Static Analysis of Connecting Rod using FEA

Connecting rod is one of the most important part in engine assembly acting as a link between crankshaft and piston. The connecting rod primarily undergoes tensile and compressive loading under engine cyclic process. So Connecting rod has to be designed to withstand these cyclic loading conditions.

The aim of this study is
a) To find out the critical stress regions and stress values due to the application of tensile and compressive load by using bolt-pretension.
b) To find out maximum deformation under tensile and compressive loading.
c) To calculate the factor of safety for highly stressed zone.

 

THEORY

The connecting rod bears the inertia load due to mixed motions of Big, Small End masses and reciprocating mass of piston. It also has to withstand the load because of gas pressure generated during compression stroke of the engine cycle.

The 1/3rd mass of the con-rod corresponding to the segment between c.g and small end is lumped on small end as it behaves like reciprocating mass along with the piston, ring and pin. The total reciprocating mass is the sum of masses of 1/3rd of connecting rod and complete piston assembly.


MATERIAL DESCRIPTION

1) Specification : AISI4140H
2) Hardness (Normalized) : 277-321 Brinell Hardness
3) Poisson’s Ratio : 0.293
4) Young’s Modulus : 2.1165e5 Mpa
5) Yield Stress : 417.1 Mpa
6) Density : 7.85 e-6 Kg/mm3

INPUTS REQUIRED FROM ENGINE ASSEMBLY FOR STRESS ANALYSIS

TOTAL RECIPROCATING MASS =2.5KG (PISTON+RINGS+PIN)

CRANK RADIUS = 80 mm

MAXIMUM RPM OF ENGINE = 2400

RATED RPM OF ENGINE = 2000

DIAMETER OF PISTON = 125mm

MAXIMUM GAS PRESSURE = 12.7 N/mm2

STROKE = 125mm

MESHED MODEL OF CONNECTING ROD
SYMMETRY BOUNDARY CONDITIONS APPLIED)

PROCEDURE FOR ANALYSIS OF BOLT PRETENSION

The aim of this analysis is to simulate the effect of bolt pretension on two parts of split type connecting rod. This is achieved by attaching both split parts by spring elements between connecting rod & bearing cap interface whose stiffness is evaluated by the procedure described in step 1, 2 & 3. This equivalent stiffness value becomes property of modeled spring element.
Thus spring element substitutes for the presence of tightened bolt in assembled condition.


STEP1: Connecting rod is modeled with bolt & bolt threads. Bolt is cut in two halves at connecting rod & bearing cap interface. Spring element (Fig1) with arbitrary stiffness is generated between bolt head & bearing cap interface. Same types of elements are generated between connecting rod threads & bolt threads. Force of 70000N (Maximum pretension allowed) is applied on bolt at cut face. The stress & deflection in bolt, connecting rod & bearing cap is noted.


STEP2: The bolt is modeled (Fig2) at half of equivalent length of bolt where bolt is subjected to predefined deflection. (No external forces applied.) The resulting stress & deflection in bolt at this stage is noted.
The results of step1 & step2 are matched in terms of stresses by taking deflections for contact element by trial & error.


STEP3: Now spring element is generated between connecting rod & bearing cap interface which represents presence of bolts. This spring element will have stiffness (k) which is calculated from the effect of compression in bearing cap & connecting rod thread in the form of deflection.
Equivalent Stiffness (k) = F/(d3+d4)
where, F= Pretension load
d3= Deflection in bearing cap due to compression
d4= Deflection in first thread of connecting rod due to compression in member between thread & constrained face.
These values of d3 & d4 are read from the analysis of step1&2.

ANALYSIS OF CONNECTING ROD WITH BOLT PRETENSION

Deflection in capside bolt = d1
Deflection in bearing cap = d3
Deflection in connecting rod bolt = d2
Deflection in first thread of connecting rod =d4

Pretension F=70000 N
(RATED TIGHTENING AXIAL LOAD)

Pretension in the step1 is now replaced by gap elements (Contact Elements) to verify stresses in bolt as shown in Fig.2.
d=FL/AE
Stiffness k= AE/L
where A= p /4*d2= p /4*(10.6)2= 88.26 mm2, E= 2E5 & L=89.77 mm
k = 1.97E5 N/mm
Total deflection=d1+d2=0.034814+0.202E-7=0.03481402 mm

BOUNDARY CONDITIONS TO FIND OUT MAXIMUM DEFLECTIONS IN BOLT SHANK, BEARING CAP AND BOLT HEAD INTERFACE AREA DUE TO PRETENSION

 

MAXIMUM DEFLECTIONS IN BOLT SHANK, BEARING CAP AND BOLT HEAD INTERFACE AREA DUE TO PRETENSION

 


MAXIMUM VON MISES STRESS IN BOLT SHANK, BEARING CAP AND BOLT HEAD INTERFACE AREA DUE TO PRETENSION

 

BOUNDARY CONDITIONS TO FIND MAXIMUM DEFLECTIONS IN CONNECTING ROD THREAD AND BOLT

MAXIMUM DEFLECTIONS IN CONNECTING ROD THREAD AND BOLT

MAXIMUM STRESS IN CONNECTING ROD THREAD AND BOLT

BOUNDARY CONDITIONS TO FIND MAXIMUM STRESS IN CONNECTING ROD THREAD AND BOLT

VON MISES STRESS PATTERN TO VERIFY STRESSES IN BOLT