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Equipment Rack Vibration

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.



RE: Equipment Rack Vibration

How are the slotted bolt connections holding up to that kind of vibration? I'd put a torque wrench on a couple of those bolts to see if they've loosened. If they are loose, then I'd consider replacing those connection with something a bit more tolerant of vibration. If they've held, I'd like to know that too.

Did you mean 1" lateral deflection?

RE: Equipment Rack Vibration

(OP)
Teguci, I have been told that all the bolted connections are still "snug", and yes 1" lateral deflection. I was also skeptical of the 2" claim, but don't have any evidence to the contrary with photo or video.

RE: Equipment Rack Vibration

There are some good documents related to foundations vs vibrating equipment that I'm aware of. But, they usually have something like avoiding frequency ratios of 0.8 to 1.2 (i.e. within 20% of the frequency of the equipment). And, you seem to be okay by that criteria.

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

(OP)
JoshPlum, that was my initial thought on the isolation pads. But there was push back because the owner did not want to unhook all the piping and equipment connected to the rack before lifting the rack to install the pads.

RE: Equipment Rack Vibration

Looking at your stiffness, 1.1 k/in, you basically have 2 cantilevered posts (3 1/4" x 1 5/8") without any frame action. Just putting some bracing in there should get rid of a lot of problems. There is no reason a strut frame, properly braced this time, couldn't handle that kind of mass. I still recommend checking the torque on the original connections. Should be 50 ft lbs for 1/2" bolts.

RE: Equipment Rack Vibration

Got pictures?

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

1" of movement, sounds like the contractor was on the right track. I bet the motor balance is way out of spec too.

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