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Bearings - 2015-04-10

Super-TF bearings

A major steel manufacturer experiencing high maintenance costs relating to bearings on a plate rolling mill has improved the situation massively with an application specific product replacement from NSK. The existing open roll bearings required a daily maintenance routine to ensure sufficient lubrication before analysis by NSK led to a proposal or the installation of its Sealed-Clean range, which proved ideal for use in this heavily contaminated environment.

The iron and steel industry is one of the harshest environments in which power transmission equipment is expected to perform. Rolling bearings for iron and steel mills have to withstand varied and extreme operating conditions, amongst which are high temperature variations, both high and low rotational speeds, heavy loading as well as exposure to moisture, hard particulates and corrosive contaminants.

In this particular case, the maintenance team was spending valuable time re-lubricating the old bearings every day. The NSK application engineers, with their considerable experience of the iron and steel industry, proposed a bearing design with increased sealing protection as well as improved bearing steel technology. Using the principles of the Added NSK Value Programme – AIP, the engineers were able to demonstrate the additional benefits of the Sealed-Clean range.

The four-row, sealed tapered roller bearings used on the roll necks of rolling mills are required to achieve long life amid severe environmental conditions such as high operating load, extreme shock loads and the presence of high pressure water and scale debris. New seal technology, developed by NSK, is enabling a longer service life and improved reliability in addition to higher load capacity over conventional sealed roll neck bearings in this application.

These advanced bearings are manufactured from NSK's patented Super-TF bearing steel, the product of research that addressed the problem of bearing life in conditions of contaminated or inadequate lubrication, which make up two of the major causes of premature bearing failure experienced in large equipment such as rolling mills.

Research by NSK has revealed that bearing life in conditions of contaminated or boundary lubrication can be improved by changing the microstructure of the bearing steel in order to enhance its ability to relax stress concentrations on the edges of indentations. This microstructure, with a high volume of retained austenite and a high hardness, produces a longer life even under conditions where there is an insufficient oil film and surface damage, such as peeling can occur.

In practice more effective results are obtained not when either retained austenite, or hardness is increased alone, but only when both properties are increased together, as is the case with Super-TF steel. In comparison with bearings made of ordinary carburized steel, Super-TF types have shown in tests to offer up to 10 times the life with contaminated lubrication and up to twice the life where the lubricant is clean. Where the problem is related to insufficient lubrication, the average life of bearings made from Super-TF is still impressive, at around 5.5 times that of ordinary carburized steel.

In NSK's new design the cage flange is included within the seal space of the Extra-Capacity Bearing, allowing for longer rollers. The cage diameter is also increased to facilitate more and larger rollers without reducing cage strength. In performance terms the optimised design raises the basic dynamic load rating capacity of the sealed roll neck bearings by up to 34 percent when compared to NSK's conventional roll neck bearings. This equates to as much as 2.7 times longer life.

For the steel mill, the cost of the daily maintenance operation was rapidly mounting up, with the additional costs of disposing of the used grease. With the introduction of the Sealed-Clean bearings from NSK, not only did the customer save over €46,000 but also significantly reduced the environmental cost of the operation.

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