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Continuous operation, high temperatures, ever increasing loads and faster speeds: those are the conditions to which bearings are subjected in paper machines. NSK has developed a number of new bearing series for this range of applications. One of the greatest challenges faced by the NSK developers is finding a solution to handle the trend towards higher and higher operating temperatures in the dryer sections of paper machines. In the latest generations of machines, temperatures in the dryer section can reach up to 200° C. This high thermal stress can cause the inner rings of the spherical roller bearings to break. The same problem arises if the temperature in the dryer systems is run up to the operating temperature very quickly. This rapid temperature increase can lead to bearing failures, which in turn lead to time-consuming and costly stoppages in the plants.
NSK has developed a method for evaluating strength, which was used as a basis to develop the TL specification ('Tough and Long-life'). An important feature of the TL specification is a special surface treatment: this creates the prerequisite for high bearing race hardness and very good permanence of size, even at temperatures up to 200° C. In addition, material treated this way offers the same reliable compressive stress as bearing races made from conventional case-hardened steel.
This combination of properties increases the resistance of the spherical roller bearing against breakage; the resistance of the inner rings of bearings that meet the TL specification is even higher than the resistance of inner rings made from bainitic steel and bearing steel. Similarly, the surface hardness of the rolling surface is higher than the hardness of rolling surfaces made from bainitic steel and case-hardened steel.
Consequently, materials that meet the TL specification have a longer service life than other materials subjected to contaminated conditions. In addition to their use throughout the dryer section, including the drying cylinder, spherical roller bearings that meet the TL specification are also suitable for use in machine parts such as calenders, guide rolls, and smoothing rolls.