Meeting the challenge of wide single tires

Most of the truck and trailer wheel ends in service today were developed several decades ago. At that time, engineers designed wheel bearings that appropriately carried the loading distribution created by dual tires.

For many decades, wheel bearings carried the loads they were expected to carry with minimal issues, as long as properly installed. An improperly adjusted wheel bearing usually won’t live as long, but some bearings are so well designed that their robustness could help compensate for some less than precise wheel end adjustment practices.

Most new-generation wide single tires are mounted on a wheel with a two-inch outset. This means the middle of the tread footprint on the wide single tire is approximately 2 inches further outboard than the middle on a set of duals. Several additional inches of the brake drum are visible, exposed on the inboard side of the wide single tire.

This tire outset affects the entire wheel end system on which they are mounted, and presents a series of challenges that make proper installation, maintenance and selection of quality products a priority.

Technical Challenges

To capture the commercial benefits of wide single tires, it is important to address the technical challenges.

Here’s why: Changing the centerline location of any tire, not just wide singles, affects how the loads are distributed on the wheel end and all of the components inside the wheel end carrying those loads.

One of the components most affected by the tire outset or shift in centerline created by the wide single tire is the bearing.

Many bearings look nearly identical, but there are significant differences in manufacturing and quality that become increasingly more important the harder the bearing works. The differences most affecting performance are raceway profiles, surface finish and steel.

Bearing Profiles

Bearing rollers and raceways are not flat. They actually have a profile. The height and consistency of these profiles are designed to distribute the load across the contact surfaces, or races.

Precisely manufactured profiles use a greater amount of the raceway to increase a bearing’s load-carrying capacity and reduce stress levels in the bearing. The two-inch outset of a wide single tire places more of the wheel end load on the outer bearing.

That means the bearing profile is especially critical to evenly distributing this increased load across as much of the raceway as possible in order to avoid premature spalling.

These profiles also make it imperative to avoid mixing a cone and cup from different manufacturers. Industry bearing standards only stipulate bore, outer diameter, overall bearing width and cup angle of a tapered roller bearing. The angle of contact for cones and rollers varies by brand.

Even though the cones and cups of different manufacturers may appear to fit together, they normally don’t, and even the most subtle profile differences will limit the performance and life of the bearing.

Bearing Surface

Bearing surface finish is another crucial factor in bearing quality. Smoother surface finishes allow better lubricant flow, helping to prevent metal-to-metal contact between the rollers and raceways.

Even though nearly all bearings appear to be smooth, there are differing levels of smoothness. Under a microscope, surface finish has asperities that resemble the peaks and valleys of a mountain range.

Rougher surfaces have higher peaks and make it difficult for the lubricant to adequately keep these peaks separated. Contact between asperity peaks will cause peeling and shorten bearing life.

Once again, the outset from a wide single tire places more of the wheel end load on the outer bearing. Higher loads work to push the rollers and raceways together with more force, making it more difficult for the lubricant to maintain the same separation achieved with the traditional loads.

Steel Quality

Steel quality - a combination of chemistry, purity and properties - is also important to bearing quality.

Clean steel is the essential foundation upon which sound profiles and surface finishes rely. Like a basement’s concrete foundation, nobody wants cracks.

Soundness hinges on the correct mixture of materials. The steel used in bearings must be carefully manufactured and heat-treated to give it the purity, hardness and durability required to handle the many stresses it faces.

Imperfections in steel reduce the bearing’s load-carrying capacity. This can lead the bearing surface to spall prematurely.

The heavier the building, the more important it is that the foundation is solid. The same is true of bearings. The heavier the loads, the more important the steel quality becomes.

The need for consistent attributes, precision manufacturing, proper lubrication and adjustment of more heavily loaded bearings make it vital to sustain the tight tolerances. For performance and safety, these considerations are especially important to fleets running wide single tires.

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