Coal Inspectorate | Alert | No.413 V 1 | 21 July 2022

Underground grader - unplanned movement

What happened?

While driving an underground grader, the operator stopped the grader to let traffic past. The operator applied the park brake, but the grader moved approximately two meters before coming to a halt. The grader service brake remained operational throughout the incident.

• Both service and park brakes had been replaced as part of normal maintenance two weeks prior to the incident.
• The grader passed the weekly scheduled brake test a day prior to the incident.
• On the day of the incident, the operator reported having used the park break a couple of times without any problems before he pulled over to let traffic past.

How did it happen?

• The checks and inspections made after the brake replacement failed to identify that the park brake was dragging (not fully releasing). This resulted in the brakes being worn out in a very short period (two weeks).
• Maintenance inspections failed to identify the failed accumulator and below specification settings which are detailed in the key issues below.
• Operators did not identify any apparent loss of power due to the brake drag.

Key issues

• The load sense stand-by pressure had been set at 35bar, although the original equipment manufacturer (OEM) specification is 60bar. This resulted in low park brake system release pressure, causing the park brake to drag and wear out. The minimum brake release pressure is 50bar.
• The main hydraulic system pressure gauge was illegible, so visual checks hadn’t identified that the standby pressure had been set at the wrong level.
• The accumulator on the hydraulic circuit had failed and would not hold residual pressure in the park brake circuit.
• There was inadequate/absent detail in the preventive maintenance service sheets for park brake pressure checks.
• The hydraulic schematic diagram listed incorrect pressure settings.
• The OEM training for maintainers on the grader had been minimal.

Recommendations

Site Senior Executives must:

• Review and update the maintenance strategy for safety systems on underground graders - in particular brake systems - to identify:
  • all maintainable items and the likelihood and consequences of their failure
  • that there is a potential for hidden failures to exist
  • settings and adjustments required to preserve system functionality
  • actions to prevent in-service failure of critical maintainable items.
• Ensure that tests are conducted to confirm system functionality after intrusive maintenance.
• Provide maintainers with specialist training from OEMs to be competent on safety critical systems.
• Ensure that technical documentation is accurate, up to date, and available to maintainers.
• Ensure adequate stocks of appropriate spare parts.

Supervisors must:

• Ensure that site maintenance strategies for plant are implemented and followed.
• Identify that maintenance work has been conducted to the required standards.

Coal Mine Workers must:

• Understand and comply with site procedures for the safe use and maintenance of graders and other mobile plant.