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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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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• Topic Covered in Previous Class
• Bearing Pressures,
• Stresses,
• Balance Weights,
• Local Balance,
• Empirical Rules for Crankshaft Dimensions,
• Six and Eight Cylinders,
• Oil holes in Crankshafts,
• Balancing Crankshaft,
• Design Procedure

Machine
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• Topics to be covered in today's Class
• Design Calculations,
• Analysis of Center Crank shaft,
• Crank at Dead Center,
• Crank at angle of maximum Twisting Moment

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        DESIGN CALCULATIONS

• In the design of the crankshafts, it is assumed
  that the crankshaft is a beam with two or more
  supports
• Every crankshaft must be designed or checked at
  least for two crank positions, one when the
  bending moment is maximum, and the other when the
  twisting moment is a maximum.
• In addition, the additional moments due to the
  flywheel weight, belt tension and other forces
  must be considered.

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        DESIGN CALCULATIONS(Contd..)

• To make the calculations simpler, without loosing
  accuracy, it is assumed that the effect of the
  bending forces does not extend two bearings
  between which a force is applied.
• There are two considerations, which determine the
  necessary dimensions of the crankpin. One is that
  its projected bearing area (diameter times
  length) must be large enough so it will safely
  sustain the bearing loads imposed upon it by gas
  pressure, inertia and centrifugal force

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        DESIGN CALCULATIONS(Contd..)
The second, that the crankshaft as a whole must
be sufficiently rigid so that it will not
vibrate voilently under the periodic forces to
which it is subjected in service
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Analysis of Center Crank Shaft
The crank shaft is to analysed for two positions
i)    Crank on Dead Center ii) Crank at
angle of maximum Twisting Moment
Machine
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Analysis of Center Crank Shaft (Contd..)
Crank on Dead Center
R3
RV3(W)
3
RH3(BELT)
y
T1T2
RV2(W)
R2
x
W
RH2(F)
2
RH2(BELT)
b
F
a
RH1(F)
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Force Analysis of Crank at Dead Center
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Analysis of Center Crank Shaft (Contd..)
Gas Load
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
RH1(F) RH2(F) -F 0
If ab, then
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Analysis of Center Crank Shaft (Contd..)
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
RH2(belt) RH3(belt) (T1T2) 0
If xy, then
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Analysis of Center Crank Shaft (Contd..)
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
Rv3(W) RV2(W) (W) 0
If xy, then
HERE DESIGN IS BASED ONLY ON BENDING MOMENT
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Analysis of Center Crank Shaft (Contd..)
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Force Analysis of Crank at Dead Center
Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
Crank Pin
N-mm
N-mm
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
Bearing pressure,
Length of the crankpin,
lp(0.8 to 1.3) dp
Or
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Analysis of Center Crank Shaft (Contd..)
Left Hand Crank Web
The thickness h0.22D to 0.32 D or
0.5 dp to 0.9 dp
0.65 dp 6.35mm
Width
(Page No 50)
or w (1.1 to 1.2) dp, mm
Empirical formulae Check for induced stresses
Check Direct stress and Bending Stress
Superimpose and check with given allowable stress
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
Right Hand Crank Web
Since the bearing 1 and 2 are usually of the same
length and symmetrical to the cylinder
centerline, therefore RH1(F) and RH2 (F) are
equal. Hence normally same dimensions are adopted
for both crank webs.   Otherwise, providing the
dimensions empirically as used in Left hand crank
web and check is made as follows
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
Superimposing the direct and bending stresses we
get total stress on the web, and check against
the allowable stresses. To avoid the
manufacturing difficulties keep the same
dimensions for both Webs by taking higher h and w
values.
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Analysis of Center Crank Shaft (Contd..)
Shaft Under the Flywheel Diameter of the shaft
between bearing 2 and 3
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center 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 Center Crank Shaft (Contd..)
Crank at angle of maximum Twisting Moment
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Analysis of Center 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 Center 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 Center Crank Shaft (Contd..)
RV3(W)
RH3(BELT)
3
y
T1T2
RV2(W)
x
RH2FR
W
RH2(BELT)
2
RH2FT
b
Ft
FP
Fr
R
Force Analysis of Crank at angle of maximum
twisting Moment
a
RH1FR
1
RH1FT
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center 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 Center Crank Shaft (Contd..)
The reactions due Radial force (Fr)
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Analysis of Center 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 Center Crank Shaft (Contd..)
The reactions due tangential force (Ft)
The reactions at the bearings 2 and 3 due to
Flywheel weight (W) and resultant belt pull
(T1T2) will be as discussed earlier.
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Analysis of Center Crank Shaft (Contd..)
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Analysis of Center Crank Shaft (Contd..)
Crank Pin
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
Crank Pin
lp(0.8 to 1.3) dp (Page No 50)
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Analysis of Center Crank Shaft (Contd..)
Shaft under the Flywheel Diameter of the shaft
between bearing 2 and 3
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
Right hand Crank Web
The bending moment due to radial component is
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
Right hand Crank Web
Z
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Analysis of Center Crank Shaft (Contd..)
Left hand Crank Web
Crankshaft bearings
The distance between bearing 1 and bearing 2 may
be assumed to be equal to twice the cylinder
diameter. From the length of the crankpin and
the thickness of the arm, the lengths of the
bearings can be found out. Bearing 2 is the most
heavily loaded, therefore, only this bearing may
be checked for the safe bearing pressure.
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Krishnaraja G. Kodancha,Assistant Professor,
BVBCET , Hubli
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Analysis of Center Crank Shaft (Contd..)
Crankshaft bearings
Where L and d are the length and diameter of the
bearing.
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Summary

• Design Calculations,
• Analysis of Center Crank shaft, Crank at
  Dead Center,
• Crank at angle of maximum Twisting Moment

Any Questions Please.
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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