DESIGN OF

CENTRE CRANKSHAFT
Presented by:
Ashvath sharma (1109122011)
Jatin mukheja (1109100023)
Vaibhav kumar banka (1109120112)
ME-B(6
th
SEM)
OBJECTIVE
To give a brief idea about the crankshaft, its types,
materials of which it is made up of, its importance in an
IC engine.
Basic designing procedure of a centre crankshaft.
To give a practical example of designing of crankshaft
for a 4 stroke diesel engine of INDICA VISTA.

CONTENTS
Introduction
Types of crankshaft.
Materials used for crankshaft.
Designing procedure.
Practical example.
References.


INTRODUCTION

The crankshaft is an important part of the IC engine that converts the
reciprocating motion of the piston into rotary motion through the
connecting rod.
The crankshaft consists of three portionscrankpin, crank web and shaft.
The big end of the connecting rod is attached to the crank pin.
The crank web connects the crank pin to the shaft portion.
The shaft portion rotates in the main bearings and transmits power to the
outside source.
TYPES OF CRANKSHAFT
The crankshaft, depending upon the position of crank,
may be divided into the following two types:
1. Side crankshaft or overhung crankshaft
2. Centre crankshaft


MATERIAL SELECTION
The crankshafts are subjected to shock and fatigue loads. Thus
material of the crankshaft should be tough and fatigue resistant.
The alloying elements typically used in these carbon steels are
manganese, chromium, molybdenum, nickel, silicon, cobalt,
vanadium, and sometimes aluminium and titanium. Each of those
elements adds specific properties in a given material.
The carbon content is the main determinant of the ultimate strength
and hardness to which such an alloy can be heat treated.
The crankshafts are generally made of carbon steel, special steel or
special cast iron.
The crankshafts are made by drop forging or casting process but the
former method is more common.
The surface of the crankpin is hardened by case carburizing,
nitriding or induction hardening.
DESIGN PROCEDURE
The following procedure may be adopted for designing a crankshaft:
1. The crankshaft must be designed or checked for at least two crank
positions.
a) When the crank-shaft is subjected to maximum bending moment
and
b) When the crankshaft is subjected to maximum twisting moment
or torque.
2. The additional moment due to weight of flywheel, belt tension and
other forces must be considered.
3. It is assumed that the effect of bending moment does not exceed two
bearings between which a force is considered.

WHEN THE CRANK-SHAFT IS SUBJECTED TO
MAXIMUM BENDING MOMENT

Piston gas load
Fp= /4*D
2
*p

Assume that the distance (b) between the bearings 1 and 2 is equal to twice
the piston diameter.
b=2D
and b1=b2=b/2

We know that due to the piston gas load, there will be two horizontal
reactions H1 and H2 at bearings 1 and 2 respectively, such that
H1=(F
p
*b1)/b
H2=(F
p
*b2)/b

Assume that the length of the main bearings to be equal, i.e., c1 = c2 = c / 2.





We know that due to the weight of the flywheel acting downwards, there
will be two vertical reactions V2 and V3 at bearings 2 and 3 respectively,
such that
V
2
=(W*c1)/c={(W*c/2)}/c=W/2
V
3
=(W*c2)/c={(W*c/2)}/c=W/2

Due to the resultant belt tension (T1 + T2) acting horizontally, there will be
two horizontal reactions H2 and H3 respectively, such that
H
2
=(T1+T2)c
1
/c={(T1+T2)c/2}/c=(T1+T2)/2
H
3
=(T1+T2)c
2
/c={(T1+T2)c/2}/c=(T1+T2)/2

Now the various parts of the crankshaft are designed as discussed below:
(a) Design of crankpin
Let d
c
= Diameter of the crankpin in mm ;
l
c
= Length of the crankpin in mm ; and

b
= Allowable bending stress for the crankpin.

We know that the bending moment at the centre of the crankpin:
M
c
=H
1
*b
2


We also know that
M
c
=/32*d
c
3
*
b


Equating equations, value of d
c
is obtained.
We know that length of the crankpin,
l
c
=Fp/(d
c
*p
b
) (by using p
b
=Fp/ l
c
*d
c
)

(b) Design of left hand crank web
We know that thickness of the crank web
t=0.65d
c
+ 6.35 mm

and width of the crank web
w=1.125d
c
+12.7 mm

We know that maximum bending moment on the crank web,
M=H
1
(b
2
-l
c
/2-t/2)

Section modulus,
Z=(1/6*w*t
2
)
















Bending stress

b
=M/Z

We know that direct compressive stress on the crank web,

c
=H
1
/w*t


Total stress on the crank web

b
+
c
If the total stress on the crank web is less than the allowable bending
stress then design is safe.

(c) Design of right hand crank web
From the balancing point of view, the dimensions of the right hand crank
web (i.e. thickness and width) are made equal to the dimensions of the left
hand crank web.

(d) Design of shaft under the flywheel
Let ds = Diameter of the shaft in mm
Since the lengths of the main bearings are equal, therefore
l
1
=l
2
=l
3
=2*(b/2-l
c
/2-t)












Assuming width of the flywheel,
Now we have ,value of c.
c
1
=c
2
=c/2

We know that bending moment due to the weight of flywheel,
M
W
=V
3
*c
1


And bending moment due to the belt pull,
M
T
=H
3
*c
1

Resultant bending moment on the shaft,
M
s
={(M
W
)
2
+(M
T
)
2
}
1/2


We also know that bending moment on the shaft (M
s
)
M
s
=(/32*d
s
3
*
b
)






PRACTICAL EXAMPLE:
TATA INDICA VISTA
4 STROKE DIESEL
ENGINE
PART SPECIFICATION
Number of Cylinders
Type of Engine ( Inline / Vee engine )
4
1248 cc, inline diesel engine
Bore/stroke(D/L)
Cylinder spacing
Power @speed
Torque @speed
Reciprocating mass
Rotating mass
Connecting rod length
Compression ratio
Engine type
Mass of piston
Mass of connecting rod
Crankpin mass
Mass of web
Center of gravity radius
Crank radius
Reciprocating mass
Rotating mass
Ratio of r/l
Cylinder pitch
Weight of flywheel
9.6/82
Assume
55KW @ 4000 rpm
190 Nm @ 1750 rpm
Assume
Assume
Assume
17.6:1
Compression ignition engine
1.36 kg
0.60 kg
0.25 kg
0.25 kg
37.96 mm
39.5 mm
1.56 kg
1.12 kg
0.31
84 mm
1 kg
p
max
= (power*no of cylinders)/(volume of cylinders*rpm)
= (55*4)/(1248*10
-6
*4000)
= 44.07 kN/mm
2


Piston gas load
Fp= /4*D
2
* p
max

= /4*(69.6)
2
*44.07=167668.5 N=167.67 kN,

Assume that the distance (b) between the bearings 1 and 2 is equal to twice
the piston diameter.

b=2D=2*69.6=139.2 N,
and b1=b2=b/2=139.2/2=69.6 mm,

We know that due to the piston gas load, there will be two horizontal
reactions H1 and H2 at bearings 1 and 2 respectively, such that

H1=(F
p
*b1)/b=(167.67*69.6)/139.2 =83.83 kN
H2=(F
p
*b2)/b=(167.67*69.6)/139.2 =83.83 kN

.


Assume that the length of the main bearings to be equal, i.e.
c1 = c2 = c / 2

We know that due to the weight of the flywheel acting downwards, there
will be two vertical reactions V2 and V3 at bearings 2 and 3 respectively,
such that

V
2
=(W*c1)/c={(W*c/2)}/c=W/2=9.8/2=4.9N
V
3
=(W*c2)/c={(W*c/2)}/c=W/2=9.8/2=4.9N

Due to the resultant belt tension (T1 + T2) acting horizontally, there will be
two horizontal reactions H2 and H3 respectively, such that

H
2
=(T1+T2)c
1
/c={(T1+T2)c/2}/c=(T1+T2)/2
H
3
=(T1+T2)c
2
/c={(T1+T2)c/2}/c=(T1+T2)/2

But in case of TATA INDICA VISTA , belt is absent so
T1+T2 = 0

Now the various parts of the crankshaft are designed as discussed below:

(a) Design of crankpin
Let d
c
= Diameter of the crankpin in mm ;
l
c
= Length of the crankpin in mm ; and

b
= Allowable bearing stress for the crankpin. It may be
assumed as 83 kg/mm
2
.
We know that the bending moment at the centre of the crankpin

M
c
= H
1
*b
2
= 83.83*69.6 = 5834.56 kN-mm

We also know that
M
c
= /32*d
c
3
*
b
= /32*d
c
3
*83 = 8.148 d
c
3
N-mm
= 8.148*10
-3 *
d
c
3
kN- mm

Equating equations ,we have
d
c
3
=62840/8.148 *10
-3

d
c
= 89.46 mm or say 90mm











We know that length of the crankpin,
l
c
= F
p
/(d
c
*p
b
) = 167.67*10
3
/(90*10) = 186.28
(by using p
b
=Fp/ l
c
*d
c
) (say p
b
= 10)

(b) Design of left hand crank web
We know that thickness of the crank web
t = 0.65d
c
+ 6.35 mm = (0.65*90)+6.35 = 64.85 say 65 mm

and width of the crank web
w = 1.125d
c
+12.7 mm = (1.125*90)+12.7 = 113.95 say 115 mm

We know that maximum bending moment on the crank web,
M = H
1
(b
2
-l
c
/2-t/2) = 83.83*(69.6-186.28/2-65/2) = -4697.83 N-mm

The bending moment is negative, therefore the design is not safe thus the
dimensions are on higher side.

Now lets assume, d
c
= 45 mm
l
c
= 373.57 mm








This is very high, which require huge length crank shaft .To have optimum
dimension of crankshaft lets assume length of crank web as l
c
=24mm and
check whether these dimension are suitable for load exerted by the piston ,,
and other forces.

Therefore , dimensions of crank web are :
t =35.6 mm
and l = 63.32 say 68 mm

But the thickness of crankshaft is also on higher side so lets assume the
thickness as t = 13.2mm

Bending moment,
M=4275.33kN mm

Section modulus,
Z=(1/6*w*t
2
)=(1/6*68*13.2
2
)=1974.72mm
3

Bending stress

b
=M/Z=4275.33/(1974.72)=2.165

kN/mm
2
=49.6N/mm
2



We know that direct compressive stress on the crank web,

c
=H
1
/w*t=83.83/(68*13.2)=.09339kN/mm
2

Total stress on the crank web

b
+
c
=2.165+.09339=2.2583N/mm
2


Since the total stress on the crank web is less than the allowable bending
stress of 83 MPa,therefore, the design of the left hand crank web is safe.

(c) Design of right hand crank web
From the balancing point of view, the dimensions of the right hand crank
web (i.e. thickness and width) are made equal to the dimensions of the left
hand crank web.

(d) Design of shaft under the flywheel
Let ds = Diameter of the shaft in mm

Since the lengths of the main bearings are equal, therefore
l
1
=l
2
=l
3
=2*(b/2-l
c
/2-t)=2*(139.2/2-24/2-13.2)=88.8 mm





Assuming width of the flywheel as 200 mm, we have
c=88.88+200 =288.88 mm

Allowing space for gearing and clearance, let us take c = 300 mm.
c
1
=c
2
=c/2=300/2=150 mm

We know that bending moment due to the weight of flywheel,
M
W
=V
3
*c
1
=4900*300=1470000 kN-mm=1.47*10
6
N-mm

We also know that bending moment on the shaft
1.47*10
6
=(/32*d
s
3
*
b
)
=(/32*d
s
3
*83)
=8.144*d
s
3

d
s
=56.51 say 60 mm.
REFERENCES
1) Design Data Hand Book, K. Mahadevan and K. Balaveera
Reddy,CBS publication, 1989
2) A text Book of Machine Design, P.C.Sharma and D.K.Aggarwal, S
K Kataria and Sons, 1993
3) A text Book of Machine Design, R S Khurmi and J1. Design Data
Hand Book, K. Mahadevan and K. Balaveera Reddy, CBS
publication, 1989
4) Automobile Mechanics, N K giri, Khanna Publishers, 2005
5) Automotive Mechanics, Crouse/Anglin, Tata McGraw-Hill, 2003