'LUBRICANTS' study notes for students.pdf

study notes

1. Lubricants
Lubricants are the substances which are used to reduce the force of friction between the two moving parts in contact with each other of a machine is
known as alubricant. The process of reducing friction between two moving, sliding surfaces by introduction of a lubricant between them
is called as Lubrication.Lubrication is the most vital singular factor in plant maintenance. It keeps the plant young, contribute to better
profits by improving the life of the wear components, equipment availability and reliability.
Function of Lubricants : The principal functions of lubricants are :
Reduce friction: Typically the lubricant-to-surfacefrictionis much less than surface-to-surface friction in a system without any lubrication. Thus use of a
lubricant reduces the overall system friction. Reduced friction has the benefit of reducing heat generation and reduced formation of wear particles as well
as improved efficiency.
Act as Coolant:It act as a coolant because it reduces production of heat between two moving parts of the machinery in contact.
Control wear:It avoids the damage of moving parts of the machinery by minimizing the production of heat. It reduces the wear and tear
of machinery by keeping moving parts apart.
1. It reduces the maintenance and running cost of machinery.
2. It helps in controlling the corrosion of the moving machinery parts.
3. Transmits power in case of hydraulics
4. Washout debris and contaminants
5. It increases efficiency of machine by decrease power requirement and loss of energy.
6. Act as a seal (Seal out contaminants) in piston.
7. It reduces noise by preventing metal to metal contact.
8. The lubrication results into extended useful life of frictional components.
9. It results into reduction in unscheduled machine down time, leading to an increased production.
10. It results into reduction in the production, maintenance, labour and replacement cost.
11. It saves energy.

2. Classification of Lubricants
1. Solid lubricants: Solid lubricants are used where components are subject to very high or very low operating temperatures
where oils and grease would be ineffective. Also used in environments where oil and grease would become contaminated and
ineffective.They are applied with a brush in powder form, spraying it on to the surface using a spray can or adding it to molten
metal in the manufacturing process.Examples are graphite, molybdenum disulphide, polytetrafluoro ethylene PTFE, soap, wax,
mica, chalk etc.
2. Semi-solid lubricants: Lubricants neither solid nor liquid come under this category. Grease and Vaseline are examples of
such lubricants. Grease is the most common semi-solid lubricant, it is a mixture of oil and thickening agents such as mineral oil
and soap. Semi-solid lubricants have an number of advantages over liquid lubricants in the fact that they do not flow freely, so
when applied they remain in place and are resistant to being displaced by centrifugal force, they form a seal against water and
other contaminants from entering the component/system Disadvantages are they do not dissipate heat as well as oils, also
because ofthe high viscosity they create a certain resistance to motion.
3. Liquid lubricants: These lubricants exist in liquid form and are used to reduce friction by providing a film between
the two surfaces in relative motion. These are very widely used in several machines and tools because they act as
i) Sealing agents
ii) Corrosion inhibitors
iii) Cooling medium
Examples of liquid lubricants are castor oil, mustard oil, synthetic oil, mineral oil, petroleum oil etc. Liquid lubricants are used:
a. Where solid and semi-solid lubricant are not suitable
b. In light duty jobs such as watches, clocks, sewing machines etc.
c.In the machines where less amount of heat is generated during motion which is insufficient to ignite or burn the lubricants.
4. Gaseous Lubricants: Compressed gas lubrication works the same way as oil lubrication in forming a thin cushion between
mating components.Examples of gaseous fuels are air, Nitrogen and Helium, carbon dioxide etc. Gas lubrication has a number of
advantages over oil and grease as they can be used for high speed applications, rotating at speeds up to 300,000 rpm, greater
temperature ranges and has very little resistance to motion and is much cleaner. Typical operating pressures range from 2-10 bar.

3. Lubricants are also classified by their VI in to low, medium, high and very high VI groups.
Characteristics of a good lubricating oil.
Following are the characteristics of a good lubricating oil:
1. High Boiling Point: In order to stay liquid with in a wide range of temperature a good lubricating oil must possess a high
boiling point.
2. Low Freezing Point:
3. High Oxidation Resistance:
4. Non-corrosive Properties:
5. Stability:

4. Type of Lubricating Oil
1.Animal and Vegetable Oils: These lubricants are obtained from animals and vegetables and possess very good oiliness
and viscosity properties. They possess good “oiliness”. However, they are costly and get deteriorated easily in the presence
of air and moisture due which nowadays they are used rarely as such. Actually they are used as “blending agents” for
mineral oils. Examples of such lubricants are whale oil, seal oil, mustard oil, cotton seed oil etc.
2.Mineral Oils: These lubricants are obtained by the fractional distillation of petroleum. These are lower molecular weight
hydrocarbons having 12-50 carbon atoms. Shorter chain oils possess lower viscosity than longer chain ones. For being cheap,
available in abundance and stable under service conditions they are most widely used lubricants. As compared to animal and
vegetable oils they possess poor oiliness. Their oiliness can be improved by the addition of blending agents such as stearic
and oleic acid.
3.Blended Oils: They are made by mixing animal or vegetable oils with mineral oils. Blending is essential since no single pure
serves satisfactorily. Blending agents are used to improve various properties such as viscosity index, oxidation stability, oiliness,
pour point etc. Blended oil are superior to vegetables and mineral oils.

5. 4. Emulsion: These lubricants are made by the mixing of two immiscible liquids. As an emulsion consisting of two immiscible
liquids is inherently unstable, therefore to increase its stability, a third agent, called emulsifier or emulsifying agent, is added.
Generally, the sodium/potassium soaps of higher fatty acids such as C₁₅H₃₁COO⁻ Na⁺/K⁺ are used as emulsifying agents as they
contain a hydrophilic head (−COO⁻ Na⁺/K⁺) and a hydrophobic tail (-C₁₅H₃₁). Depending on the relative composition of water and
oil in them, there can be two types of emulsions viz oil in-water (O/W) and water in-oil (W/O) emulsions. An emulsion
consisting of less than 26% oil, constitutes an oil-in water , emulsion. Whereas one consisting of more than 76% oil constitutes
water in oil, emulsion. An oil in-water, emulsion is obtained by mixing an oil, containing 3-20% water soluble emulsifying
agent like sodium soaps, with a suitable quantity of water. O/W emulsions are used as cooling and lubricating fluids in cutting
tools, lubricants in marine diesel engines and large internal combustion engines. An oil in-water, emulsion is obtained by mixing
water, containing 1-10% oil soluble emulsifying agent like calcium soaps, with a suitable quantity of oil. These lubricants are
used in compressors, drilling and milling machines.

6. Properties of Lubricants
Viscosity.
Viscosity or thickness is the most important property of lubricants that determines the operating condition under which it is
used. Viscosity is the property of fluid which is a measure of its resistance to flow and shear. The steady state of flow of a liquid
can be visualized to consist of series of parallel layers moving one above other. When a liquid is made to flows over a solid
surface, it stops after covering some distance. This is because of the force of attraction between solid surface and the layer of the
fluid in direct contact of it and the force of attraction between two adjacent layers of the fluid. It is due to these attractive forces
between them that a layer of fluid resists the flow of a layer moving above it. “This resistance to flow offered by a layer of fluid to
another adjacent layer moving over it is known as viscosity". TheStronger the resistance to flow higher will be the viscosity of the
fluid and vice versa. Viscosity is reciprocal of fluidity. Higher the viscosity of a liquid lower is its fluidity and vice-versa. For
example water is less viscous but more fluid than honey. Magnitude of the attractive forces decreases gradually on moving away
from the solid surface to the top most layer of the fluid thus being maximum between the solid surface and the layer of fluidin
its direct contact and minimum between the upper most layers. Velocity of the layer in the direct contact of the solid surface is
practically zero and that of upper most layer is maximum. This gradual increase in the velocity of various layers of the fluid results
into the development of a velocity gradient, ‘du/dy’ in a direction perpendicular to the direction of flow of liquid.

7. In fact a moving fluid is always under the influence of deformation forces due to which it changes its shape continuously.
Deformation forces working in opposite directions are known as shearing or splitting forces. An imbalance of shearing forces
results into the development of a shearing stress on the moving liquid. The rate of shear stress (τ)is directly proportional to the
velocity gradient i.e.
Where ‘η’ is the coefficient of viscosity, also known as ‘dynamic’ or ‘absolute’ viscosity.
Also
So dynamic or absolute viscosity is the shear stress acting on a flowing liquid per velocity gradient. SI units of dynamic viscosity
are Ns/m⁻² or kg m⁻¹s⁻¹. CGS unit of viscosity is poise (P) expressed in g cm⁻¹s⁻¹. It is more commonly expressed, particularlyin
ASTM standards, as centipoise(cP). 1 centipoise = 0.01 poise = 1g cm⁻¹s⁻¹ =0.1 kg m⁻¹ s⁻¹.
Kinematic viscosity is the ratio of dynamic viscosity to fluid density, ρ.
SI units of kinematic viscosity (ϒ) is m²s⁻¹ whereas CGS unit is the stokes (St) or cm²s⁻¹. 1 stokes = 0.0001 m²s⁻¹. However a much
smaller unit centistokes (cSt) is generally used. 1 cSt= 0.01 St = 1 mm² s⁻¹ = 10⁻⁶ m² s⁻¹.
Dynamic (or absolute) viscosity gives information about the force needed to make the lubricant flow, while kinematic viscosity
tells how fast the lubricant flows when force is applied

8. The function of lubricating oil is to form a liquid film between two moving or sliding surfaces. If the viscosity of oil is too low, the
resistance to flow between individual layer of lubrication is low and consequently the excessive wear take place. On the other
hand if the viscosity is too high, resistance to flow between different layers of fluid is high and friction between layer increases
and hence decreases efficiency. Higher is the viscosity, thicker is the oil film that separates the two sliding surfaces fromtouching
each other, lower the wear but reduced efficiency of machine. Therefore viscosity of lubricant should be optimum according to
need.
Factors affecting viscosity.
Molecular structure: in general an oil with high molecular weight possesses a high boiling point and high viscosity.
Pressure: Viscosity increases with increasing pressure of operating system.
Temperature: Viscosity of liquid decreases with increasing temperature because the lubricating oil becomes thinneras
operating temperature increases. As a rule of thumb, the viscosity of a machine oil fall about 25% with every 10 ⁰C temperature
increase.
It is desirable that viscosity of a lubricant should be consistent over a range of temperature or we can say that viscosity of agood
lubricating oil should not change much with change in temperature so that it can be used under varying conditions of
temperature. The rate at which the viscosity of an oil changes with temperature is measured by an arbitrary scale known as
viscosity index. The VI scale was set up by the Society of Automobile Engineers, SAE. If the viscosity of an oil changes rapidly over
a range of temperature it is said to possess low viscosity index. On the other hand, if over a range of temperature, the viscosity
of an oil does not show any remarkable change with rise or fall of temperature, such an oil is said to possess high viscosity index.
Determination Viscosity Index
According the ASTM standard, “viscosity index is an arbitrary number used to characterize the variation of the kinematic viscosity
of a petroleum product with temperature”. VI is a measure of change in the viscosity of a fluid with temperature fluctuations. It
tells us how does a fluid protects its viscosity under temperature fluctuations.

9. Mathematically, viscosity index represents a relative measure of a lubricant’s temperature-viscosity behaviour with respect to
two types of reference oils. VI of ‘100’ is assigned to aparaffinic (consisting of Acyclic Saturated Hydrocarbons)Pennsylvania
crude oilsand VI of ‘0’ is assigned to a naphthenic (consisting of cycloalkanes)Texas Gulf crude oils.Oils with VI of ‘100’ are
known as H-oilsand those with VI of ‘0’ are known as L-oils.
For determining the VI of an unknown oil it should be ensured that the two selected reference oils at 100 ⁰C have same
kinematic viscosity as that of unknown oil. The kinematic viscosities of the two reference oils i.e. H-and L-oils at 40 ⁰C and 100
⁰C are noted from the look-up chart ASTM D-2270-86 of kinematic viscosity.
Let the kinematic viscosity at 40 ⁰C of unknown oil = U mm²/s (cSt).
Its kinematic viscosity at 100 ⁰C = ϒmm²/s (cSt).
(An oil said to has high VI if the difference between the two values is small and poor VI if the difference is large).
Kinematic viscosity of H-oil (VI=100) at 40 ⁰C = H mm²/s (cSt).
Its kinematic viscosity at 100 ⁰C = ϒmm²/s (cSt).
Kinematic viscosity of L-oil (VI=0) at 40 ⁰C = L mm²/s (cSt).
Its kinematic viscosity at 100 ⁰C = ϒmm²/s (cSt).
Then from the under given formula VI of the unknown oil can be calculated.
As it is a ratio of kinematic viscosities, VI is a dimensionless quantity. The VI was originally measured on a scale from 0 to 100;
however, advancements in lubrication science have led to the development of oils with much higher VIs. VI
improvingadditivesand higher quality base oils are widely used nowadays which increase the VIs attainable beyond the value of
100. The Viscosity Index ofsynthetic oils ranges from 80 to over 400.

10. Numerical Problem—Measured kinematic viscosity at 40°C of an oil whose viscosity index is to be calculated is found to be
73.30 mm² /s (cSt) while its kinematic viscosity at 100°C is calculated to be 8.86 mm² /s (cSt). [Given: from ASTM chart 2270-04
Table 1, the kinematic viscosity at 40°C of an oil of 0 viscosity index having the same kinematic viscosity at 100°C as the oil whose
viscosity index is to be calculated, L= 119.94 mm²/s (cSt). The kinematic viscosity at 40°C of an another oil of 100 viscosity index
having the same kinematic viscosity at 100°C as the oil whose viscosity index is to be calculated, H = 69.48 mm²/s or cSt].
Calculate the viscosity index (VI) of the given oil.

11. Oiliness. Oiliness of a lubricating oil is the measure of its capacity to stick on to the machine parts under the conditions of
pressure or load. Oiliness may be defined as the property of lubricants by virtue of which one fluid gives lower coefficient of
friction (generally at slow speed or high pressure/load) than another fluid of the same viscosity. An oil that can withstand higher
pressures or loads is said to possess higher degree of oiliness. When a lubricant of lower oiliness is applied to machinery, under
high pressure, it squeezes out from the surface and the lubrication stops. A lubricating oil of higher oiliness can remain in place
and give lubrication even under high pressure.
Mineral oils possess very poor oiliness whereas animal and vegetable oils possess good oiliness. Blending of considerable
proportions of animal and vegetable oils with mineral oils results into the enhancement of oiliness of latter to a great extant.
Oiliness of mineral oils can also be improved by mixing very small quantities of fatty acids or oil soluble soaps There are no direct
quantitative tests available for the measurement of oiliness of various lubricants due to which comparison of oiliness of two
lubricants becomes difficult.
Cloud Point: Cloud point is defined as the minimum temperature at which a lubricating oil becomes cloudy or hazy while
cooling it.
Pour point: Pour point of a lubricant is defined as the lowest temperature at which it will cease to flow or pour when cooled
and tested under prescribed conditions. Cloud and pour point indicate the suitability of a lubricant in cold conditions. Pour point
tells the temperature below which oil can not be used as lubricant. Also, the pour point indicates the dissolved wax
concentration in the oil. Lubricant used in machine working at lower temperature should posses low pour point to avoid
solidification of oil.
Significance
•It is an indicator of lowest temperature limit for utility as lubricating oil.
•It also indicates dissolved wax concentration of lubricating oil.
Flash point. Flash point can be defined as the temperature at which the oil gives out vapours that ignite for a moment when a
small flame is brought near it. In other words flash pointis the lowest temperature at which the lubricant oil gives off enough
vapours to ignite (spark) but not burn when small flame is brought near it.

12. •Fire Point. Fire point can be defined as the temperature at which the oil gives out enough vapours which burns continuously
at least for 5 seconds when a small flame is brought near it.
•Significance
•A lubricant should have a flash point which is reasonably above its working temperature. This ensures safety against fire
hazards during the storage, transport and use.
•A good lubricating oil should not volatize under working condition and even if volatizes, the vapour formed should not catch
fire under the working temperature conditions.
•Ignition Point: Lowest temperature at which a volatile substance will ignite itself without the help of any external flame or
source of ignition. At ignition temperature enough vapours are produced by the volatile substance that are sufficient to sustain
ignition in it. Ignition point of a substance is much higher than its fire point.
Comparison of Cloud, pour, flash, fire and ignition point.
Cloud Point Pour Point Flash Point Fire Point Ignition Point
Cloud point is defined
as the minimum
temperature at which
a lubricating oil
becomes cloudy or
hazy while cooling it.
The lowest temperature
at which lubricant
ceases to flow or pour
when cooled and tested
under prescribed
conditions.
It is the lowest
temperature at which
the oil gives out enough
vapours that ignite for a
moment when a small
flame is brought near it.
It is the lowest
temperature at which
the oil gives out
enough vapours which
burns continuously at
least for 5 seconds
when a small flame is
brought near it.
It is the lowest
temperature at which a
volatile substance will
ignite itself without the
help of any external
flame or source of
ignition