The purpose of this safety alert is to highlight the risk of elastic energy being released when wear plates on excavator buckets or similar parts on other equipment are being removed or repaired.
Persons conducting a business or undertaking (PCBUs) and workers involved in similar work tasks should observe the risks identified and apply the principles outlined in this alert to manage the risks of exposing workers to the sudden release of stored energy by parts of the equipment.
In August 2017 a coal mine worker received fatal injuries while removing a large wear plate from the bottom of a large excavator bucket.
The worker was using an air arc gouging process to remove the wear plate by cutting it into smaller sections. At some point, the stored energy in the plate resulted in an uncontrolled spring-back of the plate, which sprang upwards and struck the worker.
Photographs 1 and 2 below show the underside of the excavator bucket and the wear plate involved. Similar incidents have also occurred with equipment other than excavator buckets.
An investigation into the incident by the Queensland Department of Natural Resources, Mines and Energy (DNRME) identified the primary contributing factor of the incident was an indentation of the excavator bucket. Although the wear plate was originally roll formed to the shape of the excavator bucket, the indentation (deformation) of the bucket and wear plate induced additional stresses in the wear plate and storage of elastic spring-back energy in the material.
In addition, most of the vertical welds on the sides of the plate had cracked, which did not prevent the plate from springing rapidly when the lower section of the plate was cut. It appears the larger wear plate design was a previous modification from the original equipment manufacturer’s (OEM) bucket design where multiple smaller wear plates were used. There was no evidence that any risk assessment was conducted prior to modifying the design of the external wear plates.
There are additional factors that can cause elastic spring-back in plates. The factors listed below were not necessarily all present in the August 2017 incident. However, these factors may be present in other configurations, either alone or in combination:
- Misshape: If the wear plate does not match the profile of the structure it is fitted to and is forced into shape before being welded or bolted, the residual stresses can remain in the plate until it is released.
- Welding: As molten weld metal cools after welding, it shrinks. If the parent material is constrained, forces are applied to it which may present as distortion and/or elastic spring-back.
- Wear: When a plate is newly rolled, the opposing residual stresses are in balance and maintain the desired shape of the bent plate. During operation, if one surface wears away the stress distribution is altered contributing to elastic spring-back potential.
- Poisson expansion (relating to indentation): As a plate is being indented during operations it compresses the material in the through thickness direction causing it to expand in all directions in the plane of the plate. This phenomenon adds to tension strain on the opposite side of the plate due to indentation, simultaneously increasing the spring-back potential.
- Dirt ingress: In this incident, dirt entered the space between the structural plate and the wear plate during operations after some of the welds had cracked and also after the wear plate had worn through. The presence of dirt increases the extent of deformation during impact. It also contributes to the stress profile as it tends to wedge the plates apart.
- Corrosion: The presence of rust in the space between the structural plate and the wear plate has the potential to increase the extent of deformation during impact. It also contributes to the stress profile as it tends to wedge the plates apart.
DNRME safety alerts and investigation report provides additional information about this incident and the factors that can contribute to potential spring-back (refer to Further information below).
All work tasks involving the removal or repair of wear plates on excavator buckets or other similar tasks on equipment must be risk assessed for the potential for spring-back energy to be released during the process. The level of risk of spring-back will depend on the size of the plate, the contributing factors described above and the process by which the plate is to be removed. Greater precaution and planning of the removal procedure will need to be taken for larger wear plates, even if signs of the potential for spring-back are not obvious. For example, even if there are no signs of indentations, cracking or wear, excessive force may have been applied to the plate during installation to fit it to the rest of the structure. Removal of wear plates should be done in a systematic manner to control the release of energy. This may include reinstating cracked welds or bolts or alternate bracing or restraint of the plate as the elements of the plate are released or removed.
In general, designs using several smaller wear plates instead of one large wear plate are recommended, as this will reduce the amount of stored energy and elastic spring-back potential that must be controlled at any one time during removal. People who alter the design of plant have designer duties to assess the risks of the altered design and ensure it is safe and without risks to the health and safety of workers throughout the life of the plant.
Additional considerations to control the risks of spring-back include:
- Before making alterations to the design of plant, consult with the manufacturer or supplier of the plant about the proposed alterations.
- Consult with the manufacturer or supplier about recommended maintenance and repair procedures for equipment identified with altered designs.
- Review any procedures for the identification of stored energy situations.
- Review any standard operating procedures and associated controls for hot work, cutting and welding.
- Develop specific work instructions for dealing with similar wear packages.
- Ensure any permit systems include controls for stored energy hazards.
- Ensure ‘line of fire’ is identified.
- Ensure tasks that may identify critical hazards are fully assessed by appropriate people.
- Make people who carry out similar tasks aware of the potential extent of the stored energy hazard.
- Review and assess the potential risk associated with the different types of wear packages fitted to buckets.
- As part of bucket repair tasks ensure previous repair history is assessed prior to commencement of work.
- Conduct comprehensive non-destructive testing to identify the condition of the bucket and include findings in the task assessment.