Raise your hand if you’ve ever welded on the wheel of a heavy truck or earthmoving machine—or used a torch to heat a stubborn lug nut or brake drum—without first removing the tire or, at least, pulling the bead completely off the rim on one side. If you have your hand up, thank your lucky stars you’re still here to raise it.
Welding or torching a wheel with the tire in place is a potentially deadly practice—even if the tire has been deflated. In fact, some in the tire industry suggest that heat from any source—whether dragging brakes, snagging a power line with a raised body, lightning, or running tires under-inflated, overloaded or too fast—can initiate a chemical reaction within the body of a tire called “pyrolysis.”
Although experts disagree about the precise definition of pyrolysis, the term generally is understood to mean the breakdown of organic compounds in the presence of heat, resulting in the formation of gasses that can ignite. Pyrolysis not only can pressurize the tire with gasses (reseating even loosened beads), but also can weaken the tire’s structural integrity. Lethal explosions can follow—sometimes quickly, sometimes after many hours.
If you’re not convinced of the danger involved, watch the video produced by Standards Testing Labs and Bridgestone Americas Commercial Solutions.
Using heat to repair wheels not only is potentially disastrous, the practice also is illegal. “Cracked, broken, bent, or otherwise damaged wheels shall not be reworked, welded, brazed, or otherwise heated,” says OSHA Part 1910, Subpart N, 1910.177 (g) (12). If you service large wheels in your operation and have not recently reviewed the requirements of 1910.177 (Servicing Multi- and Single-Piece-Rim Wheels), you’d be smart to do so.
Heat and inflation pressure
“Tires are vulcanized by heat, so the problem with under-inflation, overloading or pushing a haul truck too fast is that tires can become as hot as when they were manufactured,” says Steve White, marketing manager for Michelin’s Earthmover Group, specializing in quarry, construction and material handling applications. “It’s not unusual for a tire to overheat and either melt and come apart or catch fire.”
According to Kevin Rohlwing, senior vice president of training for the Tire Industry Association (TIA), the tremendous heat produced in large tires from under-inflation, overloading and over-speeding (“operational heat,” he says) results from flexing of the tire’s thick sidewall. Although not every instance of overheating results in the tire catching fire, he says, the tire always remembers the abuse.
“The tire industry often refers to this as the tire’s ‘heat history,’” says Rohlwing. “A tire with a good heat history does not suffer internal damage, but over time, a poor heat history will lead to irreversible damage.”
The experts agree that running large tires in an under-inflated condition is among the most common sources of generating excessive heat. Plus, under-inflation can result in the tire’s body cords breaking, causing “flex breaks” that destroy the casing.
Regarding proper inflation, TIA, in its Earthmover Tire Service training manual, gives this advice: “Inflation pressure must always be determined by the tire manufacturer or a technical expert. In order to establish the correct inflation pressure, the type of tire, the weight the tire will carry and the speed of the vehicle must all be considered.”
Terry Beach, director of training for Michelin’s Earthmover Group, says it this way: “Tire pressure has to be matched to the load so that the sidewall has the proper deflection—not so much deflection as to allow the tire to overheat, yet not so little as to jeopardize the contact patch.”
Cara Junkins, sales and marketing director, Titan Tire, further points out that over-inflation makes a tire vulnerable to impact damage. When tire manufacturers provide inflation pressures for a given application, says Junkins, the recommendation typically specifies the assumptions used in establishing the number. If the machine’s application changes, she says, switching a wheel loader to a significantly heavier material, for example, the manufacturer should be informed.
“In the occasional instance where a machine might be regularly switching between materials of different weights,” says Junkins, “it’s most practical to set pressures for the heaviest load. It’s much worse to under-inflate or overload, than to over-inflate and under-load. If the tire is under-inflated or overloaded, you risk degrading the casing.”
Dill Air Control Products’ Lori Joyner reminds fleet managers that inflation pressures can be only as accurate as the gauge used to make the check. Dirt and grease that work into the gauge can cause friction in the spring, she says, making the readings inaccurate. Joyner recommends that fleets regularly evaluate gauge accuracy with a gauge-check station, and notes that some gauges have an adjustment screw that allows recalibration.
Inflation gauges are available in several types, including stick or pencil, analog (having a gauge face with an indicator needle) and digital. Quality gauges of all types are accurate when maintained, says Joyner, and digital models generally are the most precise—but also are more expensive, have batteries that fail, and are not always reliable in extreme temperatures.
“Also, it’s best to choose the pressure range that best fits your application,” says Joyner. “If you’re always working with tires in the 50-80 psi range, it’s better to use a 20-90 psi gauge, rather than a 20-120 psi.”
How often should pressure checks be made?
“I’d like to see it performed daily,” says Titan’s Junkins, “but weekly is reasonable. Weekly air-pressure checks should be a minimum.”
TIA’s Rohlwing agrees, saying that daily checks are the best practice, but in an imperfect world, he’d settle for weekly.
No matter the frequency of pressure checks, Michelin’s Beach recommends a visual inspection of tires before every shift.
“An operator might not pick up on 10-percent under-inflation, but he could spot 25 percent,” says Beach. “And the inspection also will turn up any safety issues with the wheel—damage to the lock ring or flanges, for instance. It’s also good practice to keep a record of inflation pressures to pick up on any developing trends.”
If you’re a fleet manager, also keep in mind that wireless systems for monitoring both tire pressure and temperature are available. These systems can report to an in-cab monitor, alerting the operator if pre-set limits are exceeded, or to hand-held devices or to the fleet office. Some systems also can be integrated into the machine’s telematics reporting system.
“As big as some off-road tires might be,” says Beach, “they still have their limits; they can only go so far, so fast with a given load. Exceed any one of these parameters, and it could mean potential disaster. An issue for some tire users, though, is ‘Can I afford to lose a tire early if I’m, say, pulling gold out of the ground at $1,500 an ounce?’”
The user might decide to “load ‘er up, go faster and go farther,” says Beach, not recognizing that it’s not just production at stake, but also serious safety issues.
“What if the machine makes it through the day,” he says, “and as the operator is exiting the cab, he’s hit by debris when an overworked tire explodes, the result of having caught fire on the inside?”
To counter the temptation to sacrifice tires in the interest of production, tire manufacturers have established the Ton-Miles-Per-Hour (TMPH) rating system to define a tire’s safe operating zone. The TMPH rating is applicable to tires used on transport vehicles, such as off-highway haulers, scrapers and wheel loaders used in load-and-carry applications.
According to Michelin’s Earthmover Tire Limits training program, TMPH is an expression of a tire’s working capacity in terms of its allowed internal operating temperature. A specific tire’s rating depends on its design and varies with size, type and the rubber compound used in its construction.
“Basic site” TMPH is measured by multiplying average load per tire by average cycle speed (round-trip distance x number of trips per shift / number of hours in the shift) when ambient temperature is assumed to be 100F. “Real site” TMPH modifies this formula with factors for haul distances greater than three miles and actual ambient temperature. Most tire manufacturers publish TMPH ratings, but you’d be well advised to consult your tire dealer or manufacturer if you have questions regarding this critical issue.
Application and operators
To ensure tire safety, off-road tires must be properly applied. Basic tire types—C Compactor, E Earthmover, G Grader, L Loader/Dozer, S Logging Service, High-Speed (all-terrain cranes), and Industrial (material handlers) among them—are designed to perform in specific applications. Within each type, variations in tread pattern, tread depth and rubber compound allow selecting tires that match requirements at specific jobsites.
Installing the improper type—or mixing types on a vehicle—can result in poor performance and potentially dangerous situations. Some off-road tires might have a dual rating, allowing them to be used, say, in both L and E applications, but the manufacturer likely has set different limits for inflation pressure, load and speed for the two uses.
According to Michelin’s White, scrap-tire analysis, that is, investigating why tires have failed, also can contribute to overall jobsite safety. For instance, if scrap-tire analysis reveals a consistent problem with a particular wheel position on a number of vehicles, then maybe there’s a problem with the haul road—a turn that’s too tight or a grade that’s too steep—or maybe the loader operator is placing too much material on one side of the trucks.
But absolutely critical for tire safety, says White, are informed operators.
“Operators need to understand, for example, why they must adjust their speed for corners and avoid large stones in the haul road,” says White. “They need to realize that racing to the hopper—only to wait several minutes while the truck ahead finishes dumping—is not doing the tires, the truck, the company or themselves any favors.”
Making sure that operators understand the basics of tire safety and recognize tire hazards on the jobsite, says White, will keep them safer and help extend tire life.