Bearings Survive Duties on Papermaking Machines

Rolling bearings used in the production of paper have among the toughest lives of bearings used anywhere. In addition to the arduous regime of a continuous production cycle, they are required to handle high- temperature conditions, increasing loads and higher speeds; yet at the same time provide longer service life with the minimum of maintenance. The increasing demands worldwide for paper mean that failures in the continuous production cycle are extremely costly. Bearings, of course, are fundamental components in this cycle. Therefore, despite the many operating difficulties, bearing manufacturers are constantly under pressure to provide ever- increasing levels of reliability and operating life from their products. As a major supplier of rolling bearings for papermaking machines, NSK has researched the many operating problems of bearings in the industry. The result of this research is a new generation of bearing products that address the specific requirements and problems associated with all areas of the papermaking process. One of the major problems addressed by NSK is the trend to higher operating temperatures in the dryer sections of papermaking machines. These temperatures, which can be as high as 200C on newer machines, cause high thermal stresses, which can lead to fracture of the inner rings of the spherical roller bearings used in the dryer sections. The same problem can also occur as a result of rapid dryer warm up. The excessive thermal stresses that result can lead to broken bearings and costly breaks in production. 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 degrees centigrade) 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 fracture resistance performance commonly experienced with conventional spherical roller bearings. In fact TL specification bearings have higher inner ring fracture resistance compared with 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 carburised steel. This means that TL specification steel achieves a longer life, when foreign debris is present, than any of the other bearing steels. In addition to their use in dryer sections, TL Series spherical roller bearings offer operating and enhanced life benefits in drying cylinders, PV, canvas, paper and breaker stack bottom rolls, soft calendars and Yankee dryers NSK has also employed innovative materials and design technology to develop the HPS series of spherical roller bearings for use in other small diameter rolls in the papermaking process: these include canvas rolls, paper rolls, felt rolls and rider. The HPS series is a new design of spherical roller bearing (SRB) that offers twice the running life of conventional SRB of the same sise, together with a 20% increase in running speed. These performance advantages translate into benefits such as cost saving, and reduced maintenance and energy costs. The HPS design also overcomes the problem of cage wear that occurs with SRB under continuous running at high temperatures and speeds: precisely the conditions encountered in papermaking applications. In SRB, the cage plays a key role in the performance of the bearings; it must therefore have high dimensional accuracy and functionality. NSK has addressed these requirements by developing a special nitriding surface treatment, which is applied to the high-precision pressed steel cage of the HPS series bearings. compared with conventional nitriding, NSK's treatment forms a finer and harder surface with a more uniform hardness. The resulting benefit is significantly reduced wear in harsh operating environments. Also helping to transform the performance of key sections of papermaking machinery in terms of reliability, maintenance free intervals and operation in environments exposed to contamination, such as dust and/or water, are NSK's Molded Oil bearings. These bearings (standard deep groove ball and SRB) enable machine users to make real cost savings by replacing existing, and often costly, methods of lubricating machine parts, especially on raw material conveyors, carrier rope sheaves and suction rolls. Molded Oil is a solid lubricant: a plastic containing lubricating oil. As it provides a lubricating oil content of more than 50% by weight, it improves on other marketed oil impregnated plastics, where the oil content is a mere few% by weight. The characteristics of the material mean that a good lubrication regime is always maintained. This is because the mechanism of oil discharge from the matrix is temperature dependent: the higher the heat generation, the higher the oil discharge rate. Bearings equipped with Molded Oil are particularly effective in environments exposed to moisture or paper dust. They do not suffer from oil leakage and there is no loss of lubricant due to the ingress of water. In addition, Molded Oil bearings can make real differences to equipment performance in environments where lubrication is difficult to apply, and places where oil and grease absorbing dust is produced. In both cases, long term bearing performance is greatly increased by the efficient lubrication that Molded Oil provides. NSK has detailed its innovative bearing products for the papermaking industry in a new publication entitled 'Bearings for papermaking machines'. Running to 30 pages the new free booklet, provides a wealth of information regarding the choice and application of bearings for the papermaking process, tips on bearing maintenance and inspection, and also advice on effective countermeasures that can be taken to prevent bearing problems.