Most wind turbines use gearboxes to transform the relatively low rotational speed of the main shaft into higher speeds required for power generation. Because the forces a modern wind turbine gearbox has to transmit are forever increasing while the gearbox itself has to remain compact, single- or two-stage planetary gearboxes are commonly used. The bearings of the planetary gears are among the highest-loaded in this specific application. In modern gearboxes an integrated planetary bearing arrangement is used, in which cylindrical roller bearings without an outer ring are fitted in the planetary gears, with the bore of the gear forming the outer raceway.
Modular systems of cylindrical roller bearings
The modular concept is based on single-row cylindrical roller bearings of the RN range, which do not have an outer ring. These bearings are equipped with a one-piece solid brass cage, which is guided by the inner ring (MPB). This cage features inspection grooves for endoscopic inspection of the inner raceways as standard.
These single row bearings can be assembled to sets of two, three or four bearings (see Fig.) to conform with the individual load and design parameters of a specific gearbox.
Ready-to-install bearing sets
NKE supplies ready-to-install matched bearing sets. The tightly toleranced overall height of each individual bearing and their precise matching of these ensures an even distribution of the radial loads. Each bearing set includes the components required to secure the bearings axially in the planetary gears – such as retaining rings and loose rib rings. If required, the overall width of each bearing set can be adapted to the operating conditions and the available space with spacer rings fitted between the individual bearings.
Used consistently throughout the gearbox assembly, the modular concept has numerous advantages for the user: the designs of planetary bearings can be simplified; the bearing design can be standardised for the first and second gear stage and across various gearbox sizes; the required number of bearing variants is reduced; the modular design simplifies the adaptation of bearing set characteristics to the requirements of each gear stage; the one-piece solid brass cage provides excellent rolling element guidance even during rapid acceleration; and the lubrication grooves provided as standard improve the lubrication of the planetary gear bearings.
Each bearing set is packaged as a unit and is marked to prevent inadvertent swapping of matched components within each set. All sets are subjected to a stringent quality inspection of their functional characteristics before leaving NKE’s factory in Steyr.
Beside rolling bearings for wind turbine gearboxes, Austrian bearing specialist NKE develops and produces standard and special bearings for all industrial applications and provides product development, application engineering and a full range of technical, consulting, documentation and training services. NKE’s products are distributed through 15 international representative offices and more than 240 distribution outlets in 60 countries.
Daniel Thalmann, team leader in the technical service of NKE Austria GmbH, is responsible for ensuring first-rate technical support for customers.
Design team leader Faruk Duskunovic, together with his design team at NKE, is responsible for creating the drawings of the specified product features and of new design ideas for standard and special bearings.