Product category:
Bearings
News Release from: NSK UK | Subject: Super-TF
Edited by the Engineeringtalk Editorial
Team on 16 July 2002
Ultra-clean steels provide longer-living
bearings
Rolling bearings used in the production of steel have among the toughest lives of bearings used anywhere.
Rolling bearings used in the production of steel have among the toughest lives of bearings used anywhere In addition to the arduous regime of a continuous production cycle, they are routinely subjected to extremes of heat, including steam, and often have to operate with lubricant contaminated with water and scale
This article was originally published on Engineeringtalk on 22 May 2007 at 8.00am (UK)
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The ever-increasing downward cost pressures on the price of steel mean that failures in the continuous production cycle are extremely costly.
Bearings, of course, are fundamental components in this cycle.
Therefore, despite the many operating difficulties, bearing manufacturers are constantly under pressure to provide ever increasing levels of reliability and operating life from their products.
NSK is a global supplier of rolling bearings to the steel industry, providing products which deliver outstanding performance and reduced maintenance costs in a wide range of steel mill processes, including casting, rolling, finishing, raw materials handling and iron making.
NSK has studied the many operating problems of bearings in the industry and through a research programme has proved that improvements in steel cleanliness (principally reducing the number of non metallic inclusions in a steel), combined with appropriate heat treatment, significantly increases the rolling fatigue life of bearings.
Further reading
State of the art package for gearbox bearings
For gearboxes, NSK now allies the most state of the art design of any spherical roller bearing with a unique bearing material, Super-TF, which provides up to ten times longer life
Big advantages in bearings for steel industry
Super-TF bearing material greatly improves the life of bearings used in all industrial sectors including steel making and can be applied to a wide range of bearing types
Guide helps users avoid bearing failures
Free publication deals with all aspects of bearing procedures, including handling of bearings, mounting, maintenance and inspection, dismounting and bearing storage.
As a result of this research, NSK has developed ultra-clean bearing steels, which greatly improve the life of its products in all industry sectors including steelmaking.
With its commitment to produce the most durable, highest quality bearings possible, NSK is a leader in the development of ultra-clean steels for bearing rings and rolling elements.
The company's ultra-clean steel is a highly pure vacuum-degassed, high-carbon chromium steel containing a minimum of non-metallic inclusions.
Vacuum-melt or vacuum-degassing processes control the amount of non-metallic inclusions in bearing steel, enabling bearings to survive much longer than those manufactured with standard degassed steels.
NSK was instrumental in developing this technology and has pursued it in conjunction with steel manufacturers.
The result of this co-operation has been improved quality steels, with even lower inclusion levels, providing extended fatigue life.
As an indication of what has been achieved: previously bearings meeting these specifications were manufactured from aircraft-quality steel at premium prices.
Experience shows that cleaner steels make all the difference to bearing performance and operating life in applications from electric motors to racing car hubs.
In the steel industry, however, bearings regularly have to cope with exceptional operating conditions, which can impair their life.
Operating under conditions of contaminated or inadequate (boundary) lubrication are two of the major causes of premature bearing failure experienced in large equipment such as rolling mills.
Under arduous and dirty operating conditions, bearing lubricant is easily contaminated with debris.
The debris particles are then indented on the bearing raceways, causing stress concentrations at the indentation edges.
In addition, if the viscosity of the lubricant is not high enough, metal surfaces come into contact and small "peeling" damage occurs, leading to stress concentrations around the contact areas.
The result in both cases is that cracks form from the surface of the material at the stress concentration points, leading to an accelerated rate of fatigue damage and, ultimately, shorter bearing life.
Research by NSK has revealed that bearing life in conditions of contaminated or boundary lubrication can be improved by changing the microstructure of a bearing steel in order to enhance its ability to relax stress concentrations on the edges of indentations.
This microstructure - a material with a high volume of retained austenite and a high hardness - produces a longer life even under conditions where there is an insufficient oil film and surface damage, such as peeling, easily occurs.
The benefits offered by this new material, Super-TF, are many.
In comparison with bearings made of ordinary carburised steel, Super-TF bearings offer up to 10 times the life with contaminated lubrication and up to twice the life where the lubricant is clean.
Where the problem is related to insufficient lubrication, the life of Super-TF bearings is still impressive at around 5.5 times that of ordinary carburised steel.
In addition to extending service life under conditions of boundary or contaminated lubrication, Super-TF bearings also offer increased resistance to wear and seizure as a result of the dispersion of a large number of fine carbides and nitrides in the bearing material.
In comparative tests with bearings made of conventional materials, such as SUJ2 steel, the Super-TF material structure has shown less than one third the rate of wear and a 40% increase in seizure resistance.
The material specification of large Super-TF bearings means that they offer dimensional stability superior to that of SUJ2 or ASTM bearing steel.
This improvement eases the removal and refitting of bearings, reduces vibration and enables the continued running at elevated temperatures.
Super-TF is proving its pedigree in products such as NSK's unique new Extra Capacity Sealed Clean roll neck bearings, a new design that delivers up to 34% extra load capacity and longer life under severe lubrication conditions.
Super-TF technology can also be applied to other types of bearings used in steel-making processes, including cylindrical roller bearings, tapered roller bearings and spherical roller bearings.
NSK's development of the new Extra Capacity Sealed Clean roll neck bearings is an evolution from the world's first such bearing, also developed by NSK in 1977.
Bearings of this type are designed for supporting rolls in the rolling mills of steelworks.
They are required to achieve longer life while experiencing the adverse conditions of high loading, extreme shock loads, and the problem water and scale ingress, which because of its effect on lubricating conditions, probably affects bearing life the most.
The specification of NSK's new Extra Capacity KVS bearings addresses all of these adverse operating conditions.
The bearings combine the enhanced life advantages of Super-TF technology with the benefits of optimised mechanical design and advanced sealing.
The benefit of the optimised seal design is that it creates more space within the bearing to allow a greater number of larger rollers to be accommodated, without compromising strength.
This raises the basic dynamic load rating of the new bearings by up to 34% when compared to NSK's conventional roll neck bearings, equating to as much as 2.7 times longer life.
The role played of Super-TF steel in extending the life of roll neck bearings subject to contaminated lubrication is unique.
In a perfect application, however, all contamination should be denied ingress in the first instance.
This truism has been adopted by NSK in its unique seal design for the Extra Capacity roll neck bearings.
The new seal is designed primarily to combat the effects of negative pressure.
This develops inside sealed roll neck bearings because of internal temperature changes.
It causes water to more readily enter the bearing.
To overcome this, NSK has developed a new bore seal.
Located in a position relatively free from coolant water, the new seal eliminates the build-up of negative pressure and thereby minimises the entry of water, resulting in extended grease life.
Both efficiency and effectiveness of the new seal design have been proven conclusively in exhaustive simulation testing with the bearings showered in water (100litre/min) while being operated across the speed range from 100 to 700rev/min, with a load regime typical of that in the field.
An additional benefit of the new seal design is that both main seals and bore seal are easier to install and remove.
Moreover, with the outside diameter of the main seal reduced, more space is provided to accommodate the lifting jig that is required when fitting the bearings.
Unique materials technology, in the form of cleaner steels and Super-TF material, and optimised design are two elements (the third is patented sealing technology) in a unique synergised package, which NSK is introducing to revolutionise the performance, generally, of bearings used in steelmaking.
Epitomising this approach is the EA Series of spherical roller bearings, which offer the advantages of longer life, reduced heat generation and low wear, plus high strength, and improved heat resistance for operation at high temperatures.
EA Series bearings are used widely in table rollers and many wide-ranging applications in the steelmaking process.
A unique feature of these bearings is that the cage incorporates a roller guide function in place of a guide ring.
Together with other detail design initiatives, this facilitates the integration of a greater number or larger rollers than is possible with conventional spherical roller bearings.
As a result, the load capacity of the EA Series exceeds that of the conventional bearings by between 10 to 20% In addition, the operational life of the EA bearings is significantly extended by between 30 to 80%.
In user terms these twin improvements mean that in many applications bearings can downsized, leading to savings in both equipment size, weight and cost.
Another major benefit of the "next generation" design is that the cage provides excellent guidance of the rollers, thus reducing skewing and skidding.
Consequently, frictional heat produced within the bearing is substantially reduced.
Moreover, the provision of heat stabilised inner and outer rings and cages of pressed steel enable the EA Series bearings to operate at temperatures as high as 200C.
The development of Extra Capacity Sealed Clean and EA Series bearings are just two examples among many of NSK's commitment to producing "tomorrow's solutions today" for the global steel industry.
This commitment is a long term one, with NSK dedicated to providing the R and D resources necessary to ensure improved solutions to the applications challenges of tomorrow. Request a free brochure from NSK UK ...
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