White paper: Thick-walled ADI castings to boost the torque density of wind turbine gearboxes
Trend of bigger wind turbines installed on lower wind sites has become evident for onshore wind turbine component suppliers. As rotor size increases, the input torque to be transmitted through drivetrain also increases. This puts torque transmitting components under increasing strength requirements. A smaller and lighter gearbox leads to decreasing material spend, lower logistics expenses, simpler and lighter supporting structures and smaller lifting equipment.
While developing the concept for the next generation Moventas Exceed gearboxes, it was noticed that a higher strength cast material was needed to fully benefit from the new technologies. The highest strength grades of ductile irons according to standard EN 1563 have already been successfully applied in previous gearbox concepts, but there was a need for higher strength material. After concept cost value analysis, the most feasible solution was observed to be austempered ductile iron (ADI).
ADI provides excellent combination of strength and ductility with reasonable price increase compared to that of high strength cast steels. It has been successfully applied in industrial gearboxes and other industries since its invention in late 1970’s, and while having a few successful patented solutions in wind turbine applications, its implementation is very limited. To gain confidence in implementing ADI and familiarizing with austempering heat treatment cycle parameters and mechanical properties for design, simplified experiments were carried out prior to full scale prototype production.