Random Vib Isolator
Random Vib Isolator
(OP)
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
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."





RE: Random Vib Isolator
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.
RE: Random Vib Isolator
Tobalcane
"If you avoid failure, you also avoid success."
RE: Random Vib Isolator
RE: Random Vib Isolator
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
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RE: Random Vib Isolator
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."
RE: Random Vib Isolator
RE: Random Vib Isolator
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
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RE: Random Vib Isolator
RE: Random Vib Isolator
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."
RE: Random Vib Isolator
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".
RE: Random Vib Isolator
Tobalcane
"If you avoid failure, you also avoid success."
RE: Random Vib Isolator
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!
RE: Random Vib Isolator
Tobalcane
"If you avoid failure, you also avoid success."
RE: Random Vib Isolator
Cheers
Greg Locock
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RE: Random Vib Isolator
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
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