Contact ratio between two surfaces

[Ignicolist]

Hi folks,

I would like to know if there is any equation or similar to define the real contact ratio between two surfaces.
For example, we can consider two steel surfaces with a defined Rz in contact. The real contact will never be 100%, as there will be micro gaps. If we improve the Ra (and Rz) of theese surfaces, the contact ratio will increase.

Is there any information, equation or similar about this matter.

Thanks in advance!

 

[GregLocock]

I hope you get some good answers. I have a horrible suspicion that the curve of effective contact area vs roughness vs loading is very non linear. It is discussed when talking about friction, but I have no directly useful references.

Cheers

Greg Locock


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

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

 

[btrueblood]

There might be some research in the realm of heat transfer that would be helpful too (I'm thinking of thermal contact resistance between mating surfaces).

 

[Tmoose]

What are you looking to accomplish with improved contact ratio ?

Are you gently resting one component on top of the other, or clamping the beejusuzz out of the asssembly?

If the components are thick plates, and the flatness is not superb, I'm thinking the contact area and thus the global ratio will be poor even if the Ra is dead smooth.

As suggested by others Typical published curves for electrical and thermal contact show how surfaces in light contact don't make much contact at all.

https://www.researchgate.net/profil...ted-materials-and-typical-pressure-forces.png

 

[Compositepro]

You have to define exactly what you mean by contact ratio. My guess is you might get a different number if you use the effect on thermal conductivity vs electrical conductivity. Are you referring to "contact" on a microscopic or macroscopic level? Flatness affects macroscopic, while surface texture affects microscopic.

 

[Ignicolist]

Sorry, perhaps my explanation was too simple.

I have two plates in contact, both with quite good surface quality (Rz=6-10). Im expecting detailed values.
One of the plates will be hot and the other one cold. To make a very rough estimation of the heat transfer resistance, I would like to know the real contact between the plates.
In addition, there is a force that keeps both plates together, producing a contact pressure of about 5MPa.

The hot plate is made of steel and the cold one with thermal isolating material. If my plates have, for example 60% contact, I will have 60% conduction between materials and 40% convection through air gaps.
I only want to have a very rought approach.

 

[Compositepro]

Your statement "thermal isolating material" makes no sense in context of conductivity. As far as we now know the conductivity would be zero.

In any case, it seems that you need to be thinking in terms of resistivity rather than conductivity. Every layer and interface adds resistance to heat flow.

 

[dgallup]

You can't tell actual surface contact from any Ra or Rz measurement. Many quite different surface traces can give identical Ra or Rz measurements. You need to get into advanced statistical bearing ratio analysis. The Rk parameters are a common approach where the knee shaped bearing area curve is approximated by a set of straight lines.

----------------------------------------

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.

 

[dvd]

Residual stresses in the plates will result in distorted surfaces when subject to thermal gradient (and pressure distribution due to any loading). I think that an experiment to determine heat transfer with real plates would be time and money well-spent. Otherwise, choose a range of reasonable values and design to the extremes.

 

[Tmoose]

"any equation or similar to define the real contact ratio between two surfaces."

umn(x,y,t)=Xm(x)YnTmn(t)=sinµmx×sinνny×(Bmncosλmnt+B∗mnsinλmnt)

Where:
µm=mπ/a,νn=nπ/b,λmn=c√µ2m+ν2n

 

[chicopee]

Here is my take on this subject. The reason conductive past is used between surfaces is to improve for example heat transfer otherwise you are facing the dilemma mentioned in your OP being poor contact. Perhaps the subject on surface roughness could be of some help.

 

[Tmoose]

I //think// this is link discusses kind of the opposite of what the ( 6 week absent ) OP was asking.

https://www.sensorprod.com/news/white-papers/2007_ijh/index.php

But it had some cool (to me) info, so I posted it.

 

[btrueblood]

Thanks Tmoose. Would be interested to know if anybody puts thermal paste on their wheels...and how long it lasts before the bolts loosen and the wheels come off?

 

[RolMec]

there's similarity to brake design: brake disc and brake pad.
so maybe that's a place to dig.
afaik, the thermal conductivity coefficient is established in tests.
a FEA thermal modelling might add insight, for a specific case.


Roland Heilmann

 

[Tmoose]

I live in salty ∴ rusty New England USofA.
To improve the chances of drama-free spring time dis-assembly I often put a thin coating of antisieze, LPS 3, plain ol' grease etc on the hub pilot Ø and mounting faces between wheels, hubs,brake rotors etc.
I torque wheel fasteners to spec. I don't think I've ever had nicely tightened lug bolts/nuts loosen. Over the years this has included both steel wheels ( with built in Belleville effect) and alloy wheels ( that have provoked scary and threatening "must re-tighten in XXX miles" boiler plate at tire shops etc.)