For several years T-bolts have been a popular choice for joints in the field of wind energy, specifically for connecting the blade roots to the hub of the wind turbine. Their use is mandated by the geometry of the joint and they perform very well under pure axial loading. However recent analyses have shown significant bending stresses in the T-bolts preventing their use to their full capacity. These bending stresses are unavoidable due to the presence of a slew bearing between the blade root and the hub. The bending stresses generated while loading the blade root, causes the blade root-bearing joint to gradually open, causing excessive loading of the T-bolt above a certain load.
It is hypothesized that modifying the blade root design to reduce the effects of local bending can open up the possibility of reducing its mass and cost. To test this hypothesis, the blade root is initially studied and the stress ratio is identified as an appropriate joint performance parameter. The performance of the joint is boosted by increasing the pretension of the bolt. After an initial phase of over designing the joint to reduce the constraint stresses, the joint optimization is carried out using the Sequential Quadratic Programming algorithm. The optimization culminates with the mass reducing by roughly 110Kg and the material cost reducing by approximately 13% per blade root. The number of bolts reduces from 88 to 52. Thus, a simpler design is achieved, that promises simpler and cheaper manufacturability, higher reliability and lesser sites for crack nucleation in the laminates. The current design strategy at Suzlon is to employ a greater number of T-bolts with thinner shanks. Curiosity in the field of cost optimization that initiated from within Suzlon has proved that there exists a different design strategy that holds great promise for delivering structurally equivalent if not better designs with improved cost, mass and reliability.