Real time bolt measurement

Paul Boughton
An experiment on a wind farm in Texas has demonstrated new applications for Norbar’s USM-3 ultrasonic bolt meter, which provides precise measurements for the elongation (resolution to 0.0001 mm) and load in threaded fasteners from 25mm to over 15m long.

This experiment showed how it can also provide precise real-time measurements of forces affecting a bolted joint, in this case a 2m long foundation bolt on a wind turbine tower.
The initial length of the bolt was recorded using the USM-3 meter and a magnetic transducer placed onto the bolt end.  

After a while, could be noted that the elongation measurement seemed to be unstable and the value was constantly drifting. This was because, as the wind increased, it was pushing the tower in the direction of the foundation bolt, so that the joint was compressed slightly and the elongation decreased. The opposite  occurred when the wind decreased.
Norbar’s ultrasonic specialist Gregory Young explained: “This ability to read minute fluctuations in the bolt tension, real time, can be of value to many companies dealing with bolted joint issues. Many sectors such as the automotive industry will also be interested, since real time changes in bolt tension are highly important in automotive engineering.”
Ultrasonic measurement performed by the USM-3 works on a 'time-of-flight' principle, similar to sonar, with a small transducer placed on the head or stud end of the fastener sending an ultrasonic sound wave through the length of the bolt and back again, with easy to read data displayed on an LCD screen. The USM-3 incorporates digital signal processing and graphic displays of signal analysis and waveforms to make this a truly user-friendly instrument.

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