Fatigue Strength of Turbine Shafts with Shrunk-on Discs

Abstract

In 1956–57, the shafts of three large built-up steam turbine rotors developed fatigue cracks in service under very low nominal working stresses. The fatigue cracks occurred at changes in shaft section and unexpectedly large strength reduction factors were realized. The failures have been substantially explained by fatigue tests on the shaft materials, in which normal ratios of fatigue strength/ultimate tensile stress were obtained with plain specimens up to 2 5/8 in in diameter, and strength reduction factors up to 10 were obtained with model rotor shafts about 4 in in diameter with shrunk-on collars. Additional tests have verified that modifications introduced into service at the time of the failures were effective in substantially increasing fatigue strength, and have shown the effect on fatigue strength of variations in the geometry of shafts and wheel bores.