Crankshaft Final

8/12/2019 Crankshaft Final

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Side or overhung crankshaft

Side crankshaft has only one crank web & require only two bearing forsupport.

Used in medium size engine and large size horizontal engines.

Centre crankshaft

Centre crankshaft has two crank web and three bearing for support.

Used in radial aircraft engines, marine engines and more popular inautomotive engines.

Depending upon no. of crank pin used in assembly :

Single throw crankshaft

Multi throw crankshaft

Crankshaft is a central component of any internal combustion

engine and is used to convert reciprocating motion of the piston intorotary motion through connecting rod and vice versa.

Crankshaft consist of three portions:

Crank pin

Crank web

Shaft


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Crankshaft should have sufficient strength to withstand

bending and twisting moment to which it is subjected.

The crankshaft is subjected to fluctuating stresses and it should

have sufficient endurance limit stress.

Crankshaft are commonly made by drop forging process.

Popular material used for crankshaft are plain carbon steel and

alloy steel.

Plain carbon steels include 40C8, 45C8 and 50C8.

Alloy steels used for making crankshaft are nickel chromium

steels such as

16Ni3Cr2, 35Ni5Cr2 and 40Ni10Cr3Mo6.

Material Selection


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Pp = force on crank pin

D = diameter of piston

pmax = max. gas pressure inside

the cylinder

W = weight of flywheelP1= tension on tight side

P2= tension on slack side

b = distance between main

bearing 1 &2

c = distance between mainbearing 2 &3

R1, R2, R3are the reactions at the

bearings 1, 2 and 3

Crankshaft at top dead centr


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i. Bearing reaction :

Thrust on connecting rod

Pp= (D2/4)pmax

Crankshaft is simply supported between bearing 1 &2 and

subjected to force PpPp*b1= (R2)v*b or (R2)V= (Pp*b1)/b

Similarly, Pp*b2= (R1)v*b or (R1)V= (Pp*b2)/b

Crankshaft is simply supported between bearing 2 & 3 and

subjected to vertical force W and horizontal force (P1+P2)W*c1= (R3)v*c or (R3)v= (W*c1)/c

W*c2= (R2)v*c or (R2)v= (W*c2)/c

(P1+P2)*c1 = (R3)h*c or (R3)h =

{(P1+P

2)*c

1}/c

(P1+P2)*c2 = (R2)h*c or (R2)h =


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Analytical relations

Resultant reaction at the bearings are as

follows:

R1= (R1)vR2= [(R2)v+(R2)v]2+ [R2)h]2

R3= [(R3)v]2

+ [(R3)h]2

ii. Design of crank pin :

The crank pin is subjected to max.

bending moment at central plane.

(Mb)c= (R

1)vb

1I = (dc4/64) ; y=dc/2 ; b= {(Mb)c*y}/I

(Mb)c = (dc3/32)*b

Diameter of crank pin is determined

by the above equation.


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The length of crank pin is determined by bearing consideration.

pb= allowable bearing pressure at crank pin

pb= Pp/dclc

iii. Design of Left hand Crank Web :

From empirical relationships

t = 0.7dc and w = 1.14dc

The left hand crank web is subjected to direct compressive stress andbending stress due to eccentricity of reaction (R1)v.

c = (R1)v/wt

Mb

= (R1)v[ b

1-l

c/2-t/2]

I = (wt3/12) ; y = (t/2) ; c= Mby/I

b= {(R1)v [ b1-lc/2-t/2](t/2)} / (wt3/12)

b= 6(R1)v [ b1-lc/2-t/2] / (wt2)


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The total compressive stress is given by:

(c)t = c + b

The total compressive stress should be less than the allowable

bending stress.

iv. Design of Right hand crank Web :

The right hand and left hand crank webs should be identical from

balancing consideration.

v. Design of Shaft Under Flywheel :

The central plane of the shaft is subjected to maximum bending

moment.

Bending moment in the vertical plane due to wt. of flywheel -

(Mb)v= (R3)vc2

Bending moment in horizontal plane due to resultant belt tension -

(Mb)h= (R3)hc2Resultant bending moment

Mb= (Mb)v2+ (Mb)h2= [(R3)vc2]2+ [(R3)hc2]2

Similarly ,Mb = (ds3/32)*b


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Model

Referenc

e

Properties

Components

Name: chromoly steel

Model

type:

Linear Elastic

Isotropic

Default

failure

criterion:

Max von Mises

Stress

Yield

strength:

5.5e+007 N/m^2

Tensile

strength:

8.3e+007 N/m^2

Mass

density:

4020 kg/m^3

Elastic

modulus:

2e+009 N/m^2

Poisson's

ratio:

0.3 Solid Body 1(Cut-

Extrude4)(Crankshaf

t)

Total Nodes 44632

Total Elements 25685

Maximum Aspect Ratio 34.094

% of elements with

Aspect Ratio < 3

72.2

% of elements with

Aspect Ratio > 10

1.11

% of distorted

elements(Jacobian)

0

Time to complete

mesh(hh;mm;ss):

00:00:34

Computer name: SIDDHANT

Jacobian points: 4


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Mesh and Wireframe


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Buckling Displacemen


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Buckling Displacemen


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Superimposed Deformati


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Cut way section clip


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Crankshaft Final

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