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Isolating Vibrations Problem 1

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trazimcalvina

trazimcalvina

Mechanical
Nov 20, 2020
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Hi all,

Recently I was given a task of reducing the spread of vibrations through a welded steel structure. I have attached an image of the structure below. Now the structure is approximately 10 meters long and 3 meters wide, it is supporting a vibrating conveyor whose vibrations are to be prevented from spreading onwards into the steel structure below.

Now I havent had much experience in this kind of work, but after a short internet search two possible solutions immediately came to me. One involved placing Molded Steel Isolation Bearings (rubber pads with steel plates) on top of the vertical columns, or directly beneath the plate connecting the structure with the vibrating conveyor.
The second option involved installing a Restrained Spring Isolator system near the top of the vertical columns, by turning the connection with the horizontal beams into a bolted connection and adding the isolator. I have also added an image of this.

How do these ideas sound to you?

The steel structure:

Isolators in columns

Vibration pads (Molded Steel Isolation Bearings)


 
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Also if in seismic areas important to understand that the seismic load to vibration isolated equipment is increased significantly compared to rigidly attached equipment.
 
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dL said:
Also if in seismic areas important to understand that the seismic load to vibration isolated equipment is increased significantly compared to rigidly attached equipment.
Click to expand...

I didn't know that. Does the flexibility in the isolators add a whipping / dynamic amplification to the motion of the supporting diaphragm? How does one go about addressing such a thing without getting over-fancy? I'd have to think that, relative to the period of the diaphragm, there would be some isolator frequencies that would get meaningfully amplified and others that would not.
 
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Once you get your head around thing it becomes what looks like a complex problem quickly becomes much more easily solvable. I've dealt with vibrating conveyors before and seen what they can do to steel structures.

I also recently dealt with a vibrating sieve which was causing vibration that could be felt/seen/measure 100m away. See this thread for more details. [link ][/url]

Getting your steel well away from the vibrational frequency of the machine is often a good start when building new. Sometimes this means stiff, other times it means flexible. It depends on the frequencies. But the biggest gains are generally from dampers, especially when you are retrofitting.

If you read the previously linked thread you can see I started down the track of stiffening the structure. But I later calculated that if anything that could make things worse! Suitable spring & elastomer dampeners were recently installed and reduced the vibration transmission 'significantly'. The theoretical value is 92%. I've yet to get back on site to measure the actual reduction.
 
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Who is the conveyor manufacturer? It seems like they are leaving money on the table by not addressing this issue. Do they provide any guidance on mounting and support?
 
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KootK said:
Does the flexibility in the isolators add a whipping / dynamic amplification to the motion of the supporting diaphragm?
Click to expand...

This is the way that I understand it. The increased loading is evident in ASCE 7-16 Table 13.6-1. The Fp force for Vibration-isolated components is increased compared to their non-isolated counterparts.

ASCE lumps the effect into the Rp factor mostly which suggests that the ductility of these isolated components is reduced.
 
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HTURKAK said:
I would look for rubber mounts similar to the following ..
My opinion....
Click to expand...
Respectfully no.

Rubber mounts like that are too stiff to absorb much of the vibrations at the frequencies and amplitudes we need here. In my experience these frequencies at sub 30hz and often sub 20hz. I am assuming that the conveyor in question is also operating in this range.

Those sorts of mounts are often entirely useless at the lower frequencies. How do I know? Because I've seen them used many times and I've seen how ineffective they are. Oh and the calculations say so too!

I normally try to avoid mentioning specific products but I figure I will here.

amc2_scl3pb.jpg

I've found these GREATLY effective.

While I did my own calculations it was also very helpful when talking to the supplier for them to provide me with the calculated results.
temp_vtg8y0.png


Finally the product manual does provide an excellent primer in the basics of vibration absorption:

AgMechEngr said:
I have used these before. The manufacturer can assist you with sizing.
Click to expand...
Agreed. I only read your post after my long post above. That product looks like it would be quite effective. Their ideal frequency ranges seems less selectable than the other brand I linked. But the product is likely cheaper.
 
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AgMechEngr said:
human909, good post. For reference, the last time I used the isolators I suggested, the frequency of oscillation was 25 Hz.
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Thanks and thanks. Many of our items vibrate at 24hz, but the most severe ones are at 16hz and 16hz is far worse for most steel structures in my experience.

I have found a local supplier of the product you linked and will definitely be investigating these and seeing how they compare to the type I've previously used. I work in design an build so I and my colleagues are always interested in more economical or practical solutions.
 
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HTURKAK said:
Rubber mounds and cushioning plates are effective for natural frequencies btw 14-25Hz..Pls look to the attachement...

and ,
Click to expand...
True. Those rosta ones look the business. I've examined that style before though not used them. They seem to have sufficient available movement to suitably absorb the degree of motion required at the frequencies required.

The Rectangular SAW™ Mountings don't look suitable for the application in my experience nor most of these And my rudimentary reading of the charts suggest this is the case. But it depends on the loads of course. For reference the natural frequency of the setup I used to dampen a 16hz vibration by 92% was 4.4hz.

My experience comes from direct experience with many of these rubber mounts sold as antivibration mounts. And also some basic calculations.

I believe many of the standard available anti vibration mounts are more suited for higher frequencies and lower amplitudes. Things like combustion motors or other rotary procession devices. Vibrating conveyors or sieves operate at lower frequencies and higher amplitudes.
 
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Rostas are common in my area. One concern I have is the longevity of the rubber and the inclination to settle over time, so that clearances are not constant. However, they are effective.
 
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