Introduction

Ore stackers are yard machines commonly used in stockyards to form piles of material. To this end, it has a metal structure that includes translation, turning and tilting systems. It is therefore necessary to evaluate the equipment in terms of both structural criteria and mechanical systems.

Kot was responsible for checking the design of a forklift truck in order to bring it into line with a client company's regulatory standards. This is necessary in order to assess the safety of assets and thus avoid catastrophic consequences for humans and the environment.

The equipment in question transports iron ore, with a capacity of 4,200 t/h. In addition to analyzing the regulatory requirements, the study also included the structural, global stability, modal and mechanical analysis of the machine, which will be presented in this case study.

Three-dimensional modeling

The first step in the process of structural analysis using the finite element method is to model the structure. In the case of the forklift, the structural model was developed in bar and shell elements, for regions made up of structural profiles and plates, respectively. The subsystems are shown in Figure 1.

Figure 1: Complete finite element model of the structure.

The loads applied to the model include self-weight, material load, belt traction for different operating regimes, material fouling load on the structure, overloads, chute clogging, among others. These loads were combined for different considerations, as required by the standard, in order to carry out the structural and mechanical analysis.

Analysis

• Static analysis

During the static analysis of the bottom and tripper of the forklift, utilization rates were found to be higher than admissible for the structure's overload operating condition. This condition failed both the tripper 's bar elements (Figure 2) and the turntable's shell-modeled elements (Figure 3).

Figure 2: Envelope of utilization rates in tripper busbar elements for overload operation.

Figure 3: Envelope of utilization rates in turntable shell elements.

• Fatigue analysis

The fatigue analysis was carried out considering the number of cyclic repetitions required by the standard. Utilization rates above the admissible level were identified for the rear of the boom and on the turntable, as can be seen in Figure 4.

Figure 4: Fatigue utilization rates of shell elements.

• Local buckling analysis

Local buckling analysis is carried out for shell-modeled elements, in which the acting von Mises stresses, after calculating the eigenvalues, are compared to the permissible stresses. This is an important check for regions typically subjected to compressive loads, which can generate the effect of local elastic failure of the components. The structure was approved, as shown in Figure 5.

Figure 5: Buckling mode - approved structure.

• Link analysis

Welded rigid connections failed at the top of the tripper tail mast for the overload combination. The same occurs for rigid end plate connections when evaluated in combinations with chute fouling loads and wind loads.

• Global stability analysis

This type of analysis considers a stability polygon, in which each edge is considered an axis of possible overturning, as shown in Figure 6. This analysis also failed the structure for combinations with overloading on the boom.

Figure 6: Stability polygon considered in the analysis of the complete forklift truck.

• Modal analysis

Modal analysis indicates the possibility of the structure resonating with exciting sources. These sources can be, for example, wind, drums, boom conveyor rollers and belt oscillation. Some warning situations were found, as shown in the deformed view in Figure 7, which occurs when belt oscillation is an exciter source.

Figure 7: Deformation from modal analysis.

• Mechanical analysis

The mechanical analysis included the belt conveyor located on the boom and the tipping, turning and translation systems. Some of the non-conformities found were:

• One of the drums does not have a standard minimum diameter;
• The turning system was non-compliant in terms of the pressure installed to turn the machine, the brake and the bearing;
• In the drive system, slipping of some of the wheels is to be expected.

Conclusion

This case study shows the need to evaluate structures in overload situations, whether they are roofs, walkways, operating or maintenance platforms. Normative criteria must be followed and providing for this type of loading is essential to mitigate the risk of structural collapse.

Tendo isso em vista, a Kot Engenharia propôs ações a serem seguidas pela cliente para adequação do equipamento, de forma que ele opere segundo as normas requeridas. Essas ações incluem tanto reforços estruturais quanto monitoramento da máquina.

A Kot possui extenso know-how na análise estrutural e mecânica de empilhadeiras e máquinas de pátio no geral. Entre em contato com nossa equipe para mais informações!

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