rubber insulator

[autoguru]

I'm designing a device where two pieces or metal come pressing together
at a speed of 0.5m/s. I need to insulate the metal contact area with
some kind of material to dissipate the noise that would be created by
metal-metal contact. The contact area is 3mm^2.

After the two pieces of metal comes into contact, compressive force of
600 lb is applied.

Will insulating the contact area with a thin (0.2mm) layor of rubber
work? Will the compressive stress damage it? I just need something to
kill the noise from the metal to metal contact, yet not be destroyed by the stress that comes later. And the stress will be applied repeatedly.

Thanks.

 

[GregLocock]

that stress is enough to yield steelfour times over, so I think rubber will find it hard to survive.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[EricZhao]

No Greg, the stress just 129ksi. It is OK for some high strength steel. But of course, rubber is not good for this application. You should consider reducing the contact speed.

 

[autoguru]

I don't see how the rubber would get destroyed.

By yielding strength you mean something will get deformed. But deformation is OKAY. The rubber might be crushed, but it's still there. It cannnot vaporize. Deformation is not a problem, just as long as there are some "material" to cushion the sound. Even a very thin layor would make a drammatic difference.

For example, water is very easy to be "deformed", in fact, it's completely fluid. Therefore it's "yield strength" is practically zero. But when you try to compress it in a cylinder, it's strength is enough to pop the steel cylinder.

 

[EricZhao]

You example of water is somehow valid. In fact, at this pressure level, the rubber should be considered as fluid instead of solid. But the problem is how do you hold the rubber there. Since the contact speed is .5m/s. I assume you should have clearance fit. Your rubber would "flow" away.

Maybe you have a special structure. Why not do a simple test. Maybe you are right.

 

[GregLocock]

Only 129 ksi? Strange use of the word only. Yield stress of mild steel is 30 ksi



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[autoguru]

Actually, I don't know how you guys did your pressure calculation.

But 600 lb/3mm^2 = (2670N)/(0.000009 m^2) = 296666666 Pa.

That's 43000 PSI.

I can make the area twice as big with no problem. S0 then the stress would be 21000 PSI. I think even some plastics can withstand that compressive pressure.

 

[GregLocock]

Well, nice factor of 3 error in there.

So, when correcting your 'peers' do you ever bother to double check your own working?

Since we seem to often have these little discussions, could you let us know a bit of your background?




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[autoguru]

Please, correct my error if there are any.

Since you are so curious, I'm a chemical engineering student who likes to tinker wih mechanical things during free time.

 

[autoguru]

Just in case, I meant (3mm)^2, not 3mm^2. I guess I should've been more careful with the typing.

But I mentioned 0.000009 m^2 in my calculation.

 

[GregLocock]

You did not mention .0000009 in your first post, the only info we had.

What frequency noise are you trying to reduce? doesn't a layer 0.2 mm thick ring any alarm bells in the context of 600 lb forces?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SomeYahoo]

Stress aside, how about something like a Nylon 11 coating... real thin. Just enought to dampen the shock.

Or, forget the durability of the rubber and put a coating on the backside, similar to what they use on stainless kitchen sinks. I have also seen a mass placed on the backside of "ringing" metal to dampen out the noise, but I can't seem to find the information readily.

 

[RvL04]

Autoguru;

Considering your initial question: the piece of 'something' you want to place in between these metal rods, is it going to be constrained, or do the sides have an opportunity to 'bulge out', if you will? Do you want to adhere it to one of the rods (so there is rubber on metal friction after contact), or both sides (so there is rubber on rubber friction after contact)?

Which material works best depends on how exactly you're going to compress the material. If you need something to retain its shape well, silicones do just that. They can take huge stresses without growing cracks, and once you unload, they will recover very much to their initial shape. So, if you load it repeatedly, this would be a good option (think about strain offset / Mullins effect). "Yield" is the wrong term to use when dealing with rubbers, because a truly elastomeric material does not "yield" one bit, it recovers fully (the use of the term "yield" is how us rubber people recognize the plastic and metal people).

Silicone may not be very good, though, if it's constrained on all sides, since you're essentially compressing the material volumetrically / hydrostatically. Look for materials that have a very stiff bulk modulus, in that case (in other words, materials where the Poisson's Ratio is approaching 0.5).

Also, at a loading rate of 0.5 m/s, a piece of rubber of about 0.2 mm thick will need some serious damping qualities. I could imagine you wanting to do some storage modulus tests on some likely candidate materials. No particular materials come to mind, but if you google search for a paper called "measuring the dynamic properties of elastomers for analysis", the first hit describes an excellent dynamic test (not sure if I'm allowed to put the link in a post directly).

Of course, a simple high rate compression test of the candidate materials would get you quite a long way. I know there are some material testing labs out there for whom 0.5 m/s is quite easy, and some of the good labs can actually measure stress and strain quite accurately on the specimen.

Finally, the friction matters because the more friction is going on at the contact area (rubber-metal, or rubber-rubber), the more shearing is going on in the specimen (meaning, in turn, the more 'damage' you're doing to the material. You could / should consider lubricating the contact areas so as to minimize the shearing (but, if you use adhesives to stick the rubber to one of the rods anyway, you will have shearing there anyway).

Bottom line is, look at some of these variables and pick a couple of materials to try out; it sounds like you have the luxury to do so (without ruining your metal rods etc.). If not, you could still get the material samples tested on some or all of these criteria you find important, and judge based on those values.

hope this helps!

Ron