Wet-Grade Dump Truck Brake Fade and Wheel Flat
An opencast coal mine runs 190 t rear-dump trucks on a 1.8 km haul with a 12% descending grade. Dust suppression has wetted the road; 20 mm of rain fell overnight. A truck carrying 180 t reports rear brake oil at 118 °C (warning 110 °C, critical 125 °C), retarder fade, and a 0.4 m flat on the left rear inner tyre. The operator admits he stayed in a high gear and used the service brake because the retarder felt weak. Grade speed is limited to 18 km/h. Dispatch needs 14 more trips to reach the 8,000 t shift target; one of four trucks is already in the workshop. The workshop has one spare wheel assembly, and a tyre change takes 70 minutes. Sunset is in three hours.
1. What is the immediate disposition of the truck, and what should the operator do before moving it?
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Take the truck out of service; do not continue the descent or run it to the workshop on that wheel. Bring it to a controlled stop on the flattest available ground, apply the parking brake, chock the wheels and keep personnel clear of the rear tyre; a 0.4 m flat can fail violently. At 118 °C the brake oil is between warning and critical, so do not cool the brakes with water because of thermal shock. Let them cool naturally, then inspect for drag, oil level and leaks. The flat far exceeds any acceptable wheel defect and requires a wheel change. Inform dispatch immediately. If the truck cannot be moved safely, recover it on a low-bed or with a controlled tow only after the brakes have cooled and the wheel is secured.
2. Diagnose why the retarder faded and the wheel flattened, including wet-road and operating factors.
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On a 12% grade the truck has large potential energy to dissipate regardless of rain. Wetting reduces rolling resistance and traction but not the grade energy. The operator stayed in a high gear and relied on the service brake, so the retarder absorbed less energy and the service brakes absorbed more. Retarder fade begins when brake oil exceeds its limit; at 118 °C the retarder may derate, shifting even more load to the service brakes. The 0.4 m flat indicates the wheel was locked or dragged, likely from prolonged service-brake application or parking-brake drag. The 180 t payload is within the 190 t rating, but grade, wet road and high gear raised the energy. Root causes are speed/gear discipline and wet-weather haul-road controls, not simply a faulty retarder.
3. How would you recover the shift target without repeating the risk? Show the trade-offs.
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Fourteen trips at 180 t is 2,520 t. With one truck already in the workshop and this truck stopped for at least 70 minutes plus cooling and inspection, the remaining two trucks cannot realistically complete 14 trips in three hours at 18 km/h on a 1.8 km haul. Do not chase the target by allowing high-gear descents. Recover the truck, change the wheel, and return it only after a brake test. Re-plan with two trucks at reduced payload or lower grade speed, and use a grader or water cart to improve drainage. Record the cause and request a target variation if the shortfall is contractual. Check the other tyres for flats. Losing a few trips is preferable to losing a truck.