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MECHANICAL JOINT INTEGRITY


FEATURE SPONSOR


BOLTING IN THE WIND INDUSTRY PROBLEM AND SOLUTION


Bolts on wind turbines will inevitably work loose or may even fail over time, which means that preventive maintenance is a critical issue. With the number of bolts on a wind turbine numbering into the hundreds, it can be time consuming for maintenance teams to check each bolt. Juan Carlos Casas, an expert in bolting control at Norbar Torque Tools, has a solution.


THE PROBLEM


The tips of the biggest wind turbines can nowadays turn at speeds of circa 180mph but operating at this velocity can place enormous strain on the structure of the turbine, in particular the bolts. As turbines have got bigger and blades longer to capture more wind, vibration levels have increased still further which has left maintenance teams looking for better ways to measure tension (rather than torque) and prevent bolt failure.


Whilst bolting checks may not seem like a time consuming issue, the problem for the wind industry is the sheer number of bolts that need to be maintained. Specifically, there are likely to be circa 100 bolts linking the turbine base flange to the foundations with another 350 linking the middle and top section and a further 50 bolts for each blade. The time cost of further tightening each of these bolts is a significant expense and one which adds considerably to the total cost of ownership for the turbine. Tube joints, for example are checked every six months.


THE SOLUTION


So, what is the answer? A better method of preventive maintenance would be to use ultrasonic bolt measurement, which not only has the capability to improve the quality of their joint status but also increases security by adding data-logging techniques to the procedures.


In certain critical bolting applications, factors such as friction or bolt/joint geometry are such that the measurement of applied torque alone will not provide the degree of accuracy necessary to control the tension to be monitored over the operational life of the fastener. In these applications, ultrasonic measurement of fastener elongation and/or load has frequently proven to be the most accurate, reliable and cost effective solution.


For every joint type there will be a relationship between the extension and the induced tension which means that measuring the extension allows accurate calculation of load. This is the ultimate level of bolting control that safety critical bolted assembly operations need to work towards.


Ultrasonic measurement of bolt preload is made possible by introducing a sonic pulse at one end of the fastener and accurately measuring the time of flight (TOF) required for the echo to return from the opposite end. Using material constants, the ultrasonic measurement tool, such as the Norbar USM-3 converts this TOF into an "ultrasonic length" of the fastener, providing a baseline from which future measurements can be made.


When the fastener is tightened, the TOF increases and the USM-3 will again utilise material constants to eliminate the effects of stress and temperature variations on sound velocity, providing an accurate elongation or load measurement.


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www.windenergynetwork.co.uk


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