Heavy Duty Trucking, September 2019

HDT SEPTEMBER 2019 76 WWW TRUCKINGINFO COM illions of dollars worth of equipment and cargo goes up in smoke every year because of wheel end fires euphemistically called thermal events Youll 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 cant 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 theres little left pointing to a cause 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 wont 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 were told because fleets didnt 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 TMCs Fall Meeting that rubber compounds begin to break down when tire temperatures exceed 250 degrees Thats when a blowout might occur resulting from the deterioration of the tire Its 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 wont 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 its usually the result of underinflation 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 underinflation The source of that heat is friction from either a deteriorating wheel bearing or a dragging brake In the bearings case failure Preventing wheel end fires Wheel end fires often make the news with dramatic clouds of thick black smoke There are dozens of ways to set a wheel end on fire Most of them are completely avoidable usually stems from lack of lubrication There are multiple causes for dragging brakes Wheel bearings The axle hub interface is a potential source of thermal saturation High quality wheel bearings properly installed properly lubricated and operated according to the product specifications seldom fail on their own Unfortunately much can go wrong due to neglect oversight or the best intentions gone wrong Wheel bearing related fires can almost always be traced back to lack of lubrication whether its 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 By Jim Park Equipment Editor jpark@ truckinginfo com TRAILERS BODIES M PHOTO GETTYIMAGES COM THEGIFT777