Plain bearings are mechanical components responsible for supporting a load and allowing the sliding or rotational motion of two machine components relative to one another. Their primary function, as tribological components, is to prevent the wear and tear of interacting surfaces in relative motion.

In other words, plain bearings prevent the wear of the machine at the points of support that would happen if the shafts were to roll directly on the structure. Despite their use in industrial, high-load applications, plain bearings offer a simple design.

Unlike bearings with rolling elements, such as ball bearings or roller bearings, plain bearings — usually referred to as bushes, bushings, or sleeve bearings — have no moving parts. Instead, they're press-fitted into fixed parts that actually bear the loads and make contact with the moving element instead of the support component.

These cylindrically-shaped components are a fantastic choice for industrial uses requiring maintenance-free bearings with long service life.

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Types of Plain Bearings

There are five main types of plain bearings, and the choice of adequate components depends on the application operating conditions, performance requirements, and product construction. However, before we dive into different types of plain bushings, it's important to distinguish two different types of bearings:

• Rolling Bearings — rolling bearings, such as ball or roller bearings, separate the rolling surface of the supporting structure from the rotational shaft with rolling elements. These elements are housed between two grooved rings and help facilitate a rolling motion instead of a sliding one during the shaft's rotation.
• Plain Bearings — also known as friction bearings, are cylindrical components with fixed bearing surfaces that improve linear motion. This is achieved by separating the bearing surface from the rotational shaft with a thin lubricant film between the surfaces. Plain bearings are further divided into five basic types.

Sleeve Bearings

Sleeve bearings are the most widespread type of plain bearings, as they're suitable for various applications and improve oscillating, rotating, or linear motion between two components by absorbing friction.

Compared to typical ball bearings, sleeve bearings are more affordable, more reliable, more straightforward to use, and quite rugged. Their ruggedness stems from the lack of moving parts, making them more resilient to hostile environments and suitable for high-speed and low-speed applications.

More robust sleeve bearings have better wear resistance, which means they can withstand higher load capacities and correct any misalignment of the other components in the assembly. Sleeve bearings are frequently made of sintered bronze, and they occasionally have lubricant plugs inside. Depending on your application, there are various plastic bushings as well.

Flange Bearings

These bearings are fitted inside a cast iron flange, usually used for mounting purposes. They're designed to support a shaft that runs perpendicularly to a bearing's mounting surface and can support radial and light axial loads.

The addition of the flange in the bearing design allows for easier mounting and alignment of the bearing assembly during installation, prevents the bearing from moving axially, and ensures its proper positioning. They're made from various materials, including polymers, composites, and thermoplastics.

Mounted Bearings

Mounted bearings need to be precisely engineered to specs in order to ensure the best fit. For example, too-loosely mounted bearings might slip on a shaft. Or free mobility could be restricted if the press fit is too tight. This type of bearing supports high axial loads and limited radial movement, while its flanges or pillow blocks help with mounting and alignment on various surfaces.

Thrust Bearings

Thrust washer bearings are flat bearings that are commonly inserted between rotary and stationary components, providing a rotary component something to rub up against in the event that it starts to move laterally, thus keeping it in place. Thrust bearings are designed to prevent metal-to-metal contact in trust load applications. They're also quite easy to install, and they're self-lubricating, which makes them more cost-effective.

Spherical Bearings

Spherical plain bearings allow rotary motion and angular movement, making them perfect for applications requiring angular compensation to the shaft. The inner race of spherical bearings typically rotates angularly within the bounds of the outer race, while a lubricating layer between the bearing's contacting surfaces significantly decreases the coefficient of friction.

However, spherical bearings feature rolling elements between bearing races, called anti-friction spherical bearings. These are used in certain-heavy duty applications that demand the use of rolling elements to create motion with very little friction.

Plain Bearing Materials

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Depending on the application requirements, plain bearings are also made from various materials. These materials include polymers, plastics, various composites, and metals.

Metal-Polymer

Metal-polymer bearings consist of a metal backing and a running surface. The backing is usually made of steel or bronze, while the running material is made of a porous bronze layer soaked with PTFE and additives. This provides the bearing with an anti-friction, wear-resistant running layer which can operate with or without external lubrication.

Engineered Plastics

Engineered polymers offer exceptional wear resistance and low friction in both dry and lubricated conditions. They're usually formed by injection molding, allowing reproduction of practically any shape, using a variety of resins that have been blended with solid lubricants and reinforcing fibers. As a result, these bearings have great heat conductivity, low coefficients of friction, and high dimensional stability.

Composites

Fiber-reinforced composite bearings combine a range of low-friction linings within fiberglass-woven epoxy backing. Due to their design, and the materials used in their production, these bearings are capable of withstanding heavy static and dynamic loads and are resistant to corrosive operating environments due to their inert nature.

Metal

Sintered bronze, monometallic, and bimetallic plain bearings are used in heavy-duty, slow-moving industrial applications that are surface-based and submerged. While monometallic and bimetallic bearings are made for lubricated applications, solid bronze bearings treated with lubricant offer maintenance-free performance in high-temperature applications.

Plain Bearing Applications

Due to their versatility, plain bearings are used successfully across nearly all industrial sectors. Here's a list of the most common applications of plain bearings:

• Radial bearings for bearing perpendicular forces
• Axial bearings facing the center of the shaft
• Floating bearings for longitudinal displacement
• Locating bearing for absorbing transverse and longitudinal forces
• Sliding strips
• Automotive industry
• AG industry
• Off-road/construction
• Marine
• Food processing

Pros and Cons of Plain Bearings

Plain bearings offer numerous advantages over roller bearings or ball bearings, even though they perform the same function differently. With that said, selecting one type over the other greatly depends on the application requirements. Here are the advantages of plain bearings:

Pros of Plain Bearings

As stated above, plain bearings are pretty straightforward components, which aren't that difficult to produce compared to rolling element bearings. They usually consist of thin metal cylinders, and their thin walls make them lighter and easier to machine, making them cheaper to produce.

However, this doesn't mean that they're of lower quality. Plain bearings don't have rolling elements, making them much quieter during operation than ball bearings. In addition, their simple design and lack of moving parts make them impervious to shocks and impacts and grant them a longer service life.

Lastly, depending on whether or not they're self-lubricating, they usually require very little maintenance apart from the occasional lubrication for externally lubricated types.

Disadvantages of Plain Bearings

Plain bearings have their own drawbacks. Their lack of moving parts also implies higher friction during startup, requiring more axial space, and using friction-resisting materials in their production is absolutely necessary. Unfortunately, they're also more susceptible to wear and tear and generally sport approx. 20,000 hours shorter service life than ball bearing types.

Some types also rely on Myler washers and oil rings to prevent the lubricant from leaking, which puts additional friction on the shaft and stops the gasses from escaping. These gasses solidify into nitride particles, which impede the shaft's movement, and adversely affect the bearing's life expectancy.

Plain Bearings vs. Ball Bearings

When comparing plain bearings against ball bearings, it's important to note that either one isn't better than the other — they're just better suited for different applications. However, there are some notable differences between the two.

For example, due to the lack of moving parts, plain bearings tend to produce less noise than ball bearings, though if the latter has been produced with very tight tolerances, that difference becomes marginal. This is rarely done, as ball bearings are much more expensive to produce.

Theoretically, plain bearings can run indefinitely, provided they're sufficiently and adequately lubricated. In practice, however, ball bearings feature a longer life expectancy — they're usually rated for 50,000 operating hours, while plain bearings usually operate for more than 30,000.

Of course, lubricant and friction are the two most important factors that determine the service life of a bearing. For example, sleeve bearings have line contact between the shaft and the surface lining, which produces more friction than ball bearings. That's why they use thinner lubricants, like oils, instead of thicker ones, like grease.

The downside is that thinner lubricants evaporate more quickly, which leads to the aforementioned gas build-up, and failure to reapply lubrication may cause catastrophic failure.

Plain Bearing Lubrication

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Sliding one material across the other creates friction, creating heat and wear. Plain bearings employ some form of lubrication to decrease friction between assembly parts, except for very low-load applications. Although numerous liquids and gasses might theoretically be used as lubricants, mineral oil is the most typical. Water, liquid refrigerant, kerosene, gasoline, different acids, and even molten metal are other tried-and-true lubricants.

Lubrication theoretically prevents contact between two sliding surfaces, thus separating the bearing surface from the load surface. However, keeping the two surfaces entirely apart in practice is challenging. Based on the lubrication system, there are three basic types of plain bearings:

• Self-lubricating — these bearings don't require any external lubrication since they're manufactured with porous materials and impregnated with lubricants which are slowly distributed throughout the parts in motion. However, despite the advertising, you'll greatly extend their service life by lubricating them occasionally.
• Regularly lubricated bearings — these require periodic external lubrication.
• Continuously lubricated bearings — there are two subtypes of continuously lubricated bearings: hydrostatic and hydrodynamic bearings. The former is pressure-fed with lubricants via an external pump, while the latter doesn't require external injection. Instead, the moving parts of the assembly generate a hydrodynamic effect which lubricates the moving parts.

Plain Bearing Specifications

Plain bearings have several crucial dimensions you'd need to know when selecting an adequate part. Please note that these dimensions don't relate to all plain bearings and that manufacturers usually provide sizing charts.

• Clearance — the distance in the shaft's radial movement within the bushing. This should be selected based on the normal operating conditions. 
• ID and OD — these refer to the inside diameter and the outside diameter of the bushing, not accounting for the radius of the flange.
• Length — overall length of the sleeve bearing.
• Load — usually expressed in pounds per square inch.
• Rotating Speed — related to several crucial factors, like material, speed, surface finish, hardness, lubrication, alignment, etc. 
• PV value — P stands for a specific load, while V stands for sliding speed, both of which have a massive influence on the service life of the bearing. Generally speaking, a lower PV value indicates a longer service life. 

Plain Bearing Failure

To avoid unplanned downtime and higher maintenance costs, it is crucial to accurately diagnose probable bearing failures in advance. The bulk of individual bearing failures results from these primary bearing-failure causes.

• Lubrication and Contamination — as stated above, lubrication is one of the essential factors in prolonging the bearing's lifecycle. If you fail to lubricate the bearing enough, it could become contaminated, lead to excessive wear, and even heat up — all of which can result in an early bearing failure. However, it's also worth noting that high-speed applications might overheat from excessive lubrication.
• Improper Installation — Although it might seem obvious, routine wear and tear is one of the main reasons bearings fail. Although high load and vibrations cause a bearing to wear out far sooner than it should, all bearings eventually fail due to wear. However, improper installation puts more strain on the assembly, which puts the bearing at risk of early failure.
  Ultimately, all bearing eventually fail due to multiple causes rather than just one. Therefore, to keep your bearings performing at or above peak levels for as long as feasible, be aware of the problems that lead to bearing failure.

Conclusion

Plain bearings, also known as sliding contact bearings, are the simplest type of bearing, comprising of a bearing surface and no rolling elements. If you want to learn more about plain bearings or bearings in general, please visit Reid Supply and head over to their extensive library containing the latest industry news and professional literature.