Building Up Suitable Contact Forces in Integrally Shrouded Blade Rows for Reducing Vibration Amplitudes

The beneficial effects of the contact between shrouds are described extensively in recent literature: natural blade frequencies are increased and additional damping is available. Different models are proposed for analyzing its linear and nonlinear behavior, selection of optimum contact forces are proposed for reducing vibration amplitudes to a minimum. Results from different non linear analyses that use different models all based generally on a reduced modal model of the blade row and on the harmonic balance approach for modeling the non linear contact forces, are sometimes contradictory: some claim e.g. that increasing excitation amplitude leads to a reduction of the dynamic magnification factor (due to friction damping increase) some other claim the opposite. The contribution to this topic of the present paper is the analysis of the effect of a “contact shim” which can be inserted in a cavity between adjacent shrouds. The shim generates suitable contact forces between the shrouds of the blades of a row, which without shim would vibrate as free standing blades.