Blade and shaft crack detection using torsional vibration measurements Part 3: Field application demonstrations

The primary goal of the this paper is to summarize field demonstrations of the feasibility of detecting changes in blade and shaft natural frequencies (such as those associated with a blade or shaft crack) on operating machinery using non-contact, non- intrusive measurement methods. Laboratory demonstration of feasibility and some special signal processing issues were addressed in Parts 1 and 2 [1, 2]. Part 3 primarily addresses the results of application of this non-intrusive torsional vibration sensing to: a large wind tunnel fan; a jet engine high-pressure disk; a hydro station turbine; and to a large coal- fired power plant induced-draft (ID) fan motors. During the operation of rotating equipment, torsional natural frequencies are excited by turbulence, friction, and other random forces. Laboratory testing was conducted to affirm the potential of this method for diagnostics and prognostics of blade and shafting systems. Field installation at the NASA Ames National Full-Scale Aerodynamic Facility (NFAC) reaffirmed the ability to detect both shaft and blade modes. Installation on a high-pressure (HP) disk in a jet engine test cell at General Electric Aircraft Engines demonstrated that the fundamental mode of the turbine blades was clearly visible during operation. Field installation at a hydro power station demonstrated that the first few shaft natural frequencies were visible, and correlated well with finite element results. Finally, field installation on the ID fan motors also showed the first few shaft torsional modes. These field tests have resulted in high confidence in the feasibility of the application of this technique for diagnosing and tracking shaft and blade cracks in operating machinery.