Paper 9: Fretting Fatigue in Keyed Dovetail Root Fixings

Dovetail pole root fixings, consolidated by tapered keys, are a feature of many large-diameter salient pole alternator rotors. This paper describes an investigation into the failure in service of such a fixing; the failure being ascribed to fretting fatigue and resulting from a combination of tensile centrifugal loadings and superimposed oscillatory loading. The failures were reproduced on a series of full-scale joint sections and the design of a special rig, capable of applying typical oscillatory forcing superimposed upon a steady tensile load of 100 tons, is described. The rig was used to assess quantitatively the effect of various measures taken to eliminate the cause of failure. The test specimens were hand fitted to normal manufacturing methods and, despite the inevitable variations in fit, failed in a consistent manner. The results of the tests provide a practical example of fretting fatigue, and are generally in line with the findings of a fundamental investigation using small specimens and published in 1967 by Field and Waters. The dramatic improvement resulting from relatively minor geometrical changes is clearly demonstrated.