Online monitoring system of preloads for multiple bolted joints in wind turbines

Bolted joints are critical components in wind turbine structures, particularly at the interface between the hub and pitch bearings. Loosening or failure of these joints can lead to bolt fatigue fractures, posing serious safety risks and causing significant economic losses. To address this issue, a real-time online monitoring system was developed to visualize and track the preload status of multiple bolted joints in wind turbines. The system integrates an ultrasonic acquisition module, data transmission module, preload calculation module, and data processing module. The ultrasonic module measures the time-of-flight (TOF) variation within each bolt, which correlates with preload changes. To ensure reliable performance under fluctuating outdoor temperatures, a temperature compensation model was introduced in the preload calculation module. The system's accuracy and reliability were first validated in laboratory conditions. Subsequently, it was deployed on a utility-scale wind turbine in Shanxi Province, China, where it continuously monitored the preload of bolts connecting the hub and pitch bearings over a seven-month period. The results confirmed that the system could reliably detect preload variations in real time. Future improvements will focus on enhancing ultrasonic signal strength in corroded bolts, simplifying the calibration process, and reducing system cost.