Random Vib Isolator

[Twoballcane]

Hello all,

I have to pick an isolator for a box at all four corners. The input would be a random vib profile. Not knowing what the Fn of the CCAs in the box, how would you pick the Fn of the isolator? Also, would you pick the Fn where you have the hieghest g^2/Hz or the lowest g^2/hz?

Thank you in advance for your time and effort.

Tobalcane

Tobalcane
"If you avoid failure, you also avoid success."

 

[GBor]

You can do a few things, but they all start with some idea of the power spectrum and the fundamental vibratory modes of your part. You can calculate a root mean square acceleration (see

http://www.femci.gov

for some basic calculations) and design to it, but you may prefer to design isolators for specific frequencies based on the critical PSD (or ASD). It depends on how rugged your equipment is and what accelerations and frequencies may cause your part to fail.

For instance, if I have a part with a natural vibration mode at 100 Hz with 80% of mass participating at that frequency, I may not be concerned if my random vibe PSD from 80Hz to 120 Hz is virtually 0, but if I have a part that has even 10% mass participating at a frequency where I have a high PSD, it may break a particular piece off and lead to failure.

 

[Twoballcane]

Thanks GBor. Yup I agree with ya, but for this case the vendor does not know any of their Fn in the box and there is no money to do a sin sweep.

Tobalcane
"If you avoid failure, you also avoid success."

 

[GBor]

Any idea what the vibration inputs might be? I know you said random vibe, but do you know anything about where this part might be installed?

 

[IRstuff]

So what's the purpose of the isolators, if you don't even know whether they'll do any good, or harm?

As a general rule, the objective is to get the highest Fn and still achieve the desired attenuation of the PSD as a whole, and get as little collateral motion as possible. To wit, 2 Hz isolators will not attenuate as well as 1 Hz isolators, but the 2 Hz isolators will see less payload motion. This can be quite critical for some applications, ala 901 shock, since there's a requirement to keep the payload sway to less than 3 inches.

If you have no performance constraints, then you'd pick the lowest Fn that will still fit in the O&M constraints.

TTFN

FAQ731-376

 

[Twoballcane]

Thanks guys for responding,

GBor,
It is transportation PSD vib from 5 to 500Hz with your peaks and valleys.

IRStuff,
I agree with you, but this is the situation that an out side vendor design the box and guts, but have no idea what their sensitive frequencies are…which frustrates me.

In any case, If I had to pick an isolator to lower the accelerations, would I pick frequency at the highest g^2/Hz or the lowest g^2/Hz? Or, just pick an isolator that has the lowest Fn so that higher frequencies are dampened.


Tobalcane
"If you avoid failure, you also avoid success."

 

[GBor]

What type of transportation? Over the road? Ocean-going? Jet or propeller aircraft?

 

[IRstuff]

Fn is usually where there's a bit of Q, so you definitely don't want the excitation to peak there as well.

As a point of reference, we used to run 15 Hz isolators with payloads that were specifed for 250 Hz lowest resonance, with MIL-E-5400T helicopter vibration

TTFN

FAQ731-376

 

[rob768]

I'd use (rubber) dampers instead of isolators. At least, if the excitation occurs closer to natural frequencies than you like, you'll have some damping. The drawback off course is more response away from natural frequencies but without any info on frequencies, you have to do something

 

[Twoballcane]

Thanks for responding guys.

Gbor, it is over road.

IRstuff, I know this is all a shot in the dark with out knowing the sensitve Fn of the stuff in the box, But I have to make an engineering guess. So now my engineerng gut tells me to pick the isolator that has the lowest Fn so that at least the higher Fn will be isolated. What do you think?

Rob768, Yup, I am planning to use sillicon.

Tobalcane
"If you avoid failure, you also avoid success."

 

[GBor]

Twoballcane,

I don't know if you have the time or financial resources, or if the constraints are too great, but I would be inclined to build an "elastic foundation" support with a known frequency...something easy to calculate with a box weighing what yours weighs, then isolate the foundation. I may not know what is in the box, but if I mount it to a couple of pipes and attach the pipes to the isolators, I can calculate the frequency needing to be isolated based on the vibration of the pipes with the mass in the center. Obviously there are some geometric considerations, but at least this would identify what your isolators need to "isolate".

 

[Twoballcane]

Gbor, Im actually trying to get them to let me test, at least do a sin sweep to find out the Fn in the box, which is the right way to do this. In the past, this box did not need an isolator, but somebody said we need one. It is one of thoes "oh you do vib, what kind of isolator do we need?" with out any other info. Frustrating...

Tobalcane
"If you avoid failure, you also avoid success."

 

[GBor]

If they don't let you do it, I'm just not sure that you can select anything with confidence. Selecting the lowest frequency may be useless, but probably better than just arbitrarily selecting some other frequency.

Not that it will matter as low as your frequency goes, but I usually use a 20% rule of thumb for frequency effectivity. If this is appropriate, a 6Hz isolator would probably be better than a 5Hz isolator since your range stops at 5Hz.

If forced to "throw a dart", I would probably isolate the first spike of the PSD.

Good Luck!

 

[Twoballcane]

Gbor, yeah this is frustrating, but I have the same thoughts as yours...

Tobalcane
"If you avoid failure, you also avoid success."

 

[GregLocock]

5 Hz isolators are pretty soft, especially for small packages. Usually we just use soft grommets under the mounting screws.

Cheers

Greg Locock

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

 

[IRstuff]

I think you're focussing too much on Fn. The question is how much attenuation do you need at all frequencies of interest. One you determine that, the Fn is automatically determined.

Likewise, the range of disturbance frequencies is not necessarily relevant either, the objective is to reduce the overall vibration to some acceptable level, and given the single-pole rolloff characteristics an isolator, the Fn comes out as a result, not as an input.

As an example, the transportation vibe results in somewhere between 1 and 2 Hz suspension Fn for the truck, because that's what is required to attenuate the road vibe to a level tolerable to the human driver and passengers.

Most electronic equipment on ground and air vehicles have some sort of elastomeric isolators, nothing more.

TTFN

FAQ731-376