The iron and steel industry is one of the harshest environments in which power transmission equipment is expected to perform. With the ever-increasing overhead of maintenance compounded by pressure to reduce operating costs, in the bearings for steel industry publication NSK has outlined several areas for potential savings in this arduous industry and demonstrated how these savings can be achieved.
Rolling bearings for iron and steel mills have to withstand varied and extreme operating conditions, amongst which are high temperatures, both high and low rotational speeds, heavy loading as well as exposure to moisture, hard particulates and corrosive contaminants. NSK offers a complete range of products for mill processes, designed specifically to deliver reliable and continuous operation under these conditions in applications from guide rolls on continuous casting machines, to backup rolls and work rolls on rolling mills.
In 1980, NSK was the first company in the world to bring the sealed four-row tapered rolling bearing (Sealed-Clean) to the market. Since then, it has continually improved the Sealed-Clean bearing, until the latest version which now displays measurably increased load-carrying capacity through newly developed internal design and a new sealing system. Even grease consumption has been reduced, which not only leads to lower costs but also makes it more environmentally friendly. Sealed-Clean bearings can be supplied in case-hardened and through hardened steel. NSK’s various steel specifications are particularly wear and shock-resistant due to its special heat and surface treatment technology.
Each stage of steel production presents different problems and requires differing solutions to be employed.
Sintering machines - the first stage in converting the iron ore into steel products, and where NSK’s AR series of Sealed-Clean bearings for pallet wheels and the 2J Series for inboard rollers can provide increased service life, resulting in a claimed reduction in maintenance costs of 25 per cent-35 per cent.
Difficult operating environment
The operating environment sees high temperatures, heavy loads, low speed and exposure to sintered particles, all of which typically lead to premature bearing failure on traditional pallet wheels and inboard rollers. Potential contamination also leads to more frequent greasing, which also incurs additional costs.
For the best solution is, instead of a plain bearing, use a roller bearing with especially tough seals. The newly NSK developed encapsulated design effectively prevents scale from penetrating into the bearing, and high-quality grease also has outstanding resistance to heat and pressure. Re-lubrication of the bearing units is unnecessary.
Basic Oxygen Furnaces (BOFs) and converters - effectively the melting pot stage of the process which sees some very difficult operating conditions: high temperature, heavy loads, ultra low speed and oscillation. The major issue is the time required to replace the inboard bearings, with unexpected failures causing large production losses.
By introducing Ultra-Large split bearings for trunnions, the maintenance can be done without removing the bull gear. This reduces maintenance costs and production down time by approximately 35 per cent.
Continuous casting machines - here the molten steel solidifies and starts its journey towards the rolling mills. Again the operating conditions involve heat, heavy loads, water vapour, ultra low speed, scale and skewed positioning. The main issues in this area of production are uneven wear and flaking or cracking of the outer raceway. These can lead to the expansion of roll gaps and unexpected production line failure.
NSK has developed a range of sealed spherical roller bearings specifically designed to prevent the ingress of contamination and provide significant improvements in bearing durability. Rigorous testing has shown that this will reduce maintenance costs and improve overall production figures. The Tapered roller / Cylindrical roller bearings with self aligning rings (AR/RUB series), split roller bearings and SWR bearings are all designed for use in the continuous casting environment.
Rolling mills - These can include Plate mills, Table Rollers, Rolling mills, Hot Strip mills, Cold rolling mills, Multi-roll rolling cluster mills and Skin pass mills. This area of production can be a very difficult environment, and requires specialist design features to ensure that the machinery keeps moving and production continues. NSK has developed a range of bearings specific to each task and tested each one on test rigs which are designed to simulate the rigours of the real working environment before field trials were used to confirm performance.
By using its Super TF steel, the expected service life of the bearing, compared with a conventional steel component, has been extended by a factor of 2. By also improving the sealing, the ingress of water and foreign bodies is reduced and the overall bearing life is three times that of the standard item.
The use of Four-Row cylindrical roller bearings (STF-RV series), Moulded oil bearings, Sealed-Clean, EW & EM series cylindrical roller bearings, Four-Row tapered roller bearings, WTF Bearings & Extra-Capacity Sealed-Clean Four Row tapered roller bearings (KVS series) has seen vast improvements in the performance of the bearings in this area of production. Super TF steel has excellent resistance to contamination by foreign bodies, which results in longer service life, which reduces maintenance costs and lost production time.
Overall, NSK has been able to demonstrate a 30–35 per cent cost benefit in maintenance costs when using its range of specifically designed bearings over conventional sealed bearings and is happy to provide real-world examples to back up its claims. NSK has also released an industry-specific brochure which provides in-depth analysis of the issues encountered in the production and processing of steel as well as detailed solutions and specifications. There are details of failure analysis that show how the new products are developed and exactly how they can improve the performance of any steel processing plant.
Free copies of the bearings for steel industry publication are available from NSK: Freephone 0500 2327464. Callers should quote the publication reference: STE/B/E/03.11 and also the title of the publication. For further information please visit www.nskeurope.com.