While Saia reported that its incidence of vehicle fires increased dramatically after it installed trailer skirts, it found that while the skirts did not cause the problem, they aggravated a bad situation by containing the heat.
Old Dominion reported that most of its fires were due to dragging brakes or brakes that were adjusted too tightly. It found the heat generated in these instances causes the inside tire to blow first. What was surprising to me was most of these fires occurred within 30 to 50 miles of the start of the trip.
According to Michelin, tires usually provide the fuel needed for vehicle fires.
At 300 degrees Fahrenheit, the tire comes apart and blows out; at 500 F to 550 F, flammable vapors are generated; and at 650 F to 700 F, the tire will burn when provided with an ignition source, which comes from the sparks generated by the wheel dragging on the pavement.
Usually the remaining rubber-coated body plies or belt strands catch on fire first. However, in cases where the brakes are dragging or bearings freeze, heat generated by friction travels through the metal of the hub and through the wheel(s), where it comes in contact with the tire bead(s). The axle end can heat up to over 850 F to 900 F, at which point tires may combust.
The National Transportation Safety Board conducted tests several years ago and found that wheel-end components heated by frozen bearings can reach temperatures of 1,300 F. Even when tire fires are suppressed in these conditions, they will reignite over and over again because of the super heat created by the failed wheel-end components.
One would think that drivers would be able to feel when one or two brakes are dragging, but more often than not, they mistake the drag for a load that is heavier than usual, a headwind or hilly terrain that makes the engine work a little harder. High-torque engine power and weight of the vehicle also can hide any handling difference such as unusual pull or deceleration.
As proof of this, I have been working with a tanker fleet that recently had an issue of a dragging brake on one of its tractors. Fortunately, the tire-pressure-monitoring system on the tractor alerted the fleet to the hot axle end. Dispatch alerted the driver to the buildup of excessive heat, but he reported that he didn't notice anything wrong and did not feel the brakes dragging.
When the tractor returned to the yard at the end of the day, maintenance personnel examined the vehicle and found that the right side brake chamber was leaking air, which caused both right-side drive-axle brakes to drag and generate excessive heat. Had garage personnel not been alerted to this situation, a thermal event could have resulted with devastating consequences since the tanker was hauling gasoline.
Since drivers usually are not aware of a buildup of excessive heat in their trucks' axle ends, it's critical that vehicle and tire technicians are vigilant when it comes to inspecting wheel bearings and brakes. High quality wheel bearings that are properly installed and lubricated seldom fail on their own.
Wheel bearing-related fires almost always are caused by lack of lubrication, whether it's lube loss due to a seal failure or contamination of the oil by water or debris that has gotten into the bearing well and damaged the seal and caused a leak. That's why frequent inspections of the hub cap is vital.
Proper installation of the wheel bearings also is key. Over-tightening the bearing (excessive pre-load) can limit the amount of lube on the bearing which can generate heat.
Conversely, excessive end-play can affect seal alignment and shorten seal life, which then allows debris and water to contaminate the lubricant and cause a leak.
There are many fleets that use grease to lubricate their wheel bearings. Some people think that packing the cavity full of grease is better than partially filling it, but if the cavity is overfilled with grease past the recommended level, the bearing can overheat since there is no ability to dissipate the heat.
Bearings should normally run between 160 F and 175 F. They begin to run into trouble at 250 F and higher.
When lubricant is completely depleted, temperatures continue to rise until the wheel-end assembly either fails completely and separates from the truck, or it heats the surrounding materials to a point where the tire catches fire, which is around 650 F and higher.
There are a lot of different reasons a brake can drag and cause a tire fire, including: driving with the parking brake applied; brakes adjusted too tightly; drivers not turning on air valves; a failed parking brake chamber diaphragm not compressing the spring completely; brakes not releasing after a brake application; corrosion-related binding of the camshaft or disc brake caliper internal parts or sliding system; malfunctioning slack adjusters; or excessive brake lining swell.
Really, any moving brake part could be suspect from valves that do not fully exhaust application pressure due to fouling or corrosion, to broken parking brake springs that inhibit full retraction of the push rod, or lack of S-cam or slack adjuster lubrication.
A false charge also can occur when a pneumatic valve in the brake system gets plugged, air bleeds off and the brakes come on. This phenomenon usually occurs on double and triple configurations where dollies can be the source of the problem.
To prevent thermal events, proper preventive maintenance and inspection of all wheel-end components is critical. If your company provides commercial vehicle maintenance service, vehicle technicians should check oil lubricated hubs every time a trailer enters your shop. They should always look for signs of lubricant leakage on the axle ends.
The lube should be checked for signs of water contamination which gives it a milky appearance, and the oil should be sniffed to see if it smells burnt and has been subjected to high temperatures. Also, the condition of the sight glass should be checked. If it is cloudy and the oil can't be checked through it, it's no longer doing its job and should be replaced.
Wheels on trailers with grease-filled hubs where the lubricant is not visible should be jacked up, rotated and checked for signs of rough rotation, stiffness or looseness. If the hub cap is removed, technicians should verify the lube level is correct (not over-filled) and check for contamination and corrosion on the outer bearing.
Tire technicians have to be on their toes, too. When changing a tire, whether it is on the side of the road, in the shop or at a customer's location, they always should approach an axle end that appears to be hot very cautiously.
Not only should the tires on the vehicle be checked, but the oil level in the hub-sight glass should be checked, too. If the hub won't turn or is hot to the touch, the vehicle bearings have seized. If the hub is relatively cool, but the wheels are hot, it's probably a case of a seized or dragging brake.
Technicians must be very careful in servicing this equipment. Changing just the tire may get the vehicle back on the road but only for a short time until the real cause of the problem sets it ablaze.
Also, technicians should protect themselves.
Excessive heat can change the metallurgy of both steel and aluminum, which can cause wheels to lose strength and change dimension. The bead seat and rim flange area of wheels exposed to excessive heat actually can get smaller and change contour.
As a result, the flange area may no longer have the ability to hold the tire bead on the wheel under pressure, and the tire can blow off the wheel during or after inflation with explosive force.
If it is suspected that excessive axle-end temperatures were experienced, the wheel should be inspected carefully for signs of charring or discoloration caused by heat and a rim flange that has a scalloped appearance. The tire beads may be burned or deformed or have left rubber crystallization on the rim flange.
When the wheel is removed from over the brake drum, and the brake drum has an orange iron oxide color on it, that's a definite sign that the brake drum has been exposed to excessive heat, and probably the wheel has been, too. Any time a tire is suspected of having run overloaded or underinflated for an extended period of time, special attention should be paid to inspecting the wheel for heat damage as well.
I wish I could tell you that these thermal events are a passing aberration, but I think just the opposite is true.
With fleets feeling pressure to deliver freight faster, the shortage of commercial vehicle technicians and shrinking profit margins, the opportunities to conduct thorough vehicle maintenance inspections may become more limited.
We will continue to see more incidents of frozen wheel bearings, dragging brakes and flat tires that produce scorching temperatures which result in thick black smoke and flames.
However, you can play an integral part in preventing these tragedies.
Peggy can be reached via e-mail at [email protected]. Her previous columns are available at www.tirebusiness.com
