthesized in chemical plants.
In general, they possess the following properties:
1. Thermal stability in high temperature
2. Chemical stability in corrosive environment
3. High viscosity index
4. Low freezing point
Few applications are:
Chemical name Application Special quality
Diester Lubrication of turbo-jets Performs satisfactorily
between -500
C-2300
C
Phosphate esters Additive in petroleum
lubes
Improves boundary
lubrication properties
Poly-alkylene glycol Aircraft turbine lubes Thermally stable, free from
corrosive actions
Poly-glycidyl ethers Submarines High oxidation stability
Silicone Lube for clocks, timers,
electronic devices
Moisture repellent
dielectric lubes; Prone to
oxidation at high
temperatures
Basic principles
From the fundamental laws of friction, we know that when a solid body is dragged on a dry
flat surface (refer fig. 1), the resisting force due to friction is dependent on

7. 1. The normal reaction and
2. The nature of the surfaces in contact.
It is also known that the frictional force is independent
of the area of contact.
Refer fig. 1. In this case,
F = N (N = W in this particular case)
Where  = co-efficient of friction and depends upon the nature of the surfaces of
contact.
However, putting some lubricant in between the surfaces to separate them, changes the whole
picture (refer fig. 2).
Now, the frictional resistance is greatly reduced
and is independent of the nature of the surfaces
in contact, but is directly dependant on the area
of contact. In this case, the resistance to dragging
is not due to frictional forces as the surfaces are
not in contact.
Here, the resistance is mainly due to shear stress induced in the lubricant. Thus, the property
of “viscosity” of the lubricant comes into picture. From the Newton’s law of viscosity, we
know that the shear stress of a fluid is directly proportional to the velocity gradient of the
fluid. Mathematically,Shear stress 
dy
du
Where, du is the change in velocity of the fluid and y is the distance between the layers.
Putting the constant of proportionality, the above equation can be written as,
 = 
dy
du
This constant of proportionality is known as co-efficient of dynamic viscosity or simply
viscosity of the fluid.
Again, the resisting force here is the product of shear stress and the area in contact.
Mathematically, F =  A
Thus, the resisting force now is dependent on the quality of the lubricant being used and the
area of the surfaces in contact.
It may be noted that, the above statement is true only when the solid surfaces are completely
separated by a film of lubricant. But this is not possible every time. Let us suppose, a shaft,
rotating inside a journal bearing at a very low speed. In such case, it will practically not be
possible to maintain a continuous film of lubricant separating the surfaces of the shaft and the
bearing. Since the surfaces will be sometimes in contact, the nature and condition of the
surface is also of same importance as that of the quality of lubricant. Moreover, the thickness
of the film will depend not only on the quality of the lubricant but also the load on the shaft.
Based on the above statements, the condition of the surface and lube film thickness may be
classified as under:
1. Perfectly clean surface: free from lube oil or any other contaminant. Maximum
frictional resistance is encountered in this case.

8. 2. Boundary condition: Slightly lubricated surface as an absorbed film on the surface
(in the form of oxides, vapors etc.). The frictional resistance is slightly reduced in
such case.
3. Thick film condition: Complete separation of surfaces by oil film. A hydro-
dynamic wedge of lubricant develops forcing surfaces to keep apart.
4. Mixed film condition: Intermediate between boundary and thick film. This
condition is also known as quasi-hydro-dynamic condition.
5. Elasto-hydrodynamic condition: Combined effect of elastic deformation of
surfaces resulting from external load and pressure-velocity-viscosity relationship.
In brief, there are three different types of lubrication: boundary, mixed and full film. Each
type is different, but they all rely on a lubricant and the additives within the oils to protect
against wear.
Full-film lubrication can be broken down into two forms: hydrodynamic and
elastohydrodynamic. Hydrodynamic lubrication occurs when two surfaces in sliding motion
(relative to each other) are fully separated by a film of fluid. Elastohydrodynamic lubrication
is similar but occurs when the surfaces are in a rolling motion (relative to each other). The
film layer in elastohydrodynamic conditions is much thinner than that of hydrodynamic
lubrication, and the pressure on the film is greater. It is called elastohydrodynamic because
the film elastically deforms the rolling surface to lubricate it.
Even on the most polished and smooth surfaces,
irregularities are present. They stick out of the
surface forming peaks and valleys at a microscopic
level. These peaks are called asperities. In order for
full-film conditions to be met, the lubricating film
must be thicker than the length of the asperities.
This type of lubrication protects surfaces the most
effectively and is the most desired.
Boundary lubrication is found where there are
frequent starts and stops, and where shock-loading
conditions are present. Some oils have extreme-
pressure (EP) or anti-wear (AW) additives to help
protect surfaces in the event that full films cannot
be achieved due to speed, load or other factors.
These additives cling to metal surfaces and form a
sacrificial layer that protects the metal from wear.
Boundary lubrication occurs when the two surfaces
are contacting in such a way that only the EP or
AW layer is all that is protecting them. This is not
ideal, as it causes high friction, heat and other undesirable effects.
Mixed lubrication is a cross between boundary and hydrodynamic lubrication. While the bulk
of the surfaces are separated by a lubricating layer, the asperities still make contact with each
other. This is where the additives again come into play.
Lubrication methods
The following illustration explains the different methods of lubrication.

9. The characteristics for each of the above systems along with their application in general are
explained in the following table.
Table showing characteristics and application for different methods of lubrication
Sl. No. System type Characteristics Application
1. Total loss grease Assembly packed bearings Rolling element bearings
2. Hand grease Through nipple/feed pipe Small no. of application points
3. Centralized
grease
Grease pump Large no. of application points as well
as inaccessible points
4. Total loss oil Can and intermittent feed Small no. of application points also
small rate of heat removal
5. Pressure mist Aerosol spray Rolling element bearings,
mechanisms etc.
6. Splash Lubrication by mist Enclosed gear box
7. Dip (ring or disc) Ring or disc Plain bearing with moderate rotational
speed
8. Wick Pad or wick feed Plain bearing with light load
The above table gives a general description for commonly used methods of lubrication.
However, in a complex system, where the numbers of application points are too many,
particularly for oil lubricant, the force feed circulating system is preferred. Such a system
generally has the facility to continuously clean the oil by a centrifuge or filter. The
temperature of the oil can also be monitored constantly, which enables us to know if there is
any problem in the lubricating system or the equipment.
The central oil circulation system
The central oil circulating system comprises of an oil reservoir of moderate capacity with two
chambers (one for the outgoing and the other for the return) separated by a baffle wall.
Lubricating oil from this reservoir reaches the different friction interfaces of the equipment
through a strainer/filter by the help of an oil pump with a pressure gauge. Before the oil is fed

10. to the friction points, it is cooled by an appropriate cooling arrangement. A thermometer is
fitted at a suitable point on the return line through which the oil returns to the reservoir. A
continuous cleaning arrangement of the oil in the reservoir consists of a centrifuge, a filter
and an oil pump. Care is taken so that the debris coming out of the friction points through the
return line does not reach the outgoing chamber of the oil reservoir. Fig. 3 illustrates such a
circulating system. In most of the equipment now-a day, particularly for IC engines such
system of lubrication is used.
Quantity of lubricant
While deciding upon the quantity of lubricant to be used for a particular system, two
important points are taken into consideration. They are:
1. The quantity of lubricant should be sufficient to avoid starvation of lubricant
2. There should not be excessive feeding also.
In case of starvation, wearing out of the components at interacting surfaces occurs and the
purpose of lubrication is totally lost. On the other end, in case of excessive feeding there will
be loss of lubricant resulting rise in overall maintenance cost of the equipment. It is therefore,
necessary, that an optimum condition is maintained which strikes a balance between the two
extremes.
The thumb-rule for calculation of the quantity of the lubricant is based on the assumptions
that:
Replace one-third of the bearing clearance volume for
-Every two hours for oil and every four hours for grease.

11. However, for a centralized circulation system, the above practices do not hold good. Because,
in that case, frequent replacement is not necessary and the total oil in the reservoir is in
circulation. The capacity of the reservoir, in such case, depend mainly upon the number of
points to be lubricated, circulation time, heat removal rate of the particular lubricant,
concentration of wear debris etc.
Selection of lubricant
Selection of lubricant is a complex decision making process and many parameters like the
load-speed combination of the system, the quality of the interacting surfaces, the availability
of the lubricant of the nearest quality to the desired one etc. are to be taken into account. In
this process, various alternatives are considered. The simplest and cheapest solution is – put a
small quantity of mineral oil in the place of friction. But such simple solutions do not work in
most of the cases, as prevailing situations are much complicated.
But when the life required is long, the system generates more wear debris, also the heat
generated is too much, small quantity does not work and feed of oil is necessary. Similarly,
when sealing from the environment is necessary, grease is always preferred over oil.When
low carbonization and low inflammability is required, synthetic oil is preferred over mineral
oil.When product contamination is not acceptable, solid lubricant is preferred over the
oils.The following table gives the properties of different types of lubricants generally in use.
Properties of lubricant types
Properties Lube types
Plain
mineral oil
Min oil with
additives
Synthetic oil Grease Dry
lubricant
Boundary
lub.
F G-E P-E G-E G-E
Cooling VG VG F P VP
Low friction F G F F P-F
Remain on
surface
P P VP-P G VG
Seal
contaminant
P P P VG F-E
Temperature
range
G VG F-E VG E
Anti
corrosion
P-G E P-G E P
Low
volatility
F F F-E G E
Cost V. Low Low V. High Fairly high High
E - excellent; G – good; VG – very good; F – fair; P - poor
Additives
The various additional properties of the lubricant are generally contributed by the use of
many additives while the base stock of the liquid lubricant may remain same. Therefore, the
total additive package used in the lubricant is of most importance for the functional properties

12. of the lubricant. Types of equipment vis-à-vis important functional additives are given in the
following table.
Important functional additives used based on equipment type
Sl. No. Machinery Additives used*
1. Food processing None
2. Hydraulic AR, AO
3. Steam and gas turbine AR, AO
4. Air compressor AR, AO
5. Gears (steel to steel) AO, AW, EP, PPD, AF
6. Gears (steel to bronze) AO, OI
7. Machine tool sideways OI
8. I.C.Engines AO, AR, AW, PPD, VII, Dt, Ds, AN, CI, AF
*The abbreviations are as under:
CI – Corrosion inhibitor, AN – Acid neutralizer, Ds – Dispersant, Dt – Detergent,
OI - Oiliness improver, AF – Antifoam, VII – Viscosity index improver,
PPD – Pour point depressant, EP – Extreme pressure, AW – Antiwear, AR – Anti rust, AO –
Antioxidant
It may be seen that number of additives being used in I.C.Engines is comparatively higher
than the additives used in other equipment. This is because an I.C.Engine works with wide
variation of pressure and temperature condition. At the same time, no additive is used for
food processing equipment to avoid contamination of the end product.
The viscosity and additives required for a particular service is given in the following table.
From these tables it is seen that for each application, the group of additives differ. Also, for a
particular application, the property requirement must be within certain limits.Achoice has to
be made within these limits so that other requirements are also satisfied.
Additives based on services
Sl.No Service Viscosity (SUS) Additives*
1. Spindle oil > 3600rpm
1800 –3600 rpm
300 – 1800 rpm
35 – 110
100 – 150
150 - 900
RI, MD
-do-
-do-
2. Turbine oils- Direct connected
- Geared
150
300
RI, OI
-do-
3 Hydraulic oils – Vane pumps
-Radial piston pumps
-Axial piston pumps
-Gear pumps
150 – 300
150 – 900
150 – 300
150 – 600
RI, OI
-do-
-do-
-do-
4. Circulating bearing oils– Light loads
- Intermediate loads
- Heavy loads
- Extra heavy loads
150 – 400
400 – 900
900 – 2500
1500 - 3500
RI, OI
-do-
-do-
-do-
5. Engine oils – Gasoline engines
- Diesel engines
SAE30
SAE30-40
OC,CI,Dt,VI,RI,AW,Df
-do-

13. - Gas engines -do- -do-
6. Compressor oil – upto 10 kgf/cm2
- 10 – 70 kgf/cm2
- 70 – 150 kgf/cm2
- 150 – 300 kgf/cm2
- . 300 kgf/cm2
300
500 – 600
600 – 1500
1500 – 2500
2500
OI
OI, AW, CI
-do-
-do-
-do-
7. Refrigerating compressor oil 150 – 300 RI, OI
8. Automotive transmission oil,
conventional transmission and
differential oil
Hypoid gears
Fluid transmission
SAE 90 – 250
SAE 90 – 140
200
AW, Df
-do-
AW, Df, OI, RI
9. Gear oils (enclosed gears)
Spur, herringbone, bevel
Heavy/shock loading worm
Hypoid
600 –1800
1500 –2500
2500
OI, OA, AW, Df
-do- and EP
-do-
*The abbreviations are as under:
AW- Antiwear; OA – Oiliness additive; EP – Extreme pressure; OI – Oxidation inhibitor;
CI – Corrosion inhibitor; RI – Rust inhibitor; Df – Defoamant; MD – Metal deactivator
Apart from the above informations, we have some standard graphs, which are frequently
referred for the purpose of selection of lubricant for particular equipment. Fig. 4 is a pressure
vs. contact speed graph, which explains the limitations of different types of lubricant. This
graph is often referred for the purpose of selection of lubricant.
Similarly, fig. 5 is a viscosity vs. temperature graph showing characteristics of various oils
and fig. 6 is a viscosity vs shear rate graph for different oils. These graphs a are always
referred for selection of lubricant.

14. Hydrodynamic lubrication
Consider a block being dragged on a lubricated flat surface. The block, if light will float as
shown in fig. 2. But a heavy will block will sink and if dragged, there will be build-up of
lubricant in front of it. Thus, when dragged along, it will slide over oil
Keeping itself in inclined position as shown in fig.7. i.e.
an wedge is formed. If the oil is viscous, velocity gradient
will exist and there will be shearing stress induced. The
dragging force will be the product of the shearing stress
and the area of contact. The block and the surface may be
considered the development of a journal bearing. This
theory holds good for a journal bearing also provided the
journal is concentric to the bearing.
On this basis, the Petroff’s law was devised which states,
P
ZN

Where,  = Co-efficient of friction
Z = Co-efficient of viscosity
N = Speed of the journal and
P = Pressure one the lubricant
But the wedge formation makes it different and due to this, there is change of
velocity, followed by change in oil pressure. Consider a horizontal bearing with considerable
load. When stationary, the shaft sinks in the oil film (fig.(a)). But when revolution starts, it
crawls up and drags in some lubricant, and the wedge of lubricant is formed. This is
explained in fig. (b).
(a) (b) (c)

15. Therefore, change in velocity across the circumference takes place and accordingly there is
change in pressure. The journal now, floats from right to left and ultimately settles in an
eccentric position. In this condition, the hydrodynamic pressure keeps the journal afloat on
the oil and the separation between the solid surfaces is maintained.
Mining Equipment
These machines are deployed in most of the mining and construction industries. The
environmental and operating conditions of these equipment are different due to extreme
variation of the environmental conditions (temperature, humidity etc.). However, the
lubrication practice recommended by the manufacturer of the equipment remains same for
similar equipment and thus, does not take into consideration the variation of operating and
environmental condition under which the equipment operates.
These oils are in continuous circulation. Other lubricants like grease and solid lubricants
which are fed to some specific points by the method of local application do not count much
as their consumption is much less compared to those lubricating oils. Out of these oils,
maximum cost is incurred hydraulic fluids as reported by a study made. It has also been
reported that about 6.5% of the operating cost is towards the cost of lubricants used in
HEMM’s in open cast mines.
In this context, it may be mentioned that, hydraulic oil cannot be said to be a typical lubricant
as it serves more as power transmission medium than as lubricant.
Oil degradation
As every material has a span of life while in continuous use, so as the lubricants which
degrades continuously and ultimately reaches a point when it does not serve the very purpose.
This is true even if the system is maintained in best possible condition. A lubricant in use is
subjected to continuous mechanical and thermal stresses. This affects the polymer chains and
there is always some destruction and reconstruction of these chains. This causes some change
in the basic chemical structure and also loss of oxidation stability of the lubricant which is an
important property. In addition, the additives are also in continuous decay and loss of
properties. Further to these, the continuous generation of wear debris from the tribo-
components in contact of the oil further deteriorates the system. Contamination by moisture,
dust, dirt and such other materials are also unavoidable in a mechanical system. Diesel
dilution is a common phenomenon for engine oils.
As the oil degrades continuously, it is essential to drain off the oil in use after certain period
of time and recharge by fresh oil. The time lag between two such charges is the life of the
lubricant. As lubricants are very much costly and more so as it involves foreign exchange,
We must be very careful for its use. Every drop of lubricant must be used up to the last
minute it serves the purpose.