When it comes to making the wheels turn on today's tractor-trailers, wheel bearings continue to play a critical yet unheralded role.
Stemco explains that there are four critical wheel-end components: seals, bearings, adjusting nuts and hubcaps. Bearings, however, must handle much of the friction generated as truck wheels turn, which is why Stemco says research into new materials for bearings is ongoing.
For example, the company's Platinum Matched Bearing sets are sought by fleets because they are easy to use, are of high quality, and can be added to Stemco's Platinum Performance System for extended warranty coverage — a plus for vehicles that are expected to last for a million miles.
“The importance of the truck to our modern economy is inestimable,” adds W.R. Timken, Jr., chairman of the Timken Company. The company believes innovation is critical to improving bearings, and thus trucking operations, so it is in the process of converting from tube to forged slugs for its high-volume bearings.
This program hit some glitches when the Kentucky plant had to deal with start-up problems related to forging suppliers, but Timken still believes that focusing on bearing innovation is a must.
“Innovation is a key driver of success because our products must manage a critical physical phenomenon — friction,” says Salvatore Miraglia, Timken's senior vp-technology.
According to Federal-Mogul, however, managing friction isn't related solely to the composition of wheel bearings. The company points out that too help ensure proper bearing function, it is important to mount a wheel bearing with the proper fit. Generally, rotating members are installed with tight or press fits (known as interference fits) while stationary components can be either tight- or loose-fitted, depending on the application, Federal-Mogul says.
In heavy-duty truck wheel ends, the cup, or outer race, has a tight fit, while the cone, or inner race, typically has a loose fit. Tight fits keep the rotating bearing components (cup) from turning in the housing (hub). When the bearing cup turns in the hub, it wears away the hub inside diameter (ID) and backing shoulder, creating debris particles. This can lead to seal wear and leaks, weakening of the hub, and wear particle deposits that oxidize wheel bearing lubricant, Federal-Mogul warns.
If the hub ID is warn away at the bearing locations) the hub bore increases, resulting in an out-of-tolerance hub, the company notes. For this reason, the hub ID should be measured when the bearings are removed for service. If the hub ID is larger than the OEM's specification, the hub should be replaced.
Loose fits are used on the stationary component (cone), primarily to accommodate ease of assembly and bearing adjustment. The loose fit allows the cone to easily slide along the shaft as the nut is tightened during bearing adjustment.
However, excessively loose fits should be avoided or misalignment may occur within the wheel-end. This could result in bearings and seals being worn out prematurely.
Federal-Mogul explains that the spindle diameter should also be measured during wheel-end service to make sure there' not too much spindle wear It's important to inspect the top and bottom of the spindle because the under side may show wear when the top half appears new. If the spindle diameter is out of the original manufacturing specs, the spindle should be replaced.
Timken adds that many mechanics are not aware of the importance of setting the wheel bearing end-play to the proper specifications. Consequently, many bearings are adjusted too loosely or too tightly, causing them to fail prematurely.
Timken notes that poor bearing adjustment can cause downtime and rack up repair expense. While the most obvious result is shortened bearing life, poor adjustment affects much more than the bearing. It can also contribute to decreased operation and service life of the spindle, wheel seal, brake components and tires.
In addition, ABS and traction-control systems using wheel-end sensing require precision bearing adjustment to function properly.
All in all, improper wheel bearing adjustments can be a costly problem for fleets.
FOR MORE INFORMATION, CIRCLE NUMBER ON REPLY CARD:
Federal-Mogul 310
Southfield, MI Stemco 311
Longview, TX
Timken Company 312
Chicago, IL
