Roulements - 2010-04-19
Rolling bearings are key components in ensuring continuity of production in cement industries. If they fail, whole production processes can come to a halt, resulting in costly downtime. General maintenance such as re-lubrication is, of course, important in ensuring that bearings operate reliably. But in the dusty, grit and moisture-laden atmospheres encountered in cement plants, lubricant can quickly become contaminated, becoming a cause of failure in itself. Where this problem is prevalent, no amount of maintenance will solve it. Instead, the solution lies with new materials technology, in the shape of NSK’s Hi-TF and Super-TF bearing steels.
The ability of Hi-TF and Super-TF steels to provide extra life in difficult operating environments has been proven in the steel and aluminium producing industries. As a result of this success, NSK has made the material available for use in cement production, with its latest ranges of spherical roller bearings new high capacity EM and EW series of cylindrical roller bearings, and tapered roller and ball bearings.
Hi-TF and Super-TF steels are an answer to the problem of premature bearing failures resulting not only from contaminated, but also from insufficient lubricant. These failures occur because, under arduous and dirty operating conditions, bearing lubricant is easily contaminated with debris (i.e. cement dust). The debris particles are then indented on the bearing raceways, causing stress concentrations at the identation 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.
Hi-TF and Super-TF overcome these problems with a unique microstructure, which is able to relax the 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.
In comparison with bearings made of ordinary carburized steel, Hi-TF offers up to 7 times the life with contaminated lubricant, and up to 1.5 times the life with clean lubricant. Super-TF material improves on these figures by offering 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 is still impressive at around 5.5 times (4.7 for Hi-TF) that of ordinary carburised steel.
Besides extending service life under conditions of boundary or contaminated lubrication, both Hi-TF and Super TF bearing steels offer increased resistance to wear and seizure as a result of the dispersion of 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% (20% for Hi-TF) increase in seizure resistance.
The effectiveness of Hi-TF in cement production has been demonstrated by two recent solutions it has provided to persistent failures problems: the first on a screw pump conveying system at a cement plant in the USA; the second on a high temperature fan at a cement plant in Europe.
In the screw pump conveying system, a large angular contact bearing was experiencing dust contamination. Hygroscopic dust was wearing the raceways and increasing clearance to the point that, to avoid catastrophic failures, the bearings had to be changed prematurely, and on a regular basis.
NSK recommended Hi-TF Steel™ as a solution to this problem, asserting that the material would manage the problem by offering resistance to the contamination. In addition, the new bearings would run quieter and smoother and offer shock load protection.
Since installing the new Hi-TF bearings, bearing consumption at the plant has reduced by more than 50%. As a result of this success, the plant has since converted other locations to Hi-TF bearings.
In the second Hi-TF application, a cement plant in Europe was experiencing regular bearing failures on a fan that was operating at 150°C. Typically, during an 18-month period, the original split bearings would fail at least three times: a failure rate that was considered unacceptable.
Examination of the application itself, and of the split bearings, revealed that a combination of high temperature and heavy load was causing the premature failures of the split bearings. As a solution, NSK recommended the use of Hi-TF spherical roller bearings mounted in plummer blocks fitted with labyrinth seals. This combination is capable of continuous operation at 200°C under heavy load. The success of this solution has resulted in a cost savings at the plant of €9,130 per annum on the operation of the fan.
Hi-TF and Super TF are two solutions to problems of contaminated lubricant in cement plants; however, NSK has developed another solution for bearings that are difficult to access and/or are used places where oil and grease absorbing dust is produced. This technology, Molded Oil™, can transform the performance of bearings used in machinery and equipment - in terms of reliability, maintenance free intervals and operation - in environments exposed to contamination. It also enables plant operators to make real cost savings by replacing existing, and often costly, methods of lubricating machine parts.
Molded Oil™ is a plastic containing lubricating oil. By providing a lubricating oil content of more than 50% by weight it improves on other marketed oil impregnated plastics, where the oil content is a mere few percent by weight. The technology comprises a lubricating oil in a polyolefin matrix. The characteristics of the material mean that a good lubrication regime is maintained. The mechanism of oil discharge from the matrix is temperature dependent; the higher the heat generation, the higher the oil discharge rate.
Molded Oil™ bearings effectively address the problem of lubricant loss in bearings, due to water ingress. They are ideally suited to many areas of cement production including pre-heaters, conditioning towers, fans, conveyors, precipitators and conveyors.
NSK is offering Molded Oil™ technology on standard deep groove ball bearings and spherical roller bearings in outside diameter sizes up to 250mm. Prior to filling the bearings with Molded Oil™ their interior surfaces are specially treated. As a result bearing torque is not much higher than that of standard grease-lubricated bearings.
In addition to its materials and lubricant solutions, NSK has also addressed the operating problems inside cement plants with improved labyrinth sealing on its mounted bearing products. These are widely used in cement plants due to their traditional time saving, easy mounting and maintenance-free benefits.
Designed for easy fitment on conveyors, fans and general handling equipment, the mounted bearing offer a robust construction that includes high efficiency steel reinforced seals and cast iron bearing covers for added protection against dust. The combination of the cast iron dust cover and sealing mechanism provides the highest levels of protection for the spherical ball bearings in heavily contaminated environments. Moreover, the flexibility of the dust cover design enables a variety of configurations to be supplied to suit different shaft mounting requirements.
Complementing the performance of the dust cover, the reinforced rubber seal provides additional protection against dust and moisture ingress with two lips that provide comprehensive sealing upon a shaft yet minimise frictional torque. In addition to its safeguarding role, the seal also aids installation, offering the capability of absorbing up to +1% of shaft misalignment without deterioration in its performance.
In addition to highly efficient sealing, the life performance of the mounted bearing units is also optimised with a totally new type of spherical roller bearing: the HPS™ (High Performance Standard) series.
Designed for use in all areas of cement plants – raw grinding, electrostatic precipitators, kilns, finish grinding and conveyors – the HPS™ series delivers innovative performance with twice the running life (as the result of minimising slippage) compared to conventional SRB of the same size, and a maximum limiting running speed that has been improved by up to 20%. These performance advantages provide significant benefits in cement, extraction and construction industries, by reducing maintenance and energy costs and – often – enabling product downsizing.
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