Roulements - 2015-04-01
Hot wide strip rolling mills process red-hot slabs into steel strip using a high rolling pressure. An international steel processing company operates such a rolling mill in Europe, which produces steel strip with a thickness of up to 13 mm. The steel is rolled into coils which may weigh more than 30 tonnes. These coils are then used, for example, in the automotive industry.
Work rolls: extreme conditions for bearings
The entire mill train is approximately 300 metres long. The central components of the system are the work rolls, which have direct contact to the material and ultimately influence the surface quality of the strip.
The major part of the roller force is carried by the supporting rolls. However, a proportion of the radial and axial forces is transmitted through the work rolls to their bearings, so the bearings must be suitably strong. Four-row tapered roller bearings are therefore generally used, which are able to cope with forces coming from both directions.
Furthermore, when selecting bearings it must be taken into account that the speed of the strip – and therefore the speed of the bearing – at the end of the line is significantly higher than at the beginning of the line. This means the bearings on the first roll stand need to withstand high rolling forces. They are also frequently subjected to impact loads.
Switch from open to sealed bearings
The other operating conditions for bearings in hot rolling mills are also unfavourable. High temperatures prevail in the environment surrounding the rolling equipment, and it is possible for mill scale and (cooling) water to penetrate the bearings.
Working with the maintenance specialists in the steel plant, some time ago NSK initiated a project to increase the service life of work roll bearings under these conditions and additionally to extend the maintenance intervals. At this point, the company was still using open four-row tapered roller bearings. NSK recommended switching to sealed bearings with a special material quality. It delivered the first samples, which were installed by a team comprising members from both companies.
Special material for long service life in unfavourable ambient conditions
The bearings which are now in use are manufactured from Super TF (STF) steel. This super-pure special steel was developed for applications where long bearing service life under unfavourable conditions, such as increased ingress of contaminants, is desirable.
Combined with a special heat treatment, the material selected prevents cracks and damage to the bearing race caused by particles of dirt. Comparative testing with bearings made from standard materials shows that when running with contaminated lubricant, STF bearings achieve a service life that is around ten times longer. In practice, service lives of up to three times the usual service life can be expected.
In conjunction with the highly effective Sealed Clean seal, the four-row tapered roller bearings are manufactured as the KVS Sealed Clean series. These are primarily used for equipment in the steel industry.
The next step: selecting the optimum lubricant
After installation, it was quickly demonstrated that the STF tapered roller bearings with Sealed Clean seals achieved a significantly longer service life than the open bearings previously used. Therefore, further bearings were replaced.
However, the project partners were not yet completely satisfied with the result, that is, with the expected service life of the bearings. For the subsequent revisions, NSK experts went on site again and analysed the bearings. This also included lubricant analysis of the bearing grease recommended by NSK, which was developed in cooperation with the lubricant manufacturer.
The analyses showed an increased concentration of water in the lubricant at individual locations in the bearings. Although the value had not yet reached a critical level, the surrounding design was further improved and adjusted to NSK specifications.
Longer maintenance intervals, significantly higher service life
The result of the project was a significant increase in both the maintenance intervals and the service life of the work roll bearings. For the open bearings, maintenance was required after 600 to 800 hours, whereas for the sealed bearing, the interval could be extended to 1,400 hours. The figures for the service life show a similar relation: it could be increased from around 8,000 hours to around 14,000 hours.
Thanks to the use of the sealed bearings made from STF steel, the operator of the rolling mill has the advantage of improved machine availability because the service life could be increased. At the same time, maintenance costs are reduced and the impact on the environment is minimised as there is no need to relubricate the bearings during operation. The grease only needs to be replaced during planned maintenance.
AIP: consultancy with industry expertise
The higher purchase costs for the STF bearings with Sealed Clean seals can be regarded as a disadvantage. However, this is more than compensated for by the reduced maintenance costs and lower downtime. Furthermore, the seals need to be replaced every so often. In order to ensure that this can be done easily, the sealing concept was fundamentally revised. NSK also gave the maintenance personnel appropriate training.
This kind of training is part of the service provided in the Added Value Programme (AIP for short), during which NSK industry experts assess bearings on machines in operation. The aim of the programme is primarily – as is also shown by this application example – to increase the service life of the bearings and thus reduce downtime and maintenance costs.
NSK can offer extremely valuable advice here, not only in the steel industry but also for other bearing applications. This is because in its product range there are also bearing model series which have been especially developed for unfavourable ambient conditions.
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