Specifying a Spring

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abbey road d enfer

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As best I can tell from manufacturer tear sheets a static compression of 2" has a resonant frequency around 1Hz.
This doesn't make sense to me. What is "static compression"?
You need to know the stifness constant, which, if not specified can be calculated by dividing a specified force by the resulting elongation/compression.
Any serious spring mfgr/distributor should have this information ready.
As best I can tell from manufacturer tear sheets a static compression of 2" has a resonant frequency around 1Hz.
Where did you get this? Pure nonsense. Resonant frequency depends on stifness and total suspended weight.
 

Gold

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there may be other ways to skin the cat... you may be able to damp resonance with some kind of shock absorber (dash pot).

JR
Sure, I know there are fancy polymers that do that. I know Northward makes speaker stands using a fancy polymer. I’m a hack not an engineer. I’m trying to go with something cheap, plentiful and with a lot of options. That way I’m not locked into anything.
 

Gold

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This doesn't make sense to me. What is "static compression"?
You need to know the stifness constant, which, if not specified can be calculated by dividing a specified force by the resulting elongation/compression.
Any serious spring mfgr/distributor should have this information ready.
They calculate to make their isolators and tell you the recommended weight to produce the correct results.
Where did you get this? Pure nonsense. Resonant frequency depends on stifness and total suspended weight.
Which produces a specific amount of static compression. How much the spring compresses under load. It may be shorthand but I’m not convinced it’s wrong. The spring isolator manufacturers use amount of static compression as one of the main specifications. That and weight. That's all I had to specify to the spring isolator manufacturer I bought from to make a lathe inertia base.
 
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abbey road d enfer

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It may be shorthand but I’m not convinced it’s wrong.
Short hand indeed. Once one knows the rules, it's OK.
The spring isolator manufacturers use amount of static compression as one of the main specifications. That and weight.
OK, with these two informations, you get the stifness. Divide weight by "static compression". Then you can use this value to calculate the resonant frequency, using the stifness constant and the total suspended mass
That's all I had to specify to the spring isolator manufacturer I bought from to make a lathe inertia base.
What was the target then? Did you specify a resonant frequency?
 

Gold

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Short hand indeed. Once one knows the rules, it's OK.

OK, with these two informations, you get the stifness. Divide weight by "static compression". Then you can use this value to calculate the resonant frequency, using the stifness constant and the total suspended mass
That's all well and good but I haven't seen the stiffness constant specified on any spring I've looked at. There is probably a way to extract it but I can't do math.

What was the target then? Did you specify a resonant frequency?
No, 2" static compression is the maximum I've seen on any spring isolator mount. The manufacturer said the resonant frequency was sub 3Hz. Under correct loading should be less. I'll start a new thread with the inertia base I'm making for the lathe.
 

5v333

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It gets more complicated the more you learn :)

my understanding is that optimal decoupling is made with a compression of 10-25% of the non loaded spring. for ex a 1" spring should be squeesed to about a 0.8" spring when loaded. But i have not seen math or proof for this.
When a spring gets compressed to the max, it no longer funktions as a spring. and too little compression has problems to.

Also, there is damping coeficiant and damping factor (under- over- critical damping). the math is fun if you are a true nerd. With all theese components you could do the differential equations that solves alot.

As abbey says, if you know a weigth and what that causes, you basicaly have the stiffness konstant.
With that, and the mass you already knew, will give you the frequency of the system.
 

abbey road d enfer

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There is probably a way to extract it but I can't do math.
Divide weight by "static compression". Do it in metric units.
No, 2" static compression is the maximum I've seen on any spring isolator mount. The manufacturer said the resonant frequency was sub 3Hz.
Doesn't seem right to me. Does he know the actual load?
 

5v333

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ex:

1 lb = 0.45kg
2" = 0.051m

F = 0.45kg x 9.82 m/s^2
k = (0.45 x 9.82) / 0.051

f = sq root (k) / ( 2 x pi x sq root 0.45 )
 

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5v333

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Forgot to mention the acceleration due to gravity, g, which is part of the force of gravity which influences the weight.
F = m x g
Where m is the mass of the weight in kg and g is the acceleration due to gravity which is a konstant around 9.82m/s^2 depending on where you are in relation to the mass center of the earth.
 

JohnRoberts

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when trying to buy a spring off the shelf, you have to quantify it the way they are trying to sell it.

I looked at the mcmaster carr website and didn't see any obvious winners

JR
 

Gold

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Have you thought about trying sorbothane hemispheres?
I don’t think sorbothane acts as a pure spring. It’s complicated enough as is to figure this stuff out. Especially when arithmetic is a challenge. I’m somewhere in between paint by numbers and starting at material science. I think the advantage of sorbothane is its efficiency. I don’t need a thin material. I can accommodate any reasonable length spring.
 
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leftblock

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I'm assuming a coil spring, if so this PDF may help, if you want to damp resonance you might want to stuff some dense foam rubber into the middle of the spring

I've made quite a few springs - you can use piano wire from a hobby shop or Ebay, or as I have done find a local spring maker and buy a selection of spring wire lengths, I picked up 6 different thickness of wire around 5 meters of each for $25.
 

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JohnRoberts

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In this specific case, it's the absence of proper definitions that get in the way. I tried to google "static compression" and got nowhere (except for motor tuning).
McMaster Carr apparently didn't use Hooke's law.

ODIDDia.Wd.Thick.Compressed Lg.
@ Max. Load
Max.
Load, lbs.
Rate,
lbs./in.
Material

I don't know how practical it is to go to a commercial spring maker for a one off project.

JR
 

abbey road d enfer

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McMaster Carr apparently didn't use Hooke's law.

ODIDDia.Wd.Thick.Compressed Lg.
@ Max. Load
Max.
Load, lbs.
Rate,
lbs./in.
Material
Well, apparently they specify Rate in lbs/in, which is the US version of N/m.

EDIT: actually the rate is specified quite explicit on the McMaster site. All te necessary info is there.
 
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