designing of center crankshaft

8/11/2019 designing of center crankshaft

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DESIGN OF

CENTRE CRANKSHAFT

Presented by:Ashvath sharma (1109122011)

Jatin mukheja (1109100023)

Vaibhav kumar banka (1109120112)

ME-B(6th

SEM)


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OBJECTIVE

To give a brief idea about the crankshaft, its types,materials of which it is made up of, its importance in anIC engine.

Basic designing procedure of a centre crankshaft.

To give a practical example of designing of crankshaftfor a 4 stroke diesel engine of INDICA VISTA.


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CONTENTS

Introduction

Types of crankshaft. Materials used for crankshaft.

Designing procedure.

Practical example. References.


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INTRODUCTION

The crankshaft is an important part of the IC engine that converts thereciprocating motion of the piston into rotary motion through theconnecting 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.


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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


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MATERIAL SELECTION

The crankshafts are subjected to shock and fatigue loads. Thusmaterial 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 thoseelements adds specific properties in a given material.

The carbon content is the main determinant of the ultimate strengthand hardness to which such an alloy can be heat treated.

The crankshafts are generally made of carbon steel, special steel orspecial cast iron.

The crankshafts are made by drop forging or casting process but theformer method is more common.

The surface of the crankpin is hardened by case carburizing,

nitriding or induction hardening.


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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 momentor torque.

2. The additional moment due to weight of flywheel, belt tension andother forces must be considered.

3. It is assumed that the effect of bending moment does not exceed twobearings between which a force is considered.


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WHEN THE CRANK-SHAFT IS SUBJECTED TO

MAXIMUM BENDING MOMENT

Piston gas load

Fp= /4*D2*p

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

b=2D

and b1=b2=b/2

We know that due to the piston gas load, there will be two horizontalreactions H1 and H2 at bearings 1 and 2 respectively, such thatH1=(Fp*b1)/b

H2=(Fp*b2)/b

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


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

such thatV2=(W*c1)/c={(W*c/2)}/c=W/2

V3=(W*c2)/c={(W*c/2)}/c=W/2

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

H2=(T1+T2)c1/c={(T1+T2)c/2}/c=(T1+T2)/2

H3=(T1+T2)c2/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 dc= Diameter of the crankpin in mm ;

lc= 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:

Mc=H1*b2


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We also know that

Mc=/32*dc3*b

Equating equations, value of dc is obtained.

We know that length of the crankpin,

lc=Fp/(dc*pb) (by using pb=Fp/ lc*dc)

(b) Design of left hand crank web

We know that thickness of the crank web

t=0.65dc+ 6.35 mm

and width of the crank web

w=1.125dc+12.7 mm

We know that maximum bending moment on the crank web,

M=H1(b2-lc/2-t/2)

Section modulus,

Z=(1/6*w*t2)


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Bending stress

b=M/Z

We know that direct compressive stress on the crank web,

c=H1/w*t

Total stress on the crank web

b+c

If the total stress on the crank web is less than the allowable bendingstress then design is safe.

(c) Design of right hand crank web

From the balancing point of view, the dimensions of the right hand crankweb (i.e. thickness and width) are made equal to the dimensions of the lefthand 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

l1=l2=l3=2*(b/2-lc/2-t)


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Assuming width of the flywheel,

Now we have ,value of c.c1=c2=c/2

We know that bending moment due to the weight of flywheel,

MW=V3*c1

And bending moment due to the belt pull,

MT=H3*c1

Resultant bending moment on the shaft,

Ms={(M

W)2+(M

T)2}1/2

We also know that bending moment on the shaft (Ms)

Ms=(/32*ds3*b)


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PRACTICAL EXAMPLE:

TATA INDICA VISTA4 STROKE DIESEL

ENGINE


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PART SPECIFICATIONNumber of Cylinders

Type of Engine ( Inline / Vee engine )

4

1248 cc, inline diesel engineBore/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 webCenter 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 kg37.96 mm

39.5 mm

1.56 kg

1.12 kg

0.31

84 mm

1 kg


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pmax = (power*no of cylinders)/(volume of cylinders*rpm)

= (55*4)/(1248*10-6*4000)

= 44.07 kN/mm2

Piston gas load

Fp= /4*D2* pmax= /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 twicethe 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 horizontalreactions H1 and H2 at bearings 1 and 2 respectively, such that

H1=(Fp*b1)/b=(167.67*69.6)/139.2 =83.83 kN

H2=(Fp*b2)/b=(167.67*69.6)/139.2 =83.83 kN


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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, therewill be two vertical reactions V2 and V3 at bearings 2 and 3 respectively,such that

V2=(W*c1)/c={(W*c/2)}/c=W/2=9.8/2=4.9N

V3=(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 betwo horizontal reactions H2 and H3 respectively, such that

H2=(T1+T2)c1/c={(T1+T2)c/2}/c=(T1+T2)/2

H3=(T1+T2)c2/c={(T1+T2)c/2}/c=(T1+T2)/2

But in case of TATA INDICA VISTA , belt is absent so

T1+T2 = 0


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

(a) Design of crankpin

Let dc= Diameter of the crankpin in mm ;

lc= Length of the crankpin in mm ; and

b= Allowable bearing stress for the crankpin. It may be

assumed as 83 kg/mm2.

We know that the bending moment at the centre of the crankpin

Mc = H1*b2= 83.83*69.6 = 5834.56 kN-mm

We also know that

Mc= /32*dc3*b = /32*dc3*83 = 8.148 dc3N-mm

= 8.148*10-3 * dc3 kN- mm

Equating equations ,we have

dc3=62840/8.148 *10-3

dc= 89.46 mm or say 90mm


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We know that length of the crankpin,

lc = Fp/(dc*pb) = 167.67*103/(90*10) = 186.28

(by using pb=Fp/ lc*dc) (say pb = 10)

(b) Design of left hand crank web

We know that thickness of the crank web

t = 0.65dc+ 6.35 mm = (0.65*90)+6.35 = 64.85 say 65 mm

and width of the crank web

w = 1.125dc+12.7 mm = (1.125*90)+12.7 = 113.95 say 115 mm

We know that maximum bending moment on the crank web,

M = H1(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 thedimensions are on higher side.

Now lets assume, dc = 45 mm

lc = 373.57 mm


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This is very high, which require huge length crank shaft .To have optimumdimension of crankshaft lets assume length of crank web as lc=24mm andcheck 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 thethickness as t = 13.2mm

Bending moment,

M=4275.33kN mm

Section modulus,

Z=(1/6*w*t2)=(1/6*68*13.22)=1974.72mm3

Bending stress

b=M/Z=4275.33/(1974.72)=2.165kN/mm2=49.6N/mm2


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We know that direct compressive stress on the crank web,

c=H1/w*t=83.83/(68*13.2)=.09339kN/mm2

Total stress on the crank web

b+c=2.165+.09339=2.2583N/mm2

Since the total stress on the crank web is less than the allowable bendingstress 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 crankweb (i.e. thickness and width) are made equal to the dimensions of the lefthand 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

l1=l2=l3=2*(b/2-lc/2-t)=2*(139.2/2-24/2-13.2)=88.8 mm


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

Allowing space for gearing and clearance, let us take c = 300 mm.

c1=c2=c/2=300/2=150 mm

We know that bending moment due to the weight of flywheel,

MW=V3*c1=4900*300=1470000 kN-mm=1.47*106N-mm

We also know that bending moment on the shaft

1.47*106=(/32*ds

3*b

)

=(/32*ds3*83)

=8.144*ds3

ds=56.51 say 60 mm.


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REFERENCES

1) Design Data Hand Book, K. Mahadevan and K. Balaveera

Reddy,CBS publication, 19892) 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 DataHand Book, K. Mahadevan and K. Balaveera Reddy, CBS

publication, 19894) Automobile Mechanics, N K giri, Khanna Publishers, 2005

5) Automotive Mechanics, Crouse/Anglin, Tata McGraw-Hill, 2003


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designing of center crankshaft

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