Rod Design Parameters to Improve Performance


The Connecting rods are widely used in variety of engines to transmit the thrust of the piston to the crankshaft, and results into conversion of the reciprocating motion of piston to the rotational motion of crankshaft. It consists of a pin-end, a shank section, and a crank-end. Pin-end and crank-end pin holes are machined to permit accurate fitting of bearings. One end of the connecting rod is connected to the piston with the help of a piston pin. The other end revolves with the crankshaft and is split to permit it to be clamped around the crankshaft. Depending upon the size of big end, the two parts are clamped by two or four bolts. Connecting rods are subjected to forces generated by mass and fuel combustion. These two forces results in axial and bending stresses. Bending stresses appear due to eccentricities, crankshaft, case wall deformation, and rotational mass force; therefore, a connecting rod must be capable of transmitting axial tension/compression and bending stresses caused by the thrust and pull on the piston and by the centrifugal force [1].

The connecting rods of the tractor are mostly made of cast iron through the forging or powder metallurgy. The main reason for applying these methods is to produce the components integrally and to reach high productivity with the lowest cost [2] and optimized shape [3]. The connecting rod design is complicated because the engine is to work in variably complicated conditions. The connecting rod is subjected to the varying pressure by the rod mechanism and inertia forces due to the acceleration/retardation in a cycle [4]. Biancolini et al. [5] carried out fatigue analysis and proposed the design of connecting rod. A rupture due to the fatigue and the method of correcting the connecting rod design parameters is reported by Rabb [6]. Beretta et al. [7] presented a strengthening method for the connecting rod design. Finite Element Method (FEM) is a modern technique for the fatigue analysis of connecting rod for the estimation of component longevity. FEM is capable of generating the stress/strain distributions throughout the component which enables us to find the critical points authentically. This method is extremely useful particularly when the component geometrical shape is complex and loading conditions are not sophisticated. In FEM, the influential component factors are able to change such as material, cross section conditions etc. and component optimization under the fatigue cyclic loading can be performed easily and quickly [6]. In Computer Aided Design, the analysis of a component is performed in a virtual environment without any necessity of making a prototype [8]; leading to savings in terms of time and cost.

For the reason that the connecting rod failure is usually due to the fatigue phenomenon, M. Omid et al. [9] performed FE analysis of U650 Tractor connecting rod on ANSYS software and concludes that, under reverse loading (tensile and compressive) the critical point is 46 (near the big end of the connecting rod). In order to improve on fatigue life of the connecting rod this value may be increased by decreasing the stress concentration coefficient.

In the present work, the modeling of connecting rod is done using Pro/E software and finite element fatigue analysis is carried out on ANSYS workbench. The aim is to investigate the effects of connecting rod design parameters to improve the performance under the reversible cyclic loadings. It is observed that the fatigue life can be improved by reducing the stress concentration coefficient by modifying some of the design parameters of connecting rod.


Despite the fact that most engineers and designers are aware of fatigue due to reversible cyclic loadings and a large amount of experimental data has been generated on the fatigue properties of various metallic and non-metallic materials, fatigue failures of engineering components are still common. The factors which influence the fatigue life of a component in service are

  • complex stress cycles
  • engineering design
  • manufacturing and inspection
  • service conditions and environment, and
  • material of construction

The use of calculations and simulations is a key feature of the modern design process. Several properties such as stress, strength, stiffness, durability, handling, ride comfort and crash resistance, etc. can be numerically analyzed with varying levels of accuracy. Development time can be reduced by ensuring that some, or rather all, of these properties fulfil established requirements even before the first prototype is being built. Accordingly, calculations based on fatigue life and accurate loading histories permit structures and components to be optimized for durability without the need for expensive and time-consuming testing of series of prototypes. Thus, designs can be obtained that are less conservative (i.e., better optimized) than those based on traditional criteria, such as maximum load or stress for a series of standard load cases [15].

The use of Finite Element Method (FEM) for calculating stress and strain is a well established procedure in analyzing the fatigue and determining longevity of components. Del Llano-Vizcaya et al. [16] carried out finite element stress analysis in ANSYS code and performed multi-axial fatigue study of helical compression springs using the fatigue software. Biancolini et al. [5] designed a connecting rod based on fatigue analysis. Beretta et al. [7] presented a resistant method to failure on connecting rod design that improved the fatigue life slightly. They found that the occurrence of fatigue phenomenon is closely related to the appearance of cycling stresses within the connecting rod body.

Lu [17] presented an approach to optimize the shape of a connecting rod subjected to a load cycle which consisted of the inertia load deducted from gas load as one extreme and peak inertia load exerted by the piston assembly mass as the other extreme. A FE routine is first used for calculating the displacements and stresses in the connecting rod, which are further used in another routine to calculate the total life. Fatigue life is defined as the sum of crack initiation and crack growth lives, with crack growth life obtained using fracture mechanics. Rahman et al. [18] presented the finite element based fatigue life prediction of a new free piston linear generator engine mounting. The objective of investigations is to assess the critical fatigue locations due to loading conditions. They concluded that Morrow mean stress correction method gives the most conservative (less life) results for crack initiation method.

Nanaware and Pable [19] described a case study on the fatigue fracture of rear axle shafts of 575 DI tractors. The failure of rear axle shafts was due to inadequate spline root radius, which led to crack initiation and subsequent crack growth is by fatigue under the cyclic loading conditions of field operation. In general, the shafts in power plant systems run with a steady torsion combined with cyclic bending stress due to self-weight or weights of components or possible misalignment between journal bearings [20]. A similar case study [19] is reported in Fatigue Design Handbook AE 10 (Fatigue design handbook, 1988). The rear axle shaft failure of a scraper type tractor was considered as a case study. The rear axle shafts were failing within six months of service, even though durability tests were done in the laboratory. It was concluded that failure of shaft was due to the reverse torque.

Sarihan and Song [21] used a fatigue load cycle, consisting of compressive gas load corresponding to the maximum torque and tensile load corresponding to the maximum inertia load for the optimization of wrist pin end. They used the maximum loads for the entire operating range of the engine. Modified Goodman equation with alternating octahedral shear stress and mean octahedral shear stress are used for the fatigue design. They generated an approximate design surface and optimized the same. The objective and constraint functions are updated to obtain precise values. This process is repeated till the convergence is achieved. They also included constraints to avoid fretting fatigue. The mean and the alternating components of the stresses are calculated using maximum and minimum values of octahedral shear stress. Their exercise reduced the connecting rod weight by nearly 27%. The initial and final connecting rod wrist pin end designs are shown in Figure 1.1.

Rabb [6] performed a detailed FE analysis of the connecting rod that led to a disastrous failure of an engine. He modelled the threads of the connecting rod and connecting rod screws, the pre-stress in the screws, the diametric interference between the bearing sleeve and the crank end, the diametric clearance between the crank and the crank bearing, the inertia load acting on the connecting rod and the combustion pressure. The analysis clearly indicates the failure location at the thread root of the connecting rod, caused by improper screw thread profile. An axi-symmetric model was initially used to calculate the stress concentration factors at the thread root which are used to obtain nominal mean and alternating stresses in the screw. Based on the comparison of the mean stress and stress amplitude at the threads obtained from specimen the fatigue test, the adequacy of new design is checked. Load cycling is also used in inelastic FEA to obtain steady state situation.

Yoo et al. [22] used variational equations of elasticity, material derivative idea of continuum mechanics and an ad-joint variable technique to calculate shape design sensitivities of stress. The results are used in an iterative optimization algorithm, steepest descent algorithm, to numerically solve an optimal design problem of a connecting rod. The stress constraints were imposed on principal stresses of inertia and firing loads. But fatigue strength was not addressed. The other constraint was the one on thickness to bind it away from zero. They could obtain 20% weight reduction in the neck region of the connecting rod. The optimum design is shown in Figure 1.2.

Sonsino and Esper [23] described the fatigue design of sintered connecting rods. They designed a connecting rod with a load amplitude Fa = 19.2 kN and with different regions being designed for different load ratios (R), such as, in the stem Fm = -2.2 kN and R = -1.26, at the piston pin end Fm = -5.5 kN and R = -1.82, at the crank end Fm = 7.8 kN and R = -0.42. They performed preliminary FEA followed by production of a prototype. Figure 1.3 shows the prototype of connecting rod used for the fatigue tests and experimental stress analysis. In order to verify the design against fatigue, they computed the allowable stress amplitude at critical locations, taking the R-ratio, the stress concentration and statistical safety factors into account, and ensured that maximum stress amplitudes are below the allowable stress amplitude.

Bayrakceken et al. [24] performed failure analysis of crankshafts of two single cylinder diesel engines. The single cylinder diesel engines are extensively used in agricultural areas for several purposes such as water pumping or driving some auxiliary agricultural vehicles. Two different failure cases of crankshafts of these engines were analyzed. Some characterization studies and fracto -graphic analysis were also carried out to assess the failure reason. However, the cranks have some miner design differences, both failures are occurred after a fatigue process.

Ishida et al. [27] measured the stress variation at the column centre and column bottom of the connecting rod, as well as the bending stress at the column centre. The plots, shown in Figure 1.4 indicate that at higher engine speeds, the peak tensile stress does not occur at 360o crank angle or at top dead centre. It is also observed that the R ratio varies with location as well as engine speed. The maximum bending stress magnitude over the entire cycle (0o to 720o crank angle) at 12000 rpm and at the column centre is found to be about 25% of the peak tensile stress over the same cycle.

M. Omid et al. [9] performed FE analysis of U650 Tractor connecting rod on ANSYS software and concluded that under the reverse loading (tensile and compressive), the critical point is observed at node 46 (near the big end of the connecting rod). It is concluded that the fatigue life of connecting rod may be improved by reducing the stress concentration coefficient.

1.3 Manufacturing Aspects of Connecting Rod


The connecting rod manufacturing processes for the conventional fracture crack-able steel forging is shown in Figure 1.5.

Biliet Heating

Hot Forging





Machining of outer contours-top

Machining fracture splitting groove

Grinding the side face

Fracture splitting

Drilling the piston end

Assembly of CR

Broaching of crank and piston rod


Horning of crank and piston bush

Drilling of bolt Holes and tapping

Drilling of holes

Machining of bolt head seat


Figure 1.5: C-70 connecting rod manufacturing process flow chart

A comparison between the processes can be made by comparing the charts. The following steps in the manufacturing of the existing forged steel connecting rod can be eliminated by introducing C-70 crack-able steel: the heat treatment, the machining of the mating faces of the crank end, and drilling for the sleeve. The only manufacturing aspect taken into account during the optimization process was maintaining the forge ability of the connecting rod.


Literature review suggests that the connecting rod behavior affected by fatigue phenomenon due to reversible cyclic loadings and to consider the results for more savings in time and costs, as two very significant parameters relevant to manufacturing. The objective of present work is to study the fatigue behavior of connecting rod by modifying some of its critical design parameters. The process begins with identifying the correct load conditions and magnitudes. Overestimating the loads will simply raise the safety factors. The idea behind this process is to retain just as much strength as is needed. Commercial software’s such as Pro/E and ANSYS Workbench are used to estimate the fatigue life of connecting rod. The allowable number of load cycles and using fully reverse loading was gained 108. Usually, the worst case load is considered in the design process. In the present work, modeling and finite element fatigue analysis of connecting rod has been carried out. The objectives of present work are:

  • Modeling and finite element analysis of I-section CR
  • Modeling and finite element analysis of H-section and +section connecting rods, keeping the mass of three connecting rods to be equal
  • Investigations of effects of critical dimensions such as fillet radius, inner diamater and height of the big end of CR on its fatigue life
  • Comparitive studies of these parameters on the fatigue life of these three connecting rods.

The results indicate that with fully reverse loading, one can estimate longevity of a connecting rod and the effect of basic critical dimensions on the fatigue life of connecting rods, keeping all other dimensions as constant. It is concluded that results obtained can be useful to bring about modifications in the process of connecting rod manufacturing.

1.5 Concluding Remarks

In this chapter, literature review related to fatigue failure of components such as connecting rod, crankshaft, etc., manufacturing aspects of connecting rod and scope of work is described. The present work is focused on the fatigue analysis of connecting rod using Finite Element Code. Many successful studies have been carried out by the researchers to study the fatigue behaviour and failure of connecting rods. FEA software has been widely used for studying the fatigue behaviour and stress distribution to locate the critical sections of the connecting rod. Some investigators have performed the experimental work to study the fatigue phenomenon of the connecting rod. Finally, it is concluded that finite element analysis is capable of presenting stress, strain distributions in different part of the connecting rod.

In Chapter 2, detailed concept of fatigue failure is presented. The fundamentals of fatigue considerations, basic elements of fatigue design process, stress-life based approach for the fatigue design are presented. Chapter 3, describes the modeling of I-section, H-section and plus (+) section connecting rods. It includes the mesh convergence, details of loads and restraints applied at the end of connecting rod. The validation of prosed FE model is carried out with the analytical work carried out by Omid et al. [9]. Chapter 4 discusses the result obtained by the finite elemnt fatigue analysis. At first, results are presented for I-section CR. Next a comparative study has been carried out for the I-section, H-sectionand +section connecting rods. The results are summarized in the form of conclussion in Chapter 5.

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