The machine in question produces I-section beams using a series of rolls to maintain pressure on the profile as the molten steel solidifies. Shaft-mounted bearings support each set of rolls.
At the steel plant, the cylindrical roller bearings were lubricated with oil via a complex system of piping. Unfortunately, high maintenance costs were the norm because of the time required to disconnect and reconnect the pipes after every bearing failure. Due to the demanding nature of the application (high temperatures, contamination, high loading) – as well as the design constraints of the machine which prevented the fitting of heavier duty bearings – the service life of the cylindrical roller bearings was just 1-2 months.
To provide an improvement, NSK concluded its analysis by proposing to fit the rolls of one segment with sealed spherical roller bearings (60 in total).
NSK’s solution doubled the maintenance intervals to 2-4 months thanks to the high sealing performance of the sealed spherical roller bearings, resulting in substantial cost savings of €14,850 (for just a single segment of the continuous casting machine). Instead of the single-lip concept of standard sealed spherical bearings, NSK’s design takes advantage of a special outward-extending, spring-loaded lip seals made from nitrile rubber, delivering robust protection against the ingress of contamination.
NSK’s sealed spherical roller bearings use long-life, temperature-resistant grease, subsequently eliminating oil costs. The grease does not require reapplication for the operating lifetime of the sealed NSK bearing, which means that dismantling and re-installing the oil piping system is no longer needed after a bearing failure, significantly reducing the time required for maintenance. Overall, the time required for the replacement of a segment is now 10 hours, instead of 20 previously.
In terms of bearing design, NSK’s sealed spherical roller bearings feature symmetrical rollers and raceways to prevent edge loading problems at the contact points under high-load operating conditions. Furthermore, a special chamfer configuration ensures smooth axial movement of the bearing at the free end.
Such was the success of the project at the steelmaker that NSK received an order for 900 bearings with the intention of replacing the existing cylindrical roller bearings on all segments of the continuous casting machine, multiplying the savings accordingly.
A standard range of NSK sealed spherical roller bearings is today available from 50 to 130 mm internal diameter (90-280 mm external diameter). The maximum permissible misalignment is 0.5° as per the industry standard, while the maximum operating temperature is 80°C, although this can be enhanced in line with the seal material and type of grease used.
The maximum limiting speed of the NSK sealed spherical roller bearings is 8 m/s (as determined by the permissible linear speed at point of contact with the seal lip), ensuring its suitability for other applications involving higher operating speeds than continuous casting.
