Design And Analysis Of A Connecting Rod.docx

DESIGN AND ANALYSIS OF A CONNECTING ROD

ABSTRACT: A connecting rod is a rigid member which connects a PISTON to a CRANK or CRANKSHAFT in a RECIPROCATING ENGINE. Together with the crank, it forms a simple mechanism that converts reciprocating motion into rotating motion. This document deals with the designing procedure of a connecting rod using conventional methodology as well as using modelling and simulation soft wares in order to validate the obtained results.

INTRODUCTION: A connecting rod is a rigid member which connects a PISTON to a CRANK or CRANKSHAFT in a RECIPROCATING ENGINE. Together with the crank, it forms a simple mechanism that converts reciprocating motion into rotating motion. It has two ends named as SMALL END and BIG END. The small end is connected to the piston pin where as Big end is connected to crank pin. Today, the connecting rod is best known through its use in internal combustion piston engines, such as automobile engines. These are of a distinctly different design from earlier forms of connecting rod used in steam engines and steam locomotives.

KEY POINTS: Load multiplier, BLF, Mode, Von-mises stress, Factor of safety,

TERMS TO BE KNOWN: MODE: Here buckling modes are considered as failure modes in which the connecting rod gets collapsed in that form.

LOAD MULTIPLIER: The term “load multiplier” used in ANSYS refers to the load at which buckling occurs. In other words, BLF and load multiplier sense same. For example, if you apply 1N force and you obtained the load multiplier value as 3, this means that buckling occurs at (1N * 3) = 3N.

BUCKLING LOAD FACTOR (BLF): The buckling load factor is an indicator of factor of safety against buckling or the ratio of the buckling loads to the currently applied loads. While designing any structure, it’s better to have high value of BLF as much as possible. Table 1: Interpretation of the Buckling load factor (BLF)

BLF VALUE

BUCKLING STATUS

REMARKS

>1

Buckling not predicted

=1

Buckling predicted

<1

Buckling predicted

-1
Buckling possible

-1

Buckling possible

<-1

Buckling not predicted

The applied loads are less than the estimated critical loads The applied loads are exactly equal to the critical loads. Buckling is expected The applied loads exceed the estimated critical loads. Buckling will occur. Buckling is predicted if you reverse the load directions Buckling is expected if you reverse the load directions The applied loads are less than the estimated critical loads, even if you reverse their directions.

DESIGN OF CONNECTIG ROD BY CONVENTIONAL METHODOLOGY: Diameter of piston, D=100mm Mass of Reciprocating parts, M= 2025Kg Length of the connecting rod, L=300mm Stroke length=125mm Speed, N= 1500 rpm Maximum explosion pressure, P= 35N/mm2 Factor of safety= 2 Compressive yield strength= 330Mpa 

I-Section has been considered for connecting rod so that from Euler’s Equation IXX=4IYY for equal strength in both the planes (plane of motion and perpendicular plane) is satisfied. The dimensions width b=4t and Depth h=5t of I-Section give a value of Ixx=3.2Ixy, where t is the thickness of the section.

Fig 1: Dimensions of I-Section 

Treating the body of the connecting rod as a column with having both ends fixed, the stress due to axial load( Rankine-Gordon formula): σcr= Fc/A = σc/(1+K(l/k)2)

eq(1)

Where, Fc is the crippling load, i.e. axial load on the rod due to steam or gas pressure corrected for the inertia effects of piston and other reciprocating parts, N (kgf) σc= allowable unit stress for designing MN/m2 K= constant= 1/25000 for rod having both ends fixed. k= radius of gyration of cross-section about an axis parallel to the point of the end joints = (bh3- b1h13)/(12(bh-b1h1)) for an I-Section = √3.174 t mm A= Area of cross section=11t2 for I-Section DESIGN OF SMALL END AND BIG END: Length of bearing at small end = l1= 0.45D to 0.5D The ratio of length to diameter at small end= l1/d = 1.5 to 2.0 The diameter of crank pin= dp1=0.67D to 0.73D The length of crank pin bearing = lp= 0.5dp1 to 1.5dp1

Let dp1=0.70D=70mm, l1=1.2dp1=84mm Let l1=70mm, d= 25mm The outer diameter for Big end = 70+2(5) +5=85mm where “2(5) “ is for diameter of bolt and “5” is clearance. The outer diameter for small end = 25+2(5) +5=40mm. F.O.S= 2 Fc= = 𝜋𝑟 2 * P*F.O.S = 549778.7N From eq (1): 549778.7= σcA/ (1+K (l/k)2)

t= 0.012m or 12mm.

MODELLING OF CONNECTING ROD: With the obtained values of dimensions of big end and small end along with given length of connecting rod, its model can be created by using modelling soft ware’s like CATIA,SOLIDWORKS,CREO etc. Here, we have used CATIAV5 for modelling.

Fig 2: Connecting rod model in CATIAV5

DESIGN OF CONNECTING ROD IN ANSYS: Here we considered a connecting rod made of Aluminium bronze material. It is subjected to pressure of 35N/mm2 the top of the small end. Our aim is to perform buckling analysis for this connecting rod and to validate with the results obtained by conventional methodology.

Procedure: Step 1: Assign the material (Aluminium bronze) to the connecting rod. Step 3: Apply meshing to the connecting rod. Step 4: Now fix the small end and big end of the connecting rod by selecting “fixed support”. Step 5: Now apply a downward pressure of 35N/mm2 on the top of small end. Step 6: Now obtain the total deformation value and equivalent stress (von-mises stress) Now we need to link this “static structural” model to “Eigen value buckling” to obtain mode shape results.

Fig 3: meshed model of connecting rod in ANSYS

Fig 4: Factor of safety obtained in ANSYS.

Fig 5: Load multiplier values for connecting rod obtained in ANSYS.

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The load multiplier ( Buckling load factor) values are obtained in the range of -950 to +954 , implies the given connecting rod model will be subjected to buckling if the buckling load factor(BLF) of any one of these modes reaches its specified BLF value. The factor of safety induced to the given model in conventional methodology where the factor of safety obtained in simulation is 2.2. There is some deviation in values of F.O.S as the Simulation software follows Finite Element Method which gives approximate results.

CONCLUSION: A Connecting rod is modelled in CATIAV5 software with the help of certain required dimensions which are calculated by using conventional methodology provided in Design data hand book , for the given factor of safety value and this model is analysed in ANSYS software in order to validate the results obtained in conventional methodology.