Abstract
In this study, the relationship between tightening torque and axial force of bolt at the tower top flange
was investigated through a laboratory test with flange section models and a 750 kW wind turbine which had
experienced the damages of bolt at the tower top flange. Tightening torques and axial forces of bolt in these
tests were measured directly by using torque wrenches and strain gauges inserted in the bolts. Existence of
lubricant had a great influence on the axial force of bolt, while types of torque wrench had a small impact on
those. The expected axial force of bolt was obtained by torque wrenches and the variation was around 10%.
A method for quantitatively evaluating decrease in axial force of bolt was proposed by measuring rotation of
bolt. The damaged bolts in the wind turbine were also investigated and the axial forces at the damaged bolts
were completely lost, while those at the loosed bolts were more than 30% of the initial value. It was found
that the axial force of bolt decreased by about 20% after 500 hours from the exchange of bolts. This
indicates that retightening is necessary for wind turbine tower bolts.
