Kot Engenharia

Análise estrutural e mecânica de uma recuperadora de ponte: Case de sucesso

COVER 2

Introduction

A análise estrutural pelo método dos elementos finitos (MEF) é um recurso amplamente empregado na avaliação da integridade estrutural de ativos industriais. A título de exemplo, tal análise pode ser aplicada em máquinas de pátio para verificar a existência de não conformidades, eventuais falhas ou mesmo fenômenos desconhecidos.

Structural analysis

Kot has carried out the structural validation of a homogenizing reclaimer using the finite element method (FEM). The iron ore reclaimer, which has a design capacity of 1950 tons per hour, is of the overhead crane type and has two bucket wheels for reclaiming material.

Structural verification began with the creation of a finite element model of the machine in CAE (Computer Aided Engineering) software, accurately reproducing the geometry of the asset. The model generated can be seen below in Figure 1, showing the components of the equipment in different colors.

Finite element model of the reclaimer and legend with the items bridge, bucket wheel, translation system, trolley, rake

Figure 1: Finite element model of the reclaimer. SOURCE: Kot Collection.

In this model, profiled components are represented as bar elements, while those formed by plates are represented as shell elements, as shown in Figure 2 below.

Bar and shell elements in the legend of the reclaimer model

Figure 2: Bar and shell elements in the reclaimer model. SOURCE: Kot Collection.

When they are in operation, the yard machines work with high loads from the equipment's own weight, the material load on the belt conveyor, the wind load, the fouling load, among others. Once the model has been generated, the loads acting on the structure are identified and grouped into load combinations.

Using FEM, these combinations are discretized and applied to the equipment arrangement for structural verification under different operating conditions, as recommended by the standards. Static, modal, buckling, fatigue and connection analyses were then carried out on the reclaimer in question, the results of which were interpreted according to standard criteria.

The static analysis of the machine was based on the stresses obtained from the finite element model. Specific allowable stresses, safety factors and utilization rates (UIs) were considered for each component of the structure, depending on the load combinations applied and evaluated. In the analysis of the bar elements, utilization rates above the admissible level were identified at the rear of the rake, as highlighted in region D of Figure 3 below.

Utilization rates with a focus on region D

Figure 3: Bar element utilization rates in static analysis. SOURCE: Kot Collection.

With regard to the static analysis of the shell elements, UIs above the permissible level are observed in the bridge ribs (region A in Figure 4) and in the trolley stretcher support (region D in Figure 5).

Utilization rates focusing on Region A and Region C

Figure 4: Bridge utilization rates in static analysis. SOURCE: Kot Collection.

Utilization rates with a focus on region D

Figure 5: Stretcher support utilization rates in the static analysis. SOURCE: Kot Collection.

In the fatigue analysis, a normative evaluation methodology was adopted for the useful life of the shell elements, which takes into account the existing stress concentrators and predictions of the number of cycles, the magnitude of the stresses and the variation in stresses in each element. As a result, UIs were found to be higher than admissible at the same points accused by the static analysis, as can be seen in Figures 6 and 7.

Utilization rates focusing on region A, showing Detail K3

Figure 6: Bridge utilization rates in fatigue analysis. SOURCE: Kot Collection.

Utilization rates with a focus on region B, presenting Detail K2

Figure 7: Stretcher support utilization rates in fatigue analysis. SOURCE: Kot Collection.

In the local buckling analysis of the slender plates, which are mainly stressed by compression, the critical stresses were calculated. The type of stress involved, the boundary conditions, the geometric parameters, the material properties, the values of the stresses involved and other factors were taken into account for the recovery system. The results obtained were within the permissible limits for all the loading cases evaluated.

The connections, commonly classified according to their stiffness, were also analyzed according to the standards. As a result, UIs above the admissible level were observed in the connections of some of the bridge's tubular bracings, mainly due to the action of the operating wind and storm loads on the machine (the most critical condition for the structure). The stresses acting on the connections (for the storm wind condition) can be seen in Figure 8.

Utilization rates focusing on Region A and Region B, with an illustration of the crossbar lamination

Figure 8: Structure utilization rates in connection analysis. SOURCE: Kot Collection.

In the modal analysis, the possibility of resonance of the structure was verified. The frequency of material unloading onto the impact rollers of the crane conveyor was taken as the excitation source. This identified the risk of resonance in one of the structure's natural vibration modes (Figure 9), which is characterized by the rotation of the bridge and the trolleys around the machine's longitudinal axis. As a result, it was found that the vibration speeds could exceed the limits allowed by the standard, an undesirable result for the proper functioning of the equipment.

Vibration mode showing the direction of rotation of the structure

Figure 9: Evaluated natural vibration mode. SOURCE: Kot Collection.

Finally, based on the results obtained from the FEM structural analysis of the machine, some points of concern were raised. An inspection would be necessary to investigate the condition of the asset in order to determine whether or not corrective action was required.

Due to the risk of failure in the rear of the rake, in the ribs of the transverse beam supporting the bridge and in the support for the tensioner of the bucket wheel drive chain, reinforcements were proposed to bring the reclaimer's structure into line with the normative criteria, by correcting the non-conformities found.

Mechanical analysis

Mechanical analysis is used to verify the operational functioning of mechanical components, and is also relevant to the design of machinery, as systems can be dimensioned using analytical and numerical calculations.

To this end, Kot carried out a mechanical check of the belt conveyor of the iron ore homogenizing reclaimer. The model used in the analysis can be seen below, in Figure 10.

Belt conveyor model, with X and Y axes

Figure 10: Model of the belt conveyor. SOURCE: Kot Collection.

In the analysis, the capacity of the conveyor in question was checked in order to assess the possibility of material overflow during operation. It was then checked whether the belt could withstand the stresses imposed on it by means of its tensile strength. Other parameters relating to the belt were also assessed at this stage, such as transition distance, vertical curves, flapping, among others.

In addition, the power required was analyzed in order to assess whether the belt conveyor's drive system is capable of meeting the design capacity, i.e. whether it has the necessary power. Also with regard to the drive system, the sizing of its mechanical components was checked, such as motors, couplings, gearboxes, brakes and backstops, based on the catalogs of the respective manufacturers and taking into account the highest power required and/or installed. In addition to the conveyor drive system, the sizing of the rollers and drum shafts was also checked.

According to the analysis of the crane's belt conveyor, it was observed that during braking with ore loading, the tension value on the slack side of the drive drum is lower than the minimum permissible tension value. According to the standard, this can lead to the belt slipping in relation to the drive drum. The shafts of the return rollers, on the other hand, deflected more than the permissible limits set by the manufacturer and the standard, and were therefore considered to be non-compliant.

In addition to the belt conveyor, the bucket wheel and the travel systems of the machine, trolley and rake tipping were also analyzed. As a result, the braking moment of the machine's travel system is, in some cases, insufficient to stop it, and the travel drive wheels are unable to transmit the braking force without slipping on the rail. In addition, the mechanical service factor of the bucket wheel reducer is lower than the minimum recommended by the manufacturer. No non-conformities were identified in the trolley and rake tipping systems.

Finally, based on the results obtained from the mechanical analysis, it was recommended that the current counterweight be increased and that it be checked in the field to see if there was a rail-lock system to help with braking during storm winds. If not, it was recommended that this device be installed as a corrective measure.

Conclusion

It can therefore be seen that using the finite element method for the structural analysis of yard machinery provides a highly reliable assessment of the structural perspective of the asset. Using this methodology, it is necessary to know the boundary conditions, mass and position of non-modeled components, constraints, among other factors. It is also important to know the magnitude and types of stresses expected on the equipment during its useful life, as well as the standards that recommend each type of analysis.

Similarly, by using mechanical analysis, you can be more assertive about the operational functioning of the machines. This also extends to assessing the condition of their mechanical components and sizing their systems.

A Kot conta com profissionais capacitados para entender e avaliar as melhores soluções para os desafios de engenharia presentes no dia a dia da indústria, assim como para análises estruturais e mecânicas de máquinas de pátio. Consulte nossa equipe para mais informações!

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Kot Engenharia Team

With more than 30 years of history and many services provided with excellence in the national and international market, the company promotes the integrity of its clients' assets and collaborates in solving engineering challenges. To achieve this, it uses tools for the calculation, inspection, instrumentation and monitoring of structures and equipment.