Bolt Inspection Case — ST100 Inspects Bolts on Wind Turbines That Have Been in Operation for 10 Years

ST100 Safeguards In-Service Wind Turbine Units In early 2021, a customer used Bosheng Technology’s ST100 product to conduct bolt axial force inspections on wind turbine units at a northwest wind farm that had been in operation for nearly ten years. During testing, it was discovered that certain bolts on specific flange surfaces of the tower exhibited very low measured axial forces—approaching zero kN. After verifying that all instrument settings were normal, it became apparent that the actual axial forces of these bolts were likely much lower than expected. Subsequent tests revealed several additional bolts in similar conditions. When one of these low‑force bolts was tapped with a tool, it was found to be loose, and its nut could be removed. The tested unit was randomly selected from among the wind turbines at this wind farm, each operating for nearly a decade. This demonstrates that even traditional torque‑based bolt inspections—conducted as spot checks over nearly ten years—can overlook critical issues, leaving some bolts with severely insufficient axial forces and potentially compromising operational safety. The ST100 provides wind farm operators with a reliable verification tool for reviewing torque inspection results. The ST100 electromagnetic ultrasonic bolt stress meter is truly nondestructive, requires no coupling agent, and enables rapid, direct measurement of axial forces (stress) in in-service bolts. It can quickly detect under‑tightened or missed‑tightened bolts, while test data can be stored, managed, and automatically compiled into reports. Because each bolt generates unique test data, using the ST100 for axial force inspections helps prevent omissions or errors, offering customers a new approach to bolt axial force monitoring and verification. Figure 1: Axial force distribution of bolts on the first section of the tower for a tested unit. Figure 2: On-site testing reveals abnormally low axial force on a particular bolt. Figure 3: A bolt that was not properly tightened can have its nut easily loosened.

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