Bearings at the wheel end are out of sight, but they're an important part of hub and halfshaft service
For most technicians, wheel-bearing service was one of the first procedures learned in a school auto shop class, or in the driveway. It was simple, easy, and unless you left the cotter pin out, unlikely to cause trouble. Front-wheel and all-wheel drive, as well as independent rear suspension, however, mean that many vehicles on the hoist these days don’t have a repack procedure. That doesn’t mean, however, that wheel-end bearings can be ignored, since wear or damage is not only a customer service issue, but also one that the customer won’t differentiate from a defective, newly installed halfshaft or brake rotor.
Cartridges predominate for FWD
Why do manufacturers use packaged bearings? Cost and ease of assembly are always drivers, but from a service perspective, a major advantage is that settings are locked in the manufacturing process, removing guesswork. That “permanent” setting also allows manufacturers to extend the allowable load zone for the system, permitting smaller bearings than would be possible if maladjustment was a service possibility. Longer seal life is an added benefit. A disadvantage, however, is the inability to easily inspect or lubricate the cartridges, meaning a potentially expensive repair, especially if the job is packaged with CV joint or brake service.
Just because you can’t adjust it, however, doesn’t mean that it shouldn’t be inspected, and the dial indicator is the correct tool for the job. According to Jerry Landry, service engineer for Timken: “To measure bearing setting, it’s a standard procedure to mount a dial indicator on the hub, and pull or oscillate until the indicator stops moving. Then you pull in the opposite direction until the indicator stops again. If the bearing has preload, meaning a negative value, you can’t check it with an indicator. The indicator only moves with axial movement of the hub relative to the spindle.”
Does that mean that an indicator reading on a preloaded bearing is showing wear or damage? Not necessarily, says Landry: “If you were 100 percent sure that you could check the same bearing in the same way as new, it would indicate wear. Hub runout could be an issue. That’s why the dial indicator stem needs to be attached to the hub, with the indicator sensing off the exact centre of the spindle. We recommend indicating from a small ball centred in the spindle. As you rotate the hub, the indicator should not move. You should not be on the flat face of the hub, which hasn’t been ground. You have to measure carefully.”
How wheel bearing lubrication works
Good installation practices mean little if a new bearing isn’t properly lubricated. Prelubricated cartridge bearings need little more than clean, careful assembly, but in rear-wheel drive cars and light trucks, the front wheels still need a periodic repack. Grease may seem like the simplest of lubricants, but the way it works inside the hub is far from trivial. “The grease doesn’t actually do anything in the (hub) environment”, declares Jim Lackowski, product manager, automotive aftermarket for the Chicago Rawhide division of SKF: “What actually lubricates the rollers is a leachate from the grease. That’s why you need a good quality EP grease that can leach compounds to where the bearing needs it”, says Lackowski, who adds, “the grease will migrate, then stay put. Synthetics can also migrate, become a gel on cooling, and then migrate again in service. Petroleum-based greases don’t, so you need to rely on what leaches out of the compound. There are a lot of good qualities to synthetics. They reduce the amount of smearing on dry load startup and during break-in. After that the advantage is more protective. If you over lubricate the bearing, however, the grease can’t move in its cavity, resulting in “plowing” as the roller attempts to move though the semi-solid material. You can end up with a high amount of friction, causing a failure due to heat.”
Keeping lube in and dirt out
Keeping grease inside wheel bearings and excluding moisture and dirt is a fundamental problem at the wheel end, one that has been addressed in the past with leather, felt, rubber and modern polymers. Front-wheel-drive and independent rear suspension on rear-wheel-drive vehicles, however, add a rotating shaft as another potential entry point for contaminants. That disadvantage is minimized in two ways. One is by the rotating action of the shaft itself, which acts to fling dirt and debris radially outward and away from the seal. The other is the use of extensions on the back of the seals called “conical lips”, which interfere lightly with the spinning shaft to keep contaminants out. The close tolerances also act as a “labyrinth”, lengthening the path that contaminants must take to reach critical components.
Seal installation is almost too simple to describe, but an important consideration is that “conventional” shaft seals lack the conical lips that are important to exclude dirt, yet may physically fit the hub and the shaft. Installers should be careful with “equivalent to” interchanges to make sure that the seal has the full OE protection, regardless of a good fit, by comparing the old seal to the replacement. Water is notoriously difficult to exclude from many applications such as four-wheel-drives and snowmobile/boat trailers, so a “marine” grease may help segregate the water from the lube. Before experimenting with greases in a light truck application, however, replace seals and be sure that the lube meets the necessary NGLI and EP specifications for highway use.
Ball or tapered roller designs: Which is better?
Traditionally, taper roller designs are favourites for load capacity and longevity, but in automotive and light truck service, balls are not necessarily a disadvantage.
According to Chicago Rawhide’s Jim Lackowski, “it’s basically the engineer’s preference. Today’s ball bearings are angular contact types, with two rollers opposing one another allowing them to take a much greater load. Tapered roller bearings are very much the same. It could be a year-long debate about which is better, but the truth of the matter is that the load of the vehicle is below what the products are rated for, so whether you install ball or tapered roller bearing, survivability isn’t dependent on the corner spring weight of the vehicle. The designs of the two types are relatively unimportant. Most large Class-A truck hubs use tapered bearings, while most mid-weight passenger cars use double-row ball bearings in their cartridges. At extreme loads, the tapered roller bearing can take a larger load, but in automotive applications it’s moot.”
Nothing lasts forever, but considering the environment in which they operate, auto and light truck wheel bearings may be the hardest-wearing rotating parts on the vehicle. While periodic repack has become essentially a rear-wheel-drive only procedure, inspection and replacement still needs a smart approach. It’s the high technology inside that makes wheel bearings forgotten, but not gone.
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