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.