A BENCHMARK FOR TIP TIMING MEASUREMENT OF FORCED RESPONSE IN ROTATING BLADED DISKS

The Blade Tip-Timing is a well known non-contact measurement technique for the identification of the dynamic properties of rotating bladed disks. Even if it is an industrystandard technique its reliability has to be proved for the different operation conditions by comparison with other well stablished measurement techniques. Typically a strain gauges system in conjunction with radio telemetry is used as reference. This paper aims at evaluating the accuracy of a last generation Tip-Timing system on two bladed dummy disks characterized by different geometrical, structural and dynamical properties. Both the disks were tested into a spinning rig where a fixed number of permanent magnets, equally spaced around the casing, excites a synchronous resonance vibration with respect to the rotor speed. The so called beam shutter method was adopted for the Tip-Timing system. Due to the presence of shrouds a particularly set up of the probes was chosen in order to avoid that the probes look radially inward at the blade tips as in the most common configurations. he probes are optical laser sensors pointing at leading and trailing edges locations where the blade experiences the greatest magnitude of displacement. The Blade Tip-Timing measured data are post-processed by two different methods, the Single Degree of Freedom Fit (SDOF) and the Circumferential Fourier Fit (CFF). The amplitude and frequency values at resonance obtained by the Tip-Timing system are compared with those obtained by the strain gauge measurements