When proper installation procedure is not followed, commercial vehicle wheel bearings can be subject to a multitude of maintenance concerns ranging from minor challenges—irregular tire wear, noise and vibration—to severe problems such as wheel separation or, in rare cases, wheel fires. And with drivers often seated far away from developing issues, these worries tend to go undetected until it is too late.
The good news is that fleets can take proactive measures to help avoid complications down the road. This article covers proper installation and maintenance procedures for tapered roller bearing (TRB) adjustment for steer (front) nondrive, drive, and trailer axle assemblies using conventional double-nut or single-nut systems on Class 6, 7 and 8 trucks.
Understanding preload and endplay
Preventing wheel bearing problems starts by understanding the root causes of common issues, and the first thing to know is that bearing preload and endplay are opposites.
Preload is a thrust load that is applied to bearings to eliminate all axial clearance while keeping rollers aligned and operating as they should. Typically, preload is accomplished using a spring mechanism to introduce the required compressive forces and/or by incorporating a spacer to control the clearance between bearing sets. Bearings that use an adjustment nut and factory-installed spacer are commonly called preset or pre-adjusted bearings. Timken develops preset bearing assemblies and different setting techniques, giving installers a range of alternative mechanisms that reduce the risk of user error compared to manual bearing adjustment.
Commonly, problems with bearings will occur due to excessive preload, where overtightening of the adjustment nut (axle spindle nut) during installation leads to higher friction between the rollers and raceway, thereby generating excessive heat in the bearing, degrading the lubricant and triggering a range of damage modes that all equate to costly repairs.
Endplay, meanwhile, is the amount of axial or end-to-end movement in a wheel hub due to clearance in the bearings. What this means is that preload and endplay cannot exist simultaneously—the conditions are mutually exclusive (that is, a bearing having zero axial clearance offers zero endplay).
Excessive endplay, where the fit is too loose, can create misalignment in the bearing by causing a rocking motion that disrupts the even distribution of force along the rollers. Here again, damage can mean downtime for busy fleets. What starts out as minor vibrations can soon evolve into wheel wobble, seal leakage and ABS and/or other braking impairments. Excessive endplay can cause so much movement in the wheel that it can wear the seal out and shorten the life of the bearings due to the edge stresses created by misalignment.
The key is finding the sweet spot, or clearance measurement for optimal bearing performance and longevity, and to avoid preload whenever possible. With preload, there is a much greater possibility for bearing failure from increased heat generation, whereas an endplay situation introduces a subtle clearance between the rolling elements and races that can mean longer life when the bearing system is set correctly.
Ensuring proper endplay during installation
Compared to preload installation, endplay is preferred because not only can it lengthen the life of the bearing, it is also easily verifiable by the installer who can measure to determine the actual movement of the hub assembly.
The eight-step procedure described in Recommended Practice (RP) 618B from the American Trucking Associations’ Technology & Maintenance Council offers service technicians a way to achieve a verifiable wheel bearing endplay of 0.001-0.005 in. for steer, drive and trailer axle assemblies using conventional double-nut or single-nut adjustment systems. By carefully following the RP and verifying their work with the aid of a dial indicator, installers can ensure that bearings are tightened to the proper tolerances.
Verification is the most important and often most overlooked step involved. It’s vital to have a repeatable process and to hold everyone accountable to that process. The aim is to achieve a wheel bearing endplay of .001 to .005 inch. For comparison, that’s a range from the width of a human hair to the width of a sheet of office paper. That sort of precision requires the full eight-step procedure outlined in 618B.
Eliminating routine maintenance errors
Minding the details is the best way to avoid problems when setting wheel bearings or performing maintenance of any kind.
It is critical to closely follow manufacturers’ guidelines when applying torque to bearings that can look identical but have different requirements. For example, if you’re using a single-piece locknut and the bearing maker says, “torque to 300 ft./lbs.,” that’s exactly what you should do. What gets overlooked is how a minor miscalculation can lead to big problems as the miles add up. Always take time to verify the correct setting.
The following are additional recommended precautions that help ensure a successful installation:
- Handling. Take great care not to drop or damage the bearing assembly—it has been precision-manufactured to ultra-high tolerances (often to millionths of an inch).
- Cleaning. Never spin bearings with air when cleaning them (or for any purpose) as this can cause the cage to deflect, allowing rollers to be forcefully expelled from the cone assembly giving rise to the risk of serious injury.
- Tools. Never use an impact wrench when adjusting lug nuts or bolts during the setting procedure.
- Extensions. Use as few (or no) connections between the torque wrench and adjusting nut, as more connections make it harder to achieve an accurate reading.
- Lubrication. Employ the same type of lubricant on the bearings as on the axle assembly, since packing a bearing with grease can impede oil flow from the drive axle sump, for instance.
- Contamination. Take extra caution to prevent moisture and debris from contacting bearings during installation or when storing bearings.
Consider what’s at stake
Wheel bearings are subject to varying speed and load demands while handling high and low temperatures, dust and debris, moisture and other harsh environmental conditions. When properly adjusted, however, they offer many positive rewards to fleet operators: improving fuel economy, preventing excessive tire wear, and reducing the potential for wheel separation, to name a few. Furthermore, bearing seal reliability can benefit greatly when proper adjustment procedure is followed.
Regular servicing is another critical component to achieving the best results. In addition to the recommendations described in this article, visual inspections on the road are important. Many fleets have a prescribed time when they will ask drivers to inspect the backs of their rims to look for leakage, an indicator that maintenance is required. Any leakage in and around the hubcaps will cause a D.O.T. inspector to investigate further, often resulting in a ticket.
A final consideration is the quality of your wheel bearings. Remember that value goes beyond price. Compared to a stronger bearing solution, a roadside breakdown can be far more costly to your business and brand. Be sure to choose a product that will deliver for both your drivers and customers.
This article was contributed by Rick Domin, principal training and product support manager, and Rick Formisani, national fleets account manager for The Timken Co.