In the adverse and constantly changing environmental conditions, both on and offshore, efficient and continuous power generation from wind turbines demands top performance from every component – especially bearings. As a major supplier to the global wind energy sector, NSK is addressing these operating conditions by providing individually tailored, environmentally conscious bearings that deliver outstanding levels of reliability, resistance to heat and seizure, and exceptionally long life.
As part of its development of wind turbine bearings that are ever more reliable, longer lasting and robust – even at high speeds – NSK harnesses and co-ordinates the capabilities of its technology centres in Europe, America and Asia. These centres conduct research and development in four core technology areas: Tribology, Materials Engineering, Analysis Technology and Mechatronics.
Investment in these four core technologies is really paying off for the global wind industry. For example; research into materials engineering has led to the development of Z and Super Tough steels that produce much improved life compared with high carbon chromium-bearing steel (SAE52100, SUJ2), which is usually employed for rolling bearings. Z Steel, with lower oxide non-metallic inclusions, delivers significantly extended service life, up to 1.8 times longer, compared with conventional vacuum degassed (SUJ2) steel. Even greater life performance is delivered by Super Tough (Super TF) steel. As a result of higher chromium and residual austenite content, the service life of a bearing made of Super TF material is up to 10 times longer than for one manufactured from standard material.
The performance and versatility of Super TF technology means that it can be used in a wide range of bearing designs: cylindrical roller bearings, taper roller bearings, spherical roller bearings, deep groove ball bearings and angular contact ball bearings.
A further innovation by NSK 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. The ceramic is applied by plasma spraying to achieve secure bonding to the bearing steel. The 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. This 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.
The 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. The 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.
A second rig is a Test Stand for high-speed shaft rolling bearings. This 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.
In addition to testing, the nature of application specific bearing designs means that a bearing 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 developments include optimised methods and calculation procedures that increase accuracy for estimating bearing life.
STIFF Innovation software
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. This is a program 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. The resulting model divides the rolling elements into cross-sections. A modified service-life rating is determined for each cross section. These data are then integrated using the time components for each instance of loading.
The scope of the STIFF calculation software is impressive, and the application delivers results that enable rapid parametric analysis. The 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.
The net result of the design initiatives taken by NSK in the four key areas is a product range that delivers precise products for the widest variety of applications in the wind energy sector.
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 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.
The 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.
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.
The final area of wind turbine operation where NSK provides tailor-made – or standard – bearings 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.
Free Publication for Wind Energy
More comprehensive details of all NSK’s bearings Wind Energy applications are contained in a free 12-page publication. Copies are available via NSK’s Freephone service: Tel: 0500 2327464. Callers should quote the catalogue reference: Ref: WIN/A/E/08.09, and also the title of the publication: ‘Premium Technology for the Wind Industry’. For further information abut NSK’s bearings please go to www.nskeurope.com.