Bearings are broadly classified by the type of rolling element used in their construction. Ball bearings employ steel balls, which rotate in grooved raceways, whilst Roller bearings utilise cylindrical, tapered and spherical rollers running in suitably shaped raceways.
Although these notes give information on the uses of the various types of ball and roller bearings (together with general information on installation, maintenance and inspection), the Aircraft Maintenance Manual (AMM) should be the final arbiter for specific installations.
Ball bearings and tapered roller bearings accept both radial and axial loads, whilst the other types of roller bearings may accept only radial loads. Generally, those bearings are used for engine and gearbox applications with rotational speeds in excess of approximately 100rpm. Most other bearings on an aircraft/engine are intended for oscillating or slow rotation conditions and do not have a cage. They are generally shielded or sealed and pre-packed with grease, although some have external lubrication facilities.