University of Puerto Rico
Mayaguez Campus
Mechanical Engeneering Department
Machine
component design
“
Piston Rod
”
Reinaldo Santiago Bermudez
802

02

6981
Josue Cortes Irizarry
802

02

1497
Carmen Sanchez
802

03

7593
Title: Connecting Ro
d
Objetives:
Propose a new mechanic component to analyze
the efforts and load that
acts on the connecting rod.
Analyze how the effort and loads acts to propose a better design of the
piece.
Learn new study concepts and improve mechanical parts.
Analy
ze the heat effect in this component.
Description:
Our group chose the piston connecting rod because it’s exposed to a series of
loads that can be studied under the concepts of this course. Also because it is a very
important component in the operation
of automobile engine and it can establish better
designs for its construction. The piston connecting rod is connected to the crank shaft,
this mean that we denominate connecting rod to a piece that is holds by one of its ends
to a piston that makes a stra
ight movement in line, and by the other end to the crank
shaft or a wheel, by this manner being capable to transform a alternative movement in a
rotational movement.
The upper end of the connecting rod articulates whit the bolt of the piston, and
it’s
bui
lt

in to
an
antifriction socket
to avoid the wearing down caused by the alternative
and oscillating movements of the piston. The fabrication of the connecting rods has
different materials like steel, aluminum and others. Knowing it operation and of what
ma
terial we can analyze it to make de tension calculus and the deflection below the
static loads. Also realize calculations of the material index, critical sections, life utility
and safety factor, etc. It is possible to indicate that it’s exposed to heat an
d we will try to
see the effect that this cause to our component. By last, our purpose is to analyze the
component to propose a better design of this one.
First we
begin
to find normal stress acting on rings of piston:
Stresses in Piston:
Assuming Cas
t Iron
material
: E = 100 GPa
Ring width = 0.00115m
Ring depth = 0.0031m
Free ring radius approx. = 45mm
Free ring gap approx. = 10mm
Angle subtended at center
of ring by gap = 10/45 = 0.222radians
Starting with the bending equation:
M
dx
y
d
EI
2
2
Then integrating:
C
Mx
dx
dy
EI
L
ength
of Beam = 0.09
π
m
Substituting this length for x
0.22 EI = 0.09 M
m
N
E
EI
M
222
.
0
12
09
.
0
0031
.
0
00115
.
0
9
100
22
.
0
09
.
0
22
.
0
3
MPa
I
My
5
.
120
0031
.
0
00115
.
0
12
00155
.
0
222
.
0
3
Then the maximum stress
in
the ring is 120 M
Pa. How the ring encloses the
cylinder
area
we can conclude that the maximum strength executed by the pisto
n is
equal to 120
M
Pa. .
Having stress over rings in piston we assumed that the stress acting in rings
equals same at surface area on piston because of very low tolerance between cylinder
and piston rings, and therefore the pressure done by gas and air
during combustion at
certain static time is the force acting on piston rod.
The relation between the
strength
and the compressive force acting in the rod is
given by:
D
pis
ton
= 75 mm (standard bore of 1.6 honda civic
sohc
motor)
R = (7
5/2
mm)*(1m/1000mm
) = 0.0375 m
kN
P
Mpa
m
A
P
A
P
4
.
532
120
*
)
0375
.
*
(
*
2
This force is concentrate on piston by pressure of explosion by gas and air.
We assume
that this force is equal in every point of piston surface area, therefore, this force will act
directly up on
piston rod.
Assumptions:
a)
rectangular neck of piston rod
b)
angle of 20º after certain time of expa
n
sion stroke
c)
Force acting over piston rod hea
d and compression force of cran
k
shaft under
piston rod are equal
Free body diagram:
Bending moment produce b
y force P represented in the following picture:
The neutral axis and the critical zone we assume at center of rod where probably will be
the major deflection: Length of rod is 6 in, critical zone at 3in:
MPA
x
x
x
bh
m
x
in
m
in
KNxsen
I
My
bending
1954
10
51
.
4
11
.
88
12
)
0254
.
5
.
0
(
)
0254
(.
12
))
0254
)(.
4
1
((
)
/
0254
.
0
)(
3
)(
20
4
.
532
(
8
3
3
0
Then the
torsion
d
ue by torque of crankshaft:
In this part we assume that the torsion produce by the rotation of crankshaft will
affected directly the rod, and the force of compression is the same of force P acting on
head
of piston rod by static, the
torsion acting on r
od produce by crankshaft will be:
T =
R
cranckshaft
* F
(compression of crankshaft)
= (
1.5/2+.5)*0.0254*
(532.4KN
) =
16.9
KN

m
τ
torsion
=
J
r
T
*
2
/
)
254
.
0
*
2
/
5
.
1
(
*
)
0254
.
0
*
2
/
5
.
1
(
*
20
*
9
.
16
4
m
in
sen
m
KN
MPa
X
532
10
07
.
2
14
.
110
7
Selection of materials:
We wan
ted to select a material that provides an efficient work in common
engines and resist heat and misuse. Therefore we wanted to our piece be strong and
light
. For that requirements and see the tables of function, objective and constraints; we
have to maximiz
e (σ
y
2/3
/ρ). Searching in strength density diagram we choose aluminum
alloys, titanium alloys and niq
uel alloys
, also steels
. The density of aluminum is less
than others and the extrusion production cost also. Our high candidate for material will
be alumin
um.
A high performance engine for racing requires materials that can operate at high
temperatures and efforts
diminishing at the same time the weight of the motor. In the
normal automobiles, often the connecting rods are made of forged steel or a fused iro
n.
We can save a considerable weight when replacing these pieces by titanium
. Making a
heat treatment for increase hardness of material
reduces the speed of growth of any
crack by fatigue that could appear.
Our strong candidates
for material selection
are
carbon steel 1030 and aluminum
alloy 70

75, the two of them are used in some types of connecting rod
and are relatively
cheap.
We want to be light an
d
strong and
to be less cost and light, so
we analyze the
neck like a beam
, so
the i
ndex to maximize are
3
2
y
for strong and light and
m
y
C
3
2
for
less cost and light:
Diagrams used:
Properties for aluminum 70

75 Properties for carbon steel 1030
Density = 2.81 g/cc = .102lb/in
2
Density = .284 lb/in
2
σ
y
= 503
MPa
σ
y
= 50 x 10
3
psi = 345 M
Pa
=
620
102
.
503
3
2
3
2
y
2
.
173
284
.
345
3
2
3
2
y
So maximum value is 620, aluminum is the best in this case.
(strong vs. light).
In cost vs. light situation:
Aluminum:
Acero:
620
102
.
503
3
2
3
2
m
C
y
2
.
173
284
.
345
3
2
3
2
m
C
y
For the two cases the relative cost is one because the position of the cat
egory of two
materials seen in the diagram.
In the two cases the maximum value is for aluminum,
therefore we have chosen this material to our connecting rod fabrication.
Cyclic c
omponent behavior:
Applied Force
s
:
Fa =
2
min
max
F
F
= 1200 lb
–
0 lb = 600 lb
Fm =
2
min
max
F
F
= 1200 lb + 0 lb = 600 lb
Applied Torque:
Ta = F
a
a = 600 lb x 2 in x sin20 = 410.4 lb

in
Tm = F
m
a =
600 lb
x 3
in x sin20 = 410.4 lb

in
Sf = k
load
x k
size
x k
surf
x k
temp
x k
reliability
x S
´
e
A
95
= .05 bh
= .05 x 1in x 6in = 3in
2
(non

rotating)
d
equiv
=
0766
.
95
A
= 1.98 in
in
d
in
for
10
3
.
Size Factor:
k
size
=
097
.
869
.
d
= .81
Surface Factor:
k
surf
=
b
ut
aS
; as forged
k
surf
=
272 x (572)

.995
= .491
k
temp
= @ 600°C = .549 (we choose the maximum value of temp. because the heat in the
chamber)
k
reliability
= @ 99.
9999
= .62
Sf = 1 x .81 x .491 x .549 x .62 x 286 = 38.72 MPa
We used this fatigue master diagram for our chosen
material and with it we have S
m
and
S
a
for bending force assuming that connecting rod with piston are at end of combustion ,
therefore, no pressure ejected, so P
min
= 0, and forces of amplitude and mean are equal.
Also we decided to design for infinite lif
e (10
7
cycles):
Schematic FBD of our assumption:
For 82 k
si Hardened Aluminum
:
S
f
=
a
= .131
r
a
q
1
1
84
.
5
.
131
.
1
1
q
q
f
= 1 +q (k
t

1)
For K
t
of bending:
Bending
:
in
h
H
5
.
1
1
5
.
1
in
d
r
5
.
1
5
.
K
T
= 1.38
For K
t
for torsion: In this case we analyze as a rectangular piece, not like a bar,
Torsio
n
:
in
d
D
5
.
1
5
.
1
5
.
d
r
K
T
= 1.18
K
f
bending = 1 + q (k
T

1) = 1 + .84 (1.38
–
1) = 1.32
K
f
t
orsion = 1 + q (k
T

1) = 1 + .69 (1.18
–
1) = 1.1242
From master diagram;
σ
a
= 19ksi = 131.7 Mpa
σ
m
= 17 ksi = 117.2 Mpa
Amplitude and mean component do to bending and torsion:
σ
a
= k
f
x
a
= 1.32 x 131.7 MPa = 173.9 MPa
σ
m
= k
fm
x
m
= 1.12 x 117.2 MPa = 131.3 MPa
τ
a
, tosion = K
F, shear
=
J
r
T
a
=
MPa
x
x
x
MPa
x
1
.
16
10
07
.
2
)
0254
(.
)
2
5
.
1
(
)
23
.
2
(
12
.
1
3
τ
m
, tosion = 16.1 Mpa
Alternating/amplitude and mean Von Misses stresses:
2
,
,
,
2
,
2
,
,
3
a
xy
a
y
a
x
a
y
a
x
vm
a
x
Mpa
x
Mpa
vm
a
176
)
1
.
16
(
3
9
.
173
2
2
,
2
,
,
,
2
,
2
,
,
3
m
xy
m
y
m
x
m
y
m
x
vm
m
x
MPa
x
Mpa
vm
m
9
.
133
)
1
.
161
(
3
131
2
2
,
From S

N diagram:
Sm = .9 x (572) = 514.8
Sf = .4 S
ut
= 228.8
log (514.8 x 10
6
) = log(a) + (b)log x 10
3
–
log(228.8 x 10
6
) = log(a) + (b)log(5 x 10
8
)
)
(
7
.
5
357
.
10
5
10
1
log
)
(
10
8
.
228
10
8
.
514
log
8
3
6
6
b
x
x
b
x
x
b =

.062
log(a) = log x Sm

3b
log(a)
= log(514.8)
–
3(

.062)
a = 790.02
We want to design to infinite life (10
7
)
Sn = aN
b
Sn = 790.02 x (10
7
)

.062
Sn = 290.8 Mpa
And final:
Safety Factor:
Modified

Goodman
N
f
=
ut
m
e
a
S
S
1
N
f
=
2
.
1
572
9
.
133
8
.
290
9
.
173
1
We conside
red the situation in a 2

D behavior, so the stresses in x

direction are almost
null; bending forces is the most active.
Drawings:
Discussion:
There to many things that can be writing about this project. The complication of
forces acting
on piston rods is a critical issue that in real life is difficult to understand.
We make a lot of assumptions that with our knowledge acquired on class were almost
correct. Bending and torsion stuff we understand that were the most active forces on
piston
rod. Is important to understand that pressure over piston can be vary by pushing
gasoline or by increasing compression ratio in cylinder. Maximum forces can exceed
over 20000lbf on racing cars. We pretend to design a piston rod use in normal spec cars.
So
me advantages will be the selection of material, in real life piston rod are make of
many materials like steel, aluminum, calamine, etc. Aluminum according to our results
will be the best for our design. Other advantage was some assumptions that facilitate
d
some calculations as taking in mind that P
min
= 0, and therefore mean and amplitude
stresses and torques were the same.
Some disadvantages of our project are the issue of understand the acting forces,
too difficult, and some errors may appears. Also th
e effect of friction affect calculations,
we not take in count. The effect of heat is also a problem because it expands the piece a
certain measurement and forces should be greater. Some suggestions will be
to make a
contour of the piece, with a nodal distribution. This can help us to see the effect of heat
and meet some new critical areas. Others will be to analyze fatigue in misuse, bad
lubrication and dirt that wear away greater the piston rod and could be
broke under
design stipulations. Use another material alloys will improve the life of our parts, if you
have money you can spend in piston rod practically irromplible. To understand how car
engine works, the times, the rotations of crankshaft, transmissio
n, etc. improve how to
understand forces on piston rod but our knowledge in automobile mechanics helps a
little beat to analyzed these type of stuff.
Conclusion:
After finishing our project we have to mention a source of things that evaluate
ourselves
. First of all, the project is a difficult one because of the different situations
acting in motor like heat.
We pretend to show the force acting in static and cyclic
behavior. Our chosen material is the best for many reasons; aluminum rods are popular
in
vehicles specially in high rpm uses. In a high performance engine we recommended
to use steel rods because aluminum stretches more than
steel, bearing retention is a
problem
. Our assumptions probably are wrong but our results prove that the project was
doi
ng well.
The movement of the piston with crankshaft is rotational and ever in a same
cycle, so we analyze in 2

D situation to understand best the concepts. High fatigue
situations will damage the piece, cracks and notches more. The misuse, bad lubrication
and high performance applications are other factors that could damage and broke the
piece. In general machine course help us to visualize this type of behaviors and how
mechanical issues affect our daily life. Best regards and good vacations.
Refe
rences:
“
Metal
Fatigue in Engineerin”, Fuchs and Stephen, New York 1981
“Machine Component Design” BJ Hamrock
Handouts for the class
www.matweb.com
www.grapeoperacing.com
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