Safety alert issued over elastic spring-back in wear plates on excavator buckets

Workplace Health & Safety Queensland recently issued a safety alert highlighting 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.

The alert was issued following an incident in which 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.

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.

The safety alert recommended a number of actions and said 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.

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.