Solution to problem of bearing inner ring fractures
Published: 08 July, 2011
NSK has developed TL (Tough and Long Life) bearing material, to overcome the problem of damage due to inner ring fracture encountered on spherical roller bearings in the dryer sections of papermaking machines. In addition to its primary function of preventing inner ring fracture, TL also contributes to an increased surface hardness on the bearing raceways, and provides enhanced dimensional stability under high temperature conditions.
TL has been developed to address the trend to higher operating temperatures in the dryer sections of paper making machines. Today, with the moves toward higher performance equipment, the temperatures in the hollow cylinder of the dryer section, through which steam or oil passes, can reach 180 degrees C; and sometimes on newer machines, even 200-250oC.
These temperatures cause high thermal stresses, which can lead to fracture of the inner rings of the spherical roller bearings used in the dryer sections. During the drying operation, thermal expansion of the cylinder takes place. This increases the tensile stress on the bearing inner ring due to the temperature difference between the cylinder and the ring. Under these conditions there is a greater risk of failure due to inner ring fracture.
One method of preventing the increase in tensile stress is by suppressing the temperature difference between the cylinder and the inner ring. This is achieved by providing cooling to the cylinder and by the use of a heat insulated sleeve to insulate the bearing parts from the heat generated in the hollow cylinder. Some machines have already adopted this countermeasure. The actual machinery, however, becomes complicated and modifications to existing machinery are not straightforward. As a result, the industry tends to favour an approach that strengthens the bearings.
NSK has analysed the mechanism of inner ring fracture and established a strength evaluation method that has resulted in the development of the TL specification material. The TL specification is a special surface treatment of an original steel type. It attains both high raceway surface hardness and dimensional stability under high temperatures (up to 200oC), while at the same time having the same level of compressive residual stress at the raceway surface as conventional carburised steel.
The latter feature increases, substantially, the strength against fracture performance commonly experienced with conventional spherical roller bearings. In fact TL specification bearings have higher strength against inner ring fracture than bainitic steel (austempered high-carbon chrome bearing steel) and bearing steel (hardened high-carbon chrome bearing steel) units.
The raceway surface hardness of TL material also exceeds that of bainitic bearing steel and carburised steel. This means that TL specification steel achieves a longer life, when foreign debris is present, than any of the other bearing steels.
Although the TL specification was originally developed for use on spherical roller bearings used in papermaking equipment, the advantages of the material, fracture prevention, dimensional stability and longer life performance mean, that this same specification can also be applied more widely in an expanded range of applications.
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