By Christopher W. Ferrone
Typically we think wheel bearing fires occur strictly as the result of traveling for long durations at highway speeds. Not so. In September I experienced a situation so innocuous I never thought it would ultimately cause a wheel bearing fire.
One of our bus drivers called in an accident, telling the dispatcher he had just hit a parked car and damaged the center hub cap on the tag axle, which in turn allowed most of the oil from the hub cap to leak out. The impact damaged the plastic sight glass portion of the hubcap, so some hub oil remained in the hub housing.
The dispatcher immediately sent another driver with more experience to view the damage and assess whether or not the bus could drive on its own to our garage a short distance away. We made the collective decision to have the driver return the bus to the garage.
Now let’s do the math. The American Sightseeing garage is approximately three miles from the Adler Planetarium where this accident occurred. The outside temperature on that day was 70 degrees. The driver drove at a very low rate of speed over the short three-mile trip. In fact when this incident was over, I checked our satellite data for that bus and found in that period of time the driver never exceeded 15 mph. Nonetheless, the bus was on fire when it arrived at the garage.
Fortunately the mechanics were able to contain the fire to the tire and wheel bearing area and extinguish the flames.
On another note, this summer we made a fleet-wide change over to 140W gear oil in all oil bath wheel bearings. Even with the remaining 140W hub oil trapped in the portion of the hubcap, and traveling at 15 mph for only three miles, the bus was still in danger. Once we made the repairs we analyzed the wheel bearings and found the damage to be the result of extreme heat generated by the loss of hub oil and by driving the vehicle to the garage.
The damage had blued and deeply scored both the inner and outer bearings along with their races. The disc brake pads for this axle cracked and became disconnected from the metal backing plate for the disc brake pad assembly — all signatures directly related to extreme heat.
This fire was solely the result of the accident and damage to the hubcap that led to the loss of hub oil. The two drivers who inspected the damage made the wrong decision to ease the bus back to the garage. Driving even the short distance at a low speed was enough to ignite the fire. But looking at each component in this situation separately, the potential harm to the equipment or danger of fire was not that apparent. Now we know that is just not the case.
In hindsight, with this bus well within the fleet rescue range of our company, the driver should have informed the dispatcher he did not have the knowledge to assess the damage. Instead a dispatcher should have sent a mechanic to the scene to make the assessment. With the bus in damaged condition, he would have called for one of our mechanics to go to the scene, replace the hub cap and refill it with oil, consequently preventing the bearings from ever overheating on the way back to the garage.
The lesson from this situation is that no one can make light of any type of accident and mechanical failure. Carefully evaluate every incident with the same level of vigilance. Never underestimate any road failure.
Evaluate every incident with an eye to detail, potential danger and with attention to safety for the vehicle and the passengers.
Christopher W. Ferrone is president of Americoach Systems, Inc., Glenview, IL, an engineering firm specializing in transportation technology, analysis and engineering safety.