Module 5
Lubrication and Bearings
By Swaraj Lewis
Lubricant
• Friction is the resistance to relative motion between two surfaces in contact, and the
function of a lubricant is to reduce it.
• Any substance placed between the rubbing surfaces, which reduces the friction is a
lubricant.
• If the lubricant is fluid and there is enough of it between the surfaces to separate them
completely, fluid film lubrication exists.
• But if the layer of the lubricant is so thin that there is partial metal-to-metal contact,
boundary lubrication exists.
Types of Lubricant
Lubricants are classified into the following groups
• Liquid
• Semi-liquid
• Solid
• Liquid Lubricant
• The liquid lubricants usually used in bearings are mineral oils and synthetic oils. The
mineral oils are most commonly used because of their cheapness and stability. The
liquid lubricants are usually preferred where they may be retained.
• Semi-Liquid Lubricant
• A grease is a semi-liquid lubricant having higher viscosity than oils. The greases are
employed where slow speed and heavy pressure exist and where oil drip from the
bearing is undesirable.
• Solid Lubricant
• The solid lubricants are useful in reducing friction where oil films cannot be maintained
because of pressures or temperatures. They should be softer than materials being
lubricated. A graphite is the most common of the solid lubricants either alone or mixed
with oil or grease. Other solid lubricant are soap, talc, wax, mica, French chalk.
Properties of Lubricants
1. Viscosity
It is the measure of degree of fluidity of a liquid. It is a physical property by virtue of which an oil is
able to form, retain and offer resistance to shearing a buffer film-under heat and pressure. The
greater the heat and pressure, the greater viscosity is required of a lubricant to prevent thinning and
squeezing out of the film.
2. Oiliness
It is a joint property of the lubricant and the bearing surfaces in contact. It is a measure of the
lubricating qualities under boundary conditions where base metal to metal is prevented only by
absorbed film. There is no absolute measure of oiliness
3. Density
This property has no relation to lubricating value but is useful in changing the kinematic viscosity to
absolute viscosity
Properties of Lubricants
4. Viscosity index.
The term viscosity index is used to denote the degree of variation of viscosity with temperature.
5. Flash point
It is the lowest temperature at which oil gives off sufficient vapour to support a momentary flash
without actually setting fire to the oil when a flame is brought within 6 mm at the surface of the oil
6. Fire point
It is the temperature at which oil gives off sufficient vapour to burn it continuously when ignited
7. Pour point or freezing point
It is the temperature at which oil will cease to flow when cooled.
Properties of a good lubricant are
1. It should give rise to low friction.
2. It should adhere to the surface and reduce the wear.
3. It should protect the system from corrosion.
4. It should have good cleaning effect on the surface.
5. It should carry away as much heat from the surface as possible.
6. It should have thermal and oxidative stability.
7. It should have good thermal durability.
8. It should have antifoaming ability.
9. It should be compatible with seal materials.
10.It should be cheap and available in plenty
Types of Lubrication
1. Hydrodynamic lubrication
2. Hydrostatic lubrication
3. Elasto-hydrodynamic lubrication
4. Boundary lubrication
5. Solid film lubrication
Hydrodynamic lubrication
• The load-carrying surfaces of the bearing are separated by a relatively thick film of lubricant, to
prevent metal-to-metal contact, and that the stability thus obtained can be explained by the laws
of fluid mechanics
• Hydrodynamic lubrication does not depend upon the introduction of the lubricant under pressure,
though that may occur; but it does always require the existence of an adequate supply.
• The film pressure is created by the moving surface itself pulling the lubricant into a wedge-shaped
zone at a velocity sufficiently high to create the pressure necessary to separate the surfaces
against the load on the bearing.
• Hydrodynamic lubrication is also called full-film, or fluid, lubrication.
Elasto-hydrodynamic lubrication (boundary lubrication)
• It is the phenomenon that occurs when a lubricant is introduced between surfaces that are in
rolling contact, such as mating gears or rolling bearings.
• Insufficient surface area, a drop in the velocity of the moving surface, a lessening in the quantity
of lubricant delivered to a bearing, an increase in the bearing load, or an increase in lubricant
temperature may prevent the build-up of a film thick enough for full-film lubrication. This is called
boundary lubrication
Hydrostatic Lubrication
• Obtained by introducing the lubricant, which is sometimes air or water, into the load-bearing area
at a pressure high enough to separate the surfaces with a relatively thick film of lubricant. So,
unlike hydrodynamic lubrication, this kind of lubrication does not require motion of one surface
relative to another.
Solid Film lubricants
• Solid film lubricant exhibit very low shear. They are used where the oil films cannot
maintain minimum film thickness due to the high pressure and associated temperatures
Bearings
• A bearing is machine part, which support a moving element and confines its motion. The
supporting member is usually designated as bearing and the rotating member (Shaft) is termed as
journal.
• Since there is a relative motion between the bearing and the moving element, a certain amount of
power must be absorbed in overcoming friction, and if the surface actually touches, there will be a
rapid wear.
• Wear causes changes in dimensions and eventual breakdown of the machine element and the
entire machine.
• The loss of just a few milligrams of material in the right place, due to wear can cause a production
machine or an automobile to be ready for replacement
Properties of a good bearing material
• When metal to metal contact occurs, the bearing material should not damage the surface of the
journal. It should not stick or weld to the journal surface.
• It should have high compressive strength to withstand high pressures without distortion
• In certain applications like connecting rods or crankshafts, bearings are subjected to fluctuating
stresses. The bearing material, in these applications, should have sufficient endurance strength to
avoid failure due to pitting
• The bearing material should have the ability to yield and adopt its shape to that of the journal. This
property is called conformability
• The dirt particles in lubricating oil tend to jam in the clearance space and, if hard, may cut scratches
on the surfaces of the journal and bearing. The bearing material should be soft to allow these
particles to get embedded in the lining and avoid further trouble. This property of the bearing
material is called embeddability.
• The bearing material should have reasonable cost and should be easily available in the market.
• In applications like engine bearings, the excessive temperature causes oxidation of lubricating oils and
forms corrosive acids. The bearing material should have sufficient corrosion resistance under these
conditions
Classification of Bearings
• Depending upon the direction of the load to be supported
• Radial bearing.
• Thrust bearing.
• Depending upon the nature of contact between the working surfaces
• Sliding contact bearings
• Rolling contact bearings.
Radial load bearing
Bearing Materials advantage and disadvantage
Hydrodynamic Lubrication (Hydrodynamic Theory of Lubrication)
• Thick film lubrication is further divided into two groups: hydrodynamic and hydrostatic
lubrication.
• Hydrodynamic lubrication is defined as a system of lubrication in which the load-
supporting fluid film is created by the shape and relative motion of the sliding surfaces
• Assumptions
1. The lubricant obeys Newton's law of viscous flow.
2. The pressure is assumed to be constant throughout the film thickness.
3. The lubricant is assumed to be incompressible.
4. The viscosity is assumed to be constant throughout the film.
5. The flow is one dimensional, i.e. the side leakage is neglected
Hydrodynamic Lubrication (Hydrodynamic Theory of Lubrication)
• Consider a steady load F, a fixed bearing and a rotating journal.
• Stage 1 : At rest, the bearing clearance space is filled with oil,
but the load F has squeezed out the oil film at the bottom.
Metal-to-contact exists. The vertical axis of bearing and journal
are co-axial. Load and reaction are in line (Fig.a)
• Stage 2: When the journal starts rotating slowly in clockwise
direction, because of friction, the journal starts to climb the
wall of the bearing surface as in (Fig. B & c) Boundary
lubrication exists now. The wear normally takes place during
this period. However, the journal rotation draws the oil
between the surfaces and they separate.
Hydrodynamic Lubrication (Hydrodynamic Theory of Lubrication)
• Stage 3: As the speed increases, more oil is drawn in and
enough pressure is built up in the contact zone to float” the
journal (Fig. D) Further increase in speed, additional pressure
of the converging oil flow to the right of the minimum film
thickness position (ho) moves the shaft slightly to the left of
center. As a result full separation of journal and bearing
surfaces occurs. In stable operating condition, the pressure
distribution on the journal is shown in Fig. D.
• This is known as – Hydrodynamic lubrication or full film/thick
film lubrication. At this equilibrium condition, the pressure
force on journal balances the external load F.
Petroff’s Equation
• Petroff’s equation is used to determine the
coefficient of friction in journal bearings. It is based
on the following assumptions
(i) The shaft is concentric with the bearing.
(ii) The bearing is subjected to light load.
(iii) The end leakage is completely negligible.
(iv) The oil is used of high viscosity.
(v) The journal revolves at very high speeds
Note: In practice, such conditions do not exist. However, Petroff’s equation is important
because it defines the group of dimensionless parameters that govern the frictional
properties of the bearing.
Petroff’s Equation
Let,
D =Diameter of bearing
d= diameter of journal
c= diametrical clearance = D-d
Ψ =Diametrical clearance ratio =
= Speed of journal in rps=
L= length of bearing
v = velocity at the journal surface =
p = Bearing pressure =
=Coefficient of friction
Flim thickness
Shear stress in the lubricant Viscosity
Substituting h,A and v in (1)
Ψ
Coefficient of friction
Substituting F in (2), we get
Ψ Ψ Ψ
c c
Data hand book eq. 15.4 (a) page 353
Sommerfeld number
The Sommerfeld number is also a dimensionless parameter used extensively in the design of
journal bearings. Mathematically
McKee equation
= f
c
Bearing Modulus
• A plot of change in the co-efficient of friction versus the value of for full journal
bearing given as
"
Heat Generated in a Journal Bearing
The heat generated in a bearing is due to the fluid friction and friction of the parts having
relative motion. Mathematically, heat generated in a bearing
The heat generated can also be found by knowing the temperature rise of the lubricant
which is used to carry away the heat generated in the bearing.
Heat dissipated in a Journal Bearing
Let,
Problem 1: A 75 mm long full journal bearing of diameter 75 mm supports a radial load of 12kN
at a shaft speed of 1800 rpm. Assuming the ratio of diameter to diametral clearance as 1000.
The viscosity of oil is 0.01 N/m2s at the operating temperature. Determine (i) Sommerfeld
number, (ii) co-efficient of friction as calculated by the McKee equation. (iii) Amount of heat
generated.
Ψ
2
Average bearing pressure
Surface velocity of journal
i)Sommerfeld number
Data hand book eq. 15.6 (a) page 354
Ψ=
ii) co-efficient of friction as calculated by the McKee equation
= f Data hand book eq. 15.4 (b) page 353
c
iii) Heat generated by friction
Data hand book eq. 15.6 (j) page 356
Problem 2: A 75 mm long full journal bearing of diameter 75 mm supports a load of 12kN
on a journal turning at 1800 rpm. Assuming a ratio of 1000, and an oil having viscosity of
0.01 kg/m-s at the operating temperature, determine the coefficient of friction by using (i)
the Mckee equation, (ii) the Raimondi and Boyd curve (iii) also determine the heat
generated using the co-efficient of friction as calculated by the McKee equation
Average bearing pressure
i) co-efficient of friction as calculated by the McKee equation
Data hand book eq. 15.4 (b) page 353
= f
c 𝐾𝑎=0.195×106 full bearing 𝑖.𝑒.𝛽=360°
(Δ f)=0.002 for bearing having 𝑙/𝑑=0.75 𝑡𝑜 2.8
=
ii) The Coefficient of friction using the Raimondi and Boyd Curve
We need Sommerfeld number to use Raimondi and Boyd curve
Sommerfeld number
For β = 360 , S = 8.43𝑓𝑟𝑜𝑚 𝑔𝑟𝑎𝑝ℎ 15.7, 𝑝𝑎𝑔𝑒 355
ii) Heat generated using the co-efficient of friction as calculated by the McKee equation
Data hand book eq. 15.6 (j) page 356
Surface velocity of journal
Problem 3: Design a journal bearing for a centrifugal pump running at 1200 rpm. Diameter of
journal is 100 mm and load on bearing is 15 KN. / = . , bearing temperature 50°C and
ambient temperature 30°C. Find whether artificial cooling is required.
Given: 𝐵
𝐴
Average bearing pressure
Surface velocity of journal
Difference in temperature
𝑜
From data hand book table 15.7, page 366, for centrifugal pump
→ bearing pressure p = 0.7 − 1.4 𝑀𝑁/𝑚
→ viscosity Z = 0.025 𝑁𝑠/𝑚
→∴ select bearing pressure p = 1 𝑀𝑁/𝑚
i) co-efficient of friction as calculated by the McKee equation
Data hand book eq. 15.4 (b) page 353
= f
c 𝐾𝑎=0.195×106 full bearing 𝑖.𝑒.𝛽=360°
(Δ f)=0.002 for bearing having 𝑙/𝑑=0.75 𝑡𝑜 2.8
0.025 ∗ 20 1
𝑓=0.195 ∗ 10 ∗ ∗ 10 + 0.002
1 0.0013
ii) Heat generated using the co-efficient of friction as calculated by the McKee equation
Data hand book eq. 15.6 (j) page 356
Data hand book eq. 15.7 page 361
Take 𝐾 = 0.484 for light bearing
Problem 4: A full journal bearing 60 mm diameter and 150 mm long has a radial load of 2MPa
per unit projected area. The Shaft speed is 500 rpm and H9 e9 fit is to b used. The surrounding
air temperature is 20C and the oi used has viscosity 0.099 Kg/ms at its operating temperature.
Determine the probable temperature of the bearing surface, assuming all the heat generated is
dissipated by heat transfer. Use Mckee’s equation. What is the operating temperature of oil?
Given: 2
a
For e9 shaft From data hand book table 23.3, page 447
Diameter of shaft is
For H9 shaft From data hand book table 23.3, page 449
Diameter of bearing is
Diametral clearance
Ψ= =
Surface velocity of journal
co-efficient of friction as calculated by the McKee equation
Data hand book eq. 15.4 (b) page 353
= f 𝐾𝑎=0.195×106 full bearing 𝑖.𝑒.𝛽=360°
c (Δ f)=0.002 for bearing having 𝑙/𝑑=0.75 𝑡𝑜 2.8
0.099 ∗ 8.33 1
𝑓=0.195 ∗ 10 ∗ ∗ 10 + 0.002
2 3.4744 ∗ 10
For heat balance
Take 𝐾 = 0.484 for light bearing
∆
0.0043142*
.
Difference in temperature
