When the Going gets Really Hot – How Do You Choose the Right Bearings?

 

When the manufacturing process gets hot, beyond what is considered to be ‘standard operating temperatures’ the durability of machinery components and materials is critical and where 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. For example, 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 120ºC (248ºF) up to a maximum of 150ºC (302ºF).  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 330ºC (626ºF) but this doesn’t always meet the demands of all applications.  Other factors to be considered are that as 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.

How do you select the best high temperature 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 330ºC) 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 500ºC – 1200ºC, depending on the material. Typically, high temperature grease is very thick and this can increase operating torque and whilst there are dry (WS2 and MoS2) and solid lubricants (graphite solid fill) available to mitigate this.

What else to consider when choosing a high temperature bearing?

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 optimum shaft and housing fit at the likely operating temperatures. Typically this means 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 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 bearings internal clearance which should be carefully considered. Also, it’s important to being 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 special bearings for gruelling, high temperature applications in which ‘standard’ bearings cannot survive and 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. For example, ultra-high vacuum applications where operators are performing the ‘bake-out’ process at high temperatures so the compatibility of the materials is paramount to ensure that the risks of ‘outgassing’ are minimised, whilst also optimising system performance at  extreme temperatures. Carter also offers fully customisable hybrid and ceramic bearings for the most extreme applications.

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