This is a little bit of a spinoff from:
http://maintenanceforums.com/eve/forums/a/tpc/f/7161085912/m/5111080763

Do you attach any significance when you see multiple pole pass sidebands around LF in the current spectrum. For example:
LF +/- 1*Fp
Lf +/- 2*Fp
Lf +/- 3*Fp

Is you see a high sideband LF +/- 1*Fp accompanied by the +/-2*Fp, +/-3*Fp, do you assign it more importance than if it were just the LF +/- 1*Fp alone?

The EMPATH Analysis Manual states:
'If the 2nd/3rd slip harmonics are within 10% of each other: Rotor degradation is most likely in the form of cracks.

If the slip fundamental and its harmonics all decrease: Rotor degradation is most likely in the form of broken bars.

If there are more than three slip harmonics: The slip (pole-pass) appears to have more than three harmonics in the demodulated spectrum. This is most likely a mechanical frequency related to rotation of some highly stressed component, such as a tight belt, a worn gear, speed reducer, or shaft whip. Multiple harmonics can also signify blow holes in cast aluminum rotors.'

I have seen cases wherein only the lower PPF sideband was strong, inspection of these rotors did not reveal any damage. Nowadays, I ignore such cases. I guess these would have been eccentricities with no speed fluctuations.

Secondly, there are occasional cases where the 2xPPF sideband is higher than 1xPPF sideband. I will try to hunt that data & post. These turned out to be casting voids. However, I have also seen casting voids where only 1xPPF sidebands exist.

Regards,

Aditya