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How to Prevent Wheel-End
Millions of dollars’ worth of equipment and cargo goes up in smoke every year because of wheel-end fires, euphemistically called thermal events. You’ll see it on the news or social media, with dramatic footage of thick black smoke and flames consuming an entire trailer — and the tractor, if the driver can’t get the two units unhooked fast enough.
But determining how and where the fires start continues to frustrate the maintenance community, because the damage is often so severe, there’s little left pointing to a cause.
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Each fire could be a one-off incident — or it could be indicative of a pattern of events that happens to other fleets too, but fleets likely won’t be aware of what maintenance issues their competitors are experiencing. The American Trucking Associations’ Technology & Maintenance Council surveyed members to determine which, if any, contributing factors might show up as trends across a broad truck and trailer population. Nothing turned up that would give investigators something to focus on. Participation in the survey was light, we’re told, because fleets didn’t want to incriminate themselves by admitting to having had a fire, despite being promised anonymity.
WHAT IS KNOWN IS THAT TIRES DO NOT CAUSE THE FIRES.
Phil Arnold, a field engineer with Michelin North America, told attendees at TMC’s Fall Meeting that rubber compounds begin to break down when tire temperatures exceed 250 degrees. That’s when a blowout might occur resulting from the deterioration of the tire. It’s not until the rubber reaches temperatures of 500-550 that flammable vapors are emitted by the tire. If an ignition source is present, the rubber will start burning at 650-700 degrees. Spontaneous combustion won’t occur until 850-900 degrees. (The normal operating temperature range for tires is between 100 and 150 degrees).
“Tires contain a great deal of potential energy,” he said. “They are like high-grade coal when they start to burn, and they are very difficult to extinguish.”
When a tire comes apart while driving, it’s usually the result of under inflation. Heat generated within the sidewalls as the tire flexes weakens the steel belts in the casing while softening the rubber to the point it eventually breaks down and the tire blows apart. With tire fires, the heat source is the wheel end. Heat travels through the metal of the hub and through the wheel, where it comes into contact with the tire bead. Since the bead is built differently from the upper sidewall, the rubber there just gets hotter and hotter, until at some point it begins burning, rather than exploding as the casing might due to under inflation.
The source of that heat is friction from either a deteriorating wheel bearing or a dragging brake. In the bearing’s case, failure usually stems from lack of lubrication. There are multiple causes for dragging brakes.
