Fault characteristics and diagnosis method of intershaft bearing in aero-engine

Spectrum and envelope spectrum analysis of vibrations measured on aero-engines were developed in rotary-speed-difference-domain for detection of faults in inter-shaft bearings. Rotary-speed-difference of high pressure rotor and low pressure rotor is used as triggering signal. Vibration signals were sampled at the whole cycles of rotary-speed-difference, and their spectrum and envelope spectrum were analyzed in rotary-speed-difference-domain. The result shows that the position of vibration components caused by rotor imbalance, misalignment and defects in normal bearings(outer race fixed) will change in the horizontal coordinate(octave form in rotary-speed-difference-domain) when the rotary speed changes. However the side-band components in the vibration spectrum resulting from defects in inter-shaft bearings will keep constant space between any two components when the rotary speed changes. This phenomenon is referred to as "constant space" feature. Furthermore, vibration components in envelope spectrum caused by defects in inter-shaft bearings will keep constant location in the horizontal coordinate(octave form in rotary-speed-difference-domain) when the rotary speed changes. This phenomenon is referred to as "constant octave" feature. The experimental results obtained from the measurements on a test rig of inter-shaft bearing with defects in outer race show that the two features really present in the spectrum and envelope spectrum in in rotary-speed-difference-domain respectively.