Effect of variation in contact friction on the performance of the under-platform dampers

Under platform-dampers (UPDs) are commonly used devices in turbomachinery to mitigate the turbine blade vibrations caused by the periodically fluctuating stresses. These dampers are placed in the underside of two adjacent blades and vibration energy is partly dissipated by the friction at the blade/damper interfaces. As a result, the vibration amplitude is reduced with beneficial effects on the blade fatigue life. At LAQ AERMEC a novel test rig has been developed to accurately measure the response of a single turbine blade and the kinematics/dynamics of two adjacent UPDs. In this newly developed test rig, each damper is in contact with the under-platform of the blade on one side and with ground/fixed platform on its other side. The dampers are pressed against the blade platform by static forces applied by dead weights. A static force is also radially applied to root of the blade to clamp it to the rig, simulating the effect of the actual centrifugal force in operating conditions. Finally, a transverse periodic excitation is applied in order to excite the blade's first resonances. In this paper, the performance of different UPDs in terms of reduction of the blade vibration amplitude and shift in resonance frequency is studied at two different contact friction conditions (normal and low friction). Low friction conditions are obtained by introducing a thin layer of oil between the damper-blade contact interfaces. Experiments are performed on a real turbine blade to investigate the semi-cylindrical dampers. This profound study of UPDs provides a strong basis to understand the effect of damper with different contact conditions, to limit the blade vibration