ReportGroupD.doc

7/27/2019 ReportGroupD.doc

1/19

University of Puerto RicoMayaguez Campus

Mechanical Engeneering Department

Machine component design

Piston Rod

Reinaldo Santiago Bermudez802-02-6981

Josue Cortes Irizarry802-02-1497

Carmen Sanchez802-03-7593


2/19

Title: Connecting Rod

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.

Analyze the heat effect in this component.

Description:

Our group chose the piston connecting rod because its 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 straight 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

its built-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

material 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 its exposed to heat and 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.


3/19

First we begin to find normal stress acting on rings of

piston:

Stresses in Piston:

Assuming Cast 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

ydEI =

2

2

Then integrating: CMxdx

dyEI +=

Length of Beam = 0.09 m

Substituting this length for x

0.22 EI = 0.09 M

mN

E

EIM

=

=

=

222.0

1209.0

0031.000115.0910022.0

09.0

22.0

3

MPaI

My5.120

0031.000115.0

1200155.0222.03=

==


4/19

Then the maximum stress in the ring is 120 MPa. How the ring encloses the

cylinder area we can conclude that the maximum strength executed by the piston is

equal to 120 MPa. .

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:

Dpiston= 75 mm (standard bore of 1.6 honda civic sohc motor)

R = (75/2 mm)*(1m/1000mm) = 0.0375 m

kNP

Mpam

APA

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 20 after certain time of expansion stroke

c) Force acting over piston rod head and compression force of crankshaft under

piston rod are equal

Free body diagram:


5/19

Bending moment produce by 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:


6/19

MPA

x

xxbh

mxinminKNxsen

I

My

bending1954

1051.4

11.88

12

)0254.5.0()0254(.

12

))0254)(.4

1(()/0254.0)(3)(204.532(

8

33

0

=

=

=

=

=

Then the torsion due 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 rod produce by crankshaft will be:

T = Rcranckshaft * F (compression of crankshaft)

= (1.5/2+.5)*0.0254* (532.4KN) = 16.9 KN-m

torsion = =J

rT*

2/)254.0*2/5.1(*

)0254.0*2/5.1(*20*9.164

minsenmKN


7/19

MPaX

5321007.2

14.1107==

Selection of materials:

We wanted 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 maximize (y2/3/). Searching in strength density diagram we choose aluminum

alloys, titanium alloys and niquel alloys, also steels. The density of aluminum is less

than others and the extrusion production cost also. Our high candidate for material will

be aluminum.

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 iron.

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 and strong and to be less cost and light, so we analyze the

neck like a beam, so the index to maximize are

32

y for strong and light and

m

y

C

3

2

for

less cost and light:

Diagrams used:


8/19

Properties for aluminum 70-75 Properties for carbon steel 1030

Density = 2.81 g/cc = .102lb/in2 Density = .284 lb/in2

y = 503MPa y = 50 x 10

3psi = 345 MPa

= 620102.

50332

3

2

==

y 2.173284.

34532

3

2

==

y

So maximum value is 620, aluminum is the best in this case. (strong vs. light).

In cost vs. light situation:


9/19

Aluminum: Acero:

620102.

50332

3

2

==

mC

y

2.173284.

34532

3

2

==

mC

y

For the two cases the relative cost is one because the position of the category of twomaterials 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.


10/19

Cyclic component behavior:

Applied Forces:

Fa = 2

minmax FF

= 1200 lb 0 lb = 600 lb

Fm = 2

minmax FF

= 1200 lb + 0 lb = 600 lb

Applied Torque:

Ta = Faa = 600 lb x 2 in x sin20 = 410.4 lb-in

Tm = Fma = 600 lb x 3 in x sin20 = 410.4 lb-in

Sf = kload x ksize x ksurfx ktemp x kreliability x Se

A95 = .05 bh = .05 x 1in x 6in = 3in2 (non-rotating)

d equiv = 0766.95A

= 1.98 in

indinfor 103.

Size Factor:

ksize =097.869. d = .81

Surface Factor:

ksurf=b

utaS ; as forged

ksurf = 272 x (572)-.995 = .491

ktemp = @ 600C = .549 (we choose the maximum value of temp. because the heat in the

chamber)

kreliability = @ 99.9999 = .62

Sf = 1 x .81 x .491 x .549 x .62 x 286 = 38.72 MPa


11/19

We used this fatigue master diagram for our chosen material and with it we have S m and

Sa for bending force assuming that connecting rod with piston are at end of combustion ,

therefore, no pressure ejected, so Pmin = 0, and forces of amplitude and mean are equal.

Also we decided to design for infinite life (107 cycles):

Schematic FBD of our assumption:


12/19

For 82 ksi Hardened Aluminum:

Sf= a = .131

=

+

=

r

a

q

1

1

84.

5.131.1

1=

+

=q

qf = 1 +q (kt -1)

For Kt of bending:

Bending:

inh

H5.11

5.1==

ind

r5.

1

5.==

KT = 1.38

For Kt for torsion: In this case we analyze as a rectangular piece, not like a bar,


13/19

Torsion:

ind

D5.1=

5.1

5.==

d

r

KT = 1.18

Kfbending = 1 + q (kT -1) = 1 + .84 (1.38 1) = 1.32

Kftorsion = 1 + q (kT -1) = 1 + .69 (1.18 1) = 1.1242

From master diagram; a = 19ksi = 131.7 Mpa

m = 17 ksi = 117.2 Mpa

Amplitude and mean component do to bending and torsion:

a = kf x a = 1.32 x 131.7 MPa = 173.9 MPa

m = kfm x m = 1.12 x 117.2 MPa = 131.3 MPa

a, tosion = KF, shear=J

rTa= MPa

x

xxMPax

1.161007.2

)0254(.)2

5.1()23.2(12.1

3=

m, tosion = 16.1 Mpa

Alternating/amplitude and mean Von Misses stresses:


14/19

2

,,,

2

,

2

,, 3 axyayaxayaxvma x ++=

MpaxMpavma 176)1.16(39.17322

, =+=

2

,,,

2

,

2

,, 3 mxymymxmymxvmm x ++=

MPaxMpavmm 9.133)1.161(313122

, =+=

From S-N diagram:

Sm = .9 x (572) = 514.8

Sf = .4 Sut = 228.8

log (514.8 x 106) = log(a) + (b)log x 103 log(228.8 x 106) = log(a) + (b)log(5 x 108)

)(7.5357.105

101log)(

108.228

108.514log

8

3

6

6

bx

xb

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 (107)

Sn = aNb

Sn = 790.02 x (107)-.062

Sn = 290.8 Mpa

And final:

Safety Factor:


15/19

Modified-Goodman

Nf=

ut

m

e

a

SS

+

1

Nf=2.1

572

9.133

8.290

9.173

1=

+

We considered the situation in a 2-D behavior, so the stresses in x-direction are almost

null; bending forces is the most active.

Drawings:


16/19

Discussion:


17/19

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.

Some 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 facilitated

some calculations as taking in mind that Pmin = 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 the 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, transmission, 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:


18/19

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

doing 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.

References:


19/19

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
http://www.matweb.com/http://www.grapeoperacing.com/http://www.matweb.com/http://www.grapeoperacing.com/

Documents

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