Design and Comparative FE Analysis of Connecting Rod

Abstract

The connecting rod is one of the vital transmission parts of the gas engine. One of its functions is to connect the piston and the crankshaft and transmit the force acting on the piston to the crankshaft so that the reciprocating motion of the piston is converted into the rotary motion of the crankshaft, and the external output is done. The small end of the connecting rod reciprocates when it works, the big end rotates, and the rod moves in a complex plane, so the force of the connecting rod is very complicated. It is one of the most heavily loaded parts of the gas turbine. The strength of the connecting rod must be checked for the design. Currently, the calculation methods of the strength of its front generally include the conventional and finite element methods. Compared with the conventional calculation method, the finite element method has the characteristics of high precision and closer to the actual simulation situation.

Introduction

I. INTRODUCTION

The connecting rod is one of the vital transmission parts of the gas engine.[1] One of its functions is to connect the piston and the crankshaft and transmit the force acting on the piston to the crankshaft so that the reciprocating motion of the piston is converted into the rotary motion of the crankshaft, and the external output is done.

The small end of the connecting rod reciprocates when it works, the big end rotates, and the rod moves in a complex plane, so the force of the connecting rod is very complicated.[1] It is one of the most heavily loaded parts of the gas turbine. When it is working, it is simultaneously subjected to the action of the gas pressure from the piston, the reciprocating inertial force, and the inertial force generated by its swing.[2] The direction changes periodically. In long-term use, the middle l be bent and twisted due to factors such as the piston's violent thrust and the crankshaft's high-speed operation. Once the connecting rod is bent and twisted, in addition to causing the piston to pull the cylinder, it will also cause abnormal wear and tear of the piston, gas, cylinder crankshaft, and other parts, and it is easy to cause fatigue damage and breakage, resulting in engine failure. It is related to the safety of users, causing severe damage.

It has become difficult for designers to create more benefits for the enterprise.[3] When the engine bore is enlarged, the power will increase. Whether the bearing capacity of the original structure meets the requirements will become a critical link. The "design-validation-design" model in the conventional design method in the past cannot meet the requirements of the fierce market competition due to the significant investment and long verification period.

The finite element method should be used in the design process, which can improve the design efficiency and only need to carry out the necessary FEA tests in the final stage, dramatically improves the work efficiency, accelerates the speed of product development of the enterprise, and improves the market competitiveness of the enterprise while improving the scientific research ability of the enterprise.

The conventional connecting rod design involves the dynamic characteristics, but with the development trend of the engine's high speed and high power, the static design cannot meet the needs more and more. For example, a specific engine connecting rod is considered to have sufficient strength when performing static strength checks and finite element static calculations.[4] However, the 200-hour engine bench strengthening test and the 20,000-kilometer road test of the whole vehicle was found many times that the whole vehicle and the whole vehicle had sufficient strength. The engine has abnormal noise and constant motion, and in recent years, it has been found that the motorcycle products should have cracks in them, which is a fatal damage phenomenon.[5] Therefore, studying the dynamic characteristics of the connecting rod from the viewpoint of modern design has become essential in its design.

II. SOLID MODELING AND DESIGN CALCULATION

Classical elastic mechanics obtains its analytical solutions by solving differential equations. The finite element method avoids solving differential equations so that the finite element method can solve engineering problems with complex shapes, structures, and boundary conditions. It replaces the design-verification-design cycle in the conventional design method: the parametric design and feature modelling technology to build a 3D solid model. 3D contact finite element analysis method analyzes the bolts, body, and cap.

Its Big end assembly composed of bearing bush has been subjected to the nonlinear calculation under the action of the maximum tensile load. The three-dimensional finite element method analyzes the modal distribution of its dynamic characteristics and the mode shape of each mode and points out its weak link. Feature-based modelling technology builds three-dimensional solids and models and establishes a three-dimensional Model.

A. The theoretical basis of the finite element method

The boundary conditions determine the constraint boundary conditions for the rigid body displacement constraints of the connecting rod (including bolts, bearing bushes, and bushings) according to the analyzed problems and models. Different models use different constraints. When examining the deformation of the inner hole surface of the big end of the connecting rod, the inner hole surface of the small end is generally fully restrained. When examining the deformation, since the big end hole relates to the crank pin, one end face connected to the crank pin is fully constrained, the other end face can move in the axial direction, and the displacement coordination constraint is carried out at a node at the top of the small end.[6] The finite element analysis considers the influence of boundary conditions such as bearing length and elastic deformation. The elastic deformation was restoring force describes the preload load, which can more accurately reflect the change and influence of the large and small bolt preload loads. The connecting rod bearing assembly preload acts in the form of surface force on the contact surface; the inertial load is applied to the small end of it according to the cosine distribution law; the maximum burst pressure is applied to the small end of the connecting rod according to the cosine distribution law. The integral three-dimensional finite element analysis of the diesel Engine connecting rod, in which the force state of the connecting rod was fixed now of the worst working condition, and it was converted into a static force for division. In the analysis and simulation calculation, the connecting rod is regarded as a two-force rod; and the centre of the crank pin is fixed and restrained, the line exerts a load on the piston pin, and the constraint force of the crank pin is the same as that of the piston pin.

Numerical simulation technology is one of the fundamental driving forces for the formation and development of modern engineering. First, the numerical simulation methods commonly used in the engineering field include finite element, typical boundary element, usual discrete element method, and finite difference method, and the conventional element method is the most widely used one among them.

Based on element analysis, each element is assembled into a whole structure by using equilibrium conditions and continuous conditions. Divide the whole under the condition of determining the boundary and analyze it to obtain the whole parameter relationship equation system, that is, the matrix equation.

B. Constant strain tetrahedral elements

The constant-strain 4-node tetrahedral element is better for calculating object shape adaptability in spatial problem pairs, and the higher tetrahedral element is less commonly used.

1. Displacement Function

The tetrahedral element 4 has vertices i, j, m, and l as nodes. Each node has three degrees of freedom, and an element has 12 degrees of freedom, so the displacement function given by generalized coordinates is linear,

Using the coordinate transforming equation (1.21), the eight-node regular hexahedron element can be transformed into any prismatic hexahedron element.

C. Finite Element Method In Practical Calculation

The finite element method is a numerical calculation method for solving mathematical equations. It regards the solution area as consisting of many interconnected subdomains at small nodes, and its model gives approximate solutions to the subdomains of the fundamental equations because the subdomains can be divided into different sizes of various shapes and sizes, so it adapts well to complex geometries, material properties, and boundary conditions. The following takes the rigid space frame as an example to introduce how the method with the element is applied to the actual calculation.

The axis of the member is selected as the x-axis, and the other two coordinate axes are selected as the central axis of the centroid of the section. Figure 4 shows the element node and its positive direction, and Figure 5 shows the element node position and positive direction. According to the solution of the element stiffness matrix of the rigid plane frame and the unit torsion angle generated at one end of the fixed rod at both ends, the element stiffness matrix of the rigid space frame is,

Let (xx, xy, xz), (yx, yy, yz), and (zx, zy, zz) be the direction cosines of the three axes x, y, z in the Oxyz  coordinate system, respectively, to obtain the vector change matrix R

The non-nodal loads are decomposed along the local coordinate axes x, y, z, and then the fixed-end reaction is calculated. At this time, attention should be paid to the sign of the fixed-end reaction force in the local coordinate axis. Finally, these fixed-end reaction forces are transformed to the x,  y,  z Directions of the overall coordinate axis through coordinate transformation and a minus sign are added before it to be the equivalent nodal load.

III. MOTION AND FORCE ANALYSIS OF CONNECTING ROD

In the kinematics calculation of the crank connecting rod mechanism, the movement of the middle crank can be approximated as a constant speed rotation, which is entirely acceptable for a high-speed engine, because when the engine works in a stable condition, due to the unevenness of the pinch torque The resulting change in the crank rotation angular velocity is not large. The reason for this is that the angular velocity of the crank in this crank-link machine is,

IV. FINITE ELEMENT ANALYSIS OF STATIC STRENGTH OF CONNECTING RODS

The requirements for the calculation model must first have a certain degree of accuracy, and secondly, the calculation model must have a good economy. The simplified calculation model must accurately reflect the structure's actual situation. Otherwise, the structure's finite element calculation results will have no practical significance. When establishing a calculation model, a complex calculation model has high accuracy.

Appropriate selection should be based on calculation accuracy, mesh generation, and simulated boundary conditions. The solid model of the connecting rod meshes with a 10-node tetrahedral element type, and the global element size is defined as 2 according to the needs, and the mesh is refined in a place where it is easy to deform and break. The 10-node element SOLID92 is used in this paper, the number of grids is 24,787, and the number of nodes is 43,364. The model is shown in Figure 6.

A. Stress Analysis Under Compression Conditions

1. Big-end Strength Analysis

Figure 7-9 shows the stress profile and deformation diagram of the small end of the connecting rod and the big end under the compression condition. Under compression, the maximum stress value of the big end is 386MPa. The seat is at the transition fillet between the big end and the shaft, close to the centre position and the actual situation. The deformation of the big end of the connecting rod is shown in Figure 9, and the maximum displacement is 0.0852mm. Because the small end is fixed, the force value of the small end is small at this time, and the position of the rod body close to the big end is still a critical section.

B. Small end Strength Analysis

Figure 10-12 shows the stress and deformation diagram of the connecting rod's big-end and the connecting rod's small end under the compression condition. Under the compression condition, the maximum stress value of the small end is 497 MPa, located at the transition fillet between the big end and the shaft. Close to the centre. At the same time, the position of the small end near the 45o section and the transition fillet between the small end of the connecting rod and the shaft are high-stress areas, which should also be the focus of the experimental investigation. The deformation of the small end of the connecting rod is shown in Figure 10, and the maximum displacement is 0.0283mm.

C. Stress Analysis Under Tensile Condition

1. Big-End Strength Analysis

Figure 13-15 shows the stress cloud diagram and deformation diagram of the small end of the connecting rod and the big end under the tensile condition. Under the tensile condition, the maximum stress value of the big end is 104 MPa, located inside the big end hole. Meanwhile, near 90o on the big end. The section location and the transition fillet between the big end of the connecting rod and the shaft are high-stress areas. The deformation of the big end of the connecting rod is shown in Figure. 15, and the maximum displacement is 0.0135mm

2. Small end Strength Analysis

Figure 16-18 shows the stress diagram and deformation diagram of the large end of the connecting rod and the small end under the tensile condition. Under the tensile condition, the maximum stress value of the small end is 16-0 shrimp a, which is located near the oil hole of the small end hole of the connecting rod: meanwhile, in the Small end near 45. The high seat force area is the cross-section position and the transition fillet between the big end of the connecting rod and the shaft. The deformation of the small end of the connecting rod is shown in Figure 18, although the large displacement is 0.0144mm.

It can be seen from the cloud diagram provided by ANSYS after the stress calculation that the stress concentration is at the transition between the large end of the connecting rod and the shaft and the small end of the connecting rod and the shaft. This is consistent with the actual situation. The calculation result obtains the stress distribution cloud diagram of the connecting rod and analyzes the dangerous parts.

In the maximum tension condition, it can be seen from the figure 17 that the maximum stress of the connecting rod is 167.52Mpa, and the safety factor is close to 6.5, so the tensile strength of the connecting rod is sufficient; in the maximum compression condition, the stress at the small end of the connecting rod is 539.59Mpa, the stress at the transition fillet is 403-488 MPa, and the safety factor is about 1.5. The compressive strength of this connecting rod also basically meets the requirements.

From the post-processing displacement and isochromatic strain diagrams, it can be known that the deformation of the connecting rod is in tension and compression. Compared with the axial clearance between the connecting rod and the piston specified in the technical manual, the value is within the allowable range.

Table 4

Comparison of results between the conventional method and finite element method

Table no 4 compares the conventional method of connecting rod and the finite element calculation method. Since this model has not been tested, the accuracy of the calculation results cannot be correctly measured, but from the comparison results calculated by similar models, it can be seen that the connecting rods with the same structural size have the same strength as the connecting rods based on the finite element method. The calculated value is slightly smaller than that of the conventional calculation method.

Conclusion

In this paper, the performance of the engine connecting rod produced by our factory is analyzed by ANSYS software, and the static strength and modal characteristics of the connecting rod are obtained, which provides an improved basis for the low vibration and structural strength of this model and has a certain reference value for the calculation of the strength of the engine connecting rod in the future. Although the research on this subject has achieved certain results, each method has shortcomings, and some content related to this subject needs further development. The following is the prospect for this subject: The engine connecting rod is an important part of connecting the piston and the crankshaft. It is subjected to alternating loads such as bar, compression, and bending during operation. Therefore, high requirements for its rigidity and strength are put forward, which is the focus of engine design. One of the difficulties. The research on the connecting rod involves many theories and disciplines. This paper only does some preliminary work. Many problems that are too close to the actual displacement and load constraints need further research.

References

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Copyright

Copyright © 2022 A K Yadav, K. Ratre, B. Biswas. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.