Keep 'Em Rolling
Wheels may make the world go around, but any major problems in the wheel-end system can make a good day turn bad. Proper wheel-end maintenance is vital to keeping you from being stranded on the side of the road and late for a delivery.
One of the toughest things to do with the wheel as it turns is to hold it snugly in place on a vehicle without generating a lot of friction. This is the job of the wheel bearing. Tapered roller bearings have a cone shape, and they roll in cone-shaped bearing cups. This allows them to carry vertical loads and handle end thrust. A wheel-bearing assembly consists of two tapered roller bearings situated opposite one another and turned in opposite directions. While they work together in supporting the wheel and handling the friction, they work in opposite directions in handling the end thrust. One of the two bearings takes the end thrust generated when the wheel pushes toward the chassis in corners, and the other takes thrust when the wheel pushes away from the chassis. The distance the wheel can be shifted from side to side, from the point at which one bearing begins to resist the end thrust until the other does, is called “end-play.”
Adjusting the bearing assembly involves controlling how far apart the two bearings are when they are assembled. If the two bearings are forced closer and closer together they will eventually reach the point where both are pressed tightly against the bearing cup that they roll against. Such a situation would eliminate end-play and produce “pre-load,” a situation where a slight tension would be maintained on the bearings.
Under controlled manufacturing conditions, where bearing adjustment can be performed with absolute precision, the bearing parts may be adjusted up to the precise point that all the end-play is eliminated and a small amount of tension is maintained. This is considered an ideal situation. But it’s considered impractical to achieve on manually adjusted systems.
A wheel-end consists of the two bearings and their grease or oil, together with the seals that hold the lube in. Not only are wheel-ends difficult to maintain, but the consequences of failing to keep them in proper working order can be severe. Under extreme neglect and abuse, wheels may fall off while a truck is on the highway.
Wheel-end maintenance was made a little easier during the 1990s as more trucks and trailers were manufactured with outboard brake drums. These are drums bolted to the outside of a flange on the hub that contains the wheel-end. They can be unbolted and pulled off the hub without disturbing the wheel-end components. After outboard drums were introduced, only wheel-ends needed to be accessed to ensure proper lubrication, a big savings for fleets where brake jobs must be done frequently.
But you can’t just ignore your long-life wheel systems. ArvinMeritor experts stress that “Maintenance-free doesn’t mean inspection-free.”
An annual inspection replaces routine adjustment and “repacking” or regreasing. With all long-life systems, safely lift each wheel-end well off the road. Slowly rotate the wheel to check for any roughness in the bearing. Also, firmly tug the wheel in and out to check for significant end-play. On greased systems, remove the hubcap to check for grease leaks. If the bearing operates smoothly and without end-play and there’s no sign of leakage, you can run it for another year. If there is any question in your mind, get it checked by a technician to locate excessive end-play.
Steve Slesinski, product manager for drive axles and wheel-ends at Dana Corporation’s Commercial Vehicle Systems Division, recommend frequent inspection of conventional systems.
The fastening system for the Meritor Triad wheel-end system is used with a relatively simple and inexpensive traditional bearing and seal system that can easily be disassembled and serviced. Because of a patented spindle nut installation procedure, it works at an unusually tight tolerance and offers longer life than typical conventional wheel-ends.
Annual disassembly, repacking, replacement of seals, and readjustment is “the ideal,” in the view of the ArvinMeritor experts. They say that if you do brake jobs at 100,000-mile intervals, this wheel operation should be performed at least every second brake job, or 200,000 miles. You should consult the instruction book and go by any manufacturer-specified interval if your situation is different. Of course, a frequent check for leaks is essential, especially when using oil rather than grease. Leaks have to be fixed before oil leaks onto brakes or ruins the bearings. If you are routinely adding oil, replace the seals.
Whenever a wheel-end must be disassembled for any reason (including just to do a brake job if you have inboard drums) it needs to be cleaned, repacked with grease and given new seals.