Rolling-element bearing
Rolling-element bearing

A rolling-element bearing, also known as
a rolling bearing, is a bearing which carries a load by placing rolling
elements between two bearing rings called races. The relative motion of the
races causes the rolling elements to roll with very little rolling resistance
and with little sliding. One of the earliest and best-known
rolling-element bearings are sets of logs laid on the ground with a large
stone block on top. As the stone is pulled, the logs roll along the ground
with little sliding friction. As each log comes out the back, it is moved to
the front where the block then rolls on to it. It is possible to imitate such a
bearing by placing several pens or pencils on a table and placing an item
on top of them. See “bearings” for more on the historical development of
bearings. A rolling element rotary bearing uses a
shaft in a much larger hole, and cylinders called “rollers” tightly fill
the space between the shaft and hole. As the shaft turns, each roller acts as the
logs in the above example. However, since the bearing is round, the rollers
never fall out from under the load. Rolling-element bearings have the
advantage of a good tradeoff between cost, size, weight, carrying capacity,
durability, accuracy, friction, and so on. Other bearing designs are often
better on one specific attribute, but worse in most other attributes, although
fluid bearings can sometimes simultaneously outperform on carrying
capacity, durability, accuracy, friction, rotation rate and sometimes
cost. Only plain bearings are used as widely as rolling-element bearings.
Design There are five types of rolling-elements
that are used in rolling element bearings: balls, cylindrical rollers,
spherical rollers, tapered rollers, and needle rollers.
Most rolling-element bearings feature cages. The cages reduce friction, wear,
and bind by preventing the elements from rubbing against each other. Caged roller
bearings were invented by John Harrison in the mid-18th century as part of his
work on chronometers. Typical rolling-element bearings range
in size from 10 mm diameter to a few metres diameter, and have load-carrying
capacity from a few tens of grams to many thousands of tonnes.
=Ball bearing=A particularly common kind of
rolling-element bearing is the ball bearing. The bearing has inner and outer
races between which balls roll. Each race features a groove usually shaped so
the ball fits slightly loose. Thus, in principle, the ball contacts each race
across a very narrow area. However, a load on an infinitely small point would
cause infinitely high contact pressure. In practice, the ball deforms slightly
where it contacts each race much as a tire flattens where it contacts the
road. The race also yields slightly where each ball presses against it.
Thus, the contact between ball and race is of finite size and has finite
pressure. Note also that the deformed ball and race do not roll entirely
smoothly because different parts of the ball are moving at different speeds as
it rolls. Thus, there are opposing forces and sliding motions at each
ball/race contact. Overall, these cause bearing drag.
=Roller bearings=Cylindrical roller
Common roller bearings use cylinders of slightly greater length than diameter.
Roller bearings typically have higher load capacity than ball bearings, but a
lower capacity and higher friction under loads perpendicular to the primary
supported direction. If the inner and outer races are misaligned, the bearing
capacity often drops quickly compared to either a ball bearing or a spherical
roller bearing. Roller bearings are the earliest known
type of rolling-element-bearing, dating back to at least 40 BC.
Spherical roller Spherical roller bearings have an outer
ring with an internal spherical shape. The rollers are thicker in the middle
and thinner at the ends. Spherical roller bearings can thus accommodate
both static and dynamic misalignment. However, spherical rollers are difficult
to produce and thus expensive, and the bearings have higher friction than an
ideal cylindrical or tapered roller bearing since there will be a certain
amount of sliding between rolling elements and rings.
Gear bearing Gear bearing is roller bearing combining
to epicyclical gear. Each element of it is represented by concentric alternation
of rollers and gearwheels with equality of roller(s) diameter(s) to gearwheel(s)
pitch diameter(s). The widths of conjugated rollers and gearwheels in
pairs are the same. The engagement is herringbone or with the skew end faces
to realize efficient rolling axial contact. The downside to this bearing is
manufacturing complexity. Gear bearings could be used, for example, as efficient
rotary suspension, kinematically simplified planetary gear mechanism in
measuring instruments and watches. Tapered roller
Tapered roller bearings use conical rollers that run on conical races. Most
roller bearings only take radial or axial loads, but tapered roller bearings
support both radial and axial loads, and generally can carry higher loads than
ball bearings due to greater contact area. Tapered roller bearings are used,
for example, as the wheel bearings of most wheeled land vehicles. The
downsides to this bearing is that due to manufacturing complexities, tapered
roller bearings are usually more expensive than ball bearings; and
additionally under heavy loads the tapered roller is like a wedge and
bearing loads tend to try to eject the roller; the force from the collar which
keeps the roller in the bearing adds to bearing friction compared to ball
bearings. Needle roller
Needle roller bearings use very long and thin cylinders. Often the ends of the
rollers taper to points, and these are used to keep the rollers captive, or
they may be hemispherical and not captive but held by the shaft itself or
a similar arrangement. Since the rollers are thin, the outside diameter of the
bearing is only slightly larger than the hole in the middle. However, the
small-diameter rollers must bend sharply where they contact the races, and thus
the bearing fatigues relatively quickly. Toroidal roller bearing
Toroidal roller bearings are bearings that accommodate both angular
misaligment and axial displacement. The radius of the outer ring is much larger
than a spherical roller bearing… Toroidal roller bearings were introduced
in 1995 by SKF as “CARB bearings”. The inventor behind the bearing was the
engineer Magnus Kellström. Configurations
The configuration of the races determine the types of motions and loads that a
bearing can best support. A given configuration can serve multiple of the
following types of loading.=Thrust loadings=
Thrust bearings are used to support axial loads, such as vertical shafts.
Common designs are Thrust ball bearings, spherical roller thrust bearings,
tapered roller thrust bearings or cylindrical roller thrust bearings. Also
non-rolling element bearings such as hydrostatic or magnetic bearings see
some use where particularly heavy loads or low friction is needed.
=Radial loadings=Rolling element bearings are often used
for axles due to their low rolling friction. For light loads, such as
bicycles, ball bearings are often used. For heavy loads and where the loads can
greatly change during cornering, such as cars and trucks, tapered rolling
bearings are used.=Linear motion=
Linear motion roller-element bearings are typically designed for either shafts
or flat surfaces. Flat surface bearings often consist of rollers and are mounted
in a cage, which is then placed between the two flat surfaces; a common example
is drawer-support hardware. Roller-element bearing for a shaft use
bearing balls in a groove designed to recirculate them from one end to the
other as the bearing moves; as such, they are called linear ball bearings or
recirculating bearings. Bearing failure
Rolling-element bearings often work well in non-ideal conditions, but sometimes
minor problems cause bearings to fail quickly and mysteriously. For example,
with a stationary load, small vibrations can gradually press out the lubricant
between the races and rollers or balls. Without lubricant the bearing fails,
even though it is not rotating and thus is apparently not being used. For these
sorts of reasons, much of bearing design is about failure analysis. Vibration
based analysis can be used for fault identification of bearings.
There are three usual limits to the lifetime or load capacity of a bearing:
abrasion, fatigue and pressure-induced welding. Abrasion occurs when the
surface is eroded by hard contaminants scraping at the bearing materials.
Fatigue results when a material becomes brittle after being repeatedly loaded
and released. Where the ball or roller touches the race there is always some
deformation, and hence a risk of fatigue. Smaller balls or rollers deform
more sharply, and so tend to fatigue faster. Pressure-induced welding can
occur when two metal pieces are pressed together at very high pressure and they
become one. Although balls, rollers and races may look smooth, they are
microscopically rough. Thus, there are high-pressure spots which push away the
bearing lubricant. Sometimes, the resulting metal-to-metal contact welds a
microscopic part of the ball or roller to the race. As the bearing continues to
rotate, the weld is then torn apart, but it may leave race welded to bearing or
bearing welded to race. Although there are many other apparent
causes of bearing failure, most can be reduced to these three. For example, a
bearing which is run dry of lubricant fails not because it is “without
lubricant”, but because lack of lubrication leads to fatigue and
welding, and the resulting wear debris can cause abrasion. Similar events occur
in false brinelling damage. In high speed applications, the oil flow also
reduces the bearing metal temperature by convection. The oil becomes the heat
sink for the friction losses generated by the bearing.
ISO has categorised bearing failures into a document Numbered ISO 15243.
=Life calculation=The prediction of bearing life is
described in ISO 281 and the ANSI/American Bearing Manufacturers
Association Standards 9 and 11. The traditional method to estimate the
life of the rolling-element bearings uses the basic life equation:
Where: is the ‘basic life’ for a reliability of
90%, i.e. no more than 10% of bearings are expected to have failed
is the dynamic load rating of the bearing, quoted by the manufacturer
is the equivalent dynamic load applied to the bearing
is a constant: 3 for ball bearings, 4 for pure line contact and 3.33 for
roller bearings Basic life or is the life that 90% of
bearings can be expected to reach or exceed. The median or average life,
sometimes called Mean Time Between Failure, is about five times the
calculated basic rating life. Several factors, the ‘ASME five factor model’,
can be used to further adjust the life depending upon the desired reliability,
lubrication, contamination, etc. The major implication of this model is
that bearing life is finite, and reduces by a cube power of the ratio between
design load and applied load. This model was developed in 1924, 1947 and 1952
work by Arvid Palmgren and Gustaf Lundberg in their paper Dynamic Capacity
of Rolling Bearings. The model dates from 1924, the values of the constant
from the post-war works. Higher values may be seen as both a longer lifetime
for a correctly-used bearing below its design load, or also as the increased
rate by which lifetime is shortened when overloaded.
This model was recognised to have become inaccurate for modern bearings.
Particularly owing to improvements in the quality of bearing steels, the
mechanisms for how failures develop in the 1924 model are no longer as
significant. By the 1990s, real bearings were found to give service lives up to
14 times longer than those predicted. An explanation was put forward based on
fatigue life; if the bearing was loaded to never exceed the fatigue strength,
then the Lundberg-Palmgren mechanism for failure by fatigue would simply never
occur. This relied on homogeneous vacuum-melted steels, such as AISI
52100, that avoided the internal inclusions that had previously acted as
stress risers within the rolling elements, and also on smoother finishes
to bearing tracks that avoided impact loads. The constant now had values of 4
for ball and 5 for roller bearings. Provided that load limits were observed,
the idea of a ‘fatigue limit’ entered bearing lifetime calculations: if the
bearing was not loaded beyond this limit, its theoretical lifetime would be
limited only by external factors, such as contamination or a failure of
lubrication. A new model of bearing life was put
forward by FAG and developed by SKF as the Ioannides-Harris model. ISO 281:2000
first incorporated this model and ISO 281:2007 is based on it.
The concept of fatigue limit, and thus ISO 281:2007, remains controversial, at
least in the US. Constraints and trade-offs
All parts of a bearing are subject to many design constraints. For example,
the inner and outer races are often complex shapes, making them difficult to
manufacture. Balls and rollers, though simpler in shape, are small; since they
bend sharply where they run on the races, the bearings are prone to
fatigue. The loads within a bearing assembly are also affected by the speed
of operation: rolling-element bearings may spin over 100,000 rpm, and the
principal load in such a bearing may be momentum rather than the applied load.
Smaller rolling elements are lighter and thus have less momentum, but smaller
elements also bend more sharply where they contact the race, causing them to
fail more rapidly from fatigue. Maximum rolling element bearing speeds are often
specified in ‘nDm’, which is the product of the mean diameter and the maximum
RPM. For angular contact bearings nDms over 2.1 million have been found to be
reliable in high performance rocketry applications.
There are also many material issues: a harder material may be more durable
against abrasion but more likely to suffer fatigue fracture, so the material
varies with the application, and while steel is most common for rolling-element
bearings, plastics, glass, and ceramics are all in common use. A small defect in
the material is often responsible for bearing failure; one of the biggest
improvements in the life of common bearings during the second half of the
20th century was the use of more homogeneous materials, rather than
better materials or lubricants. Lubricant properties vary with
temperature and load, so the best lubricant varies with application.
Although bearings tend to wear out with use, designers can make tradeoffs of
bearing size and cost versus lifetime. A bearing can last indefinitely—longer
than the rest of the machine—if it is kept cool, clean, lubricated, is run
within the rated load, and if the bearing materials are sufficiently free
of microscopic defects. Note that cooling, lubrication, and sealing are
thus important parts of the bearing design.
The needed bearing lifetime also varies with the application. For example,
Tedric A. Harris reports in his Rolling Bearing Analysis on an oxygen pump
bearing in the U.S. Space Shuttle which could not be adequately isolated from
the liquid oxygen being pumped. All lubricants reacted with the oxygen,
leading to fires and other failures. The solution was to lubricate the bearing
with the oxygen. Although liquid oxygen is a poor lubricant, it was adequate,
since the service life of the pump was just a few hours.
The operating environment and service needs are also important design
considerations. Some bearing assemblies require routine addition of lubricants,
while others are factory sealed, requiring no further maintenance for the
life of the mechanical assembly. Although seals are appealing, they
increase friction, and in a permanently sealed bearing the lubricant may become
contaminated by hard particles, such as steel chips from the race or bearing,
sand, or grit that gets past the seal. Contamination in the lubricant is
abrasive and greatly reduces the operating life of the bearing assembly.
Another major cause of bearing failure is the presence of water in the
lubrication oil. Online water-in-oil monitors have been introduced in recent
years to monitor the effects of both particles and the presence of water in
oil and their combined effect. Designation
Metric rolling-element bearings have alphanumerical designations, defined by
ISO 15, to define all of the physical parameters. The main designation is a
seven digit number with optional alphanumeric digits before or after to
define additional parameters. Here the digits will be defined as: 7654321. Any
zeros to the left of the last defined digit are not printed; e.g. a
designation of 0007208 is printed 7208. Digits one and two together are used to
define the inner diameter, or bore diameter, of the bearing. For diameters
between 20 and 495 mm, inclusive, the designation is multiplied by five to
give the ID; e.g. designation 08 is a 40 mm ID. For inner diameters less than 20
the following designations are used: 00=10 mm ID, 01=12 mm ID, 02=15 mm
ID, and 03=17 mm ID. The third digit defines the “diameter series”, which
defines the outer diameter. The diameter series, defined in ascending order, is:
0, 8, 9, 1, 7, 2, 3, 4, 5, 6. The fourth digit defines the type of bearing:
0. Ball radial single-row 1. Ball radial spherical double-row
2. Roller radial with short cylindrical rollers
3. Roller radial spherical double-row 4. Roller needle or with long
cylindrical rollers 5. Roller radial with spiral rollers
6. Ball radial-thrust single-row 7. heavy vehicle bearings prepared
7. Roller tapered 8. Ball thrust, ball thrust-radial
9. Roller thrust or thrust-radial The fifth and sixth digit define
structural modifications to the bearing. For example, on radial thrust bearings
the digits define the contact angle, or the presence of seals on any bearing
type. The seventh digit defines the “width series”, or thickness, of the
bearing. The width series, defined from lightest to heaviest, is: 7, 8, 9, 0, 1,
2, 3, 4. The third digit and the seventh digit define the “dimensional series” of
the bearing There are four optional prefix
characters, here defined as A321-XXXXXXX, which are separated from
the main designation with a dash. The first character, A, is the bearing
class, which is defined, in ascending order: C, B, A. The class defines extra
requirements for vibration, deviations in shape, the rolling surface
tolerances, and other parameters that are not defined by a designation
character. The second character is the frictional moment, which is defined, in
ascending order, by a number 1–9. The third character is the radial clearance,
which is normally defined by a number between 0 and 9, in ascending order,
however for radial-thrust bearings it is defined by a number between 1 and 3,
inclusive. The fourth character is the accuracy ratings, which normally are, in
ascending order: 0, 6X, 6, 5, 4, T, and 2. Ratings 0 and 6 are the most common;
ratings 5 and 4 are used in high-speed applications; and rating 2 is used in
gyroscopes. For tapered bearings, the values are, in ascending order: 0, N,
and X, where 0 is 0, N is “normal”, and X is 6X.
There are five optional characters that can defined after the main designation:
A, E, P, C, and T; these are tacked directly onto the end of the main
designation. Unlike the prefix, not all of the designations must be defined. “A”
indicates an increased dynamic load rating. “E” indicates the use of a
plastic cage. “P” indicates that heat-resistant steel are used. “C”
indicates the type of lubricant used. “T” indicates the degree to which the
bearing components have been tempered. While manufacturers follow ISO 15 for
part number designations on some of their products, it is common for them to
implement proprietary part number systems that do not correlate to ISO 15.
See also References
Further reading Johannes Brändlein, Paul Eschmann,
Ludwig Hasbargen, Karl Weigand. Ball and Roller Bearings: Theory, Design and
Application. Wiley. ISBN 0-471-98452-3. External links
Technical publication about bearing lubrication
NASA technical handbook Rolling-Element Bearing
NASA technical handbook Lubrication of Machine Elements
How rolling-element bearings work Kinematic Models for Design Digital
Library – Movies and photos of hundreds of working mechanical-systems models at
Cornell University. Also includes an e-book library of classic texts on
mechanical design and engineering.

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