Using 'minimum static deflection' to choose a vibration isolator from a manufacturer's catalog

[mrev23]

Which vibration isolator complies with a specification that requires a "minimum static deflection of 1 inch" (where the load imposed on the isolator is 300 pounds):
[ul]
[li]Proposed by the contractor: An isolator with a rated deflection of 1 inch at a rated load of 500 pounds in the manufacturer’s literature?[/li]
[li]Found in the manufacturer’s catalog: An isolator with a rated deflection of 2 inches at a rated load of 600 pounds?[/li]
[/ul]
The contractor seems to believe that the proper selection is found by:
[ul]
[li]Comparing the “rated deflection” in the manufacturer’s literature with the “minimum static deflection” in the specification.[/li]
[li]Ensuring that the isolator’s rated load provides an adequate safety factor over the load imposed on the isolator.[/li]
[/ul]
When I pointed out that the isolator rated for 1-inch deflection at 500 pounds will deflect only 0.6-inch when loaded to 300 pounds, this was dismissed as "an interpretation."

I haven’t found a reference that plainly states how to select a vibration isolator based on the required “minimum static deflection.” I believe the procedure includes:
[ol 1]
[li]Determine the imposed load at the isolator’s location.[/li]
[li]Determine spring rate needed to provide the specified static deflection (300 pounds per inch in the example above).[/li]
[li]Find an isolator in the manufacturer’s catalog that has:[/li]
[ol a]
[li]A spring rate no greater than (not stiffer than) the value determined above (300 pounds per inch).[/li]
[li]A load capacity no less than the imposed load plus a safety factor.[/li]
[/ol]
[/ol]
Many web pages go into theory, with equations for natural frequency, transmissibility, etc. They also include other considerations for an engineering analysis: the disturbing frequency, how the floor under the equipment is supported, etc.

These pages do not seem to be useful for a discussion with the contractor about what the project specification requires.

By whatever means, the engineer concluded that a “minimum static deflection of 1-inch” is needed.

Which of the two selections at the top of this post is more appropriate, given the direction from the engineer to the contractor via the specifications?

 

[GregLocock]

It's a funny way of specifying it but the requester is actually specifying a maximum resonant frequency, hence the rate is what is important, so the second of your two options is correct.

Cheers

Greg Locock


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[Tmoose]

The ASHRAE Handbooks (Applications Volume) have long referred to minimum static deflection for isolators for equipment depending on mounting conditions ( slab on grade, floor span) and equipment type and rpm.

More recently, NEMA has included a chart "minimum elastic displacement as a function of nominal test speed" for an "easy determination" of the elasticity of the support for "resilient mounting" for vibration testing.

Way better than talking about isolator load ratings, as has surely tripped up mrev's contractor.

 

[IRstuff]

How does the contractor "interpret" that? Since he's proposing it, he should be able to prove it by taking a 300-lb load and measuring the deflection; if it's at least 1 inch, then his solution is compliant.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[electricpete]

I haven’t found a reference that plainly states how to select a vibration isolator based on the required “minimum static deflection.”

I didn't study the question or responses closely. I apologize in advance if I'm off in left field.

I'll just point out (in case it's not obvious) that specifying a minimum static deflection (sd)is equivalent to specifying a maximum resonant frequency under sdof assumption. Presumably we want to keep that below excitation frequency by some margin.

sd = Weight / K
sd = M*G / K
sd = G * (M/K) = G / wn^2
wn = sqrt(G/sd)
Fn = 1/(2pi) * sqrt(G/sd)

By the way in real world even with something like four isolators supporting a rigid platform, we still have a variety of rocking modes possible which makes the sdof assumption a little hokey. I guess you'd need to estimate a ratio of the highest frequency rocking mode to the simple vertical mode (which is the one that meets wn = sqrt(G/sd)) and include an allowance for that in whatever frequency separation margin is applied between resonance frequency and excitation frequency.


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(2B)+(2B)' ?