When the manufacturing process gets hot, beyond what are considered to be ‘standard operating temperatures’, the durability of machinery components and materials is critical, and, in extreme cases, the potential for failures and potential environmental issues increases exponentially. Bearings are often an essential component in any kind of machine that requires motion and this includes those that need to operate safely and efficiently in high temperature industrial applications. These include, heat treatment furnaces, kilns, steel production, automotive paint baking ovens along with other specialist applications, such as high-pressure turbines, butterfly valves and nuclear energy.
With the global push to reduce waste and improve the environmental footprint, it is critical to ensure bearings have the longest possible operational life.
Why use a high temperature bearing?
Bearings for ‘standard’ operating temperature applications like those offered by precision bearing specialists Carter Manufacturing are usually made from chrome steel (52100) and depending on the specifics of each application, are rated from 120⁰C (248⁰F) up to a maximum of 150⁰C (302⁰F). However, when the going gets hotter, standard bearing materials (such as 52100) have a very significant reduction in hardness which can cause premature bearing failure. Special heat treatment processes are available which can increase the high temperature of 52100 bearing for operations up to 330⁰C (626⁰F) but this doesn’t always meet the demands of all applications.
As well as this, because lubrication is essential for all bearings, regardless of application temperatures, standard bearing greases are not rated to high temperatures. Furthermore, there is the risk that the specified radial internal clearance within bearings can be eliminated at higher temperatures due to a mismatch of coefficient of thermal expansion between the shaft, rings, balls and housing.
The best bearing configuration
Speed ratings are a huge factor when choosing between different types of high temperature bearings, with the trade-off being load capacity, speed rating and clearly temperature ratings.
For higher temperature applications (above 330⁰C), hybrid and ceramic bearings can be effective, although high temperature steels such as M50, BG42 and XD16N can all be used as ‘hybrid’ bearings as they offer outstanding performance at high speeds. Other high temperature ceramic bearing materials include zirconia, silicon nitride and silicon carbide which can withstand between 500⁰C - 1200⁰C, depending on the material. Typically, high temperature grease is very thick, and this can increase operating torque – although there are dry (WS2 and MoS2) and solid lubricants (graphite solid fill) available to mitigate this.
What else to consider?
Once the temperature ranges, speed and load requirements are determined for your application, other important challenges to think about are fits and tolerances. The bearing design needs to ensure it has the best shaft and housing fit at the most common operating temperatures. Typically, this means that the fit at ambient temperature is unusually loose, or tight, depending on the coefficient of thermal expansion (CTE) of the mating components. In addition, it’s critical to consider that the hoop stress of the bearing rings never exceeds the yield stress of the bearing material when choosing a bearing fit. The fit at operating temperature can also have an impact on the bearing’s internal clearance – this should also be carefully considered. Also, be aware that some ceramic bearings are sensitive to the rate at which the temperature increases or decreases within the system.
Carter has been pushing the boundaries in developing new materials which has resulted in the company launching special bearings for gruelling, high temperature applications in which ‘standard’ bearings cannot survive – this includes equivalents from other manufacturers. These bearings are manufactured in Europe and conform to ISO standards and are being constantly developed to be the most effective option for high temperature applications.
Future growth areas for high temperature bearings have also been identified. This includes ultra-high vacuum applications where operators are performing the ‘bake-out’ process at high temperatures – here the compatibility of the materials is paramount to ensure that the risks of ‘outgassing’ are minimised, whilst also improving system performance at extreme temperatures. Carter also offers fully customisable hybrid and ceramic bearings for the most extreme applications.
www.carterbearings.co.uk
