The wheel bearing on a vehicle allows rotating parts to turn freely without rubbing against their rigid supports. Each of the wheels on a vehicle will have at least one bearing each, some will have two.
How a Wheel Bearing Works
Wheel bearings work by allowing balls or cylinders to spin between two rigid items. Some bearings are ìsealedî while others are not, but all work on the same principle. When the rotating item spins, it turns the balls in the bearing, which sandwich between the spinning item and the rigid support (usually an outer shaft pipe or eye).
The spinning bearings are lubricated, often with petroleum-based grease such as axle grease or bearing grease. The lubricant keeps the bearings from rubbing directly against the metal on the spinning shaft or its support by providing a thin barrier between them.
Wheel bearings eventually require lubrication, maintenance, or replacement.
When To Lubricate Bearings
Most late-model vehicles have sealed bearings, which require no lubrication or maintenance. These are closed, inaccessible bearings that have grease inside a sealed housing. Older or non-sealed bearings will require regular maintenance and most mechanics recommend that they be done every other year or every 30,000 miles.
Some vehicles have grease points - small ports that a grease gun can be attached to. During regular service or tune ups, these are often filled. This does not replace re-packing (removing and re-greasing) the bearings, however.
How To Pack Bearings
Packing or greasing bearings is not difficult. On most vehicles, removing the tire and wheel exposes the hub (center of the axle). This will usually have a cap, though on some models it will be a switch (on 4WD and some AWD models). Refer to your manual for safe removal of these.
Removing the cap reveals the axle nut, which will require heavy torque to remove and possibly a special tool, depending on your vehicle make/model. Removing this will expose the axle's end and the bearing(s) surrounding it, likely embedded in a thick wad of grease.
Remove the bearings by carefully grabbing them with needle nose pliers, a strong magnetized rod, or other means. Take care not to scratch or bend the bearings. Using shop rags, clean as much of the old grease out of the axle housing as you can and thoroughly clean the bearings off. Do not use water or a parts washer to clean bearings. Inspect the bearing for undue wear or signs of weakness. Spin it a few times to listen for squeaking.
Once clean, wad a large amount of grease in the palm of your hand and thoroughly coat the bearing with it. Replace the bearing in the housing and stuff as full as possible with more grease. Replace the nut to recommended torque specs, then the cap and wheel.
Repeat for all wheels on the vehicle and consult your manual for more possible bearing locations.