Evaluarea structurală a comportamentului unui piston

Objective

In this paper, we attempt to evaluate the structural behavior of the piston compressor con-rod (stress and displacement magnitude values) based on dynamic stress simulation, for two situations:

1. The con-rod length is constant, the compressor speed is increased from idle speed to nominal speed;

2. For a constant compressor speed (nominal speed), the con-rod length has the different values.

The analyses were made using specific module of the MSC software – MSC Visual Nastran Desktop 4D.

Keywords: finite elements, stress, strain, optimization, rigidity, virtual prototyping

Introduction

The actual tendency of the engineers and researchers people from worldwide are focused both on the research-developed-optimization activities for some components or parts of system assemblies or sub-assemblies and on the increasingly quality, reliability and durability of products in order to remain competitive and profitable or to obtain the marketplace.

The manufacturers are pressed to refresh their product models much faster than before. Shortening time to market and reducing costs are simply not good enough anymore. They have to penetrate new markets with innovative new products that support and cultivate strong brand values. This calls for a high performance development process – with high throughput and high precision. A process that delivers the right products – designed right first time. Engineering departments now more than ever are challenged to tune the functional performance characteristics of their new designs.

Today, the industry evolution towards more virtual prototyping results in fewer prototypes that must be tested in greater detail. Virtual prototyping method won it an important role in the different high industry domains, such as: automotive, aircraft, biomechanical industry. The truth virtual prototyping is created by means of MBS.

An interest domain of the MBS systems for the researchers is the analysis of the flexible bodies in motion from an assembly or subassembly.

Flexible bodies provide a very efficient way to represent structural flexibility for even the most complicated geometry. The method uses an advanced numerical algorithm based on FEA (Finite Element Analysis).

This functionality allows visualizing the stress field of a part undergoing dynamic motion.

Problem description

The processes and phenomena that develop in mechanical system are quite complex and difficult to represent faithful in virtual models, which represent in fact an approximation of the real systems.

Theoretical interpretation of the phenomena and processes from mechanical systems gets useful in order to make a final decision with regard to adoption some optimal functional parameters.

The analyzed model was made for a piston compressor, and block diagram of the debate study is presented in the figure 1.

The input values known are mass, material & geometrical properties for all components of the assembly, joint positions of the mechanism assembly, generating motion (piston compressor speed) and force pressure, which acts on the piston head per a complete cycle.

Based on input kinematics parameter adopted and the restraints imposed by joints applied, it can be obtained the motion equations of the mechanical system, resulting the kinematics behavior of the model. The motion equations obtained form kinematics behavior of the mechanical system along with external and internal loading applied on the study model leads on to obtained the dynamic model and implicit to the dynamic behavior of the system.

Finally, based on summing kinematics and dynamic behavior effects of the mechanical system, it obtained the dynamic stress and strain state of the marked component.

Geometry model

The con-rod is a known component from a crank mechanism that delivers the gas force pressure of the piston head to crankshaft in order to generate torque motor. Also, the con-rod converts alternative translation piston motion in a turning motion of the crankshaft.

In the figure 2, it shows the CAD model of system analyzed that contains: piston, con-rod and crankshaft.

The mechanism has one degree of freedom: rotating crankshaft. The input kinematics parameter of the mechanism known is the piston compressor crankshaft speed; based on crankshaft speed it will result the angular velocity of the crankshaft: