In this paper, a long-term fatigue damage analysis method for gear tooth root bending in wind turbine’s drivetrains is presented. The proposed method is established based on the ISO gear design codes which are basically developed for gears in general applications, not specifically for wind turbine gears. The ISO procedure is adapted and further improved to include the long-term fatigue damage of wind turbine’s gears. The load duration distribution (LDD) method is used to obtain the short-term stress cycles from the input load time series of global response analysis. Dynamic loads and load effects in the gearbox are obtained by two dynamic models; a simplified approach and Multi Body Simulation (MBS) method. A good agreement between these two methods is observed. The long-term fatigue damage is then calculated based on the SN-curve approach by considering all short-term damages and the long-term wind speed distribution. Finally, the reliability and service life probability of failure considering load and load effect uncertainties is calculated. The procedure is exemplified by a 5 MW gearbox designed for a pitch controlled, bottom-fixed offshore wind turbine.
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