Expert Guide 2026: What Are Plain Bearings Made Of? 7 Key Material Types

Abstract

Plain bearings, fundamentally simple in concept yet complex in material science, are constructed from a diverse spectrum of materials tailored to specific operational environments. The selection of a material is not arbitrary; it is a deliberate engineering decision that dictates the bearing's performance characteristics, including its load-bearing capacity, frictional properties, thermal resistance, wear life, and chemical inertness. An inquiry into what are plain bearings made of reveals a landscape of metals, polymers, composites, and ceramics. Metallic options like bronze and babbitt offer high strength, while advanced polymers such as PTFE and PEEK provide self-lubricating properties for maintenance-free operation. Composite bearings synergize these benefits, combining metal backings with polymer linings. Ceramic materials are reserved for the most extreme conditions of temperature and corrosion. A thorough examination of these materials provides engineers, technicians, and procurement specialists with the foundational knowledge required to select the optimal bearing, thereby enhancing machinery efficiency and longevity.

Key Takeaways

• Material selection is the single most defining factor for a plain bearing's performance and service life.
• Polymer-based bearings enable self-lubrication, which is ideal for maintenance-free or contamination-sensitive systems.
• Metal alloys provide the necessary compressive strength and thermal conductivity for heavy-duty, high-load applications.
• A clear understanding of what are plain bearings made of allows for precise system optimization and cost reduction.
• Composite materials offer a balanced solution, combining the strength of metals with the low friction of polymers.
• Ceramics and cermets are reserved for niche applications involving extreme temperatures, speeds, or corrosive agents.
• The operational environment directly dictates the suitability of a particular bearing material.

Table of Contents

• Understanding the Essence of Plain Bearings
• A Comparative Overview of Primary Plain Bearing Materials
• Metallic Plain Bearings: The Foundation of Strength
• Polymer Plain Bearings: The Rise of Self-Lubrication
• Metal-Polymer Composites: The Best of Both Worlds
• Bimetallic and Sintered Bearings: Porosity and Power
• Ceramic and Cermet Bearings: For the Harshest Frontiers
• Specialized and Exotic Materials: Pushing the Boundaries
• Selecting the Right Material: A Practical Framework
• Frequently Asked Questions (FAQ)
• Final Considerations on Material Selection
• References

Understanding the Essence of Plain Bearings

To embark on a meaningful exploration of what plain bearings are made of, one must first grasp the philosophical and mechanical principle they embody. A plain bearing, in its most elemental form, is a surface. It is a deceptively simple component designed to support a load and permit relative motion between two machine parts, typically through a sliding action. Unlike their rolling-element counterparts, such as ball bearings or roller bearings, plain bearings have no moving parts themselves (SKF, 2024). They are the quiet, unassuming heroes of mechanical engineering, performing their duty through the intrinsic properties of the materials from which they are formed.

The Fundamental Role of a Plain Bearing

Imagine trying to spin a heavy steel shaft directly inside a hole drilled into a cast iron machine frame. The friction would be immense. Heat would build up rapidly, the surfaces would gall and seize, and the entire assembly would fail in short order. The plain bearing is the intermediary that prevents this catastrophe. It introduces a material with more desirable properties—lower friction, better wear resistance, or the ability to embed contaminants—between the shaft and the housing. Its function is one of sacrifice and facilitation. It bears the load and absorbs the punishment of friction so that the more expensive and integral components, like the shaft and the housing, are preserved. This concept is central to understanding why the material composition is so paramount. The bearing is not just a spacer; it is a carefully chosen functional surface.

Sliding Friction vs. Rolling Friction: A Core Distinction

A helpful thought exercise is to compare pushing a heavy box across a floor versus pulling it on a wheeled cart. Pushing the box involves sliding friction, where the entire bottom surface of the box resists the motion. Pulling the cart involves rolling friction, where the contact points are the small areas where the wheels touch the floor. Rolling friction is significantly lower, which is why rolling-element bearings like deep groove ball bearings are suited for high-speed, low-friction applications (NTN Corporation, 2024).

Plain bearings operate on the principle of sliding friction. This might seem like a disadvantage, but it brings unique benefits. A plain bearing has a much larger contact area, allowing it to support significantly higher loads and withstand greater shock and vibration than a similarly sized ball bearing. However, this large contact area also means that the material properties are under constant scrutiny. Without the right material, sliding friction would lead to rapid failure. The entire field of plain bearing material science is dedicated to minimizing the negative aspects of sliding friction while maximizing the load-carrying benefits of the large surface area.

Why Material Choice Defines the Bearing

Because a plain bearing has no rolling elements to distribute load or motion, its entire capability is derived from its substance. The question, "What are plain bearings made of?" is not one of mere curiosity but the central inquiry for anyone designing, maintaining, or purchasing these components. A material must be chosen based on a complex interplay of factors: the magnitude and type of load (radial, axial, or both), the rotational speed, the operating temperature, the presence of corrosive chemicals, the availability of lubrication, and the required service life. A soft, conformable material like Babbitt might be perfect for an engine's connecting rod where it can embed debris, but it would be utterly destroyed in a high-temperature, unlubricated conveyor system. Conversely, a hard ceramic bearing that thrives in a chemical pump would be too brittle and expensive for a simple hinge. Therefore, the material is the bearing. Its identity and its function are one and the same.

A Comparative Overview of Primary Plain Bearing Materials

To provide a clear, at-a-glance understanding, the following table contrasts the general characteristics of the main families of materials used in plain bearing construction. Consider this a map for the more detailed exploration that will follow.

Material Family	Typical Load Capacity	Max. Temperature	Friction Coefficient (Dry)	Key Advantage	Common Applications
Bronze Alloys	High to Very High	~250°C (480°F)	0.15 – 0.25	High strength, good wear resistance	Heavy machinery, pivots, construction equipment
Babbitt (White Metal)	Low to Medium	~120°C (250°F)	0.30 – 0.40 (requires lubrication)	Conformability, embeddability	Internal combustion engines, turbines
Polymers (PTFE)	Low	~260°C (500°F)	0.04 – 0.10	Extremely low friction, self-lubricating	Food processing, chemical pumps, textiles
Polymers (PEEK)	Medium to High	~260°C (500°F)	0.25 – 0.35	High strength at high temperatures	Aerospace, medical devices, automotive
Metal-Polymer Composite	High	~280°C (536°F)	0.03 – 0.20	Combination of high strength and low friction	Automotive suspension, hydraulic pumps
Sintered Bronze (Oil-Impregnated)	Medium	~100°C (212°F)	Varies (lubricated)	Self-lubricating via stored oil	Small electric motors, home appliances
Ceramics (Silicon Carbide)	High	>1000°C (1832°F)	0.15 – 0.25	Extreme temperature and corrosion resistance	Pumps for aggressive media, magnetic drives

Metallic Plain Bearings: The Foundation of Strength

When mechanical systems demand brute strength, resilience against shock, and effective heat dissipation, metallic plain bearings are the first and most traditional line of defense. Their history is deeply intertwined with the Industrial Revolution, where the need to support massive, rotating iron and steel shafts gave rise to the development of specialized bearing alloys. The primary virtue of a metallic bearing is its high compressive strength and modulus of elasticity, meaning it resists deformation under immense loads.

Bronze and Copper Alloys: The Traditional Workhorses

Bronze, an alloy primarily of copper, is arguably the most recognized material for plain bearings. The addition of other elements like tin, lead, aluminum, or zinc creates a vast family of bronzes, each with a unique personality.

• Tin Bronze (e.g., C90700): Adding tin to copper increases its strength and hardness. Tin bronzes are excellent general-purpose bearing materials, offering a good balance of wear resistance, load capacity, and corrosion resistance. They are the dependable soldiers of the bearing world.
• Leaded Bronze (e.g., C93200 / SAE 660): The introduction of lead, which does not alloy with the copper but disperses as soft particles, provides a remarkable quality: a degree of self-lubrication. Under pressure or heat, the lead can smear over the bearing surface, providing a thin, slick film that protects against seizure during momentary losses of lubrication. The lead particles also make the material more conformable and better at embedding small dirt particles that might enter the bearing, protecting the shaft from scoring. However, due to environmental regulations in many regions, the use of leaded alloys has been declining in favor of lead-free alternatives.
• Aluminum Bronze (e.g., C95400): These alloys are the heavyweights. They offer superior strength, hardness, and wear resistance, especially under heavy loads and at slow speeds. Their excellent corrosion resistance makes them a prime choice for marine applications, such as in ship propellers and underwater fittings. They do, however, require clean operating conditions and good alignment, as they are less forgiving than their leaded counterparts.
• Manganese Bronze: Despite the name, this is technically a high-strength brass. It is known for its exceptional tensile strength and is often used in applications requiring both bearing properties and structural integrity, such as bridge components and heavy-duty gear systems.

The choice among these bronze alloys is a classic engineering trade-off. Do you need the forgiving nature of a leaded bronze for an environment with potential contamination, or the raw strength of an aluminum bronze for a clean, high-load application?

Babbitt (White Metal): Sacrificial Protection

Babbitt metal, named after its inventor Isaac Babbitt in 1839, is a soft, slippery alloy with a low melting point. It is typically based on either tin or lead, with additions of antimony and copper. The idea of using a soft material for a bearing might seem counterintuitive. One expects hardness and strength. Yet, the genius of Babbitt lies in its weakness.

Babbitt bearings are designed to be sacrificial. In large, critical machinery like steam turbines or the crankshaft bearings of an internal combustion engine, the shaft is an incredibly expensive and precise component. If a lubrication failure occurs or a piece of hard debris enters the bearing clearance, a hard bearing material would grind against the shaft, destroying it. A Babbitt bearing, however, does something remarkable. Its soft surface allows hard particles to press into it, becoming harmlessly embedded. In a catastrophic lubrication failure, the Babbitt layer will melt and wipe away before the shaft temperature reaches a critical point, sacrificing the inexpensive bearing to save the invaluable shaft. This property, known as embeddability and conformability, makes Babbitt an enduringly relevant material even in 2026.

Aluminum Alloys: Lightweight Strength

Aluminum alloys have carved out a significant niche in the world of plain bearings, particularly in automotive engines for connecting rods and main bearings. Compared to traditional bronzes, they offer lower weight, higher thermal conductivity (they are better at pulling heat away from the bearing surface), and excellent corrosion resistance. Modern aluminum-tin alloys, often with silicon, provide a strong matrix for load-carrying with soft tin particles dispersed for seizure resistance, mimicking the function of lead in bronze but in a more environmentally friendly package. They represent a modern compromise, offering performance that often rivals older materials but with advantages in weight and thermal management.

Cast Iron: Cost-Effective Durability

For slow-speed, high-load applications where cost is a primary driver, simple gray cast iron can serve as an effective plain bearing material. The graphite flakes within the cast iron's microstructure provide a degree of self-lubrication, reducing the coefficient of friction. It is not a high-performance material by any measure. It is brittle and lacks the conformability of Babbitt or the seizure resistance of bronze. Yet, for agricultural machinery, simple pivots, and other non-critical applications, its low cost and adequate performance make it a viable, pragmatic choice.

Polymer Plain Bearings: The Rise of Self-Lubrication

The second half of the 20th century witnessed a revolution in material science that profoundly impacted bearing technology: the development of high-performance engineering plastics. Polymer plain bearings answer a question that has plagued engineers for centuries: What if a bearing needed no external lubrication at all? For applications in the food processing industry, where grease contamination is unacceptable, or in remote, inaccessible locations where regular maintenance is impossible, polymer bearings are not just an alternative; they are an enabling technology. They operate on the principle of a low-friction polymer sliding against a shaft, often transferring a thin film of that polymer to the shaft surface, creating a highly efficient polymer-on-polymer sliding interface.

Polytetrafluoroethylene (PTFE): The King of Low Friction

PTFE, most famously known by the DuPont brand name Teflon, possesses the lowest coefficient of friction of any known solid material. It is the closest thing engineers have to a frictionless surface. Pure PTFE is soft, has poor wear resistance, and tends to "cold flow" or deform under load. Therefore, it is almost always used with fillers. Mixing in particles of glass fiber, carbon, graphite, or bronze dramatically improves its mechanical strength, wear resistance, and thermal conductivity while retaining its signature slipperiness. PTFE-based bearings are masters of dry-running applications and exhibit exceptional chemical resistance, making them ideal for pumps and valves in the chemical processing industry.

Polyamide (PA) and Polyoxymethylene (POM): Engineering Plastics

PA (Nylon) and POM (Acetal/Delrin) are tough, resilient, and cost-effective engineering plastics that bridge the gap between commodity plastics and high-performance polymers.

• Polyamide (PA): Nylons are known for their excellent toughness and ability to withstand high shock loads and vibration. They have good wear resistance and a low coefficient of friction, especially when lubricated. Their main drawback is their tendency to absorb moisture, which can cause dimensional instability (swelling) and a reduction in mechanical properties. This makes them less suitable for precision applications in humid environments.
• Polyoxymethylene (POM): POM is a more stable alternative. It is strong, rigid, and has excellent dimensional stability, absorbing very little moisture. Its natural lubricity and wear resistance are very good, making it a go-to material for a wide variety of bearing applications, from automotive components to consumer goods. It is a well-rounded performer.

Polyetheretherketone (PEEK): High-Temperature Performance

PEEK is a high-performance thermoplastic that operates in a different league. It retains its exceptional mechanical strength, stiffness, and wear resistance at continuous operating temperatures up to 260°C (500°F). It also exhibits excellent resistance to a broad range of chemicals and hydrolysis (degradation by hot water or steam). Like PTFE, PEEK is often compounded with fillers like carbon fiber or graphite to enhance its bearing properties further. Its high cost restricts its use to the most demanding applications where other polymers would fail, such as in aerospace components, downhole drilling equipment in the oil and gas industry, and medical implants.

Ultra-High-Molecular-Weight Polyethylene (UHMWPE): Wear Resistance Champion

UHMWPE is a fascinating material. It is a type of polyethylene with extremely long molecular chains, which gives it incredible abrasion resistance and impact strength. It is tough, has a low coefficient of friction, and is highly resistant to corrosive chemicals. Its wear resistance is so high that it often outperforms steel in sliding abrasion scenarios. It is widely used for conveyor belts, star wheels in bottling plants, and dock fenders, where its ability to withstand constant scraping and impact is invaluable. It is not a high-temperature material, but for rugged, room-temperature applications, it is a champion of durability.

Metal-Polymer Composites: The Best of Both Worlds

What if one could combine the immense strength and rigidity of a steel backing with the unparalleled low friction and self-lubricating properties of a polymer like PTFE? This is the elegant solution offered by metal-polymer composite bearings. These are not simple mixtures but sophisticated, layered structures designed to deliver performance characteristics that no single material could achieve on its own. They represent a pinnacle of material synergy and are among the most versatile plain bearings available today. For a deeper dive, exploring a catalog of high-performance plain bearing solutions can reveal the vast array of options in this category.

The Layered Structure: A Symphony of Materials

A typical metal-polymer composite bearing consists of three distinct layers bonded together into a single, robust unit:

1. The Steel Backing: The outermost layer is usually made of carbon steel, though bronze or stainless steel can be used for enhanced corrosion resistance. This metal shell provides the structural integrity. It gives the bearing its shape, allows it to be press-fit into a housing with high interference, and handles the high compressive loads without deforming.
2. The Sintered Bronze Interlayer: This is the clever part. A thin layer of porous bronze powder is sintered onto the steel backing. This bronze layer serves two purposes. First, it creates a strong metallurgical bond between the steel backing and the polymer liner. Second, its porous, sponge-like structure acts as a mechanical anchor for the polymer overlay, locking it firmly in place.
3. The Polymer Sliding Layer: The innermost layer, the surface that contacts the shaft, is a thin overlay of a low-friction polymer, typically a PTFE-based mixture. During the manufacturing process, this polymer paste is rolled into the pores of the bronze interlayer and then heated to create a solid, coherent liner approximately 0.01 to 0.03 mm thick.

Common Configurations: Steel Backing with PTFE or POM

The most common variant of this composite structure uses a PTFE-based sliding layer. During a brief initial run-in period, a microscopic film of this PTFE mixture transfers from the bearing to the shaft. From that point on, the sliding occurs between the PTFE on the bearing and the transferred PTFE film on the shaft, resulting in an exceptionally low-friction interface. This type of bearing is perfect for completely dry-running, maintenance-free applications.

Another popular configuration uses a POM (acetal) layer. These bearings are designed for lubricated or marginally lubricated applications. The surface of the POM layer is often indented with a pattern of "grease pockets." These pockets hold grease or oil, providing a lubricant reservoir that ensures consistent lubrication over long periods, significantly extending maintenance intervals. They combine the wear resistance of POM with the strength of steel, making them ideal for oscillating or rotating movements under heavy loads, such as in construction equipment pivots and automotive steering systems.

Applications and Advantages of Composite Design

The brilliance of the composite design is its ability to decouple mechanical and tribological (friction and wear) properties. The steel handles the load, and the polymer handles the sliding. This allows for bearings that are:

• Strong and Compact: They can handle high loads in a very small radial space.
• Lightweight: Much lighter than a solid bronze bearing of equivalent size.
• Maintenance-Free: The PTFE-lined versions require no lubrication for their entire service life.
• Versatile: Suitable for a huge range of applications, from the delicate mechanisms inside a printer to the rugged suspension joints of a heavy truck.

Bimetallic and Sintered Bearings: Porosity and Power

Within the family of metallic bearings, there are two particularly ingenious sub-categories: sintered bearings and bimetallic bearings. Both leverage clever manufacturing techniques to create properties that go beyond what a solid, homogenous metal could offer. They address the age-old problem of lubrication in unique ways.

The Manufacturing Process of Sintered Bearings

Sintered bearings are born from powder. The process, known as powder metallurgy, begins with fine metallic powder, most commonly bronze or iron. This powder is poured into a die shaped like the final bearing and compacted under immense pressure. This "green" compact is fragile but holds its shape. It is then moved to a high-temperature furnace with a controlled atmosphere, where it is heated to a temperature just below the melting point of the metal. At this temperature, the individual metal particles fuse together at their contact points, a process called sintering. The result is a solid metallic part that is intentionally porous, typically containing 15% to 30% interconnected porosity by volume.

Oil Impregnation: A Built-in Lubricant Reservoir

This porosity is the secret to the sintered bearing's success. After sintering, the bearings are submerged in a bath of warm lubricating oil under a vacuum. The vacuum removes the air from the pores, and when the vacuum is released, atmospheric pressure forces the oil deep into the bearing's porous network. The bearing effectively becomes a metal sponge saturated with lubricant.

In operation, as the shaft begins to rotate and generate heat, the oil's viscosity decreases, and it is drawn out of the pores onto the bearing surface by capillary action and thermal expansion. This creates a hydrodynamic or boundary lubrication film, separating the shaft from the bearing metal. When the rotation stops and the bearing cools, the oil is drawn back into the porous structure, ready for the next cycle. This makes sintered bearings self-lubricating for their entire operational life under normal conditions. They are ubiquitous in small electric motors, fans, printers, and home appliances where external lubrication is impractical.

Bimetallic Bearings: Steel Shells with Bronze Linings

Bimetallic bearings are conceptually similar to metal-polymer composites but use a metallic lining instead of a polymer one. They typically consist of a strong steel backing onto which a layer of a bearing alloy, such as leaded bronze or an aluminum-tin alloy, is bonded. This construction offers the high load capacity and structural integrity of steel while providing the superior bearing surface properties of the bronze or aluminum alloy.

They are a step up in load capacity from sintered bronze bearings and are often used in more demanding applications like automotive engine camshafts, connecting rods, and transmission components. The steel backing ensures a tight, reliable press-fit into the engine block or connecting rod, while the bearing alloy liner provides the necessary seizure resistance and embeddability. They are a cost-effective way to get high-performance bearing characteristics in a mass-produced component.

Ceramic and Cermet Bearings: For the Harshest Frontiers

When operating conditions become so extreme that even the most robust metals and high-performance polymers falter, engineers turn to a class of materials defined by their incredible hardness and stability: ceramics. Ceramic plain bearings are not for everyday applications. They are specialized components for the frontiers of technology, where punishing temperatures, corrosive chemicals, and high speeds create an environment hostile to conventional materials.

Silicon Carbide (SiC) and Zirconia (ZrO2): Material Properties

Two of the most prominent ceramics in bearing technology are Silicon Carbide and Zirconia.

• Silicon Carbide (SiC): SiC is one of the hardest and most wear-resistant materials commercially available. It maintains its strength at temperatures exceeding 1400°C (2550°F) and is virtually immune to all common acids and alkalis. Its thermal conductivity is also very high, helping to dissipate frictional heat. This makes it an outstanding choice for bearings in pumps handling highly abrasive and corrosive slurries, or in high-speed applications where minimal wear is paramount.
• Zirconia (ZrO2): While not as hard as SiC, Zirconia has a significant advantage: higher fracture toughness. This means it is more resistant to cracking under sharp impacts or high stress concentrations. It also has a coefficient of thermal expansion closer to that of steel, which can simplify designs when fitting a ceramic bearing into a metal housing. Zirconia is often the choice for applications where some degree of toughness is required alongside the traditional ceramic benefits of corrosion and temperature resistance.

The primary challenge with ceramic bearings is their inherent brittleness. They do not deform plastically like metals; they fracture. This requires extremely careful design, handling, and installation to avoid chipping or cracking.

Cermets: Combining Ceramic Hardness with Metal Toughness

To overcome the brittleness of pure ceramics, a class of composite materials called cermets was developed. A cermet (from "ceramic" and "metal") consists of ceramic particles bonded together by a metallic binder. A common example is tungsten carbide (a ceramic) bonded with cobalt (a metal). The resulting material has a hardness approaching that of the pure ceramic but with significantly improved toughness and fracture resistance contributed by the metallic matrix. Cermets bridge the performance gap between metals and ceramics, offering extreme wear resistance with a reduced risk of catastrophic brittle failure.

When to Choose Ceramics: Extreme Conditions

The decision to specify a ceramic or cermet bearing is driven by necessity, not preference. Their high cost and design constraints mean they are used only when no other material will suffice. Typical scenarios include:

• Highly Corrosive Environments: Pumps for transferring strong acids, bases, or other aggressive chemicals.
• Extreme High Temperatures: Furnace components, gas turbines, or any machinery operating where metals would lose their strength or polymers would melt.
• Non-Magnetic Requirements: Applications like MRI machines or sensitive electronic equipment where metallic components would interfere with magnetic fields.
• High-Wear, Abrasive Media: Bearings in slurry pumps, mixers, or grinding equipment.

Specialized and Exotic Materials: Pushing the Boundaries

While metals, polymers, and ceramics form the mainstream of what plain bearings are made of, a fascinating group of other materials serves niche but important roles. These materials, some ancient and some cutting-edge, demonstrate the boundless ingenuity applied to the fundamental problem of reducing friction.

Carbon Graphite: Dry Running and High Temperatures

Carbon graphite is a unique material with a self-lubricating ability that actually improves as temperature increases, a behavior opposite to most materials. The layered, graphitic crystal structure allows planes of atoms to shear easily, providing a low-friction sliding surface. It can operate completely dry at temperatures far exceeding the limits of polymers, up to 500°C (930°F) in an oxidizing atmosphere and even higher in inert environments. Carbon graphite is also chemically inert to most substances, with the exception of strong oxidizing agents. This combination of properties makes it the ideal bearing material for high-temperature ovens, kilns, food processing equipment (where it is non-toxic), and pumps for volatile or corrosive fluids. It is relatively brittle and has a lower load capacity than metals, but in its specific niche, it is unrivaled.

Wood and Lignum Vitae: A Historical Perspective

It may be surprising to learn that for centuries, one of the best high-performance bearing materials was wood. Specifically, a dense, oily wood called Lignum vitae. This "wood of life" is one of the hardest and densest woods in the world, and it is naturally impregnated with a waxy resin that gives it self-lubricating properties, especially when wet. For hundreds of years, it was the material of choice for the stern tube bearings of ships, which support the propeller shaft as it exits the hull. The combination of seawater lubrication and the wood's natural oiliness created a nearly perfect, long-lasting bearing. While largely replaced by modern composites and elastomers, Lignum vitae stands as a powerful testament to how nature can provide elegant engineering solutions. Other woods, like oil-impregnated maple, are still used in some light-duty agricultural or conveyor applications today.

The Future of Bearing Materials in 2026

The quest for the perfect bearing material is ongoing. Research in 2026 is focused on several exciting fronts. Nanocomposites, where nanoparticles of materials like graphene or carbon nanotubes are dispersed in a polymer matrix, promise to dramatically enhance the strength and thermal conductivity of plastic bearings without compromising their low-friction properties. Self-healing materials, which can repair microscopic wear and cracks automatically, could one day lead to bearings with virtually infinite lifespans. Additive manufacturing (3D printing) is also changing the game, allowing for the creation of bearings with complex internal cooling channels or functionally graded materials that transition from a hard, wear-resistant surface to a tough, impact-resistant core. The fundamental question of what are plain bearings made of will only become more complex and fascinating as material science continues to advance.

Selecting the Right Material: A Practical Framework

Choosing the correct plain bearing material from this vast array of options can seem daunting. However, the process can be systematized by considering a hierarchy of operational parameters. It is an exercise in matching the material's capabilities to the demands of the application. A methodical approach will almost always lead to a successful and reliable outcome.

Application Example	Dominant Challenge	Primary Material Candidate	Secondary Candidate	Rationale
Excavator Arm Pivot	High Load, Shock, Contamination	Metal-Polymer Composite (POM)	Hardened Steel with Grease	Composite handles high load and its sealed nature resists dirt. The POM surface is ideal for oscillating, lubricated motion.
Food Conveyor Roller	Dry Running, Washdown, Hygiene	UHMWPE or PTFE-filled Polymer	Stainless Steel (Unlubricated)	Polymers offer self-lubrication, eliminating grease contamination. They are also resistant to cleaning chemicals.
Automotive Engine Crankshaft	High Speed, High Load, Lubricated	Bimetallic (Aluminum-Tin on Steel)	Babbitt on Steel	Bimetallic bearings offer high fatigue strength for engine dynamics. Babbitt is a classic choice for its superior embeddability.
Chemical Centrifugal Pump	Aggressive Media, High Speed	Silicon Carbide (SiC) Ceramic	PEEK with Carbon Fiber	SiC is nearly inert and handles high speeds with minimal wear. PEEK is a less brittle, high-performance polymer alternative.
Small Appliance Motor	Low Load, Quiet Operation, Low Cost	Sintered Bronze (Oil-Impregnated)	Acetal (POM)	Sintered bronze is extremely cost-effective and its self-lubricating nature ensures a long, maintenance-free life.

Analyzing Load, Speed, and Temperature (PV Value)

The most fundamental performance metric for a plain bearing is its PV value. 'P' stands for the specific pressure on the bearing (load divided by the projected area), and 'V' stands for the surface velocity (speed). The product, PV, represents the energy being dissipated as heat due to friction. Every bearing material has a maximum PV rating, beyond which frictional heat builds up faster than it can be dissipated, leading to rapid wear and failure.

• High P, Low V: Applications with very heavy loads but slow or oscillating movement (like a bridge bearing or a crane pivot) require materials with high compressive strength, such as bronze or metal-polymer composites.
• Low P, High V: High-speed, lightly loaded applications (like a guide for a textile machine) can use materials with very low friction, such as PTFE compounds, to minimize heat generation.
• High P, High V: This is the most challenging scenario and often requires forced lubrication (oil or grease) or materials with excellent thermal conductivity and high-temperature resistance, such as PEEK or even ceramics.

Temperature is the third critical factor. The operating temperature must be well within the material's continuous service limit. Polymers will soften and lose strength, and even metals can lose hardness at elevated temperatures.

Considering Environmental Factors: Corrosion, Contamination, and Chemicals

The operating environment can be just as destructive as mechanical loads.

• Corrosion: In marine, chemical, or outdoor applications, materials must be chosen for their corrosion resistance. Stainless steel backings, bronze alloys, and polymers are excellent choices. A standard carbon steel bearing would quickly rust and fail.
• Contamination: If the environment is dusty or dirty (e.g., agriculture, construction), the bearing choice is critical. One can either use a sealed bearing (like a composite bearing with seals) to keep contaminants out, or use a material like Babbitt or leaded bronze that can embed small particles without damaging the shaft.
• Chemicals: In the process industries, bearings must be compatible with the fluids they are exposed to. PTFE and PEEK offer the broadest chemical resistance among polymers. Certain ceramics like SiC are virtually inert. A careful review of chemical compatibility charts is non-negotiable in these applications.

Lubrication Requirements: Greased, Oil, or Dry-Running?

The final piece of the puzzle is lubrication. The choice of material is inextricably linked to the lubrication strategy.

• Dry-Running: If lubrication is impossible, undesirable, or too expensive, the choice is limited to self-lubricating materials. This includes PTFE-lined composites, solid polymer bearings (PTFE, PEEK, PA), and carbon graphite.
• Marginally Lubricated: For applications with infrequent lubrication or "lube-for-life" designs, sintered bearings, POM composites with grease pockets, or leaded bronzes are excellent.
• Fully Lubricated: For high-speed, high-load applications, a continuous supply of oil or grease is necessary to form a hydrodynamic film that completely separates the surfaces. Here, the material's role is to provide a strong, compatible substrate for the lubricant film and to survive momentary contact during startup and shutdown. Bimetallic bearings, bronzes, and Babbitt excel in these regimes. Consulting a catalog of specialized plain bearings can often provide specific PV ratings for both lubricated and dry conditions.

By systematically evaluating these factors—Load, Speed, Temperature, Environment, and Lubrication—one can confidently navigate the complex material landscape and select the ideal plain bearing for any given task.

Frequently Asked Questions (FAQ)

What is the primary difference in function between a plain bearing and a ball bearing?

The core difference lies in their method of handling friction. A plain bearing uses a sliding motion between two surfaces, relying on the material's properties (and often a lubricant) to reduce friction. A ball bearing, a type of rolling-element bearing, uses spherical balls to separate the moving rings, replacing high-friction sliding with low-friction rolling. This makes ball bearings ideal for high-speed, precision applications, while plain bearings excel in high-load, high-impact, or contaminated environments due to their large contact area and simpler construction.

Can modern polymer bearings completely replace traditional metal bearings?

In many applications, yes, but not all. Polymer bearings are superior in applications requiring dry-running, corrosion resistance, or zero maintenance. However, they generally have lower load capacities and temperature limits than metallic bearings. For extremely heavy-duty machinery, high-temperature engines, or applications where high thermal conductivity is needed to dissipate heat, robust metallic bearings like bronze or bimetallic types remain the superior choice. The selection is always application-dependent.

How do I know if I need a self-lubricating bearing?

You should opt for a self-lubricating bearing in several key scenarios: 1) When the bearing location is inaccessible for regular maintenance. 2) In applications where lubricant contamination is unacceptable, such as food processing, medical, or textile industries. 3) In highly contaminated environments where grease would attract abrasive dust. 4) For very slow or intermittent motions where a stable hydrodynamic film cannot be formed.

What does the 'PV value' of a bearing material signify?

The PV value is a critical performance metric representing the bearing's ability to handle the heat generated by friction. 'P' is the surface pressure (load in psi or N/mm²) and 'V' is the surface velocity (speed in fpm or m/s). The product, PV, is a measure of the frictional work. Every material has a maximum PV limit for a given condition. Exceeding this limit means the bearing will generate heat faster than it can dissipate it, leading to a rapid temperature rise, material degradation, and premature failure.

Are plain bearings suitable for high-speed applications?

Yes, but with specific conditions. While rolling-element bearings are generally better for high speeds, plain bearings can perform exceptionally well in high-speed applications provided there is a reliable, clean supply of lubricant (usually oil). Under these conditions, a hydrodynamic film forms, completely separating the shaft and bearing. This is how the crankshaft bearings in a high-revving car engine function. For dry-running high-speed applications, specialized materials like carbon graphite or certain PTFE composites are necessary.

Why is Babbitt metal, a very soft material, still used in heavy-duty engines in 2026?

Babbitt's softness is its greatest strength. It has two key properties: conformability and embeddability. Conformability allows it to deform slightly to match minor misalignments of the shaft, ensuring even pressure distribution. Embeddability allows hard contaminant particles (dirt, metal shavings) to press into the soft Babbitt surface, preventing them from circulating in the oil and scoring the much harder and more expensive crankshaft. It acts as a sacrificial layer, protecting the most critical components.

What is the difference between a bimetallic bearing and a metal-polymer composite bearing?

Both have a strong steel backing for structural support, but their sliding layers differ. A bimetallic bearing has a metallic sliding layer, such as a bronze or aluminum alloy, bonded to the steel. It is designed for high-load, lubricated applications like engines. A metal-polymer composite bearing has a polymer sliding layer, typically PTFE or POM, bonded to the steel via a porous bronze interlayer. These are often designed for self-lubricating, maintenance-free operation.

Final Considerations on Material Selection

The examination of what plain bearings are made of reveals a profound truth about engineering: there is no single "best" material. There is only the most appropriate material for a given set of circumstances. The optimal choice is a carefully considered compromise, a balancing act between strength and friction, temperature and cost, environment and maintenance. The soft, sacrificial nature of Babbitt is as brilliant in its context as the unyielding hardness of silicon carbide is in another. The self-lubricating whisper of a PTFE composite is as valuable in a cleanroom as the rugged durability of an aluminum bronze is in the depths of the ocean. A deep appreciation for the properties, strengths, and limitations of each material family empowers an engineer or technician to move beyond simply replacing a part and toward truly optimizing a mechanical system for a longer, more efficient, and more reliable operational life.

References

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

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

NTN Corporation. (2011, May 24). Needle roller bearings technical data. NTN Global.

NTN Corporation. (2024, September 6). Ball and roller bearings deep groove ball bearings. NTN Global.

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

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