The investigation was aimed at evaluating the structural response of existing crawler excavator cab to the loads imposed by the ROPS test as proposed in the revision of ISO 12117 (TOPS) as drafted in Doc. ISO/TC 127/SC2 WG5 N4.
Though it is clearly understood that the analysis shows limits imposed by the model size and that the geometry will be significantly simplified, this investigation should highlight problems, limits and constraints in the procedure proposed by the standards being developed. It could be used as a general guideline for future improvements of the design and for assessing limits of products being put into service in the next years.
The entire structure was divided into its constitutive parts, using a criteria based on the optimization of geometric shape (small parts) and efficient handling (larger assemblies). This resulted in a model decomposed into 18 different components, individually modelled, transferred and meshed.
The geometry import, model reparation and meshing phases were performed using Gambit 2.0.4, a dedicated pre-processor developed by Fluent Inc., which offers an option for mesh export towards Ansys 5.7.
The model includes a total of 18 nodal components and 18 element components.
The generation of the constraint equations was semi-automatic, and followed the following step-by-step procedure:
1. Selection of all components of the model
2. Selection of nodal components only for the part to be connected to the model
3. Selection of nodes located on the exterior of the selected part
4. Unselection of the elements belonging to the component whose nodes were in the selection list
5. Automatic generation of constraint equations between all nodes (first region) and elements (reminder of the model) laying within a prescribed tolerance
A further improved strategy for automatic generation of constraint equations could consider the actual relative thickness of the members to be connected, according to the top-, bottom- or mid-surface approximation used to generate shell elements.
However this automatic procedure would conflict with the actual spot-welding process used to assemble the final structure, generating a set of constrained surfaces with unrealistic coupling of degrees of freedom. The best procedure to be used for spot welded structures is still to a large extent unknown, and for this reasons various options will be considered. Only a criticism based on at least a partial knowledge of cab overall stiffness would guide the choice between different options.
The computation shows that the overall performance of the structure is such that it cannot withstand the loads imposed by the draft EOPS standard (proposed revision of ISO 12117 - part 2), even in the case of the lighter machine, and the difference indicates that a design evolution would hardly match the requirements. The F.E.M. investigation, however, proved to be a valuable tool for at least concept design of an improved cab.
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