The importance of having two temperature probes is to continuously monitor both the temperature of the inner and outer bearing races throughout the entire warming process of a bearing. By doing this you mitigate possible workpiece damage due to stresses caused by the temperature differential between the ID and OD.
Most induction heaters only monitor the temperature of the inner race, neglecting the temperature of the outer race. The problem with this is that the two races will not expand at the same rate (see fig. 1). If the ID heats faster than the OD, it will create large stresses on the roller elements that get sandwiched between the inner and outer races. These stresses will create cracks, fractures, and/or pitting that may not be noticed, reducing the life span of the bearing. This also holds true for gears, as the membrane between the ID and OD will also be stressed and this may create cracks lead to fractures.
In response to an increasing demand for reliability and quality, we not only monitor the ID but also the OD’s temperatures, and the SURETHERM heater compensates with corresponding power cycle adjustments to maintain an optimal expansion ratio. This is critical for high precision bearings that have a much tighter clearance tolerances.
In order to satisfy QA/QC documentation requirements for the proper heating of a bearing, it would be necessary to have a graphing capability: a graph that will show both the temperature of the ID and OD over the time that it took the bearing to reach the desired preset temperature and the deviation between the two temperature curves in order to document that the two did not deviate from the maximum permissible difference entered between the ID and OD temperatures. In addition, the induction heater should also have the capability to export and print the graph.
Finally, the last capability should be a safety feature that allows the user to set a delay once the start button is pressed, in order to protect users with medical conditions such as heart pacemakers and other implants.
Download our Basics of Induction Heating white paper to learn the basic principle behind induction heating, the diverse range of applications and advantages, and the different types of equipment you can use to reach your goals.
Induction Heating by Bernd Seidenthal CRL