Ceramics for extreme bearings

Ceramic bearings for cryogenic applications

Jonathan Newell talks to Erick Sloan of Carter Manufacturing about the rise of ceramic bearing materials and the applications they are used in.

The choice of materials used in ball and roller bearings depends on many factors and manufacturers are keen to make sure that those who specify bearing products have the right level of knowledge at their fingertips to be able to choose between traditional steel products and more modern polymer, ceramic and hybrid material bearings.

To understand more about the factors influencing the choice of ceramic or hybrid ceramic bearings, I spoke to Erick Sloan of Carter Manufacturing, one of the UK’s manufacturers of high technology bearing products.

Material properties

The difference between steel and ceramic bearings all comes down to the material properties. There are advantages and disadvantages to each, but these mainly come down to mechanical load resistance, thermal properties, surface finish and friction characteristics.

The fundamental difference between metals and ceramics is the way molecules are bonded together, which is ionic in metals and covalent in ceramics, the difference being that covalent bonds share electrons. This difference at the atomic level is behind most of the differences in properties.

Mechanical properties

In terms of mechanical properties, ceramic bearings enjoy a high hardness of 70-90HRc, compared to between 50 and 70 for hardened tool steels.  They also have a very high elastic modulus and are resistant to changes in shape under load. However, ceramic materials are able to take less load then steel, especially shock loads. For ceramic/steel hybrid bearings, both radial and axial dynamic loads are similar but static loads are about 30% less for ceramics.

It is also more difficult to achieve the same level of surface finish as on a steel bearing although improvements in finishing technology are changing this. “Recent technological advances have made higher precision classes than P5 possible,” Sloan told me.

Thermal properties

The thermal expansion characteristics are also very different from steel and so ceramic bearings are highly suitable for operating at constant high or extremely low temperatures. For this reason, ceramic materials are often used in scientific, cryogenic and space exploration applications. However, these materials have a low resistance to thermal shock. According to Sloan, the differential thermal expansion due to temperature gradients within the material result in internal stresses which form cracks if they exceed the material strength.

To determine whether ceramic or hybrid bearings are the right choice in extreme temperature environments, it is the temperature profile which is important, according to Sloan. “Thermal expansion coefficient is really important, especially with hybrid designs. If you take a hybrid with ceramics in an aluminium housing as an example, there can be temperature profiles that can result in a 200 micron difference in expansion between the two materials. This is significant,” he said.

Lubrication-free running

Unlike steel bearings, there is no tendency in ceramics to “micro-weld”, a phenomenon that occurs when surface imperfections on the rolling elements and raceways interact to cause an electrical arc. This micro-welding degrades the surface and reduces the life of the bearing. To prevent this, steel bearings need lubrication. Without this disadvantage, ceramic bearings can be used in applications which require the bearing’s operation without lubrication.

Other advantages of ceramics over steel include high resistance to corrosion, non-magnetism and insulation characteristics.

Bearing speed

Although the low density of ceramic materials when compared to steel enables them to operate at higher speeds, the relatively poor surface finish tends to detract from high speed applications. However, the latest developments in hybrid bearing design overcome this with designs which have been engineered to take the advantages of both materials to suit very high speed applications. Generally, according to Sloan, hybrid bearings have a 20-30% higher speed rating than steel equivalents.

When asked to clarify this, Sloan told me, “People tend to choose ceramics due to their material properties rather than their speed characteristics. However, hybrids are faster due to the combination of race speed and the low mass of the ceramic balls along with the ability to run them lubrication free.”

Carter Manufacturing supplies such high speed hybrid bearings into applications where speeds of up to 200k rpm need to be achieved in a lubrication free environment.

Sloan went on to say that hybrid bearings are being used in more industrial and contaminant-free applications because they don’t outgas and in motors where arcing from metal balls could be a problem.

Manufacturing

One of the disadvantages of ceramic bearings is the cost associated with the manufacturing process. Very high energies are needed to reach the required temperature for the sintering process of high-grade raw materials. Since ceramics are so hard, the machining and grinding costs add up quickly when manufacturing precision bearings. All of this must be done in a clean environment with a skilled workforce.

Cleanroom manufacturing is needed because ceramics are incredibly sensitive to impurities in their pores so any contaminates could cause a premature failure. Contaminants also have an effect on the ability to achieve a good surface finish on the bearing.

As the size increases, the price also increases exponentially because of the requirement of high cost processing methods. These include a slower sintering process required to overcome the temperature gradient in the green body, the amount of evenly applied pressure over a larger volume and the resulting machine costs.

Geometry also has an effect on cost since although all standard bearing configurations are possible, such as taper, needle or roller bearings, some are more expensive to manufacture, especially if the rollers are tapered.

As with many examples of providing specifications for engineering components, the choices aren’t always simple and there are many trade-offs but the choice of materials for bearing design is such that there is the ability to supply steel, polymer, ceramic or hybrid bearings to suit every application from the mundane to the extreme.

Read more advice and guidance on designing with ceramic bearings http://environmentalengineering.org.uk/news/advice-and-guidance-on-designing-with-ceramic-bearings-2391/

Hybrid Ceramic Bearings for use in cryogenic pumps

* Steel bearings have a high thermal expansion coefficient which means at the extremely low temperatures seen in cryogenic applications, the bearings contact which takes out the clearance in the bearings and making them seize.

* The significantly lower thermal expansion coefficient of ceramic materials means that the bearings will not contract to the same extent as steel ones and so they retain their geometry at low temperatures enabling them to run smoothly.

* The need for lubrication in steel bearings presents a problem at cryogenic temperatures as the lubricant will change its consistency and freeze. Since hybrid bearings do not need any lubrication, they offer a significant advantage when operating at the extremely low temperatures in which lubricants freeze.

* Cryogenic pump bearings have been used with great success in a submerged pump. The pump works in a vertical orientation and is submerged in liquid Argon or Nitrogen at operating temperature of between -155C and -196C.

* The configuration is an angular contact bearing pair with stainless steel Races, silicon nitride ceramic balls and a custom PEEK cage. A dry film lubricant was also applied to meet with the demands of extreme low temperature as well as extremely high loading and speed conditions.

Jonathan Newell

Jonathan Newell

Jonathan Newell is a graduate of Loughborough University and has three decades of experience in engineering as well as broadcast and technical journalism.
Jonathan Newell

Latest posts by Jonathan Newell (see all)

About Jonathan Newell

Jonathan Newell is a graduate of Loughborough University and has three decades of experience in engineering as well as broadcast and technical journalism.

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