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?

 

[GEspo]

Ok gotcha, so we'd say a spring has a damping constant, or would we say a material has a damping constant? You would have 2 springs same material different geometry, same damping constant..

 

[SwinnyGG]

We would not say 'a spring has a damping coefficient'

We would say 'Steel alloy ASTM XXXX has a damping coefficent of 0.0XX'

Any spring mads of that specific alloy will have that specific damping coefficient, regardless of size/type/diameter/weight/load/whatever.

 

[GEspo]

I will consider and test appropriately in the software. Starting to lean on the side of real world testing to estimate any values I set.. thx again

 

[swimfar]

a damper is used in parallel

Why do you think you need to add the damping characteristics of the spring if you're using it with a separate damper? I would think that any damper you add will be multiple orders of magnitude stronger than the damping from the spring. I would be surprised if adding damping to the spring would have any measurable effect on your simulation.

 

[GEspo]

Thats a good question, and it's not obvious when initially using the software, however this specific element is idealized so the spring available to use in simulation has NO damping, which is why I included a short video of someone coding a piece of hair bouncing way above.. in non idealized settings everything has damping etc, so you must add damping and the way to do that in this software is to run a parallel damper(you end up with an ideallized spring with NO damping parallel with a damper) thx

 

[3DDave]

Which software?

 

[GEspo]

Maple

 

[3DDave]

Maple is a general purpose solver, which suggests you are using a particular add-on package for which there should be documentation. Seeing as it is unlikely to be an FEA solution, it will be unable to cope with actual spring geometry.

These math packages simply do what you would normally do by hand; that is, the user should understand how each element functions before applying the software. You should have had a course in machine elements before this.

 

[RRiver]

""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.""

A spring collapsed 100% can still vibrate. The natural frequency of a static spring has nothing to do with a spring in use where its length is constantly changing.

Mix the colors blue and yellow and the result is green. The color isn't blue and yellow mixed. In an odd way this is the same problem. Colors have a natural frequency in visible light. Do anything to effect the natural frequency of a color it's a different color and what it started as is irrelevant.

""A coil spring has mass M, length L, stiffness K, and an internal damping coefficient C. The displacement along the x
direction is u(x,t). An infinitesimal element dx from the spring length is shown in Figure 2. Thus, the force F created in
the spring is related to deflection . . . ""

https://docs.lib.purdue.edu/cgi/vie...tpUkw-&httpsredir=1&article=2189&context=icec

But you have already been told this.

This recently came up in another discussion. Look up a train's secondary suspension to maintain a car's natural frequency. What would be the point of maintaining a spring's static natural frequency? Use a solid bar.

 

[GEspo]

"course in machine elements", can you recommend some literature, preferably online and free.. I'm always up for learning for the sake of accuracy.. thanks

 

[onatirec]

It seems we all want this thread to be more involved than it actually is. OP is just modeling a simple spring-mass system and, in order to make it more realistic, damped the model so the response would die out. My reading is that the actual, physical response of the system isn't as important as roughing something out that looks right. E.g. the video posted early on is not about accurately modeling a spring-mass system, but modeling hair that behaves in a visually appealing way for a Pixar movie.


OP, I'd suggest looking more into Transient Responses

https://en.wikipedia.org/wiki/Transient_response

and follow the links for settling time, overshoot, etc.
I'd also recommend working through a section of a Vibrations textbook on "Free Vibration with Viscous Damping" to get a better look at deriving equations that describe the relationships between stiffness/damping/mass and system response. I'm sure there are a ton of free videos and sites with good info on it.

 

[3DDave]

How did you come to a position where you are trying to simulate what you don't understand?

 

[GEspo]

I'm simulating a coilover shock setup... I have degrees in Math and Comp Sci, so I'd hardly say it's something I can't understand, however I'm teaching myself Vehicle Dynamics and designing a 3D program for race vehicles.. Most of the focus of the program is on geometry and optimization, which I have a background in(worked for a genetics co doing AI, a fancy term for mathematical optimization).. I'd like to be close on simulating the shocks since they play a large part in any long travel system, which at this stage is what the program is geared toward..

 

[RRiver]

WOW!

Please explain what you've already done in designing the chassis. Do you have any one of the chassis design handbooks? What are the X, Y, and Z angles of the coil over from vertical? Each angle changes any of the coil over's static specs. "Effective Spring Rate"" This is just one of the many dimensional relationships that will effect what you're trying to model.

 

[GregLocock]

I suggest you reread my answers since I model suspensions all day every day. I can guarantee that for any normal vehicle the internal damping of the steel coil spring material is so small as to be ignored. You may need some damping in there for numerical stability of the solver, but that is all.

Cheers

Greg Locock


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

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

 

[GEspo]

I’m definitely learning as much as I can on suspension at this point, and have a ways to go.. I can build nearly any geometry but the one I’m currently getting fine tuned is your typical ifs with 4 link rear.. typical Trophy Truck or Ultra 4 stuff.. the geometry and optimization is done, just putting it all together.. The software has a hydraulics package by Modelon so I’ve been successful in modeling a monotube w/ PGA, and also a triple bypass too(took way too long)..

I have a good vehicle dynamics book with plenty of matrix eqn.. covers all the basics, some good stuff on slip etc.. looks to be about college senior level w/ some grad level..

 

[GregLocock]

Here's an old one of mine.

https://www.carsguide.com.au/car-advice/whats-a-watts-linkage-34480

Cheers

Greg Locock


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

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

 

[RRiver]

Greg, You want to add roll center and cornering to this? LOL

 

[GregLocock]

?

Cheers

Greg Locock


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

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

 

[RRiver]

Primary benefits of a Watts Link. Add a Watts link means removing the leaf springs and going to coil springs. Roll center and cornering, another yet to be discussed force on springs.