Consider Advanced Bearing Materials

This ore crusher cylinder is used in a cement plant and features a bronze bearing with graphite plugs. Photos courtesy Motion

Today’s bearing manufacturers provide alternatives that could optimize asset performance.

Evaluating alternative materials when selecting bearings is a worthwhile exercise. Properly implemented, today’s advanced bearings can give your assets a competitive edge. Understanding the advantages of these newer materials and how they can improve your system’s operation is key to making an informed choice. 

Graphite: At its most basic, graphite is the crystalline form of carbon, structured in single carbon atom layers (graphene) with a hexagonal pattern consisting of six atoms. This material has been commonly used in pencils for centuries and can even be found in nature.

Graphite’s properties include high-temperature and electrical conductivity, stability (chemically inert), high resistance to thermal shock, a high-temperature rating, and good lubrication. This lubricating property can be explained by the weak van der Waals bonds between the layers of the crystal lattice. These bonds allow the graphene layers to easily slide and separate from one another. This same slippery property that benefits bearings lets graphite leave marks on paper.

Graphite is also easy to work with. It can be formed into shapes that hold their structure and can be bonded with other materials, allowing the endless potential to develop new alloys. A good example is the impregnation of metals into graphite to form the carbon brushes commonly used in DC motors. The ease of forming and impregnation, good electrical conductivity, high-temperature rating, and lubricating properties make it ideal for this application.

This can also be seen in several different bushing/babbitt bearings readily available today. Such bearings are typically used in higher- or lower-temperature applications or where environmental sensitivity exists, and when the use of oil or grease as a lubricant should be eliminated. These applications include, but are not limited to, turbines, ovens/kilns, industrial dryers, pumps, washdown systems, packaging machines, electrical swivels, and welding equipment.

Ceramic: Use of ceramic materials in bearings is still relatively new, at least when compared to traditional technologies. Ceramics tend to be extremely hard materials with high compressive strength and are very stable (chemically inert, corrosion resistant) with a low coefficient of thermal expansion. They have low ductile and tensile strengths and are poor conductors of heat and electricity because of a lack of free electrons. Ceramics are lighter than steel and this lower density significantly benefits higher-inertia/centrifugal/higher-speed applications. Categories of this type of material are typically referenced as an oxide, nitride, or carbide.

When applied to bearing technology, you often hear about ceramic or hybrid bearings. Unlike full ceramic, the hybrid model combines ceramic and traditional (often steel) components. You might see a traditional-looking deep-groove roller bearing with only ceramic rolling elements. This arrangement has many benefits, but one newer application is using a hybrid bearing to help combat fluting that can result from capacitive-induced currents typically experienced with a variable-frequency drive in a motor application. Other applications include furnaces (high temperatures), aerospace/performance racing vehicles (low weight), laboratory equipment, and underwater equipment (corrosion resistance). 

Bronze: Innovation in bronze (an alloy comprised primarily of copper) bearings is expanding with the formulation of the alloy itself, resulting in manufacturing technologies using bronze in new and unique ways. Some properties of bronze worth considering when designing an application are its low frictional coefficient, high fatigue strength, stability (corrosion resistance), low shear strength (a forgiving material), and extremely low cost. Other beneficial qualities include the ease of being formed into unique or complex shapes, excellent heat and electrical conductivity, thermal shock and contamination resilience, and consistent performance across a wide range of temperatures. From a material-science perspective, bronze is also very adaptable. Slight modifications to the alloy can greatly enhance its benefits. 

The white bearing surfaces, which comprise alternative plastic materials, play a large role in conveyor applications such as this in which two surfaces repeatedly slide over one another in linear motion. This photo was taken during installation before adding the chain on top of the bearing surfaces.

Alloy advances

Some of the most interesting advances in bearing technology have resulted in various alloys. These include, but are not limited to, aluminum (improving strength), manganese (improving strength and corrosion resistance), leaded and high-leaded tin (improving machinability/lubricity), and tin (adding strength). Besides the adaptability of alloy composition, bronze can be combined with other materials, thanks to advances in manufacturing, machining, and sintering technologies.

These include, but are not limited to, thin-oil impregnation, the addition of graphite solid lubricant plugs, and adhesion to other materials for unique applications, i.e., saddle/sliding/journal bearings and valve plates. Applications for bronze bearings include agricultural and construction equipment, hydraulic cylinders, medical equipment, textile machinery, filling/packaging equipment, iron and steel manufacturing equipment, pumps, combustion engines, and food-processing equipment.

Plastic: Plastics are another set of materials that have started proliferating through bearing technology. All plastics are polymers, so these two terms are synonymous in the bearing industry. However, not all polymers are plastics. 

The properties that make plastics ideal for bearings vary greatly by material type. Some common properties include a low frictional coefficient, wide temperature range, strength, durability, stability (corrosion resistance), light weight, good electrical and temperature insulation, and a relatively low cost.

One of the primary reasons plastic bearings are selected is their chemical resistance. You’ll often find these bearings in applications exposed to chemical or corrosive elements, aggressive cleaning agents, and water spray. Applications include food and beverage processing, pumps, light conveyance, packaging, semiconductor manufacturing, and medical equipment. 

In a light-duty conveyor application (see photo), for example, ultra-high molecular weight (UHMW) bearing surfaces were installed between the steel frame and the chain. A table-top chain is dragged over that surface. Without the UHMW, a high frictional coefficient would make the chain wear through the steel structure. 

Alternative materials are crucial in many similar bearing applications, with two surfaces repeatedly sliding over one another (linear as opposed to rotary motion). This arrangement is common in food and beverage plants, which require frequent and easy wash downs. Riding over the UHMW material, the chain will carry boxes, cans, or whatever else the facility wants to move. 

These material types have a wide range of individual benefits. Aside from the benefits of each polymer type, plastic bearings can generally be loosely described as “free from maintenance,” when selected properly. This is particularly true in the plastic bearing lines called “self-lubricating” or “solid lubricant.” Like bronze, plastic material offers flexibility to combine with other technologies or material types to develop a hybrid solution.

As the material-science industry progresses, the bleed-through into the bearing industry will continue. Understanding these technologies and their applications in industry will be key to future innovation and growth. EP

Based out of Calgary, Ian Miller, P.Eng, is the Manager for Mi Fluid Power Solutions in Canada. He has more than 15 years of hydraulic and electrical experience in the field, including system design, troubleshooting, on-site installations, and technical training/support. For more information, visit motion.com/efficientplant.