This week: James Walker: Can the industry afford not to check or be late in critical bolt checking?

Woking Surrey, 16/08/2011. Assuring the integrity of all the joints in a wind turbine and tower is essential if reliability is to be maximised, and maintenance and downtime reduced. As wind turbines are becoming larger and heavier, bolted connections are becoming more critical and the industry has begun to realise the importance of tension rather than torque (and hydraulic pressure) when tightening bolts.

Joints can fail if the correct bolt load is not achieved during installation; bolts can fatigue, self loosen or slip during the operation of a wind turbine. Industry guidance recommends that bolt tension should be checked approximately 500 working hours after initial installation; this is called the ‘A’ test and typically 10% of bolts are checked annually although this varies by wind turbine manufacturer and operator. Checking bolt tension is critical as bolts over time are affected by temperature changes, joint relaxation, fatigue, pressure fluctuations and vibration. These factors can cause initiation of fatigue cracks in a bolt or self loosening causing a loss of tension leading to bolt failure and ultimately, downtime. Bolted joint design calculates a bolt tension/joint clamp force that will withstand these working loads incorporates known factors to ensure that a bolted joint is designed to the correct tension. Then in practice, the key factor is being able to achieve the theoretical design tension of the bolted joint on installation tightening.

As 90-95% of all bolted joint failures in industrial applications are caused by insufficient bolt tension on installation, attaining the correct tension at installation is essential to achieve reliability. Traditional torque tightening procedures mean that bolted joints are not tightened in a tension controlled manner and the bolt tension achieved that is unknown. Methods, such as torque and hydraulic tensioning, measure the effort applied and not the tension achieved across the bolted joint.

New bolt tension technologies can help improve the reliability of bolted joints. There is a bolting solution that delivers improved tension control at installation, and throughout the lifetime of the bolted joint. Through smart technology, RotaBolt® converted fasteners consistently indicate within ḟ5% of the selected load. Its accuracy is not affected by friction, as in torque control, or by load transfer errors in hydraulic tensioning. Such technology means that turbine builders and operators have more confidence in the bolted joint and any loss of tension can be much more easily observed and this significantly speeds up bolt checking whilst also reducing the need to carry heavy equipment.

Significant tension can be lost before bolts even become loose or nuts rotate. It has been observed that lines have been marked on standard bolts to give an indication when the bolt has loosened, however this can be a dangerous practice as bolt tension has usually been lost before any noticeable movement has occurred. Lines on bolts may give an indication that vibration loosening has occurred, but bolts can be affected by other factors and no indication will be given. For examples bolts can fatigue, lose their tension and there will be no movement and this loss of bolt tension will not be visible through this technique.

“Tension control is critical to the reliability and safety of bolted joints”, said Rod Corbett, managing director of James Walker RotaBolt. “As wind turbines become larger, achieving the correct design tension of a bolted joint becomes even more important due to the increased service loads. By providing a simple, accurate and continuous measure of bolt tension, innovative bolting solutions provide increased assurance to the wind industry.”