The bearing industry uses different materials for the production of the various bearing components. Bearing materials are processed to achieve desirable properties to maximize bearing performance and life. Here, we examine the materials most commonly used and why.
Bearings made of chrome steel — SAE 52100
The most common material used to produce the load-carrying components in precision ball bearings, roller bearings, and tapered roller bearings is 52100 chrome steel. These components are the bearings’ inner and outer rings, balls, and rollers. The chemical composition of this steel has high carbon and about 1.5% chromium content. Using controlled processing and heat-treating methods, engineers create finished bearing components that have enough strength to resist cracking and a surface hard enough to resist subsurface rolling contact fatigue.
Extra clean 52100 chrome steel bearings
The raw steel used to produce high precision miniature bearings is processed with additional melting steps. The result is a type of steel with a uniform fine grain material structure; the bearing contact surfaces can be super finished very smooth so the bearing is quiet.
The most common heat-treating method for chrome steel is to thru harden the steel in a controlled atmosphere furnace. Bearings manufactured from chrome steel can operate at continuous temperatures up to 120°C.
Bearings made of stainless steels
Stainless steel materials are used to make bearing components because it is more resistant to surface corrosion due to the higher content of chromium (~18%) with the addition of nickel. The chromium reacts with oxygen to form a layer of chromium oxide on the surface, creating a passive film.
Martensitic stainless steel bearings — AISI 440C
The carbon content in 400 series stainless steels is high enough to be hardened using standard heat-treating methods up to Rc58. With lower hardness, the load-carrying capacity is 20% lower in bearings made from this material than with 52100 chrome steel bearings. The level of carbon content means the components are magnetic.
Martensitic stainless steel bearings — ACD34 / KS440 / X65Cr13
Many miniature bearing manufacturers make their rings and balls with a stainless steel material with slightly lower carbon and chromium content than AISI 440C, known under several names including ACD34, KS440, and X65Cr13. This material has smaller carbides after the heat treatment, so the bearing will have superior low noise characteristics while offering the same corrosion resistance as 440C.
Martensitic stainless steel bearings — SV30
Martensitic stainless steel can be modified during the processing of the raw steel by lowering the carbon content and introducing nitrogen as an alloying element. The nitrogen increases the saturation of the chromium, which transforms into chromium nitrides instead of chromium carbides. The result is a high strength, high hardness steel with a superior microstructure that extends fatigue life by as much as 100% (double) in certain applications.
AISI316 austenitic stainless steel bearings
Bearing components made from 300 series stainless steel materials have greater corrosion resistance and are non-magnetic because of the low carbon content. However, the tradeoff is that this material cannot be hardened, so the bearings can only operate under low loads and speeds. The bearing’s surfaces undergo a chemical reaction with the oxygen called a passivation process; the passive film developed on the surface protects the bearing from corrosion.
Other 300 Series stainless steel used for bearing components
The bearing shields, seal washers, and ball retainers are sometimes made from AISI303 or AISI304 stainless steel because they have moderate corrosion resistance and are better for forming into various shapes.
Plastic & Non-Metal Material Bearings
Rolling Element Bearings Made of Plastics
Usually, when users think of bearing materials, the first option they will imagine is steel. While this is undoubtedly the most common material, there is a range of other choices, each with their own unique advantages and limitations. Depending on the application type, bearing experts may advise on the use of plastic ball bearings.
The most common plastic ball bearings have both inner and outer rings constructed of an acetal base material. The balls are most often made of stainless steel, but glass or plastic balls are also readily available. Various types of plastic ball bearings are available including, deep groove or Conrad type, miniature, angular contact, self-aligning, and thrust.
Plastic bearings provide numerous advantages compared to steel or ceramic bearings. General characteristics of plastic ball bearings are:
• Grease lubrication is not required
• Excellent performance in chemical environments. It can be used (completely immersed) in water
• Good temperature range
• Non-magnetic and non-conductive
The disadvantage of plastic bearings, when compared to traditional steel bearings, is load capacity, deflection, and lack of stiffness. They are typically used in lightly loaded mechanisms and equipment, running at low to moderate speeds.
A variety of other plastic materials are available as well. Consult with a bearing professional to ensure you make the right choice for your application.
Phenol plastic — Phenolic resin consists of phenol and formaldehyde thermosetting resin as raw materials, which are artificially synthesized. Since it is produced from resins similar to phenols cresol, these types of materials are frequently called “phenolic resins.” This material is highly resistant to oils and chemicals; it has strong heat resistance but is poor in alkalis. It is non-conductive and non-magnetic.
PTFE is commonly known as Teflon R polytetrafluoroethylene. TEFLON is a registered trademark of DuPont. Characteristics include low surface friction, resistance to corrosion and chemicals, high resistance to heat with a wide temperature range of 196 ~ 260 °C, and a water absorption rate of 0%. This material is often used in medical equipment.
UHMW (ultra-high-molecular-weight polyethylene) is a high molecular-weight thermoplastic resin. This special kind of polyethylene is produced via advanced polymerization techniques. Characteristics include excellent chemical resistance, low water absorption, excellent abrasion resistance, retains its properties at low temperatures, and lightweight.
PEEK is a crystalline thermoplastic. It is a type of special, synthetic aromatic-polyetherketone. Characteristics include very high heat-resistance, excellent fatigue-resistance, tough abrasion-resistance, strong dimensional stability, excellent chemical resistance, and excellent insulation and radiation-resistance. PEEK is very expensive.
Applications for Plastic Ball Bearings
• Conveyor systems used in cleaning systems, surface treatment, or plating equipment. Corrosive liquids are often encountered in these applications.
• Food processing equipment
• Medical imaging equipment
• Pumps and spray equipment
Plain, or Sliding Bearings, Made of Plastics
Plain bearings are cylindrical sleeves that have no rolling elements. They are the most straightforward and lowest cost type of bearing. When sliding radially or axially over shafts, they can accommodate both rotary motion and linear motion. They can handle light to moderate radial loads.
One of the more common types is plastic sleeve bearings (which often depend on a hydrodynamic or a full film of lubrication). Self-lubricating plastic bearings in various geometries for bushings, thrust bearings, and integral-slide applications are also available.
There are hundreds of different types and blends of plastic material available for plain sleeve bearings. They provide a wide range of mechanical and physical properties that enhance the suitability of the bearing for different applications, such as low friction, anti-wear, high temperature, food-grade, and high load.
Advantages of plastic plain bearings include:
• High production volume and low cost
• Maintenance-free operation (run dry)
• Lower friction
• Excellent wear resistance
• Chemical resistance and low water absorption
Bearings made of ceramic materials
Bearings made with ceramic materials fall into a specialty niche in the bearing industry. The most common arrangement is a hybrid bearing, usually with stainless steel rings and ceramic balls. The most common ceramic material used is silicon nitride. Balls made from this material are hard, up to Rc78, and have a very smooth surface. Hybrid bearings are more expensive than all stainless-steel bearings.
Today’s engineers have many options available to them when faced with extreme or demanding operating conditions. This abundance of choices includes the use of ceramic materials that have many benefits in a variety of applications. The most common use is producing the rolling elements — the balls — from a ceramic material with the inner and outer rings made from traditional, heat-treated, chrome, or stainless steel.
With improvements in manufacturing technology, Full Ceramic Bearings are also readily available. These bearings have both the inner and outer rings and the rolling elements manufactured from ceramic material. While hybrid and full ceramic bearings demand a premium price, they are often the most economical solution when considering the cost of a failure and the total cost of ownership for the end-user.
The most common configuration is with chrome or stainless steel rings and ceramic balls. Nitrogen enhanced stainless steel ring material is also available.
The most common ceramic material for the balls is silicon nitride, Si3N4. Compared to traditional steel balls, this material has desirable properties for rolling bearings, such as:
• lightweight (approximately 60% lighter than bearing steel)
• high hardness — over 75 HRc
• higher elastic modulus — stiffer
• lower coefficient of thermal expansion
• higher maximum operating temperature — 10000C
• highly polished surface finish resulting in lower friction
Balls made of zirconia, ZrO2, are also available. This ceramic material has some properties similar to silicon nitride but differs in that it has some mechanical and thermal properties much closer to that of the alloy steel that the rings are made from. This can be desirable in maintaining the preload and contact angle over a broad range of operating temperatures.
Benefits of Hybrid Ceramic Bearings:
• higher stiffness and rigidity reduces deflection
• increased the limiting speed, RPM
• less adhesive wear, there is no cold welding
• better lubricant life
• lower noise levels and vibration
• no electrical arcing through the balls
Applications for Hybrid Ceramic Bearings:
• machine tool spindles
• pumps – submersible, fuel, vacuum
• dental drills
• optical scanners
• electric motors
• aerospace applications
• laboratory equipment
• applications that require higher speeds, longer life, and lower friction
Full Ceramic Bearings
Full Ceramic Bearings are often the answer for extreme environments, such as high temperatures, corrosive chemicals, steam, and hard vacuum. These bearings have both the inner and outer rings and the rolling elements manufactured from ceramic material. Like the balls in hybrid bearings, the rings in full ceramic bearings are typically manufactured from silicon nitride, Si3N4, or zirconia, ZrO2. Retainers in full ceramic bearings are usually made from high-performance plastics such as PEEK or PTFE.
Benefits of Full Ceramic Bearings:
• resistant to acid, salt and water, and blood
• lower coefficient of friction
• resistant to electricity
• lighter, smoother and stiffer
Applications for Full Ceramic Bearings:
• MRI equipment
• vacuum environments
• semiconductor manufacturing
• food processing industries
• any extreme environment requiring non-corrosive, non-conductive, or non-magnetic bearings