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20th Annual Technical Forum: Wheel Bearings


9/6/2012

 

Q: Do wheel bearings require any maintenance?

A. The wheel bearing cartridges and hub assemblies on late-model vehicles do not require any maintenance. They are sealed for life and cannot be lubricated.

However, there are many older vehicles on the road that may still require wheel bearing maintenance. On these applications, the wheel bearings are not sealed and do require periodic inspection, cleaning and repacking with grease. The recommended service interval is typically every 30,000 to 50,000 miles (or more frequently if the vehicle is driven off-road or through hub deep water). The wheel bearings also should be inspected and lubricated when the brakes are serviced, or if a wheel bearing problem is suspected (noise, roughness or looseness).

 Serviceable wheel bearings should be disassembled, cleaned and allowed to air dry. Roller bearings should not be force-dried with compressed air as spinning the bearings with no lubrication risks damaging them. The bearings should then be visually inspected for hairline cracks, flat spots, dimples, flaking, wear or damage. If a bearing race, roller or cage is bad, it has reached the end of the road and must be replaced.
Note: The entire wheel bearing (inner and outer races, and the ball or roller bearings and cage) must be replaced as a complete assembly. Parts are precision-matched and should never be mixed with old parts or parts from another bearing.

Wheel bearings should only be lubricated with high-temperature wheel bearing grease, never ordinary chassis grease. A No. 2 NLGI lithium-based grease or a synthetic wheel bearing grease should be used. The hub cavity needs to be about two-thirds full to leave some air space for thermal expansion. Also, the old grease seals in the hub should be discarded and replaced with new ones.

After the bearings have been installed, bearing end play must be adjusted to specifications by tightening and then backing off slightly the spindle nut. Most older cars and trucks allow a small amount of end play, but some require a slight preload. The grease cap also must fit tightly to keep water and contaminants out of the hub.

Q. What’s the difference between tapered roller bearings and ball bearings?
A. Tapered bearings have cylindrical rollers between the inner and outer race. The rollers are held in place by a steel or phenolic cage. The rollers are larger on one end than the other and rotate in a cone-shaped path. This allows the bearing to handle sideways loads and well as vertical loads. The larger surface area of tapered bearings allows them to support greater loads than ball bearings, which is why tapered bearings are often used in larger, heavier vehicles. Tapered roller bearings are also used in most older vehicles with serviceable wheel bearings.

Double row ball bearings have round steel balls between the inner and outer race. Like roller bearings, the balls are held in place by a steel cage. Ball bearings are less expensive to manufacture than tapered bearings, and produce less friction, which helps improve fuel economy. This type of wheel bearing is often used the sealed hub assemblies on many late model cars. But ball bearing wheel bearings have a smaller surface contact area than roller bearings and can’t handle severe side loads as well as tapered bearings. Consequently, the wheel bearings may not stand up well to the forces that are generated by hard cornering in applications such as police cars or performance cars that are driven aggressively.













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