Equipment Rack Vibration
Equipment Rack Vibration
(OP)
Preface the post with the post with only a fundamental (pun intended) understanding of dynamics. I was tasked with reinforcing an existing planar unistrut equipment rack (all members are back-to-back), designed by others, comprised of 2 posts 54" tall with 3 cross beams 54" long supporting 680# of equipment/rack in an industrial facility. There's a large compressor engine that, when operating (I was told it's running at 330 rpm), causes the rack to vibrate up to 2" (1" +/- from vertical). The posts are orientated in the strong axis to this vibration and the stiffness is approximately 1.1 k/in. A local contractor has already specified a redesign of the entire frame, but the owner thinks it is excessive. There is room behind the unit for angled braces from the floor to the top of the rack.
Looking just at the frequencies, a basic hand calc for the frequency of the rack, f = 1/T = 2*pi*sqrt(m/k) = 4.1 Hz, compared to the engine running at 5.5 Hz. If I remember back to university, these two shouldn't be so close together. Isn't there a ratio between the these frequencies to limit the vibration? 2x? 3x? Basically, there is going to be added bracing, but I want make sure the logic is there before specifying anything. And if the frequency of the rack is, say 3x the engine, does this take out the vibration? Am I looking at this the right way? Also, would neoprene pads under the unit help without bracing?
Probably spending more time than necessary, but it's an interesting task that I have not yet encountered. Thanks in advance.
Looking just at the frequencies, a basic hand calc for the frequency of the rack, f = 1/T = 2*pi*sqrt(m/k) = 4.1 Hz, compared to the engine running at 5.5 Hz. If I remember back to university, these two shouldn't be so close together. Isn't there a ratio between the these frequencies to limit the vibration? 2x? 3x? Basically, there is going to be added bracing, but I want make sure the logic is there before specifying anything. And if the frequency of the rack is, say 3x the engine, does this take out the vibration? Am I looking at this the right way? Also, would neoprene pads under the unit help without bracing?
Probably spending more time than necessary, but it's an interesting task that I have not yet encountered. Thanks in advance.






RE: Equipment Rack Vibration
Did you mean 1" lateral deflection?
RE: Equipment Rack Vibration
RE: Equipment Rack Vibration
This could also be a case where the actual frequency of the structure is different from the theoretical frequency. Any chance you could put the equipment on isolation pads or something? That's often the easiest solution.
RE: Equipment Rack Vibration
RE: Equipment Rack Vibration
RE: Equipment Rack Vibration
What kind of compressor? centrifugal, reciprocating or ????
Without pictures etc this is absolutely a wild guess, but I'd be kind of surprised if well done diagonals did not reduce the motion of the treehouse mezzanine significantly.
4 (or more) spindly legs stuck up in the air ain't much of a machine base. Unacceptable Horizontal motions are likely even when resonance is not in the vicinity.
http://36.media.tumblr.com/374cc3da38e8364cb890ed9...
https://s-media-cache-ak0.pinimg.com/236x/0a/97/15...
Is the rack adjacent to a nice concrete wall that could receive a few braces?
RE: Equipment Rack Vibration