A study of mechanical behaviour of the double-row tapered ring bearing for the main shaft of a direct-drive wind turbine

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Double-row tapered roller bearings (TRBs) are one of the main components of a modern direct-drive wind turbines, TRBs are commonly used to support the main shafts of wind turbines since fault scan lead to the malfunctions and downtime of wind turbines. In recent decades, some numerical approaches have been proposed for calculating the contact force and pressure distribution of double-row TRBs. Nevertheless, most of the existing studies failed to consider angular misalignment between the inner and outer rings as well as the friction force between the rollers and raceways. A fatigue life analysis of roller bearings is typically performed for bearings under constant rotating speed and invariant loading conditions. The bearings used in floating direct-drive wind turbines, they often experience oscillating motions with varying loading patterns ; thus, for which the standard fatigue life analysis is not valid in this case due to the presence of fluctuating loads. Notably, aquasi-static state does not exist for bearing in the actual operating condition. Since the dynamic model is unable to show the detailed dynamic mechanic al behaviour of double-row TRBs such as the contact are a stress, total displacement of bearing components of bearings, velocity and acceleration of rolling elements (by considering the combined radial and axial load), the angular misalignment of roller and inner ring, roller skewing conditions, components vibration characteristics and roller-end flange friction a new general dynamic model was proposed in this section based on the previous studies. This is because most of the previous studies are unsuitable for real working conditions. However, to verify the proposed dynamic model, a simplified finite element analysis (FEA) model was also established using the commercial software ANSYS Work bench. Hence, the data obtained from this dynamic behaviour analysis can be used to implement the fatigue life prediction for the double-row TRBs, which can significantly benefit their design and manufacturing. This thesis presents a comprehensive quasi-static model to investigate the internal load and contact pressure distribution in a double-row TRB by considering the angular misalignment, the combined external loads and friction force. Most importantly, the presented numerical model was validated by other published references and the simplified FEA model. First of all, it was found that a small misalignment angle between the inner and outer rings can cause a significant change in the magnitude and distribution of the contact force and pressure. A double-row TRB with a crowned roller profile exhibits a substantial improvement in contact pressure distribution by eliminating the contact pressure. Peak contact pressure can be significantly reduced on a roller with crowned profile, even with a misaligned bearing. Comparisons of the simulated contact loads and pressure distributions demonstrate that need to consider angular misalignment and friction force in the modelling of large size and heavy-load double-row TRBs. Furthermore, this thesis presents a fatigue life analysis for double-row TRBs under oscillating external load and speed conditions in which the double-row TRB was used to support the main shaft of a large modern direct -drive wind turbine. Meanwhile, the proposed comprehensive and quasi-static model of the double-row TRB outlines the internal load distribution of rollers. The contact pressure of rollers is then provided based on an iterative scheme using the elastic contact model. Thereafter, the basic rating life of the double-row TRB under an oscillating external load and speed is provided to calculate the fatigue life. Numerical simulations were also performed to investigate the effects of the oscillating load and speed, angular misalignment, and internal clearance on the fatigue life of this bearing. Finally, the simulation results of the dynamic model analysis indicated that the combined radial load and pure radial load have a significant effect on the vibration of rollers and the inner ring of double-row TRBs in a floating direct-drive wind turbine. Meanwhile, the angular misalignment of the inner ring also affects the vibration of the rollers and the inner ring itself. With an increase in the misalignment angle, the vibration of roller elements became increasingly apparent. The vibration frequency of rollers and the inner ring gradually decreased with an increasing misalignment angle. Additionally, the vibration of components in the double-row TRBs is sensitive to the initial axial preload.
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