Alert no. 456 | 12 May 2025 | Version 1

Uncontrolled movement of coal stacker boom

What happened?

• A raw coal stacker boom was in the process of lowering into position when it rapidly and unexpectedly luffed-up fully (see figures 1 and 2).
• Although a coal mine worker (CMW) was on board at the time, he was not injured.
• Initial investigations indicate the hydraulic luffing cylinder (HLC) had detached at the bottom, allowing the mass of the counterweight to drive the boom into the air.

How did it happen?

• The bottom end of the hydraulic cylinder that luffs the boom up or down was attached with two clevises and bolts (see figure 3). When the bolts failed, the clevises and the bottom end of the hydraulic cylinder detached. There was subsequently no down force to counter-act the motion of the large counterweight on the opposite end of the boom. The weight moved downwards, rotating the boom end upwards (see figure 4).
• From preliminary observations, it appears that the western clevis bolts had failed prior to the event. The incident occurred later when the eastern bolts failed.
• The failed bolts were worn thinner than original. The thread on some nuts was stripped off.
• Similar incidents have in the past led to fatalities.
• In one scenario, the counterweight accelerates the boom upwards, potentially catapulting persons or objects into the air.
• Design standards were revised after these incidents.

Recommendations

• It is recommended that stackers and other mobile equipment for handling bulk materials be assessed by a structural specialist against the requirements of the latest standard.
• Develop and implement plans to meet the intent of current design and operations standards.
• In particular, pay attention to the configuration where a luffing hydraulic cylinder (or the parts it is attached to) is operating in tension and a component fails.
• Note that components such as bolts tend to wear or corrode over time. Safety factors that had been built in when new reduce accordingly.
• A dual cylinder design is often safer if, in case of a failure, the second cylinder is able to hold the boom.

References and further information

• The stacker was designed in 1990 to German Standard BG1986 Regulations, calculations and dimensioning of large machines in open cuts.
• In 1997, BG1986 was replaced by German Standard DIN22261 Excavators, spreaders and auxiliary equipment in opencast lignite mines
• The relevant Australian Standard ‘AS 4324.1 Mobile equipment for continuous handling of bulk materials’ was published in 1995 and revised in 2017. It will again be revised in 2025.
• Another related standard is ISO 5049-1: 1994; Mobile equipment for continuous handling of bulk materials – Part 1: Rules for the design of steel structures.