Dealing with Vibrating Machinery

[bootlegend]

I'm curious how everyone handles designing structures around vibrating machinery. For example, in the situation shown in the attached sketch. I have a small structure supported on a larger structure that also supports a large vibrating screen. The screen weighs about 93,000 lbs and operates at 800 rpm. Currently I'm interested in how you would approach the design of the small structure to avoid vibration issues due to the screen. My approach has been to model the structure and check for natural frequencies that fall within plus or minus 30% of the operating frequency. I then modify design if necessary to change the natural frequency. Additionally, I usually use vibration damping pads and bushings beneath the baseplates.

For those of you with access to RISA, you can see in the attached file that this particular structure has a couple natural frequencies that are close to the operating frequency. I'm limited on support schemes and bracing schemes so I'm having no luck with that approach.

Is it possible to determine whether the damping pads will prevent the vibrations in the structure? I've talked to two suppliers in the past who claim a really high percentage of isolation, but I've never had to use the pads on a structure that shows a natural frequency really close to an operating frequency.

What would be your approach to avoiding vibrations in a structure like this?

 

[bootlegend]

RISA file attached. Was unable to add two files in one post.

 

[SwinnyGG]

I don't have RISA so I can't look at your CAD..

But do you have a vibration signature for this machine, either measured or spec'd by the manufacturer, or are you forced to just assume based on machine speeds??

Much can be done to isolate machinery like this, but having a reasonably good estimate of the true vibration signature is critical- the poorer your estimate is, the more difficult it will be to properly isolate.

 

[bootlegend]

What I have from the equipment manufacturer is the estimated vibrated and static weight, the estimated operating speed and stroke, and the estimated spring resonance. I'm also given estimated vertical and horizontal dynamic loads at each corner to design the support structure. I'm not sure if I'll have any luck with getting a vibration signature but I will see if they have that. Is that the same thing as a time history function of the machine?

 

[Shu Jiang PEng SE]

I am not familiar with RISA.
You may need to perform a response spectra analysis for the screen, small structure and its supporting structure. You need to simulate the damping pad with some kind of “damper” element to cushion the vibration effect on the main structure.

 

[WARose]

Is it possible to determine whether the damping pads will prevent the vibrations in the structure?

Depends of the stiffness of the pad. Modeling that will tell you it's impact on overall performance.

 

[Agent666]

If you have an issue that you cannot avoid via due to resonance of one of the building modes, then an option that might be available is installing a tuned mass damper to dampen out the particular resonance issue you are experiencing.

 

[bootlegend]

simulate the damping pad with some kind of “damper” element

Modeling that will tell you it's impact on overall performance.

Do you model this as a spring support? I have the deflection criteria for the pads based on stress so I could come up with a spring stiffness.

 

[WARose]

Do you model this as a spring support?

Yep.