Research into materials engineering has been particularly productive, with the development of Z and Super Tough (Super TF) steels that yield better lifetime performance than high carbon chromium bearing steel (SAE52100, SUJ2), which is usually employed in the construction of rolling bearing elements.
For example, Z Steel, with lower oxide non-metallic inclusions, delivers significantly extended service life (up to 1.8 times longer, according to NSK) compared to that of conventional vacuum degassed (SUJ2) steel.
Even greater life performance is delivered by Super TF steel. As a result of its higher chromium and residual austenite content, the service life of a bearing made of Super TF material is claimed to be some ten times longer than that of a bearing manufactured from standard materials.
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 coating is applied by plasma spraying to achieve secure bonding to the bearing steel, and is covered with an acrylic resin to ensure high electrical resistance.
Non-electrically conducting hybrid bearings that use ceramic balls are similarly suitable for generator shafts. These lightweight bearings also have good thermal resistance, low thermal expansion and longer life. NSK also uses the same ceramic material - silicon nitride (Si3N4) - for the rolling elements in its hybrid bearings.
Fit for purpose? Putting it to the test
With so much at stake out in the field, it is essential that the properties of these rolling bearings are tested on application-specific test rigs that simulate the actual operating conditions to which they will ultimately be subjected.
The first of these rigs is the Rotor Shaft bearing test stand, where rolling bearings designed for use in multi-megawatt turbines can be thoroughly analysed. Radial and axial loads and bending torques can be applied, in static and dynamic modes.
The second rig is a high-speed shaft rolling bearings test stand. Just as with the rotor shaft bearing test stand, static and dynamic forces as well as torque can be applied to the rolling bearing.
In addition to testing, there is one other essential parameter that must be assessed –an estimation of bearing life. In order to ensure the reliability of these estimations, NSK has developed appropriately optimised methods and calculation procedures.
STIFF 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 a software application called ‘STIFF’.
This is a program that takes into account various key parameters, including precise 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, and 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 method also offers a time-saving means of testing special rolling bearing adaptations.
Finite element analysis (FEA) is also employed by NSK to examine the distribution of stress factors within the bearing and its components and thus provide optimum support for non-standard applications, while frequency analysis is applied to the bearing’s acoustic emissions. From the analysis results, performance and operating characteristics of the bearing system can be more readily understood.
Matching bearings to applications
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.
Megawatt-class wind energy systems often combine a planetary 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.
The 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.
Not even the oil pumps in turbine installations escape NSK’s attention. Here, 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 that are provided for this application.
Need more information?
More comprehensive details of all NSK’s bearings for wind energy applications are contained in a free 12-page publication. Copies are available via NSK’s Freephone service: 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’.