Wheel-End Fires

Wheel Bearings

Wheel-bearing-related fires can almost always be traced back to lack of lubrication, whether it’s lube loss due to a seal failure or contamination related to water ingress, or debris in the bearing well damaging the seal and causing a leak.

Any situation that might increase friction between the axle spindle and the hub needs to be addressed in the installation and maintenance of the wheel-end assembly.

“Over-tightening (excessive preload) the bearing can limit the lube film, which will generate heat, though perhaps not to the level where there’s a risk of a fire,” says Ean Dickerhoof, an application engineer for mobile on-highway products at Timken. “Conversely, excessive end play can affect seal alignment compromising seal life, which can allow debris to enter the system or the lubricant to leak out.”

DRIVERS SHOULD BE TRAINED TO INSPECT HUB CAPS FOR PROPER CONDITION. THIS ONE IS OVER-FILLED WITH LUBRICANT.

Obviously, seal condition and lube levels and condition need to be inspected at regular intervals, Dickerhoof adds, being careful to maintain the proper lube level.

“Some people think packing the cavity full of grease is better than partially filling it,” says Michael Gromosiak, Timken chief application engineer for mobile products. “There’s a certain percentage fill that’s recommended. If you overfill the cavity you can overheat the bearing, because there’s no heat dissipation.”

Timken says bearings ought to run somewhere in the 160-175 degree range. Running at 250 and higher is getting into risky territory. In the case of complete lube depletion, temps will 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.

Dragging Brakes

“There are a lot of different reasons a brake can drag to a point of a thermal event occurring,” says Joseph Kay, Meritor director of brake engineering. “In general, it takes a rather large force applied to the brake shoes on a drum brake or brake pads in a disc brake to generate substantial rubbing forces that cause the brake system to get hotter and hotter as the vehicle is driven.”

Kay points to several potential sources, including driving with the parking brake applied, a failed parking chamber diaphragm not compressing the parking 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.

“In most cases the driver will not be able to detect one or maybe two brakes that are dragging because of the engine power and weight of the vehicle,” Kay says. “This is where the driver needs to be aware of excessive smoking from the brakes or any handling differences, such as unusual pull or deceleration.”

Literally 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, says Keith McComsey, director of marketing and customer solutions – wheel-end at Bendix Spicer Foundation Brake. “Proper preventive maintenance and inspection of all wheel-end components is critical, as is the proper specification and condition of brake linings per the OEM literature.”

As for TMC’s survey results, they revealed 88% of reported fires occurred on vehicles equipped with drum brakes and self-adjusting brake adjusters, while 25% of respondents reported fires on disc-brake-equipped vehicles.

Drivers are the last line of defense against wheel-end fires, but they can’t be expected to notice everything or even know what to look for without proper training in wheel-end inspection.

Drivers should touch and smell the wheel-hub area for excessive temperatures and tell-tale odors of overheated brakes on wheel-ends. On top of that, drivers should “GET OUT AND LOOK” if something doesn’t feel right. A dragging brake can be difficult to detect from the driver’s seat.

Drivers won’t always feel the brake dragging and its not uncommon for a driver to mistake a dragging brake for a heavier-than-usual load, a headwind, or even terrain that seems to make the engine work a little harder.”

Determining the cause of a wheel-end fire might not always be possible, but you can certainly reduce the chances by proper pre-trip procedures and common sense.