Based on the paper “Fatigue lifetime calculation of wind turbine blade bearings considering blade-dependent load distribution”
Predicting the fatigue lifetime of rotor blade bearings is a particularly difficult task as the use of existing calculation methods requires a comprehensive understanding of the systems and the methods are scarcely validated for the unique blade bearings with their extraordinary load situations.
Hardly any other types of bearings in the industry are subject to such complex loads as rotor blade bearings in wind turbines. Forces and moments in all six degrees of freedom occur stochastically depending on the wind – with almost uniquely bearing dimensions. The bearing itself only rotates a little in order to adapt to the wind, which is exactly what renders the use of existing calculation methods difficult. While reliable calculation methods for fatigue in conventional roller bearings have been around since the 1950s, they cannot be used for blade bearings without further adjustment: they are based on tests of smaller bearings, which rotate fully and experience recurring loads.
For this reason, the National Renewable Energy Laboratory (NREL) evaluated the state of the art in the “Wind Turbine Design Guideline DG03” in 2009, with the aim of being able to calculate fatigue lifetimes for blade bearings as well as azimuth bearings – a calculation also prescribed by the DNV GL since 2016. There are adaptation methods for many special conditions which occur with rotor blade bearings and two methods are proposed for the equivalent load in particular – a decisive value in lifetime calculation. The International Organization for Standardization (ISO) also offers another means of calculating this important value with the ISO 16281.
A comprehensive understanding of the system is required for consideration of all relevant factors of the calculation. At the Fraunhofer IWES, the IWT 7.5 is a system model of a nearshore turbine with 7.5 MW nominal power. For even more application proximity, Enercon has provided an IPC controller for the system model within the scope of the publicly funded Highly Accelerated Pitch Bearing Test (HAPT) project. This makes it possible to calculate the loads affecting a turbine over 20 years with great precision.
There are also sophisticated simulation models available for the loads that the bearing will experience. We are aware that connecting components cannot be ignored when performing the calculation and include them accordingly, so as to allow precise simulation of the deformation of the bearing.
These prerequisites allow the performance of analyses which would otherwise not be possible. For example, the equivalent load can be analyzed in a wide variety of blade pitch angles and compared between the different calculation methods. In this way, it is possible to investigate which blade position damages the bearing and which spares the raceways.
This makes it possible to perform the calculations, which are otherwise usually simplified, in a way which is highly detailed and uncompromising. Nevertheless, results are achieved similar to those seen in other publications on the topic: the calculated lifetime is very short. Too short to be realistic – as rotor blade bearings in the field generally last longer than just 107 days.
Clearly, there is a need for further research here: If detailed, extensive calculations in line with the state of the art are insufficient, the scope must be expanded. At the Fraunhofer IWES, there is already a comprehensive testing infrastructure available for large rolling bearings of various dimensions, which could be used to satisfy the needs of the industry.
Germanischer Lloyd: DNVGL-ST-0361 – Edition September 2016, Machinery for wind turbines, 2016
ISO: DIN 26281:2010-11, Rolling bearings – Methods for calculating the modified reference rating life for universally loaded bearings (ISO/TS 16281:2008 + Cor. 1:2009), 2010a.
Harris, T., Rumbarger, J., and Butterfield, C. P.: Wind turbine design guideline DG03: yaw and pitch rolling bearing life, 2009