Structural dynamic response of a track chain complete undercarriage system using a virtual proving ground approach

Fig.1: CAD model of the complete system
Fig.2: Track with shoes and complete chain FEM models
Fig.3: Frame FEM model

The ITM Group Engineering Department uses advanced tools as finite element methods for static structural analyses of undercarriages, side frames or undercarriage components, such as track chain, rollers and tension devices. In order to integrate the recent prototype concepts into the design process, a new design procedure is proposed to design and to develop complete undercarriage systems.

The procedure, which is briefly outlined in this article, will include full system real time dynamic simulations, able to represent a typical situation in operation manoeuvre, experimental test information, and 30 years experience of the ITM group. The goal of the activities described here is to build up a new design procedure able to predict accurately structural responses of a track chain undercarriage system, starting from 3D parametrical CAD models, and exploiting the FEM analyses to investigate and point out characteristic dynamic phenomena under working conditions. The activities’ logic flow was developed taking into account the ITM group technical and logistic requirements with the aim to maximize time efficiency of the new procedure and not just its effectiveness. For these reasons, the initial set-up of the FEM models has been carried out by using the ANSYS WB environment, which provides a parametrical CAD data integration and an easy-to-use hexahedral mesh tool. For model implementation and analyses launch preparations, the eta/VPG set of tools are used, in particular for mechanical system creation, contact definition, boundary and initial conditions conception.

Fig.4: Model assembly of all meshed parts

Dynamic analyses have been performed by using the explicit finite element code LS-DYNA® while, for prediction of durability due to critical proving ground events and final fatigue response assessment, the eta/VPG fatigue/durability tool was applied. The basic idea behind the development of a new design procedure for a track chain undercarriage system, was to build up a methodology able to point out which CAE tool is needed to perform every single step of the design, how these tools can be integrated into a single work flow and if the whole procedure can be fitted into the ITM group design process. The activities were divided into four main phases:

• Assessment of the numerical models meshing all components and complete model implementation (component or sub-system joining, vehicle dynamic fixing, etc.)
• FEM simulation of a typical event of the working conditions by using the explicit finite element code LS-DYNA®
• Fatigue validation of the more critical parts by using the eta/VPG code
• Methodology feasibility study

In the first step, the original CAD models were fixed with the aim to get their compatibility with the FEM analyses (Fig. 1). This required to import into and repair in ANSYS WB each single CAD model of the track group with shoes (Fig. 2), roller, sprocket, idler, frame and tension devices. Later on, all the different parts were meshed with hexahedral elements (mesh parts are shown in Fig. 2-4).

Fig.5: Gravity unloaded and loaded complete system

Fig.6: Fatigue life cycles

Once imported into eta/VPG, the entire mesh model can be sketched as in Fig. 4, while Fig. 5 sketches the two rollers located in the inner part of the frame. In the last step of this first phase, the working conditions of the track chain undercarriage system were assessed.
The initial forces acting on the system were the external gravity and the internal spring preload. Therefore, before evaluating the system’s dynamic response, a preliminary analysis was performed with the aim to get the system equilibrium at time zero; Fig. 5 shows a comparison between the unloaded and loaded system.

Once the initial and boundary conditions were assessed, as a typical working application, impact conditions were chosen and “the climbing over a step” phenomenon was studied.

In the third phase, the fatigue life of the system frame was investigated by using the eta/VPG code. Fatigue is a common failure mechanism of various components under cyclic loading. An accurate analysis of fatigue damage requires not only knowledge of the stress/strain history to which the component is subjected, but also a suitable method for cumulative damage summation.

The eta/VPG Fatigue Post-Processor analyses and processes LS-DYNA® analysis results, predicting the life cycles that the selected system can sustain under given loading conditions. Different approaches exist according to the type of element used to mesh the component to be investigated. The frame was modelled by using solid elements, and therefore, the so-called Stansfield’s Approach had to be applied to predict the fatigue life.

Fig.7: Side 1 of fatigue life cycles	Fig.8: Side 2 of fatigue life cycles

The test case to be investigated is “the climbing over a step” (height of 30 mm) with the computation of frame acceleration and roller forces in the system dynamic response and the following evaluation of the frame fatigue life. Fig. 6 points out the two most critical regions of the frame, while Fig.7 and Fig.8 show a more focused area for both frame sides.

Fig. 9: Process integration into the ITM design chain

One area corresponds to the hole connecting the undercarriage system with the remaining part of the machine (zone A), while the second one (zone B) is spread over the inner part of the drivetrain hole. The minimum fatigue life is equal to 4.35*104 cycles. These results are in good agreement with the ITM Group Engineering Department experiences.
Finally, a merits and limits analysis is performed in terms of quality simulation results, numerical model complexity, design procedure efforts and computational time needed. The results are then compared to the experiences of the ITM Group En-gineering Depart-ment.
The following remarks are applicable:

• No severe geometric complexity of the components - The building of FEM models is practicable
• The complete model can be easily built up in the ANSYS WB environment
• The phenomenon to reproduce is non linear dynamic
• Boundary and initial conditions can be easily applied in the eta/VPG environment

An evaluation of time required to perform the whole activity is described in the diagram in Fig. 9

Marco Perillo, Vito Primavera
EnginSoft Spa

Giorgio Bonello, Marco Cavedoni
Italtractor ITM Spa