Plain bearing

Simplest type of bearing, with no rolling elements

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Plain bearing on a S-Motor locomotive showing the axle, bearing, oil supply and oiling pad A sliding table with four cylindrical bearings (1)

A plain bearing, or more commonly sliding contact bearing and slide bearing (in railroading sometimes called a solid bearing, journal bearing, or friction bearing[2]), is the simplest type of bearing, comprising just 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.[4]

Design

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

Integral

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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.

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, which is both a thrust bearing and a journal bearing.

Bushing

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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.

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.[11]

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.[12]

The thrust form of a bushing is conventionally called a thrust washer.

Two-piece plain bearings, known as full bearings in industrial machinery,[13] are commonly used for larger diameters, such as crankshaft bearings. The two halves are called shells.[14] 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.[14]

Materials

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Split bi-material bushings: a metal exterior with an inner plastic coating A graphite-filled groove 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. The hard constituent supports the load while the soft constituent supports the hard constituent.[citation needed] 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]

Babbitt

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Babbitt is usually used in integral bearings. It is coated over the bore, usually to a thickness of 0.25 to 2.5 mm (9.8 to 98.4 thou), 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.[13]

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.[17] 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.)
[(MPa) psi] V (max.)
[m/s (sfm)] PV (max.)
[MPa m/s (psi sfm)] Steel-backed PTFE-coated bronze &#;200&#;280 °C or &#;328&#;536 °F 248 MPa or 36,000 psi 2.0 m/s (390) 1.8 MPa m/s (51,000) Aluminum-backed frelon &#;240&#;204 °C or &#;400&#;400 °F 21 MPa or 3,000 psi 1.5 m/s (300) 0.70 MPa m/s (20,000)

Bronze

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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.)
[MPa (psi)] V (max.)
[m/s (sfm)] PV (max.)
[MPa m/s (psi sfm)] SAE 841 &#;12&#;104 °C (10&#;220 °F) 14 MPa (2,000 psi) 6.1 m/s (1,200) 1.75 MPa m/s (50,000) SAE 660 &#;12&#;232 °C (10&#;450 °F) 28 MPa (4,000 psi) 3.8 m/s (750) 2.6 MPa m/s (75,000) SAE 863 &#;12&#;104 °C (10&#;220 °F) 28 MPa (4,000 psi) 1.14 m/s (225) 1.23 MPa m/s (35,000) CDA 954 Less than 260 °C (500 °F) 31 MPa (4,500 psi) 1.14 m/s (225) 4.38 MPa m/s (125,000)

Cast iron

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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

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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.)
[MPa (psi)] V (max.)
m/s ([sfm)] PV (max.)
[MPa m/s (psi sfm)] Graphalloy &#;268&#;399 °C or &#;450&#;750 °F

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5 MPa or 750 psi 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.[22]

Jewels

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

Plastic

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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

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).

Bearing properties of various plastics Type Temperature range P (max.) [MPa (psi)] V (max.) [m/s (sfm)] PV (max.) [MPa m/s (psi sfm)] Frelon

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&#;240 to 260 °C (&#;400 to 500 °F)

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10 MPa (1,500 psi) 0.71 m/s (140) 0.35 MPa m/s (10,000) Nylon &#;29 to 121 °C (&#;20 to 250 °F) 3 MPa (400 psi) 1.83 m/s (360) 0.11 MPa m/s (3,000) MDS-filled nylon blend 1* &#;40 to 80 °C (&#;40 to 176 °F) 14 MPa (2,000 psi) 2.0 m/s (393) 0.12 MPa m/s (3,400) MDS-filled nylon blend 2* &#;40 to 110 °C (&#;40 to 230 °F) 2 MPa (300 psi) 0.30 m/s (60) 0.11 MPa m/s (3,000) PEEK blend 1** &#;100 to 249 °C (&#;148 to 480 °F) 59 MPa (8,500 psi) 2.0 m/s (400) 0.12 MPa m/s (3,500) PEEK blend 2** &#;100 to 249 °C (&#;148 to 480 °F) 150 MPa (21,750 psi) 1.50 m/s (295) 1.32 MPa m/s (37,700) Polyacetal &#;29 to 82 °C (&#;20 to 180 °F) 7 MPa (1,000 psi) 5  m/s (100) 0.09 MPa m/s (2,700) PTFE &#;212 to 260 °C (&#;350 to 500 °F) 3 MPa (500 psi) 0.5 m/s (100) 0.04 MPa m/s (1,000) Glass-filled PTFE &#;212 to 260 °C (&#;350 to 500 °F) 7 MPa (1,000 psi) 2.0 m/s (400) 0.39 MPa m/s (11,000) Rulon 641 &#;240 to 288 °C (&#;400 to 550 °F) 7 MPa (1,000 psi) 2.0 m/s (400) 0.35 MPa m/s (10,000)

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Rulon J &#;240 to 288 °C (&#;400 to 550 °F) 5 MPa (750 psi) 2.0 m/s (400) 0.26 MPa m/s (7,500) Rulon LR &#;240 to 288 °C (&#;400 to 550 °F) 7 MPa (1,000 psi) 2.0 m/s (400) 0.35 MPa m/s (10,000) UHMWPE &#;129 to 82 °C (&#;200 to 180 °F) 7 MPa (1,000 psi) 0.5 m/s (100) 0.07 MPa m/s (2,000) MDS-filled urethane* &#;40 to 82 °C (&#;40 to 180 °F) 5 MPa (700 psi) 1.00 m/s (200) 0.39 MPa m/s (11,000) Vespel &#;240 to 288 °C (&#;400 to 550 °F) 34 MPa (4,900 psi) 15.2 m/s (3,000) 10.5 MPa m/s (300,000)

Others

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Lubrication

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

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.

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[34] and release the oil when pressure or heat is applied.[35] 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. 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.[37]

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

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A lemon bore A pressure dam Miba tilting pad bearing used in turbomachinery

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.[13] 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.[38]

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.[39]

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.[13]

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.[13][40]

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.[13]

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.[13]

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

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An early pillow block bearing with a whitemetal plain bearing

Other components that are commonly used with plain bearings include:

• Pillow block bearing: 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

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• Bearing modulus &#; Dimensionless number used in journal bearing design
• Computer fan &#; Miniature fan used in a computer for active cooling
• Hot box &#; Overheating of railway rolling stock
• Pillow block bearing &#; Bracket used to provide support to rotating shafts
• Plastigauge
• Roller bearing &#; Bearing which carries a load with rolling elements placed between two grooved rings

  Pages displaying short descriptions of redirect targets

• Stave bearing

References

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Bibliography

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Journal or plain bearings consist of a shaft or journal which rotates freely in a supporting metal sleeve or shell. There are no rolling elements in these bearings. Their design and construction may be relatively simple, but the theory and operation of these bearings can be complex.

This article concentrates on oil and grease-lubricated full fluid film journal bearings; but first a brief discussion of pins and bushings, dry and semilubricated journal bearings, and tilting-pad bearings.

Low-speed pins and bushings are a form of journal bearing in which the shaft or shell generally does not make a full rotation. The partial rotation at low speed, before typically reversing direction, does not allow for the formation of a full fluid film and thus metal-to-metal contact does occur within the bearing. Pins and bushings continually operate in the boundary lubrication regime.

These types of bearings are typically lubricated with an extreme pressure (EP) grease to aid in supporting the load. Solid molybdenum disulfide (moly) is included in the grease to enhance the load-carrying capability of the lubricant.

Many outdoor construction and mining equipment applications incorporate pins and bushings. Consequently, shock loading and water and dirt contamination are often major factors in their lubrication.

Figure 1. Kingsbury Radial
and Thrust Pad Bearing

Dry journal bearings consist of a shaft rotating in a dry sleeve, usually a polymer, which may be blended with solids such as molybdenum, graphite, PTFE or nylon.

These bearings are limited to low-load and low-surface speed applications. Semilubricated journal bearings consist of a shaft rotating in a porous metal sleeve of sintered bronze or aluminum in which lubricating oil is contained within the pores of the porous metal. These bearings are restricted to low loads, low-to-medium velocity and temperatures up to 100°C (210°F).

Tilting-pad or pivoting-shoe bearings consist of a shaft rotating within a shell made up of curved pads. Each pad is able to pivot independently and align with the curvature of the shaft. A diagram of a tilt-pad bearing is presented in Figure 1.

The advantage of this design is the more accurate alignment of the supporting shell to the rotating shaft and the increase in shaft stability which is obtained.1

Journal bearings are meant to include sleeve, plain, shell and babbitt bearings. The term babbitt actually refers to the layers of softer metals (lead, tin and copper) which form the metal contact surface of the bearing shell. These softer metals overlay a stronger steel support shell and are needed to cushion the shell from the harder rotating shaft.

Simple shell-type journal bearings accept only radial loading, perpendicular to the shaft, generally due to the downward weight or load of the shaft. Thrust or axial loads, along the axis of the shaft, can also be accommodated by journal bearings designed for this purpose. Figure 1 shows a tilt-pad bearing capable of accepting both radial and thrust loads.

Figure 2. Layers of Journal Bearing Structure

Journal bearings operate in the boundary regime (metal-to-metal contact) only during the startup and shutdown of the equipment when the rotational speed of the shaft (journal) is insufficient to create an oil film. It is during startup and shutdown when almost all of the damage to the bearing occurs.2

Hydrostatic lift, created by an external pressurized oil feed, may be employed to float large, heavy journals prior to startup (shaft rotation) to prevent this type of damage. During normal operation, the shaft rotates at sufficient speed to force oil between the conforming curved surfaces of the shaft and shell, thus creating an oil wedge and a hydrodynamic oil film.

This full hydrodynamic fluid film allows these bearings to support extremely heavy loads and operate at high rotational speeds. Surface speeds of 175 to 250 meters/second (30,000 to 50,000 feet/minute) are common. Temperatures are often limited by the lubricant used, as the lead and tin babbitt is capable of temperatures reaching 150°C (300°F).

It is important to understand that the rotating shaft is not centered in the bearing shell during normal operation. This offset distance is referred to as the eccentricity of the bearing and creates a unique location for the minimum oil film thickness, as illustrated in Figure 3.

Figure 3. Shaft Motion During Startup

Normally, the minimum oil film thickness is also the dynamic operating clearance of the bearing. Knowledge of the oil film thickness or dynamic clearances is also useful in determining filtration and metal surface finish requirements.

Typically, minimum oil film thicknesses in the load zone during operation ranges from 1.0 to 300 microns, but values of 5 to 75 microns are more common in midsized industrial equipment. The film thickness will be greater in equipment which has a larger diameter shaft.

Persons requiring a more exact value should seek information on the Sommerfeld Number and the Reynolds Number. Discussion of these calculations in greater detail is beyond the scope of this article. Note that these values are significantly larger than the one-micron values encountered in rolling element bearings.

The pressures encountered in the contact area of journal bearings are significantly less than those generated in rolling bearings. This is due to the larger contact area created by the conforming (similar curvature) surfaces of the journal and the shell.

The mean pressure in the load zone of a journal bearing is determined by the force per unit area or in this case, the weight or load supported by the bearing divided by the approximate load area of the bearing (the bearing diameter times the length of the bearing). In most industrial applications, these values range from 690 to 2,070 kPa (100 to 300 psi).

At these low pressures, there is virtually no increase in the oil viscosity in the bearing contact area due to pressure. Automotive reciprocating engine bearings and some severely loaded industrial applications may have mean pressures of 20.7 to 35 MPa (3,000 to 5,000 psi). At these pressure levels, the viscosity may slightly increase. The maximum pressure encountered by the bearing is typically about twice the mean value, to a maximum of about 70 MPa (10,000 psi).

Oil whirl is a phenomenon that can occur in high-speed journal bearings when the shaft position within the shell becomes unstable and the shaft continues to change its position during normal operation, due to the fluid forces created within the bearing. Oil whirl may be reduced by increasing the load or changing the viscosity, temperature or oil pressure in the bearing.

A permanent solution may involve a new bearing with different clearances or design. Oil whip occurs when the oil whirl frequency coincides with the system&#;s natural frequency. The result can be a catastrophic failure.3

Oil Lubrication

Oils are used in journal bearings when cooling is required or contaminants or debris need to be flushed away from the bearing. High-speed journal bearings are always lubricated with oil rather than a grease. Oil is supplied to the bearing by either a pressurized oil pump system, an oil ring or collar or a wick. Grooves in the bearing shell are used to distribute the oil throughout the bearings&#; surfaces.

The viscosity grade required is dependent upon bearing RPM, oil temperature and load. The bearing speed is often measured strictly by the revolutions per minute of the shaft, with no consideration of the surface speed of the shaft, as per the &#;ndm&#; values calculated for rolling bearings. Table 1 provides a general guideline to selecting the correct ISO viscosity grade.

The ISO grade number indicated is the preferred grade for speed and temperature range. ISO 68- and 100-grade oils are commonly used in indoor, heated applications, with 32-grade oils being used for high-speed (10,000 RPM) units and some outdoor low-temperature applications.

Note in the table that the higher the bearing speed, the lower the oil viscosity required; and that the higher the operating temperature of the unit, the higher the oil viscosity that is required. If vibration or minor shock loading is possible, a higher grade of oil than the one indicated in Table 1 should be considered.

Bearing Speed

Bearing / Oil Temperature (°C)

(rpm)

0 to 50

60

75

90

300 to 1,500

-

68

100 to 150

-

~1,800

32

32 to 46

68 to 100

100

~3,600

32

32

46 to 68

68 to 100

~10,000

32

32

32

32 to 46

Table 1. Journal Bearing ISO Viscosity Grade Selection

Another method of determining the proper viscosity grade is by applying minimum and optimum viscosity criteria to a viscosity-temperature plot. A generally accepted minimum viscosity of the oil at the operating temperature for journal bearings is 13 cSt, although some designs allow for an oil as thin as 7 or 8 cSt at the operating temperature.

The optimum viscosity at operating temperature is 22 to 35 cSt, for moderate-speed bearings if no shock loading occurs. The optimum viscosity may be as high as 95 cSt for low-speed, heavily loaded or shock-loaded journal bearings.

Using this method requires some knowledge of the oil temperature within the bearing under operating conditions, which can be difficult to determine. Fortunately, an accurate oil temperature is not needed for most viscosity determinations. It is common to determine the temperature of the outer surface of the pipes carrying oil to and away from the bearing.

The temperature of the oil inside of the pipes will generally be higher (5 to 10°C, 10 to 18°F) than the outer metal surface of the pipe. The oil temperature within the bearing can be taken as the average of the oil entering versus the temperature exiting the bearing.4

A third and more complex method is to calculate the oil viscosity needed to obtain a satisfactory oil film thickness. Persons wishing to learn more about this method should seek information regarding the Sommerfeld equation and either eccentricity ratios or Reynolds Numbers.4

If the oil selected is too low in viscosity, heat will generate due to an insufficient film thickness and some metal-to-metal contact will occur. If the oil is too high in viscosity, heat will again be generated, but due to the internal fluid friction created within the oil. Selecting an oil which is too high in viscosity can also increase the likelihood of cavitation.

The high- and low-pressure zones, which are created within the oil on each side of the area of minimum film thickness, can cause oil cavitation in these bearings. Cavitation is a result of expansion of dissolved air or a vapor (water or fuel) in the low-pressure zone of the bearing.

The resulting bubble implodes, causing damage, as it passes through the high-pressure portion of the bearing. If the implosion or collapse of the vapor bubble occurs next to the metal surface, this can cause cavitation pitting damage to the metal. If the implosion of the bubble occurs within the oil, a micro hot spot or micro-dieseling can occur, which may lead to varnishing within the system.

Typically, a rust and oxidation (R&O) inhibited additive system is used in the oils employed in these applications. Antifoam and pour point depressant additives may also be present. Antiwear (AW) hydraulic oils may also be used as long as the high-temperature limit of the zinc AW component is not exceeded and excessive water is not present.

R&O oils tend to have better water separation characteristics, which is beneficial, and the AW properties of a hydraulic oil would be beneficial only during startup and shutdown, assuming a properly operating bearing.

Grease Lubrication

Grease is used to lubricate journal bearings when cooling of the bearing is not a factor, typically if the bearing operates at relatively low speeds. Grease is also beneficial if shock loading occurs or if the bearing frequently starts and stops or reverses direction.

Grease is almost always used to lubricate pins and bushings because it provides a thicker lubricant than oil to support static loads and to protect against vibration and shock-loading that are common in many of these applications.

Lithium soap or lithium complex thickeners are the most common thickeners used in greases and are excellent for most journal bearing applications. The grade of grease used is typically an NLGI grade #2 with a base oil viscosity of approximately 150 to 220 cSt at 40°C.

Greases for low-speed, high-load, high temperatures and for pins and bushings may use a higher viscosity base oil and be formulated with EP and solid additives. Greases for improved water resistance may be formulated with heavier base oils, different thickeners and special additive formulations.

Greases for better low-temperature dispensing may incorporate a lower viscosity base oil manufactured to an NLGI #1 specification. Bearings lubricated by a centralized grease dispensing systems typically use a #1, 0 or 00 grade of grease.

The apparent viscosity of grease changes with shear (pressure, load and speed) that is, greases are non-Newtonian or thixotropic. Within a rotating journal bearing, as the bearing rotates faster (shear rate increases), the apparent viscosity of the grease decreases and approaches the viscosity of the base oil used in grease.

At both ends of the bearing shell, the pressure is lower and therefore the apparent viscosity remains higher. The resulting thicker grease at the bearing ends acts as a built-in seal to reduce the ingression of contaminants.

Greasing Procedures

The greasing procedures for journal bearings and pins and bushings are not as well-defined or as critical as for rolling bearings because the grease is not subjected to the churning action created by the rolling elements.

The volume of grease to inject and the frequency of application are dictated more by trial and error. Generally, most journal bearings cannot be overgreased. Caution must be taken when pumping grease into a bearing that is fitted with seals, so they are not damaged or displaced by the force and volume of the incoming grease.

The harshness of the environment, shock loading and especially the operating temperature will be major factors in determining the frequency of relubrication.

Journal bearings are generally a simpler design and not as difficult to lubricate as rolling element bearings. The proper viscosity matched to the operating conditions and a clean and dry lubricant will usually suffice to form a full fluid lubricating film and provide excellent bearing life.

References

1. Strecker, William. &#;Troubleshooting Tilting Pad Thrust Bearings.&#; Machinery Lubrication magazine, March-April .

2. Strecker, William. &#;Failure Analysis for Plain Bearings.&#; Machinery Lubrication magazine, July-August .

3. Berry, James. &#;Oil Whirl and Whip Instabilities within Journal Bearings.&#; Machinery Lubrication magazine, May-June .

4. Tribology Data Handbook. Chapter 61, Journal Bearing Design and Analysis. Khonsari, M. CRC Press, .

Editor&#;s Note:
Portions of this article have been previously published in the Society of Tribologists and Lubrication Engineers (STLE) Alberta Section, Basic Handbook of Lubrication, Second Edition, .

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