Engineering

Assembled ViewIntroduction:

The Connecting rod is the intermediate member between the piston and the Crankshaft. Its primary function is to transmit the push and pull from the Piston pin to the Crank pin and thus converts the reciprocating motion of the piston into rotary motion of the Crank.

A Connecting rod is subjected to several repetitive cyclic loadings,which generates from high compressive loads due to gas pressure during combustion process from TDC to BDC.

Additionally high tensile inertial loads also come into action which are generated due to rotating motion of the Crankshaft. Therefore, to validate design for tensile and compressive loading, it is necessary to analyze a connecting rod load parameters, not only in the segment of static analysis but also quasi dynamically.

In this Case study,  Connecting rod model  (Ref.Dynamic Load Analysis and Optimization of connecting rod - P S Shenoy) is analyzed axially for its tensile and compressive loading by considering Static and Quasi-Dynamic Case.

The Connecting rod was modeled in CATIAV5R20 and further imported and analyzed for different conditions in Ansys Workbench.

Problem Statement:

A Static Structural and Quasi – Dynamic Analysis of a “Connecting rod” was carried out axially for assessing its strength against compressive and tensile loading. The Connecting rod is made of “Forged Steel” with given material properties:

 

Density 7820 kg/m3
Young’s Modulus 206.7 GPa
Poisson’s Ratio 0.3
Tensile Yield Strength 700 MPa
Ultimate Tensile Strength 938 MPa

 

The Connecting rod is designed to withstand a maximum axial load of 26.7 KN at a maximum revolution of 5700 rev/min. in both tension and compression. The connecting rod was modeled into two different parts and then assembled. For the purpose of analysis both the parts of connecting rod were assumed to be “Bonded”. An exploded and assembled View of the connecting rod is shown below:

Exploded View

Fig. 1. Exploded View

 

Assembled View

 Fig. 2. Assembled View

Load Analysis: Engine configuration to which the typical connecting rod is analyzed is shown in Table: 1.

 

Crankshaft Radius (mm) 48.5
Connecting Rod Length (mm) 141
Piston Diameter (mm) 86
Mass of the Piston Assembly (kg) 0.434
Mass of the Connecting Rod (kg) 0.439
Izz about the center of Gravity (kg m2) 0.00144
Distance of CG from crank-end center (mm) 36.4
Maximum Gas Pressure (bar) 37.3

                                                                                                  Table: 1

 

Calculation of Forces exerted on Connecting Rod:

The following calculations were performed to obtain the forces exerted on the connecting rod:

Static FEA:

The normal Gas Pressure exerted on the Piston inside the Piston Cylinder is given by:

P=P0 cosΦ 

This pressure (P) is used to find out the Force on the connecting rod (Fc) which is equal to maximum force on the piston (FL) due to gas pressure,

i.e.

Pressure

 

When Connecting Rod completes a cycle of 180° from TDC (Top Dead Center) to BDC (Bottom Dead Center), during this phase it will be under the influence of Tensile Loading and Compressive Loading as well. The Load on the Connecting Rod is distributed over an angle of 180°. The Total resultant of the tensile load is given by:

Total Resultant

 

Where,

P0 = normal pressure constant

r = radius of crank

t = thickness of rod at crank end.

By using the above formula the value of “P0” was calculated for Tensile Loading acting on Crank End and Piston Pin End.

Tensile Load acted on Crank End:

By putting the following values normal pressure (P0) constant was calculated at “Crank End”.

 

Pt 26700 N
r 24 mm
t 20 mm

The normal pressure constant (P0) obtained at crank end is 35.41 MPa.

Tensile Load acted on Piston Pin End:

By putting the following values normal pressure constant (P0) was calculated at “Piston Pin End”.

 

Pt 26700 N
r 12 mm
t 20 mm

The normal pressure constant (P0) obtained at piston pin end is 70.82 MPa.

Now,

The Total resultant of the Compressive load is given by:

compresive Load

 

Where,

P0 = normal pressure constant

r = radius of crank

t = thickness of rod at crank end.

By using the above formula the value of “P0” was calculated for Compressive Loading acting on Crank End and Piston Pin End.

 

Compressive Load acted on Crank End:

By putting the following values normal pressure constant (P0) was calculated at “Crank End”.

Pt 26700 N
r 24 mm
t 20 mm

The normal pressure constant (P0) obtained at crank end is 32.11 MPa.

 

Compressive Load acted on Piston Pin End:

By putting the following values normal pressure constant was calculated at “Piston Pin End”.

Pt 26700 N
r 12 mm
t 20 mm

The normal pressure constant (P0) obtained at piston pin end is 64.23 MPa.

 

Quasi-Dynamic FEA:

During regular operations, a connecting rod undergoes complex motion. And the inertial loads acted onto it. Therefore Angular Velocity, Angular Acceleration and Linear Acceleration will also be considered along with the loads under Boundary Conditions. While applying the tensile loads, the load will be applied 90° on either side of the direction of the resultant load, totally 180°.

In compressive loading, the load will be applied 60° on either side of the direction of the resultant load, totally 120° (Ref. Webster et al 1983).

 

Inputs for FEA of connecting rod using dynamic analysis results at crankshaft speed of 5700 rev/min.

 

Table

 

Meshed Model:

Meshed Model Of The Connecting Rod 

 

 

The problem was solved once for Static FEA and then for Quasi-Dynamic FEA by applying above mentioned Load values.

Static FEA:

In Static Analysis the problem was solved for four different cases.

 

Static Structural Analysis for Tensile Loading (Load at Crank End):

In this case the connecting rod was analyzed against tensile loading at BDC at an angle of 180°. The boundary condition for this case is mentioned in Fig.4.

Tensile load at crank end

 

Results:

Equivalent Stress:

equivalent stress plot

 

Static Structural Analysis for Tensile Loading (Load at Piston Pin End):

In this case the connecting rod was analyzed against tensile loading at TDC at an angle of 360°. The boundary condition for this case is mentioned in Fig.6.

Tensile load at piston pin end

 

Results:

Equivalent Stress:

equivalent stress plot 2

 

Static Structural Analysis for Compressive Loading (Load at Crank End):

In this case the connecting rod was analyzed against compressive loading at TDC at an angle of 360°. The boundary condition forthis case is mentioned in Fig.8.

Compressive Load at Crank pin End

Results:

Equivalent Stress:

equivalent stress plot 3

 

Static Structural Analysis for Compressive Loading (Load at Piston Pin End):

In this case the connecting rod was analyzed against compressive loading at BDC at an angle of 180°. The boundary condition for this case is mentioned in Fig.10.

Compressive Load at Piston Pin End

 

Results:

Equivalent Stress:

equivalent stress plot 4

 

Conclusion:

After analyzing all four cases for static loading, we observed that the Von-Mises stress for all four cases was well below than the prescribed material yield stress limits i.e. 700 MPa. Therefore the Connecting Rod is safe against the static loads acted onto it.

 

Quasi-Dynamic FEA:

In Quasi-Dynamic Analysis the problem was solved for four different cases.

 

Quasi-Dynamic Analysis for Tensile Loading (Load at Crank End):

In this case the connecting rod was analyzed against dynamic tensile loading at BDC at an angle of 180°. The boundary condition for this case is mentioned in Fig.12.

Tensile Load at Crank 2

 

Results:

Equivalent Stress:

equivalent stress plot 5

 

Quasi-Dynamic Analysis for Tensile Loading (Load at Piston Pin End):

In this case the connecting rod was analyzed against dynamic tensile loading at TDC at an angle of 360°. The boundary condition for this case is mentioned in Fig.14.

Tensile load at piston pin end 2

 

Results:

Equivalent Stress:

equivalent stress plot 6

 

Quasi-Dynamic Analysis for Compressive Loading (Load at Crank End):

In this case the connecting rod was analyzed against dynamic compressive loading at TDC at an angle of 360°. The boundary condition for this case is mentioned in in Fig.16.

Compressive Load at Crank pin End 2

 

Results:

Equivalent Stress:

equivalent stress plot 7

 

Quasi-Dynamic Analysis for Compressive Loading (Load at Piston Pin End):

In this case the connecting rod was analyzed against dynamiccompressive loading at BDC at an angle of 180°. The boundary condition for this case is mentioned in Fig.18.

Compressive Load at Piston pin End. 2

 

 

Results:

Equivalent Stress:

equivalent stress plot 8

 

Conclusion:

After analyzing all four cases for Quasi-Dynamic loading, we observed that the Von-Mises stress for all four cases was well below than the prescribed material yield stress limits i.e. 700 MPa. Therefore the Connecting Rod is also safe against the Dynamic loads acted onto it.

 

Final Deduction:

“Static Structural” and “Quasi – Dynamic” Analysis of the “Connecting rod” was carried out axially, and in both the cases, it is observed that the maximum Von-Mises stress was well below than the Tensile Yield Strength of the material used, i.e. Forged Steel (700MPa).

Thus, the connecting rod design is validated  for “Static and Dynamic” Loads at 5700 rpm.