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 only a bearing surface and no rolling elements. Therefore, 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 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.
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. It is also referred to as a sleeve bearing, especially in applications where a cylindrical bushing supports the shaft. In locomotive and railroad car applications journal bearing specifically referred to the plain bearings once used at the ends of the axles of railroad wheel sets, enclosed by axleboxes (also called journal boxes). Axlebox bearings today are rolling-element bearings rather than plain bearings. Linear bearing: 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). Thrust bearing: A thrust bearing provides a bearing surface for forces acting axial to the shaft. One example is a propeller shaft.
Integral An integral plain bearing is 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.
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 door hinge, which is both a thrust bearing 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 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 (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.
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 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. Flanged bushings are designed for enhanced resistance to both radial and axial loads.
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 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 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.
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.
Materials
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. The hard constituent supports the load while the soft constituent lubricates the hard constituent. 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.
Babbitt
Babbitt is usually used in integral bearings. It is coated over the bore, usually to a thickness of , depending on the diameter. Babbitt is made using soft material when compared to the material of composition of the journal or the rotating shaft. 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-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-backed Frelon are commonly used in corrosive environments because the Frelon is chemically inert.
+ Bearing properties of various bi-material bearings Type Temperature range P (max.) V (max.) PV (max.)[MPa m/s (psi sfm)] Steel-backed PTFE-coated bronze 2.0 m/s (390) 1.8 MPa m/s (51,000) Aluminum-backed frelon 1.5 m/s (300) 0.70 MPa m/s (20,000)
Bronze
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.
+ Bearing properties of various bronze alloys Type Temperature range P (max.) V (max.) PV (max.) [MPa m/s (psi sfm)] SAE 841 6.1 m/s (1,200) 1.75 MPa m/s (50,000) SAE 660 3.8 m/s (750) 2.6 MPa m/s (75,000) SAE 863 1.14 m/s (225) 1.23 MPa m/s (35,000) CDA 954 Less than 1.14 m/s (225) 4.38 MPa m/s (125,000)
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 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 Type Temperature range P (max.) V (max.)m/s ( PV (max.)[MPa m/s (psi sfm)] Graphalloy 0.38 m/s (75) 0.42 MPa m/s (12,000) Graphite ? ? ? ?
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, ruby, and garnet. Jewel bearings are used in precision instruments where low friction, long life, and dimensional accuracy are important. Their main use is in mechanical watches.
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. These materials can heat rapidly when used in applications outside the recommended pV limits.
Solid polymer type bearings are limited by the shapes possible with the injection molding process. Plastic bearings are also 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).
+ Bearing properties of various plastics Type Temperature range P (max.) V (max.) PV (max.) [MPa m/s (psi sfm)] Frelon 0.71 m/s (140) 0.35 MPa m/s (10,000) Nylon 1.83 m/s (360) 0.11 MPa m/s (3,000) MDS-filled nylon blend 1* 2.0 m/s (393) 0.12 MPa m/s (3,400) MDS-filled nylon blend 2* 0.30 m/s (60) 0.11 MPa m/s (3,000) PEEK blend 1** 2.0 m/s (400) 0.12 MPa m/s (3,500) PEEK blend 2** 1.50 m/s (295) 1.32 MPa m/s (37,700) Polyacetal 5 m/s (100) 0.09 MPa m/s (2,700) PTFE 0.5 m/s (100) 0.04 MPa m/s (1,000) Glass-filled PTFE 2.0 m/s (400) 0.39 MPa m/s (11,000) Rulon 641 2.0 m/s (400) 0.35 MPa m/s (10,000) Rulon J 2.0 m/s (400) 0.26 MPa m/s (7,500) Rulon LR 2.0 m/s (400) 0.35 MPa m/s (10,000) UHMWPE 0.5 m/s (100) 0.07 MPa m/s (2,000) MDS-filled urethane* 1.00 m/s (200) 0.39 MPa m/s (11,000) Vespel 15.2 m/s (3,000) 10.5 MPa m/s (300,000)
MDS (molybdenum disulfide) PEEK (polyether ether ketone)
Others
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 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. Aluminum alloys can be used for low load applications
Lubrication
The types of lubrication system can be categorized into three groups: