Fretting Fatigue Conditions for two Types of Turboalternator Rotor

By means of a finite element model, the mating conditions between the end thread, located on the body of a shaft, and the thread of a ring, which supports the stud bolt that connects the internal and external conductors of a turboalternator in use at the ENEL (the Italian State Energy Corporation), are simulated. The contact pressures are calculated between the threads and their relative displacement, taking friction also into account, and conditions are identified that are characteristic of the phenomenon of fretting fatigue. Some considerations on the local risk situation allow the conclusion to be made that, in all probability, the structure is not in danger, even if the safety margin cannot be assessed precisely enough. The same analysis has been carried out for Darlington-type turboalternators that, as is known from the literature, have fractured. The two situations are compared and the safety coefficients for both types of rotor are deemed to be reliable.