BEARINGS - ThomasNetRoller bearings employ a host of different shapes for their rolling elements,...
Transcript of BEARINGS - ThomasNetRoller bearings employ a host of different shapes for their rolling elements,...
BEARINGSBUYING GUIDE
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BEARINGS A ThomasNet Buying Guide
Bearings are mechanical assemblies that consist of
rolling elements and usually inner and outer races
which are used for rotating or linear shaft
applications. There are several different types
including ball and roller bearings, linear bearings, as
well as mounted versions that may use either rolling
element bearings or plain bearings. Ball bearings have
spherical rolling elements and are used for lower load
applications, while roller bearings use cylindrical rolling elements for heavier load carrying
requirements. Linear bearings are used for linear movements along shafts and may also have rotational
capabilities. Mounted bearings are assemblies in which the bearings are pre-assembled in mountings
that, in turn, are bolted to frames, stanchions, etc., and are used for supporting the ends of shafts,
conveyor rollers, etc. In addition to ball and roller bearings in their radial, linear, and mounted forms,
bearings include those for civil engineering applications, which are called slide bearings; those used in
small instruments and the like, known as jewel bearings; and very specialized bearings known
collectively as frictionless bearings which includes air and magnetic varieties. Sleeve bearings, journal
bearings, and other fluid-film bearings are addressed in the Bushings family.
Types of Bearings
Ball Bearings Ball Bearings are mechanical assemblies that consist of rolling spherical elements
that are captured between circular inner and outer races. They provide a means
of supporting rotating shafts and minimizing friction between shafts and
stationary machine members. Ball bearings are used primarily in machinery that
have shafts requiring support for low friction rotation. There are several
configurations, most notably shielded or sealed. Ball bearings are standardized to
permit interchangeability. Ball bearings are also known as rolling element
bearings or anti-friction bearings. Considerations include
First choice for high speeds or high precision apps
Large range of standardized forms
Handle radial and axial loads with specific configurations
Roller Bearings Roller Bearings are mechanical assemblies that consist of cylindrical or
tapered rolling elements usually captured between inner and outer races.
They provide a means of supporting rotating shafts and minimizing
friction between shafts and stationary machine members. Roller bearings
are used primarily in machinery with rotating shafts that require support
of heavier loads than ball bearings provide. Tapered roller bearings are
often used to accommodate higher thrust loads in addition to the radial
loads. Types range from cylindrical to spherical rollers. Roller bearings are
standardized like ball bearings, albeit to a lesser degree. Considerations
include
Higher load capacities than ball bearings
Can withstand high axial loads
Mounted Bearings Mounted Bearings are mechanical assemblies that consist of bearings
housed within bolt-on or threaded mounting components and
include pillow blocks, flanged units, etc. They provide means of
supporting rotating shafts and minimizing friction between shafts
and stationary machine members. Mounted bearings are used
primarily in machinery with exposed rotating shafting. They are used
as take-up devices on the ends of conveyors and as flanged units
along intermediate points. The bearings can be rolling element or
journal bearing configurations. Mounted bearings are designed for
bolt-on mounting and ease of replacement. Other varieties of
mounted bearings include rod end bearings and cam followers.
Considerations include
Housed units reduce mounting concerns, protection issues
Cartridge designs ease replacement
Shafts usually held in place with set screws
Allow adjustment of the supported components
Mainly used for low/mid speed applications
Linear Bearings Linear Bearings are mechanical assemblies that consist of ball or roller
elements captured in housings and used to provide linear movement
along shafts. Linear bearings are used primarily in machinery that requires
linear movement and positioning along shafts. They also may have
secondary rotational features depending on the design. Considerations include
Lower friction and higher accuracies compared with bushings
Costlier and more complex than bushings
Slide Bearings Slide bearings are mechanical assemblies designed to provide free
motion in one dimension between structural elements. Slide bearings
are used primarily in the structural support of bridges as well as
commercial and industrial buildings. These parts accommodate
thermal movement, allow for end-beam rotation, and isolate
components of the structure against vibration, noise, and shock.
Other types of slide bearings include those used on truss base plates, heat exchangers, and process
equipment.
Jewel Bearings Jewel bearings are mechanical devices used in
light rotating applications such as watches, meter
movements, gyroscopes, etc. where loads are
small and the supported rotating shafts are tiny.
Jewel bearings are constructed from a range of
synthetics, with ruby and sapphire being
particularly common.
Frictionless Bearings Frictionless bearings are mechanical or electro-mechanical alternatives to
conventional bearings that provide controllable shaft support through air,
magnetic fields, etc. for critical, high precision applications.
Applications and Industries Ball and roller bearings are used in machinery of all kinds, from boiler feed pumps to automotive
transmissions. Mounted bearings are especially common on conveyors, in shaft linkages, and
particularly where long lengths of shafting must be supported by housed units where the bearing is not
protected by another housing such as a transmission case. Linear bearings are used exclusively in linear
applications such as slide tables. Slide bearings are used primarily for load bearing application in large
civil engineering projects such as bridges where they accommodate a limited range of movement, unlike
the other bearings here, where motion—either radial or linear—is the main concern. Jewel bearings are
restricted to very small devices and movements and do not rely on any rolling elements. Frictionless
bearings are any of the other special purpose designs that include air bearings, magnetic bearings, etc.
While bearings are used nearly everywhere, there are some industries that use so many or have specific
requirements for durability, cleanliness, etc. that they warrant mentioning here. Some of these
industries are
Aerospace
Agricultural
Automotive
Machine Tools
Medical
Mining
Considerations Radial ball bearings are used primarily for radially loaded shafts and those with light axial loads. Angular
contact ball bearings are designed to take higher axial loads in one direction in addition to their radial
capacities. Ball thrust bearings are available which are specifically intended to take axial loads alone. The
most common configuration for radial ball bearings is the single row version, which could be shielded or
sealed depending on whether it is to be used within a housed area—a transmission, say—or in an
exposed environment such as on a bicycle wheel. The seals and shields keep lubricant in the bearing and
dirt and debris out of it. Ball bearings are usually fitted with retainers which space the balls evenly
between and around the perimeters of their outer and inner races. Full capacity bearings dispense with
retainers in order to fill as many balls as possible between the races, adding to the bearing’s load
handling capacity.
Roller bearings employ a host of different shapes for their rolling elements, including straight rollers,
needle rollers, tapered rollers, spherical rollers, etc. Roller bearings are able to take higher radial loads
than their ball bearing counterparts due to the higher contact area between the rollers and the races.
Some roller bearings are designed to take high thrust loads using tapered elements and races.
Mounted bearings are ball, roller, or sleeve bearings which are furnished in housings, flanges, etc. and
usually installed with seals and/or shields for environmental protection. Common mounting styles
include pillow blocks, flanges, take-ups, etc. They are often used on conveyors where take-up
assemblies provide adjustment for conveyor belt tension.
In selecting rolling element bearings, either ball or roller or as mounted units, designers usually consider
a number of factors including loads, both their quantities and directions, the accuracy requirements of
the shaft system, misalignment factors, speeds, noise, and friction. Where radial loads are high, a
designer may opt for a roller bearing over a ball bearing and might do the same where high axial loads
are anticipated. Where the bearing needs to be able to accommodate some shaft misalignment, the
designer may elect a ball bearing where loads are normal or go to a spherical roller bearing which is also
very capable of handling misalignment. Ball bearings tend to be better at handling high speeds than
roller bearings, and in some cases where accuracy and low friction are paramount, such as machine
tools, a ball bearing may be the only way of meeting the requirements.
Of particular interest in considering bearings are their static and dynamic load ratings. Bearing that are
subject to high loads when they are not rotating can undergo a phenomenon known as brinelling, where
the balls dent the races in the same place repeatedly. The same loads applied to the bearing when
running may cause less concern because any indentations will distribute around the bearing races and
not pile up in the same spots each time.
Bearing makers list bearing rated capacities for their bearings, which for ball bearings are identified as
extra-light, light-, medium-duty, and so on, where the dimensions of the bore or shaft requirements
increase to accommodate increasing loads. The rated capacity is based on a statistical measure which
states that a certain percentage of bearings will complete a stated number of revolutions without failing.
These catalog numbers can be massaged to better pick the bearing suited to the actual conditions of
use.
Linear bearings are sized according to linear travel, total linear distance, load, precision requirements,
etc., with many parameters being analogous to the radial bearing considerations. Linear bearings are
used with ground shafting for dimensional accuracy and low-friction sliding.
Slide bearings are used to accommodate expansion and contraction in stationary structures such as
bridges and building. Often they consist of two Teflon plates which are sandwich between major
structural members. Sometimes stainless steel is used instead of Teflon for one of the two facing
bearing surfaces. Of principal concern with slide bearings is the force per unit area they can withstand.
Jewel bearings are used in very light loading applications. Jewel bearings provide very accurate, hard
surfaces which can support lightly loaded shafts that see mostly intermittent motion.
Frictionless bearings are bearings that use air or other gases or magnetic fields to support rotating
journals and are so-called to distinguish them from anti-friction bearings—another term for rolling
element bearings, which in itself was coined to distinguish these from original journal bearings which
used friction developed through shaft rotation to create films of fluid for supporting shaft journals.
Frictionless bearings represent a small slice of the bearing world and are usually applied only in very rare
situations.
ABMA The ABMA (American Bearings Manufacturers Association) provides standards for many bearing types
and is affiliated with the so-called ABEC system which rates bearing precision.
Important Attributes
Bearing Type For ball bearings, the Conrad, or non-slot fill, bearing is the most common, whose design dispenses with
a filling slot and instead relies on displacing the inner race to load the balls and a cage to keep them
evenly spaced. For roller bearings, bearing type requires a selection of roller type, be it cylindrical,
tapered, spherical, etc. Mounted units require a type selection of ball, roller, or spherical, as well, and
then an additional selection of style, as defined below. Linear bearing types range from ball bearing
cages—essentially bare cages holding balls that are often used as die-post bushings—to recirculating
ball designs where the balls roll linearly along the shaft and then return to their starting points through
channels on the non-shaft sides of the bearings.
Style This attribute applies solely to mounted units, where a distinction must be made between the housing
for the bearing, among them the choice of pillow blocks, flanges, take ups, etc.
Material Material choice for ball and roller bearings is generally limited to a few steel alloys, some plastics,
occasionally ceramics, etc. while mounted units have more material choices owing to the additional
materials available for housings.
Seal/Shield Ball bearings exposed to the environment can be ordered with seals and/or shields
where shields provide some protection of the bearing elements from dirt ingress with
minimal added friction and seals provide shaft contacting lips that exclude moisture but
do add to the friction on the bearing. Seals and shields can be added on both sides,
either side, alone or in combination. The image to the right shows a radial bearing in
cross section with shields on both sides.
Race Ball bearings races fall generally into two designs: angular contact and radial. Angular
contact bearings (image right) load the balls at angles to the perpendicular radial
planes, whereas radial contact bearings (image above) load the balls through the
perpendicular planes. Angular contact bearings are generally preferred where axial
loading is a consideration. Deep groove bearings are commonly associated with
radial contact bearings. Cup and cone bearings are common on bicycle wheels where the bearings are
loosely packed between cones and the cones are adjusted for play.
Locating Mounted pillow block units are classified as expandable and non-expandable and in situations where
two are more pillow block bearings are installed for shaft support one will ordinarily be specified as non-
expandable and the other as expandable which allows the bearing to accommodate slight growth of the
shaft. Some units are configured to allow either option.
Maximum Static and Dynamic Loads Bearing loading is based on static and dynamic values and the choice of which governs is a function of
the operating conditions the bearing will see.
Related Product Categories Shafts (Shafting) are mechanical components, usually metallic, that typically rotate axially to
transmit motion.
Hydraulic/Pneumatic/Radial Shaft Seals are mechanical devices used for sealing the
components of reciprocating and rotating shafts where fluid is used as an actuating force or
where oil/grease are used as lubricants.
Lubricating Greases are semi-solid mixtures of lubricants and thickeners usually made from
minerals and soaps to produce a higher viscosity than oil and used to prevent wear on contact
surfaces.
Lubricating Oil is a slippery and viscous liquid made of any of numerous mineral, vegetable,
animal, or synthetic substances. It is often a mixture of gaseous, liquid, and solid hydrocarbons
used for lubricating. It is also available in synthetic and edible forms.
Plain Bearings (Bushings) are mechanical elements used to reduce friction between rotating
shafts and stationary support members. Typically, a bushing relies on soft metal or plastic and
an oil film to support the rotating shaft on the hardened shaft journal.
Bearing Isolators are mechanical devices designed to seal and protect bearings from infiltration
by fluid and airborne contaminants.
Chain and Bearing Lubricators are mechanical devices used to deliver oils, grease, or other
lubricants to moving or contacting parts or joints for the purpose of reducing friction.
Induction Heaters are devices that use electromagnetic energy to induce heating in electrically
conductive materials. Heaters are sometimes used for bearing installation.
Resources
General Ball Bearings http://www.timken.com/en-us/products/bearings/productlist/ball/Pages/default.aspx Needle Bearings http://www.ntnamericas.com/en/website/documents/brochures-and-literature/tech-sheets-and-supplements/needle_roller_bearings_handbook_cat_9013-e_lowres.pdf Mounted Units http://www.ktntn.com/pdfs/Introduction%20to%20Mounted%20Unit%20Bearings.pdf Linear Bearings http://www.bsahome.org/tools/pdfs/Linear_Bearings_web.pdf Trade Associations http://www.americanbearings.org/ http://www.bsahome.org/default.aspx
Manufacturers and Suppliers http://www.intechbearing.com/ http://www.spb-usa.com/ http://www.tpa-us.com/index.html http://www.emersonbearing.com/index.html http://www.kmsbearings.com/ http://www.nomabearing.com/index.html http://www.timken.com/en-US/Pages/Home.aspx