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?

 

[desertfox]

Hi there

The damping constant would normally come from a dashpot or more commonly known on vehicle’s as shock absorbers.


“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[BrianE22]

I'm just guessing, but I'd think the mounting of the ends of the springs to the load and ground produce most of the damping. Also, air resistance against whatever is shaking might add a bit.

 

[GEspo]

Just from an empirical standpoint it makes sense that some oscillating will occur in any spring.. this is what I'm trying to get my hands on... As far as damping of a "system" composed of spring and damper/dashpot(a coilover shock for example), the engineer tries to get as close to critical damping as possible... of the system. Systems can be overdamped, underdamped and critically damped.. Here we are concerned with the spring only. The spring MUST have some damping as noted in the equation for motion from the site I included in the first post here.. thanks for the responses so far..

From the link above:

"A mass suspended from a spring, for example, might, if pulled and released, bounce up and down. On each bounce, the system tends to return to its equilibrium position, but overshoots it. Sometimes losses (e.g. frictional) damp the system and can cause the oscillations to gradually decay in amplitude towards zero or attenuate. The damping ratio is a measure describing how rapidly the oscillations decay from one bounce to the next."

** the damping constant may have something to do with the damping ratio which is: damping coefficient(actual damping)/critically damped

"For a damped harmonic oscillator with mass m, damping coefficient c, and spring constant k, it can be defined as the ratio of the damping coefficient in the system's differential equation to the critical damping coefficient"

HERE'S a VIDEO explaining simulating an actual spring:

https://www.khanacademy.org/computing/pixar/simulation/hair-simulation-code/v/sim8-fix

 

[GEspo]

I should add this: the spring rate may help me with the damping constant in simulation.. If we say the spring rate is 500, so that if a 500lb weight is placed on the spring it compresses 1”.. in this compression does it exactly drop to 1” and stop with NO oscillation.. so it NEVER passes 1”.. answering this may help LOTS! I’d like to assume it passes 1”, oscillates, and ends up there as an equilibrium.. This may be wrong though.. thx!

 

[3DDave]

A spring element by itself has little damping; an ideal spring element has no damping. In most calculations the spring rate and damping coefficient are 100% entirely separate/independent.

In real life most of the damping is just air friction or friction in bearings or sliding surfaces. The amount of energy dissipated in the spring is close enough to zero as to be negligible.

 

[GEspo]

Thanks for that.. So the correct way approach this is to say the spring mass system has a damping constant.. I can get numbers on a springs natural freq(I’m not too familiar with this yet), and use it in the equations for motion..

 

[3DDave]

A spring-mass system has no damping. A spring-damper-mass system does. A spring has no particular natural frequency.

This may be of interest:

https://ocw.mit.edu/courses/physics...and-waves/lecture-2/copy9_of_lecture-2-video/

looking at the 49:00 minute mark.

 

[GEspo]

That would disagree with the wiki link in the first post, no? How does a spring with mass attached STOP oscillating after some F is applied?

I’ll check your link.. thx for posting btw

 

[GregLocock]

Many years of bashing steel structures with (instrumented) hammers tells me that for pure steel, no interactions, a damping coefficient of 0.002 is a good place to start. cast iron, 0.01


Cheers

Greg Locock


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

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[GEspo]

Thanks for the numbers.. also found this..

https://www.reddit.com/r/engineerin..._source=amp&utm_medium=&utm_content=post_body

“Per the Barry Isolator selection guide, the damping of a steel spring is .005 (0.5%).”

 

[handleman]

Do you not have a professor that can help you?

 

[dgallup]

A spring has no particular natural frequency

A spring most definitely does have a natural frequency and you want to avoid operating too close to it to avoid surging.

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

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[RRiver]

Aren't you mixing apples and oranges with your question? Spring rate is a scalar quantity. Spring dampening is about rate of change (speed)and time. A vector quantity. The third law of motion. For every action there is an equal and opposite reaction. Damping is about mitigating the action, reaction or both of the effect of the force velocity on the spring.

My experience with oscillating springs had to do with vibration and not rate.

 

[3DDave]

It only has a natural frequency in a particular mounted position with a particular forcing direction. Surge makes multiple assumptions.

 

[GEspo]

"Spring rate is a scalar quantity. Spring dampening is about rate of change (speed)and time."

Yes definitely.. the units for my damping constant N*s/m... units for my spring constant N/m.. We are definitely discussing two different values when we look at damping constant and damping coefficient, however its possible there is some relation? From what I found so far the damping ratio for a passenger car(w/ physical damper(mono tube etc) added w/ spring etc) is around .3 and for race vehicles is .5 to .7..

Lets setup an experiment: we preload the spring I mention (3x12x500), hang a 500lb weight from it so it stretches 1" and comes to rest(would be the same as placing it on top while the spring is on the ground).. Next pull down(or push up) on the weight with 500lbs force and let go..

According to the spring rate the spring will stretch and additional inch(as I visualize this the force is constantly applied(constant v) until the attached weight stops at 1" lower, then force is removed) We guess some oscillations occur... at what t do the oscillations stop? (Beauty of this is I can test this in my software and post the results)

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

the above link(pg 3) defines the oscillations we are guessing occur as the natural frequency: "defined as the number of oscillations that a system will carry out in unit time if displaced from it equilibrium position and allowed to vibrate freely"

 

[moon161]

OP, It seems like you could be more familiar with the damped harmonic oscillator concept, and that, practically describes as system instead of one component. Even though a component like a spring can be seen to have a spring rate and (very small) damping coefficient (or constant, or rate, IME, the terms are interchangeable), engineering DHO systems (like suspensions, or door closers) are usually made of separate damping and storage elements that have their own spring and damping rates.

Instead of wikepedia, you might find a copy of say a dynamics text or a dynamic systems text for something more thourough.

https://www.wiley.com/en-us/Modeling+and+Analysis+of+Dynamic+Systems,+3rd+Edition-p-9780471394426

 

[GEspo]

Thanks for the book recommendation... I'll check it out.

I see also defined in the optics.arizona link above the damping coefficient: "Damping for a material is
expressed by its damping coefficient" which is C= lbs*s/in so as noted by the post above what I'm calling the damping constant is also called the damping coefficient..

.. making progress so thanks very much to all the posting!

 

[RRiver]

Honestly, I'm not sure what it is you're trying to get at. If you're trying to identify some inherent dampening component of a coil spring I'm not sure what value that would have in a suspension design and furthermore you would have to at least identify the spring steel grade and material. That's not going to be a "spring steel constant."

For a suspension you would also have to know the effective rate of the springs. If the spring is getting compressed on a control arm, what is the lever component length of the control arm? What's that doing to your dampening constant? How does the mechanical advantage of the lever effect the spring rate at one inch?

It would take a professor with no real experience to think there was some value to the question you're asking.

EDIT: I would say the spring rate is the dampening rate adjusted for any angles of the applied force on the spring and it won't be linear.

 

[GEspo]

As my first post states, I’m trying to find a value to put in my modeling software.. what I’ve found that may be helpful are eqns for natural freq w/ damping, damping ratio etc.. It looks like I’ll have to work backwards using the period(T) of the system when c=0=damping coefficient (when modeled).. this way I get a nat freq of the system with no damping, solve for a few variables with that and use those values to solve for a set of values of c.. this will assume I can find a value to use for critical damping which won’t be too hard with modeling. If anyone has experience using Advanced Systems Modeling Software please chime in.. thx