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Equipment vibrations

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Equipment is a new 500 HP electric motor hung below a steel framed free
standing platform with a concrete deck. All members at 30% stress level or
less and short spans. There are no eccentricities or reciprocating load
items. I have been called to do a inspection due to vibrations. Been here
before and for electric motors unloaded its always been a bent shaft which
vendor refuses to admit. Needless to say vibration analysis is tedious and
not that exact anyway. I plan to have the motor go from stop to operating
and am of the opinion that if frame resonance is the issue I should see very
little vibration until I get to the problem frequency. If it is a shaft or
motor problem I believe it will be a linear vibration increase as RPMs
increase. Next step will be to disconnect the gear box and turn on the motor
only for same type observation. I would like some experienced guidance, on
what else to look for, comments on the validity of this inspection, and
would really appreciate any feedback.
Bob Hanson, S.E.
Torrance, Calif.

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