Mines freak out over hot tires. It's easy to understand why. When tires work too hard, the associated heat breaks down their rubber. At best, this breakdown may reduce tire lifespan. At worst, it could lead to a catastrophic failure. With tires forming one of mining's top costs, of course mines are careful to track tire temperature.
But, what if tracking tire temperature matters less than mines thought? Tire specialist Adam Gosling thinks just that. He represents TyreSafe Australia, a consulting firm that assists clients in getting better returns from their tires."
[Temperature] is a bit vague, really," he says. "What's the actual temperature you're talking about? At the tread? The sidewall? The air inside? It's quite nebulous."
Consider a standard OTR tire used in mining. Many sites equip these tires with internal sensors that check the temperature in the tire's air chamber. That's great, says Gosling, but temperature readings can be really inconsistent.
"Some of those tires are 14 feet high," he says." Through convection, the air at the bottom will be cooler than the air at the top.
"[Plus], the air inside a tire is always spinning." As a tire moves along, the air inside picks up momentum that continues even after the vehicle stops. "There are vortexes and eddies. As the tire bounces, the air is being thrown about every which way. It's not a homogenous environment, like people think it is.
"[Sensors] are measuring the pressure and temperature inside that turbulent atmosphere. At best, they're taking an average."
Not only that, but other parts of the wheel may influence certain temperature monitoring systems. Brakes, bearings, and wheel motors can generate substantial heat that mars the readings from an on-wheel thermometer, even if the tire's condition is perfect.
Gosling says part of the problem lies with mining's reliance on the ideal gas equation for its tire work thresholds. This law of physics indicates a close relationship between pressure and temperature in a rigid vessel with a fixed volume, like a scuba tank.
Tires are different, though. They're flexible, composite vessels that behave differently than rigid tanks do. They take a lot of punishment on a rugged mine site and each bump can cause them to deform. (This video shows how tires change shape as they move through the rough-and-tumble conditions of a mine.) As a tire works, its different parts respond to changes in pressure and temperature differently. With so many variables involved, using the ideal gas equation to calculate a tire's work threshold just doesn't prove accurate.
That accuracy can make a huge difference. If a tire's temperature exceeds its threshold too often, the mine may not get the full value of its asset. Gosling says mines may only get 65% of their tire lifespan, in part due to inconsistent measurements.
"Now, they'd scream bloody murder if they were only getting 65% of their fuel they paid for," he says. "But for some reason, they think tires are different."
He suggests a better way to measure tire performance: Check the pressure at the valve stem."
The temperature changes, but pressure is more consistent — especially pressure at the valve stem, where the air is trying to escape," he says. Although pressure also changes as tires work, it's much more regular than the range of temperature readings possible under the same conditions.
All mines want to extend the life of their tires. They invest heavily in systems like Wenco's TireMax that do prove helpful to the bottom line. But, if the initial data entering these systems is off, results fall short. To get the best performance available from their tires, mines need to make sure their data stays accurate. Really, they need to use a tire pressure monitoring system to keep tire measurements consistent.
Of course, it all starts with using the right measurement in the first place.