crankshaft final
TRANSCRIPT
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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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