Application of Torsional Vibration Measurement to Blade and Shaft Crack Detection in Operating Machinery

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, nonintrusive measurement methods. This paper primarily addresses the results of application of this nonintrusive torsional vibration sensing to: a large wind tunnel fan; a jet engine high-pressure disk; a hydro station turbine; and to large induced-draft (ID) fan motors at a coalfired power plant. 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. Field installation at a hydro power station demonstrated that the few shaft natural frequencies were visible, and correlated well with finite element results. Field installation on the ID fan motors also showed the first few shaft torsional modes. Finally, 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. The results of these field tests have resulted in high confidence that this technique is practical for diagnosing and tracking shaft and blade cracks.