That moment as an operator that you realise you’ve just become a passenger

in a loaded articulated dump truck when you have gone for the brakes, there’s nothing there and there is no way to stop. The feeling of what these operators were confronted by is what we have empathised with over the two serious incidents that occurred in a matter of months last year.

These incidents triggered us to delve into the braking systems of articulated dump trucks (ADTs) and the failure modes that exist for these incidents to have been able to happen, what we found is concerning.

Before I get into the details, The reason this has bothered me so much is because I have had over 20 years exposure to ADTs as a mechanic and have always prided myself on identifying unique circumstances that could lead to serious incidents, yet I never saw the full consequences of this situation until now.

To understand why ADTs, have such a high likelihood of failure and a high consequence when they do fail, I’ll do my best to explain braking systems on similar types and purposes of equipment and vehicles as reference. For the purposes of this article, I’ll explain; heavy - on highway trucks (road trucks), scrapers, graders, loaders, and ADTs and what the fundamental differences of braking systems are.

On a road truck, the service brake, the park brake, and the emergency brake use the same brake components (or mechanism) which results in the overall braking capacity of the truck for the; service brake, park brake and emergency brake being roughly the same and the system is known as a "spring applied - pressure release" braking system (a few variables being adjustment/ air pressure/ temperature as some). The service brake is applied via air pressure from the cabin to the booster which applies the brake. In a resting state the park brake is applied via a spring in the booster which requires air pressure to release the booster before the truck can move and the emergency brake is applied when the air pressure that is releasing the park brake is exhausted and uses the park brake spring to apply the brakes (which is very heavy and applies approx. the same forces as the air pressure from the service brake).

Scrapers use this same system, however with a scraper you have an additional fail safe that you can drop the bowl that will help to slow you down.

Off-highway dump trucks have a system that uses the same principle of "spring applied - pressure release" brakes, however it uses hydraulic oil to apply the brakes which are known as "brake packs" and are made up of multiple discs that are cooled with oil. This is to make the system more effective due to the weight of the trucks, the limiting size of the braking components and the life that is needed to be obtained from these components.

Unfortunately, with a combination of companies keeping equipment running for longer than intended, lower competence of maintenance crews and tighter maintenance time frames, non-tangible maintenance items are being put overlooked.

When the service brake is applied air pressure from the cabin is supplied to a “roto chamber” which then converts air pressure to hydraulic pressure that is supplied to the brake pack to apply the brakes, in a resting state the park brake is applied via multiple springs that directly apply spring force the piston then onto the brake pack and requires hydraulic oil pressure to release it. When the emergency brake is applied the brake is applied via this spring pressure as the hydraulic oil is released from the "park brake release" chamber. There are no adjustment only routine brake pack thickness wear checks, a slack adjuster to ensure the roto chamber does not run out of stroke in normal operating conditions and an “over stroke” light to tell the operator if the roto chamber has "over stroked" and that the truck should not be driven.

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Articulated dump trucks, graders and loaders mostly have a similar system (except for older Volvo trucks which if you’re interested in those failure modes I’ll explain in the comments), which uses the same hydraulicly actuated "brake packs", only these do not have the direct acting spring park brake.

When the service brake is applied hydraulic oil is supplied from the cabin directly to the brake pack (which is usually supplied by a dedicated brake charge pump). The park brake is completely independent to the service brake and does not have one fitted on each wheel like the above examples. There is only one park brake, and it is usually on the transmission or centre drive shaft, and it is usually spring applied and released by either a hand brake, switch, or valve on the dash. Although the appearance of this design is like the previous braking types described the performance of this system is usually less than 25% of service braking capacity (for loaders), 16% for graders and ADTs as opposed to greater than 66% on trucks and scrapers.

The emergency brake is applied only using service brake pedal (not the park brake) using a reserve of charged hydraulic oil from a component called the brake accumulator.

Note: As the consequence and likelihood of brake failure for graders and loaders is lower to that of ADTs for the remainder of the article these equipment types will not be referred to. This is due to these equipment types having an additional failsafe by being able to lower implements as a last line of braking and typically operate at much lesser speeds and on more level grades than ADTs.

If we were to use the park brake as an emergency brake it would simply burn out before the truck would be able to be pulled up. The park brake in ADTs is not designed as an emergency brake and its capacity is simply not great enough to pull up a loaded truck at speed; it is only designed to hold a truck – loaded, stationary and on a maximum incline of 15%.

This forms a direct relationship between the emergency braking performance and the charge of the brake accumulators. The issue is with brake accumulators as a failsafe is that they deteriorate and lose their pressure over time without indication and when this happens the reserve charged oil volume required in a braking emergency is not available which results in no emergency braking and no way of stopping the truck. Which brings us to the incidents that occurred.

The OEM maintenance procedures ALWAYS require brake accumulators to be checked and are usually in the form of two checks: a functional accumulator test and an accumulator charge test. The functional test must be undertaken regularly (usually noted to be every 250 hours but due to the simplicity should be undertaken once a week) and will tell you how many brake applications are available in the event of any emergency - which can be done with minimal training. The second is the accumulator charge test (and is usually performed every 2000 hours) and requires special tooling to perform. If during a service there is not enough time to complete all the preventative maintenance tasks, the brake accumulator pressure checks are usually (even though they are highly critical) one of the first items not completed.

This second check becomes critically important as not all ADTs are fitted with the components to be able to perform the first functional test – I am personally perplexed over why they are not fitted.

Over the thousands of assessments, we have completed we have noted a common strategy that rapidly growing "yellow gear" hire companies dispose of equipment at under 1000 hours. The issue is that if that if this strategy changes (due to varying factors out of the company’s control) and they are required to continue to own the equipment for say more than four thousand hours, they may not be equipped to perform maintenance to the level required to effectively deliver safe plant. These pressures can come in the form of; limited maintenance budgets, limited access to highly competent trades persons, limited margins, and importance of keeping customers happy in an ultra-competitive market over the duty of operating and maintaining safe plant.

To manage this risk CPS now require evidence of brake accumulator pressure checks having been undertaken for all plant assessments (not just for ADTs but all equipment that have brake accumulators) in the form of a service sheet (when the hours exceed what the OEM recommended brake accumulator check interval is). Since starting this, we have found that more than 50% of equipment with over than 3000 hours have discharged brake accumulators – in short 50% of trucks with over 3000 hours had no emergency braking function.

Both of these incidents were easily avoidable from correct maintenance and I am sure there are many other incidents we do not know of that were similar to these.

From a hirer or projects perspective, where does the danger lie?

• Prioritise hire from companies that DO NOT purchase high hour trucks for dry hire only (above say 10,000 hours). As there is a lower vested interest in the production performance of the trucks the maintenance quantity and quality is typically much less frequent and lower which reduces the probability that critical items such as brake accumulator checks will be performed.
• Be vigilant of companies that rely purely on subcontractors for maintenance. Where companies outsource all maintenance to mechanical subcontractors, be mindful that those companies that own the equipment are likely not to be reviewing maintenance tasks completed by the subbies vs maintenance tasks required. The benefits of using subbies as being convenient is usually offset by those maintenance tasks not completed and the unidentified resulting exposure.
• Mandate a requirement that service sheets are supplied prior to hire that are congruent to or exceed the maintenance task intervals as recommended by the OEM, review these checklists and understand the severity of the items on these checklists and if there were actually performed or not, if a service work order notes "completed as per OEM" it is likely it has not been.

If you would like to know any more about the above, please reach out, we will not be discussing particulars of the incidents that occurred but are more than happy to talk about the fundamentals to prohibit this from happening in the future.

Some further information on the above discussed components:

How an accumulator works.

How a brake booster works.