Feature Seals & bearings Better bearing life boosts efficiency
The working conditions of a cement plant do not foster long service life for machinery, due to dust, heat, moisture and vibration. Therefore it is essential that every opportunity is taken to improve the efficiency and reliability of the plant. Bearings are common to most pieces of equipment, so by concentrating on improving the service life of this critical component, significant savings can be made. NSK Europe has developed several materials and technologies to improve bearing performance
oor lubrication and high contamination levels are respon- sible for 80 per cent of premature bearing failure and therefore make up the two key factors in determining bearing life. The cement industry uses both wet and dry processes to manufacture the finished product, resulting in both dust and moisture causing problems for the bearings. Cement dust is hygroscopic, meaning that it absorbs the lubricating oil from the bearing grease, resulting in a grinding paste inside the bearing. In such environments, general maintenance and re-lubrication is essential to ensure continued reliable performance, however, fresh lubricant can quickly become contaminated and cause failure itself. Where this is the case, pure maintenance will not resolve the issue; there is a require- ment for materials technology, which can take the form of NSK's Hi-TF and Super-TF bearing materials.
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NSK has developed these special bearing steels specifically for harsh operating conditions and they have proven effective already in both the steel and mining industries. Following on from this success, they have been made available for products used in the cement industry in the latest ranges of spherical roller bearings, high capacity EM and EW series cylindrical roller bearings, as well as tapered roller andball bearings. NSK has examined the main issue of contamination of lubricant, literally under a microscope. The debris parti- cles become indented on the bearing raceway, causing stress concentrations at the edges of these indentations. Secondly, if the viscosity of the lubri- cant is not high enough, then the metal surfaces come into direct contact, leading to stress concentrations around the contact areas. In both cases cracks form on the surface of the mate- rial at the stress concentration points which leads to an accelerated rate of fatigue damage and, ultimately, shorter bearing life.
Factory Equipment JULY/AUGUST 2013
Tough steel microstructure The solution to this issue lay in the microstructure of the steel used to make the bearings and the heat treat- ment process. NSK has developed Hi-TF and Super-TF with a unique microstructure which is able to relax the stress concentrations on the edges of the indentations. These materials have a high volume of retained austen- ite and improved hardness when com- pared with conventional bearing materials. All of this innovation has resulted in greatly increased bearing life when operating in contaminated conditions. Based on customer requirements, NSK will suggest which material is right for the application. When compared with bearings made of ordinary carburised steel, Hi-TF offers up to seven times the expected life with contaminated lubricant, while Super-TF even improves on that by offering up to 10 times the expected bearing life with contaminated lubricant. Where the service life is reduced by insufficient lubrication, Super-TF can still provide around 5.5 times (4.7 times for Hi-TF) the life expected from a similar bearing made of carburised steel. However, results from a laboratory need to be proven in the field and NSK has been pro- viding solutions to many industries including mining, steel production as well as cement production, all of which offer extremely harsh operating environments. Two recent solutions for the cement industry demonstrated the effectiveness of Hi-TF to resolve persis- tent bearing failures: the first, a screw pump conveying system and the second, a high temperature fan.
NSK Super TF range bearings have been developed specially for harsh operating conditions
In the case of the screw pump con- veying system, a large angular contact bearing was suffering from contamina- tion with cement dust. This resulted in wear to the bearing raceways and increased clearances to such an extent that the bearings had to be changed to avoid a catastrophic failure. NSK carried out a detailed site survey resulting in a recommendation to install Hi-TF bearings, which would provide increased resistance to the contamination as well as running quieter and smoother, while also offer- ing increased shock load protection. Since the installation of the Hi-TF bearings, the plant has reduced bearing consumption by more than 50 per cent and saved on production losses and maintenance time as a result. In the second case, a cement plant was experiencing regular bearing failures on a fan that was operating at 150˚C. Typically, the plant had to replace the original split bearings at least every six months, which was considered to be unacceptable. Examination by NSK of the original bearings showed that high temperature and heavy load were the root cause for the premature failure. NSK recom- mended the use of Hi-TF spherical roller bearings, mounted in plumber blocks and fitted with labyrinth seals – a combination capable of continuous operation at 200˚C under heavy load. The result was a cost saving for the plant of over €9.130 per year on the operation of the fan.
A further example of the benefits of these new materials comes from an Australian iron ore mine where a drive pulley on a critical conveyor would only survive for six months before replacement. The mining environment contains a lot of contaminants and in this case it caused premature bear- ing failure to a critical pro- duction line component. The solution was a bearing design using Super-TF mater- ial, which offers an effective countermea- sure to the wearing action caused by the contaminated
lubricant.The operating life of the bearing was increased by over four times that of the original design. NSK UK T: 0500 2327464
www.nskeurope.com Enter 302
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