Plain bearing

A plain bearing, or more commonly sliding bearing and slide bearing (in railroading sometimes called a solid bearing or friction bearing), is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore, the journal (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[1] or the ways on the bed of a lathe.

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.[2]

NYC 100-driving-axle-friction-bearing
Plain bearing on a 1906 S-Motor locomotive showing the axle, bearing, oil supply and oiling pad


The design of a plain bearing depends on the type of motion the bearing must provide. The three types of motions possible are:


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 or babbitt and a hardened steel shaft is used in the bearing.[6][7]

Integral bearings are not as common because bushings are easier to accommodate and can be replaced if necessary.[1] 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, which is both a thrust bearing and a journal bearing.


A bushing, also known as a bush, is an independent plain bearing that is inserted into a housing to provide a bearing surface for rotary applications; this is the most common form of a plain bearing.[8] 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 (or clinch) 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.[9]

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.[10] 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 rings, 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 thrust washer.


Two-piece plain bearings, known as full bearings in industrial machinery,[11] are commonly used for larger diameters, such as crankshaft bearings. The two halves are called shells.[12] There are various systems used to keep the shells located. The most common method is a tab on the parting line edge that correlates with a notch in the housing to prevent axial movement after installation. For large, thick shells a button stop or dowel 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 that keys the shell to the housing through a hole or slot in the shell.[13]

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.[12]


Linear-table with detail numbered

A linear table with four linear bearings (1)

GWR Spoked wagon wheels

A wheelset from a GWR wagon showing a plain, or journal, bearing end[3]

Archbar ACL143

Archbar type truck with journal bearings in journal boxes as used on some steam locomotive tenders. A version of the archbar truck was at one time also used on US freight cars

Journal box

Diagram of a railroad Journal box[3]

Plain-bearing DIN-ISO4379 Type-G

A solid sleeve bushing

Plain-bearing DIN-ISO4379 Type-F

A flanged bushing

Clenched bushing

A clenched (or clinched) bushing


Plain bearings must be made from a material that is durable, low friction, 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 or to seize when lubrication fails.[8][14][15]


Babbitt is usually used in integral bearings. It is coated over the bore, usually to a thickness of 1 to 100 thou (0.025 to 2.540 mm), depending on the diameter. Babbitt bearings are designed to not damage the journal during direct contact and to collect any contaminants in the lubrication.[11]


Assorted Doppelmayr glacier bushings
Split bi-material bushings: a metal exterior with an inner plastic coating

Bi-material bearings consist of two materials, a metal shell and a plastic bearing surface. Common combinations include a steel-backed PTFE-coated bronze and aluminum-backed Frelon.[16] 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-backed frelon are commonly used in corrosive environments because the Frelon is chemically inert.[17]

Bearing properties of various bi-material bearings[17]
Temperature range P (max.)
[psi (MPa)]
V (max.)
[sfm (m/s)]
PV (max.)
[psi sfm (MPa m/s)]
Steel-backed PTFE-coated bronze −328–536 °F or −200–280 °C 36,000 psi or 248 MPa 390 (2.0 m/s) 51,000 (1.79 MPa m/s)
Aluminum-backed frelon −400–400 °F or −240–204 °C 3,000 psi or 21 MPa 300 (1.52 m/s) 20,000 (0.70 MPa m/s)


A common plain bearing design utilizes a hardened and polished steel shaft and a softer bronze bushing. The bushing is replaced whenever it has worn too much.

Common bronze alloys used for bearings include: SAE 841, SAE 660 (CDA 932), SAE 863, and CDA 954.[18]

Bearing properties of various bronze alloys[18]
Temperature range P (max.)
[psi (MPa)]
V (max.)
[sfm (m/s)]
PV (max.)
[psi sfm (MPa m/s)]
SAE 841 10–220 °F (−12–104 °C) 2,000 psi (14 MPa) 1,200 (6.1 m/s) 50,000 (1.75 MPa m/s)
SAE 660 10–450 °F (−12–232 °C) 4,000 psi (28 MPa) 750 (3.8 m/s) 75,000 (2.63 MPa m/s)
SAE 863 10–220 °F (−12–104 °C) 4,000 psi (28 MPa) 225 (1.14 m/s) 35,000 (1.23 MPa m/s)
CDA 954 Less than 500 °F (260 °C) 4,500 psi (31 MPa) 225 (1.14 m/s) 125,000 (4.38 MPa m/s)

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.[6]


In harsh environments, such as ovens and dryers, a copper and graphite alloy, commonly known by the trademarked name graphalloy, is used. The graphite is a dry lubricant, therefore it is low friction and low maintenance. The copper adds strength, durability, and provides heat dissipation characteristics.

Bearing properties of graphitic materials
Temperature range P (max.)
[psi (MPa)]
V (max.)
[sfm (m/s)]
PV (max.)
[psi sfm (MPa m/s)]
Graphalloy[17] −450–750 °F or −268–399 °C 750 psi or 5 MPa 75 (0.38 m/s) 12,000 (0.42 MPa m/s)
Graphite ? ? ? ?

Unalloyed graphite bearings are used in special applications, such as locations that are submerged in water.[19]


Known as jewel bearings, these bearings use jewels, such as sapphire, ruby, and garnet.


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. These materials can heat rapidly when used in applications outside the recommended pV limits.

Solid polymer type bearings are limited by the injection molding 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, tills, farm equipment, textile machinery, medical devices, food and packaging machines, car seating, and marine equipment.

Common plastics include nylon, polyacetal, polytetrafluoroethylene (PTFE), ultra-high-molecular-weight polyethylene (UHMWPE), rulon, PEEK, urethane, and vespel (a high-performance polyimide).[20][21][22]

Bearing properties of various plastics[20][21][23]
Temperature range P (max.) [psi (MPa)] V (max.) [sfm (m/s)] PV (max.) [psi sfm (MPa m/s)]
Frelon[24] −400 to 500 °F (−240 to 260 °C)[25] 1,500 psi (10 MPa) 140 (0.71 m/s) 10,000 (0.35 MPa m/s)
Nylon −20 to 250 °F (−29 to 121 °C) 400 psi (3 MPa) 360 (1.83 m/s) 3,000 (0.11 MPa m/s)
MDS-filled nylon blend 1 −40 to 176 °F (−40 to 80 °C) 2,000 psi (14 MPa) 393 (2.0 m/s) 3,400 (0.12 MPa m/s)
MDS-filled nylon blend 2 −40 to 230 °F (−40 to 110 °C) 300 psi (2 MPa) 60 (0.30 m/s) 3,000 (0.11 MPa m/s)
PEEK blend 1 −148 to 480 °F (−100 to 249 °C) 8,500 psi (59 MPa) 400 (2.0 m/s) 3,500 (0.12 MPa m/s)
PEEK blend 2 −148 to 480 °F (−100 to 249 °C) 21,750 psi (150 MPa) 295 (1.50 m/s) 37,700 (1.32 MPa m/s)
Polyacetal −20 to 180 °F (−29 to 82 °C) 1,000 psi (7 MPa) 1,000 (5.1 m/s) 2,700 (0.09 MPa m/s)
PTFE −350 to 500 °F (−212 to 260 °C) 500 psi (3 MPa) 100 (0.51 m/s) 1,000 (0.04 MPa m/s)
Glass-filled PTFE −350 to 500 °F (−212 to 260 °C) 1,000 psi (7 MPa) 400 (2.0 m/s) 11,000 (0.39 MPa m/s)
Rulon 641 −400 to 550 °F (−240 to 288 °C) 1,000 psi (7 MPa) 400 (2.0 m/s) 10,000 (0.35 MPa m/s)[26]
Rulon J −400 to 550 °F (−240 to 288 °C) 750 psi (5 MPa) 400 (2.0 m/s) 7,500 (0.26 MPa m/s)
Rulon LR −400 to 550 °F (−240 to 288 °C) 1,000 psi (7 MPa) 400 (2.0 m/s) 10,000 (0.35 MPa m/s)
UHMWPE −200 to 180 °F (−129 to 82 °C) 1,000 psi (7 MPa) 100 (0.51 m/s) 2,000 (0.07 MPa m/s)
MDS-filled urethane −40 to 180 °F (−40 to 82 °C) 700 psi (5 MPa) 200 (1.02 m/s) 11,000 (0.39 MPa m/s)
Vespel −400 to 550 °F (−240 to 288 °C) 4,900 psi (34 MPa) 3,000 (15.2 m/s) 300,000 (10.5 MPa m/s)


  • igus, iglidur Specially developed polymer bearing materials with life prediction
  • Ceramic 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[27]
  • Lignum vitae is a self lubricating wood and in clocks it gives extremely long life. Also used with bronze wheels in ship rigging.
  • In a piano, various (usually) wooden parts of the keyboard and action are linked together by center pins typically made of German silver. These linkages usually have felt, or more rarely, leather bushings.


Graphite filled groove bushing
A graphite-filled groove bushing

The types of lubrication system can be categorized into three groups:[10]

  • 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 Oilites and plastic bearings made from polyacetal; examples of the third type are metalized graphite bearings and PTFE bearings.[10]

Most plain bearings have a plain inner surface; however, some are grooved, such as spiral groove bearing. The grooves help lubrication enter the bearing and cover the whole journal.[28]

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[29] and release the oil when pressure or heat is applied.[30] An example of a sintered metal bearing in action can be seen in self-lubricating chains, which require no additional lubrication during operation. Another form is a solid one-piece metal bushing with a figure eight 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.[31] 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.[32]

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

Hydrodynamic lubrication attitude angle
A schematic of a journal bearing under a hydrodynamic lubrication state showing how the journal centerline shifts from the bearing centerline.

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 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.[11] Hydrostatic bearings usually use oil, 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.[33]

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 forces can counteract gravity loads, causing the journal to take up unusual positions.[11]

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.[11][34]

Plain bearing lemon bore
A lemon bore

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, shims 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.[11]

Plain bearing pressure dam
A pressure dam

Another design is the pressure dam or dammed groove,[35] 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.[11]

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.

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 lubricating mechanism used in the first half of the 20th century for medium speed applications.
  • Stuffing box: A sealing system used to keep fluid from leaking out of a pressurized system through the plain bearing.

See also


  1. ^ a b c d e Bearings and bearing metals 1921, p. 1
  2. ^ Journal Bearings, archived from the original on 2009-12-30, retrieved 2009-12-29.
  3. ^ a b c CSX Dictionary J Archived 2014-07-29 at the Wayback Machine
  4. ^ Car and Locomotive Cyclopedia Of American Practice
  5. ^ "The Evolution of Railway Axlebox Technology". Evolution. SKF. 2010-12-07. Retrieved 2014-09-18.
  6. ^ a b Bearings and bearing metals 1921, p. 15
  7. ^ Bearings and bearing metals 1921, p. 18
  8. ^ a b Brumbach, Michael E.; Clade, Jeffrey A. (2003), Industrial Maintenance, Cengage Learning, p. 199, ISBN 978-0-7668-2695-3.
  9. ^ Neale 1995, p. A12.1.
  10. ^ a b c Weichsel, Dick (1994-10-03), "Plane bearings" (PDF), ESC Report, 5 (1): 1–2, archived from the original (PDF) on 2009-12-10.
  11. ^ a b c d e f g Journal Bearings, archived from the original on 2001-05-02, retrieved 2010-05-08.
  12. ^ a b Mobley, R. Keith (2001), Plant engineer's handbook (5th ed.), Butterworth-Heinemann, p. 1094, ISBN 978-0-7506-7328-0.
  13. ^ Neale 1995, p. A11.6.
  14. ^ Bearings and bearing metals 1921, p. 29
  15. ^ Bearings and bearing metals 1921, p. 30
  16. ^ Frelon bushings Archived 2010-11-26 at WebCite
  17. ^ a b c McMaster-Carr catalog (115th ed.), McMaster-Carr, p. 1115, retrieved 2009-12-21.
  18. ^ a b McMaster-Carr catalog (115th ed.), McMaster-Carr, p. 1116, retrieved 2009-12-17.
  19. ^ Glaeser, William A. (1992), Materials for tribology, Elsevier, ISBN 978-0-444-88495-4.
  20. ^ a b McMaster-Carr catalog (115th ed.), McMaster-Carr, p. 1110, retrieved 2009-12-22.
  21. ^ a b McMaster-Carr catalog (115th ed.), McMaster-Carr, p. 1114, retrieved 2009-12-21.
  22. ^ McMaster-Carr catalog (115th ed.), McMaster-Carr, p. 1121, retrieved 2009-12-21.
  23. ^ McMaster-Carr catalog (115th ed.), McMaster-Carr, p. 1111, retrieved 2009-12-22.
  24. ^ Frelon lined linear bushings (PDF), March 1997, archived from the original (PDF) on 2010-11-26, retrieved 2010-11-26.
  25. ^ Frelon lined linear bearings, archived from the original on 2010-11-27, retrieved 2010-11-26.
  26. ^ Rulon 641, retrieved 2015-10-26
  27. ^ Silano, Louis (1993). Bridge Inspection and Rehabilitation. Wiley. p. 185. ISBN 978-0-471-53262-0.
  28. ^ McMaster-Carr catalog (115th ed.), McMaster-Carr, p. 1119, retrieved 2009-12-20.
  29. ^ Oilite (PDF), archived from the original (PDF) on 2009-12-16, retrieved 2009-12-16.
  30. ^ Curcio, Vincent (2001), Chrysler: The Life and Times of an Automotive Genius, Oxford University Press US, p. 485, ISBN 978-0-19-514705-6.
  31. ^ McMaster-Carr catalog (115th ed.), McMaster-Carr, p. 1118, retrieved 2009-12-20.
  32. ^ Iglide (PDF), pp. 1.2–1.3, archived from the original (PDF) on 2009-12-10, retrieved 2009-12-10.
  33. ^ "Converting Ships to Seawater Lubricated Shaft Lines". Gallagher Fluid Seals. Retrieved 21 July 2017.
  34. ^ "Fundamentals of Rotating Machinery Diagnostics", pps480 - 489. (2002), Bently.D. & Hatch.C. The Bently Pressurised Bearing Co. ISBN 0-9714081-0-6
  35. ^ Neale 1995, p. A10.4.


External links

Babbitt (alloy)

Babbitt, also called Babbitt metal or bearing metal, is any of several alloys used for the bearing surface in a plain bearing.

The original Babbitt alloy was invented in 1839 by Isaac Babbitt in Taunton, Massachusetts, United States. He disclosed one of his alloy recipes but kept others as trade secrets. Other formulations were developed later. Like other terms whose eponymous origin is long since deemphasized (such as diesel engine or eustachian tube), the term babbitt metal is frequently styled in lowercase. It is preferred over the term "white metal", because the latter term may refer to various bearing alloys, lead- or tin-based alloys, or zinc die-casting metal.

Babbitt metal is most commonly used as a thin surface layer in a complex, multi-metal structure, but its original use was as a cast-in-place bulk bearing material. Babbitt metal is characterized by its resistance to galling. Babbitt metal is soft and easily damaged, which suggests that it might be unsuitable for a bearing surface. However, its structure is made up of small hard crystals dispersed in a softer metal, which makes it a metal matrix composite. As the bearing wears, the softer metal erodes somewhat, which creates paths for lubricant between the hard high spots that provide the actual bearing surface. When tin is used as the softer metal, friction causes the tin to melt and function as a lubricant, which protects the bearing from wear when other lubricants are absent.

Internal combustion engines use Babbitt metal which is primarily tin-based because it can withstand cyclic loading. Lead-based Babbitt tends to work-harden and develop cracks but it is suitable for constant-turning tools such as sawblades.

Bearing (mechanical)

A bearing is a machine element that constrains relative motion to only the desired motion, and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or to the directions of the loads (forces) applied to the parts.

Rotary bearings hold rotating components such as shafts or axles within mechanical systems, and transfer axial and radial loads from the source of the load to the structure supporting it. The simplest form of bearing, the plain bearing, consists of a shaft rotating in a hole. Lubrication is often used to reduce friction. In the ball bearing and roller bearing, to prevent sliding friction, rolling elements such as rollers or balls with a circular cross-section are located between the races or journals of the bearing assembly. A wide variety of bearing designs exists to allow the demands of the application to be correctly met for maximum efficiency, reliability, durability and performance.

The term "bearing" is derived from the verb "to bear"; a bearing being a machine element that allows one part to bear (i.e., to support) another. The simplest bearings are bearing surfaces, cut or formed into a part, with varying degrees of control over the form, size, roughness and location of the surface. Other bearings are separate devices installed into a machine or machine part. The most sophisticated bearings for the most demanding applications are very precise devices; their manufacture requires some of the highest standards of current technology.


Bushing may refer to:

Bushing (bearing), a type of plain bearing

Bushing (electrical), an insulated device that allows a conductor to pass through a grounded conducting barrier

Bushing (isolator), a mechanical device used to reduce vibrations

Threaded bushing, a metal sleeve with screw threads

Ducati L-twin engine

The next new Ducati engine to appear after the Ducati Apollo was the 90° V-twin, initial Grand Prix racing versions being 500 cc, and the production bikes were 750 cc. There was also the Ducati 750 Imola Desmo that won at Imola in 1972. These engines had bevel gear shaft drive to the overhead camshaft, and were produced in round, square, and Mille crankcases. In the 1980s these gave way to the belt drive camshaft engines that have continued to this day, in air-cooled and liquid-cooled form. The Mille used a plain bearing crank, like the belt models.

Frelon (material)

Frelon is a polytetrafluoroethylene (PTFE) based material with other proprietary fillers to increase bearing characteristics, such as low wear, low friction, and high strength. It is chemically inert and self lubricating. It qualifies as a class III plain bearing. The load capacity of a frelon-lined bearing is typically four to eight times that of a comparable ball bearing; for instance, a 0.5 in (13 mm) Frelon-lined bearing can support the same load as a 1 in (25 mm) ball bearing.

Jewel bearing

A jewel bearing is a plain bearing in which a metal spindle turns in a jewel-lined pivot hole. The hole is typically shaped like a torus and is slightly larger than the shaft diameter. The jewel material is usually synthetic sapphire or ruby (corundum). Jewel bearings are used in precision instruments where low friction, long life, and dimensional accuracy are important. Their largest use is in mechanical watches.


Komotini (Greek: Κομοτηνή; Turkish: Gümülcine) is a city in the region of East Macedonia and Thrace, northeastern Greece. It is the capital of the Rhodope regional unit. It was the administrative centre of the Rhodope-Evros super-prefecture until its abolition in 2010, by the Kallikratis Plan. The city is home to the Democritus University of Thrace, founded in 1973. Komotini is home to a sizeable Turkish-speaking Muslim minority.

Built at the northern part of the plain bearing the same name, Komotini is one of the main administrative, financial and cultural centers of northeastern Greece and also a major agricultural and breeding center of the area. It is also a significant transport interchange, located 795 km NE of Athens and 281 km NE of Thessaloniki. The presence of the Democritus University makes Komotini the home of thousands of Greek and international students and this, combined with an eclectic mix of Western and Oriental elements in the city's daily life, have made it an increasingly attractive tourist destination.

Linear-motion 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, XY tables, roller tables and some dovetail slides are bearings moved by drive mechanisms. Not all linear slides are motorized, and non-motorized dovetail slides, ball bearing slides and roller slides provide low-friction linear movement for equipment powered by inertia or by hand. All linear slides provide linear motion based on bearings, whether they are ball bearings, dovetail bearings, linear roller bearings, magnetic or fluid bearings. XY Tables, linear stages, machine slides and other advanced slides use linear motion bearings to provide movement along both X and Y multiple axis.


Nylatron is a tradename for a family of nylon plastics, typically filled with molybdenum disulfide lubricant powder. It is used to cast plastic parts for machines, because of its mechanical properties and wear-resistance.Nylatron is a brand name of DSM Plastics and was originally developed and manufactured by Nippon Polypenco Limited.Nylatron is used in several applications such as:

rotary lever actuators where unusual shapes are required

heavy-duty caster wheels, normally as a replacement for cast iron or forged steel

plain bearing material, especially in screw conveyor applications


Plastigauge is a measuring tool used for measuring plain bearing clearances, such as in engines. Other uses include marine drive shaft bearings, turbine housing bearings, pump and pressure system bearings, shaft end-float, flatness and clearance in pipe-flanges and cylinder heads. Wherever it is required to determine the separation between hidden surfaces. Plastigauge is a registered trademark of Plastigauge Ltd., West Sussex, United Kingdom. Plastigauge was introduced to US retail sales in 1948.Plastigauge consists of a strip of soft material with precise known dimensions and deformation characteristics. This is sandwiched between a clean bearing surface on a shaft and the bearing shell itself. The plastigauge flattens after the bearing cap is tightened. The dimensional clearance is then determined by comparing the amount that the gauge material has flattened using a template. Letter designation describes the range of measurement use for each gauge.

PL-A (0.001" - 0.007 in. (0.025mm-0.175mm)), PL-B (0.004" - 0.010 in. (0.100mm-0.250mm)), PL-C (0.007" - 0.020 in. (0.175mm-0.500mm)), PL-D (0.020" - 0.040 in. (0.500mm-1.00mm)), PL-E (0.030" - 0.070 in. (0.75mm-1.75mm)), PL-X Plastigauge (0.018mm - 0.045mm)

Porsche Intermediate Shaft Bearing issue

Most models of the 996 generation of the Porsche 911 sports car were afflicted with a vulnerability in the intermediate shaft (IMS) that drove their engines' camshafts. Failure of the ball bearing of the IMS generally leads to varying degrees of engine failure. Generally, after IMS bearing failure, the engine internals are contaminated with debris from the failure that requires the engine to be stripped and rebuilt. In severe failure modes, cam timing may be affected, leading to valve-piston impact, necessitating replacement of the entire engine.

Rotary stage

A rotary stage is a component of a motion system used to restrict an object to a single axis of rotation. The terms rotary table or rotation stage are often used interchangeably with rotary stage. All rotary stages consist of a platform and a base, joined by some form of guide in such a way that the platform is restricted to rotation about a single axis with respect to the base. In common usage, the term rotary stage may or may not also include the mechanism by which the angular position of the platform is controlled relative to the base.

Spherical bearing

A spherical plain bearing is a bearing that permits angular rotation about a central point in two orthogonal directions (usually within a specified angular limit based on the bearing geometry). Typically these bearings support a rotating shaft in the [bore] of the inner ring that must move not only rotationally, but also at an angle.

Self-aligning spherical bearings were first used by James Nasmyth around 1840 to support line shaft bearings in mills and machine shops. For long shafts it was impossible to accurately align bearings, even if the shaft was perfectly straight. Nasmyth used brass bearing shells between hemispherical brass cups to align the bearings to self-align.

Spring pin

A spring pin (also called tension pin or roll pin) is a mechanical fastener that secures the position of two or more parts of a machine relative to each other. Spring pins have a body diameter which is larger than the hole diameter, and a chamfer on either one or both ends to facilitate starting the pin into the hole. The spring action of the pin allows it to compress as it assumes the diameter of the hole. The force exerted by the pin against the hole wall retains it in the hole, therefore a spring pin is considered a self retaining fastener.

Spring pins may be used to retain a shaft as a journal in a plain bearing, as a type of key to fasten one shaft to another, or to precisely fasten flat faces of mating parts together through symmetric hole locations.

Steam locomotive components

This is a glossary of the components found on typical steam locomotives.

Guide to steam locomotive components (The image is of a composite imaginary locomotive, not all components are present on all locomotives and not all possible components are present and/or labelled in the illustration above).

1 Tender — Container holding both water for the boiler and fuel such as wood, coal or oil for the fire box.

2 Cab — Compartment where the engineer and fireman control the engine and tend the firebox.

3 Whistle — Steam powered whistle, located on top of the boiler and used for signalling and warning.

4 Reach rod — Rod linking the reversing lever in the cab (often a johnson bar)) to the valve gear.

5 Safety valve — Pressure relief valve to stop the boiler pressure exceeding the operating limit.

6 Generator — electrical generator driven by small steam turbine, for locomotive lighting and headlight.

7 Sand dome — Holds sand that is dropped on the rail in front of the driving wheels to improve traction, especially in wet or icy conditions.

8 Throttle Lever/Regulator — sets the opening of the regulator/throttle valve (#31) which controls the pressure of steam entering the cylinders.

9 Steam dome — Collects the steam at the top of the boiler so that it can be fed to the engine via main steam pipe, or dry pipe, and the regulator/throttle valve

10 Air pump or compressor — compresses air for operating the brakes (train air brake system). This is sometimes called a Westinghouse pump or Knorr pump after George Westinghouse and Georg Knorr. Single stage steam-driven air compressor or higher capacity two-stage, cross-compound compressors were used

11 Smokebox — Collects the hot gases that have passed from the firebox and through the boiler tubes. It may contain a cinder guard to prevent hot cinders being exhausted up the chimney. Usually has a blower to help draw the fire when the regulator is closed. Steam exhausting from the cylinders is also directed up the chimney to draw air through the firebed while the regulator is open.

Blower a circular pipe below the chimney petticoat pipe, with holes to blow steam upwards. Provides a draught to maintain adequate combustion when locomotive is stationary and the blastpipe is not effective. This draught also prevents smoke and flames from entering the cab.

Petticoat pipe is a pipe with a bellmouth-shaped end extending into the smokebox and the other end in the smoke stack. Its function is to enhance and equalize draft through the boiler tubes.

12 Steam pipe — carries steam to the cylinders.

13 Smoke box door — Hinged circular door to allow service access to the smoke box to fix air leaks and remove char.

14 Hand rail — Support rail for crew when walking along the foot board.

15 Trailing truck/Rear bogie — Wheels at the rear of the locomotive to help support the weight of the cab and fire box.

16 Foot board/Running board — Walkway along the locomotive to facilitate inspection and maintenance. UK terminology is Footplate.

17 Frame — Carries boiler, cab and engines and is supported on driving wheels and leading and trailing trucks. The axles run in slots in the frames. American locomotives usually have bar frames (made from steel bar) or cast steel frames (see Bury bar frame locomotive), while British locomotives usually have plate frames (made from steel plate).

18 Brake shoe and brake block — rub on all the driving wheel treads for braking.

19 Sand pipe — Deposits sand directly in front of the driving wheels to aid traction.

20 Side rods/Coupling rods — Connects the driving wheels together.

21 Valve gear/motion — System of rods and linkages synchronising the valves with the pistons and controls the running direction and power of the locomotive.

22 Main rod/Connecting rod — Steel arm that converts the horizontal motion of the piston into a rotary motion of the driver wheels. The connection between piston and main rod is a cross-head which slides on a horizontal bar behind the cylinder.

23 Piston rod — Connects the piston to the cross-head.

24 Piston — Driven backward and forward within the cylinder by steam pressure, producing motion from steam expansion.

25 Valve — Controls the supply of steam to the cylinders, valve position relative to piston determined by valve gear connected to driving wheel. Steam locomotives may have slide valves, piston valves or poppet valves.

26 Valve chest/steam chest — Valve chamber adjacent to cylinder, contains passageways to distribute steam to the cylinders.

27 Firebox — Furnace chamber that is built into the boiler and surrounded by water. Various combustible materials can be used as fuel but the most common were coal, coke, wood or oil.

28 Boiler tubes — Carry hot gasses from the fire box through the boiler, heating the surrounding water.

29 Boiler — container almost full of water with air space above. The water is heated by hot gases passing through tubes, producing steam in the space above the water.

30 Superheater tubes — Pass steam back through the boiler to dry and superheat the steam for greater efficiency.

31 Regulator/Throttle valve — Controls the amount of steam delivered to the cylinders (also see #8), one of two ways to vary power of the engine (throttle governing)

32 Superheater — Feeds steam back through boiler tubes to superheat (heat beyond boiling temperature of water at boiler pressure) the steam to increase the engine efficiency and power.

33 Chimney/Smokestack — Short chimney on top of the smokebox to carry the exhaust (smoke) away from the engine so that it doesn't obscure the footplate crew forward view. Usually extended down inside the smokebox - the extension is called a petticoat. Some railways, e.g. the Great Western Railway, fitted a decorative copper cap to the top of the chimney.

34 Headlight — Light on front of the smoke box to illuminate track ahead and warn approach of locomotive to other track occupants.

35 Brake hose — Air or vacuum hose for transmitting brake system pressure/vacuum to train brakes. See air brake and vacuum brake.

36 Water compartment — Container for water used by the boiler to produce steam.

37 Coal bunker — Fuel supply for the furnace, may be wood, coal/coke or oil. Fed to the firebox either manually or, for bigger fire grates, by mechanical stoker.

38 Grate — Holds the burning fuel and allows ash to drop through.

39 Ashpan hopper — Collects the ash from the fire.

40 Journal box — Housing for the plain bearing on a Driving wheel axle.

41 Equalising beams/Equalising levers/Equalising bars — Part of the locomotive suspension system, connected to leaf springs, free to pivot about their centre which is fixed to the frame. Function is to even out weight carried on adjacent axles on uneven or poorly laid tracks.

42 Leaf Springs — Main suspension springs for the locomotive. Each driver wheel supports its share of the locomotive weight using a leaf spring which connects the axle journal box to the frame.

43 Driving wheel/Driver — Wheel driven by the pistons to move the locomotive. Drivers are balanced with weights to reduce unwanted motion of the locomotive. There are 3 sets of driving wheels in this example.

44 Pedestal or saddle — Connects a leaf spring to a driver wheel journal box.

45 Blast pipe — Directs exhaust steam up the chimney, creating a draught that draws air through the fire and along the boiler tubes.

46 Pilot truck/Leading bogie — Wheels at the front to support weight of boiler front end/smokebox and reduce flanging forces between front driving wheels and rail when rounding curves.

47 Coupling/Coupler — Device at the front and rear of the locomotive for connecting locomotives and rail cars together.

Snifting valve (not shown) — An anti-vacuum valve which permits air to be drawn through the superheater and cylinders which allows the engine to coast freely when the regulator is closed.

Wheel bearing

Wheel bearing may refer to:

Ball bearing

Bearing (mechanical)

Fluid bearing

Jewel bearing

Journal bearing

Needle roller bearing

Plain bearing

Rolling-element bearing

Self-aligning ball bearing

Spherical roller bearing

Tapered roller bearing

Thrust bearing

Wheelset (rail transport)

A wheelset is the wheel–axle assembly of a railroad car. The frame assembly beneath each end of a car, railcar or locomotive that holds the wheelsets is called the bogie (or truck in North America). Most North American freight cars have two bogies with two or three wheelsets, depending on the type of car; short freight cars generally have no bogies but instead have two wheelsets.

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