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Non-Intrusive Stress Measurement System (NSMS Blade Tip Timing)

Blade tip timing (BTT) uses probes of various designs to detect the arrival time of individual rotor blades at a number of points around the rotor casing. From these times it is possible to calculate the tip displacement and, using mathematical algorithms developed specifically for the application, the blade frequency and amplitude. By referring to previous correlation data or the blade FEM the blade stresses can be deduced. The off-line analysis software requires experienced, but not specialist, operators. Probe location in the engine giving sight of the blade tip is essential.

Rotadata manufactures various BTT items of parts and equipment which can be used by the customer to build up his own BTT system.

Rotadata manufactures various BTT items of parts and equipment, which can be used by the customer to build up his own BTT system. The use of BTT systems provides a quick and reliable alternative to the strain gauging of rotating engine blades to evaluate the likely functionality of the blades under operating conditions. The NSMS system is an alternative to strain gauge instrumentation for measurement of blade dynamic stresses. NSMS instrumentation consists of a set of probes mounted on the engine casing that accurately determine the arrival time of a blade tip at a particular case location. Optical, capacitive and eddy current probes can be used for the arrival time measurement. Rotadata provide equipment for two alternative NSMS systems one for the ‘cold’ end of the engine (fan and LT compressor) and the ‘hot’ end of the engine (HT compressor and turbine). Our ‘cold’ BTT system utilises eddy current (optical) probes and the ‘hot’ BTT system utilises capacitance probes. At present, the technology for BTT is changing all the time and we suggest that customers contact us with their specific requirements in order that we can provide the most suitable system for any BTT application.

NSMS probes can be configured to measure any component of the dynamic deflection such as axial, tangential, radial.

Once the amplitude, frequency and nodal diameter are determined, the dynamic blade deflection can be converted to stress from mode shape information that is obtained via a lab test or Finite Element Model (FEM).

Rotadata provide the following two BTT systems.

1. BTTF8 - a BTT system for use up to 300ºC operating gas conditions (typically engine fan an LT compressor arrangements). Scope includes eight off fibre optic probes and laser optic modules, laser junction box, timing card, interconnecting cables and a rack mounted PC configured with BTTF08 software.
2. BTTCAP8 - a BTT system for use in 300ºC up to 1,000ºC operating gas conditions (typically engine HT compressor and turbine arrangements). Scope includes eight off capacitance probes and modules, timing card and connections, interconnecting cables and a rack mounted PC configured with BTTFCAP8 software.