Rolling and Plain Bearings
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.
Features are application
Every roller bearing design has characteristic features that make it especially suitable for specific bearing applications. It is not possible to draw up generally valid rules for the selection of the bearing type as several factors usually have to be considered and weighed up. In addition to load and speed, attention must also normally be paid to influences such as temperature, lubrication, vibrations, fitting, maintenance etc. In many cases, at least one of the main dimensions of the bearing – usually the bore diameter – is already defined by the design of the adjacent construction.
Rolling bearings for predominantly radial loads are described as radial bearings. Most radial bearings can support combined loads, e.g. deep groove ball bearings, angular contact ball bearings, tapered roller bearings or spherical roller bearings. Cylindrical roller bearings N, NU, most needle roller bearings, drawn cup needle roller bearings and needle roller and cage assemblies can only support radial loads.
Rolling bearings for predominantly axial loads are described as axial bearings. Axial spherical roller bearings and single direction axial angular contact ball bearings can support combined axial and radial loads. The other types of axial bearings are only suitable for axial loads.
If there is little radial space available, bearings with a low cross-sectional height must be selected, such as needle roller and cage assemblies; needle roller bearings with or without an inner ring, deep groove ball bearings and spherical roller bearings of certain series.
If there is little axial space available, bearings series including single row cylindrical roller bearings, deep groove ball bearings or angular contact ball bearings are suitable for radial and combined loads. For axial loads, axial needle roller and cage assemblies, axial needle roller bearings or axial deep groove ball bearings are used.
A further feature is how the bearings guide a shaft. There are bearings that allow axial displacements, bearings that guide a shaft in one or both axial directions and bearings that allow angular adjustment and thus tolerate misalignment of the adjacent construction.
The bearing size is determined primarily by the magnitude and type of load – dynamic or static – the bearing load carrying capacity and the requirements for operating life and operational reliability of the bearing arrangement. Rotating bearings are subjected to dynamic stresses. Bearings are subjected to static stresses if there is only very slow relative motion between the bearing rings, if swivel motion occurs or if loads occur in a stationary condition. Where external dimensions are identical, roller bearings can in general be subjected to higher loads than ball bearings. As a result, ball bearings are usually used for small and moderate loads, whilst roller bearings are frequently used for higher loads and larger shaft diameters.
The function of plain bearing arrangements, like rolling bearing arrangements, is to support or guide parts that are movable relative to each other. They must support and transmit the forces occurring in this case. Whereas the bearing arrangement elements in rolling bearing arrangements are separated from each other by means of rolling parts – rolling elements – the movable component in plain bearing arrangements – normally a shaft, stud or strip – slides on the sliding surface of a static bearing bush, bearing cup or sliding strip. The sliding motion occurs directly between the sliding layer of the bearing body and the part that is supported. Lubrication is ensured by embedded lubricants or a solid layer applied to a supporting body. In radial motion, the clearance between the shaft and sliding layer ensures the mobility of the sliding partners.
Plain bearings are available as radial bearings, axial bearings, strips, cups and in many other designs. They run quietly and are particularly suitable where high loads must be supported under relatively slow rotary and swivel motion and at low and high temperatures. Due to their versatile specific characteristics, they are therefore used in almost all areas of industry and especially where the space available for the bearing arrangement is very limited.