The trend to use higher operating temperatures in the dryer sections of paper making machines poses severe problems of bearing reliability for engineers designing such machines. High temperatures cause high thermal stresses which can lead to fracture of the inner rings of the spherical roller bearings used in the dryer sections. What is more, the same problem can occur as a result of business pressures which cause some producers to rush the dryer warm up. In fact, the risks and the potential costs far outweigh the benefits of rapid warm up. The excessive thermal stresses that result can lead to broken bearings and costly breaks in production.
The high-temperature equipment for the dryer section of a papermaking machine comprises a hollow cylinder through which high temperature steam or oil passes (Figure 1). Today, with the move toward higher performance equipment, the temperatures of the steam and oil can reach 180 C, and sometimes, on newer machines, even 200-250 C.
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 cracking.
Inner ring cracking is a phenomenon caused by hairline fractures that originate at the surface or near the surface of stress concentrations caused by a combination of high inner ring hoop stress and stress caused by repeated passing of rolling elements over the raceway. The hair line fractures grow in the axial direction until they eventually become cracks. Non-metallic inclusions, pitting from rust and minute flaking are the sources of stress concentrations. Because inner ring cracks quickly lead to damage that causes machine stoppage, crack damage must be prevented, even if the bearing raceway ring has stress concentration sites.
One method of preventing the increase in hoop stress is by suppressing the temperature difference between the cylinder and the inner ring. This is achieved by providing cooling to the shaft and by the use of a heat insulated sleeve to insulate the bearing parts from the heat generated in the hollow shaft. 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 instead.
NSK has analysed the mechanism of inner ring fracture, and established a strength evaluation method that has enabled the company to develop the TL (Tough and Long-Life) specification.
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 200 C) while, at the same time, having the same level of compressive residual stress at the raceway surface as conventional carburized steel. The latter feature increases substantially the strength against fracture performance commonly experienced with conventional spherical roller bearings. In fact, the TL specification bearings have higher strength against inner ring fracture than both bainite steel (austempered high-carbon chrome bearing steel) and bearing steel (hardened high-carbon chrome bearing steel) units.
The raceway surface hardness of the TL material also exceeds that of bainite bearing steel and carburized steel. NSK claims that this means that a TL specification steel achieves a longer life, when foreign debris is present, than any of the other bearing steels.