Bushing (bearing)
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A plain bearing, or more commonly sliding contact bearing and slide bearing (in railroading sometimes called a solid bearing, journal bearing, or friction bearing), is the simplest type of bearing, comprising just a
bearing surface A bearing surface in mechanical engineering is the area of contact between two objects. It usually is used in reference to bolted joints and bearings, but can be applied to a wide variety of engineering applications. On a screw the bearing area ...
and no rolling elements. Therefore, the
journal A journal, from the Old French ''journal'' (meaning "daily"), may refer to: *Bullet journal, a method of personal organization *Diary, a record of what happened over the course of a day or other period *Daybook, also known as a general journal, a ...
(i.e., the part of the shaft in contact with the bearing) slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole. A simple linear bearing can be a pair of flat surfaces designed to allow motion; e.g., a drawer and the slides it rests on or the ways on the
bed A bed is an item of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many beds ...
of a
lathe A lathe () is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, and turning, with tools that are applied to the workpiece to c ...
. Plain bearings, in general, are the least expensive type of bearing. They are also compact and lightweight, and they have a high load-carrying capacity.


Design

The design of a plain bearing depends on the type of motion the bearing must provide. The three types of motions possible are: * ''Journal'' (''friction'', ''radial'' or ''rotary'') ''bearing'': This is the most common type of plain bearing; it is simply a shaft rotating in a hole. In
locomotive A locomotive or engine is a rail transport vehicle that provides the Power (physics), motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as a multiple unit, Motor coach (rail), motor ...
and
railroad car A railroad car, railcar (American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (British English and UIC), also called a train car, train wagon, train carriage or train truck, is a ...
applications a ''
journal bearing A plain bearing, or more commonly sliding contact bearing and slide bearing (in railroading sometimes called a solid bearing, journal bearing, or friction bearing), is the simplest type of bearing, comprising just a bearing surface and no roll ...
'' specifically referred to the plain bearing once used at the ends of the
axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In the former case, bearing ...
s of railroad wheel sets, enclosed by ''
journal box A bogie or railroad truck holds the wheel sets of a rail vehicle. Axlebox An ''axle box'', also known as a ''journal box'' in North America, is the mechanical subassembly on each end of the axles under a railway wagon, coach or locomotive; ...
es'' (
axlebox A bogie or railroad truck holds the wheel sets of a rail vehicle. Axlebox An ''axle box'', also known as a ''journal box'' in North America, is the mechanical subassembly on each end of the axles under a railway wagon, coach or locomotive; ...
es). Axlebox bearings today are
rolling-element bearing In mechanical engineering, a rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements (such as balls or rollers) between two concentric, grooved rings called races. The relative m ...
s rather than plain bearings. * ''
Linear bearing A linear-motion bearing or linear slide is a bearing designed to provide free motion in one direction. There are many different types of linear motion bearings. Motorized linear slides such as machine slides, X-Y tables, roller tables and som ...
'': This bearing provides linear motion; it may take the form of a circular bearing and shaft or any other two mating surfaces (e.g., a
slide plate A slide plate is a linear bearing that may be part of the expansion joints of bridges, high temperature horizontal ducts of water-tube boilers and other mechanical or structural engineering applications. In each case one plate is fixed and the oth ...
). * ''
Thrust bearing A thrust bearing is a particular type of rotary bearing. Like other bearings they permanently rotate between parts, but they are designed to support a predominantly axial load. Thrust bearings come in several varieties. *''Thrust ball bearings ...
'': A thrust bearing provides a bearing surface for forces acting axial to the shaft. One example is a propeller shaft.


Integral

''Integral'' plain bearings are built into the object of use as a hole prepared in the bearing surface. Industrial integral bearings are usually made from
cast iron Cast iron is a class of iron–carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuriti ...
or
babbitt Babbitt may refer to: Fiction * ''Babbitt'' (novel), a 1922 novel by Sinclair Lewis ** ''Babbitt'' (1924 film), a 1924 silent film based on the novel ** ''Babbitt'' (1934 film), a 1934 film based on the novel *Babbit, the family name of the titl ...
and a
hardened steel The term hardened steel is often used for a medium or high carbon steel that has been given heat treatment and then quenching followed by tempering. The quenching results in the formation of metastable martensite, the fraction of which is reduce ...
shaft is used in the bearing. Integral bearings are not as common because bushings are easier to accommodate and can be replaced if necessary. Depending on the material, an integral bearing may be less expensive but it cannot be replaced. If an integral bearing wears out, the item may be replaced or reworked to accept a bushing. Integral bearings were very common in 19th-century machinery, but became progressively less common as interchangeable manufacture became popular. For example, a common integral plain bearing is the
hinge A hinge is a mechanical bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation: all other ...
, which is both a
thrust bearing A thrust bearing is a particular type of rotary bearing. Like other bearings they permanently rotate between parts, but they are designed to support a predominantly axial load. Thrust bearings come in several varieties. *''Thrust ball bearings ...
and a journal bearing.


Bushing

A ''bushing'', also known as a ''bush'', is an independent plain bearing that is inserted into a housing to provide a
bearing surface A bearing surface in mechanical engineering is the area of contact between two objects. It usually is used in reference to bolted joints and bearings, but can be applied to a wide variety of engineering applications. On a screw the bearing area ...
for rotary applications; this is the most common form of a plain bearing. Common designs include ''solid'' (''sleeve'' and ''flanged''), ''split'', and ''clenched'' bushings. A sleeve, split, or clenched bushing is only a "sleeve" of material with an inner diameter (ID), outer diameter (OD), and length. The difference between the three types is that a solid sleeved bushing is solid all the way around, a split bushing has a cut along its length, and a clenched bearing is similar to a split bushing but with a
clench Clench may refer to: People *Clench (surname) Places in England * Clench, Wiltshire, in the parish of Milton Lilbourne ** Clench Mill, a windmill in Wiltshire * Clench Common, a hamlet in the parish of Savernake, Wiltshire In fiction * Clench ...
(or
clinch Clinch may refer to: * Nail (fastener) or device to hold in this way * Clinching, in metalworking * Clinch fighting or the clinch, a grappling position in boxing or wrestling, a stand-up embrace * Clinch County, Georgia, USA * Clinch River, near T ...
) across the cut connecting the parts. A flanged bushing is a sleeve bushing with a flange at one end extending radially outward from the OD. The flange is used to positively locate the bushing when it is installed or to provide a thrust bearing surface. Sleeve bearings of inch dimensions are almost exclusively dimensioned using the SAE numbering system. The numbering system uses the format -XXYY-ZZ, where XX is the ID in sixteenths of an inch, YY is the OD in sixteenths of an inch, and ZZ is the length in eighths of an inch. Metric sizes also exist. A linear bushing is not usually pressed into a housing, but rather secured with a radial feature. Two such examples include two
retaining ring A retaining ring is a fastener that holds components or assemblies onto a shaft or in a housing/bore when installed - typically in a groove - for one time use only. Once installed, the exposed portion acts as a shoulder which retains the speci ...
s, or a ring that is molded onto the OD of the bushing that matches with a groove in the housing. This is usually a more durable way to retain the bushing, because the forces acting on the bushing could press it out. The thrust form of a bushing is conventionally called a '.


Two-piece

''Two-piece'' plain bearings, known as ''full bearings'' in industrial machinery, are commonly used for larger diameters, such as
crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...
bearings. The two halves are called ''shells''. There are various systems used to keep the shells located. The most common method is a tab on the
parting line A parting line, in industrial casting of molds, is the border line in which draft angles change direction. One can check the parting line in the mould or product which divides the two half, i.e; the core and the cavity of a molded part. It is so ...
edge that correlates with a notch in the housing to prevent axial movement after installation. For large, thick shells a button stop or
dowel A dowel is a cylindrical rod, usually made of wood, plastic, or metal. In its original manufactured form, a dowel is called a ''dowel rod''. Dowel rods are often cut into short lengths called dowel pins. Dowels are commonly used as structural ...
pin is used. The button stop is screwed to the housing, while the dowel pin keys the two shells together. Another less common method uses a
dowel pin A dowel is a cylindrical rod, usually made of wood, plastic, or metal. In its original manufactured form, a dowel is called a ''dowel rod''. Dowel rods are often cut into short lengths called dowel pins. Dowels are commonly used as structural ...
that keys the shell to the housing through a hole or slot in the shell. The distance from one parting edge to the other is slightly larger than the corresponding distance in the housing so that a light amount of pressure is required to install the bearing. This keeps the bearing in place as the two halves of the housing are installed. Finally, the shell's circumference is also slightly larger than the housing circumference so that when the two halves are bolted together the bearing ''crushes'' slightly. This creates a large amount of radial force around the entire bearing, which keeps it from ''spinning''. It also forms a good interface for heat to travel out of the bearings into the housing.


Gallery

File:Linear-table with detail numbered.png, A linear table with four linear bearings (1) File:GWR Spoked wagon wheels.jpg, A wheelset from a
Great Western Railway The Great Western Railway (GWR) was a British railway company that linked London with the southwest, west and West Midlands of England and most of Wales. It was founded in 1833, received its enabling Act of Parliament on 31 August 1835 and ran ...
(GWR) wagon showing a plain, or journal, bearing endCSX Dictionary J
File:Archbar ACL143.JPG, Archbar type
truck A truck or lorry is a motor vehicle designed to transport cargo, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame construction ...
with journal bearings in
journal box A bogie or railroad truck holds the wheel sets of a rail vehicle. Axlebox An ''axle box'', also known as a ''journal box'' in North America, is the mechanical subassembly on each end of the axles under a railway wagon, coach or locomotive; ...
es as used on some steam locomotive tenders. A version of the archbar truck was at one time also used on US
freight cars A railroad car, railcar (American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (British English and UIC), also called a train car, train wagon, train carriage or train truck, is a ...
File:Journal box.png, Diagram of a railroad
journal box A bogie or railroad truck holds the wheel sets of a rail vehicle. Axlebox An ''axle box'', also known as a ''journal box'' in North America, is the mechanical subassembly on each end of the axles under a railway wagon, coach or locomotive; ...
File:Plain-bearing DIN-ISO4379 Type-G.png, A solid sleeve bushing File:Plain-bearing DIN-ISO4379 Type-F.png, A
flange A flange is a protruded ridge, lip or rim (wheel), rim, either external or internal, that serves to increase shear strength, strength (as the flange of an iron beam (structure), beam such as an I-beam or a T-beam); for easy attachment/transfer of ...
d bushing File:Clenched bushing.png, A clenched (or clinched) bushing


Materials

Plain bearings must be made from a material that is durable, low
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
, low wear to the bearing and shaft, resistant to elevated temperatures, and corrosion resistant. Often the bearing is made up of at least two constituents, where one is soft and the other is hard. In general, the harder the surfaces in contact the lower the coefficient of friction and the greater the pressure required for the two to
gall Galls (from the Latin , 'oak-apple') or ''cecidia'' (from the Greek , anything gushing out) are a kind of swelling growth on the external tissues of plants, fungi, or animals. Plant galls are abnormal outgrowths of plant tissues, similar to be ...
or to seize when
lubrication Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in a contact between two surfaces. The study of lubrication is a discipline in the field of tribology. Lubrication mechanisms such as fluid-lubric ...
fails.


Babbitt

Babbitt is usually used in integral bearings. It is coated over the bore, usually to a thickness of , depending on the diameter. Babbitt bearings are designed to not damage the journal during direct contact and to collect any contaminants in the lubrication.


Bi-material

Bi-material bearings consist of two materials, a metal shell and a plastic bearing surface. Common combinations include a steel-backed
PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemour ...
-coated bronze and aluminum-backed Frelon. Steel-backed PTFE-coated bronze bearings are rated for more load than most other bi-metal bearings and are used for rotary and oscillating motions.
Aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
-backed Frelon are commonly used in corrosive environments because the Frelon is
chemically inert In chemistry, the term chemically inert is used to describe a substance that is not chemically reactive. From a thermodynamic perspective, a substance is inert, or nonlabile, if it is thermodynamically unstable (positive standard Gibbs free en ...
.


Bronze

A common plain bearing design utilizes a hardened and polished
steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
shaft and a softer
bronze Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (including aluminium, manganese, nickel, or zinc) and sometimes non-metals, such as phosphorus, or metalloids such ...
bushing. The bushing is replaced whenever it has worn too much. Common bronze alloys used for bearings include:
SAE 841 Oilite is a brand of self-lubricating bearing that is made from metal alloys with pores that channel lubricants between the bearing itself and the shaft. It is manufactured from different types of material. Traditional Oilite is mostly made of copp ...
, SAE 660 ( CDA 932),
SAE 863 Oilite is a brand of self-lubricating bearing that is made from metal alloys with pores that channel lubricants between the bearing itself and the shaft. It is manufactured from different types of material. Traditional Oilite is mostly made of copp ...
, and CDA 954.


Cast iron

A cast iron bearing can be used with a hardened steel shaft because the coefficient of friction is relatively low. The cast iron glazes over therefore wear becomes negligible.


Graphite

In harsh environments, such as
oven upA double oven A ceramic oven An oven is a tool which is used to expose materials to a hot environment. Ovens contain a hollow chamber and provide a means of heating the chamber in a controlled way. In use since antiquity, they have been us ...
s and dryers, a
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
and
graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
alloy, commonly known by the trademarked name graphalloy, is used. The
graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
is a
dry lubricant Dry lubricants or solid lubricants are materials that, despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid oil medium. The two main dry lubricants are graphit ...
, therefore it is low friction and low maintenance. The copper adds strength, durability, and provides heat dissipation characteristics. Unalloyed graphite bearings are used in special applications, such as locations that are submerged in water.


Jewels

Known as ''jewel bearings'', these bearings use jewels, such as
sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, chromium, vanadium, or magnesium. The name sapphire is derived via the Latin "sapphir ...
,
ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sa ...
, and
garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...
.


Plastic

Solid plastic plain bearings are now increasingly popular due to dry-running lubrication-free behavior. Solid polymer plain bearings are low weight, corrosion resistant, and maintenance free. After studies spanning decades, an accurate calculation of the service life of polymer plain bearings is possible today. Designing with solid polymer plain bearings is complicated by the wide range, and non-linearity, of
coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape A shape or figure is a graphics, graphical representation of an object or its external boundary, outline, or external Surface (mathematics), surface, as opposed to other pro ...
. These materials can heat rapidly when used in applications outside the recommended pV limits. Solid polymer type bearings are limited by the
injection molding Injection moulding (U.S. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Injection moulding can be performed with a host of materials mainly including metals (for ...
process. Not all shapes are possible with this process, and shapes that are possible are limited to what is considered good design practice for injection molding. Plastic bearings are subject to the same design cautions as all other plastic parts: creep, high thermal expansion, softening (increased wear/reduced life) at elevated temperature, brittle fractures at cold temperatures, and swelling due to moisture absorption. While most bearing-grade plastics/polymers are designed to reduce these design cautions, they still exist and should be carefully considered before specifying a solid polymer (plastic) type. Plastic bearings are now quite common, including usage in photocopy machines,
till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...
s,
farm equipment Agricultural machinery relates to the mechanical structures and devices used in farming or other agriculture. There are many types of such equipment, from hand tools and power tools to tractors and the countless kinds of farm implements that the ...
, textile machinery,
medical devices A medical device is any device intended to be used for medical purposes. Significant potential for hazards are inherent when using a device for medical purposes and thus medical devices must be proved safe and effective with reasonable assura ...
, food and packaging machines, car seating, and marine equipment. Common plastics include
nylon Nylon is a generic designation for a family of synthetic polymers composed of polyamides ( repeating units linked by amide links).The polyamides may be aliphatic or semi-aromatic. Nylon is a silk-like thermoplastic, generally made from petro ...
,
polyacetal Polyoxymethylene (POM), also known as acetal, polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic pol ...
,
polytetrafluoroethylene Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemour ...
(PTFE),
ultra-high-molecular-weight polyethylene Ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) is a subset of the thermoplastic polyethylene. Also known as high-modulus polyethylene, (HMPE), it has extremely long chains, with a molecular mass usually between 3.5 and 7.5 million amu. T ...
(UHMWPE), rulon,
PEEK Polyether ether ketone (PEEK) is a colourless organic thermoplastic polymer in the polyaryletherketone (PAEK) family, used in engineering applications. The polymer was first developed in November 1978, later being introduced to the market by Vic ...
, urethane, and
vespel Vespel is the trademark of a range of durable high-performance polyimide-based plastics made by DuPont. The one shown in the structure on the right was the first to be commercialized. Characteristics and applications Vespel is mostly used in ...
(a high-performance
polyimide Polyimide (sometimes abbreviated PI) is a polymer containing imide groups belonging to the class of high-performance plastics. With their high heat-resistance, polyimides enjoy diverse applications in roles demanding rugged organic materials, e.g ...
). * MDS (
molybdenum disulfide Molybdenum disulfide (or moly) is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is . The compound is classified as a transition metal dichalcogenide. It is a silvery black solid that occurs as the mineral molybdenit ...
) ** PEEK (
polyether ether ketone Polyether ether ketone (PEEK) is a colourless organic thermoplastic polymer in the polyaryletherketone (PAEK) family, used in engineering applications. The polymer was first developed in November 1978, later being introduced to the market by Vic ...
)


Others

* Igus, iglidur - Specially developed polymer bearing materials with life prediction *
Ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
bearings are very hard, so sand and other grit that enters the bearing are simply ground to a fine powder that does not inhibit the operation of the bearing. * Lubrite *
Lignum vitae Lignum vitae () is a wood, also called guayacan or guaiacum, and in parts of Europe known as Pockholz or pokhout, from trees of the genus ''Guaiacum''. The trees are indigenous to the Caribbean and the northern coast of South America (e.g: Col ...
is a self lubricating wood and in clocks it gives extremely long life. Also used with bronze wheels in ship rigging. * In a
piano The piano is a stringed keyboard instrument in which the strings are struck by wooden hammers that are coated with a softer material (modern hammers are covered with dense wool felt; some early pianos used leather). It is played using a keyboa ...
, various (usually) wooden parts of the keyboard and action are linked together by center pins typically made of
German silver Nickel silver, Maillechort, German silver, Argentan, new silver, nickel brass, albata, alpacca, is a copper alloy with nickel and often zinc. The usual formulation is 60% copper, 20% nickel and 20% zinc. Nickel silver does not contain the eleme ...
. These linkages usually have
felt Felt is a textile material that is produced by matting, condensing and pressing fibers together. Felt can be made of natural fibers such as wool or animal fur, or from synthetic fibers such as petroleum-based acrylic or acrylonitrile or wood ...
, or more rarely,
leather Leather is a strong, flexible and durable material obtained from the tanning, or chemical treatment, of animal skins and hides to prevent decay. The most common leathers come from cattle, sheep, goats, equine animals, buffalo, pigs and hogs, ...
bushings.


Lubrication

The types of
lubrication Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in a contact between two surfaces. The study of lubrication is a discipline in the field of tribology. Lubrication mechanisms such as fluid-lubric ...
system can be categorized into three groups: * Class I — bearings that require the application of a lubricant from an external source (e.g., oil, grease, etc.). * Class II — bearings that contain a lubricant within the walls of the bearing (e.g., bronze, graphite, etc.). Typically these bearings require an outside lubricant to achieve maximum performance. * Class III — bearings made of materials that are the lubricant. These bearings are typically considered "self-lubricating" and can run without an external lubricant. Examples of the second type of bearing are
Oilite Oilite is a brand of self-lubricating bearing that is made from metal alloys with pores that channel lubricants between the bearing itself and the shaft. It is manufactured from different types of material. Traditional Oilite is mostly made of copp ...
s and plastic bearings made from
polyacetal Polyoxymethylene (POM), also known as acetal, polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic pol ...
; examples of the third type are metalized graphite bearings and
PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemour ...
bearings. Most plain bearings have a plain inner surface; however, some are grooved, such as
spiral groove bearing Spiral groove bearings (also known as ''Rifle bearings'') are self-acting (journal and thrust), or hydrodynamic bearings used to reduce friction and wear without the use of pressurized lubricants. They have this ability due to special patterns of ...
. The grooves help lubrication enter the bearing and cover the whole journal. Self-lubricating plain bearings have a lubricant contained within the bearing walls. There are many forms of self-lubricating bearings. The first, and most common, are sintered metal bearings, which have porous walls. The porous walls draw oil in via
capillary action Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces li ...
and release the oil when
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
or
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...
is applied. An example of a sintered metal bearing in action can be seen in
self-lubricating chain Self-lubricating chains, also referred to as lube-free chains, are commonly found in both roller chain ( ANSI Standards, British Standards, and DIN Standards) and conveyor chain varieties, with specialty self-lubricating chains also available. The ...
s, which require no additional lubrication during operation. Another form is a solid one-piece metal bushing with a
figure eight Figure 8 (figure of 8 in British English) may refer to: * 8 (number), in Arabic numerals Entertainment * ''Figure 8'' (album), a 2000 album by Elliott Smith * "Figure of Eight" (song), a 1989 song by Paul McCartney * '' Figure Eight EP'', ...
groove channel on the inner diameter that is filled with graphite. A similar bearing replaces the figure eight groove with holes plugged with graphite. This lubricates the bearing inside and out. The last form is a plastic bearing, which has the lubricant molded into the bearing. The lubricant is released as the bearing is run in. There are three main types of lubrication: ''full-film condition'', ''boundary condition'', and ''dry condition''. Full-film conditions are when the bearing's load is carried solely by a film of fluid lubricant and there is no contact between the two bearing surfaces. In mix or boundary conditions, load is carried partly by direct surface contact and partly by a film forming between the two. In a dry condition, the full load is carried by surface-to-surface contact. Bearings that are made from bearing grade materials always run in the dry condition. The other two classes of plain bearings can run in all three conditions; the condition in which a bearing runs is dependent on the operating conditions, load, relative surface speed, clearance within the bearing, quality and quantity of lubricant, and temperature (affecting lubricant viscosity). If the plain bearing is not designed to run in the dry or boundary condition, it has a high coefficient of friction and wears out. Dry and boundary conditions may be experienced even in a fluid bearing when operating outside of its normal operating conditions; e.g., at startup and shutdown.


Fluid lubrication

Fluid lubrication results in a full-film or a boundary condition lubrication mode. A properly designed bearing system reduces friction by eliminating surface-to-surface contact between the journal and bearing through fluid dynamic effects. Fluid bearings can be ''hydrostatically'' or ''hydrodynamically'' lubricated. Hydrostatically lubricated bearings are lubricated by an external
pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they u ...
that maintains a ''static'' amount of pressure. In a hydrodynamic bearing the pressure in the oil film is maintained by the rotation of the journal. Hydrostatic bearings enter a ''hydrodynamic state'' when the journal is rotating. Hydrostatic bearings usually use
oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
, while hydrodynamic bearings can use oil or grease, however bearings can be designed to use whatever fluid is available, and several pump designs use the pumped fluid as a lubricant. Hydrodynamic bearings require greater care in design and operation than hydrostatic bearings. They are also more prone to initial wear because lubrication does not occur until there is rotation of the shaft. At low rotational speeds the lubrication may not attain complete separation between shaft and bushing. As a result, hydrodynamic bearings may be aided by secondary bearings that support the shaft during start and stop periods, protecting the fine tolerance machined surfaces of the journal bearing. On the other hand, hydrodynamic bearings are simpler to install and are less expensive. In the hydrodynamic state a lubrication "wedge" forms, which lifts the journal. The journal also slightly shifts horizontally in the direction of rotation. The location of the journal is measured by the ''attitude angle'', which is the angle formed between the vertical and a line that crosses through the center of the journal and the center of the bearing, and the eccentricity ratio, which is the ratio of the distance of the centre of the journal from the centre of the bearing, to the overall radial clearance. The attitude angle and eccentricity ratio are dependent on the direction and speed of rotation and the load. In hydrostatic bearings the oil pressure also affects the eccentricity ratio. In electromagnetic equipment like motors,
electromagnetic force In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of ...
s can counteract gravity loads, causing the journal to take up unusual positions. One disadvantage specific to fluid-lubricated, hydrodynamic journal bearings in high-speed machinery is ''oil whirl''—a self-excited vibration of the journal. Oil whirl occurs when the lubrication wedge becomes unstable: small disturbances of the journal result in reaction forces from the oil film, which cause further movement, causing both the oil film and the journal to "whirl" around the bearing shell. Typically the whirl frequency is around 42% of the journal turning speed. In extreme cases oil whirl leads to direct contact between the journal and the bearing, which quickly wears out the bearing. In some cases the frequency of the whirl coincides with and "locks on to" the critical speed of the machine shaft; this condition is known as "oil whip". Oil whip can be very destructive. Oil whirl can be prevented by a stabilising force applied to the journal. A number of bearing designs seek to use bearing geometry to either provide an obstacle to the whirling fluid or to provide a stabilising load to minimize whirl. One such is called the ''lemon bore'' or ''elliptical bore''. In this design,
shim Shim may refer to: * Shim (spacer), a thin and often tapered or wedged piece of material ** CPU shim, a spacer for a computer heat sink ** Shim (fencing), a device used in the sport fencing ** Shim (lock pick), a tool used to bypass padlocks * Sh ...
s are installed between the two halves of the bearing housing and then the bore is machined to size. After the shims are removed, the bore resembles a lemon shape, which decreases the clearance in one direction of the bore and increases the pre-load in that direction. The disadvantage of this design is its lower load carrying capacity, as compared to typical journal bearings. It is also still susceptible to oil whirl at high speeds, however its cost is relatively low. Another design is the ''pressure dam'' or ''dammed groove'',. which has a shallow relief cut in the center of the bearing over the top half of the bearing. The groove abruptly stops in order to create a downward force to stabilize the journal. This design has a high load capacity and corrects most oil whirl situations. The disadvantage is that it only works in one direction. Offsetting the bearing halves does the same thing as the pressure dam. The only difference is the load capacity increases as the offset increases. A more radical design is the tilting-pad design, which uses multiple pads that are designed to move with changing loads. It is usually used in very large applications but also finds extensive application in modern turbomachinery because it almost completely eliminates oil whirl. File:Plain bearing lemon bore.svg, A lemon bore File:Plain bearing pressure dam.svg, A pressure dam File:Miba Journal Bearing.jpg, Miba tilting pad bearing used in turbomachinery


Related components

Other components that are commonly used with plain bearings include: * '' Pillow block'': These are standardized bearing mounts designed to accept plain bearings. They are designed to mount to a flat surface. * ''
Ring oiler A ring oiler or oil ring is a form of oil-lubrication system for bearings. Ring oilers were used for medium-speed applications with moderate loads, during the first half of the 20th century. These represented the later years of the stationary s ...
'': A lubricating mechanism used in the first half of the 20th century for medium speed applications. * ''
Stuffing box A stuffing box or gland package is an assembly which is used to house a gland seal. It is used to prevent leakage of fluid, such as water or steam, between sliding or turning parts of machine elements. Components A stuffing box of a sailing boat ...
'': A sealing system used to keep fluid from leaking out of a pressurized system through the plain bearing.


See also

* * * * *
Roller bearing In mechanical engineering, a rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements (such as balls or rollers) between two concentric, grooved rings called races. The relative m ...
*
Stave bearing A stave bearing is a simple journal bearing where a shaft rotates in a bearing housing. Rather than the usual arrangement where the fixed part of the bearing surrounds most of the circumference of the shaft in one or two pieces, a stave bearing uses ...


References


Bibliography

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External links


Kinematic Models for Design Digital Library (KMODDL)
- Movies and photos of hundreds of working mechanical-systems models at Cornell University. Also includes a
e-book library
of classic texts on mechanical design and engineering.
CSX Dictionary J
{{DEFAULTSORT:Plain Bearing Bearings (mechanical)