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Re: 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.
It's not clear what the problem is--that is, what you're expected to fix. 
I gather someone's noticed that the platform shakes when the motor runs. 
I guess I'd start by finding whether the culprit is the motor or what 
ever the motor is driving. If it happens when only the motor runs there's 
your culprit. If everything needs to be turning, and the motor's OK it's 
in the machinery. You can look for a bent shaft by turning over the 
system slowly and checking the runout. If there's an imbalance in 
anything that's turning, it won't show up as a deformation. What happens 
in that case depends on the location of the machinery with respect to 
some soft point in the structure. Probably the best solution is to try to 
find exactly what's vibrating and stiffen that. If the motor is mounted 
on a soft floor it'll vibrate when the rotational frequency matches the 
corresponding frequency. Or the motor may be exciting some other point. 

>From what you've mentioned, my first guess is that the motor is mounted 
to a soft floor. Sounds a little like a re-fit--someone adding machinery 
after the building was built. Try jumping near the motor and get an idea 
of the floor frequency. Then run up the motor and see where you get the 
resonance. If you find to imbalance in the motor, you'll probably end up 
stiffening the floor. Make sure you aren't adding mass at the same rate 
as you're adding stiffness, o rit won't do much good.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw


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