Engine & Turbine Technology
A further innovation in materials technology, ceramic- coating of bearings, ensures protection and longer working life in applications where standard bearings are exposed to electrical corrosion: for example, in the generators of wind turbines. Te ceramic is applied by plasma spraying to
achieve secure bonding to the bearing steel. Te ceramic coating is then covered with an acrylic resin that ensures high electrical resistance. Hybrid bearings that use ceramic balls feature outstanding performance characteristics, such as thermal resistance, longer service life, light weight and low thermal expansion. Moreover, the ceramic balls are not electrically
conductive. Tis makes these hybrid bearings also particularly well-suited for use as generator bearings. NSK also uses the same ceramic material silicon nitride (Si3N4) for the rolling elements in its hybrid bearings. Te constantly increasing performance of today’s tailor-made bearing designs for wind turbines means that it is essential to test the properties of these rolling bearings on application-specific test rigs that simulate actual operating conditions. Te first of these rigs is the rotor shaft bearing test stand. On this rig, rolling bearings designed for use in multi-megawatt turbines can be thoroughly analysed. Radial and axial loads and bending torques can be applied, whereby the loads can act statically and dynamically.
Te second rig is a test stand for high-speed shaft
rolling bearings. Tis enables the rolling bearings used on high-speed shafts to be tested on a specifically designed test rig. Just as with the rotor shaft bearing test stand, static and dynamic forces as well as torques can be applied to the rolling bearing.
Estimating bearing life In addition to testing, the nature of application specific bearing designs means that a bearing solution must consider all aspects for high capacity and efficient design. One criterion is of paramount significance in this respect: estimating bearing life. For a long time experts have been aware of the fact that many features need to be considered for an estimation to be reliable. NSK’s latest developments include optimised methods and calculation procedures that increase accuracy for estimating bearing life. In ISO 281, annex 4, the calculation of modified
service life rating is based on simplified rolling bearing geometry. In order to increase the accuracy of the results of these calculations, NSK has developed the so-called STIFF software. Tis is a programme that takes into account key parameters, including exact interior geometry, operating clearance and pre-load, deformation of the shaft bearing system, load area and the load distribution between rolling elements and raceway. Te resulting model divides the rolling elements into cross sections. A modified service-life rating is determined for each cross section. Tese data are then integrated using the time components for each instance of loading.
Te scope of the STIFF calculation software is impressive, and the application delivers results that enable rapid parametric analysis. Te system also makes it possible to save time when testing special rolling bearing adaptations.
Another example of the design tools employed
by NSK is FEM analysis. Finite Element analysis examines the distribution of stress factors within the bearing and its components and thus provides optimum support for non-standard applications. Frequency analysis, on the other hand, examines noise generation of the rolling bearing within the application. From the analysis results, performance and operating characteristics of the bearing system can be understood. Te net result of the design initiatives taken by NSK in the four key areas is a product range that delivers precise solutions for the widest variety of applications in the wind energy sector.
Bearing stiffness In rotor shaft bearing arrangements, where bearing stiffness is paramount, NSK supplies spherical, cylindrical and taper roller bearings mounted either in the classic fixed end/loose end bearing arrangement or in a floating bearing arrangement. Both are particularly well-suited for this application. NSK is also at the forefront in megawatt-class wind energy systems that often combine a planet gear stage with multiple spur gear stages to facilitate differential transmissions. In these systems, deep groove ball bearings, spherical roller bearings, cylindrical roller bearings, taper roller bearings and four-point contact ball bearings are used, depending on the location of the bearing.
Te same set of bearing types is also provided for yaw gearboxes that turn the nacelle into the wind or away from it. Pitch gearboxes are required for rotor blade adjustment. Deep groove ball bearings, cylindrical roller bearings, spherical roller bearings and taper roller bearings are typically installed in these gearboxes.
Te generators used in wind turbines primarily use deep groove ball bearings and cylindrical roller bearings. It is in these units that transmission of electrical current can damage the rolling bearings and shorten their service life. In order to avoid this damage, the use of insulated
rolling bearings is usually recommended: hybrid bearings with ceramic rolling elements, or coated rolling bearings where an insulating coating has been applied to the bearing outer rings. Te final area of wind turbine operation where NSK
provides tailor-made – or standard - bearing solutions is oil pumps. In this application, the gear teeth used to drive the pump exert radial and axial forces on the deep groove ball bearings, cylindrical roller bearings or taper roller bearings in this application. ●
For more information,
www.nskeurope.com www.engineerlive.com 43
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