Vibration Characteristics of Steam Turbine Blades with Straddle Mount (Effect of Contact Condition on Vibration Characteristics)

Abstract

There are several kinds of blade root structures to support centrifugal forces acting on steam turbine blades such as straddle mount type root, T-root, axial or side entry root and fork type root. Steam turbine blades are surrounded by contact surfaces at a cover and the above blade root. It is known that contact conditions affect vibration characteristics of turbine blades. In this paper, the effects of contact surfaces on vibration characteristics of steam turbine blades with straddle mount type root were investigated. For that purpose, developed analysis method was applied, and natural frequencies and non-linear frequency responses of a steam turbine blade with contact surfaces at the cover and straddle mount type root were obtained using this method. From these results, it was concluded as follows. (1) Natural frequencies of the blade gradually decrease with increasing rotation speed due to the variation of contact area of the blade root. (2) The effect of friction damping at the cover was dominant and the effects at others were small. To verify these results, natural frequencies and dynamic responses of the blades during rotation were obtained in a spin pit test. Calculated results showed good agreement with experiment results and the analysis method was verified.