When haul roads are kept in top condition, trucks run faster and more safely, cycle times improve and more ore is produced. Good road conditions also reduce truck maintenance, lower fuel costs and reduce tire damage.Download PDF
It used to be that tires were just a cost of doing business. Mine owners were accustomed to spending the initial purchase price of a large mining truck in tire-related costs over the lifetime of the machine. Today, with an unprecedented demand and production that can’t keep up, tires are no longer considered just a cost. They’re mission-critical to keeping your haulage fleet working.
“This shortage isn’t going to end this year, or next year, or even the year after that,” says Pete Holman, senior consultant in Caterpillar Global Mining. “It’s going to take a while for the manufacturers to catch up. So mines have to do what they can to take care of the tires they have. And the only way to do that is to change the site culture. Now tires are getting the care they should’ve been getting all along.”
About 80 percent of all large tires fail before they wear out. Cuts are responsible for about 45 percent of failures, with impacts causing nearly 30 percent. One of the most important things a mine can do to prevent these premature failures is to take care of its haul roads.
“Cuts and impacts are mostly caused by spillage on the haul road,” says Caterpillar application consultant Kent Clifton. “When you’re driving a car, it’s easy to dodge a large rock. With a mining truck, by the time you see it, it’s too late.”
Clifton, who has visited sites all over the world and witnessed best practices, says the first step is proper design. The best haul roads have crowned straight sections, superelevated curves, safety berms and drainage ditches on both sides.
Negotiating curves can generate high lateral tire forces, which contribute to high tire wear and ply separation. Superelevation—the difference in height between the inside and outside edges of the bed of a banked road—helps eliminate these forces.
When properly designed, superelevation keeps loads level and square on the tires, decreasing side forces on the tire casing, and reducing scuffing and wear on the treads. In addition, superelevation allows trucks to operate at more consistent speeds, which means less braking and less heat—another cause of shortened tire and component life.
The amount of superelevation depends on the curve’s radius and the speed at which it is negotiated. Because superelevated turns can prevent a danger when slippery, those over 10 percent should be used with caution.
Another approach to superelevated curves is to determine the safe speed for negotiating a turn at a certain lateral tire force. In general, a 20 percent lateral coefficient of traction is safe for all but slippery conditions.
Grade is also important because it affects where the load sits on the tires. The goal is for 33 percent of the load to be in the front of the truck and 66 percent to be in the rear, Clifton says. The ideal grade is between 8 and 10 percent with low rolling resistance of 2 percent or less.
It’s also important to remember that the haul road begins at the loading face and ends at the dump point. Proper design of bench and dump areas can increase tire life as well by reducing spillage that can become a hazard to tires. Trucks should be able to quickly come in, turn around, and then back up. “You don’t want to turn the steering wheel and move the tires while the truck is not in motion because when tires twist on top of the rocks, it causes wear,” Clifton says.
Road width is another concern. “Roads should be three times the width of the widest truck, so tires aren’t bumping into the safety berms or dropping into ditches,” he says.
Using the proper material to prepare and maintain the road bed also is important. If the surface under the haul road is soft or moist, rolling resistance is high and tires are allowed to sink—which means contact with dirt along the sidewall. This can wear away the rubber from the tires.
Usually in cases of high rolling resistance, the ground is wet—another threat to tire life. “Moisture causes tires to slip. And when tires slip, they are not biting the surface—which means they are burning rubber,” warns Clifton.
While haul road design has a significant effect on tire life, by far the biggest threat is cuts and impacts due to rocks on the haul road. When a tire comes in contact with a rock, it can cut the tread or cause the belting of the tire to separate, which reduces the tire’s structural strength.
Making sure haul roads are free from rocks and other debris should be a top priority on every mine site, Clifton says. And that dedication does make a difference. “Some mines will get just 4,000 to 5,000 hours from a tire,” Clifton says. “But Pine Branch Coal, in the Eastern Coal area of the United States, is averaging 11,400 hours from a tire. They operate in one of the most severe environments for tires. If you can extend tire life at Eastern Coal, you can do it anywhere.”
Pine Branch owner Dave Duff, who has been in the mining business for over 50 years, says proper haul road maintenance comes down to making it a priority. “It’s a lot of work, but you just have to keep after it,” he says. “We try to do it the best it can be done. People have to care about it. And we do.”
“The key is communication,” he continues. “We all have radios, and whenever anyone sees a rock, they tell someone and it gets picked up. It’s teamwork.” Pine Branch has machines dedicated exclusively to removing rocks from the haul roads. “We call them the rock chasers,” Duff says. “We have little tractors that are outfitted with a box scraper. And they run wherever a rock has fallen.” In addition to picking up rocks, the box scraper also pushes dirt and smaller rocks into low spots that need to be filled.
Pine Branch also focuses on proper watering techniques. In addition to causing slippage, water also makes fallen rocks even more dangerous to tires. “A wet rock cuts twice as quickly as a dry rock,” Duff says. “We try to find a happy medium. We have to keep the dust down so you can see the rocks in the road. But we can’t get it too wet either.”
Operator training is important to every aspect of mine operations—and extending tire life is no exception. Motor grader operators need training in how to properly maintain haul roads to maximize tire life. Truck operators need to be aware of how their driving habits have an impact on tires.
“Watch your loads and watch your speeds,” says Holman. “Check your tire pressures. Be aware of rocks and debris. Be careful on turns.”
“A lot of what impacts the life of tires comes down to who is driving your truck,” says Duff.
Haul road maintenance sometimes takes a back seat to moving material. “Instead of putting an operator on a motor grader to maintain roads, mines will have him drive a truck—because what’s in that truck is what makes money for the mine,” says Clifton. “But if poorly maintained roads lead to trucks up on blocks because they don’t have tires, then you’ve lost the production you could have gotten from that truck.”
“Sure, haul road maintenance is a cost, but it’s worth it,” he says. “Good roads improve production, extend tire life and reduce overall operating costs.”
The lack of availability for large tires has been a challenge in the mining industry for the last four years, with a continuing supply gap of 20 to 30 percent. But what's causing the shortfall? Industry experts cite a skyrocketing demand for mining equipment due to high commodity prices—a situation exacerbated by a long period of underinvestment in the 1990s and early 2000s.
For most tire manufacturers, large earthmoving tires are just a small percentage of their business. For some time, these companies couldn’t get a high enough return on investment to build new plants that produce large tires. At the same time, commodity prices had been weak and on a flat trend for nearly 20 years. When the demand for mined products suddenly peaked, tire manufacturers weren’t ready.
“Everybody missed it,” says Holman. “The global demand is unprecedented—and it wasn't matched by an increase in supply.”
Mining companies point to the rapid industrialization of China, India and other developing countries as a main reason for the expanding appetite for mined materials. “It takes material for countries to develop,” says Holman. “And a lot of these commodities come from mining.”
As mining companies work around the clock to meet the demand for materials, they’re expanding their truck fleets. According to The Parker Bay Company, a mining equipment industry research specialist, the active population of mining trucks increased by nearly 40 percent over the three years ending Dec. 31, 2006.
Parker Bay identified 16,425 large mining trucks operated by more than 1,000 large surface mines in more than 70 countries. This increase was accomplished by the commissioning of more than 4,400 new trucks shipped during 2004-2006—the largest total for a three-year period ever recorded by the mining truck industry. At the same time, many mines chose not to retire existing machines or placed older units (either their own or ones acquired on the used equipment market) back into service to meet the surging demand.
Caterpillar believes that the shortfall will exist for at least another three to four years. “This will be around for a while—at least through 2010. Current inventories are gone. Our customers expect to weather the shortfall, but many anticipate having trucks parked at some time,” says Holman.
“Major tire manufacturers have expansions currently underway to meet the demand,” says Dan Gove, Caterpillar component product manager. “These expansions are capital intensive and require a significant investment in specialized machinery. It can take two to three years for an expanded facility to be up and running.”
Caterpillar is doing what it can to help customers find tire solutions—initiating projects to get maximum life from existing tires, sourcing radials, using bias tires and looking into repairing and retreading options.
“Tires are critical to the success of our customers,” says Holman. “And even though we’re not in the tire business, they’re critical to our success as well.”
Motor graders, wheel dozers and track-type tractors are recommended for the following haul road applications:
Motor graders and wheel dozers
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