Resistance of two clamped chunks of metal

"BobWanker "

On thing to note is that the initial resistance will be much higher until you pass full current through the contact, then the value will quickly fall ( cos local heating will soften the surface and create more contact area) and then stay at the new value.

Ref: Materials and Processes, James F. Young, 2nd Ed, page 192.

.... Phil

Here's a paper on the topic. Probably a bit more theoretical than the answer you want. But you can try chasing down a few of the references for further information

The contact resistance will be less than a microohm if you do it right, but in practice will be limited by oxide on the surfaces and any roughness.

The clamping force's main effect is to increase the contact area by making the material flow--contact area ~ (normal force)/(yield strength).

Cheers

Phil Hobbs

The contact resistance will be less than a microohm if you do it right, but in practice will be limited by oxide on the surfaces and any roughness.

The clamping force's main effect is to increase the contact area by making the material flow--contact area ~ (normal force)/(yield strength).

Cheers

Phil Hobbs

The contact resistance will be less than a microohm if you do it right, but in practice will be limited by oxide on the surfaces and any roughness.

The clamping force's main effect is to increase the contact area by making the material flow--contact area ~ (normal force)/(yield strength).

Cheers

Phil Hobbs

Simply the resistance of the individual pieces + the contact resistance.

You can measure the contact resistance by measuring the resistance of the individual pieces and substracting that from the resistance when they are mated. Of course the contact resistance depends on many variables so...

You can easily calculate the resistance theoretically for the individual pieces.

The contact resistivity should be independent of the bar's size so you can probably get a profile by testing out. It might slightly depend on the contact area because it takes a larger force for the same pressure. So technically you would need use the same pressure. (because the pressure effects the contact resistance)

Again, you can estimate all that relatively accurately if you can measure the resistance accurately. If you can only compute the resistances then you'll have trouble Since the contact resistance and total resistance will be unknown. (I guess you could estimate the contact resistance but it would be difficult I suppose)

I imagine, but have no idea if it is true, that if you passed a very large current through the contacts that you could "fuse" them to some degree. It would have to basically melt the metal at the contacts, assuming it has a much higher resistance than the pieces individually, which would fuse them together to some degree.

It might be possible to correlate the surface friction with the contact resistance. High friction would suppose less coupling and higher resistance. Hence it might be "proportional" to the friction between the surfaces.

Probably your best bet is to "weld" the two metals together if you can(basically the fusing idea).

What kind of metal? What type of clamp? What level of clamping force? What surface finish on the metal?

Good answer, but for some materials (aluminum for example) that oxidize to an insulator, there are bound to be other factors... That's why I asked Bob about the type of metal...

(Yes, I see he say's tin plated copper...)

: :Anyone have an idea how to calculate the resistance between two chunks of :metal clamped together? : :Here's what I'm doing. I have to connect a power supply that will deliver :12V at 600A to a system. I plan to clamp two pairs of bus bars together as :the "connectors" between the power supply assembly and the backplane bus :bars. : :So, I'd like to be able to estimate the dcr of the two mated surfaces. :Obviously the resistance will be a function of materials used, surface :flatness, area of contact, and (I would assume) force applied to keep the :two surfaces together. : :The plan is to have each bus bar be 1" wide by 1/2" thick. There will be 1" :of contact area to a similar bus bar along the 1" dimension. The bars will :be tin plated copper. : :The big question is - how to estimate the dcr of such a system? : :Any help would be greatly appreciated. : :Bob

I don't think that a 1 inch square contact area will be good enough for 600A if you want minimum volt drop. Typically, in an old Strowger exchange a 500A busbar would be 2" x 0.5" bare copper with either Prespan or heatshrink insulation.

It is also important to ensure that the clamping mechanism applies even pressure over the whole of the contact surface area. A system with two external clamp plates with a bolt on each corner should be used. The resulting joint will be virtually airtight thus preventing subsequent oxidation at the contact area.

"Ross Herbert" "BobWanker "

The OP's 1/2 sq inch cross section copper bar will dissipate about 20 watts

However, a join of 5 micro-ohms will dissipate less than 2 watts.

I suspect the engineers that speced the buss bars for Strowger exchanges did not like to have them running warm and so waste precious energy......

..... Phil

busbar

pressure

OK I know nothing of this, but isn't 2" by 0.5" equal to one square inch?

George H.

RH was referring to the CROSS SECTION of the bar - not the overlap area.

In making an over-lap join, one would not use only 1/2 inch of 2 inch wide busbar.

..... Phil

The point was that a 1" wide bar might be found lacking at these current levels.

The most I've ever done was 100A, and that was done with ordinary lugs, albeit large ones (like #2 wire).

Hope This Helps! Rich

I've got 300A running through 1/4" copper tubing and flare joints. But when it's water cooled, you don't care about mating surfaces. ;-)

Tim

Thanks, sorry Ross, I misunderstood.

George H.

Yeah sorry about that. A buss bar with a 2" by 0.5" X-sectional area is a 'serious' piece of copper! I'd sure want more overlap than just that area.

George H.

Thanks, Paul. That article was very helpful.

I think that our tin-plated copper bars should yield less than 12 micro ohms at the clamping force we plan to apply. That will kee the voltage and power losses well within our limits.

Bob

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Didn't look at the paper, but evenness of the clamping pressure over the contact area is worthwhile as it normally results in more contact area. And correspondingly, flatness and smoothness of the mating surfaces.