Evaluation of Turbine Blade Root Damping

A test program has been conducted to measure the vibration damping of three designs of steam turbine blades. These blades were tested in their root attachments, sectioned from retired rotors. Blade loading was accomplished in a new type of blade damping test rig and damping was evaluated in the lowest tangential and axial blade modes. The test environment was room temperature air. The damping rig concept and development is presented in detail as well as results of an intensive test program. The blade designs are designated as Types A, B, and C, where Types A and B were of a fir tree root design and Type C was a ball and shank design. A total of 86 blades were tested and the influence of vibration amplitude, applied axial (simulated centrifugal) load, and vibration mode on damping logarithmic decrement was studied. For the designs studied, the most important findings were: a) Logarithmic decrement was inversely related to blade centrifugal load. High centrifugal loads resulted in low damping ratios. b) Logarithmic decrements varied in an almost linear manner with vibration amplitude for most tests. Constant logarithmic decrement conditions were not observed. c) The damping test rig worked well in this test series. Long LP blades may cause problems without some design changes. d) Results were highly reproducable per each blade group tested. Typical scatter was observed when comparing the results of one blade group to another. e) Changing the initial excitation amplitude did not cause any changes in the vibration decay. There is one characteristic decay trace for each centrifugal load. Test data was reduced by hand, which caused some difficulty as the damping ratio approached 1.0. An error in the third decimal place could greatly effect the logarithmic decrement value. It is recommended that a micro-processor be used in future tests