A broken or cracked shaft is a typical source of nonlinear vibration phenomena, so you should expect a broadband increase of the spectrum level in the whole frequency range. Depending on the nature of the damage, you could also notice peaks at rotating frequency and its integer multiples, probably with exponentially decreasing amplitude.
Goodbye
hello michelle. our fitters thought it was a cracked shaft or maybe a loose commutator. the motor was run at 1500rpm/25hz no load conditions.there is no previous spectra. the spectra shows multiple harmonics of 2x 3x 4x high 5x line frequency,possible electrical problem?i am going to run the motor and adjust the speed to 1400 rpm and see if the these peaks follow the motors rotational speed
if you want to see the spectra just let me know.
thanks for your reply
"First rule of cracks" per Bently Nevada: "If a shaft is cracked, it almost certainly is bowed." If you can get a dial gage on the inboard end of the motor, check for excessive radial runout. A crack usually shows up as 1X vibration, sometimes 2X. I suspect you have some sort of looseness (commutator or bearings?) based on your description of the spectral plots.
