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Bolted joint resistances.

Bolted joint resistances.

Bolted joint resistances.

(OP)
Hello,

  I am trying to calculate roughly the value for bolted copper busbar resistance.

I have viewed this website

http://www.copperinfo.co.uk/busbars/pub22-copper-for-busbars/sec7.htm#Joint

But the information is less consistent than reality.

I am seeing more like 5-7 micro-ohms resistance across a bolted connection. About half inch thick copper on both ends overlapping about 2 inches, with belleville washers.

Does that sound reasonable?

 Thanks

RE: Bolted joint resistances.

5 to 7 micro ohms seames reaosonable.

I think that national gid say 5 ohms for a fixed connectin and 10 micro ohms for a flexible connection.

I did some testing last week and it was stainless steel lugs bolted to copper busbar, and saw 1.7 to 3.3 micro ohms.

Got one reading of 14micro ohms because the swong washer was used.

What size  are the bolts, what torque is used?

RE: Bolted joint resistances.

(OP)
Thank you for this valuable information.

Do you have any documentation that says what the lug connection value should be? You were mentioning something about the national gid, is this a code I can follow? Do you have a link?

Regards

RE: Bolted joint resistances.

Hi dbecker

What is your joint area? how many bolts? what torque are you using and finally which belleville washer are you using?
If you can provide more information like that above we may be able to help further.

desertfox

RE: Bolted joint resistances.

(OP)
Hello desertfox,

  The contact area is 25.4 mm by 25.4 mm with two holes centered, each hole is 12.7 mm in diameter.

Hardware, two M12 bolts with one belleville washer on each side. I do not know which kind, which are most typically used in industry? We can assume that for now.

I do not have a torque value, assume a typical value of 15 ft-lbs? Does that sound legitimate?

Any kind of ballpark answer with the above information will greatly help me at this time.

Material is fresh copper on copper, no oxidation.

Anything between 3 to 7 micro-ohms is what I am looking for, I just need confirmation.

Thanks again,

  - D

RE: Bolted joint resistances.

Hi dbecker

Yes your figure sound reasonable to me, I calculate your pressure to be around 13N/mm^2

desertfox

RE: Bolted joint resistances.

(OP)
Thank you desertfox, that is much appreciated information. I thought I was going crazy for a second there.

RE: Bolted joint resistances.

(OP)
Hi desertfox, can you please walk me through the hand calculations you performed? I would like to know how you did that.

Thank you,


 - D

RE: Bolted joint resistances.

15 ftlbs looks light for an M12, that's more like what I'd put on an M8.

M12 end up at 30 or 40 ftlbs

 

RE: Bolted joint resistances.

(OP)
Hello, we can assume 30-40 ft-lbs. I just needed a starting number for the calculations, can someone walk me through how the calculation was performed? I am in dire need of this and cannot find much information on the web.

The website i posted up in the beginning of the thread is giving me a result thats 100x what desertfox and I agreed on. Which is why I am curious how to do this calculation. Please help, thank you.

RE: Bolted joint resistances.

Hi dbecker

Haven't got time now to go into great detail, however I got the 13N/mm^2 pressure by calculating the axial load from the bolts based on the first torque figures you gave, then dividing that by your joit area ie:- 50.8mm overlap X 25.4
face area.
Now what does look in question is the units on the graph because it gives micro ohms per mm^2, which implys that you multiply the calculated pressure figure by the value obtained off the graph, not sure thats correct.
I'll look at it again tomorrow when I'm home.

Regards

desertfox

RE: Bolted joint resistances.

Hi dbecker

Okay I have sorted it I think, first calculate the clamping load of each bolt from the 15 ft-lbs using the equation:-

F= T/(0.2*d)    where T= torque
                       d = bolt dia
                        0.2 = friction factor

F = 15*12/(0.2*0.5) = 1000lbf

now you have two bolts :- 2 * 1000lbf = 2000lbf

joint pressure = 2000/(2"*1") = 1000lbf/in^2

convert to N/mm^2 = 12.44 N/mm^2

Now using the graph on the link you provided go along the horizontal axis till you get to 12.5N/mm^2, then go up vertically till you intersect the curve, then read across on the vertical axis a value for Y, in your case I read approx 2500 micro ohms/mm^2, finally divide this figure by the cross sectional area of the joint:-

2500/(2"*1"*25.4)^2 = 1.93 micro ohms.

Okay so I calculate 2 micro ohms for your joint, however a lot of factors can effect that figure ie: variation in bolt clamping load, surface finish of the copper, plating or tinning of the mating surface.
The tightening of the bolts is important, however you should not base it on the strength of the bolt material but on the stress in the copper joint due to bolt torque.
Your pressure on the copper looks about right to me but you should also calulate the stress in copper after the joint as expanded due to the normal service current generating a temperature rise.

desertfox

RE: Bolted joint resistances.

(OP)
Excellent desertfox, I will review this calculation and I may come back with a question. Thank you for taking the time with this it is extremely helpful for our project.

 - Dan

RE: Bolted joint resistances.

Hi dbecker

Your welcome its difficult to respond in the week as I am away from my main computor, however weekends I have good access.
Anyway this paper on bolted joints should help, see section 2.4 it analyses at busbar joint.

http://docs.google.com/viewer?a=v&q=cache:WtP95ocdxHoJ:www.solonmfg.com/springs/pdfs
/DavetPaper.pdf+plain+washer+stiffness+in+bolt+joint+
analysis&hl=en&gl=uk&pid=bl&srcid=ADGEESip_PZWUsnbT-9G_fUtUf1zAMSIDfJcKn7LaYnA5xE1iwWA7Ms6Vzxa_
EVG6DjqkPbEbMjXnldawFuMj5giR2C8dL58Q1NJ4Am_MfQeZ3UKWH-mZpcrsOUGKnvuORY9f5x-9C9h&sig=AHIEtbQB1IPCSyo4zySPTKJ-MkKt7StKCQ

desertfox

RE: Bolted joint resistances.

(OP)
Hello desertfox,

 I read through that document and there was no explicit way to compute resistance for a bolted joint. Although it was very helpful in understanding the belleville washer, it was more geared towards universal usage of them and only one segment devoted to electrical bus connections without any examples on contact resistance.


I will use your technique which you outlined using the website I first posted. That works fine for my case.

Thanks!
 

RE: Bolted joint resistances.

(OP)
desertfox,

When you say divide by the cross sectional area of the joint to obtain the 2 micro ohms, which cross sectional area do you mean? The face to face contact area? Or the area of the thickness of the entire joint (imagine a plane section through the joint).

Thanks,

 D

RE: Bolted joint resistances.

Hi dbecker

the area for the calculation should be the area of contact and from your earlier posts I believed your coppers overlapped by 2" and in the other plane 1".
The link I left wasn't to help you calculate resistance but to show how to calculate bolt and copper stresses due to a temperature rise and also to indicate that bolt clamping forces can vary a lot which in turn effects the electrical contact pressure and hence the joint resistance.
I think you can estimate the resistance of the lug by working out the bolt load due to tightening torque and hence the pressure by dividing in by its contact area.
I assume this is linked to your earlier posts in the Heat transfer Forum relating to some lug failures you experienced.

desertfox

RE: Bolted joint resistances.

(OP)
desertfox you are right this does pertain to an old post I made about lug temp failures.

This is what's confusing me though;

"Now using the graph on the link you provided go along the horizontal axis till you get to 12.5N/mm^2, then go up vertically till you intersect the curve, then read across on the vertical axis a value for Y, in your case I read approx 2500 micro ohms/mm^2, finally divide this figure by the cross sectional area of the joint:-

2500/(2"*1"*25.4)^2 = 1.93 micro ohms."

I dont understand why you squared the area term, if the overlap is 1X2 inches it shouldnt be squared? I am missing something.

I did the bolt calc and got the 12.5 N/mm2, that is right. I found the 2500 micro ohm/mm2 from the graph; I just need to understand which area I divide by, the contact face area or the cross sectional area of the joint (thickness of the joint).


Thanks again,

  - D

RE: Bolted joint resistances.

A slight error on my part i should of squared the 25.4 to convert imperil to metric

RE: Bolted joint resistances.

(OP)
ahhh!!

I thought I was loosing my mind... :)

RE: Bolted joint resistances.

(OP)
I forgot to add, do you happen to have any suggestions for my post on crimped lugs? Looking for a typical value of resistance at the crimp for 646 crimped lug connections.

Thanks again you are great help

RE: Bolted joint resistances.

Sorry, can't help with that one, however I would have thought the electrical resistance at the crimp would be very low if the crimp has been correctly made.

You said in your earlier post that this is related to the failed lugs so I have a question - where exactly did the failure take place - at the crimp or at the lug face?
If the failure did occur at the lug face then you need to look at the link I gave you earlier where it shows how to calculate stresses due to thermal expansion, because if the bolt was incorrectly tightened there is a very good chance that your failure occurred due to this fact.

If it occurred at the crimp then you need to go and have a look at the tooling and make sure that the tool is the correct one for the particular type of crimp. Also, there are standard pull tests for crimped joints, so you need to review these and ensure that your specimen crimp joints passed this test.

Regards
desertfox

RE: Bolted joint resistances.

(OP)
Hello desertfox,

  The failure was a temperature value that was exceeded, not mechanical failure in that sense. I believe it was excess of 35C over a 55C ambient, which means that failure (according to code) occurs at 90C.

I do not know where exactly it occured (could have been under-torqued bolts, wrong washers used etc...), I am hundreds of miles away from the actual unit that was tested and they do not have very good data, just the fact that it hit 90C and failed.

In any case, the failure could also have been caused by poor ventilation of the system, inadequate cooling, too much current; we can speculate. My job at this point is to try and understand what the effects are of bolted joint design and temperature rise for a new design being implemented.

  Would you be comfortable to say that the crimped lug connection is on the order of 1 micro-ohm or less? I would imagine so, they use a 12 ton hydraulic press to crimp the cables and its a permanent ordeal.  

RE: Bolted joint resistances.

Hi dbecker

No I wouldn't want to say I would be comfortable with guessing a figure of resistance of the crimp joint.
What I would say though is do a calculation of stress due to thermal expansion on the connection, you may have to assume a torque to get a clamping load but it might give you an idea of how tight your connections should or shouldn't be.
I also think you are more likely to encounter bigger problems with differential thermal expansion at the bolted joint than at the crimp joint, as the materials at the latter joint will be similar.

desertfox

RE: Bolted joint resistances.

If the crimp joint is made properly, there will be a solid mass of copper or aluminum.  The resistance should be lower than the resistance of an equivalent length of conductor.
 

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