1Introduction
Pistons and connecting rods are moving components of air compressors. Their function is reversed and force is transferred from the crankshaft for the purpose of compressing air in the cylinder Therefore, the load applied on piston and connecting rod is very complicated. The quality of piston and connecting rod directly are effects on the air compressor’s performance as well as their durability and longevity.
In the past, traditional dynamics study of piston and connecting rod accurately unable to include variation of loading conditions and the calculation results and actual vary widely. Moreover, the cost in designing or producing for real pistons and connecting rods which serve for testing is high.
Nowadays, the development of computing technology and finite element methods supplied some software help for design and analysis structures becoming more convenient and more economical.
Therefore, the main objective of the presented work is to perform the Structural Analysis of pistons and connecting rods of specific air compressor model, so as to determine and evaluate the mechanical strength in pistons and connecting rods. Finally, the simulation results can be used to estimate the maker’s design of pistons and connecting rods.
2Methodology
2.1Finite element method
The finite element method (FEM) is a numerical technique for finding approximate solutions to boundary value problems for partial differential equations. It subdivides a large problem into smaller, simple parts. The simple equations that model these finite elements are then assembled into a larger system of equations that models the entire problem. The finite element method (FEM) applied in engineering is a computational tool for performing engineering analysis (Manjunatha et al., 2013). It includes the use of mesh generation techniques, as well as the use of software program coded with FEM algorithm. In software program, numerical methods for ordinary differential equations are methods used to find numerical approximations to the solutions of ordinary differential equations. Many differential equations cannot be solved using symbolic computation. For practical purposes, a numeric approximation to the solution is often sufficient. The algorithms studied can be used to compute such an approximation. An alternative method is to use techniques from calculus to obtain a series expansion of the solution. In addition, some methods in numerical partial differential equations convert the partial differential equations into an ordinary differential equation, which must then be solved.
2.2Computational model and setup parameters
Firstly, the 3D model of air compressor are designed in Solidworks and the general parameters are showed in Table 1 (Malhotra at al., 2014). The model of air compressor are then imported in ANSYS (Fig. 1). In ANSYS finite element model for the air compressor are built. By using mesh generation in ANSYS, geometry of air compressor is divided. The calculation model adopted hexa and tetra mesh style which consists of 1.1 million nodes (Fig. 2) (ANSYS, 2016).
After meshing for air compressor geometry, setup mechanical properties of the different main parts of air compressor is shown in Table 2 (Shenoy and Fatemi, 2006).
Before performing full transient dynamic analysis, the loads are applied for the air compressor. The first load is joint load rotation applied for connecting rod cam and the load value is 1,750 rpm. The second load which is pressure on piston No. 1, piston No. 2. The loads are illustrated in Fig. 3 and Fig. 4 (Hanlon, 2001; Lingaiah and Narayana Iyengar, 2006). The initial loads are corresponding with crank angle 0°.
3Results and discussions
3.1Equivalent stress on pistons and connecting rods
After solving in structural analysis, maximum equivalent stress on pistons and connecting rods are shown on Fig. 5 and Fig. 6. The X axis is crank angle and Y axis is maximum equivalent stress. actually, after solving, the minimum and maximum equivalent stress are identified at each point of crank angle but the minimum equivalent stress is dramatically lower than maximum equivalent stress. It is the cause that the minimum equivalent stress is not considered.
According to Fig. 5 and Fig. 6, the maximum stresses on piston and connecting rod No.1 when the crank angle is 135° while in case of piston and connecting rod No.2, is 315°. corresponding with the pressure on piston No. 1 and piston No.2 are maximum. Moreover, the maximum stress on two pistons and connecting rods values are little different.
The maximum equivalent stress value on piston and connecting rod No. 1 shown on Fig. 7 and Fig. 8, obtained are 205.32 MPa and 123.51 MPa. The maximum equivalent stress value on piston and connecting rod No. 2 shown on Fig. 9 and Fig. 10, indicated are 195.9 MPa and 123.07 MPa. The figures also identify the location of maximum stress concentration. Obviously, the maximum stress on the pistons and connecting rods are lower than tensile yield strength of A6061T6 material. It mean that the pistons and connecting rods deform elastically.
The stress concentration is distributed on different area are indicated by different colors. The maximum stress is viewed by red color.
3.2Total deformation of pistons and connecting rods
The maximum total deformation value of pistons and connecting rods are illustrated on Fig. 11 and Fig. 12.
As Fig. 11 and Fig. 12 show, the maximum total deformation of pistons reach the top at crank angle 180° while the highest value of connecting rods are at crank angle 135° and 225°.
The maximum deformation is used to evaluate the shape deformation of pistons and connecting rods under stress effects. The maximum deformation of pistons at crank angle 180° is 11.6 mm for both pistons that are shown on the Fig. 13 and Fig. 14. The maximum deformation of connecting rods at crank angle 135° and 225° are 12.6 mm respectively that are shown on the Fig. 15 and Fig. 16. The maximum total deformation are not corresponding with the pressure on piston No. 1 and piston No.2 are maximum. Because, the pistons and connecting rods move on the plane and in the program, the total deformations is calculated from the intial position corresponding with crank angle is 0°.
3.3Safety factor of pistons and connecting rods
Safety factor of pistons and connecting rods are shown on Fig. 17, 18, 19 and Fig. 20. Safety factor is a term describing the load carrying capacity of a system beyond the expected or actual loads. Essentially, the factor of safety is how much stronger the system is than it usually needs to be for an intended load.
The Fig. 17 and Fig. 18 described that the range of piston No.1's safety factor near with 1 is from 135° to 180° and these of piston No. 2 is from 315° to 360°. In this case, additional load will cause pistons to fail.
For case of connecting rods No. 1 and No. 2 as shown on Fig. 19 and Fig. 20, their safety factor always are higher than 2. The safety factor of No.1 and No.2 of connecting rods meet the safety requirement. And the connecting rods will fail at twice the pressure load.
4Conclusion
This paper introduce part of author’s work on long term study about the reciprocating compressor as well as the structural analysis. To summarize the work presented above, conclusions are made below

1) The maximum equivalent stress value on piston and connecting rod No. 1 are 205.32 MPa and 123.51 MPa. The maximum equivalent stress value on piston and connecting rod No. 2 are 195.9 MPa and 123.07 MPa. These highest value are found at joint of the pistons and piston pin. Because maximum stress on the pistons and connecting rods lower than tensile yield strength of design material, the pistons and connecting rods deform elastically.

2) The maximum total deformation of pistons reach the top at crank angle 180° while the highest value of connecting rods are at crank angle 135° and 225°. Beside, the maximum deformation on piston No. 1 is different from piston No. 2. However, the maximum total deformation are not corresponding with the pressure on piston No. 1 and piston No. 2 are maximum.

3) As Fig. 17 and Fig. 18 shown, the safety factor of piston No. 1 and No. 2 are near with 1, therefore it is necessary to consider about design of pistons. While, in case of the connecting rod No. 1 and No. 2 are higher than 2 as shown in Fig. 19 and Fig. 20, and meet the requirement of safety (Ilman and Barizy, 2015).