Innovative Design in Expander Wheel-Shaft Attachment Replaces Hirth Coupling for High Speed, High Temperature and High Power Applications
Expander-Generator unit for a geothermal power plant application experienced ongoing failures during a ten year period. The longest run cycle for the machine was 18 months yet with less than 10 MW power production. The average time between failures was less than six months and with each failure; the wheel was completely damaged and separated from the shaft. A complete metallurgical study on previous failures showed the hirth coupling attachment was the first to fail in all cases. Using fully coupled non-linear contact point FEA analysis and benefiting from advancement in machining techniques, new four-lobe polygon shaft-wheel attachments were designed for high speed, high power and high temperature applications. The redesign reduced the hub centrifugal stress by 30 percent, allowed the shaft to be inserted inside the wheel to hold the wheel C.G. on the shaft, and increased the power transfer capacity of the rotary parts. By holding the wheel on the shaft, other problems like high vibration, electrical current passing through the unit, and incorrect blade resonance could be addressed and resolved. For more than two years, the redesigned machine has run continuously without any failures and with constant low vibration producing up to 12 MW, at minimum 20% above any previous operating points.
Shokraneh, Houman (2013). Innovative Design in Expander Wheel-Shaft Attachment Replaces Hirth Coupling for High Speed, High Temperature and High Power Applications. Texas A&M University. Turbomachinery Laboratories. Available electronically from