Krishnaraja G Kodancha,

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• Krishnaraja G Kodancha,
• Assistant Professor,
• Department of Automobile Engineering,
• B V B College of Engg. Tech.,
• HUBLI
• 09886596953
• email krishnaraja_at_bvb.edu

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AU51, Theory and Design of Automotive Engines
Chapter No 06 CRANK SHAFT
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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CRANK SHAFT
Plan of Execution Session 24 Introduction and
Basics, Power Transmitting, torsion and bending
formulae, Methods of obtaining Torque, Bending
moment, Shear stress, Bending Stress, Definition
of Crank shaft, Types, Materials used,
Manufacturing. Session 25 Bearing Pressures,
Stresses, Balance Weights, Local Balance,
Empirical Rules for Crankshaft Dimensions, Six
and Eight Cylinders, Oil holes in
Crankshafts,Balancing Crankshaft, Design Procedure
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Plan of Execution
Session 26 Design Calculations, Analysis of
Center Crank shaft, Crank at Dead Center, Crank
at angle of maximum Twisting Moment Session 27
Analysis of side Crank Shaft, Crank at Dead
Center, Crank at angle of maximum Twisting
Moment, Simple numericals Session 28 Numericals,
Software demo, References, Questions from
Previous Question Papers, Model Questions, and
Summary
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• Topics covered in Previous Class
• Design Calculations,
• Analysis of Center Crank shaft,
• Crank at Dead Center,
• Crank at angle of maximum Twisting Moment

Machine
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• Topics to be Covered in this Class
• Analysis of side Crank Shaft,
• Crank at Dead Center,
• Crank at angle of maximum Twisting Moment,
• Simple numericals

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Analysis of side Crank Shaft
The crank shaft is to analysed for two positions
i)    Crank on Dead Center ii) Crank at
angle of maximum Twisting Moment
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Analysis of side Crank Shaft (Contd..)
Crank on Dead Center
RV2(W)
RH2(F)
2
RH1(BELT)
y
T1T2
RV1(W)
x
Machine
W
RH1(F)
1
RH1(BELT)
b
F
Force Analysis of side Crank at Dead Center
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Analysis of side Crank Shaft (Contd..)
Gas Load,
To find the reactions RH1(F) and RH2(F)
 
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Analysis of side Crank Shaft (Contd..)
RH1(F) - RH2(F) -F 0
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Analysis of side Crank Shaft (Contd..)
Reactions at bearing 1 and 2 due to Belt Pull,
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
RH1(belt) RH2(belt) (T1T2) 0
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Analysis of side Crank Shaft (Contd..)
Reactions at bearing 1 and 2 due to Weight of the
Flywheel (W)
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
Rv2(W) RV1(W) (W) 0
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Analysis of side Crank Shaft (Contd..)
CRANKPIN
bearing pressure
Allowable bearing pressure is available in Table
3.6/49
length of the crankpin is approximately taken as
(0.8 to 1.1) diameter of the crankpin. Refer
page no 50 of the data hand book
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
The crankpin is now checked for bending stress
Get ?b and compare with given allowable bending
stress
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Analysis of side Crank Shaft (Contd..)
Design of Bearing
l11.5d1 to 2d1
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
OR
Thickness of web th(0.5 to 0.9) dp (Page No
50) Let us take h0.6dp Length of the bearing
l11.7dp
We know that bending moment,
We know that bending stress,
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Analysis of side Crank Shaft (Contd..)
Design of Shaft under the flywheel
Horizontal Bending Moment Gas load and Belt
pull Vertical Bending Moment - Flywheel Weight
Because of Horizontal Loading
MHORMGasMBelt
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
Because of Vertical Loading
Resultant
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Analysis of side Crank Shaft (Contd..)
Design of Crank Web
Thickness of web h(0.5 to 0.9) dp
Width of crank web w(1.1 to 1.2 )ds
Maximum bending moment on the crank web
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
Design of Crank Web
Direct stress
Superimposing the stresses we get,
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Analysis of side Crank Shaft (Contd..)
Crank at angle of maximum Twisting Moment
The twisting moment on the crankshaft will be
maximum, when the tangential force Ft is maximum
and this can be calculated graphically by taking
pressures from the net effort diagram for
different crank angles. The angle usually lies
between 250 to 350 from the dead center for a
constant volume combustion engines and between
300 to 400 for constant pressure combustion
engines. At this angle, the gas pressure will not
be maximum. (FIG 3.1/50 in design data book)
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
Crank at angle of maximum Twisting Moment
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Analysis of side Crank Shaft (Contd..)
Crank at angle of maximum Twisting Moment
3.12/45
3.13/45
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
Crank at angle of maximum Twisting Moment
3.14/45
In this position of the crankshaft, the different
sections will be subjected to both bending and
torsional moments and these must be checked for
combined stress. At this point, Shear stress is
taken as failure criteria for crankshaft.
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Analysis of side Crank Shaft (Contd..)
RV2(W)
RH2FT
RH2(BELT)
RH2FR
2
y
T1T2
RV1(W)
x
RH1FR
W
RH1(BELT)
1
RH1FT
b
Ft
FP
Fr
R
Force Analysis of Side Crank at angle of maximum
twisting Moment
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
The reactions due Radial Force (Fr)
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
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Analysis of side Crank Shaft (Contd..)
The reactions due tangential force (Ft)
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
The reactions due tangential force (Ft)
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The reactions at the bearings 1 and 2 due to
Flywheel weight (W) and resultant belt pull
(T1T2) will be same as discussed earlier.
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Analysis of side Crank Shaft (Contd..)
Design of Crank Web

• The dimensions of the crankpin and crank web are
  taken same as obtained in crank shaft at dead
  center.

• The most critical section is where the web joins
  the shaft. This section is subjected to the
  following stresses

Bending stress due to the tangential force FT
Bending stress due to the radial force Fr
Direct compressive stress due to radial force
Fr Shear stress due to the twisting moment of FT.
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
Bending stress due to the tangential force FT
Bending moment due to tangential force,
Therefore bending stress due to tangential force
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
Bending stress due to the radial force Fr
Bending moment due to the radial force,
Therefore bending stress due to radial force
 
Direct compressive stress due to radial force Fr
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Analysis of side Crank Shaft (Contd..)
Shear stress due to the twisting moment of FT.
Twisting moment due to the Tangential force,
Therefore shearing stress due to Tangential force
Superimposing the stresses we get, Total
compressive stress,
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
Now the total or maximum normal and maximum shear
stresses are given by,(1.11b/2) and (1.12/2)
This total maximum stress should be less than the
maximum allowable stress.
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Analysis of side Crank Shaft (Contd..)
Design of Shaft under the flywheel
Horizontal bending moment acting on the shaft due
to piston gas load,
Horizontal bending moment acting on the shaft due
to belt pull,
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of side Crank Shaft (Contd..)
Therefore total horizontal bending moment,
Vertical bending moment due to flywheel,
Since these bending moments act at right angles
to each other, the combined bending moment is
given by
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In addition to this moment there will be a
twisting moment because of tangential force Ft.
Therefore Equivalent twisting moment,
We have
Diameter of the shaft under flywheel dW can be
obtained.
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Problem No.1
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Design an overhung crank pin for an engine
having the following particulars Cylinder
diameter 300mm Stroke
500mm Maximum explosion pressure in the
cylinder 1.8MPa Engine
Speed 200rpm Permissible bending stress for
pin 1000MPa Permissible Bending stress 85MPa
(Academic Problem)
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Given data Cylinder diameter
D300mm Stroke L500mm Maximum explosion
pressure in the cylinder Pmax1.8MPa Engin
e Speed N200rpm Permissible bending stress
for pin ?b800MPa Permissible Bearing
stress pb85MPa
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Solution We know that bearing pressure
..P1.1
Where lp and dp are length and diameter of the
crankpin respectively. pb is the allowable
bearing pressure on the pin, MPa The length of
the crankpin is approximately taken as (0.8 to
1.1) diameter of the crankpin. Refer page no 50
of the data hand book Let us take lp1.1dp
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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We know that gas load
Substituting the values in equation (P1.1) we
get Diameter of the crank pin dp36.88mm
Referring the table 3.5a/48, standard diameter
of dp40mm is taken. Length of the crankpin
lp(1.1)(40)44mm
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Check We know that,
(1.16/3)
Substituting the values in Equation 1.16 and
solving for ?b we get,
?800MPa, hence safe
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Problem No.2
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A force of 120kN acts tangentially on the crank
pin of an overhang crank. The axial distance
between the centre of the crankshaft journal and
the crank pin is 400mm and the crank is 500mm
long. Determine a)     Diameter and length of
the crankpin journal. b)      Diameter of the
shaft journal Given that Safe bearing
pressure 5MPa Bending stress 65MPa Princ
ipal stress in the shaft journal 65MPa

FEB 2005,
12M VTU
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Given Data
b400mm and R500mm
pb5MPa, ?b65MPa, ?max 65MPa, F120(10)3N
Solution a)
We know that, Bearing pressure
And assuming ratio of length to diameter of the
crank pin as 1.3,
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Solving we get, diameter of the crank pin dp
35.87mm. Adopting the standard diameter dp
140mm T3.4/48
Minimum length of the crankpin,
171.4mm
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Check
b) Bending moment at the shaft journal
MF(b)120(10)3(400)48(10)6, N-mm
Twisting moment at the shaft journal,
TF(R)120(10)3(500) 60(10)6, N-mm
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According to maximum normal stress theory,
----------(3.5a/42)
Here, because of solid shaft, K0, Substituting
the values of M, T and ?max in equation 3.5a we
get
213.85mm
Taking ds220mm as standard diameter (T3.4/48)
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Problem No.3
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Determine the maximum normal stress and the
maximum shear stress at section A-A for the crank
shown in Figure 3 when a load of 10kN is assumed
to be concentrated at the center of the crank
pin.
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Bending moment M10(10)3(402525)9(10)5,
N-mm Twisting moment T10(10)3(125)12.5(10)5,
N-mm
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Maximum Principal Stress(?1)
.(1.11a/2)
29.46MPa
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Maximum Shearing Stress(?max)
(1.12/2)
18.60MPa
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Summary

• Analysis of side Crank Shaft, Crank at Dead
  Center,
• Crank at angle of maximum Twisting Moment,
• Simple numericals

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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Thank You
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli