Module 4. Lubricant & Lubrication PDF

Module 4 : LUBRICATION
Importance of lubrication
It is known since ages that oils and greases reduce the friction between sliding surfaces, by
filling the surface cavities and making the surfaces smoother. Action of liquids/greases
known as lubrication. In other words, lubrication is a process by which the friction and wear
rates in a moving contact are reduced by using suitable lubricant. Lubricant is a substance
introduced between relatively moving parts to reduce friction (μ = 0.1 to 0.0001) and wear
rate. The progress in scientific research indicated that reduction in friction occurs due to
decrease in adhesion component of friction compared to abrasion component of friction.
Almost every relatively moving component in an assembly requires lubricant.
Examples :
(1) A standard lock(Fig. 4.1) : On turning the key, the bolt slides into a notch on the door
frame. Force required to turn key and move bolt will be reduced by lubrication.
Fig. 4.1: Simple lock and key.
(2) Window lifting mechanism(Fig. 4.2) : The windows in most of cars use linkages to lift the
window glass. A small electric motor is attached to a worm gear to lift the window. For
smooth functioning lubrication is used.
Fig. 4.2: Window lifting mechanism[1].
Module 4: Lubricant & Lubrication 1

Fig. 4.3: Pendulum clock.
Sometimes the choice of lubricant type depends the properties of system. For example, in
watches or instruments(Fig. 4.3), any lubricant type could meet the load and speed
requirements, but because of need for low friction it is normal to use a very low viscosity oil.
However, for open gears, wire ropes, or chains the major problem is to prevent the lubricant
from being thrown off the moving parts, and it is necessary to use a thick bituminous oil or
grease having special adhesive properties.
Advantages of lubrication in addition to reducing friction and wear rate are :
• Reducing instant failures.
• Reducing fatigue failure (Lubricant reduces the force required in tangential direction
so reduces the Fatigue Failure)
• Reducing surface failures.
• Reducing stress concentration.
Applications of Lubricant :
1. Transmission parts.
2. Bearings.
3. Cams and followers.
4. Journals.
5. Seal faces.
6. Any situation involving metal to metal contact.
Lubricants are often classified as "Newtonian and "Non-Newtonian" fluids. This classification
is on basis of relation between shear stress and shear strain rate(Fig. 4.4).

Fig. 4.4: Newtonian and non Newtonian fluid.
For Newtonian fluid, shear stress is given by Eq.(4.1)
....Eq.(4.1)
In this relation, η is known as dynamic viscosity, which is one of the important lubrication
parameters. Method of replenishing lubricant decides overall performance of the system.
What do we expect from lubricant :
Required lubricant properties are specific to applications. We expect some requirements

from the lubricant which can be explained by consider few examples :
(1) Lubricant between cylinder liner and rings(Fig. 4.5).
Requirements are :
• Lubricant must form a film to separate the surfaces and reduce the friction between
metal to metal contact in order to improve the efficiency of the system.
• Needs to adhere to the surfaces (attachment of thin lubricant layer on the surfaces).
• Must neutralize the corrosive products of combustion.
• Withstand high temperature inside the cylinder.

Fig. 4.5: Lubricant between cylinder liner and rings.
(2) Lubrication in journal bearings (Fig. 4.6) :
Requirements are :
• Lubricant should support heavy shaft and loads.
• Lubricant should avoid contact stresses.
• Lubricant should have ability to dampen vibrations.

Fig. 4.6: Radial journal bearing hydrodynamic pressure profile.
Lubrication in bone joints(Fig. 4.7) :
Requirements are :
• Contain proteins that stick to cartilage layer resulting in smooth sliding.
• Coefficient of friction ~ 0.01.
• Minerals that nourish the cartilage cells.
• Increase viscosity with increase in applied pressure.
Fig. 4.7: Lubrication in bone joint.
Lubricant thickness between two solid surfaces must be thick to avoid wear, but thin enough
to minimize lubricant shearing. In other words, lubrication can be thick or thin based on the
application.
Thick & Thin Lubrications(Fig. 4.8) :
• Thick lubrication is governed by Reynolds theory. Thick lubrication is not

advantageous because lesser the quantity of oil gives the lesser friction.
• Thin lubrication is far more complex. Requires scientific study at nano- to micro-
level. From friction point of view, it is advantageous than thick lubrication.

Fig. 4.8: Fluid film lubrication.
Lubrication Mechanisms :
Although fluid film lubrication relies heavily on fluid mechanics and kinematics, yet it is still
ultimately a problem of two surfaces that are either in partial-contact or separated by a thin
fluid film. Further, Reynolds equation that governs fluid film lubrication is based on the
assumption of thin film. Therefore it is necessary to understand the importance of these
lubrication mechanisms relative to the surface texture of tribo-pair. A dimensionless film
parameter Λ (often referred as “specific film thickness”) is used to classify the
aforementioned four lubricant regimes. Rrms,a is root mean square (rms) surface
roughness of surface a, and Rrms,b is rms surface roughness of surface b. Interestingly
here rms value is used, while generally arithmetic avg. roughness is used. To clarify this, let
us examine Fig. 4.8(a). From tribological point of view, a surface without any asperity but
with a number of valleys (that retains lubricants and provide a room for debris collection) is
always preferred. Measurement of average roughness imposes a linear penalty on all points
whether a point is too close to nominal line or too far. However, rms roughness parameter
uses square term. If there are three points: A one unit, B two units and C three units away
from nominal line, RMS roughness parameter put penalty of one, four and nine on points A,
B, C respectively. Therefore rms value is a better roughness parameter compared to
average roughness.
Fig. 4.8(a): Comparison between average and root mean square roughness.
Based on the value of dimensionless film parameter(Λ), Eq.(4.2) lubrication mechanisms

are classified as follows :
• Boundary lubrication, Λ < 1

• Hydrodynamic lubrication, Λ > 5
• Mixed lubrication, 1 < Λ < 3
• Elastohydrodynamic, 3 < Λ < 5
Peak surface roughness is generally two to three times of the rms surface roughness.
Therefore Λ > 1 does not indicate the clear separation between tribo-pair. This is a main
reason to keep film parameter lesser than 3 but greater than 1 to identify mixed lubrication
mechanism. To avoid any wear and minimize friction, a complete separation, between
asperities of two relative moving surfaces is essential. This requires film parameter more
than 3. Film parameter depends on film thickness and composite surface roughness of
tribo-pair. Often foreign particles or wear debris changes the
hydrodynamic/elastohydrodynamic lubrication to mixed or boundary lubrication mechanism,
as shown in Fig. 4.9.
Fig. 4.9 : Foreign particles/wear debris shift hydrodynamic/elastohydrodynamic lubrication in

boundary/mixed lubrication.
....Eq.(4.2)
In presence of debris(loose particles) lubrication mechanism may be changed as shown in

Fig. 4.9.
References :
1. http://auto.howstuffworks.com/power-window1.htm Accessed on 19th February 2013.

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Boundary Lubrication
'Boundary Lubrication' term was coined by English Biologist “Sir Hardy” in 1922. He quoted
that “Very thin adsorbed layers, about 10 A° thick, were sufficient to cause two glass
surfaces to slide over each other”. The layer of lubricant separates sliding surfaces, i.e. no
direct contact of the sliding parts. This situation is required for many applications such as
steel gears, piston-rings and metal -working tools, to prevent severe wear or high
coefficients of friction and seizure.
How thin layer is able to separate surfaces ?

Fig. 4.10: Boundary lubricants "Oiliness additives"[1].
The best boundary lubricants are long chain molecules with an active end group, typically
fatty acids. Representative molecules of these types are shown in Fig. 4.10. These consist
of a hydrocarbon backbone of carbon atoms and an active end group. In fatty acid active
group is COOH, known as the carboxyl group. Such a material, dissolved in a mineral oil,
meets a metal or other solid surface with active end group attaches itself to the solid and
gradually builds up a surface layer.
Characteristics required for Thin Film Lubrication :
(1) Required long chain molecules(Fig. 4.11), with an active end group, which by
attaching itself to the solid surface builds a surface layer. More number of layers
reduce lubrication in friction coefficient as shown in Fig. 4.12.
(2) It should be dissolvable in mineral/lubricating oils.
(3) Temperature stability : It is important because increase in operating temperature may

cause reduction in molecular attraction that may lead to detachment of boundary
additives from surface.

Fig. 4.11: Long chain boundary additives[1].
Fig. 4.12: Number of layers vs friction coefficient[1].
In sliding contact under air or water, the protective oxide is torn away, exposing the pure
metal of both surfaces. These may weld together before oxygen can reform the protective
layer. Therefore, boundary lubricants are required when metals are covered with natural
protective layer of oxide.
Effect of length of molecule on boundary lubrication :
Longer hydrocarbon chain, will give more effective separation between solid surfaces. This
brings low coefficient of friction. Sir William Hardy’s experimental results are :
(1) Normal fatty acids gives less coefficient of friction than normal paraffin
hydrocarbons(Fig. 4.13).
(2) Bismuth surfaces have less friction than the steel surfaces etc(Fig. 4.14).

Fig. 4.13: Effect of lubricant on friction coefficient.
Fig. 4.14: Effect of material in pressure of boundary additives.
Mechanisms of Boundary Lubrication :
Boundary lubrication comprises of two mechanisms :
(1) Physisorption.
(2) Chemisorption.
Physisorption :

First boundary lubrication mechanism is physisorption or “physical adsorption’ (physical
bonding by van der Waals force). In this mechanism, surface active molecules of oiliness
additives are attracted to surface by electrostatic (dipole) forces(Fig. 4.15).
Fig. 4.15: Physical adsorption of solid additives on boundary surface.
Energy is lowered when the molecules adsorb on the surface by physical attraction of
additives on the surfaces. It requires some properties like
• Additives should dissolve in solute.
• Attachment and detachment is a process encouraged by dilute concentrations and

hindered by high concentration of polar molecules. Hence too much of additives
should not be present.
Table 4.1: Percentage of boundary additive vs friction coefficient.
The Table 4.1 indicates the effect of additives (added to the mineral oil) on coefficient of
friction . As we increases the percentage of additives, the coefficient of friction reduces, but
upto a certain limit after that coefficient does not change.
As the temperature increases, the viscosity reduces so that friction reduces . But as the
temperature reaches critical value the friction increases, this is shown in Fig. 4.16. Fig. 4.17
indicates disruption of boundary lubricants at critical temperature, which results in increase
in friction coefficient(Fig. 4.16).

Fig. 4.16: Temperature vs μ.
Fig. 4.17: Effect of temperature on adsorption[1].
At a somewhat higher temperature, physically absorbed molecules get desorbed. In other

words, molecules still present on the surface but lose their attachment. Consequently
wherever the surfaces come together, the lubricant molecules are pushed away and
intimate metal-metal contact is able to occur.
Chemisorption :
Chemisorption is a form of corrosion.To form a chemically bound layer, three things are
needed :
- Chemically active group.

- Reactive surface of material.
- Surface free from physisorbed material so that chemical reaction occurs. This
creates a gap(Fig. 4.18), where boundary additives become ineffective.
Fig. 4.18: Temperature gap.

During each contact, the chemical layer is rubbed off the surface and has to be reformed
before next contact come round. Surface is therefore, slowly worn away so the additive
must be chosen with care. Fig. 4.19 indicates effective boundary lubrication requires a
combination of physisorption and chemisorption.
Fig. 4.19: Combination of Physisorption and Chemisorption of effective lubrication.
Desirable properties from a boundary lubricant :
• Dissolvability in lubricating oils.
• Reactivity with metals at high temperature to form the metal soap (higher melting
points).
• Low shear strength to give a low friction.
• High melting point so that it provides solid-film protection up to a high temperature.
• Resist penetration by surface asperities.
Boundary(BL) vs Extreme Pressure(EP) lubrication :
Often there is doubt about BL and EP lubrication. BL is restricted to those systems where
there is thermodynamic reversibility. A small change in temperature or concentration, up or
down, brings a related change in film coverage. If lubricant reacts chemically (chlorine,
sulfur, phosphorus) with metal, then it must known as EP lubricant. At low load/temperature,
E.P. Lubricants are ineffective and BL remain active.
Major difficulty with EP lubricants is their carcinogenic nature and environmental pollutant,
therefore EP additives should be avoided as far as possible.
Comparative Study :
To understand lubrication in all respect, a comparative study among dry, boundary

lubricated, and fluid film lubricated has been provided.
Wear rate vs. time(Fig. 4.20a) :

As the time increases, wear rate decreases and remain constant up to certain time then
increases for dry lubrication. For boundary lubrication, the wear rate decreases up to certain
time then decreases or increases depends on improvement in surface smoothness. If
surface smoothness occurs, boundary lubrication turn out to fluid film lubrication it means
wear rate decreases otherwise wear rate increases. For fluid film lubrication wear rate
drastically decreases, then remain constant up to certain limit are then increases.
Fig. 4.20: Comparative study among Dry(1), Boundary(2) and Hydrodynamic(3) lubrication
mechanisms.
Wear rate vs. load(Fig. 4.20b) :
For dry lubrication, as the load increases wear rate increases. For boundary lubrication,
wear rate increases and the rate of increase in wear rate is lesser than the dry lubrication.
For fluid film lubrication also wear rate increases, but rate of increase in wear rate is initial
lower. Same process does occurs with increase in the temperature.
Wear rate vs. temperature(Fig. 4.20c) :
Wear rate increases for all three lubrication mechanisms with increase in temperature. One
of the common element in all machine is spur gear. Spur gear generally operates under
boundary lubrication regime. To understand the effect of time and load on wear rate of spur
gears, an experimental(Fig. 4.21) study was performed.
Experimental Study :

Fig. 4.21: Online monitoring of spur gears.
Geometric data related to single stage gearbox(Fig. 4.21) are given below :
• Pinion speed (Np = 3000 rpm)
• Gear speed (Ng = 1500 rpm)
• Pitch circle diameter of pinion (Dp = 55 mm)
• Pitch circle diameter of gear (Dg = 110 mm)
• No of teeth on pinion (Tp = 22)
• No of teeth on gear (Tg = 44)
• Module = 2.5 mm
To measure the wear rate, online sensors(Fig. 4.22) were used.
Fig. 4.22: Online sensor instrument.
Experimental Results :

Results obtained from experimental study are tabulated in Tables 4.2 to 4.6 :
Table 4.2: Fe concentration.
Results listed in Table 4.2, show running in behaviour of spur gear at speed 500 rpm. But
chasing speed from 500 to 1500 rpm, Fe concentration increases. This concludes that
running in behaviour occurs at every speed. Result of Table 4.3 illustrate that on changing
lubricating oil(using fresh oil), Fe concentration(compared to results shown in Table 4.2)
decreases.
Experimental Results after changing oil :
Table 4.3: Changing operating condition changes the dynamics of B.L.L
Experimental Results :
Tables 4.4 and 4.5 shows wear rate versus time. Both of these tables indicate reduction in
wear rate with time.

Table 4.4: Particle size vs time.
Table 4.5: Particle size vs time.
Experimental Results under Load :
Table 4.6 indicates reduction in wear rate on increasing load from zero load to very low
load. This means some load is required to reduce the vibration of gears and reduce the
wear rate.
Table 4.6: Fe concentration vs load.

References :
1. Stachowiak G W & Batchelor A W, Engineering Tribology, Third Edition, Elsevier Inc.,

2005.
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Mixed Lubrication
In bearings and other lubricated devices where relative motion occurs, lubrication is divided
into three types : fluid film, wherein the load is supported by the film pressure of a fluid
lubricant; boundary, in which the load is supported by direct surface films of the moving
solid members; and mixed, where the load is carried by some mixture of the first two. All
three lubrication mechanisms can be compared using stribeck curve.
Stribeck curve :

Fig. 4.23: Stribeck curve.
This above plot(Fig. 4.23) for a hypothetical fluid lubricated bearing system presents friction
coefficient as a function of sliding speed, fluid viscosity and unit load. Three lubrication
mechanisms; boundary, mixed and hydrodynamic lubrications has been marked on this plot.
This plot defines the stability of lubrication. Suppose we are operating to the right of
minimum friction and something happens, say an increase in lubricant temperature. This
results lower viscosity and hence a smaller value of bearing number. The coefficient of
friction decreases, not as much as heat is generated in shearing the lubricant, and
consequently the lubricant temperature drops. Thus the region to the right of minimum
defines stable lubrication because variations are self correcting. To the left of line, a
decrease in viscosity would increase the friction. As temperature rise would increase, the
viscosity would be reduced still more. The result would be compounded. Thus, the region to
the left of minimum represents Unstable lubrication.
Load sharing :
Fig. 4.24 shows two machine elements in lubricating contacts. These are four possible
mechanisms; asperities in contact may experience dry or boundary lubrication and
asperities in contact with liquid lubricant may experience elasto-hydrodynamic or
hydrodynamic lubrication.
Fig. 4.24: Load sharing
W1 load carried by dry(no intended lubrication).

W2 load carried by boundary lubrication (Physical/chemical lubrication).
W3 load carried by elasto-hydrodynamic lubrication.
W4 load carried by fluid Lubrication.
ω1 + ω2 + ω3 + ω4 = 1.0....Eq.(4.3)
ω1, ω2, ω3 and ω4 are the weights given to the different lubrication mechanism, and sum
of all weights must be equal to one.
Total load capacity W = ω1W1 + ω2W2 + ω3W3 + ω4W4....Eq.(4.4)
Coefficient of Friction :
In "Friction" coefficient of friction is expressed as
....Eq.(4.5)
Where
• τi : = shear strength at interface.
Interface shear strength is expressed as
Ti = ω1Tmi + ω2Tbli + ω3TEHLi + ω4TFFLi....Eq.(4.6)
Where
• τi : = shear strength at interface.
• τbli : = shear strength at boundary lubrication interface.
• τFFLi : = shear strength at fluid film lubrication interface.
• τmi : = shear strength at mixed lubrication interface.
• τEHLi : = shear strength at elasto hydrodynamic lubrication interface.
Generally, interface shear strength τEHLi and τFFLi are much lesser than τbli and τmi,
therefore
τi ≈ ω1τmi + ω2τbli....Eq.(4.7)
It is often difficult to evaluate ω1, τmi, ω2 and τbli. One way is to express
ω1 = α
ω2 = 1 - α

Table 4.7: Effect of boundary lubricant.
Assuming a heavily load tribopair(Fig. 4.24) is lubricated with mineral oil. Due to high load,
lubricant will squeeze out and asperities will be in metal to metal contact. Table 4.7 lists the
coefficient of friction in presence and absence of boundary lubricants. If we assume shear
strength of soft material is τy = 200 MPa, then
It is interesting to note that the shear strength of boundary lubricant, when attached to solid
surface is very close to a solid surface (or between two solid surfaces) compared with the
bulk fluid, which is generally lesser than one MPa.
Mixed Lubrication : Wear
The complexity of the wear processes, prevents real testing of the concept based on
superposition of individual components of wear, which is often espressed by Eq.(4.8)
V = va + vc + vf + vaf + vcf + vabrasion....Eq.(4.8)
Where V is the wear volume , and the subscripts f, a, c, refer to fatigue, adhesion, corrosion
respectively. There is a recognition that some of the mechanisms are interactive, therefore
Vaf indicates wear volume from fatigue and adhesion combined. In the above equation,
abrasion has a unique role; since all of the mathematical models for primary wear assume
clean parts and lubricant, there will be no abrasion until wear particles have accumulated.
Therefore, V abrasion becomes a function of va, vc and vf, probably a step function. In most
cases Vaf is negligible because adhesion and fatigue rarely coexist. However, corrosion is
known to accelerate fatigue, therefore, Vcf may be significant.

It has been observed that the sensitivity of wear reduction in the presence of boundary
lubricant is higher than reduction in friction coefficient.
A crude assumption is
β = α3/2....Eq.(4.9)
where, β = (Wear lubricated)/(Wear unlubricated)
α = (μ - μ1)/(μm - μ1)
μ = τi/[2√(τ2y - τ2i)]
τi = ατmi + (1 - α)τbli
μm = 0.6; μ1 = 0.05.
Table 4.8
Using these, approximately wear in the pressure of boundary lubricant may be estimated as
shown in Table 4.8.
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Hydrodynamic Lubrication
Hydrodynamic lubrication occurs if tribo-surfaces are perfectly rigid and retained their
geometric shape during operation. This mechanism is extensively used to support load
without causing any wear of machine components. In this mechanism, fluid completely
separates relatively moving surface. The fluid may be liquid or gas. In case gas as
separating fluid, lubrication mechanism is termed as aerodynamic lubrication. This kind of
lubrication mechanism was first explored in Tower’s experiment.
Towers experiments :
Tower performed experiments on a partial arc bearing by imposing load using bearing cap
on rotating journal as shown in the Fig. 4.25. Lower part of journal was immersed in
lubricating oil and friction resistance on bearing was obtained by measuring friction moment
acting on bearing cap. He found reduction in the coefficient of friction in presence of liquid
lubricants in rotation.
Tower’s observations :
1. He found very small (~0.001) coefficient of friction.

2. Increase in frictional resistance on increase in sliding speed. This was different from the
Coulomb frictional resistance, which does not depend on the sliding velocity.
3. Decrease in friction resistance on increase in operating temperature.
Fig. 4.25: Sketch of Tower's test setup.
Conclusions based on Tower's experiments :
(1) In oil bath lubrication, friction resistance follows the laws of “liquid friction” compared to
“solid friction (coulomb, adhesion)”.
(2) Floating is possible, when we use lubricating oil.
(3) Fluid pressure is maximum in middle of bush and it is more than double the mean
pressure. It means pressure is not constant but varying.
(4) Unstable friction resistance occurs during lubrication other than oil bath lubrication,
probably due to insufficient amount of oil.
(5) Tower’s experimental results motivated researchers to understand the effect of various
parameters responsible for generation of fluid film pressure.
Fluid mechanics concepts :
To understand the Tower's experiment, we can take an example of parallel plate (Fig. 4.26)
and use fluid mechanics concepts :

Fig. 4.26: Lubrication between parallel plate.
• Consider two parallel plates, AB and A’ B’, separated by fluid film of thickness h in y-
direction. Assume plate A’ B’, is sliding at velocity ua relative to plate AB. No
pressure will develop within the parallel surfaces.
• Now assume plate AB is inclined at angle α and film thickness h is function of

coordinate x. Due to inclination exit area (B B’) will be smaller compared to entrance
area (A A’) as shown in Fig. 4.27. To conserve the mass flow rate, a positive
pressure gradient will be generated at exit and negative pressure gradient will
generated at entrance as shown in Fig. 4.28.
Fig. 4.27: Lubrication between inclined plates.
In other words, positive pressure(to support load) is developed between inclined and
relatively moving surfaces.
Fig. 4.28: Positive pressure gradient at exit and negative pressure gradient at entrance.

Fig. 4.29: Pressure profile between inclined plates.
To develop hydrodynamic lubrication mechanism, two features are essential :
(1) The liquid must be viscous.

(2) The geometry of the surfaces must be such that as one surface moves over the other
and they must produce a convergent wedge of liquid as shown in Fig. 4.30.

Fig. 4.30: Convergent wedge of liquid.
Fig. 4.31: Journal bearing.
Details of the geometry of a journal bearing presented in a polar coordinate system (r, θ, z)
with θ = 00, being aligned with the line of centres is shown in Fig. 4.31. In this analysis, the
subscript “b” is used for bush and “j” for journal. The bearing dimensions are: rb, the radius
of the bush; rj, the radius of the journal; l, the length of the bearing; c, the radial clearance,
equal to rb - rj. Under stable conditions with an applied load F, the journal operates with an
eccentricity e in relation to the bush, the eccentricity ratio ε being given by e/(rb-rj). The
operating speed of the journal is n, its surface velocity being U = (2 π n/60)rj. The film
thickness is dependent on θ and has the value.
h(θ) = C + e cos(θ)....Eq.(4.10)
When the load is relatively low and the relative speed is high, a fluid wedge is created by
two approaching surfaces. Being incompressible, the fluid generates sufficient pressure to

support the load completely, keeping the two surfaces apart. This is generally referred to as
the hydrodynamic lubrication. Increasing load or decreasing speed will reduce the film
thickness. Depending on the elasticity of the solids, these surfaces can be elastic in nature
and deflect under fluid pressure. This is commonly referred to as elastohydrodynamic
lubrication. Increasing the load further or decreasing the speed, plastic deformation occurs.
When the fluid film thickness is less than the surface roughness, commonly referred to as
boundary lubrication which was studied in previous subheading.
Characteristics features of hydrodynamic lubrication :
(1) In hydrodynamic lubrication, the total friction arises from shearing of the lubricant film so
that it is determined by the viscosity of the oil. We can observe that, lower the viscosity
of the oil, the lower the friction. However, the distance of nearest approach (minimum
film thickness) between sliding surfaces places a limit to the lowest possible viscosity.
(2) The great advantages of hydrodynamic lubrication are that the friction can be very low
(μ ~ 0.001) and, in the ideal case, there is no wear of the moving parts.
Viscosity :
Viscosity is the property of the fluids, which is due to internal friction and molecular
phenomena . This causes resistance to fluid flow. Fig. 4.32 illustrates very viscous to
viscous liquids trying to flow under same gravitational force. There are two representations
for viscosity. They are Dynamic Viscosity and Kinematic Viscosity. Dynamic or absolute
viscosity is the ratio of the shear stress to the resultant shear rate when a fluid flows. The
unit of dynamic viscosity in SI units in Pascal and in CGS units it is centipoises. The
kinematic viscosity is equal to the dynamic viscosity divided by density. The SI unit is
square meter per second, but CGS units centistokes is more widely accepted. The
centistokes is the unit most often quoted by lubricant supplier and users as listed in Table
4.9.
Fig. 4.32: Viscosity comparison.
Table 4.9: Typical operating viscosity ranges.

Fig. 4.33: Lubricant shearing of Newtonian fluid.
Fig. 4.34: Viscosity shear rate relationship.
In hydrodynamic lubrication, liquid friction is given by Friction = Shear stress * Area
Assuming Newtonian fluid behaviour, F = (Viscosity * V/h) * Area
Viscosity shear rate behaviour for various lubricant models are plotted in Fig. 4.34.
Film thickness depends on the applied load and relative velocity, and is largely affected by
viscosity of lubricant. Increase in viscosity can increase the film thickness and establish the
elasto hydrodynamic or hydrodynamic lubrication. As most industrial lubricants are liquids,
the viscosity is considered to be the best single index for use in full film lubrication. In most
of tribo-pairs such as, gear boxes, hydraulic systems, engines, it is the viscosity which

determines the friction loss, load carrying capacity, film thickness, lubricant flow rate, and in
many cases, wear rate. Viscosity is mostly measured in centistokes (1cSt = 1mm2/s) and
expressed in centipoises (1cP = 0.001 Pa.s). To standardize lubricant oils, International
Organization for Standardization (ISO) has setup grading system. This system is based on
the kinematic viscosity of the oil, in centistokes, at 400C, as shown in Table 4.9(a).
Table 4.9(a): ISO Viscosity grades.
Oil viscosity is a strong function of temperature; therefore, it is always quoted with

temperature. The viscosity-temperature variation is expressed as η = ηref e-β.(T - Tref), or
Vogel’s equation(η = ηref eb/(t + θ), or Walther’s equation(loglog[η/(ρ + 0.6)] = constant - c log
T). Fig. 4.34(a), shows the variation in oil viscosity with temperature.
Fig. 4.34(a): Variation in oil viscosity with temperature.
For better evaluation of the relationship between viscosity and temperature, a comparator
“Viscosity Index” is used. High viscosity index indicates less sensitivity of viscosity on
temperature. Fig. 4.34(b), shows lubrication regimes for three oils with low, medium and
high viscosity index. Merits of oil with high viscosity index are clearly illustrated in this Fig.
4.34(b). As viscosity is a very influencing parameter in designing tribo-pair based on the
fluid film lubrication, it is important to keep this parameter within certain limits. Use of multi-
grade oil is a way to reduce sensitivity of viscosity-temperature. Therefore, most oils on
shelf today are MULTIGRADE oils, such as 10W30 or 20W50.

Fig. 4.34(b): Stribeck diagram.
The two numbers in oil grade indicate two separate grades: one grade at 00F and in a
higher grade at 2100F. For example 10W30 indicates 2100 cP at 00F & viscosity of SAE30
at 2100F. Lower the first number better is the performance in extremely cold conditions.
Higher the second number the oil will protect better at higher temperatures. These oils are
made by adding polymers in mineral oils to enhance viscosity indices (about 150). At cold
temperatures, the polymers are coiled up and allow the oil to flow as their low numbers
indicate. As the oil warms up, the polymers begin to unwind into long chains that prevent the
oil from thinning as much as it normally would. Table 4.9(b), shows viscosity index of few
commonly used lubricating oils.
Table 4.9(b): Variation of viscosity with temp for commonly used engine oil.
Multigrade oil exhibits a non-newtonian behavior. Viscosity behavior of non-Newtonian fluids

depends on the rate of shear. Higher relative velocity of tribo-pairs causes shear thinning
effect, as shown in Fig. 4.34(c). Such a behavior is represented by expression
where γ is shear rate, and µ is dynamic viscosity.

Fig. 4.34(c): Shear thinning effect of multi-grade oils.
To understand the role of shear stability parameter K, let us consider two monograde oils
(μ1 = 0.0111 Pa.s; μ2 = 0.0063 Pa.s), and two multi-grade oils: K for oil A = 1500 Pa, and K
for oil B = 20000 Pa. We use these four oils to evaluate the performance of a journal
bearing operating at 5000 rpm, under radial load of 21110 N. The results of minimum film
thickness and maximum pressure are listed in Table 4.9(c).
Table 4.9(c): Shear stability of multi-grade oils.
Table 4.9(c) shows multigrade oil with a high value of K performs better than the oil with low
value of K. Therefore even if oils have same grade, they may perform differently because of
their shear stability parameter. In previous paragraphs, qualitative behavior of four different
mechanisms of fluid film lubrication were highlighted. Temperature and shear thinning effect
of lubricants were described. Thin film hydrodynamic and elastohydrodynamic lubrications
were treated as ideal lubrication mechanisms. Dependence of elastohydrodynamic
lubrication on behavior of hydrodynamic lubrications was remarked. This makes it worth to
investigate the quantitative behavior of hydrodynamic lubrication.
Viscosity Temperature Relationship :

Fig. 4.35(a): Viscosity temperature relationship.
For all liquids viscosity decreases as the temperature increases, but rate of decrease varies
considerably. The sensitivity of viscosity thinning on increase in temperature is expressed
by VI = Viscosity Index.
Viscosity Index :
Relates change of viscosity with temperature (at 37.80C and 98.90C) to two arbitrary oils
(having viscosities L & H at 37.80C), one based on a Pennsylvania crude oil and one on gulf
coast oil(Fig. 4.35). Higher value of VI is always aimed. High VI from mineral oils can be
obtained by :
- Remove aromatics (low VI) during refining stage.

- Blending with high viscous oil.
- Using polymeric additives.
Fig. 4.35(b): Viscosity index.

In the 1920s it was known that Pennsylvania oils were good with minimum temperature
effect, while oils from gulf coast (Texas) varied very much with temperature. Viscosity index
is given by VI = 100[(L - U)/(L - H)]
Viscosity Temperature Relationship :
To model viscosity temperature relationship, dynamic viscosity is expressed by Vogel(Eq.

4.11) relation and kinematic viscosity is expressed by Walther`s(Eq. 4.12) relation.
Vogel`s equation η = ke b/(t + θ)....Eq.(4.11)
k gives inherent viscosity, b has units of temperature. b increases with increase in viscosity.
Walther`s equation loglog(cS + 0.6) = constant - clog T....Eq.(4.12)
Table 4.10: VI of various lubricants.
Most oils on shelf today are multigrade oils, such as 10W30 or 20W50 as listed in Table
4.10. These oils are made by adding polymers in mineral oils to enhance viscosity indices
(about 150). At cold temperatures the polymers are coiled up and allow the oil to flow as
their low numbers indicate. As the oil warms up the polymers begin to unwind into long
chains that prevent the oil from thinning as much as it normally would.
Two systems for oil classification are given in Table 4.10:
SAE– (Society of Automotive Engineers)
API-- (American Petroleum Institute) Designation “S” for gasoline and “C” for diesel engines.
----------------------------------------------------------------------------------------------------------------------
Elasto-Hydrodynamic Lubrication
In rolling contact elements (bearings, gears), generated fluid film is minutely small slightly
greater than irregularities of the surfaces, but serve much longer than predicted by mixed
lubrication theories. Increased viscosity under the action of extreme local pressure leads the
generation of thicker film.

Many times using only classical hydrodynamic lubrication theory predicts a negligible fluid
film in high-pressure non-conforming contacts such as exist in rolling element bearings and
gears, but on considering the influence of pressure on both elastic deformation and lubricant
viscosity, a significant oil-film thickness becomes possible. It is interesting to note this
difference between elastohydrodynamic lubrication(EHD) and hydrodynamic lubrication(HD)
occurs as HD is based on the assumption of a fluid continuum , while EHD shows significant
increase in local (limited to few molecular thickness) viscosity compared to bulk viscosity.
Pressure Viscosity Relationship(Barus Relation) :
• According to the Barus relation, lubricant viscosity increases with pressure.
• Barus Relationship: η = η0 exp(αp)
where
η = fluid viscosity at pressure p.
η0 = fluid viscosity at ambient pressure.
α = piezo-viscous coefficient.
α for oil ~ 1 - 2 * 10-8/pa
• Using Barus relation, the effect of pressure on viscosity thickening is listed in Table
4.11. This is important in lubrication of heavily loaded concentrated contacts. At high
pressure, the molecules take considerable time to re-arrange themselves, following
pressure change. This means viscosity thickening takes sometime, and it does not
happen instantaneously.
Table 4.11: Effect of pressure on relative viscosity thickening.
Elasto - hydrodynamic Lubrication :
In addition to viscosity thickening under mechanical load, every surface gets deform. The
applied lubricant gets dragged into the interface and builds pressure. For example; rubber
seals, gear teeth. Film pressure is greater than 10 MPa is sufficient to deform the tribo-
surfaces by sub-micron to micron level. Fig. 4.36 shows elastic deformation of rubber under
load. On relative motion, lubricant is dragged and builds pressure and supports more load.

Fig. 4.36: Elastic deformation of rubber.
In other words, three mechanisms help to support the load under elastohydrodynamic
lubrication.
- Elastic deformation of tribo-surfaces.

- Effect of increase in viscosity with pressure.
- Hydrodynamic lubrication.
To get first hand experience on elastohydrodynamic lubrication, experimental studies on

brass and acrylic bearings(Table 4.12) were performed.
Table 4.12: Material data of brass and acrylic bearing materials.
Fig. 4.37: Experimental setup.

Fig. 4.38: Acrylic bearing.
Experimental study on soft & hard bearing materials :
Experimental setup used to test soft(Acrylic) and hard(brass) bearings is shown in Fig. 4.37.
Loading lever with appropriate weights was used to displace submerged(Acrylic or Brass)
bearing relative to fixed vertical shaft. A photograph of acrylic bearing is given in Fig. 4.38.
Experimental results are tabulated in Table 4.13.

Table 4.13
The results of Table 4.13 clearly demonstrate the advantages (low friction coefficient and
low values of maximum pressure) of Acrylic(soft) bearing compared to Brass(hard) bearing.
Based on these results it can be said that design goal must be "Elastohydrodynamic
Lubrication".

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Types & Properties of Lubricants
Wear coefficients K for different lubrication mechanisms are provided in Table 4.14. Here
unlubricated wear mean, no intentional lubricant at the interface. If we compare unlubricated
and solid lubricated cases, we find wear rate in the presence of solid lubricants will be
lesser than 1% of wear rate observed under unlubricated case. These data(Table 4.14)
motivate us to use lubricants.
Table 4.14: Wear coefficient for lubricated sliding[1].
Lubricant Classification
Lubricant is substance that reduces friction and wear at the interface of two materials. The
lubricant at interface reduces the adhesive friction by lower the shear strength of interface.
Based on the shear strength of lubricant or molecular state, lubricants are classified in four
categories.
(1) Gaseous lubricants
(2) Liquid lubricants
(3) Semi-solid lubricants
(4) Solid lubricants
Fig. 4.39: Molecular state of lubricants.
Solid Lubricants :
A solid lubricant is basically any solid material which can be placed between two bearing
surfaces and which will shear more easily under a given load than the bearing materials

themselves. The coefficient of friction in dry lubrication is related to the shearing force and
the bearing load. Two primary property requirements are :
1. Material must be able to support applied load without significant distortion, deformation
or loss in strength.
2. Coefficient of friction and the rate of wear must be acceptably low.
Advantages & disadvantages of solid lubricants are listed in Table 4.15 :
Table 4.15: Advantages and Disadvantages of solid lubricants.
One way to apply solid lubricant is powder coating. We can use powder form of solid
lubricant and rub against the tribo surfaces. Two eccentric cam are shown in Fig. 4.40. Left
hand side cam(Fig. 4.40a) is rubbed with MoS2 powder. After three hours of operation,
MoS2 coating gets detached from the cam surface and worn out cam is shown in Fig. 4.40b.
In short; if self healing mechanism is missing, then operating life of solid lubricated
component will be very short. Therefore, often carrier fluids are used to coat MoS2 under
operating conditions and replenish MoS2 wherever required.
Fig. 4.40: Eccentric cam.
Solid lubricants as Bonded Coating :
To increase the durability of solid lubricants coated on surfaces often binders along with
lubricating pigments are used. Bonded coatings provide greater film thickness and
increased wear life and are more reliable and durable method for applying solid lubricants.
Under carefully controlled conditions, coatings consisting of a solid lubricant and binding

resin agent are applied to the material by spraying, dipping, or brushing. Dipping is less
expensive method. Resins, binder agents, remain effective if operating temperature is
lesser than 3000C. Inorganic binders, such as metal salts or ceramics, are used for higher
temperature application(> 3000C).
Surface preparation is very important to remove contaminants and to provide good surface
topography for lubricant adhesion. Air-cured coatings are temperature sensitive, therefore,
heat-cured coatings, which can tolerate higher temperature are used for inorganic binders.
Typical applications of bonded coating of solid lubricants are :
(a) Cylindrical bushes (Plain bearings)

(b) Separator (Cage of rolling bearing)
(c) Electrical brushes (Additive to carbon-graphite)
Classification of Solid Lubricants :
Solid lubricants in use are self-lubricating composites. These composites are classified as
polymer, metal-solid, carbon and graphite, and ceramic and cermet.
POLYMERS :
These lubricants are suitable to bear light loads. With recent advances in polymers,
polymers make the largest group of solid lubricants.
There are two main limitations of solid lubricants which must be accounted before selecting
polymers as solid lubricants.
• Low thermal conductivity of polymers inhibits heat dissipation, which causes

premature failure due to melting.
• Two polymers in sliding contact will normally operate at significantly at reduced

speeds than a polymer against a metal surface.
In polymer, sub class of solid lubricants, PTFE, Nylon and Synthetic polymers are common
solid lubricants.
Poly Tetra Fluoro Ethylene (PTFE) :
Polytetrafluoroethylene is a polymer produced from ethylene in which all the hydrogen

atoms have been replaced by fluorine atoms. Teflon is trade name of PTFE given by Du
Pont. Very light load applications. Poor adhesion of PTFE to other materials is responsible
for very low μ (< 0.1).
Strengths of PTFE :
• High chemical stability, great chemical inertness, because of carbon fluorine bonds.
• Very low surface energy, low friction (0.1), high P, low V.
• Nontoxic- useful in pharmaceutical and food industries.
Weaknesses of PTFE :

• Too soft, high wear rate.
• Poor creep resistance, Low load capacity.
• Poor thermal conductivity, high thermal expansion, temp. limit(2500C).
• Vacuum is detrimental to performance.
• Nylon :- Similar to PTFE, but slightly harder (Specific wear rate; 10-6 - 10-5 mm3/min).
• Synthetic polymers :- Most of disadvantages of PTFE can be overcome by using

fillers (glass, carbon) & impregnating it with metal (bronze, lead) structures.
With a suitable rigid (metal) backing, PTFE can withstand wear under extremely high loads
(100 MPa) or more, with a friction coefficient of 0.1 or less and virtual freedom from stick-
slip sliding. The wear rate of polymer composites is highly dependent upon the surface
roughness of the metal counter faces. In the initial operating stages, wear is significant but
can be reduced by providing smooth counter faces. As the run-in period is completed, the
wear rate is reduced due to polymer film transfer or by polishing action between the sliding
surfaces. The low thermal conductivity of polymers inhibits heat dissipation, which causes
premature failure due to melting.
Metal - solid lubricant :
These metal solids lubricants containing lamellar solids rely on film transfer to achieve low
friction. But continous transfer of film may reduced the life of component, therefore often "no
lamellar solids" are added to lamellar solids low friction characteristics. To achieve these
objectives, holes are drilled in machine parts and those holes are packed with solid
lubricants. Various manufacturing techniques are used in the production of metal-solid
composites. These include powder metallurgy, infiltration of porous metals, plasma
spraying, and electrochemical code position.
Molybdenum Disulfide :
It is one of the most common used metal-solid lubricant.

• Strengths of MoS2 :
1. High Load Carrying (> 700

MPa)
2. Low Friction
3. High temperature lubricant

particularly in space.
• Weaknesses of MoS2 :
1. Moisture detrimental to
performance
2. Film thickness ~ 15 μ m.
Fig. 4.41: MoS2
Molybdenum disulphide starts to oxidize significantly above 3500C in oxygen and 4500C in
air, but the main oxidation product is molybdic oxide, which is itself a fair high temp
lubricant. In high vacuum, the disulphide is said to be stable to 10000C and it evaporates
very slowly, so that it has been widely used in space.
The thickness of the film when applied is generally 15 microns, which provides the longest
life. Thicker films apparently do not last as long, apparently because it becomes easier for
wear particles consisting of the MoS2-resin material to come off in lose form.
Carbon and 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 6000C enable use of this solid lubricant at high temperatures and high sliding
speeds.

Fig. 4.43: Carbon transfer layer on stainless steel.
Fig. 4.42: Carbon graphite seal.
Carbon graphite seals(Fig. 4.42) are the most common example "Carbon and Graphite"
solid lubricant group. These seals transfer layers of graphite on mating surface(Fig. 4.43)
and provide low friction, but tight seal.
• Strengths of graphite :
1. Moderate loads (< 275 MPa).

2. Low friction.
4. High temp. stabilty.
• Weaknesses of graphite :
1. Corrosion.
2. Vacuum detrimental to performance.

Fig. 4.44: Structure of graphite.
Fig. 4.44 indicates that graphite is lamellar solid.
Fig. 4.45: Perfect and distorted structure of graphite.
Mechanical distortion of graphite is shown in Fig. 4.45, which limits its usage to moderate
load(< 275 MPa). It is intersting to note that presence water helps graphite in lubrication,
white presence of water detrimental to MoS2. On other hand vacuum is detrimental to
graphite, but favorable for MoS2.

Fig. 4.46: Graphite seal in water.
To observe the performance of graphite, an experiment on graphite seal submerged in

water, as shown in Fig. 4.46, was performed. Experiment results indicated excessive wear
rate of mechanical seal(at 100-300 rpm). Second experiment on same seal but much lesser
water, as shown in Fig. 4.47, was performed and much lesser wear but almost same friction
performance was observed.

Fig. 4.47: Maximum seal wear occurs under complete water environment, and minimum
wear occurs under vapor lubrication.
It can be said that low friction behavior of graphite relies on adsorbed moisture or vapors to
achieve. At temperature lesser than 1000C, possibility of adsorbed moisture or vapors is
reduced, therefore graphite may not be effective lubricant. Lubrication performance of
graphite increases with increase in temperature, but beyond 5000C the possibility of
corrosion also increases.
- During world war II, aircraft flew at higher altitude and electric motor brushes
failed. Research into this problem revealed that graphite requires an adsorbed
layer of water vapor to lubricate effectively.
- Graphite promotes electrolysis. Graphite has a very noble potential of + 0.25V,

which can lead to severe galvanic corrosion of copper alloys and stainless steels.
Ceramic and Cermet (metal bonded ceramic)coatings :
Ceramics and cermet can be used in applications where low wear rate is more critical than
low friction. These composites can be used at temperatures up to 10000C. Ceramic/Cermet
coating up to 0.5 mm thick on metal substrates offer a convenient way of utilizing the wear
resistance of metal with a minimum processing cost. The coating of these material can be
applied using :
• Plasma spraying.
• Impingement coatings from a detonation gun.

- Improved adhesion to the substrate metal and lower porosity.
• Electrolytic deposition from electrolyte containing ceramic particles (Tribomet coating).

- Ability to coat small internal surfaces inaccessible by any other technique.
Semi-Solid Lubricant : Grease
In layman’s language Grease is: A black or yellow sticky mass used in the bearings for
lubrication purpose. Lubricating greases consist of lubricating oils, often of quite low
viscosity, which have been thickened by means of finely dispersed solids called thickeners.
It consist of base oils(75 to 95%), additives(0 to 5%) and minute thickener fibers(5 to 20%).
Fig. 4.48: Semi solid lubricant.
Base Oil :- Many different types of base oil may be used in the manufacture of a grease,
including petroleum (napthenic: more popular, parafinic) and synthetic (PAO's, esters,
silicones, glycols). The viscosity of the base oil is the most significant property. A lighter,
lower viscosity base oil is used to formulate low temperature greases, while heavier, higher
viscosity base oil is used to formulate high temperature greases.
Additives :- Chemical additives are added to grease in order to enhance their performance.
Performance requirements, compatibility, environmental considerations, color and cost all
factor into additive selection. Solid lubricants such as graphite, MoS2, EP additives are few
examples.
Thickener :- The two basic types of thickeners are organic thickeners and inorganic
thickeners. Organic thickeners can be either soap-based or non-soap based, while
inorganic thickeners are non-soap based. Simple soaps are formed with the combination of
a fatty acid or ester (of either animal or vegetable origin) with an alkali earth metal, reacted
with the application of heat, pressure or agitation through a process known as
saponification. The fiber structure(Fig. 4.48) provided by the metal soap determined the
mechanical stability and physical properties of the finished grease. In order to take on
enhanced performance characteristics, including higher dropping points, a complex agent is
added to the soap thickener to convert it to a soap salt complex thickener. The greases are
then referred to as "complexes“. A classification greases based on simple and complex

soap thickeners is listed in Table 4.16. The most commonly economic grease is
lime(calcium) base grease (max. temperature 55-800C). Soda(Sodium) base grease (max.
temperature 90-1200C) is preferred over lime based grease in rolling bearings.
Complex Grease :- Complex grease is similar to a regular grease except that the thickener
contains two dissimilar fatty acids, one of which is the complexing agent. This imparts good
high temperature characteristics to the final product.
To make a lithium-based complex grease, part of the fatty acid is replaced with another acid
(usually a diacid), which makes the complex soap. This type of mixed soap structure has
special properties that enable the grease to be heated to a higher temperature without
losing its structure or oil separating from the thickener. This maximum temperature is
referred to as the dropping point. The dropping point is critical because it is the point at
which the grease reverts back to a liquid (the oil separates from the thickener).
Table 4.16: Classification based on thickeners.
Comparison among various types of greases has been provided in Table 4.17. The role of
base oil to decide the operating temperature is given in Table 4.18.
Table 4.17: Comparative chart.
Table 4.18: Role of base oil.
Advantages Of Greases :
• Remains at application point & adhere to surface.

• Less-frequent application needed.
• Good for inclined/vertical shafts.
• Seal out contaminants & less expensive seals needed.
• Water resistant & reduce oil vapor problems.
• Prolong the life of worn parts by filing irregularities as shown in Fig. 4.49.
• Provide better mechanical lubrication cushion for extreme conditions such as

shock loading, reversing operations, low speeds & high loads.
• Reduce noise and vibration.
Fig. 4.49: Grease filing irregularities.
Disadvantages of Greases :
• Because of semi-solid nature of greases, it does not perform the cooling, so poor
dissipation of heat.
• Once dust or dirt enters the grease, it cannot be easily removed and would act as
deterrent in performance.
• No filtration.. So contaminants/wear-debris cannot be separated.
Grease Characteristics :
Consistency : Degree of grease hardness

1. Grease surface (maintained at 250C) is smoothed out to make it uniform.
2. Cone release mechanism is activated and cone is allowed to sink for 5 seconds.
Table 4.19: National lubricating grease

institute(NLGI) grease classification
Fig. 4.50: Cone arrangement to measure

consistency.
Grease - Characteristics :
• Greases are a type of pseudo - plastic (shear-thinning) fluid which means after
sufficient shear force, the viscosity of grease drops and approaches to the base
lubricant viscosity. The most important physical characteristics of a grease is its
relative hardness or softness, which is called consistency. Consistency is
assessed by measuring the distance in tenths of mm to which a standard metal
cone penetrates the grease(Fig. 4.50) under a standard load; the result is know
as the penetration. A widely used classification of grease is that the American
National Lubricating Grease Institute (NLGI). Table 4.19 lists the NLGI grade of
greases.
Yield stress is measured using Rheometer and using Eq.(4.13) is taken as the 3 rpm
reading. The value of n and ηb are calculated from the 300 or 600 rpm values.
....Eq.(4.13)
Grease Additives :
A number of additives may used in grease. Extreme pressure and friction modifiers are the
two very commonly used additives.
Extreme-pressure additive :
Soluble compounds of sulphur, chloride, phosphorous chemically react with sliding metal
surfaces to form films which are insoluble in the lubricant. Most widely used EP
additives(Fig. 4.51) are :
 Tricresyl phosphate (TCP)

 Synthetic lub, Dibenzyl disulphide.
 Zinc dialkyl dithiophosphate(ZDDP).

Fig. 4.51: E.P Additives.
Friction modifier :
These additives reduce friction by increasing the adhesive film strength to avoid surface to
surface contact. In other words these additives provide a cushioning effect and keep metal
surfaces apart from each other :
The most common friction modifiers are :
- MOLY Grease.
- Teflon grease.
- Graphite grease.
Liquid Lubricants
There are two systems for oil classification. The SAE (Society of Automotive Engineers)
viscosity grade and the API (American Petroleum Institute) classification that designates the
type of engines for which the oil was designed. The SAE viscosity grade is known as the
“W” number when classifying winter oils. In general, the lower the first number, the better
the oil performance in extremely cold conditions. Conversely, the higher the second number
the better the oil protection at higher temperatures.
The API designation is typically “S” designation for gasoline engines and a “C” designation
for diesel engines. Most of today’s oils carry an SH,CF or SJ,CF designation signifying that
they are suitable for use in all gasoline or diesel automotive applications.
Classification of Liquid Lubricants :
• Vegetable (Castor, Rapeseed) oils :

- Less stable (rapid oxidation) than mineral oils at high temp

- Contain more natural boundary lubricants than mineral oils.
• Animal fats : These are fatty substances extracted from animals, and fish. They are
composed of fatty acids and alcohols. They are called fixed oils because they do not
volatilize unless they decompose. This process is known as drying. The fixed oils which
are slow to dry(slow in oxidation) are used for lubrication. Fixed oils are usually added
to mineral oils to improve film formation as these lubricants have extreme pressure
properties. Common examples of these lubricants are tallow, castor oil and fish oil. One
of major problem of this class of lubricants in the availability.
• Mineral oils.
Fig. 4.52: Stribeck curve.
Extracted from crude oil. Mineral oil consists of hydrocarbons (Composed of 83-87% carbon
and 11-14% hydrogen by wt.) with approximately 30 carbon atoms in each molecule
(composed of straight & cyclic carbon chains bonded together). Also contain sulphur,
oxygen, nitrogen. Based on sulphur contents these oils are classified as Pennsylvanian oil
(< 0.25%), Middle east (~1%), Venezuelan (~2%), Mexican (~5%) 0.1% to 1.0% preferred.
This means Mexican and Venezuelan are least preferred. Fig. 4.52 indicates "Zone 1",
"Zone 2" and "Zone 3" based on product of viscosity, speed and inverse of apparent
pressure. Mineral oils are suitable for "Zone 3", while grease may be used for "Zone 2" and
solid lubricants for "Zone 1".
Classification based on chemical forms of mineral oils :
Mineral oils are classified as paraffins, naphthene and aromatic. Paraffins are preferrable
choice compared to napthenes or aromatics.

Fig. 4.53: chemical forms of mineral oils.
Paraffinic Oils :
These oils have good natural resistance to oxidation. But on oxidation it forms acids, which
means when burnt, leaves a hard carbonaceous deposit.
• Good thermal stability :

- Low volatility.
- High viscosity index (VI=90-115)
- High flash point.
- Pour point higher than naphthenic or aromatic.
Naphthenic Oils :
• Lower VI (15-75)
• Less resistant to oxidation.
• Lower flash points than paraffinic.
• Lower pour point than paraffinic therefore good for low temperature applications.
• When burnt soft deposits are formed, therefore abrasive wear is lower.
• Oxidation leads to undesirable sludge type deposits.
Multigrade Oils :
Most oils on shelf today are multigrade oils, such as 10W30 or 20W50. These oils are
made by adding polymers in mineral oils to enhance viscosity indices. At cold temperatures
the polymers are coiled up(Fig. 4.54) and allow the oil to flow as their low numbers indicate.
As the oil warms up the polymers begin to unwind into long chains(Fig. 4.54 and Fig. 4.55)
that prevent the oil from thinning as much as it normally would. In other words, in the
uncoiled form, they tend to increase the viscosity thereby compensating for the decrease in
viscosity of the oil.

Let us consider multigrade 10W30 oil. This oil has one grade beahvior at 00F and second
grade behavior at 2100F. This oil (10W30) has viscosity 2100CP at 00F and behavior of
SAE30 at 2100F.
- Lower the first number, better performance in extremely cold conditions.
- Higher the second number better the oil will protect at higher temperatures.
- 20W50 may be good in Mumbai, but 0W30 will be preferred in Kashmir.
Fig. 4.54: VI Improvers.
Fig. 4.55: VI improvers in action.
Effectiveness of multigrade oils is affected by the shear rate, the rate at which the oil has to
pass through confined spaces. At high shear rate, viscosity of multigrade oil may be little or
no different from that of base oil as shown in Fig. 4.56.
Fig. 4.56: Viscosity model of multigrade oil.
Synthetic Oils :
Synthetic oils were originally developed more than 50 years ago and became widely used in
jet engines. Less than -1200F ambient temperatures, 60000 shaft rpm, and 5000+F exhaust
temperatures proved too much for conventional oils. Synthetics were created specifically to

withstand these harsh conditions and to date every jet engine in the world uses synthetic
lubricants. Synthetic oils are engineered specifically in uniformly shaped molecules with
shorter carbon chains which are much more resistant to heat and stress.
- Viscosity does not vary as much with temperature as in mineral oil.

- Rate of oxidation is much slower.
- Cost(expensive, but applied where mineral oils are inadequate).
Common synthetic oils are :
• Polyglycols (Polyalkylene glycol) :

- Originally used as brake fluids VI = 200, absorb water.
- Distinct advantages as lubricants for systems operating at high temperatures
such as furnace conveyor belts, where the polyglycol burns without leaving a
carbonaceous deposit. Used in textile industry.
• Esters :
- Better (in reducing friction, resisting oxidation, prolong draining period, volatility)
than mineral oils.
- Costs only a little more than mineral oils.
• Silicon : VI 300, chemically inert, poor boundary lubricant, low solubility, space
application, high production cost.
• Perfluoropolyalkylether : Good oxidation & thermal stability VI= 200. In vacuum used for
thin film lubrication.
• Perfluoropolyethers :
- High oxidation (3200C) & thermal (3700C) stability.
- Low surface tension & chemically inert.
 Few Remarks on usage of lubricants :
• Grease : provides excellent protection against environmental contamination but

restricted to a speed of 2 m/s for the reason of inadequate heat dissipation.
• Liquid : Low viscosity oils have low fluid friction losses(provided metal to metal contact
is avoided) and consequently low heat generation. Liquid can carry away heat.
Under high loads and slow rubbing speed a hydrodynamic film cannot form, hence mineral
oils are combined with fatty oils to give a boundary lubrication layer.
Gas Lubrication :
Gas(i.e, Air, Nitrogen, and Helium) lubrication is used for ultra thin film thickness(separation)
between tribo-pairs.
ADVANTAGES :
• Temperature range– (-2000C) to (20000C). No vaporization, cavitation,

solidification, decomposition.

• Very low viscosity (1000 times less viscous than even the thinnest mineral oil),
therefore ultra low friction. Possible high speed.
• Cleanliness.
• No seal requirement for lubrication.
DISADVANTAGES :
• Very low load capacity. Low damping. Ultra low film thickness.
• Smooth surfaces & very low clearance (to maximize load capacity & minimize
flow rate) needs a specialist designers & manufacturer (close tolerance).
• Less forgiving of errors in estimating loads or of deviations from specifications

during manufacture and installation.
Selection of Lubricant Type :
Load and speed are two major factors(as shown in Fig. 4.57) which affect selection of
lubricants environment and sealing requirements are additional factors which affect lubricant
selection. Apparent area, material conductivity and friction coefficient decide the operating
temperature.
Topt α (μFV/A.K).
where
Topt = operating temperature.
μ = coefficient of friction.
F = normal force.
V = velocity.
A = Area of contact.
K = Thermal conductivity.

Fig. 4.57: Lubricant selection.
Reference :
1. Peterson M B and Winer W O, Wear control handbook, ASME, 413-473, 1980.

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Lubricant Additives
Need of Additives :
In applications where hydrodynamic lubrication films are formed there is no need of

additives, but to counteract high speed, high temperature, high load, etc. additives are
required. Additives are incorporated into either a liquid base (mineral oil, synthetic fluid, etc.)
or grease need to be soluble or uniformly dispersed throughout the carrier media. In
practice, a formulated lubricant comprises a base fluid and a performance additive package.
Practically, all lubricants contain additives to enhance existing properties, or to impart new
properties.
• Gears are subjected to very high contact pressure(Fig. 4.58), and experience
metal to metal contact at gear teeth. Lubricants with extreme pressure (EP)
additives are required
Fig. 4.58: High contact pressure in gears.
• I.C. Engine parts are subjected to high temperatures. Lubricants need to be

oxidation resistant.

• Detergent and dispersant additives to remove combustion and breakdown
products of the oil from the surfaces.
• Corrosion inhibitors to prevent corrosion caused by combustion and oxidation

products.
• Refrigeration system lubricants encounter the low temperatures (below 00C), the
oils need to have low pour points.
Importance of lubricant additives is indicated in Table 4.20. Engine oil(base oil + detergent
and dispersant additives + corrosion inhibitors + antiwear additives) reduces wear rate to
mild wear regime.
Table 4.20: Importance of lubricant additives.
Desired properties of lubricants :
• Adequate film strength.
• Chemical stability.
• Adequate lubricity (adhesiveness to surfaces).
• Purity (freedom from contaminants).
• Non-corrosive.
• Good sealing properties.
• High VI (change in viscosity with temperature).
• Minimum volatility or out-gassing.
Additives classified based on their functionality are listed in the first column of Table 4.21.
Most common elements used for each type of additive are listed on the right column of
Table 4.21.
Table 4.21: Lubricant additives.

Detergent additives :
Detergents, like dispersants, are blended into lubricants to remove and neutralize harmful
products. Detergents form a protective layer on the metal surfaces to prevent deposition of
sludge and varnish. The metallic basis for detergents includes barium, calcium, magnesium
and sodium. In engines, this can reduce the amount of acidic materials produced. Protective
ability of detergent is measured by its total base number or its reserve alkalinity.
Detergent additives are soaps of high molecular weight, soluble in oil (functional) group
attracts particulate contaminants in the lubricant.
Fig. 4.59: Detergent additive action
Fig. 4.60: Detergent additives in action.
Dispersants additives :

Purpose of dispersant additives is to suspend or disperse harmful products (i.e. dirt, water,
fuel, process material, and lube degradation products such as sludge, varnish, oxidation
products) within the lubricant. These compounds have a large hydrocarbon tail and a “polar
group” head. Tail section serves as a solubilize in the base oil, while polar (functional) group
attracts particulate contaminants in the lubricant.
Detergents, like dispersants, are blended into lubricants to remove and neutralize harmful
products. In addition, detergents form a protective layer on the metal surfaces.
Anti-wear additives :
• Prevent metal to metal contact. Useful under lighter to moderate loads (bearings).
• Anti-wear additives typically contain zinc and phosphorus compounds. With increase in
load anti-wear additive may be ineffective and EP additives are required in heavy load
applications such as gearboxes.
• Chemical active —- Coat a protective layer on the metal surface by chemical

decomposition and absorption. Zinc dithiophosphate (ZDP): Probably the most widely
used in formulated engine oils, also acts as a corrosion inhibitor and antioxidant.
- Molybdenum disulfide and graphite additives are a special form of anti-wear
additives known as anti-seize agents. ~~ Depletion
- Zinc compounds.
- Stearic acid is also used as antiwear additive.
Anti-foaming agents (Foam Inhibitors) :
Lubricant foams due to agitation and aeration that occurs during operation. Foaming
interfere with flow rate and heat transfer and increase oxidation.
• Detergent and dispersant additives tends to promote foam formation.
• The additives (usually long chain silicon polymers are used in small quantities of about
0.05% to 0.5% by weight) lower the surface tension between the air and liquid to the
point where bubbles collapse.

Fig. 4.61: Formation of air bubbles in lubricant.
Anti-oxidant additives (Oxidation Inhibitors) :
Oxidation due to high temp. and pressure in lubrication oil occurs. Products of oxidation
gummy, deposits on surface, corroded cadmium, copper & lead alloys form.
- Power loss due to increased viscous drag & difficulties in pumping increases.
- Therefore it is recommended to replace oil, if TAN > 3.
• Corrosion inhibitors : Used for non-ferrous metals (copper, aluminum, tin, cadmium, etc.
used in bearings, seals), protect surfaces against any corrosive agents (sulphur,
phosphorus, chlorine, and oxidation products) present in oil.
• Rust inhibitors : Needed for ferrous metals particularly to trap(Fig. 4.62) oxygen
dissolved in oil & water. These additives of polar type adsorbed strongly upon metal
surface and neutralize acids. Sulphonate, phenate and amines are few examples of rust
inhibitors additives.
Fig. 4.62: Rust prevention additives.
• Rust inhibitors neutralize acids formed by oxidation.
• Chemically react with the metal surfaces to form a protective film.
Pour point depressants :
Pour point depressants reduce the pour point and are therefore required when operating at
lower temperatures. Pour point is the lowest temperature at which the lubricant will flow.

Waxy crystals are formed at lower temperature, therefore additives methacrylate polymers,
polyalkylphenol esters. Encapsulate crystal so that it cannot grow.
Interference between additives :
• Additives often interfer with each other, for example.
• Detergents & rust inhibitors can significantly suppress lubricating action of ZDDP.
• Corrosion inhibitors contaminated with ammonia lead to extensive plant damage.
• Dispersants accelerate the oxidation of oil & anti-oxidants must be included when
these additives are used.
Lubricant Selection :
Often lubricants are chosen only to meet the friction and wear requirements. But lubricant
must be able to solve the cooling, contamination and corrosion problems.
• First step toward selection :

- Lowest initial cost without any facility for re-lubrication. Ex: watches, clocks, door
locks, sewing machines, etc.
• Second step :
- Is there any need to change lubrication system due to excessive load and/or
speed, heat or debris? If yes, refer Table 4.22.
Table 4.22: Lubricant selection.
• Third Step : If lubricant selection is complete assembly (i.e. I.C. Engine), then it will be
preferable to use same lubricant for all tribo pairs of that assembly.
(a) Single oil reservoir & circulation system can be used :

- Economic.
- Reliable.
- Storage lesser chances of wrong usage of lubricant.

(b) Due to self compensating behavior of oil viscosity, slightly higher value of viscosity can
be selected.
----------------------------------------------------------------------------------------------------------------------
Tribology (MCQ : MODULE - 4) Lubricant & Lubrication
Question 1: The purpose of lubrication is

A. To reduce friction.
B. To reduce wear.
C. Transfer heat produced.
D. All of above.
Question 2: Which of the following is NOT a function of lubricant in IC engine?

A. Form a film to separate the surfaces.
B. Adhere to surface.
C. Withstand high temperature inside the cylinder.
D. Reduce the size of the asperities and improve the surface finish.
Question 3: Synovial fluid is a lubricant that is found in

A. Human bone joints.
B. Gearboxes.
C. IC engines.
D. Rolling element bearings.
Question 4: Which one of them is a correct combination?
1. Boundary lubrication.
2. Hydrodynamic lubrication.
3. Mixed lubrication.
4. Elastohydrodynamic lubrication.
(i) Dimensionless film thickness < 1.
(ii) Dimensionless film thickness lies between 1 and 3.
(iii) Dimensionless film thickness lies between 3 & 5.
(iv) Dimensionless film thickness is greater than 5.

A. 1-(i), 2-(iv), 3-(ii), 4-(iii).
B. 1-(iv), 3-(iii), 2-(i), 4-(ii).
C. 2-(i), 3-(iv), 4-(iii), 1-(ii).
D. 3-(iv), 2-(iii), 1-(i), 4-(ii).
Question 5: As the temperature is increased, the coefficient of friction

A. Increases.
B. Reduces.
C. Remains unchanged.
D. Increase or decrease based on the lubrication regime.
Question 6: Which of the following is a desirable property of boundary lubricant?

A. Dissolvability in lubricating oils.

B. Affinity to metallic surfaces.
C. Low shear strength and high melting point.
D. All of above.
Question 7: The major disadvantage with extreme pressure lubricants is

A. Carcinogenic nature of the lubricant.
B. Low melting point.
C. It is ineffective.
D. All of above.
Question 8: In hydrodynamic lubrication the major source of friction is

A. Shearing of lubricant film.
B. Abrasion due to asperities on tribo-surfaces.
C. Abrasion of tribo-surfaces due to free particles.
D. All of the above.
Question 9: Which of the following statements is true about viscosity?

A. Dynamic viscosity is the ratio of shear stress to the resultant shear rate.
B. Kinematic viscosity is equal to dynamic viscosity divided by density.
C. The CGS unit of dynamic viscosity is Centipoise and CGS unit of kinematic viscosity is
Centistokes.
D. All of above.
Question 10: Film thickness in elastohydrodynamic lubrication depends on

A. Applied load and relative velocity.
B. Lubricant properties.
C. Properties of contacting materials.
D. All of above.
Question 11: Viscosity of multigrade oils

A. Reduces with temperature but at higher sensitivity compare to monograde oil.
B. Increases with temperature but at higher sensitivity compare to monograde oil.
C. Reduces with temperature but at lower sensitivity compare to monograde oil.
D. Increases with temperature but at lower sensitivity compare to monograde oil.
Question 12: Viscosity Index denotes

A. Relationship between the dynamic and kinematic viscosities.
B. Sensitivity of lubricants viscosity with respect to temperature.
C. Both (a) and (b).
D. There is no sliding and only rolling motion involved between cage and balls.
Question 13: Which of the following is true in modelling viscosity temperature relationship?
A. Dynamic viscosity is expressed by Vogel relation.
B. Kinematic viscosity is expressed by Walther’s relation.
C. Both (a) & (b).
D. None of the above.
Question 14: Viscosity Index of the mineral oil can be improved by?
A. Removing aromatics components during refining stage.
B. Blending with high viscous index oils.
C. Using polymeric additives.

Question 15: Which one is the common system for oil classification?
A. SAE (Society of Automobile Engineers).
B. API (American Petroleum Institute).
C. ISO (International Organization for Standardization).
Question 16: Barus relation, shows the relationship between

A. Lubricant viscosity and temperature.
B. Lubricant viscosity and pressure.
C. Dynamic viscosity and kinematic viscosity.
D. Lubricant temperature and lubricant pressure.
Question 17: Which of the following is not an advantage/benefit of solid lubricant?

A. More effective at high loads.
B. Resistance to deterioration.
C. Good heat dissipation.
D. Highly stable in extreme temperature and environment.
Question 18: Out of the following which is NOT an example of solid lubricant?
A. Graphite lubricant.
B. Molybdenum Sulphite lubricant.
C. Polytetrafluoroethylene lubricant.
D. Multigrade lubricant.
Question 19: Which of the following is/are the constituents of grease?

A. Base oil.
B. Additive.
C. Thickness fibre.
D. All of above.
Question 20: Which of the following is NOT the advantage of grease?

A. Remains at application point and adhere to the surface.
B. Less frequent application needed.
C. Good for inclined/vertical shaft.
D. Good dissipation of heat.
Question 21: The common friction modifiers used in grease is

A. Tricresyl phosphate (TCP).
B. Dibenzyl disulphite.
C. Zinc dialbyl dithiophosphate (ZDDP).
D. Molybdenum disulphide.
Question 22: Synthetic oils are used in aerospace applications because

A. They can withstand very high range of temperature from -1200 F to 5000 F.
B. Very high shaft rpm of the order of 60,000 rpm.
C. They have shorter carbon chains which are more resistant to heat and stress.
Question 23: Identify the INCORRECT statement about the additives.

A. The purpose of dispersant is to suspend harmful products like dirt and sludge.
B. Anti-wear additives typically contain zinc and phosphorus compounds.
C. Anti-foaming agents tend to lower the surface tension between air and liquid to the point
where bubbles collapse.

D. Pour point additives increase the pour point of the lubricants.
Question 24: Apart from reducing friction and wear, the secondary purpose(s) of lubricants
is/are
A. Heat dissipation.
B. Reducing corrosion.
C. Both (a) & (b).
D. None of these.
---------------------

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Module 4 : LUBRICATION

Fig. 4.1: Simple lock and key.

Fig. 4.2: Window lifting mechanism[1].

Module 4: Lubricant & Lubrication 1

Advantages of lubrication in addition to reducing friction and wear rate are :

• Reducing instant failures.

• Reducing surface failures.

• Reducing stress concentration.

3. Cams and followers.

6. Any situation involving metal to metal contact.

Module 4: Lubricant & Lubrication 2

For Newtonian fluid, shear stress is given by Eq.(4.1)

What do we expect from lubricant :

Required lubricant properties are specific to applications. We expect some requirements

• Must neutralize the corrosive products of combustion.

• Withstand high temperature inside the cylinder.

Module 4: Lubricant & Lubrication 3

(2) Lubrication in journal bearings (Fig. 4.6) :

• Lubricant should support heavy shaft and loads.

• Lubricant should avoid contact stresses.

• Lubricant should have ability to dampen vibrations.

Module 4: Lubricant & Lubrication 4

Lubrication in bone joints(Fig. 4.7) :

• Contain proteins that stick to cartilage layer resulting in smooth sliding.

• Coefficient of friction ~ 0.01.

• Minerals that nourish the cartilage cells.

• Increase viscosity with increase in applied pressure.

Fig. 4.7: Lubrication in bone joint.

Thick & Thin Lubrications(Fig. 4.8) :

• Thick lubrication is governed by Reynolds theory. Thick lubrication is not

Module 4: Lubricant & Lubrication 5

Based on the value of dimensionless film parameter(Λ), Eq.(4.2) lubrication mechanisms

• Boundary lubrication, Λ < 1

Module 4: Lubricant & Lubrication 6

• Mixed lubrication, 1 < Λ < 3

• Elastohydrodynamic, 3 < Λ < 5

Fig. 4.9 : Foreign particles/wear debris shift hydrodynamic/elastohydrodynamic lubrication in

In presence of debris(loose particles) lubrication mechanism may be changed as shown in

1. http://auto.howstuffworks.com/power-window1.htm Accessed on 19th February 2013.

How thin layer is able to separate surfaces ?

Module 4: Lubricant & Lubrication 7

Characteristics required for Thin Film Lubrication :

(2) It should be dissolvable in mineral/lubricating oils.

(3) Temperature stability : It is important because increase in operating temperature may

Module 4: Lubricant & Lubrication 8

Fig. 4.12: Number of layers vs friction coefficient[1].

Effect of length of molecule on boundary lubrication :

Module 4: Lubricant & Lubrication 9

Fig. 4.14: Effect of material in pressure of boundary additives.

Mechanisms of Boundary Lubrication :

Boundary lubrication comprises of two mechanisms :

Module 4: Lubricant & Lubrication 10

Fig. 4.15: Physical adsorption of solid additives on boundary surface.

• Additives should dissolve in solute.

• Attachment and detachment is a process encouraged by dilute concentrations and

Table 4.1: Percentage of boundary additive vs friction coefficient.

Module 4: Lubricant & Lubrication 11

Fig. 4.17: Effect of temperature on adsorption[1].