Open Cut » Maintenance & Equipment
With mine operators moving to re-rate their draglines to 125% RSL, any structural problems are likely to be exagerated and need investigation. This project sought to evaluate higher loads on the dragline revolving frame and tub structure and develop responsive structural improvements and repair strategies. This project was undertaken with the following objectives:
- Develop a clear understanding of the reasons for the past failures
- Critically review the current repair and modification strategies and methods
- Evaluate the influence of higher loading (e.g. 125%RSL) on the revolving frame and tub structures
- Identify possible upgrades (where necessary) and improved repair methods
- Develop engineering drawings for identified upgrades.
The investigation was based on two BE1370 machines and two Marion 8050 machines. Inspection of the four machines indicated that comparatively the Marion revolving frame structures perform better from fatigue point of view. Therefore, it was decided that the instrumentation and measurement and the finite element (FE) analysis should be based on a BE1370 machine.
The drawings of the selected revolving frame were obtained from Bucyrus and an accurate FE model of the revolving frame structure was generated. The structure was analysed for 100%RSL and 125% RSL load combinations within this FE model. The results of the analysis, maintenance records collected from the sites and the observations from the inspections were used to identify the critical areas of the structure. These observations helped to identify the locations for instrumentation.
The results of the FE model could be calibrated using the measured results. The remaining component of the investigation was undertaken using the FE model. The influence of various load combinations at 100% RSL and 125% RSL were investigated to study the influence of 125% RSL loading on the revolving frame structure.
Two areas which are most critical in the BE1370 revolving structure were identified as the mast anchor area and the centre pin area. Both these areas carry concentrated loading and hence are important from a structural point of view. Due to the same concentrated loading, the welded connections in this area tend to generate high stress concentrations that can create fatigue issues.
Finite element sub-models were created for the two areas and different modification options were studied to increase strength and reduce fatigue.
The following conclusions have been made from the investigation:
- It was found that the general total stress levels increased by approximately 12% when the machine is working at 125% RSL compared to 100% RSL.
- The critical areas in the structure for strength as well as fatigue were identified using the results of the FE analysis. The stress ranges for fatigue at these locations were also investigated for 100% RSL and 125% RSL. This analysis indicated that the stress ranges at the critical locations increased approximately by 28% for 12% RSL compared to 100% RSL.
- According to this assessment the fatigue damage accumulation at these locations is likely to double when working at 125% RSL compared to working at 100%RSL.
- The area where the mast anchors are located is the most deficient area of the structure for 125% RSL loading. This area in some BE1370 draglines has had fatigue issues even before the loading levels of the draglines were increased to 125% RSL. The measured and calculated general stresses (i.e. not stress concentrations) in the area of mast anchor show that the structure can be considered marginal for strength at 125% RSL loading. The stress levels in the area are certainly too high to give reasonable fatigue performance.
- Apart from the area discussed above, the remaining structure is satisfactory for strength. There are several other areas which can be considered as deficient from fatigue point of view.
- The mast anchor area and the centre pin area were selected for detailed review and design of structural improvements. The engineering drawings for the recommended modifications are provided in the report as appendices A and B.
- An overall strategy for maintenance of revolving frames and tubs have been developed and presented in section 8 of the final report.