Open Cut » Maintenance & Equipment
Electric shovels are the primary production machine for many mine sites. They offer mine operators relatively low-cost production and a much higher degree of mobility than draglines. The industry monitors nominated the P&H XPB shovel at Goonyella Riverside (GYRS) mine for the investigation. The dipper handle and boom was nominated for a detailed investigation.
Significant delays to the project occurred due to prolonged negotiations between BMA and P&H MinePro to obtain structural drawings of the shovel and digital control set up parameters required for the project.
In June 2006, the industry monitors decided to change the scope of the project and adopt an alternative approach which included scanning the boom and dipper handle to get the geometry and measurement of plate thicknesses to develop the FE model. The revised scope of work and objectives for the study was changed and approved in September 2006.
This project finally focused mainly on instrumentation, data collection and testing, development of stress/load spectrums experienced at the various locations and identification of the root causes of the various failure modes that have been experienced by the dipper handle and boom.
The following conclusions can be made from the study:
- The root cause for the failures experienced at the boom foot, internal diaphragms, boom bottom plate at change in direction, shipper shaft support etc. were mainly due to the high stresses experienced in these areas during normal operations.
- The boom foot casting to plate connections to casting recorded stress ranges up to 153MPa during normal operations with the upgraded plate thickness of 50mm. The results clearly indicate that the implementation of the thicker plate had reduced the stresses in this region by 55%. The estimated design life after upgrades (50mm plate) is 22,000 hours. This upgrade has been implemented in the XPC shovel as well.
- The original boom foot casting to plate connection would have experienced equivalent stress ranges of 130MPa per load cycle which is extremely high for a welded connection. The estimated design life for this location with original plate thickness was 3,500 hours. The results clearly indicate the high stresses observed were the main reason for the failures observed in this location.
- The internal diaphragms (lower third of the boom) recorded tensile stresses of up to 285MPa. These high stresses clearly indicate the reason for the cracking observed in these diaphragms.
- The boom at the change in direction recorded stress ranges of up to 195MPa. These high stresses would be the main cause for the cracking observed in this area.
The estimated design life for the welded connection was 20,000 hours. Cracks have been observed in this location in all three shovels investigated.
- The results obtained at the shipper shaft boss area clearly indicate a significant reduction in stresses, indicating improved fatigue performance after the modifications. This modification has been implemented on the XPC shovel as well.
- The stresses in the dipper handles at the contact region of the shipper shaft were extremely high and would be the main reason for the cracking observed at this connection.
- Significant stresses were observed at the torque tube to dipper handle connection due to side loading. Cracking would be expected at this connection.
- The analysis indicates that the magnitude of the clearance between the wear plates and dipper handle is critical for minimising stresses in the dipper handle and the boom.
- One of the main reasons for the hairline cracking observed in the side plate at the dipper handle could be the high contact stresses experienced during operations, which was observed in the FE analysis.
- The original stiffener shape experiences significant tensile stress due to twisting of the dipper handles. These high stresses would be the main causes for the cracking observed in these stiffeners.
- Swinging while digging and hoisting simultaneously causes significant stresses in the boom and dipper handles.
- Sudden deceleration at dumping can also cause significant cyclic lateral loads in the boom and dipper handle as seen during operations.
- At several instances, the application of a small hoist reference and/or swing reference caused application of very high fluctuating hoist, crowd and swing armature currents, which are proportional to the hoist/crowd/swing forces. These forces cause significant stresses in the boom and dipper handle structures and can not be controlled by the operator.
The following recommendations are made from the study to improve the structural fatigue performance and reliability:
- A detailed study should be undertaken to clearly understand the behaviour of the control system in P&H XPB shovels with a view of tuning/optimising critical control system parameters to minimise stresses in the structure. At present, the application of small hoist references and/or swing references by the operator when digging and swinging with load can cause very high fluctuations in the hoist, crowd and swing armature currents. This causes extremely high stresses in the boom and dipper handle.
- The study should be extended to develop modifications/weld procedures to minimise the cracking at the rack to vertical weld connection of the dipper handle.
- A similar study should be undertaken to improve the structural reliability of other areas in the shovel structure, such as the revolving frame, carbody and track frames.