Failure Mechanisms of Main Bearings in Vertical-Axis Wind Turbines: A Comparative Review

Vertical-axis wind turbines (VAWTs) have emerged as a promising alternative for wind energy generation, particularly in offshore environments. However, their reliability continues to be limited by a critical component: the main bearing, which constitutes a major bottleneck in operation and maintenance (O&M). Analysis of its loading and operational conditions reveals complex scenarios, notably uneven radial load distributions on bearing elements, aerodynamic vibrations, and particular rotational dynamics, which render the VAWT main bearing highly susceptible to failure. To identify the damage mechanisms influencing its reliability, this review adopts a systematic comparative approach that leverages extensive failure data from various bearings used in horizontal-axis wind turbines (HAWTs). The findings indicate that these bearings are prone to failure during both operational and stationary periods, primarily due to contact fatigue and fretting damage, with stationary failures in offshore environments potentially progressing into tribocorrosion phenomena. By clarifying the domain of applicability of life prediction existing models and mitigation strategies, this review provides a structured framework for interpreting reported bearing failures in VAWTs and for guiding future experimental, modelling, and design efforts aimed at improving bearing reliability.