Overloaded: The Risks and Rewards of Increasing Payload Size
February 3, 2020 - In mining, small changes can add up to big results. With dozens or hundreds of machines running around the clock, each handling or carrying hundreds of tons of material, even minor adjustments to things like payload and cycle times can result in substantial productivity gains.
With that in mind, it can be tempting for some mines to try loading trucks beyond their recommended payload capacity. Overloading trucks to move more material with every trip to the crusher may look like an easy way to increase truck productivity. But looking at the total productivity of the loading and hauling system reveals that this is not always the case.
According to Caterpillar Performance Manager John Ingle, loading trucks beyond their recommended payload cannot only increase risks, but does not guarantee higher system production. It can even decrease the productivity of some operations, as well as increasing operating costs.
“Item number one is safety, of course,” says Ingle. “Trucks are designed to be able to carry loads that are within a reasonable amount larger than their rated payload values. But a truck that’s overloaded will not handle the same. If you add too much, it’s not going to respond, maneuver or brake as well.”
According to Ingle, trucks are designed around an optimal balance of factors to provide the lowest possible cost per ton. Payload is just one of these factors — increasing it upsets that balance and can have consequences that can wipe out any productivity gained by hauling more material.
“A truck carrying more weight than it’s supposed to can’t move as quickly,” says Ingle. “It’s also taking extra wear and tear, which can reduce component and tire life and increase fuel burn.”
Some of the negative impacts of loading trucks beyond their payload capacity are easy to predict. Cycle times and fuel consumption will increase if the machine is carrying more weight. Likewise, component life and ultimately machine life can be affected by a machine working harder than it was intended to.
According to Ingle, another drawback is that it is often impractical for a loader and hauling operation to achieve an increased payload.
“Any increase in target payload has to be accomplished with tighter load distribution. And if the loading tool is not matched that’s hard to do without partial passing, which means your loader is taking more time to fill the truck,” says Ingle. “This affects not only truck cycle times, but the total system fleet match, and you may have trucks just sitting at the face waiting to be loaded.”
In one theoretical example conducted by Caterpillar, experts simulated a truck’s performance with payload increases of 5% and 10%. According to Ingle, this study did reveal that a 5% increase in payload could drive as much as a 2.5% increase in tons moved per hour. A 10% increase, however, actually reduced the truck’s tons per hour by more than 3%.
In a controlled study conducted with a customer in the field, Caterpillar found that a 4% increase in target payload increased cycle times by 2.4%.
Another controlled study found that the difficulty in achieving a 5% payload increase caused load and exchange times to rise by 27% — and a total truck cycle time increase of 5.2%.
“Ultimately, you’re doing all this work to hit a higher payload and most of the time it’s not even helping the fleet move more,” says Ingle.
Ingle acknowledges that some mines will still want to attempt to increase payload, either because they believe it will work well in their specific conditions or because they believe that higher payloads will result in higher production. Caterpillar offers a number of recommendations for companies that choose to pursue an increased average payload:
- Never exceed the maximum allowable payload.
- Operate within tire size nominal and maximum load capacity.
- Maintain correct tire pressures.
- Maintain proper strut charges and performance.
- Inspect and maintain proper body shimming.
- Monitor frame conditions.
- Set and manage proper speed limits.
- Maintain an onboard payload management system.
- Maintain upper front strut bearings.
- Keep haul roads in good condition and use the machine’s road analysis monitor to identify opportunities for improvement.
- Avoid uneven loading and dumping.
- Follow proper loading practices.
- Monitor and track machine event data and use data analytics.
- Follow proper towing procedures.
- Minimize payload variance around your target.
“We don’t recommend overloading your trucks, but both Caterpillar and our dealers are willing to work with you on a plan to help mitigate the impact on your operation if you do,” says Ingle. “But in the end, the mine has to accept the consequences — good and bad — of loading trucks beyond their recommended payload.”