Spring Damping Constant 1

[GEspo]

Hello. I'm working on modeling a coil spring in my Advanced Systems Modeling software. There is a damping constant value that needs to be set so that the spring will stop oscillating at some t.

(heres info on damping ratio

https://en.wikipedia.org/wiki/Damping_ratio)

Suppose we take a compression coil spring of 3(ID) x 12(resting L) x 500(lb rate).. if we place a 500lb weight on it the spring will compress 1". The damping constant, as far as I can tell, changes how many times the spring will oscillate before reaching equilibrium.

I have reached out to the manufacturer of the springs I'm modeling and they said they didn't have info on damping constants.

Thoughts?

 

[RRiver]

Natural frequency has nothing to do with dampening or applied force. It's the absence of dampening or applied force. There is no dampening constant except the spring rate. You could find the dampening coefficient for a spring if it's not spec'd. Look up dampening ratio.

 

[GEspo]

https://wp.optics.arizona.edu/optom...3/2016/08/Barry-isolators-selection-guide.pdf

Pg 3 nicely outlines what’s going.. defines a number of things..

for damping ratio I posted a wiki link on one of my first posts..

 

[RRiver]

I mentioned vibration in my first post. This is from your link and on page one. You seem to be trying to define some program parameters based on a constant that doesn't exist. You need the coefficient for the source which is a constantly moving target based on the application and conditions. An automotive spring on a road compared to a spring off road has very few shared constants. The oscillating characteristics or dampening coefficients are determined by the driving surface, not the spring. Suspension failures could be greatly curtailed if constants were involved.

""When compared to stationary applications, vehicular
installations subject equipment to much more severe shock
and vibration. Vibration from a propulsion engine is present
in air, sea and road vehicles as well as shock and vibration
effects from the media in which they travel.""

""As my first post states, I’m trying to find a value to put in my modeling software.. "" That value you're looking for is actually an infinite number of possibilities.

 

[GEspo]

I’m reaching the same conclusion but this way: the spring mass systems damping constant has to do with K the spring rate(which is all the info we really get from a spring since material and composition will directly effect K), and the attached mass.. so the vibrating/oscillating of the spring/mass motion(ie natural frequency with damping) is really dependent on whatever spec we have on the mass.. which can be near infinite in its application

 

[GregLocock]

Steel, in itself has a small, but measurable damping. A coil spring also has small but measurable additional damping from its end condition, usually as the end coil scrapes across the spring seat. In a typical suspension neither of those are even slightly important compared with the contribution of the other elements in the suspension, unless you are considering very high frequency effects.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[RRiver]

""I’m reaching the same conclusion but this way: the spring mass systems damping constant has to do with K the spring rate(which is all the info we really get from a spring since material and composition will directly effect K), and the attached mass.. so the vibrating/oscillating of the spring/mass motion(ie natural frequency with damping) is really dependent on whatever spec we have on the mass.. which can be near infinite in its application.""

You're still fighting this:

Dampening Constant: This does not exist. There is a Dampening Coefficient.

Natural Frequency with Dampening: Again, does not exist. Natural Frequency is the frequency which a system tends to oscillate in the absence of force or dampening.

Does natural frequency effect suspension frequency? By the math it does but suspension guys will often tell you it doesn't do much or help. What matters is the periodic changes in the surface being driving on which is a resonant frequency that's constantly changing. Sometimes greatly and sometimes minimally. When looking at a suspension in terms of mass and a system you also have to consider the differences between sprung and un-sprung mass and weight. There's also rotating mass to consider which can be effected by the simple property of brake location.

 

[GregLocock]

Here's the formula for spring surge. The relevant equation is probably surge between two fixed plates.

https://roymech.org/Useful_Tables/Springs/Springs_Surge.html

In a vehicle suspension spring surge has not ever caused me a problem, the only time I remember it as being important is in the valve springs. Coil to coil contact is a problem with variable rate springs, that's one of the reasons they are not preferred.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[GEspo]

From the link, "The spring , however not weightless and thus it has vibration characteristics of its own".. After coming to the conclusion that the damping coefficient has to do with the mass, I decided to add mass to the spring and adjust the damping constant so the spring has no vibration of its own at rest.. I've yet to test this thoroughly however it seems to be an ok starting point to allow the spring to stop oscillating..

Thanks posters for the assistance with understanding this..

 

[3DDave]

Who is the software publisher of the Advanced Systems Modeling software?

 

[onatirec]

The best way to get a value for realistic spring damping would be through experimentation. Pull the spring-mass system away from equilibrium, measure the time it takes for oscillations to die out. Damping coefficient doesn't really depend on spring mass. Damping is all about energy loss from the system and depends upon a variety of factors, which I don't think could be calculated very easily nor reliably. I.e. I think you'd be better off adjusting (guessing at) the damping coefficient and double-checking through the simulated response of your system. Here's a source with a few numbers:

https://syont.files.wordpress.com/2007/05/damping-properties-of-materials.pdf

As mentioned above: springs, in models, usually aren't attributed a damping coefficient (even though they do have a small amount of damping in reality). I recommend just adding an independent damper in parallel with the spring, as this is generally how models are constructed in system dynamics.

I think drawing a line between damping constant and damping coefficient is splitting hairs; I've heard them used interchangeably. Damping ratio, however, is the ratio of the damping coefficient (what you need) to the critical damping coefficient (which is easily calculated analytically from spring rate and mass).

 

[SwinnyGG]

You are way, way, way overthinking this.

Damping coefficient is a material property. It's what tells the software how much energy is absorbed by the deflecting component (the spring) and not returned to the system as kinetic energy (i.e. how much energy is 'damped' from the system per oscillation). All materials have some level of internal damping- they convert some percentage of strain energy to heat. For steel this number is going to be very low. Sensible values for steel and cast iron have already been suggested in this thread.

It has nothing to do with the attached mass, the spring's own mass, air resistance, the system's critical damping (i.e. nothing to do with damping ratio) or anything else. It's a material property based solely on what material your spring is made from. That's it.

 

[GEspo]

Thanks for posting the material damping info. I'm using a spring/damper and have found its best to do a few trials, as you mention, to find the most reasonable value for the programs damping constant. It looks like 1/60th of the spring rate does an nice job to stop it from vibrating(gives it critical damping) when the spring is given mass of 5kg and is supposed to be at rest w/ no forces acting on it.

 

[SwinnyGG]

I don't know exactly what you're simulating, but falsely giving your springs exact critical damping is the definition of garbage in. Your results aren't going to mean very much.

I'll say again: damping coefficient is a material property.

 

[GEspo]

Swinny... if I place a spring in a system w/ no damping constant it will vibrate freely.. infinitely.. If I attached that spring to any mass it will oscillate heavily... infinitely.. What makes the most sense to me is that a spring does have mass and at rest there is NO vibrating.. how would you explain differently?

 

[onatirec]

Swinny is right. Garbage in, garbage out. You should also not change the mass of the spring itself. Use the known value for that from supplier/manufacturer. In simple modeling, springs are considered massless (just as they are considered to have no damping). If the software you're using includes masses for the springs themselves, you are unnecessarily affecting the simulated response of your system by using values that you know to be incorrect.

Put all the known values in, then add a damper in parallel. Damping is a material property, but there are other effects mentioned which would affect the effective damping (air resistance, the boundary conditions of the spring, temperature etc).

You should maybe ask your teacher to clarify the scope. Modelling a simple spring-mass system would result in an idealized, infinite harmonic oscillation. If you want to get something close to realistic, buy a spring and experimentally measure effective damping. Otherwise, know you're just making up numbers and don't worry so much about being "right".

 

[GEspo]

The spring mass I mention above IS the actual mass, so we are all agreeing here and yes a damper is used in parallel. What also makes sense regarding my process is that the damping I'm setting is related to the spring rate(all material info is here). Yes, real world testing is a great idea, which led me to the basic idea that the spring has mass and is not vibrating at rest.. my results look good when applying force(additional to the springs mass), and could be compared to a real world trial.

 

[3DDave]

The mass of the spring isn't helpful unless one knows both the distribution of that mass and the distribution of elasticity. Not knowing the actual software in use makes dealing with this problem indecipherable.

 

[SwinnyGG]

What also makes sense regarding my process is that the damping I'm setting is related to the spring rate(all material info is here).

This may make sense to you, but it doesn't make sense from the standpoint of physics.

Damping coefficient and spring rate are completely unrelated. Damping coefficient is a material property, spring rate is a product of geometry. They are completely unrelated.

 

[GEspo]

Given the same geometry..

 

[SwinnyGG]

No, not given the same geometry... damping coefficient is a material property, just like young's modulus or poisson's ratio. None of them change with geometry. This is a thing that's really important to understand if you ever work with detailed engineering of springs, especially springs made from elastomers.