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
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 SpeedBearing / Oil Temperature (°C)
(rpm)0 to 50
60
75
90
300 to 1,500-
68
100 to 150
-
~1,80032
32 to 46
68 to 100
100
~3,60032
32
46 to 68
68 to 100
~10,00032
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 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.
For more information, please visit Hangzhou Journal Bearing.
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.
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
Strecker, William. &#;Troubleshooting Tilting Pad Thrust Bearings.&#; Machinery Lubrication magazine, March-April .
Strecker, William. &#;Failure Analysis for Plain Bearings.&#; Machinery Lubrication magazine, July-August .
Berry, James. &#;Oil Whirl and Whip Instabilities within Journal Bearings.&#; Machinery Lubrication magazine, May-June .
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, .
There are two main categories of bearing, plain and anti-friction (rolling). Plain bearings operate on the principle of sliding motion whilst rolling bearings operate on the principle of rolling motion.
Plain bearings operate on the principle of sliding friction and employ no rolling elements; they are generally quieter than anti-friction bearings as they have no moving parts. Plain bearings may be used for rotational or linear applications. For example, a shaft rotating in a hole represents a plain bearing being used to constrain rotational motion; a sliding drawer represents a linear application. Plain bearings are generally chosen for high load, low speed applications.
Plain Bearing
Plain bearings are also called sliding bearings, thin film bearings, film bearings, journal bearings, white metal bearings, Babbitt metal bearings, hydrodynamic bearings and hydrostatic bearings. Unfortunately, these terms are not always applied correctly, which leads to confusion. For example, Babbitt metal bearings are bearings that have been manufactured from Babbitt metal (a soft metal alloy), but people often label all plain bearings &#;Babbitt bearings&#;, which is incorrect.
Plain metal bearings are utilised for many large machinery items, including steam turbines and large marine two stroke vessels, but they are also used for almost all internal combustion (IC) engines, as they allow the bearing to be installed in two -or more- separate pieces.
Sliding one material across another creates friction, heat, and wear. Apart from when used for very low load applications, a plain bearing will always employ some form of lubricant to reduce friction. It is theoretically possible to use many liquids and gases as lubricants, although the most common is mineral oil. Other tried and tested lubricants include water, liquid refrigerant, kerosene, petrol, various acids and even molten metal. See our bearing lubrication article for further details.
Plain bearing lubricants separate the entire bearing surface from the load surface, thus there is theoretically no contact between the two sliding surfaces. Although there is theoretically no contact between the surfaces, in reality, it is very difficult to completely separate the two surfaces at all times.
Plain Bearing Lubrication
Most plain bearings utilise liquid lubricant. For example, almost all automobiles use some form of lubricating oil system. Oil is pumped into the space between the bearing and load surfaces (hydrostatic lubrication) and this creates a thin film of oil that completely separates -theoretically- the two sliding surfaces. Once the engine is running (four stroke or two stroke), it is possible to lubricate the engine due to the motion of the shaft (hydrodynamic lubrication).
Plain bearings should never be put into service without some form of lubrication being supplied, wear at start-up is usually more than the wear during the entire period when the engine is running.
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Fluid bearings are bearings that utilise a thin layer of oil or gas to completely separate the bearing surfaces from the load. This type of bearing is used for high load and high-speed applications. Fluid bearings are classified as hydrostatic and/or hydrodynamic.
Hydrostatic bearings are lubricated using an external pump to maintain a static head of pressure. For example, a lubrication system may consist of an oil sump, pump and bearings. The system is hydrostatic because the oil pump provides the static pressure head needed for lubrication.
Hydrodynamic bearings are lubricated by the movement of the shaft itself. The moving shaft rotates and creates an oil wedge between the bearing contact surfaces.
It is possible for a bearing to be both hydrodynamic and hydrostatic lubricated. For example, an oil pump may lubricate bearings prior to the engine being started (hydrostatic lubrication), but the pump will shut down once the engine is in operation (hydrodynamic lubrication).
The material of plain bearings is chosen to be sacrificial compared to the journal (area on shaft where bearing sits) to which it is mounted. If two materials are rubbed together, the sacrificial material is the material that begins to wear first. Plain bearings are sacrificial because otherwise the journal would wear rather than the bearing. This is undesirable because the journal (part of a shaft) is usually more expensive and more difficult to replace than the bearing.
The area on a shaft that mates with a bearing is known as a journal. A journal bearing is a bearing that is mounted onto a journal, but it could also be called a plain bearing, or Babbitt metal bearing etc. (if manufactured from Babbitt metal), which would be correct also. Because many names are correct, but sometimes incorrectly applied, there is often confusion concerning plain bearing nomenclature. Journal bearings may be manufactured from a single piece (solid), or multiple pieces (split or multi-part), and they may be lubricated or non-lubricated.
Plain bearings may be solid, split or Multi-Part. A solid plain bearing is the simplest form of bearing, it consists of a cylinder with a hole bored through the centre axis. Split bearings are similar to solid bearings, but the bearing consists of two pieces. Multi-part bearings consist of more than two pieces. Examples of solid plain bearings include bushings and sleeves.
Examples of split plain bearings include crankshaft journal bearings.
Engine Crankshaft
A plain bearing that has 360-degree contact with its mating journal, is called a &#;full contact bearing&#;, or, &#;full bearing&#;. A plain bearing that has 180-degree contact -or less- with its mating journal, is called a &#;partial contact bearing&#;. If the shaft&#;s diameter is larger than that of the bearing&#;s internal diameter, the bearing is classed as a &#;fitted journal bearing&#;.
Full contact bearings are used when the load varies and may act in several directions; an engine crankshaft uses full contact bearings. Partial bearings are used when the load is constant and applied in only one direction.
Plain cylindrical bearings are the simplest design of plain bearing. This type of bearing consists of a cylinder with a hole bored through the centre axis, it has no lubrication channels. Non-lubricated cylindrical bearings are only suitable for low load applications. Many gas lubricated bearings are cylindrical plain bearings, this includes those used for computer hard drives (the old kind that utilised a rotating magnetic disc).
The applications of plain cylindrical bearings are limited unless the design is modified. Adding some means of introducing lubricant to the bearing surfaces allows the bearing to be used at higher loads and speeds. Oil holes and oil grooves are used to distribute lubricant to the bearing&#;s load carrying surfaces. The most common types of lubricated cylindrical bearings are the four-axial groove bearing and elliptical groove bearing.
Split plain bearings are used for several reasons:
Sleeve bearings are plain cylindrical bearings that have been manufactured from two separate materials. This type of bearing has a thin lining on the surface of the bearing that mates with the journal. The bearing surface that faces the journal is usually coated in a sacrificial material e.g. a soft metal, whilst the main bearing body (not in contact with the journal) is made of a different material e.g. steel.
Bushing bearings are plain cylindrical bearings manufactured from a single piece of material. There is only one difference between a sleeve bearing and bushing bearing; sleeve bearings use a thin lining of bearing material that mates with a shaft, whilst bushing bearings are manufactured entirely from a single material. Bushings are usually thinner than sleeves and are structurally weaker.
Spherical plain bearings are used when misalignment may make using other bearing designs difficult. The inner ring of the bearing is free to rotate within a spherical outer ring, which allows the bearing to pivot/rotate in order to match the orientation of the shaft. Other types of spherical bearing include the spherical roller bearing.
Spherical Plain Bearing
Spherical plain bearings are classed as &#;requiring maintenance&#; or &#;maintenance free&#;.
Spherical plain maintenance free bearings use a PTFE material to separate the inner and outer rings; this type of bearing is sometimes called a &#;PTFE on metal&#; bearing. PTFE self-lubricates and the bearing requires no maintenance (no lubricant must be added).
Spherical plain bearings requiring maintenance need to be lubricated periodically; this type of bearing is sometimes called a &#;metal on metal&#; bearing.
Note
No component or machine is ever &#;maintenance free&#;. The phrase is often used for marketing purposes, but it is inaccurate. If an engineer performs any condition-based maintenance of the bearing (visual inspection, ultrasonic testing, vibration analysis, thermography), the bearing is being maintained. If the bearing is inspected and/or replaced periodically, it is part of a maintenance programme and is thus being maintained. The only occasion when an item is &#;maintenance free&#; is when it is installed and operated until destruction, without any checks or maintenance occurring between installation and failure.
This 3D model shows all major components associated with a typical plain metal bearing, these include:
https://en.wikipedia.org/wiki/Plain_bearing
https://clr.es/blog/en/plain-bearings
https://www.ggbearings.com/en/tribou/internet-bearings/bearings
https://www.maschinenmarkt.international/the-plain-bearing--function-types-and-applications-a-
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