Prior to the downturn in the global economy in 2009, market demand resulted in the release of various models of “ultra-class” mining haul trucks. Ultra-class haul trucks are considered those that have a payload capacity of 300 US short tons or greater. By leveraging ultra-class trucks, mining operations can move more material per haul, hence improving efficiencies in haulage and loading.
Haulage and loading operations have process variations that are typically due to poor process control. These variances include the amount of material being moved per load, how it is loaded, how it is hauled and unloaded. Haulage and loading variances directly affect efficiency results in open pit operations, but they also directly affect costs. Hauling and loading efficiencies are going to be dictated primarily by the effectiveness of the shovel operator. This is determined by how they load the haul truck, and how fast. Under-loading the haul truck is not efficient, raising the cost of hauling and loading. One would rather over-load the haul truck to move more material in minimal runs, in lieu of under-loading and running more loads. The problems with persistent over-loading of haul trucks are the increased costs that impact the mining operation.
Overloading of haul trucks imposes stresses on all the components and materials that support that load. These stresses lead to failures that need to be repaired or replaced, which are costly. These costs come in the form of vehicle maintenance, haul road maintenance, and among other concerns directly impact safety. The example shown on the previous page is the result of overloading a CAT797. This haul truck, which costs at least $3.0 million and is designed to carry 400 US short tons of material (363mt), was overloaded and resulted in a catastrophic failure.
The effects of overloading can also be realized from unbalanced loading. Unbalanced loading can apply more undue pressure and stress on one side of the haul truck than another. This will lead to premature failure and increased costs just as well as overloading a truck will.
Fleet Managers are being posed with lowering the cost of ownership of haul trucks. When stresses are imposed on the haul truck, they cause premature wear and failure of key components. Amongst the components and systems are the suspension, steering, brakes, and more importantly the tires. Off-the-road (OTR) tires, for the largest haul trucks, carry a price tag of $50,000 each (May 2010) to nearly $160,000 each (May 2011). Tires are estimated to account for nearly 35% of the truck’s operating costs. OTR tire suppliers are posed with high demand and low supply of the larger haul truck tires, requiring that mining operations maximize tire life in order to avoid equipment downtime. Overloaded trucks will add undue stress on tires causing premature wear, and in worst cases, overloading will cause the tires to become so hot that a fire will ignite inside the tire causing the tire to explode. Tire explosions are known to cause severe injuries, and in extreme circumstances death, to the driver or personnel near the equipment at the time of failure. Optimizing the amount of material being loaded into haul trucks, without compromising safety, is one strategy to improving efficiencies. This can be achieved by using properly calibrated on-board weighing systems that are integrated into shovels, loaders and haul trucks.
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