Vibration Isolation

[Parsnip]

All - hope someone can help.
I am isolating a piece of medical equipment on a lab bench. The mass of the equipment is 65kg and mounted on four vibration isolators.
As far as I know the steps to follow are:
1) Find the lowest disturbing frequency
2) Calculate the minimum isolator natural frequency (disturbing frequency & 0.707)
3) Determine static deflection
Now this is where the problem arises - how do I proceed from here?
I think I have to do something with Transmissibility and/or % isolation required but I am not sure how these relate to the final design.
Any help to determine the requirements of the isolators would be much appreciated as I am seriously STUCK!!!
Thanks in advance
Parsnip

 

[tomaspin]

Parsnip,

I would suggest the following initial approach.

The key factor is the ratio between the forcing (disturbing) frequency, and the natural frequency of the system along a particular axis.
A natural frequency 2x the forcing frequency will give a transmissibility of about 0.35 (65% isolation), and 3x will give a transmissibility of about 0.15 (85% isolation).

Let's say the forcing frequency is 20Hz therefore, we will be looking for a natural frequency around 7Hz to get 85% isolation.

Natural frequency (f) = 15.8/sq.root(d)
so d = (15.8/7) squared, or about 5mm.

Our static deflection is therefore 5mm under a static load of 160N (16kg) per mounting, so a stiffness of approx 30N/mm would be required.
We then start looking for an isolator of this stiffness, with a maximum deflection capapability of 7.5-8mm, as we typically use only about 75% of the rated maximum deflection capacity.

This is the 'quick and dirty' approach I use for isolator selection, and will give a reasonable starting point for further work.

Does this help?

Let me know if you need any more info

Regards,

Tom

 

[sms]

I found this article, might help you along...

http://www.vibrationmounts.com/NewProducts/VibrationIsolation.pdf

"Why don't you knock it off with them negative waves? Why don't you dig how beautiful it is out here? Why don't you say something righteous and hopeful for a change?" Oddball, "Kelly's Heros" 1970

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[Parsnip]

Thank you both for the information - the quick and dirty approach has allowed me to select an isolator for some trial work.The document was also very useful, but i think i need to read it in more depth.

Tom: what additional information can you provide?

Thanks again

Parsnip

 

[tomaspin]

Parsnip,

I design/specify industrial rubber vibration control components, and and look after the UK market for a large German company. So if you're looking at rubber isolators I can offer some advice.

If you go to :

http://intl.simrit.de/web/public/products/technicalmanual

and go to the download section at the bottom of the page, the vibration control technical manual can be downloaded (4MB approx).

85% would be considered to be a good level of isolation for an engine or a cabin, but may not be sufficient for really sensitive equipment. Mounting equipment on an inertia base (basically a big suspended mass) can be useful in some circumstances, especially where low frequencies are involved.

It may be worth talking to the lab equipment manufacturer, as they may well be able to offer some guidance on the maximum permitted acceleration limits. This would allow for a slightly more scientific approach.

Let me know how you get on.

Tom

 

[IRstuff]

Nowhere do you state what level of vibration is acceptable nor what level of disturbance there is.

What other constraints are there? Are there limits on the amount sway allowed?

What is the purpose of the isolation, i.e., prevention of damage, or, angular line of sight stability?

TTFN

FAQ731-376

 

[Tmoose]

The basic isolator charts assume the structure supporting the isolator is very stiff or immense. HVAC equipment installed on flexible roofs or above grade floors, even in the simplest models usually qualifies as two or even 3 mass systems. I'd expect equipment on a typical (flexible) lab bench to qualify as a 2 mass system, and, if the bench is not on a slab on grade, as a 3 mass system with several resonant frequencies of its own. The lab bench and the lab floor each are part of the "system." Below is a link to an article about selecting isolators multi-mass systems. Basically a lot more static deflection in the isolator is needed to achieve expected results. And sensitive equipment may need isolation over 95% even in a low vibe envrionment.

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

 

[spongebob007]

Certainly you must be mindful of the foundation you are mounting to. Ideally you want to be as close to infinitely stiff as possible. Flexibility in your mount foundation will reduce the attenuation of your mounts, and in sever cases actually lead to amplification.

My former employer was a machine bulder for the automotive industry. Like many US companies we outsourced a lot of our work to the lowest bidder. One of our machines had the isolators mounted to a steel channel that had a resonant frequency that was merely 10Hz above the highest machine operating frequency!!! Over the years this caused cracked welds in the machine frames and even structural damage caused by vibrations transmitted through the machine frame and into the floor. Not to mention noise pollution. You could actually hear the machine hit resonance and it was nearly 100dB.

I quickly diagnosed the problem by running an FEA model of the machine. The FEA model actually showed two resonances of the foundation over the frequency range of the machine where there was amplification of the vibration in the frame. Field testing with accelerometers verified my models. The acceleration levels on the frame foundation where higher than those of the reference accelerometer mounted on the machine itself! FFT analysis of the data showed the peak values occured at the frequencies corresponding the the FEA model.