Purpose of a bearing
All bearings support loads generated by either weight or torque while allowing parts to rotate with minimal friction. Friction reduction is important for bearings as it increases efficiency and extends the life of the bearing. Bearings reduce friction by virtue of their shape, material they are made of, usage of a lubricant, or some combination of those three.
Different bearings are used in different locations. Two of the most common are pilot bearings and unit bearings.
Differentials with a pinion support that is bolted in from the front require pilot bearings. Pilot bearings help with pinion deflection as the pinion has a short stub that will ride on the pilot bearing in the housing or drop out.
Unit bearings are also known as hub bearing assemblies. Found on vehicles with auto hubs only, unit bearings have wheel studs and sealed bearings which are bolted to the front spindle.
To ensure a long life, bearings needs to be clean and well lubricated. On an assembled axle, maintenance issues come down to clean lubricating oil that is changed on a regular basis. Wheel bearings that are packed in grease need the same care, but they must be manually cleaned and repacked at regular intervals.
With a disassembled axle cleaning the bearings properly is a critical part of whatever repair operation is underway. Timken recommends using only clean kerosene or mineral oil to clean bearings, then drying them with compressed air but not spinning the bearings dry as the can literally grenade in your hands. After the bearing is clean and dry, it needs to be protected by covering it with a clean cloth or by putting it into a plastic bag.
The best practice for new bearings is to keep them in their packaging until they are needed. When you remove a new bearing from its packaging take care to clean it just like you would an older one.