The Bolt-Check tool reduces preventative maintenance time, resulting in service cost reductions while maximizing uptime; Reduced health and safety costs.

Share article

Hinnerup, Denmark, 17 August, 2021 – A necessary but time-consuming task performed by the wind turbine technicians is to check for any loss of tension in the thousands of bolts holding the structures together. Using conventional methods, each bolt must be re-tensioned individually, a tedious exercise often taking days; but now, with “Bolt-Check” developed by Danish wind turbine tools expert, R&D, the bolt tension can be checked much faster, with higher accuracy and full traceability. This results in a time saving of up to 50 percent, and technicians additionally benefit ergonomically thanks to the reduction in the repetitive actions involved when continually handling heavy tightening tools during maintenance.

In Wind turbines the bolted joints need to withstand higher centrifugal and bending forces, as well as stresses and vibrations. Incorrect bolt tension in wind turbines is a common complication that can both result in expensive call-out charges and downtime which, at worst, can result in the failure of the entire turbine. Checking the condition of the bolts to ensure safe and reliable performance is therefore an important maintenance task. Each manufacturer provides service instructions that detail when preventative maintenance should occur. This maintenance work can be time consuming and cumbersome as the tightening tools are often large and heavy and may not be optimal for health and safety. Furthermore, maintenance can be costly as the tools and the subsequent wear and tear resulting from retightening can be significant. Capable of checking more than 100 bolts every hour, the Bolt-Check tool is light and hand-held and thus offers significant health and safety improvements. Maintenance costs are significantly reduced as only the ultrasonic sensor head gets worn over time.

“An offshore wind turbine standing idle can cost as much as 8,000 dollars a day,” says Selmer Nielsen, Senior Project Engineer, R&D Test Systems.  “Multiply that by 200 for a good-sized offshore wind park and the costs quickly become sky-high. With the Bolt-Check tool, we can reduce the costs of preventative maintenance as the service time can be halved. Because the turbine is back up and running quicker, it can generate energy and therefore money sooner. That is a simple cost-saving calculation,” explains Selmer Nielsen.

Competitively priced with traceability feature

Unlike other ultrasonic-based systems, the Bolt-Check tool does not require specially manufactured bolts but can be used with any bolt. Bolt-Check can be implemented at a cost of three to four euros per bolt. This compares to specialized bolts costing 10 times that or more, depending on the bolt size, that are required for use with competing systems. A unique traceability tag is added to each bolt in the Bolt-Check system, providing a detailed documentation trail that includes the location of each bolt as well as its service and maintenance schedule.


Accurate to better than 5 percent, the “Bolt-Check Standard” version of the tool offers a superior advantage compared to traditional torque-and-tension methods, for which the error tolerance can be as much as 30 percent. “This improved accuracy has the potential to lead to reduced failure rates, with the related benefits of reduced repair costs and increased uptime,” adds Selmer Nielsen.

Service technicians benefit and human errors reduced

“If this saves technicians burnout from repetitive torqueing by simply “ping” testing, [it] could be a game changer,” one service technician commented. Bolt-Check offers the simplicity of the “ping” test while using ultrasound to obtain a highly accurate result. Because Bolt-Check documents measurements electronically, potential human errors are correspondingly reduced, and full traceability is achieved.

As the number of wind power plants increases year-on-year, so does the need for service technicians, which means their time is a scarce resource. Using the Bolt-Check tool, valuable manpower can therefore be used more efficiently since the time spent on bolt inspection is halved.

The principles behind the patent

This unique, patented approach provides bolt tension results to the necessary level of accuracy required in a fraction of the time of conventional techniques. While using ultrasonics alone for tensioning bolts is not new, the patent, awarded to R&D Test Systems in 2020, makes use of the fact that the speed of sound in a bolt under tension is slower than an unloaded bolt, meaning the measured distance is longer than it should be. For a given, calibrated batch of bolts, the difference between the ultrasonic and mechanically measured distances is directly related to the tension in the bolt. This enables accurate tension measurement without having to loosen each bolt first.

Bolt-Check won an innovation award in 2019 at the Wind Energy Denmark 2019, and it has obtained DNV-GL product certification, allowing the equipment to be used for service and maintenance inspections as an alternative to torque or tension tools. The Bolt-Check tool is currently in operation in both offshore and onshore plants.

An offshore wind turbine standing idle can cost as much as 8,000 dollars a day.

Flemming Selmer Nielsen
Senior Project Engineer, R&D Test Systems