A plain bearing, also known as a plane bearing or a 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 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.
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 bearing.
- Linear bearing: This bearing provides linear motion; it may take the form of a circular bearing and shaft or any other two matching surfaces (e.g., a slide plate).
- Thrust bearing: A thrust bearing provides a bearing surface for forces acting axial to the shaft.
Integral plain bearings are built into the object of use. It is a hole that has been prepared into a 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 easy to accommodate and if they wear out then they are just replaced. Depending on the material an integral bearing may be less expensive but it cannot be replaced. If an integral bearing wears out then 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 permeated the industry.
An example of 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. 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 across the cut. A flanged bushing is a sleeve bushing with a flange extending radially outward from the ID. 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 eights 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.
The thrust form of a bushing is conventionally called a thrust washer.
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.
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 supports 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 seize.
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 2.5 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.
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 Temperature range P (max.)
[psi sfm (MPa m/s)]
Steel-backed PTFE-coated bronze -328–536 °F / -200–280 °C 36,000 psi/248 MPa 390 (2.0 m/s) 51,000 (1.79 MPa m/s) Aluminum-backed frelon -400–400 °F / -240–204 °C 3,000 psi/21 MPa 300 (1.52 m/s) 20,000 (0.70 MPa m/s)
Common bronze alloys used for bearings include: SAE 841, SAE 660 (CDA 932), SAE 863, and CDA 954.
Bearing properties of various bronze alloys Temperature range P (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)
A cast iron bearing is commonly used with a hardened steel shaft because the coefficient of friction is relatively low. The cast iron glazes over therefore wear becomes negligible.
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 sfm (MPa m/s)]
Graphalloy -450–750 °F / -268–399 °C 750 psi/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.
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 research 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 the shapes which 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, 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 everywhere from photocopy machines to the tills in the supermarket. Other applications include farm equipment, textile machinery, medical devices, food and packaging machines, car seating, marine equipment and many more.
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 Temperature range P (max.) [psi (MPa)] V (max.) [sfm (m/s)] PV (max.) [psi sfm (MPa m/s)] Frelon -400–500 °F (-240–260 °C) 1,500 (10) 140 (0.71) (dry) 10,000 (0.35) Nylon -20–250 °F (-29–121 °C) 400 psi (3 MPa) 360 (1.83 m/s) 3,000 (0.11 MPa m/s) MDS-filled nylon blend 1 -40–176 °F (-40–80 °C) 2,000 psi (14 MPa) 393 (2.0 m/s) 3,400 (0.12 MPa m/s) MDS-filled nylon blend 2 -40–230 °F (-40–110 °C) 300 psi (2 MPa) 60 (0.30 m/s) 3,000 (0.11 MPa m/s) PEEK blend 1 -148–480 °F (-100–249 °C) 8,500 psi (59 MPa) 400 (2.0 m/s) 3,500 (0.12 MPa m/s) PEEK blend 2 -148–480 °F (-100–249 °C) 21,750 psi (150 MPa) 295 (1.50 m/s) 37,700 (1.32 MPa m/s) Polyacetal -20–180 °F (-29–82 °C) 1,000 psi (7 MPa) 1,000 (5.1 m/s) 2,700 (0.09 MPa m/s) PTFE -350–500 °F (-212–260 °C) 500 psi (3 MPa) 100 (0.51 m/s) 1,000 (0.04 MPa m/s) Glass-filled PTFE -350–500 °F (-212–260 °C) 1,000 psi (7 MPa) 400 (2.0 m/s) 11,000 (0.39 MPa m/s) Rulon 641 -400–500 °F (-240–260 °C) 1,000 psi (7 MPa) 400 (2.0 m/s) 10,000 (0.35 MPa m/s) Rulon J -400–500 °F (-240–260 °C) 750 psi (5 MPa) 400 (2.0 m/s) 7,500 (0.26 MPa m/s) Rulon LR -400–500 °F (-240–260 °C) 1,000 psi (7 MPa) 400 (2.0 m/s) 10,000 (0.35 MPa m/s) UHMWPE -200–180 °F (-129–82 °C) 1,000 psi (7 MPa) 100 (0.51 m/s) 2,000 (0.07 MPa m/s) MDS-filled urethane -40–180 °F (-40–82 °C) 700 psi (5 MPa) 200 (1.02 m/s) 11,000 (0.39 MPa m/s) Vespel -400–550 °F (-240–288 °C) 4,900 psi (34 MPa) 3,000 (15.2 m/s) 300,000 (10.5 MPa m/s)
- Ceramic bearings are very hard and sand and other grit which enter the bearing are simply ground to a fine powder which does not inhibit the operation of the bearing.
- Lignum vitae is a self lubricating wood and in clocks it gives extremely long life.
- 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.
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 and release the oil when pressure or heat is applied. Another form is a solid one-piece metal bushing with a figure eight groove channel on the ID that is filled with graphite. A similar bearing replaces the figure eight groove with holes that are plugged with graphite; this allows the bearing to be lubricated 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 will wear out and have a high coefficient of friction. 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 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 which always keeps 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 almost always use oil, while hydrodynamic bearings can use oil or grease. An example of a hydrostatic bearing is the heavily-loaded bearings (main, connecting rod big-end and camshaft) in an automobile engine, which are usually fed oil via a hole in the bearing.
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 are often aided by secondary bearings which support the shaft during start and stop periods, protecting the fine tolerance machined surfaces of the journal bearing.
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 angle formed between the horizontal and a line that crosses through the center of the journal and the center of the bearing. The attitude angle is dependent on the direction of rotation, oil pressure (in hydrostatic bearings), and electromagnetic forces (in electromagnetic equipment).
One disadvantage specific to fluid-lubricated journal bearings is oil whirl, also known as oil whip. Oil whirl is when a lubrication wedge cannot form, but instead "whirls" around the bearing. This leads to direct contact between the journal and the bearing, which quickly wears out the bearing. Moreover, the journal precesses in the opposite direction of rotation, causing the friction to increase.
One design used to minimize this problem 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.
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 pad that are designed to move with changing loads. It is usually used in very large applications.
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.
- Hot box
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