Bearings for rotary motion are designed as rolling bearings or plain bearings. A distinction is drawn as to whether the forces occurring between the parts movable relative to each other are transmitted by rolling or sliding elements.
Rolling bearings generally comprise two bearing rings with integral raceways. Rolling elements are arranged between the rings and roll on the raceways. Rolling elements can be balls, cylindrical rollers, needle rollers, tapered rollers or barrel rollers. The rolling elements are generally guided by a cage that keeps them at a uniform distance from each other and prevents them coming into contact with each other. In needle roller bearings and ribless spherical roller bearings, the cage also ensures that the rolling element axis is positioned correctly. Where bearings can be dismantled, the cage holds the rolling elements together and gives easier fitting of the bearings. For particular applications, rolling bearings with a full complement of balls, cylindrical rollers or needle rollers may be used.
The standard material for sheet metal cages is steel, while brass is also used for some applications. Solid cages are made from brass, steel, laminated fabric and other materials. Cages made from thermoplastic materials are also widely used, especially those made from polyamide reinforced by glass fibre.
Rings and rolling elements are predominantly made from through hardened chromium steel, although case hardening steel is also used. Special bearings for extreme operating conditions – load, speed, temperature, corrosion – are made from temperature-resistant and/or corrosion-resistant steels, plastic, ceramics or other materials.
Rolling bearings are available in open versions or with seals on one or both sides. The most common types of seals are gap seals and lip seals.