Wälzlager - 2013-04-08
Costly production stoppages
Conventional bearings simply cannot offer a satisfactory service life in these conditions, and even heavy-duty bearings are not ideal from the machine operator’s point of view. In practice, the guide roller bearings sometimes have to be replaced after less than two months. What makes this situation even more aggravating for the steel processing firm is that the machinery’s availability and productivity are severely impaired every time a bearing needs replacing. Restarting a continuous casting machine after downtime is no easy task and a new slab has to be cast on. On top of all this, CCMs have a complex coolant and lubricant supply system which has to be reconnected following any shutdown. This is also a lengthy process.
The development of heavy-duty bearings
With the steel industry in mind, NSK’s development team worked tirelessly to come up with bearings which would last well even in these adverse conditions. This work was based on an in-depth examination of the factors which cause wear in conventional bearings.
Because the bearings in the guide rollers of continuous casting machines run at very low speeds, an insufficient load-carrying oil film is formed. Metal-to-metal contact occurs between the rollers and the inner/outer ring. As mill scale and cooling water can also penetrate the bearing, the component is subject to abrasion-related wear.
The working surfaces between the rollers and the outer ring are particularly prone to this because wear always affects the same area due to the fixed outer ring. Consequently, the outer ring’s contact surface becomes uneven. There is little wear in the areas with rolling friction. By contrast, cracks form where sliding friction occurs – i.e. the oil film is insufficient – and the material is worn away.
Highly wear-resistant bearing steel for the outer rings
Based on these findings, NSK developed a bearing steel with high core strength which is three times more resistant to wear than conventional bearing steel (AISI 52100). The material is also much less prone to pitting in these extreme conditions – five times less, in fact. This is due in no small part to the special kind of heat treatment used.
Because this material is only used to manufacture the outer rings, the developers ensured that wear was also reduced in conjunction with the steel used for the rolling elements. The tougher material used for the outer ring does not shorten the rolling elements’ lifespan. In fact, tests and extensive field experience show that the likelihood of pitting between the outer ring’s raceway and the rolling elements is five times lower with the new type of steel. On top of all this, the risk of the outer ring breaking is five times lower than in components made from conventional bearing steel (AISI 52100) thanks to the improved material properties.
All of these outstanding test results back up the new material’s name: the bearings are marketed as ‘Super Wear Resistant’, or SWR for short.
Seals available on request
Most of the SWR components ordered are open-type bearings. However, some machine designers and users prefer enclosed bearing units because the seal enables optimum grease management and prevents lubricant from getting into the water. Instead of using standard seals, NSK has developed a particularly effective seal for heavy-industry applications which is utilised for the SWR bearings.
Compatible with standard bearings
Because continuous casting machines run for a very long time, it was important to NSK’s development team not to have to change the bearings’ dimensions in conjunction with the improved material and bearing properties. They achieved this too: the SWR bearings are 100% interchangeable with the corresponding standard bearings from the same series. This means that they can be installed in existing machines during routine bearing replacements.
Real-life example: considerable increase in service life
A real-life example shows how these bearings perform in practice. The operator of a continuous casting machine found that he had to replace the cylindrical roller bearings in his guide rollers every month or two and wanted to extend the components’ service life. He contacted NSK’s industry experts for help, who analysed bearing use on site as part of NSK’s Asset Improvement Programme (AIP).
They found that the premature wear was caused by contamination penetrating the bearings and by the cylindrical roller bearings’ inability to compensate for the deflection of the guide rollers. NSK then suggested replacing the bearings with sealed spherical roller bearings from the SWR series. The operator acted on the experts’ recommendation.
The advantages of spherical roller bearings
Unlike cylindrical roller bearings, spherical roller bearings are able to compensate for misalignment caused by the shaft deflection, meaning that the bearings are subject to an even load. The bearings can also be supplied as sealed, pre-lubricated units. One of the advantages of this option is that these components do not have to be connected to the central lubricant supply system.
Outcome: operating life more than doubled, annual savings of €14,850
The new bearings recommended by NSK led to a significant improvement in the bearing performance, an increase in the average service life to four months, and lower costs for lubricants, spare parts and maintenance work. All in all, using SWR spherical roller bearings now saves the operator approximately €14,850 a year. This is not the only case where SWR bearings have improved the productivity and availability of continuous casting machines.
SWR spherical roller bearings are not the only solution available for these demanding applications. There are also alternative concepts, such as cylindrical roller bearings with a self-aligning outer ring and corresponding tapered roller bearings. NSK’s portfolio for heavy industry includes bearing designs of this kind too, which can also withstand severe strain. Users can therefore choose whichever option suits them best.