Laser Triangulation System Measures Jet Turbine Blade Vibration - MTI Instruments - #1

Text version of the page

MTI Instruments, Inc. 325 Washington Avenue Extension Albany, NY 12205 PH: +1-518-218-2550 OR USA TOLL FREE: 1-800-342-2203 FX: +1-518-218-2506 sales@mtiinstruments.com www.mtiinstruments.com APPLICATION NOTE Microtrak 7000 Non-contact Laser Displacement System Measures Jet Turbine Blade Vibration Introduction Whenever we board an airplane we hardly give much thought to the jet engine sitting on the wing. As it comes to life and spins effortlessly we hear little, if any, vibration or noise in the cabin. Seldom do we realize how much testing and effort goes into ensuring trouble free operation through thousands of takeoffs and landings. Take the turbine blades for example (see fi gure 1). Under normal operating conditions they are subjected to extreme temperatures, pressures, vibrations and forces. To make certain they can withstand these conditions extensive testing is performed in a simulated setting. One such test is the accelerated fatigue test. This experiment excites the turbine blade and determines the number of vibration cycles it can withstand prior to failure. The Test In order to simulate real world conditions turbine blades are vibrated in a sound chamber at a variety of elevated temperatures. A siren directs a blast of pulsed air at the leading edge of the turbine blade creating a tremendous amount of force. This force excites the turbine blade at its resonant frequency, inducing strain in the blade structure. The test continues for months or years to determine how many vibration cycles a particular blade confi guration or design will endure. This provides valuable information to the designer as to how long an engine can operate before experiencing a catastrophic failure. The Problem To accurately monitor vibration amplitude and frequency a non-contact measurement sensor is required. Non-contact eliminates any “mass loading” or dampening of the turbine blade vibration and is not prone to failure or wear. Customarily a CCD camera system is used to view the blade displacement at the leading edge. Although these work well at determining major displacement or frequency changes associated with catastrophic failures they are not able to detect small changes that signify the formation of a tiny fatigue crack. Additionally, their long term stability is poor, making them less than ideal for lengthy tests. A proven high resolution, high frequency response sensor is required. MTII appnote: jetturbinebladevibrations.pdf - Page 1 of 2