Bolt tension on screen
Bolt tension on screen. Image credit: R&D

Danish engineering company R&D A/S has developed the Bolt-Check solution that accurately measures bolt tension using ultrasonic sensors and software analysis.

The system can digitally track individual bolts over their lifetime, which helps ensure that each bolt remains at its optimal tension.

The system is a response to the challenge to achieve correct bolt tension by using the traditional torque and tension methods. It is critical to measure and maintain the right tension of bolted joints in wind turbines to increase safety and reliability and minimise the risk of failure and downtime.

Now that wind turbines increase in size, it is more important than ever to maintain the integrity of the bolted joints in these structures, which need to withstand higher centrifugal and bending forces, as well as stresses and vibrations.

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“We noticed our engineers were often called to perform root cause analysis in relation to loose or broken bolts. We started analysing this issue and found out that the clamping load obtained by applied torque or tension often was found to be insufficient. So, we decided to find a solution for that and this is how Bolt-Check was developed,” says Søren Schmidt Kellenberger, sales director, technology & solutions, R&D A/S.

A wind turbine consists of thousands of different sized bolts holding structures together, often interdependently. Correct bolt tensioning is critical, as one loose bolt can trigger a domino effect that, could result in the failure of the entire turbine.

How the system works

The Bolt-Check system measures the length of the bolt as it lengthens in response to the application of load – torque or tension. These values are converted into accurate clamping force. During the bolt tensioning process at the assembly level, the operator observes the data live on the screen, and stops applying load once the desired tension has been achieved.

Once in service, measurement of the bolt tension is possible without the need to loosen the bolts. This is because the system uses ultrasonic and mechanical length measurements, as well as information about the material properties of the bolt batch. Combining these three values makes it possible to measure and calculate the actual clamping force.

A unique traceability tag is added to each bolt, supporting pre-failure diagnosis and traceability, including the location of each bolt as well as its schedule of service and maintenance. No pre-treatment of the bolts is required.

The system is accurate to within five percent and can be used directly on standard bolts, when installing new bolts or when evaluating existing bolts as part of determining the lifespan of wind turbines.