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Roulements - 2010-02-16

Steel Industry

Targeted Application Engineering

Service Life of Rolling Bearings Increased by Selection of Optimum Design and Material

 

The steel industry is an example of an industry that places high demands on rolling bearings. Bearings are subjected to especially high loads and temperatures and the environment is frequently contaminated. If problems arise during operation, the availability of the plant can be maximised with targeted application engineering and optimisation of the bearing specification.

As one of the world's leading manufacturers of rolling bearings, NSK offer its customers a complete selection of bearings. For some areas of application that place high demands on bearings, the range far exceeds the standard offering. Special bearings have been especially developed for these areas, and the plant manufacturers are extensively supported by NSK Application Engineers. This applies to wind energy technology, pump and compressor manufacturers and to the steel industry.

NSK have developed extremely pure materials that achieve high service life under, for example, extreme conditions in rolling mills.

Service for users

In addition to plant manufacturers, operators can also directly benefit from the expertise of the bearing manufacturer. NSK application engineers attend the site if premature failures occur during operation as a result of a bearing related issue Application Engineer refers to a bearing expert with knowledge and experience specific to the required industry.The problem is investigated on site and, if necessary, with the assistance of the NSK Technology Centre, a recommendation is made to extend the service life of the bearings. Recommendations may include substitution of the bearing for a different design, alternative lubrication or additional services. In this way, users can significantly increase the productivity of their plants and avoid unscheduled downtime with minimal expenditure.

Bearing failures eliminated by optimised material selection

A practical example best demonstrates the advantages of NSK Application Engineering. A company in the steel industry turned to NSK engineers because bearing failures were occurring with increased frequency on the supporting rollers in a Sendzimir cold rolling mill; typically after 1,500 to 3,000 hours of operation. This resulted in downtime of approximately three hours per breakdown. Supporting rollers in such facilities are subject to a great deal of stress because they absorb the entire rolling force and are thus subject to high mechanical strains with only moderate lubrication. Examination of 3PL type backing roll bearings in the NSK laboratory showed that the failures were caused by increased wear on the inner ring raceways. The wear was unexpected, since the Z steel used by NSK as the base material is extremely pure, and offers much higher service life than standard bearing steel under load and with poor lubrication.

As an alternative to Z steel, NSK offers bearings made from EP steel, in which the content of non-metallic inclusions is reduced to a minimum thanks to an improved grain structure. Higher resiliency and longer fatigue life are achieved. This material has been developed by NSK in cooperation with a steel producer; NSK has also developed a new imaging method that can be used to assess the purity of the steel in the lab.

Life test: 5,000 hours without failure

Given the available operating data, NSK recommended the use of EP steel. The effect of this recommendation was tested in practice: the user exchanged a complete set of bearings for one axle. After this bearing set was in operation for more than 5,000 hours without a failure and without signs of fatigue, it was decided to replace the bearings on the other seven axles.

This example shows that not only the bearing design, but also the materials used play an important role in the selection of bearings and can significantly affect the service life of the bearings and the entire machine. NSK offers many bearing types, including the NSK standard material Z steel, and other materials that have been developed for special operating conditions. Examples include the extremely pure EP steel mentioned here and the heat-resistant SHX steel for high-speed applications.

Bearing service life increased threefold

In another very similar case, the work rolls on a cold rolling mill had to be exchanged after an average of just 800 hours in operation. NSK was commissioned to examine the application and make suggestions for extending the service life. The examination included the operating conditions of the bearings on the rolling frame, the bearings themselves and the lubricant. In the NSK laboratory, an elevated water content and contaminants were found in the lubricant. In rolling mills, in order to achieve high rolling speeds, the rolling gap is lubricated with a mixture of oil and water. Water is required for cooling the cold strip.

To improve the situation, NSK engineers suggested the use of WTF type bearings. These bearings are manufactured from the material WTF, a development of the proven TF material, which is used if the environmental conditions are associated with the risk of contamination (picture 4). The 'W' in the name of the material stands for water because WTF has been specially developed for uses in which increased moisture content along with contaminants occurs in the bearing application. After the bearings were exchanged, the service life of the work roll bearings increased by a factor of 3 to approximately 2,700 hours.

Summary

These examples show how the service life of bearings can be greatly increased by the optimum selection of materials, particularly for critical operating conditions. This applies not only to rolling mills, but also to any branch of industry where bearings are subjected to extraordinarily high loads.

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