Specifying a Spring

Help Support GroupDIY:

Gold

Well-known member
Joined
Jun 23, 2004
Messages
2,743
Location
Brooklyn
Does anyone here know how to spec a spring? Say I want a spring that has 2” static compression under a 1 lb load. I also want the spring to be 3” long when under load?
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,003
Location
Marcelland
Spring manufacturers over here ask:
Length without load, Length with specified load, type of action (compression/traction), OD and composition (steel, stainless, bronse...). You can specify Preload, if any and shut length if it matters.
 

Gold

Well-known member
Joined
Jun 23, 2004
Messages
2,743
Location
Brooklyn
I recently bought spring isolators for an inertia base. They did the engineering work. I’m looking at the wide selection of compression springs available at McMaster-Carr and wonder how to figure it out.

The parameters available are Length unloaded. Rate of Compression in pounds per inch. Length Under Load. OD, ID and Wire Diameter.

I guess it’s the Rate of Compression parameter I don’t understand. Do I specify the length unloaded and loaded and figure out the Rate of Compression from there. If I did that how do I know the spring won’t compress so the coils touch each other and ‘bottom out’ with the specified amount of compression?
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,003
Location
Marcelland
The parameters available are Length unloaded. Rate of Compression in pounds per inch. Length Under Load. OD, ID and Wire Diameter.
"Length unloaded. Rate of Compression in pounds per inch. Length Under Load", there's one redundant parameter here; At least they are interactive, as long as the elasticity limits are not exceeded.
Length under load= Length unloaded-(load.Rate) (for a compression spring - for a traction spring use sign plus instead of minus).
how do I know the spring won’t compress so the coils touch each other and ‘bottom out’ with the specified amount of compression?
If "shut length" is not spec'd, use number of turns x wire diameter
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
20,973
Location
Hickory, MS
Does anyone here know how to spec a spring? Say I want a spring that has 2” static compression under a 1 lb load. I also want the spring to be 3” long when under load?
sounds like math (algebra).

looking at mcmaster carr about the best specified vendor for onsey-twosey.

I didn't see anything close to your spec.

JR
 

Gold

Well-known member
Joined
Jun 23, 2004
Messages
2,743
Location
Brooklyn
Thanks. These springs are cheap enough that I can make a few mistakes and it won’t break the bank.
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,003
Location
Marcelland
Hookes law.
For TMI, beware that Hooke's law is significantly inaccurate for helicoidal springs when the pitch changes under strain.
The force is the elastic torque divided by the sinus of the angle between a turn and the axis.
As the turns get closer, the sinus of the angle increases, so the compression rate also decreases.
 

5v333

Well-known member
Joined
Jun 30, 2013
Messages
499
Location
Gothenburg
From a basic lab exercise i had in school last year :)

A spring was elongated (expanded) by different weigths. I havnt recorded how long the unloaded spring was but i think it was around 5-10cm long..

The angle of the graph tells us what the stiffness is for every wight/force applied to the spring.
 

Attachments

  • Skärmavbild 2021-07-30 kl. 16.35.49.png
    Skärmavbild 2021-07-30 kl. 16.35.49.png
    83.9 KB · Views: 6
Last edited:

Gold

Well-known member
Joined
Jun 23, 2004
Messages
2,743
Location
Brooklyn
This inquiry is for a transfer/QC turntable cart. I want the cart on wheels and the turntable plinth spring isolated. I’ll have to do some experiments but I think I have a nice way of doing this.
 

Gold

Well-known member
Joined
Jun 23, 2004
Messages
2,743
Location
Brooklyn
If your goal is sound isolation don't use a reverb spring... ;)

JR
They don’t have anything called a reverb spring at McMaster so I should be okay😎

I’m planning on using 1” PVC sheet for the plinth. Seems appropriate. I’m not sure why manufacturers use resonant material like wood or high transmissibility material like stone. PVC is dead. If you tap it you get nothing. If you put a speaker on it with mid to high frequencies you still get nothing. If its good enough to make records out it should work as a plinth.

I like plastic. I have a bunch of Kartel furniture. High end Italian plastic furniture with classic designs back to the 1960’s. It’s mostly sold as commercial furniture but i love it. People don’t generally like to pay a lot for plastic though. 1” PVC sheet isn’t cheap but probably looks it.
 

5v333

Well-known member
Joined
Jun 30, 2013
Messages
499
Location
Gothenburg
Hookes law gives
F = k x delta l
Where F = the static force applied to a spring by a wieght in newtons
k = stiffness konstant of the spring in Newtons per meter
Delta l = the leghth or elongation, of which the spring has been alterd by the applied force, in meters.

The angle velocity gives
2 x pi x f = sq root(k / m)
Where f = the resonant freq of the system.
m = the mass of the system in kg.

This is equivalent to LCR circuits
 

Gold

Well-known member
Joined
Jun 23, 2004
Messages
2,743
Location
Brooklyn
Hookes law gives
F = k x delta l
Where F = the static force applied to a spring by a wieght in newtons
k = stiffness konstant of the spring in Newtons per meter
Delta l = the leghth or elongation, of which the spring has been alterd by the applied force, in meters.

The angle velocity gives
2 x pi x f = sq root(k / m)
Where f = the resonant freq of the system.
m = the mass of the system in kg.

This is equivalent to LCR circuits
Armed with this information I have roughly a 0% chance of getting it right. As best I can tell from manufacturer tear sheets a static compression of 2" has a resonant frequency around 1Hz. I haven't found any spring isolators with more than 2" of static compression. Using more than one equally specified spring doesn't seem to change the resonant frequency. Using 10 springs with 2" static compression has the same cutoff frequency as using 100 springs with 2" static compression. I'm flying blind like I do with Ohm's Law. I can rarely apply that correctly.
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
20,973
Location
Hickory, MS
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
 

Latest posts

Top