A Practical Buyer’s Guide for 2026: What is a Plain Bearing & 5 Key Selection Criteria

Резюме

A plain bearing is a mechanical element engineered to support a load and reduce friction between moving parts through a sliding action. Unlike rolling-element bearings that utilize balls or rollers, a plain bearing's functionality relies on direct surface contact, often separated by a thin film of lubricant. These components, also known as bushings or journal bearings, consist of a stationary outer surface and a rotating or sliding inner surface. Their design simplicity belies a sophisticated interplay of material science, tribology, and fluid dynamics. They are particularly adept at handling high static and dynamic loads, including shock and vibration, making them suitable for demanding industrial environments. The performance of a plain bearing is contingent on factors such as the material composition (metallic, polymer, or composite), the type and application of lubricant (hydrodynamic, boundary, or solid), and operational parameters like load, speed, and temperature. Their inherent robustness, compact size, and cost-effectiveness ensure their continued relevance in applications ranging from heavy construction equipment to precision automotive systems.

Основни изводи

• A plain bearing operates on the principle of sliding motion, not rolling.
• They excel in high-load, high-impact, and low-speed applications.
• Material choice is a determinant of performance in specific environments.
• Proper lubrication is fundamental to the bearing's lifespan and efficiency.
• Consider the total cost of ownership, not just the initial purchase price.
• Understanding the PV value helps in selecting the right plain bearing.
• They are naturally quieter and more compact than rolling-element bearings.

Съдържание

• Demystifying the Plain Bearing: A Foundational Overview
• The Anatomy of Simplicity: Core Components and Materials
• The First Criterion: Understanding Load Capacity and Direction
• The Second Criterion: Navigating Speed and Frictional Challenges
• The Third Criterion: Material Selection and Environmental Resilience
• The Fourth Criterion: The Nuances of Lubrication Strategies
• The Fifth Criterion: Installation, Maintenance, and Lifecycle Costs
• Plain Bearings in Action: Real-World Applications and Case Studies
• Често задавани въпроси (ЧЗВ)
• Заключение
• Препратки

Demystifying the Plain Bearing: A Foundational Overview

To truly grasp the function and value of any mechanical component, one must first appreciate its fundamental principles. It is not enough to know what something is; we must seek to understand how and why it works. The plain bearing is a perfect subject for such an inquiry. Its deceptive simplicity masks a world of engineering elegance, a testament to the power of fundamental physics applied with precision. It is a component that does its work silently, often in the harshest of conditions, enabling the motion that powers our world.

The Essence of Sliding Motion: What is a Plain Bearing?

At its core, a plain bearing facilitates motion between two surfaces by allowing them to slide against one another. Imagine pushing a heavy wooden crate across a polished floor. The interaction between the crate and the floor is a form of sliding motion. A plain bearing refines this concept to an industrial art form. It is typically a sleeve, tube, or pad—often called a bushing—that is fitted into a housing. A shaft or pin then rotates or slides within this sleeve. The term "bearing" itself derives from the verb "to bear," meaning to support a load (NSK, 2024). A plain bearing bears the load of a shaft, constraining its movement while permitting the intended motion with as little resistance as possible.

Unlike their more complex cousins, rolling-element bearings (like ball bearings or roller bearings), plain bearings have no moving parts within them. There are no balls, no rollers, no cages. There are only two surfaces in relative motion. This elegant minimalism is both their greatest strength and the source of their design challenges. The entire science of the plain bearing revolves around managing the friction and wear that occurs at this sliding interface. The goal is to separate the two surfaces, or, if direct contact is unavoidable, to ensure the materials are robust enough to withstand the interaction over millions of cycles.

Historical Context: From Ancient Wheels to Modern Machinery

The concept of using a sliding interface to enable motion is as old as civilization itself. Archaeologists have found evidence of rudimentary plain bearings in ancient potter's wheels and the axles of Egyptian chariots. These early engineers understood that placing a dissimilar, often softer material between a rotating axle and its housing could reduce friction and prevent the axle from seizing. They might have used lubricated wood or animal fat on bronze.

The Industrial Revolution propelled the development of the plain bearing into a true science. As steam engines and factory machinery demanded higher speeds and heavier loads, engineers like Isaac Babbitt developed new alloys—now known as Babbitt metal—specifically designed for bearing surfaces. These soft, tin- or lead-based alloys could deform to conform to the shaft and embed abrasive particles, protecting the more expensive steel shaft from damage. This period established the foundational principles of material selection and lubrication that still guide plain bearing design in 2026. The evolution from a simple lubricated sleeve to a high-performance, self-lubricating polymer composite represents a continuous journey of refinement, driven by the ever-increasing demands of technology.

Plain Bearings vs. Rolling-Element Bearings: A Comparative Analysis

To appreciate the unique role of the plain bearing, it is helpful to contrast it with the rolling-element bearing. The choice between these two fundamental types is one of the first decisions an engineer makes when designing a rotating system. They are not interchangeable; each possesses distinct characteristics that make it suitable for different tasks. A rolling bearing introduces elements—balls or rollers—to transform sliding friction into rolling friction (SKF, 2024). Think of the difference between dragging a heavy box across the floor versus rolling it on logs. The logs are the rolling elements. This transformation drastically reduces startup friction and allows for very high rotational speeds with low energy loss.

A plain bearing, however, sticks with the original principle of sliding. It manages friction through material choice and lubrication. This fundamental difference leads to a cascade of divergent properties, which dictate their respective applications. The following table offers a direct comparison to illuminate these distinctions.

Функции	Плъзгащи лагери	Rolling-Element Bearing (Ball/Roller)
Принцип на работа	Sliding Motion	Rolling Motion
Friction at Start-up	High	Low
Friction at High Speed	Can be extremely low (with hydrodynamic lubrication)	Low, but increases with speed
Капацитет на натоварване	Very high, distributed over a large surface area	High, but concentrated at points or lines
Shock & Vibration	Excellent damping, absorbs impact well	Susceptible to damage from shock (brinelling)
Възможност за скорост	Generally lower, limited by heat generation	Very high
Ниво на шума	Very low, virtually silent operation	Generates some noise due to rolling elements
Space Requirement	Very compact, especially radially	Requires more space for inner/outer rings and elements
Поддръжка	Can be maintenance-free (self-lubricating) or require periodic lubrication	Typically requires periodic relubrication (greasing)
Cost	Generally lower initial cost	Generally higher initial cost
Failure Mode	Gradual wear, providing warning	Often catastrophic and sudden (fatigue spalling)

As the table illustrates, the plain bearing is not an inferior alternative to the rolling bearing; it is a different solution for a different set of problems. Its ability to handle immense loads and absorb punishing shocks makes it the component of choice for the pivot points on a bulldozer's arm, the suspension system of a heavy truck, or the massive hinges of a dam gate. Its silent operation is valued in office equipment and medical devices. Its simplicity and low cost make it ubiquitous in countless consumer goods. The decision to use a plain bearing is a deliberate engineering choice that prioritizes strength, durability, and compactness over pure speed.

The Anatomy of Simplicity: Core Components and Materials

The functional elegance of the plain bearing arises from its minimalist construction. While variations exist for specific applications, the fundamental structure is reducible to a few key elements. Understanding this anatomy is the first step toward appreciating how material science and tribology—the study of friction, wear, and lubrication—converge to create a component capable of withstanding incredible forces. We will dissect the plain bearing, examining each part not as a static object, but as a functional contributor to the system's overall performance.

The Bearing Housing: The Stationary Foundation

The housing is the fixed component into which the plain bearing is mounted. It provides the structural support and alignment for the entire assembly. While sometimes seen as a simple mounting point, the housing plays a critical role in the bearing's performance, particularly in heat dissipation. The friction generated within a plain bearing, even a well-lubricated one, produces heat. This heat must be conducted away to prevent the lubricant from degrading and the bearing materials from softening or melting.

The material of the housing (typically steel, cast iron, or aluminum), its mass, and its exposure to airflow all contribute to its effectiveness as a heat sink. In high-performance applications, the housing may even incorporate cooling channels for oil or water circulation. Furthermore, the precision of the housing bore is paramount. An improperly sized or out-of-round housing can squeeze the plain bearing, reducing its operational clearance and leading to premature failure. The housing is not merely a container; it is an active partner in the thermal and mechanical stability of the bearing system.

The Sliding Layer: Where the Magic Happens

This is the heart of the plain bearing. The sliding layer is the material surface that directly interacts with the moving component. In a simple solid bushing, the entire body of the bearing is the sliding layer. In more complex, multi-layered bearings, the sliding layer may be a very thin coating applied to a stronger backing material. For example, a common type of high-performance plain bearing consists of a steel backing for strength, a sintered bronze interlayer for heat conductivity and lubricant retention, and a final, thin overlay of a polymer like Polytetrafluoroethylene (PTFE) for its extremely low coefficient of friction.

The properties of this layer dictate the bearing's performance. It must possess a low coefficient of friction to minimize energy loss and heat generation. It must have excellent wear resistance to ensure a long operational life. In some cases, it must be soft enough to embed contaminants that could otherwise score the shaft. In others, it must be hard enough to withstand high contact pressures. The engineering of this single layer is a delicate balancing act of competing properties, tailored to the specific demands of the application.

Material Science: Choosing the Right Composition for the Job

The choice of material for the sliding layer is perhaps the most significant decision in plain bearing design. There is no single "best" material; there is only the "right" material for a given set of conditions regarding load, speed, temperature, and chemical environment. Materials are broadly categorized into three families: metals, polymers, and composites. Each family offers a unique profile of strengths and weaknesses. The selection process requires a deep understanding of not just the application, but also the fundamental properties of the materials themselves. For engineers and buyers, comparing these options is a foundational step.

Material Family	Key Properties	Типични приложения	Limitations
Metallic (e.g., Bronze, Babbitt)	High load capacity, good thermal conductivity, conformability (Babbitt)	Engine crankshafts, heavy industrial pivots, gearboxes	Requires consistent lubrication, susceptible to corrosion, can damage shaft if lubrication fails
Polymer (e.g., PTFE, PEEK, Nylon)	Self-lubricating, excellent chemical resistance, low friction, quiet operation	Food processing equipment, office machines, automotive interiors, marine applications	Lower load capacity than metals, poor thermal conductivity, can swell with moisture (Nylon)
Composite (e.g., Metal-Polymer)	Combines strength of metal backing with low friction of polymer liner, maintenance-free	Automotive suspension, hydraulic pumps, construction equipment pivots	Higher initial cost than simple polymers, can be susceptible to liner damage from severe misalignment
Ceramic (e.g., Silicon Carbide)	Extreme hardness, high temperature resistance, superior corrosion resistance	Chemical pumps, high-temperature process equipment	Brittle, very low tolerance for shock loads or misalignment, high cost

This table serves as a starting point for the complex process of material selection. For instance, a bronze plain bearing might be ideal for a high-load, low-speed pivot in a piece of mining equipment where regular greasing is part of the maintenance schedule. In contrast, a PTFE-lined composite bearing would be the superior choice for a car door hinge, where it must operate for years without any maintenance in a corrosive environment while providing smooth, silent motion.

The Role of Lubrication: The Unsung Hero

If the sliding layer is the heart of the plain bearing, lubrication is its lifeblood. The primary purpose of lubrication is to reduce friction and prevent wear by separating the two sliding surfaces (NSK, 2024). This separation can be partial or complete. The lubricant also serves other vital functions: it helps to cool the bearing by carrying away heat, it protects against corrosion, and it can help to flush out contaminants.

Lubricants come in three primary forms: liquids (oils), semi-solids (greases), and solids. Solid lubricants, such as graphite and molybdenum disulfide, can be embedded directly into the bearing material itself. This is the principle behind self-lubricating or "maintenance-free" bearings. These bearings are "lubricated for life," with the solid lubricant being transferred to the shaft during initial operation to create a stable, low-friction film.

The interaction between the bearing, the shaft, the lubricant, and the operating conditions creates a specific "lubrication regime." Understanding these regimes—hydrodynamic, boundary, and mixed-film—is essential for predicting bearing performance and longevity, a topic we will explore in greater detail later. For now, it is sufficient to recognize that without effective lubrication, whether supplied externally or integrated into the material, the plain bearing could not function.

The First Criterion: Understanding Load Capacity and Direction

When selecting a plain bearing, the first and most fundamental consideration is the load it will be asked to support. A bearing that is undersized for its load will fail prematurely, while an oversized bearing represents unnecessary cost and weight. The analysis of load is not merely a question of "how much," but also "in what direction." The geometry and material of a plain bearing are intimately linked to the nature of the forces it must resist. This requires us to think about forces not as simple magnitudes, but as vectors with both size and direction.

Radial Loads: Supporting Perpendicular Forces

Imagine a wheel turning on an axle. The weight of the vehicle pushes down on the axle, and the axle pushes down on the inside of the wheel's hub. This force, acting perpendicular to the axis of rotation, is a radial load. The vast majority of plain bearings are designed primarily to handle radial loads. A simple sleeve bearing, or bushing, is a classic example. The load is distributed over a projected area within the bottom half of the bearing.

The capacity of a plain bearing to support a radial load is determined by its "projected area" (the bearing's diameter multiplied by its length) and the "specific load capacity" of its material, typically expressed in megapascals (MPa) or pounds per square inch (psi). For example, a hardened steel shaft running in a bronze plain bearing might handle specific loads of 10-15 MPa, while a polymer bearing might be limited to 3-5 MPa. However, the polymer bearing can often operate without lubrication, a trade-off that highlights the complexity of selection. The goal is to choose a bearing with a material and size combination that keeps the operational pressure well within the material's allowable limit.

Axial (Thrust) Loads: Managing Parallel Forces

Now, consider a spinning bar stool. When someone sits on it, their weight is a radial load on the support column. But when they push off a table to spin, the force that keeps the seat from flying off its pedestal is an axial load, acting parallel to the axis of rotation. This is also known as a thrust load.

Simple sleeve bearings have very limited capacity to handle axial loads. If a significant thrust force is present, it will act on the small end faces of the bearing, leading to high localized pressure and rapid wear. To manage these forces, specialized plain bearings are used. A "thrust washer" is the simplest form—a flat, donut-shaped washer against which a rotating component pushes. More sophisticated designs include "flanged bearings," which are sleeve bearings with an integrated flange at one end. This flange provides a much larger surface area to support axial loads, effectively combining a radial bearing and a thrust washer into a single component.

Combined Loads: The Balancing Act

In many real-world applications, bearings must endure both radial and axial loads simultaneously. Think of the wheel of a car as it goes around a corner. The weight of the car is a constant radial load, while the cornering force adds a significant axial load. This is a combined load scenario.

Managing combined loads requires careful consideration. A flanged plain bearing is often a good solution. Alternatively, a system might use a separate radial sleeve bearing and an axial thrust washer. A particularly elegant solution for combined loads is the spherical plain bearing. This type of bearing features a spherically ground inner ring nested within a conforming outer ring. This geometry allows it to support very high radial loads while also accommodating significant axial loads and, crucially, tolerating misalignment between the shaft and housing. The ability to "self-align" makes spherical plain bearings invaluable in applications like vehicle suspensions and construction equipment linkages, where flexing and twisting are unavoidable.

Calculating Load Requirements for Your Application

Determining the precise loads in an application can be a complex task, often requiring engineering analysis. However, a simplified approach can provide a solid foundation for bearing selection. The process involves identifying all forces acting on the shaft and resolving them into their radial and axial components.

1. Identify Static Loads: These are forces that are constant, such as the weight of components. For a conveyor belt roller, this would be the weight of the roller itself plus the weight of the material it is carrying.
2. Identify Dynamic Loads: These are forces that result from operation, such as tension in a belt, pressure from a hydraulic cylinder, or inertial forces from acceleration and deceleration.
3. Resolve Forces: Use basic trigonometry to break down any angled forces into their pure radial (perpendicular to the shaft) and axial (parallel to the shaft) components.
4. Calculate Specific Load (Pressure): Once you have the maximum radial load (P), you can calculate the specific load on the bearing using the formula: Specific Load = P / (D * L), where D is the inner diameter of the bearing and L is its length.
5. Compare with Material Limits: Compare this calculated value with the maximum specific load capacity specified by the bearing manufacturer for the chosen material. A safety factor is always recommended, especially when shock or vibration is present. For example, if a material is rated for 10 MPa, it might be wise to design for a maximum operating pressure of 5-7 MPa to ensure a long and reliable service life.

This systematic approach demystifies the process of load calculation, transforming it from an abstract concept into a practical tool for making informed decisions about which plain bearing is right for your machinery.

The Second Criterion: Navigating Speed and Frictional Challenges

After load, the second critical parameter governing plain bearing selection is speed. The rotational or sliding velocity at the bearing interface has a profound impact on friction, heat generation, and the lubrication mechanism. A plain bearing that performs flawlessly in a low-speed pivot might fail catastrophically in a high-speed spindle. Understanding the relationship between speed, pressure, and material properties is essential for ensuring the longevity and efficiency of the bearing. This relationship is often encapsulated in a single, powerful concept: the PV-value.

The PV-Value: A Key Performance Indicator

The "PV-value" is a fundamental metric in plain bearing design. It is the product of the specific load on the bearing (P, in MPa or psi) and the sliding velocity (V, in m/s or ft/min). The resulting value, PV, represents the rate of energy dissipation due to friction, which manifests as heat. Every bearing material has a maximum permissible PV-value, a limit beyond which the heat generated cannot be adequately dissipated. Exceeding this limit leads to a rapid increase in temperature, causing the lubricant to fail, the bearing material to soften or melt, and eventual seizure of the joint.

This concept is incredibly useful because it shows the trade-off between pressure and velocity. You can have a high-pressure, low-speed application or a low-pressure, high-speed application, and both might have the same PV-value. For example, a bronze bearing might have a limiting PV of 1.75 MPa·m/s, while a high-performance, self-lubricating polymer composite might reach 3.5 MPa·m/s or more.

When selecting a plain bearing, you must calculate the PV-value for your application (PV = Specific Load × Velocity) and ensure it is comfortably below the manufacturer's stated limit for the chosen material and operating conditions. This single calculation provides one of the most reliable indicators of a bearing's suitability for a given task.

Heat Generation and Dissipation

The PV-value is essentially a measure of heat generation. The challenge then becomes one of thermal management. The bearing assembly must be able to dissipate this heat to the surrounding environment at a rate equal to or greater than the rate at which it is generated. If it cannot, the temperature will rise until a component fails.

Several factors influence heat dissipation:

• Bearing Material: Metallic bearings (bronze, steel) have high thermal conductivity and are very effective at transferring heat from the sliding surface to the housing. Polymer bearings have very low thermal conductivity, which can lead to heat buildup at the sliding interface if the PV-value is too high.
• Housing Design: As mentioned earlier, a massive housing with a large surface area acts as an effective heat sink. Fins can be added to increase the surface area and improve air cooling.
• Lubrication: In a circulating oil system, the oil itself becomes a primary method of heat removal, carrying thermal energy away from the bearing to a sump or cooler. Even grease helps conduct heat away from the contact zone.
• Ambient Temperature: A bearing operating in a hot environment will have a harder time dissipating heat than one operating in a cool, well-ventilated area. The maximum allowable operating temperature of the bearing material must always be considered in relation to the ambient conditions and the heat generated.

Stick-Slip Phenomenon: Avoidance and Mitigation

At very low speeds, particularly under high loads, plain bearings can exhibit a behavior known as "stick-slip." This is a jerky, start-stop motion that can cause noise, vibration, and increased wear. It occurs because the coefficient of static friction (the force needed to start motion) is higher than the coefficient of dynamic friction (the force needed to maintain motion).

Imagine trying to slide a heavy object. It takes a large initial push to get it moving (the "stick" phase). Once moving, it slides more easily. As it slows down, the friction increases again, and it "slips" to a halt, ready for the cycle to repeat. This is stick-slip. It is particularly common in unlubricated or poorly lubricated systems.

Mitigating stick-slip involves several strategies:

• Material Selection: Certain materials, especially PTFE-based composites, have static and dynamic coefficients of friction that are nearly identical. This inherent property makes them highly resistant to stick-slip.
• Lubrication: A consistent lubrication film, even a very thin one, can prevent the direct surface contact that leads to stick-slip.
• Surface Finish: A very smooth finish on both the shaft and the bearing can reduce the interlocking of microscopic surface asperities that contributes to high static friction.

For applications requiring precise, smooth motion at low speeds, such as robotic arms or scientific instruments, selecting a plain bearing material specifically designed to combat stick-slip is a primary consideration.

The Third Criterion: Material Selection and Environmental Resilience

The soul of a plain bearing resides in its material composition. The choice of material dictates not only its load and speed capabilities but also its ability to survive in its intended operating environment. A bearing is rarely afforded the luxury of a clean, climate-controlled room. It must often perform its duties while being assaulted by moisture, chemicals, abrasive dust, and extreme temperatures. This criterion moves beyond simple mechanical properties to embrace a holistic view of the bearing as a component that must endure a specific, often hostile, world.

Metallic Bearings: Bronze, Steel, and Babbitt

Metals were the original high-performance bearing materials, and they remain indispensable in many applications today.

• Bronze Alloys: These are the workhorses of the metallic plain bearing world. Different alloys offer different properties. Tin bronzes are strong and wear-resistant. Leaded bronzes offer a degree of self-lubrication and conformability, making them more forgiving of misalignment. Aluminum bronzes provide excellent strength and corrosion resistance, making them suitable for marine applications. Bronze bearings are valued for their high load capacity and good thermal conductivity, but they almost always require a reliable supply of grease or oil.
• Steel: Hardened steel on hardened steel can be used in plain bearing arrangements, but it is unforgiving of lubrication failure and misalignment. More commonly, steel is used as a backing material for other, softer bearing layers, providing structural rigidity. Porous, sintered steel, impregnated with oil, is another common form, creating a self-lubricating bearing for moderate-load applications.
• Babbitt (White Metal): These tin- or lead-based alloys are relatively soft. This softness is their key advantage. In a large, high-speed rotating machine like a steam turbine or an engine crankshaft, a Babbitt plain bearing provides a sacrificial surface. If a lubrication failure occurs or a hard contaminant enters the bearing, the Babbitt will melt or deform, protecting the vastly more expensive steel shaft from catastrophic damage.

Polymer Bearings: The Rise of PTFE and PEEK

The development of advanced engineering polymers has revolutionized the plain bearing industry. These "plastic" bearings offer a combination of properties that metals cannot match.

• PTFE (Polytetrafluoroethylene): Best known by the trade name Teflon, PTFE has one of the lowest coefficients of friction of any known solid. It is also nearly chemically inert and has a wide operating temperature range. However, pure PTFE is very soft and has poor wear resistance (a phenomenon known as "cold flow"). Its true power is unlocked when it is used as a liner in a composite bearing or when fillers like glass fiber, carbon, or bronze powder are added to create a reinforced PTFE compound. These compounds offer extremely low friction and can operate completely dry.
• PEEK (Polyether Ether Ketone): PEEK is a high-performance thermoplastic that sits at the upper end of the polymer hierarchy. It offers excellent mechanical strength, high temperature resistance (up to 250°C), superior chemical resistance, and good wear properties. It is often used in demanding applications in the aerospace, medical, and chemical processing industries where other polymers would fail.
• Other Polymers: Materials like Nylon, Acetal, and UHMW-PE (Ultra-High-Molecular-Weight Polyethylene) offer good all-around performance at a lower cost. They are often used in less demanding applications where their specific properties (e.g., Nylon's toughness, UHMW-PE's abrasion resistance) are beneficial. However, one must be mindful of their limitations, such as Nylon's tendency to swell as it absorbs moisture.

Composite Bearings: The Best of Both Worlds

Composite bearings represent a sophisticated synthesis of materials, designed to capture the best properties of each. The most common type is the metal-polymer plain bearing. These typically feature:

1. A Steel Backing: Provides high mechanical strength and allows for a secure press-fit into a housing.
2. A Sintered Bronze Interlayer: This porous layer is bonded to the steel backing. It provides a strong mechanical link for the polymer layer and its porous structure can act as a reservoir for lubricant. It also has excellent thermal conductivity, helping to draw heat away from the sliding surface.
3. A PTFE-Based Sliding Layer: A thin layer of a proprietary PTFE-based compound is impregnated into and overlaid on the bronze interlayer. During a brief run-in period, a minute amount of this layer transfers to the shaft, creating a stable, low-friction solid lubricant film.

This composite structure results in a plain bearing solution that is strong, compact, maintenance-free, and has a very low coefficient of friction. It is this type of bearing that has replaced rolling bearings in many automotive suspension pivots and other high-load, oscillating applications.

Corrosion, Contamination, and Temperature Extremes

The operational environment often dictates material choice as much as load or speed.

• Корозия: In marine environments, food processing plants (with frequent chemical washdowns), or outdoor equipment, corrosion resistance is paramount. Stainless steel backings, bronze alloys, and nearly all polymer bearings offer excellent performance in these conditions. Standard steel-backed bearings would require protective plating (like tin or zinc) or would rust quickly.
• Contamination: Abrasive particles like dust, sand, or dirt are the mortal enemy of many bearings. Rolling-element bearings are particularly vulnerable, as a single hard particle can dent a raceway and initiate failure. The conformability of softer plain bearing materials (like Babbitt or some polymers) allows them to embed these particles, removing them from the critical sliding interface and protecting the shaft.
• Температура: Extreme cold can make some materials brittle, while extreme heat can cause them to soften or degrade. Standard polymers may be limited to around 80-100°C. High-temperature polymers like PEEK or specialized composites can operate continuously at 250°C. For even higher temperatures, metallic bearings made from special alloys or even graphite-based materials may be required.

The Fourth Criterion: The Nuances of Lubrication Strategies

Lubrication is the art and science of controlling friction and wear. In the context of a plain bearing, it is the determining factor that separates a smooth, efficient system from a screeching, seizing failure. The choice of lubricant and the method of its application are not afterthoughts; they are integral to the bearing's design. The strategy can range from a sophisticated, continuously circulating oil system to a bearing material that contains its own lifetime supply of lubricant. Understanding the different lubrication "regimes" provides a framework for appreciating how this works.

Hydrodynamic Lubrication: Creating a Fluid Film

This is the ideal state for a lubricated plain bearing. Imagine a car aquaplaning on a wet road. A wedge of water builds up under the tire, lifting it completely off the pavement. Hydrodynamic lubrication works in a similar way. As the shaft begins to rotate within the oil-filled clearance of the plain bearing, it "drags" oil into the load zone. As speed increases, the pressure in this oil wedge builds until it is strong enough to lift the shaft entirely, creating a complete separation between the shaft and the bearing surface.

In this regime, the two metal surfaces never touch. The only friction is the fluid friction from shearing the oil film. This is incredibly low, resulting in very high efficiency and, in theory, infinite life, as there is no mechanical wear. This is the principle that allows the massive crankshafts in power plant turbines and large ship engines to rotate for decades. Achieving a stable hydrodynamic film requires a continuous supply of clean lubricant, sufficient rotational speed, and precise geometry. It is a state of mechanical grace, but one that is not always achievable, especially during startup, shutdown, or under slow, oscillating motion.

Boundary and Mixed-Film Lubrication

What happens when the speed is too low, the load is too high, or the oil is too thin to create a full hydrodynamic film? The system enters the realms of mixed-film or boundary lubrication.

• Mixed-Film Lubrication: In this transitional state, the hydrodynamic film is carrying most of the load, but the highest peaks (asperities) on the shaft and bearing surfaces are beginning to make contact. There is a mix of fluid friction and solid friction. Wear is low, but it is occurring.
• Boundary Lubrication: When the hydrodynamic film collapses completely, the load is carried entirely by the surfaces themselves. This occurs at very low speeds, during startup, or under extreme shock loads. In this regime, the physical properties of the lubricant itself become secondary. The "lubrication" is now provided by chemical additives in the oil or grease—known as "anti-wear" or "extreme pressure" (EP) additives. These additives form a thin, protective chemical layer on the metal surfaces, a boundary film that prevents catastrophic welding and seizure. The coefficient of friction is much higher here, and wear is significant. A plain bearing operating continuously in the boundary regime will have a finite life.

Self-Lubricating and Maintenance-Free Bearings

The need for external lubrication (and the associated plumbing, seals, and maintenance) is a significant drawback in many applications. This led to the development of self-lubricating, or "maintenance-free," plain bearings. These ingenious components eliminate the need for grease or oil by incorporating the lubricant directly into the bearing structure.

There are two main types:

1. Sintered Metal Bearings: These are made from powdered metal (typically bronze or iron) which is compacted and then heated (sintered) to fuse the particles together, leaving a network of interconnected pores—up to 30% of the volume. This porous structure is then impregnated with oil under a vacuum. In operation, as the bearing heats up, the oil expands and flows to the surface to provide lubrication. When it cools, the capillary action of the pores draws the oil back in, preventing leakage. They offer a simple and cost-effective self-lubricating solution for moderate loads and speeds.
2. Polymer and Composite Bearings: This is where the most significant advances have been made. As discussed previously, materials like PTFE have inherently low friction. When used in a composite structure (e.g., the metal-polymer bearings), they provide a solid lubricating film that constantly replenishes itself. Fillers like graphite or molybdenum disulfide can also be blended into polymers to enhance their self-lubricating properties. These bearings are designed to operate completely dry and are the default choice for applications where maintenance is impossible or undesirable, such as in consumer products, automotive interiors, and remote equipment.

Grease, Oil, or Solid Lubricants: Making the Right Choice

When external lubrication is required, the choice between grease and oil is a primary one.

• Oil: Oil is a superior lubricant in terms of reducing friction and a far superior coolant. It can be circulated, filtered, and cooled, making it the only choice for high-speed, high-temperature applications. However, it requires a more complex system of seals and reservoirs to contain it.
• Grease: Grease is essentially oil suspended in a thickener (like a soap). The thickener acts like a sponge, holding the oil in place. Grease is excellent for sealing out contaminants and for applications where re-lubrication is infrequent. It does not leak out as easily as oil, making it ideal for sealed-for-life units or for vertical or oscillating applications. However, it is a poor coolant and is not suitable for very high speeds as the thickener can generate too much heat.
• Solid Lubricants: These are used in extreme environments—high vacuum, extreme temperatures, or where any form of liquid or grease contamination is unacceptable. They can be applied as coatings or integrated into the bearing material itself.

The selection of a lubrication strategy is a complex decision, weighing the ideal performance of a hydrodynamic film against the practical, maintenance-free reality of a self-lubricating composite plain bearing.

The Fifth Criterion: Installation, Maintenance, and Lifecycle Costs

The final piece of the puzzle in selecting a plain bearing extends beyond its technical specifications to encompass its entire lifecycle, from the moment it is installed to the day it is replaced. A technically perfect bearing can fail rapidly if installed incorrectly. An inexpensive bearing may lead to exorbitant costs through frequent downtime and replacement. A truly wise selection considers the total cost of ownership (TCO), a concept that balances initial price against long-term reliability and maintenance needs.

Proper Installation Techniques: Press-Fitting and Clearances

Unlike a rolling bearing that is often a slip-fit onto a shaft, most plain bearings (especially bushings) are installed with a "press-fit" or "interference fit" into their housing. This means the outside diameter of the bearing is slightly larger than the inside diameter of the housing bore. This interference holds the bearing securely in place and prevents it from rotating with the shaft.

This press-fitting process is critical.

• Sizing: The act of pressing the bearing into the housing slightly reduces its inside diameter. This "press-fit closure" must be accounted for. Manufacturers provide detailed specifications. If you measure the bearing's inner diameter before installation, you will find it is too large. It only achieves its correct operational size after being pressed into the correct size housing.
• Installation Tools: A proper arbor press and a mounting tool that applies force only to the strong outer circumference of the bearing are essential. Hammering a bearing into place can distort its shape and damage the delicate sliding layer.
• Clearance: The "operational clearance" is the small gap between the shaft and the installed bearing's inner diameter. This space is vital for thermal expansion and for the formation of a lubrication film. Too little clearance, and the bearing will seize. Too much, and the shaft will vibrate, leading to noise and impact loads. The required clearance depends on the materials, temperatures, and lubrication type.

Monitoring and Maintenance Schedules

For bearings that require external lubrication, a disciplined maintenance schedule is non-negotiable.

• Relubrication Intervals: Manufacturers provide guidelines for how often to re-apply grease or check oil levels, based on operating hours, speed, temperature, and contamination levels. A bearing in a hot, dirty environment may need daily attention, while one in a clean, cool application might go for months.
• Inspection: Regular visual inspection can reveal early warning signs. Look for extruded grease (which may indicate a seal failure), excessive heat (using an infrared thermometer), or unusual noise.
• Lubricant Analysis: In critical oil-lubricated systems, taking periodic oil samples for analysis can predict failures. The presence of wear metals (like bronze or steel) or contaminants can signal a developing problem long before it becomes catastrophic.

For maintenance-free, self-lubricating bearings, the focus shifts from active maintenance to condition monitoring. The goal is to monitor the wear of the bearing and replace it before it fails. This might involve periodically measuring the clearance to check for an increase, which would indicate wear of the sliding layer.

Failure Analysis: Learning from Wear and Tear

When a plain bearing does reach the end of its life, the worn component is a valuable source of information. A careful examination can reveal the cause of failure, allowing for corrective actions to improve the life of the next bearing.

• Normal Wear: A properly functioning bearing will show a smooth, polished, and evenly worn area in the load zone.
• Абразивно износване: Scoring, scratching, or a dull, lapped appearance indicates that hard contaminants have entered the bearing. This points to a need for better sealing or filtration.
• Adhesive Wear (Seizure): Areas where material has been transferred from the bearing to the shaft (or vice versa) indicate a lubrication failure and metal-to-metal contact. This could be caused by excessive heat, incorrect lubricant, or a load that was too high.
• Fatigue: Cracking or flaking of the bearing surface, often seen in Babbitt bearings, can indicate cyclic overloading.
• Корозия: Pitting and discoloration of the bearing surface point to chemical attack from moisture or aggressive fluids.

Understanding these visual cues allows one to diagnose problems in the system—misalignment, contamination, overloading—rather than simply blaming the bearing itself.

Total Cost of Ownership: Beyond the Initial Purchase Price

The cheapest plain bearing is rarely the most cost-effective. The TCO calculation provides a more complete financial picture. It includes:

• Initial Purchase Price: The cost of the bearing itself.
• Installation Costs: The labor required to install the bearing.
• Maintenance Costs: The cost of lubricants and the labor for relubrication over the bearing's life. For self-lubricating bearings, this is zero.
• Downtime Costs: This is often the largest and most overlooked cost. What is the cost to the business for every hour the machine is out of service for a bearing replacement? In a production line, this can be thousands or even millions of dollars.
• Replacement Costs: The price of the new bearing plus the labor to perform the replacement.

When viewed through the lens of TCO, a more expensive, high-performance, maintenance-free composite plain bearing can often be vastly more economical than a simple bronze bushing that requires frequent greasing and replacement. The initial investment pays for itself many times over in reduced maintenance and increased uptime. This holistic financial perspective is the final, and perhaps most important, step in making an intelligent bearing selection.

Plain Bearings in Action: Real-World Applications and Case Studies

The theoretical principles of plain bearings come to life when we examine their application in the machines that define our modern world. From the earth-moving giants that build our cities to the silent mechanisms in our homes, these components are ubiquitous. Their selection in each case is a deliberate choice, reflecting a careful balance of the criteria we have discussed: load, speed, environment, lubrication, and cost.

Heavy Machinery: Construction and Agriculture

This is the classic domain of the high-load, low-speed plain bearing. Consider the main pivot point of a large hydraulic excavator's boom. This joint must support the immense weight of the arm, bucket, and load, while enduring constant shock and vibration as it digs into the earth. The motion is slow and oscillating, not continuous rotation. This is a hostile environment for a rolling bearing, which would be susceptible to brinelling (denting) from the shock loads.

Here, a hardened steel pin rotating in a heavy-duty composite or bronze plain bearing is the ideal solution. The large surface area of the plain bearing distributes the immense load, and its material composition is designed to absorb impact. Self-lubricating composite bearings are increasingly favored in these applications because they eliminate the need for daily greasing of dozens of pivot points, drastically reducing maintenance time and costs for the equipment owner.

Automotive Systems: From Suspension to Engines

Your car is filled with dozens of plain bearings, performing a variety of roles.

• Suspension System: The wishbone pivots, strut mounts, and stabilizer bar links all use plain bearings. These are typically metal-polymer composite bushings. They must withstand high shock loads from potholes, operate in a highly corrosive environment (road salt and water), and provide smooth, silent operation for the life of the vehicle without any maintenance.
• Engine: The crankshaft and connecting rods of an internal combustion engine rotate on specialized multi-layer plain bearings, often of the Babbitt type. Here, the conditions are high speed and high load, and the system relies on a meticulously designed hydrodynamic lubrication system, with pressurized oil being constantly supplied. The soft Babbitt layer provides the critical sacrificial surface needed to protect the expensive crankshaft in case of temporary oil starvation.
• Interior and Ancillaries: Simpler polymer bushings are used everywhere: in seat adjustment mechanisms, windshield wipers, and brake pedal pivots. Here, the loads are low, and the primary requirements are low cost, silent operation, and freedom from maintenance.

Aerospace and Marine: Where Reliability is Paramount

In aerospace, every component's weight and reliability are scrutinized. Plain bearings are used in landing gear pivots, flight control surface actuators, and cargo door mechanisms. These are often highly specialized components made from advanced materials like PEEK or proprietary composites to save weight while meeting extreme temperature and load requirements. Failure is not an option, so these bearings undergo rigorous testing and certification.

In the marine world, corrosion resistance is the dominant challenge. Propeller shafts on large vessels are often supported by massive plain bearings lubricated by the water itself. The bearing material might be a specialized rubber or composite designed to operate in seawater. On deck, winches and cranes use bronze or polymer plain bearings that can withstand constant exposure to salt spray.

Renewable Energy: Wind Turbines and Hydropower

The renewable energy sector presents unique challenges. In a large wind turbine, the blades must be able to pitch (rotate along their long axis) to control speed and power generation. These blade pitch bearings are large-diameter plain bearings that must operate under extreme loads, endure constant vibration, and function reliably for 20-25 years with minimal maintenance high up in the nacelle. Specialized composite materials are often used here.

In hydropower plants, the massive wicket gates that control water flow to the turbines pivot on large, self-lubricating bronze or composite plain bearings. These bearings are often submerged in water and must operate reliably for decades. Their ability to handle immense loads and resist corrosion makes them the only viable choice for such a demanding application.

Често задавани въпроси (ЧЗВ)

What is the main difference between a plain bearing and a ball bearing? The fundamental difference is their method of operation. A plain bearing uses a sliding motion between two surfaces, often separated by a lubricant. A ball bearing uses spherical rolling elements to separate the moving parts, converting sliding friction into much lower rolling friction. This makes ball bearings better for high-speed, low-friction applications, while plain bearings excel at handling high loads and shock in more compact spaces.

Why would I choose a plain bearing when ball bearings have less friction? You would choose a plain bearing for its specific strengths: superior load capacity (especially for shock and impact), greater tolerance for contamination, silent operation, more compact design (particularly in the radial direction), and often a lower initial cost. They are ideal for high-load, low-speed, or oscillating applications like pivots and hinges on heavy equipment.

Are all plain bearings "maintenance-free"? No. This is a common misconception. Plain bearings fall into two main categories: those requiring external lubrication (like traditional bronze or Babbitt bearings that need regular grease or oil) and those that are self-lubricating. Self-lubricating or "maintenance-free" bearings have solid lubricants like PTFE or graphite integrated into their material, eliminating the need for external lubrication for their entire service life.

What does the "PV-value" mean, and why is it important? The PV-value is a key performance metric calculated by multiplying the specific load (Pressure, P) by the sliding velocity (Velocity, V). It represents the amount of frictional heat the bearing will generate. Every bearing material has a maximum PV limit. Exceeding this limit will cause the bearing to overheat and fail. Checking your application's calculated PV against the manufacturer's limit is a critical step in ensuring you have selected a suitable bearing.

Can a plastic plain bearing really be strong enough for industrial use? Absolutely. The term "plastic" can be misleading. Modern engineering polymers and composites are highly advanced materials. While they generally have lower load capacities than metallic bearings, high-performance polymers like PEEK and reinforced composite bearings (with steel backings and polymer liners) can handle very high loads and offer significant advantages in terms of corrosion resistance, weight, and self-lubricating properties, making them ideal for many demanding industrial applications.

How do I install a plain bearing bushing correctly? Most plain bearing bushings require a press-fit into their housing. This should be done using an arbor press and a proper installation tool that applies force evenly to the outside edge of the bearing. Never use a hammer. It is also vital to remember that the bearing's inside diameter is designed to be correct after it has been pressed into the housing; this "press-fit closure" must be accounted for in your design.

What causes a plain bearing to fail? Failure is most often caused by a breakdown of the lubrication system. This can be due to excessive heat, contamination (dirt, grit), overloading, or simply the depletion of the lubricant. Other causes include incorrect installation, which can eliminate the necessary operating clearance, and severe shaft or housing misalignment, which concentrates the load on the edge of the bearing.

Заключение

The plain bearing, in its elegant simplicity, embodies a profound engineering truth: often, the most robust solutions are those that adhere to fundamental principles. Eschewing the complexity of moving parts, it masters the physics of sliding motion through a sophisticated interplay of material science, lubrication dynamics, and thoughtful design. It is not merely a "low-tech" alternative to a rolling bearing; it is a distinct and powerful tool, purpose-built for a world of immense forces, punishing impacts, and unforgiving environments.

From the silent pivots in an office chair to the colossal joints of a mining shovel, the plain bearing demonstrates a remarkable versatility. Our exploration of the five key selection criteria—load, speed, material, lubrication, and lifecycle cost—reveals that choosing the right plain bearing is a process of holistic analysis. It demands that we look beyond a single specification and consider the complete operational context. By understanding the trade-offs between a bronze bushing's raw strength and a composite bearing's maintenance-free endurance, we empower ourselves to make choices that enhance reliability, reduce long-term costs, and improve the performance of our machinery. The humble plain bearing, a component with ancient roots, continues to be an indispensable element of modern technology, silently shouldering the loads of our industrial world.

Препратки

Changzhou Hecb Bearing Co., Ltd. (2025, June 25). What are roller bearings? Types and applications. HECBearing. https://ru.hecbearing.com/new_detail/What-Are-Roller-Bearings-Types-and-Applications.html

NSK. (2024, July 11). Bearing basics (uses, types, and components). https://www.nsk.com/tools-resources/abc-bearings/bearing-basics/

NSK. (2024, July 11). Introduction to bearings.

RS Components. (2024, October 9). Comprehensive guide to ball bearings. RS.

SKF. (2024, February 28). Bearing basics.