Support framework for rotating centrifuge

[Hemifun]

First off, I am not a mechanical engineer. I'm a structural engineer and my last exposure to anything like this was a course in structural dynamics in grad school 35 years ago. What I have is a centrifuge which rotates at 3200 rpm which is going into a wastewater plant. The centrifuge, which comes on its own skid and has vibration isolators, will be set on 2 steel WF beams which will be simply supported and have a span of about 14 feet. The centrifuge will be placed in an existing building which has a suspended concrete slab floor and the proposed beams will transfer the loads to walls in the basement. Intuitively, I tried to design the beams to be as stiff as possible. Based on loads provided by the centrifuge manufacturer, I should only have about .04" of deflection. However, the question has been raised about possible vibrations and/or effect of resonance. I have calculated the natural frequency of my steel beams and it appears to be about 940 cycles/min. The centrifuge engineer is not much help but he thinks that the only time we might have a problem is at shut down as the centrifuge slows down from 3200 rpm to zero. I'm thinking of putting vibration isolators under midspan of each beam to help damp any possible vibrations. Am I going to have a problem with vibrations and possible resonance??? Any help or guidance here will be greatly appreciated.

 

[Tmoose]

Machinery on above-grade installations often has resonance problems. The 3D stiffness of your structure must be MUCH stiffer than the isolators.

Was the "steel beam resonance" a vertical resonance of the beam with centrifuge aboard?

Here's a link to an old paper that discusses multi-mass systems.

http://www.mason-industries.com/masonind/downloads/spec/complete/ashlec.pdf

I expect the centrifuge would demonstrate a "critical speed" or two when slowing down on its own isolation mounts on a perfect floor. Even if running speed is OK on you new system, I think there will likely be a few extra critical speeds on shut down that personnel nearby will talk about.

 

[GregLocock]

A spin dryer operates above resonance. You may have noticed a certain amount of vibration from them when they slow down. A 14ft spin dryer at 3400 rpm would be testing the mettle of the designer and the metal of the structure. Sorry!

If you can't design the structure so that all modes are >115% of the operating speed then an isolator approach may work. I don't think harmonic dampers are a good idea in this case as the power available is so great, and continuous.

Do you know what the maximum out of balance is likely to be?





Cheers

Greg Locock


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[dktoao]

Even if your machine is not operating at the first resonance of your beams, it could be operating near the 2nd or 3rd resonance. There are empirical ways to calculate these but I could not tell you right off hand what they are. There are a few things you need to consider with your design:

If you intend to stiffen your structure so that it's resonant frequency is > 3680 Hz (3200 * 115% as mentioned by Greg) you will most likely need to get creative (i.e. curved or tapered beams as you are a long way off at 940.

Keep in mind that as the natural frequency increases the displacement (and therefore the stresses) will decrease by a factor of w^2. So Even if you can't stiffen your structure to eliminate all resonances, maybe you can push it past the point where it is harmful to your structure.

Hope this help.

-Daniel Kuntz

 

[GregLocock]

Oh btw I misread the dimensions. A 14 ft centrifuge at 3000 rpm would be awesome.

Cheers

Greg Locock


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