LUBRICANTS

FRICTION AND WEAR

• All material surfaces, no matter how smooth they are, show many irregularities in
the form of peaks (or asperities) and valleys, which are large when considered on a
molecular scale.
• When two solid surfaces are pressed over each other, a real contact between these
surfaces occur only at a limited number of asperities, i.e. peaks of the upper
surface are in contact with peaks of the lower surface.

Fig. 1. Contact between asperities get fattened under high local load.
• Even under small loads, the local pressure at the asperities may be

sufficiently great to cause appreciable deformation in ductile metals.

• This causes the formation of weed junctions, between the asperities. It is

these junction areas that carry all the load between the two surface.

• Thus, the real or true area of contact is only a small fraction of the

apparent contact area between the two surfaces.

Sliding friction : If two materials of different hardness slide at one

another, the peaks of the softer metal get broken more easily than the

peaks of the harder metals (Fig 2). The sliding friction of the harder

material, and (b) the interlocking of the surface impurities.

Fig. 2. Surface wear during sliding is due to shearing of asperities.

Rolling friction :

▪ when a loaded sphere or cylinder rolls over a flat face of the other body. Generally
the coefficient of rolling friction is very low as compared to the sliding friction.

▪ Sliding friction much larger for the static condition than for the kinetic condition.

▪ Rolling friction is believed to be caused by elastic deformation of the two surfaces as

a result of the development of a contact area between loaded sphere and a flat
surface.

▪ The rolling friction is therefore not detectably affected by the presence of lubricants.
However, in practical ball bearings, there is always certain amount of sliding
occurring between the hells and the cages, which makes lubrication necessary.
Effects of frictional heats :
▪ During a motion of the sliding surface, a considerable amount to

frictional heat is evolved at the rubbing surfaces. This results in high

local temperature, even under relatively light loads and spends.

▪ This is because the liberated frictional heat is not uniformly distributed

over the apparent contact area between the rubbing material, but it is

highly localized, particularly at the surface asperities.

▪ This may even raise their temperature to melting point of the

material, thereby accounting for the formation of welded

junctions.

▪ The generation of the frictional heat is a self-accelerating

process, since increased friction and adhesion at the localized

hot spots increases the rate of beat evolution on continuous

sliding, thereby leading to a large scale seizure or welding of the

two surfaces.
 NEED OF LUBRICANT

• In all types of machines, the surfaces of moving or sliding or rolling parts rub

against each other. Due to mutual rubbing of one part against another, a

resistance is offered to their movement. This resistance, is known as friction.

• Friction cruses a lot of wear and tear of surfaces of moving parts; and a

Large amounts of energy are dissipated in the form of heat, thereby causing

loss in the efficacy of machine. Moreover, the moving parts get heated up,

damaged and even sometimes results in seizure ( i.e., welding of two

surfaces due to heat).

• The ill-effects of frictional resistance can be minimized by using a suitable

substance, which forms a thin layer in-between the moving parts.

• Any substance introduced between two moving / sliding surfaces with a view

to reduce the frictional resistance between them, is known as lubricant.

• The main purpose of a lubricant is to keep the sliding / moving surfaces

apart, so that frictional resistance and consequent destruction of material is

minimized. The process of reducing frictional resistance between moving /

sliding surfaces, by the introduction of lubricants in-between them, is called

lubrication.
 Functions of a lubricant
(1) reduces surface deformation, wear and tear, because the direct contact

between the rubbing surfaces is avoided.

(2) It reduces loss of energy in the form of heat, in other words, it acts as a

coolant.

(3) It reduces waste of energy, so that efficiency of machine is enhanced

(4) It reduces expansion of metal by local frictional heat.

(5) It avoids seizure of moving surfaces, since use of lubricant minimizes the

liberation of frictional heat.

(6) It avoids or reduces unsmooth motion of the morning/sliding

parts.

(7) It reduces the maintenance and running cost of the machine

(8)It also, sometimes, at as a seal. For example, lubricant used

between piston and the cylinder wall of an internal combustion

engine acts as a seal, thereby preventing also the leakage of

gases under high pressure from the cylinder.

 MECHANISM OF LUBRICATION
There are mainly time types of mechanism by which lubrication is done :
(1) Fluid-film or thick-film or hydrodynamic lubrication:

• In this, the moving / sliding surfaces are separated from each other by a

thick-film of fluid (at least 1,000 A thick, so that direct surface to surface

contact and welding of junctions rarely occurs.

• The lubricant film covers / fills the irregularities of the moving / sliding

surfaces and forms thick layer in-between them, so that the then the trial

surfaces [see Fig. 3 (a)].

• This consequently reduces wear. The resistance to movement of moving /

sliding parts is only due to the internal resistance between the particles of

the lubricant moving over each other.

• Therefore, the lubricant chosen should have the minimum viscosity under

working conditions and at the same time, it should remain in place and

separate the surfaces, In such a system, friction depends on the viscosity,

thickness of the lubricant, the relative velocity and area of the moving /

sliding surfaces.
• The coefficient of friction in such cases is as low as 0.001 to 0.03.

• Hydrodynamic friction occurs in the case of a shaft running at a fair speed as

well as in well-lubricated bearing with not too high load.

• In a journal bearing (see Fig. 4), a film of the lubricating oil covers the

irregularities of shaft as well as the bearing surfaces; and the metal

surfaces do not come into direct contact with each other.

• Thus, the resistance to movement is only due to the internal resistance

of the lubricant.

• Delicate instruments, light machines like watches, clocks, guns, sewing

machines scientific instruments, etc. are provided with this type of

lubrication.
 Fig. 4. Hydrodynamic lubrication

• Hydrocarbon oils are considered to be satisfactory lubricants for fluid-film

lubrication.

• In order to maintain viscosity of the oil in all seasons of year, ordinary

hydrocarbon lubricants are blended with selected long chain polymers.

• Moreover, hydrocarbon petroleum fractions, generally, contain small quantities of

unsaturated hydrocarbons, which get oxidised under operating conditions, forming

gummy products. So antioxidant (like amino-phenols) are used in journal bearing.

(2) Boundary lubrication or thin-film lubrication is done, when a continuous film of

lubricant cannot persist and direct metal-to-metal contact is possible due to certain

reasons. This happens when:

(i) a shaft starts moving from rest, or

(ii) the speed is very low, or
(iii) the load is very high, and
(iv) viscosity of the oil is too low.
• Under such conditions, the clearance space between the moving/sliding surfaces

is lubricated with an oil lubricant, a thin layer of which is adsorbed, (i.e., surface

attached) by physical or chemical forces or both on both the metallic surfaces.

• These adsorbed layers avoids direct metal-to metal contact. The load is carried

by the layers of the adsorbed lubricant on the both metal surfaces [see Fig. 3 (b)].

• The coefficient of friction in such is, usually, 0.05 to 0.15. The oil-film keeps the

distance apart between of the meeting surfaces of the order of the height of the

asperities.
• Vegetable and animal oils (glycerides of higher fatty acids) and their

soaps possess property of adsorption (or surface attachment), either

physically adsorbed to metal surfaces or react chemically at the metal

surfaces, forming a thin film of metallic soap, which acts as lubricant.

• The load is carried by the two layers of adsorbed lubricant. Although the

fatty oils possess a greater adhesion property (called oiliness) than

mineral oil, yet they tend to break down at high temperatures.

• In order to improve the oiliness of mineral oils (which are thermally

stable), small amounts of fatty oils or fit acids are added.

• Graphite and molybdenum disulphide either alone or as stable

suspension in oil are also used for boundary lubrication.

• These materials form films on the metal surfaces, which possess i

internal friction and can bear compression as well as high temperatures.

 For boundary lubrication, the lubricant molecules should have:

(i) long hydrocarbon chains

(ii) polar groups to promote spreading and orientation over the metallic

surfaces at high pressure;

(iii) lateral attraction between the chains;

(iv) active groups or atoms, which can form chemical linkages with the

metals or other surfaces. High viscosity-index, resistance to heat and

oxidation, good oiliness, and low pour-point are some of the good

qualities of boundary lubricants.

(3) Extreme-pressure lubrications:
▪ When the moving/sliding surfaces are under very high pressure

and speed, a high local temperature is attained and under such

conditions, liquid lubricants fail to stick and may decompose

and even vaporize.

▪ To meet these extreme-pressure conditions, special additives

are added to mineral oils. These are called "extreme-pressure

additives".
▪ These additives form on metal surfaces more durable

films, capable of withstanding very high loads and high

temperatures.

▪ Important additives are organic compounds having active

radicals or groups such as chlorine (as in chlorinated

esters), sulphur (as in sulphurized oils) or phosphorus (as

in tricresyl phosphate).
▪ These compounds react with metallic surfaces, at prevailing high

temperatures, to form metallic chlorides, sulphide or phosphides.

▪ These metallic compounds possess high melting points (eg., iron

chloride and iron sulphide melts respectively at 650°C and 1,100°C) and

serve as good lubricant under extreme-pressure and

extreme-temperature conditions.

▪ If by chance, the low shear strength films are broken by the rubbing

action of moving parts, they are immediately replenished.

 PROPERTIES OF LUBRICATING OILS
1. Viscosity and viscosity index: Viscosity is the property of a

liquid or fluid by virtue of which it offers resistance to its own

flow.

• A liquid in a state of steady flow on a surface may be

supposed to consist of a series of parallel layers moving one

above the other.

where n (pronounced eta) is a constant of the liquid, called
coefficient of viscosity. If v=1 unit cm/s), d=1 unit (e.g., cm.) then

▪ Hence, coefficient of viscosity (n) may be defined as:

“The force per unit area required to maintain a unit velocity gradient

(i.e., velocity difference of one unit fluid or liquid layers, which are

unit distance apart) between two parallel layers’.

▪ The unit of viscosity is poise.

▪ If a force of 1 dyne is required to maintain a relative velocity
difference of 1 cm/s between two parallel layers 1 cm apart, its
coefficient of viscosity is 1 poise. A smaller corresponding unit
is centipoise, which is equal to 1/100 of poise.

▪ viscosity is the most important single property of any

lubricating oil, because it is the main determinant of the
operating characteristics of the lubricant :
(i) If the viscosity of the oil is too low, liquid oil filling can not be
maintained between two moving/ sliding surfaces and
consequently excessive wear will take place.
On the other hand,
(II) If the viscosity is too high, excessive friction will result.

The lubricating oil becomes thinner as the operating temperature

increases.
The rate at which the viscosity changes with temperature is known as
viscosity index (V.I.). The viscosity of the oil changes with temperature
is measured by an arbitrary scale, known as the "viscosity-index.
If the viscosity of an oil falls rapidly as the temperature is raised, it has
a low viscosity-index.
On the other hand, if viscosity of an oil is only slightly affected on
raising the temperature, its viscosity-index is high.
 Determination of viscosity-index:

For this purpose, we use a series of two types of standards oils.

(1) Paraffinic-base Pennysylvanian oils (V.I = 100)
(2) Naphthalic-base Gulf oils (V.1. = 0).
Against each of these is marked their viscosities at 100°F and 210°F. The
former are known as ‘H'-oils and the latter as 'L'-oils.

Method of measurement of viscosity index:

Step I: The viscosities of the oil under-test at 100°F and also at 210°F are first
found out, Let these values be 'U' and 'V' respectively. The difference between
the two values should be low, if the oil is good; and high, if the oil is poor.

Step II: Now from the list of H-oils (with V.I. = 100), the oil which has the same
viscosity at 210°F as the oil under-test is selected, and its corresponding
viscosity at 100°F is read off. Let it be H.
Step III: Then, from the list of L-(i.e., with V.I.= 0), the oil which
has same viscosity at 210°F as the oil under-test is selected, and
its corresponding viscosity at 100°F is read off. Let it be L. Then :

V.I. = L-Ux100/L-H

where U = Viscosity at 100°F of the oil under-test.

1. L = Viscosity at 100°F of the low-viscosity standard oil (i.e.,
Gulf, oil) having a V.I. of 0 and also having the same viscosity at
210°F as the oil under-test.

2. H = Viscosity at 100°F of the high-viscosity standard oil (i.e.,

Pennysylvanian oil) having a V.I. of 100 and also having the same
viscosity at 210°F as the oil under-test.
 How to increase V.I. of an oil ?
V.I. of lubricating oils can be increased by adding certain polymers,
which are only partially soluble in the oil.

(i) At low temperature, when the solubility of the added polymer in oil
is slight, so the effect of the polymer on the viscosity of the oil is
also slight.

(ii) At high temperature, when the solubility of the polymer in oils is

considerable, its effect will be to increase viscosity of the oil.
Thus, by the correct addition of organic polymers, it is possible
to produce oil-polymer blends, which have a very slight
temperature coefficient of viscosity or even none at all, i.e.,
viscous-static.

Measurement of viscosity of a lubricating oil is made with the help

of an apparatus called the "viscometer.

In a viscometer, a fixed volume of the liquid is allowed to flow,

from a given height, through a standard capillary tube under its
own weight and the time of flow in seconds is noted. The time
in seconds is proportional to true viscosity.
Redwood viscometers and Saybolt viscometer are used,
respectively in Commonwealth countries and U.S.A., for
measuring viscosities of lubricating oils.

The results are expressed in terms of time taken by oil to flow

through particular instrument. For example, if time of flow of an
oil through Redwood viscometer at 20°C is 100 seconds, then its
viscosity is "100 Redwood seconds" at 20°C.
A brief description of Redwood viscometer, used in this country, is given
below:
Redwood viscometer is of two types: "Redwood viscometer No. 1" is
commonly used for determining viscosities of thin lubricating oils and is
shown in Fig. 6 and it has a "jet" of bore diameter

Fig. 6. Redwood viscometer No. 1. (Diameter 1.82 mm and length 10 mm).

 On the other hand, "Redwood viscometer No. 2." is used for
measuring viscosities of highly viscous oils like fuel oil. It has a jet
of diameter 3.8 mm and length 15 mm.

Redwood Viscometer No. 1 consists of the following essential parts:

(1) Oil cup is a silver-plated brass cylinder (90 mm in height and

46.5 mm in diameter). The upper end of the cup is open. The bottom
of the cylinder is fitted with an agate jet (with bore of diameter 1.62
mm and length 10 mm).
The jet is opened or closed by a "valve rod", which is a small
silver-plated brass ball fixed to a stout wire.

The level to which the cylinder is to be filled with oil is

indicated by a "pointer", which is a stout, tapered,
upwards-pointing wire fixed on the inner side of the cylinder.

The lid of the cup is fitted with a thermometer, which

indicates the oil temperature.
(ii) Heating bath: Oil cup is surrounded by a cylindrical copper bath,
containing water. It is provided with a tap (for emptying water from it)
and a long side-tube projecting outwards (for heating the bath water by
means of a gas burner or a spirit lamp). A thermometer indicates the
temperature of the water.

(iii) Stirrer: Outside the oil cylinder is stirrer, carrying four blades, for
stirring the water in the bath for maintaining uniform desired
temperature. The stirrer is provided with a circular shield at the top, to
prevent any water splashing into the oil cylinder.
(iv) Spirit level: The lid of the cup is provided with a spirit level for
vertical levelling of the jet.

(v) Levelling screws: The entire apparatus rests on three legs,

provided at their bottom with levelling screws.

(vi) Kohlrausch flask is a specially-shaped flask for receiving the

oil from the jet outlet. Its capacity is 50 mL up to the mark in its
neck.
▪ When the oil is at the desired temperature, heating is stopped
and the ball valve is lifted and suspended from thermometer
bracket. The time taken for 50 mL of the oil to collect in the
flask is noted and then, the valve is immediately closed, to
prevent any overflow of the oil.

▪ The result is expressed in "Redwood No. 1 seconds" at the

particular temperature. Higher the time of flow lesser is the
viscosity of the oil.
2. Flash and Fire-points:

Flash-point is "the lowest temperature at which the oil lubricant gives off
enough vapors that ignite for a moment, when a tiny flame is brought near
it"; while fire-point is "the lowest temperature at which the vapors of the oil
burn continuously for at least five seconds, when a tiny flame is brought
near it". In most cases, the fire-points are 5 to 40° higher than the
flash-points.
 The flash and fire-points do not have any bearing with the
lubricating property of the oil, but these are important when oil is
exposed to high-temperature service.

A good lubricant should have flash-point at Least above the

temperature at which it is to be used. This safeguards against
risks of fire, during the of lubricant.

The flash and fire-points are, usually, determined by using

Pensky-Marten's apparatus (see Fig. 7.), which essentially
consist of:

(1) An oil cup is about 5 cm in diameter and 5.5 cm deep. The level to
which oil is to be filled is marked inside the cup.
The cup lid is provided with four openings of standard sizes.

(1)Through one of these passes a thermometer.

(2)while the second opening is used for introducing test flame.

(3)Through third opening passes stirrer carrying two brass

blades; while the fourth is meant for admission of QIT.

(ii) Shutter is a lever mechanism, provided at the top of the cup. By
moving the shutter, opening in the lid opens and flame (carried by a
flame exposure device) is dipped into this opening, thereby bringing
the flame over the oil surface.

(iii) Flame exposure device is a tiny flame, connected to the shutter

by a lever mechanism.

(iv) Air bath: Oil cup is supported by its flange over an air-bath, which
is heated by a gas burner.

(v) Pilot burner: As the test-flame is introduced in the opening, it gets

extinguished, but when the test flame is returned to its original
position, it is automatically lighted by the pilot burner.
Working:
Oil under examination is filled upto the mark in the oil cup and
then heated by heating the air bath by a burner.

Heat is applied so as to raise the oil temperature by about 5 °C

per minute. At every 1 °C rise of temperature, test flame is
introduced for a moment, by working the shutter.

The temperature at which distant flash appears inside the cup,

is recorded as the flash point.

The heating is continued thereafter and the test flame is

applied as before. When the oil ignites and continues to burn
for at least 5 seconds, the temperature reading is recorded as
the fire-point of the oil.
3. Cloud and Pour-points:

When an oil is cooled slowly, the temperature at which it becomes cloudly or

hazy in appearance, is called its "cloud-point‘’.
While the temperature at which the oil ceases to for pour, is called its
"pour-point".
Cloud and pour-points indicate the suitability of lubricants

cold conditions.

The Lubricant used in a machine working at low

temperatures should possess low pour point; otherwise

solidification of lubricant will cause jamming of the machine.

It has been found that presence of waxes in the lubricating

oil raise the pour-point of the lubricating oil.

Determination of pour-point
It is carried out with help of pour-point apparatus shown in Fig.

It consists essentially of a flat-bottomed tube (about 3 cm in

diameter and 2 cm high) enclosing an air-jacket.

The jacket is surrounded by freezing mixture (Ice+CaCl₂)

contained in a jar.

The neck of flat-bottomed tube is half-filled with oil. A

thermometer is introduced in the oil. As the cooling proceeds
slowly via air-jacket, the temperature falls continuously with
every degree fall of temperature of the oil.
Working:

The tube is withdrawn from the air-jacket for a moment (about 2-3
seconds) and examined.

It is then replaced immediately.

The temperature at which cloudiness is noticed is recorded as the

cloud-point.

After this, cooling is continued and the test-tube is withdrawn after

every 3°C fall temperature and tilted to observe the flow or pour of oil.

The temperature at which oil does not flow in the test-tube, even
when kept horizontal for 5 seconds, is recorded as the pour-point.