Exploration of a New Method for Dynamic Load Testing of Bolts

Bolts in rotating components of wind turbines—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 operating loads experienced by these bolts under different operating conditions.

Traditional testing methods involve using pressure‑pad sensors or strain gauges bonded to bolts.

Figure 1: Pressure Washer Sensor

Figure 2 Schematic diagram of the pressure washer sensor and its connections

As shown in Figure 2, this illustrates the washer-type sensor and its field installation. For bolts of different specifications, the pressure-washer sensor must be custom‑made to match the bolt size.

When using a washer-type pressure sensor, as shown in Figure 2, it is installed between the nut (or bolt head) and the flange face. Because the sensor has a certain thickness, washers must also be placed between the sensor, the flange, and the bolt. Consequently, the original bolt length of the tower structure typically cannot accommodate this mounting configuration and requires… Customize bolts of the appropriate length. Therefore, although the pressure sensor boasts high accuracy, the actual structure of the bolt differs from its final operating condition, leading to deviations in the measured bolt load.

Figure 3: Bolt for attaching the strain gauge during tensile testing machine calibration

This method involves bonding strain gauges directly to the bolts, eliminating the need for custom‑made extension bolts. However, it requires pre‑installing the gauges on the bolts and performing tensile calibration for each bolt. During field testing, the gauge‑equipped bolts must be installed in the designated bolt holes. Consequently, this approach necessitates gauge application and calibration prior to testing, as well as precise positioning of the gauge‑fitted bolts during on‑site installation.

Both of these methods require dismantling and reinstalling the bolts; for bolts that have already developed issues or for the surrounding bolts, It is not possible to measure the original bolt condition. 。

Is there a method to measure the dynamic load on a bolt without removing or disassembling it?

We propose a measurement method that combines dual‑wave electromagnetic ultrasonic techniques with single‑wave electromagnetic ultrasonic techniques:

Ready to use Electromagnetic Ultrasonic Dual-Wave Bolt Stress Meter ST100 Axial force measurements are performed on in-service bolts, followed by dynamic load testing using electromagnetic ultrasonic single‑wave technology. This approach enables the determination of the original axial force of the bolt without disassembly, while also allowing for real-time measurement of the bolt’s operating loads, thereby providing a novel testing methodology for investigating bolt fatigue and loading conditions.

Figure 4 In-situ measurement of the axial force in the pitch bearing bolts