Choosing the Right Bearing Material for High Strength and Shock Resistance

Selecting the right material with sufficient strength and shock resistance is essential for industrial equipment, particularly in the construction of bearings. Material requirements vary depending on the industry and specific application, as bearing must perform critical functions that reduce resistance between moving parts. Traditionally, metals and alloys have been used for their durability. However, those materials require proper lubrication to minimize surface friction, which can otherwise cause components to seize, even under relatively low pressures or surface loads. Bearings made from metals or alloys require frequent maintenance, leading many industries to adopt alternative materials better suited for specific applications.

Bearings are designed to reduce friction and guide the motion of machinery by effectively managing physical force. The standard design typically includes inner and outer rings forming a raceway and rolling elements supported by a guiding cage. These rings are often made from high-carbon chromium steel for maximum durability and strength. However, aerospace, automotive, and construction industries increasingly favor engineered composite materials. These composites offer lightweight yet durable solutions, often incorporating self-lubricating properties to reduce maintenance requirements and enhance performance. These sectors can achieve long-lasting, efficient solutions tailored to demanding operational environments by leveraging advanced material technologies. 

A bearing mainly aims to reduce friction between moving components, guiding a machine’s motion by focusing physical force. Typically, bearing design is simple, consisting of inner and outer rings that create a raceway, along with rolling elements that rely on a cage that guides the movement of the bearing. The material used for these rings must be durable and strong, which is normally a type of high-carbon chromium steel. However, industries like aerospace, automotive, construction, and other sectors that require lightweight yet durable equipment increasingly use engineered composites, some with self-lubricating properties. 

Selecting the Right Bearing Material 

Without bearings, most machinery with moving parts wouldn’t operate smoothly or last long. Durability is, therefore, a key requirement in the performance of a bearing. Materials like steel, plastic resins, or ceramics that can withstand intensive operation are thus used for bearings in modern equipment. Steel is still used for its durability and strength, primarily with the rings and rolling elements that make up a bearing. Materials for cages affixed to these rolling elements are often made from copper alloys, steel plates, or synthetic resins.

Each type of bearing material has unique characteristics that make it advantageous for such uses. Steel offers strength, while copper alloys resist corrosion well and offer excellent electrical conductivity. Lighter-weight synthetic resins reduce the possibility of contamination of the lubricant. While these properties make certain bearing materials more suitable for specific bearing parts, material selection should consider various factors.

When choosing bearing materials, the following dynamics should be considered: 

• Compressive strength: Bearing materials that resist compression help prevent failure in applications that apply significant constriction to the bearing’s mechanisms.
• Environmental conditions: Corrosive substances, heat, humidity, low temperatures, and other environmental factors require bearing materials with certain properties.
• Friction: All bearing materials must deal with friction, so they should be appropriately strong and durable and have a low friction coefficient. However, this often requires the addition of a lubricant.
• Loads: If it needs to withstand higher loads, durable and strong substances like ceramic or steel should be used to prevent the bearing material from deforming under pressure and eventually failing.
• Seals: Seal and bearing materials must complement each other to ensure that neither heat caused by friction nor other environmental factors degrade the seal.
• Speed: Higher-speed applications require bearing materials that can handle heat caused by elevated friction, so it’s important to understand the speed at which the bearing must run. 
• Type: Whether a ball, roller, thrust, or other type of bearing, materials require certain properties so that the bearings function appropriately. 

The choice of bearing material typically depends on the specific application. Food processing equipment requires bearing materials that are both corrosion-resistant and nontoxic, so bearings for these applications tend to be made from certain plastics or stainless steel. Heat-resistant and lightweight bearing materials, such as high-performance ceramics or composites that can withstand high temperatures, are often used for aircraft applications. 

Bearing Material Properties

As a vital component found in many pieces of industrial equipment, it’s important to consider the properties of materials that make up a bearing. Material properties directly affect the performance of both the bearing and the machinery they’re a part of.  

Characteristics that should be considered for bearing materials include: 

• Chemical inertness: This trait keeps bearing materials from degrading or otherwise reacting to contact with harsh chemicals.
• Corrosion resistance: Bearing materials that don’t corrode are required in wet environments, as corrosion can lead to premature failure.
• Hardness: This basic property relates to how well-bearing materials resist indentations or other deformation.
• Heat resistance: When operating in high-temperature environments, bearings must resist thermal stress that could affect a material’s structural integrity and other mechanical properties.
• Thermal expansion: The degree to which a bearing material expands when heated affects the bearing’s dimensional stability, so those materials with lower thermal expansion coefficients are preferred.
• Wear resistance: This bearing material feature shows how well it can resist wear due to friction.

Depending on the application, bearing materials must also exhibit different degrees and types of strength.

Measuring Bearing Material Strength

The strength of bearing materials can be measured in multiple ways.

Bearing strength is often determined by measuring:

• Compressive strength: This definition of strength determines how well a bearing material can withstand loads that would compress it, with higher compressive strength denoting that the material can handle a higher load without breaking.
• Flexural strength: Also sometimes referred to as bend strength or modulus of rupture, this determines how well a bearing material can resist bending when under a load.
• Tensile elongation: This refers to the degree to which bearing material can be stretched before it breaks when under tension; noted as a percentage, tensile elongation and tensile strength together relate to how tough a bearing material is.
• Tensile strength: This measures how well a bearing material can withstand loads under tension without failing, with higher tensile strengths making materials more resistant to cracking; also referred to as ultimate strength, it’s measured in pounds per square inch (PSI) or bar, with 14.5 psi equaling 1 bar. 

Combining various elements of bearing material strength, this covers tensile, flexural and compressive strength. Modulus is defined by a ratio between force and stress over a specific measured area. A bearing material’s modulus therefore helps engineers predict how it will react, as long as the stress is less than the material’s yield.

Various Types of Bearing Materials

Though bearing materials must present adequate strength so they don’t deform when loads fluctuate, they must also have enough elasticity to bend to changing shaft alignments. Hardness is also a factor, as this property in bearing materials corresponds to better wear resistance. However, bearings must not be so hard that they become brittle, which reduces shock resistance from impacts. While they require corrosion-resistant properties in most situations, bearing materials must also retain a lubricating film.

Many of these properties oppose each other, meaning bearing materials must present sufficient hardness and strength to withstand wear while also conforming enough to resist shock. Choosing the right bearing material depends largely on the application and environmental conditions, along with the load and speed of the process.

Bearings are generally made from metal alloys, ceramics, plastics, or composites, though certain applications require just the right material. For example, ceramics perform better than other bearing materials in high-speed applications, while steel and other metal alloys can handle high loads well. Additionally, stainless steel and certain polymers and composites offer corrosion-resistant properties that allow them to operate effectively in harsher environments.

Metal Alloy Bearing Materials 

Carbon and stainless steel are the most commonly used bearing materials due to their strength and wear resistance. Carbon steel bearings are often the first choice for general applications due to their durability and strength, though they don’t do well under highly corrosive conditions. On the other hand, stainless steel bearings work well with abrasive chemicals and in humid environments like those found in chemical processing and marine applications, where corrosion is an issue. Chromium steel is high in carbon, resists wear, and offers high hardness, so it is often used for ball or roller bearings in the aerospace and automotive sectors.

Plain and sleeve bearings are frequently made of bronze due to their cost-efficiency, low friction coefficient, and resistance to wear, with aluminum and phosphorus sometimes added to augment corrosion resistance and strength. Cast iron bearings are often preferred when rigidity is needed due to their lower cost. The aerospace industry uses beryllium copper for its conductivity and strength for more high-performance applications. It also uses titanium in certain applications because of its corrosion resistance and ratio of strength to weight. Titanium bearings are also used in many medical applications as well.  

Ceramic Bearing Materials 

With a ceramic bearing, materials like silicon carbide, silicon nitride, and zirconia are used for high-speed applications because of their heat resistance and lighter weights. Silicon carbide is a good bearing material for high heat applications due to its hardness and thermal conductivity while resisting corrosion and wear. With a lower density and high thermal conductivity, bearings made with silicon nitride decrease centrifugal force when operating at high speeds, reducing bearing wear. Zirconia resists cracking, corrosion, and high heat conditions and wears well while also having good insulation properties. Many ceramic bearings additionally use steel raceways to make them more durable, while ceramics are often used in composite bearings.

Plastic Bearing Materials

Numerous plastics are used for bearings, largely because of their lighter weight, though other properties make them useful in specific environments and for certain applications.

Some commonly used plastic-bearing materials include: 

• Nylon: Also described as polyamides, various types of nylon offer bearings suitable tensile strength and allow them to resist wear better; these bearing materials are used in machines for packaging food and textile manufacturing, applications in which lubrication is difficult to maintain.
• Polyetheretherketone (PEEK): The difficult conditions under which bearings serve in the aerospace, automotive, and chemical processing sectors benefit from PEEK’s chemical and heat resistance, along with its excellent mechanical strength.
• Polyethylene (PE): PE bearings resist chemicals and impact extremely well. They are often used in agricultural and marine applications, where their durability and resistance to water are requisite.
• Polyoxymethylene (POM): More commonly known by the brand name Acetal, this bearing material is known for its low friction coefficient while also providing significant resistance to chemicals and wear, which make it best for applications like automative parts and consumer electronics where smooth running and quiet operation are preferred.
• Polytetrafluoroethylene (PTFE): This bearing material, known more commonly by its brand name Teflon, is often used for its chemical resistance and very low friction coefficient, which allows it to be used without the need for lubricants.
• Polyvinylidene fluoride (PVDF): With excellent mechanical properties added to its chemical and heat resistance, PVDF makes a commonly used bearing material for the chemical processing sector.

Many high-performance resins are often included in composite materials used for making bearings.

Composite Bearing Materials

The combination of materials that can be made into composites for making bearings is limited only by engineers’ imaginations. Composite bearing materials are mostly limited to two types: those reinforced with fibers and those combining metals with polymers. Fiber-reinforced bearing materials offer high corrosion resistance and strength, tending to be used for chemical and marine applications. Composite bearing materials that combine metals with polymers normally use layers of alloys like bronze or steel with polymer coatings that provide low friction coefficients.  

Advantages of High-Performance Composite Bearing Materials

The advantages of composite bearing materials are many and varied. Bearings made from a combination of different materials are often the most effective for many applications. Unlike metals and alloys, composite bearing materials tend to resist corrosion from both chemicals and water better. Composites also often have self-lubricating characteristics with substantially lower friction coefficients that provide cost savings due to not needing to be continually lubricated. Like plastic, the elasticity of composite bearing materials enables them to better endure heavier loads without permanently deforming, while their lower weight enhances energy efficiency.

Composites often have properties that make them more advantageous than metal alloys, ceramics, or plastics. For example, certain bearing composite materials used in pumping systems only absorb moisture at a rate of 0.01 percent. Other composite bearing materials can handle extreme temperatures in the cryogenic range, down to -196°C (about 321°F) and up to about 165°C (329°F). In such extreme conditions, metal-based bearing materials fail more readily.

Additionally, when it comes to government regulations and industry standards, composite bearing materials often help manufacturers more cost-effectively maintain compliance. This is especially true in the food processing, healthcare, and pharmaceutical industries, which must meet strict guidelines. Plastic resins can also be suitable for application by reinforcing and layering different materials to create a whole new composite bearing material. Though numerous composites exist, polymer matrices with fibers for reinforcement are commonly used for bearings.

Some examples of composite bearing materials include: 

• Glass fiber reinforced polyamides: Reinforced by glass fibers, polyamides are a class of polymers that include various nylons and amides; this type of bearing material includes construction equipment, industrial machinery, and motor vehicle engines.
• Advantages include low friction coefficient, decent rigidity, high strength, and good wear resistance.
• Disadvantages include poor resistance to chemicals and high-heat environments.
• Carbon fiber reinforced polyimides: Made with a polyimide matrix containing carbon fibers, its properties make it suitable for use in the aerospace industry and with industrial equipment.
• Advantages include dimensional stability, heat resistance, high strength, and rigidity.
• Disadvantages include difficulty in machine and steep price differentials from other bearing materials.
• Glass fiber reinforced PTFE: These PTFE composites, reinforced with glass fibers, work well for machinery used in the food and beverage industry, as well as pumps and valves.
• Advantages include its resistance to chemicals, moisture, wear, and low friction coefficient.
• Disadvantages include poor dimensional stability and carrying capacity of loads.
• Carbon fiber reinforced PEEK: A thermoplastic reinforced with carbon fibers, this composite-bearing material is often used for aerospace, automotive, and medical applications.
• Advantages include resistance to high temperatures and wear, along with good strength.
• Disadvantages include high costs and difficulty in machining.

Other composite bearing materials include bronze with an overlay of PTFE, heat-resistant phenolic resin contained within a cotton fabric, specialty resins incorporating aramid fabrics, and many others. The types of composite bearing materials are nearly limitless.

High Strength & Shock-Resistance Bearing Materials for Cages

Different materials are normally used to make the cage, seal, and shield of a bearing. Materials for the cage usually differ from those used for the balls and rings of a bearing. Cage material is often made from various steel and other metal alloys, composites, or plastics. Composite bearing materials used for bearing cages are often made from phenolic resins reinforced with fiber. Certain stainless or other steels are used for bearing shields, whereas the seals are often made with synthetic rubber or PTFE.

Considerations surrounding bearing material strength focus on the maximum load it can handle without failing structurally. While the different types of strength are important for making a bearing more shock-resistant, the hardness of bearing material also comes into play. In particular, bearing materials used for the cage need to handle shock loads and rotational vibrations, so require a low friction coefficient. Additionally, they must be sufficiently lightweight and operational within specific temperature parameters, while also requiring the rolling elements be properly spaced.  

Bearing materials for cages differ depending on their design and construction: 

• Machined cages: Made from aluminum, bronze, phenolic resins, or steel, machined cages are used in high-speed operations to reduce wear and augment balance. 
• Molded cages: Certain types of nylon combined with carbon and glass for strength are used as bearing materials for molded cages; nylon-based bearing cages are often used for higher speed applications, as they present low operational torque.
• Riveted cages: These are essentially two stamped pieces riveted together into a cage normally made from steel, though some use bronze instead.
• Stamped cages: Normally made from steel or bronze, these cages aid help preserve the rolling element.

Several different polymers are generally used as bearing materials for cages, with polyamide being a good choice due to its formability, heat resistance, strength, and wear resistance. PEEK resists heat better than any thermoplastic resin, plus it provides self-lubricating properties and chemical and shock resistance. Due to its expense, PEEK tends to be used for higher-speed cylindric roller bearings within machine tools.

Phenolic resin reinforced by fabric as a bearing material helps reinforce phenolic resin, which doesn’t resist shock well on its own. However, its ability to self-lubricate and lighter weight, combined with other mechanical properties, make it a good choice for angular contact ball bearings used in machine tools. As phenolic resin is a thermoset, cages made with this bearing material must be machined rather than molded.

Composite Bearing Materials from Spaulding

Spaulding Composites Inc. manufactures bearing materials and custom composite bearings. These bearings are used in all heavy equipment, including propeller shafts for civilian and military maritime vessels. Spaulding uses composite bearing materials with phenolic resins woven with cotton and/or canvas, with our composite ball bearings known for their compressive and flexural strength, and machinability. To learn more about our capabilities with bearing materials and composite bearings, contact the composite experts at Spaulding today.