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
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.
Plain bearings
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.
Permaglide® plain bearing materials
The sliding materials available are Permaglide P1 and P2. Permaglide® P1 is maintenance-free and intended for dry
running. Products made from this material are particularly suitable where the bearing position must be
maintenance-free, there is a risk of inadequate lubrication or the presence of lubricant is either undesirable or
unacceptable. P1 can be used not only for rotary and oscillating motion but also for linear motion with short
strokes. The low-wear material has good sliding
characteristics, a low coefficient of friction and is highly resistant to
chemicals. Permaglide® is not hygroscopic - highly resistant to swelling - and does not tend to weld to
metals. It is also suitable under hydrodynamic operating
conditions. Maintenance-free Permaglide® materials are available in the variants P10 and P11, as well as the lead-free variants P14 and P141.
Permaglide® P2 is a low-maintenance, low-wear material with good damping characteristics and long relubrication
intervals. It can be used for rotary and oscillating motion, has little sensitivity to edge loading and is insensitive to
shocks. Low-maintenance Permaglide® materials are available in the variants P20, P21, P22 and P23.
Permaglide products are available as bushes, flanged bushes, thrust washers and
strips. Bushes, washers and strips are supplied in the maintenance-free Permaglide® P1 or the low-maintenance P2, flanged bushes are made from P1. There are also a wide variety of special
designs.
The life of a plain bearing is essentially dependent on the
load, sliding speed, mating tracks, temperature and operating
duration. There are other limiting factors such as
contamination, corrosion in dry running or possible lubricant ageing in cases of inadequate
lubrication. The basic rating life is therefore only ever a guide
value. The bearing size is determined on the basis of the load
occurring, the bearing load carrying capacity and the requirements for operating life and operational reliability of the bearing
arrangement.
Since there are many external influences on plain bearings that cannot be included in
calculation, tests under operating conditions therefore give the most reliable information on use and life in the specific
application.
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