UNIT I

Journal and Roller Bearing

Bearings:
The main function of bearing is to permit constrained relative motion of rigid parts. It
is the machine part which supports the rotating shaft, axles etc. Shaft rotates smoothly in the
bearing. Hence, loss of power due to friction is reduced. Bearing also confines the motion of
shaft.
Types of bearings
Bearings are classified into two types. They are
1. Sliding contact bearings (plain bearings).
2. Rolling contact bearings (Anti-friction bearings).
Sliding contact bearings
They are also called as plain bearings. Lubricant oil flim is kept between shaft and the
bearing. The mating surfaces are in sliding contact. Hence, it is known as sliding contact
bearing.
It is classified into two
1. Based on oil film thickness.
2. Based on direction of load.
1. Based on thickness of lubricant oil film
It further classified into four types
a) Zero film bearing
b) Thin film bearing
c) Thick film bearing (or) hydro dynamic bearing
d) Externally pressurised bearing (or) hydro static bearing.
Zero film bearing
It does not use lubricant oil. Metal to metal contact take place. So, its application is
limited.
Thin film bearing
It is also named as boundary lubricated bearing. Even though thin oil film is lying
between mating surfaces, metal to metal contact take place.
Thick film bearing
It is also named as hydrodynamic bearing. Thick oil film is lying between mating
surfaces. During running position, metal to metal contact does not occur. The relative motion
between shaft and bearing creates positive oil pressure which supports the load. Hence
separate pump is not required. But at the starting position, metal to metal contacts take place.

Externally pressurised bearing
It is also named as hydro static bearing. External pump supplies oil to the bearing at
high pressure. This high pressure oil supports the load. At any position, metal to metal
contact does not occur.
2. Based on direction of load
a. Radial bearings (or) Journal bearing
b. Thrust bearings
In sliding contact bearing if load acts perpendicular to the axis of shaft, it is called journal
bearing.

Shaft: Rotating part is called shaft.
Bearing: Shaft supporting member is called bearing.
Journal: The enclosed portion of shaft by the bearing is called journal.
Journal bearings
Hydrodynamic journal bearings are a bearing operating with hydrodynamic
lubrication in which the bearing surfaces are separated from the journal surface by the
lubricant film.

Hydrodynamic journal bearing and a journal rotating in a clockwise direction. Journal
rotation causes pumping of the lubricant (oil) flowing around the bearing in the rotation
direction. If there is no force or load applied to the journal, its position will remain concentric
to the bearing position. However, a loaded journal will be displaced from the concentric
position and forms a converging gap between the bearing and journal surfaces. The pumping
action of the journal forces the oil to squeeze through the wedge shaped gap generating a
pressure. The pressure falls to the cavitations pressure (close to the atmospheric pressure) in
the diverging gap zone where cavitations forms.
The displacement of the shaft centre with respect to the bearing centre is known as
eccentricity. The eccentric position of the shaft is governed by the radial load carried by it
and is adjusted by itself until the load is balanced by the pressure generated in the converging
lubricant film between the bearing and the journal.
The line joining the centre joining the shaft and the sleeve centre is known as the line
of centre. The load carrying capacity depends on the amount of eccentricity (e), angular speed
(), viscosity of lubricant (), bearing dimensions and the clearance.
Types of Journal Bearings
i. Full Journal bearing
ii. Partial Journal bearing
Full journal bearing
Journal is surrounded by bearing fully. Covered angle is 360
o
. It may be
represented as a cylindrical sleeve (bearing) wrapped around shaft. If the wrapping extends
around the full 360
o
of the journal, it is termed as full journal bearing.

Partial journal bearing
Journal is surrounded by bearing partially. Covered angle is 120
o
.

Application of journal bearing
[Refer to PSGDB P.NO: 7.30]
Conveyors
Cam shafts
Motor shafts
Turbine shaft
Materials Used For Sliding Contact Bearings
[Refer to PSGDB P.NO: 7.30]
All the desirable characteristics of bearing materials are not to be found to a high
degree in any particular bearing material. Hence the choice of a material for any application
must represent a compromise.
1. Tine base Babbit and lead- base babbit are in widespread use since they satisfy most
requirements for general application.
2. Where loads are very high, bronze or brass bearings may be used. Bronze is an alloy
of copper and tin. This bronze bearing is suitable for heavy loads at slow speeds.
3. Where high compressive and fatigue strength are required, copper, lead, and tin alloys
may be used.
4. Gun metal is an alloy of copper, tin, and zinc. Gun metal bearing is suitable for high
speeds.
5. Nylon and rubber are used as bearing materials. It is used in water turbine bearings
and water pump bearings.

Properties of good sliding contact bearing materials
[Refer to PSGDB P.NO: 7.30]
Required properties of good bearing material are given below
1. To reduce wear, co-efficient of friction must be less.
2. To maintain the clearance between shaft and bearing, co-efficient of thermal
expansion must be less.
3. Low cost
4. To adjust the alignment error, youngs modulus must be less.
5. To remove the heat generated due to friction, thermal conductivity must be more.
6. To prevent rusting, corrosion resistance must be high.
7. To withstand hydro-dynamic pressure, compression strength must be high.
8. To withstand varying load, fatigue strength must be high.
Lubricant
A lubricant (sometimes referred to as "lube") is a substance (often a liquid) introduced
between two moving surfaces to reduce the friction between them, improving efficiency and
reducing wear. It may also have the function of dissolving or transporting foreign particles
and of distributing heat.
Lubricants perform the following key functions.
- Keep moving parts apart
- Reduce friction
- Transfer heat
- Carry away contaminants & debris
- Transmit power
- Protect against wear
- Prevent corrosion
- Seal for gasses
- Stop the risk of smoke and fire of objects
Types of lubricants
- Solid
- Semi-solid
- Liquid
Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces
in close proximity, and moving relative to each another, by interposing a substance called
lubricant between the surfaces to carry or to help carry the load between the opposing
surfaces. The interposed lubricant film can be a solid, a liquid, and gas.
Types of Lubrication
o Hydrodynamic Lubrication
o Hydrostatic Lubrication
o Elastro hydrodynamic Lubrication
o Boundary Lubrication
The regimes of lubrication
As the load increases on the contacting surfaces three distinct situations can be observed with
respect to the mode of lubrication, which are called regimes of lubrication:
- Fluid film lubrication is the lubrication regime in which through viscous forces the
load is fully supported by the lubricant within the space or gap between the parts in
motion relative to one another (the lubricated conjunction) and solid-solid contact is
avoided.
[2]

o Hydrostatic lubrication is when an external pressure is applied to the
lubricant in the bearing, to maintain the fluid lubricant film where it would
otherwise be squeezed out.
o Hydrodynamic lubrication is where the motion of the contacting surfaces
and the exact design of the bearing is used to pump lubricant around the
bearing to maintain the lubricating film. This design of bearing may wear
when started or stopped, as the lubricant film breaks down.
- Elastohydrodynamic lubrication: The opposing surfaces are separated but there
occurs some interaction between the raised solid features called asperities, and there
is an elastic deformation on the contacting surface enlarging the load bearing area
whereby the viscous resistance of the lubricant becomes capable of supporting the
load.
- Boundary lubrication (also called boundary film lubrication): The bodies come into
closer contact at their asperities; the heat developed by the local pressures causes a
condition which is called stick-slip and some asperities break off. At the elevated
temperature and pressure conditions chemically reactive constituents of the lubricant
react with the contact surface forming a highly resistant tenacious layer, or film on the
moving solid surfaces (boundary film) which is capable of supporting the load and
major wear or breakdown is avoided. Boundary lubrication is also defined as that
regime in which the load is carried by the surface asperities rather than by the
lubricant.
Besides supporting the load the lubricant may have to perform other functions as well, for
instance it may cool the contact areas and remove wear products. While carrying out these
functions the lubricant is constantly replaced from the contact areas either by the relative
movement (hydrodynamics) or by externally induced forces.
Lubrication is required for correct operation of mechanical systems pistons, pumps, cams,
bearings, turbines, cutting tools etc where without lubrication the pressure between the
surfaces in close proximity would generate enough heat for rapid surface damage which in a
coarsened condition may literally weld the surfaces together, causing seizure.
Lubrication of bearings
Journal bearing is lubricated for the following reasons.
1. To reduce friction
2. To reduce wear
3. To transfer the heat due to friction
4. To prevent the rusting of bearing surface.
5. To prevent the damage of bearing surface.
Properties of good lubricant
The essential properties required for lubricant are
1. High viscosity index
2. High flash point
3. High fire point
4. High corrosion resistance
5. Low freezing point
6. Low cost.
Factor to be considered for the selection of type of bearing
While selecting the type of bearing, the following factors are considered
1. Type of load
2. Speed of shaft
3. Space required
4. Vibrations
5. Temperature
6. Stating torque
Advantages of sliding contact bearing
Low cost
Silent in operation
Long life
Withstands shocks
Not breaks easily
Not affected by fatigue load
Simple design
Less radial space is enough
Not damaged by impurities.

Disadvantages of sliding contact bearing
More friction
More loss of power
High maintenance cost
Large amount of lubricant is required
Replacement is not easier
Not operate in inclined position
Less accuracy

Application of sliding contact bearing
Diesel engine
Gas engines
Pumps
Compressors
Turbines
Aircraft engines
Conveyors
Typewriters

Terminology of hydro-dynamic journal bearing
When shaft is running at high speed, some terms are defined
Diameteral clearance (C) = bearing diameter journal diameter
Where
D= Diameter of journal in mm
C= Diameteral clearance in mm
D + C = Diameter of bearing in mm
L= length of bearing in mm
N= speed of journal in RPM
N= speed of journal in RPS
O= Centre of journal
O=Centre of bearing
Diametral clearance (C)
It is the difference between bearing diameter and journal diameter.
Radial clearance (C
r
)
Radial clearance = diameteral clearance
2

2
r
C
C =

Diametral Clearance Ratio (D.C.R)
It is the ratio of diametral clearance to diameter of journal.

D.C.R =
Diameteral clearance
Diameter of journal


D.C.R =
C
D

Generally ,

D.C.R = 0.001
Eccentricity (e)
It is the radial distance between centre of bearing and centre of journal.
Eccentricity (e) =
0
2
C
h
Minimum oil film thickness (h
0
)
In completely lubricanted condition, the minium distance between bearing and journal
is called h
0
0
2
C
h e =

Eccentricity ratio ( c )

It is the ratio of eccentricity to radial clearance.

( )
2
e
C
c = or
2e
C

Short, square, long bearing

L= length of journal

D= diameter of journal

If
L
D
< 1, bearing is called short bearing
If
L
D
= 1, bearing is called square bearing
If
L
D
> 1, bearing is called long bearing.

Bearing characteristic number

In the design of journal bearing, calculation of loss of power due to friction is very
important. Loss of power depends on the amount of bearing friction.

It is experimentally found that co-efficient of friction depends on three variables.

1.
b
ZN
p
2.
D
C
3.
L
D


Z = Absolute viscosity of lubricant
N = Speed of journal

b
p = Bearing pressure
Sommer Field Number
It is a dimensionless quantity. It is an important parameter in the design of journal
bearing.
Sommer Field Number,
2
6
'
10
b
ZN D
S X
p C
| |
=
|
\ .

Where,
Z = Absolute viscosity of oil in Kg/m-s
N= speed of journal in RPS
p
b
= bearing pressure in N/mm
2


D = diameter of journal in mm
C = diameter clearance in mm.
Values of sommerfield number is taken from table

Note:
If other parameters are not given to calculate sommerfield number, table cannot be
used.
In such case, sommerfield number is calculated as follows

2
6
'
14.3
10
b
ZN D
X
p C
| |
=
|
\ .

Critical pressure of the journal bearing [p
c
]

The minimum pressure at which oil film breaks and metal to metal contact takes place
is called critical pressure (or) minimum operating pressure of the bearing.

Critical pressure,
2
2
6
X
/
4.75 10
X
c
ZN D L
p N mm
C L D
| | | |
| |
+
\ . \ .


L = Length of journal in mm

Co- efficient of friction for journal bearing
[McKees equation]
McKee brothers conducted many experiments on hydrodynamic journal bearing and
established a relationship between co-efficient of friction and other parameters as follows.
6
0.326
10
X X
b
ZN D
K
p C

| |
= +
|
\ .

This is called McKees equation
K= correction factor for end leakage
Value of k is noted from graph
Generally, K=0.002 for
L
D
ratio of from 0.75 to 2.8
[Refer to PSGDB P.NO: 7.34]
Heat generated in journal bearing
In journal bearing, power lost due to friction is generated in the form of heat.
Temperature of lubricant oil and bearing surface is increased.
Heat generated,
g
Q = P V J/sec (or) watts
Where,
Q
g
= heat generated
= co-effecient of friction (calculated from McKees equation)
P or W = load on the bearing in Newton
V= rubbing velocity of journal in m/sec
Where,

60 1000 X
DN
V
t
= m/sec
Heat dissipated by journal bearing
In journal bearing, heat is generated due to friction. Heat generated must be removed.
Otherwise, babbit liner melts due to high temperature.
Heat dissipated by journal bearing depends on the following factors.
1. Temperature difference between bearing and surrounding air
2. Area of bearing surface exposed to air.
3. Amount of air contacting the bearing surface.
Heat dissipated naturally is,
. . .
6
10
C L D t
d
Q
d
A
= J/sec (or) watts
Where,
Q
d
= heat dissipated
C
d
= heat dissipation co-efficient in W/m
2
/
0
C
L x D = projected area of bearing in mm
2
( )
0
1
2
b a a
t t t t t A = =

Where,

b
t = bearing surface temp in
0
C

0
t = operating temp of oil film in
0
C (60
0
to 70
0
C)

a
t = ambient temp in
0
C
Another method
By lasches equation

2
6
( 18)
10
d
X
d
LD t
Q
K
A +
= J/sec or watts
K
d
= constant for heat dissipation in
0
C/m
2
/W
Design procedure for journal bearings
1. Calculation of torque (T) from power (P
0
)
0
2
60 1000 X
NT
P
t
=
Where,
P
0
= power transmitted in watts
N= speed of journal in RPM
2. Calculation of diameter of journal (D)
3
16
T D
t
t = N-mm
t = shear stress in N/mm
2

3. Suitable
L
D
ratio is selected and bearing length L is determined.
4. Calculation of bearing pressure (p
d
)
Load in newtons
Projected area of bearing
b
p =
P or W
LD
b
p =
5. Clearance ratio
D
C
is selected.
6. Select suitable viscosity(Z) of oil corresponding to oil temperature .