Bolt Inspection Case Study — An OEM Uses the ST100 to Investigate Axial Force Degradation After Bolt Assembly in the Workshop

A well-known wind turbine manufacturer, leveraging the Bosheng Technology ST100 electromagnetic ultrasonic dual‑wave bolt stress meter, is investigating the axial force degradation of bolts connecting the pitch bearing to the hub after in‑shop assembly.

[Video] Solution — In-Service Bolt Dynamic Load Testing in Situ

Bolts in rotating components of wind turbine generators—such as blade bolts, pitch bearing bolts, and main shaft bolts—are subjected to cyclic loading during operation, and the magnitude of this operational load varies with the turbine’s operating conditions. When assessing bolt fatigue life, it is typically necessary to measure the actual operational loads experienced by these bolts under different operating conditions.

Bolt Inspection Case Study — An OEM Uses the ST100 to Investigate Axial Force Degradation After Bolt Assembly in the Workshop

A well-known wind turbine manufacturer, leveraging the Bosheng Technology ST100 electromagnetic ultrasonic dual‑wave bolt stress meter, is investigating the axial force degradation of bolts connecting the pitch bearing to the hub after in‑shop assembly.

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.