Bolted Busbar Joint & Temperature Rise 2

[desertfox]

Hi

I am looking to do a bolted joint analysis on a copper busbar similar to the one on page 13 of this paper.(link below)
I need no help with the analysis as such, but an interesting point was brought to my attention and that was the fact that the stiffness of the plain washers was not included in the analysis, at the bottom of page 16 it says that the "spring rate of a flat washer is negligible" however I not sure whether thats just a bad term of phrase or my understanding, because those plain washers are infinitely stiff when compared to the bolt and the joint material, I assume there not included in the analysis because they need so much force to generate a small strain that the rest of the joint would have gone well beyond its yield stress in doing so.
Basically the plain washers are just treated like solid lumps with no compression,I tried doing a search on bolt joint analysis but every example I found, whether it be thermal stress or otherwise did not include the plain washers in the analysis.
Any constructive comments welcome, all I am trying to do is get a perspective on my thinking right or wrong.

http://docs.google.com/viewer?a=v&q=cache:WtP95ocdxHoJ:

http://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_
EVG6Djqk
PbEbMjXnldawFuMj5giR2C8dL58Q1NJ4Am_MfQeZ3UKWH-mZpcrsOUGKnvuORY9f5x-9C9h&sig=AHIEtbQB1IPCSyo4zySPTKJ-MkKt7StKCQ

desertfox

 

[dhengr]

desertfox:

Is this the logic and line of thinking you are looking for?

In the scheme of things the flat washers contribute little, so they are neglected to simplify the analysis. The plain washers are not “lumps with no compression,” they just have a negligible elongation or shortening. Their length (thus, their deltas) in the direction of your analysis is very small in comparison to all the other members in the joint. Their modulus of elasticity and coefficient of thermal expansion are about the same as the nut and bolt, so I suppose you could factor their bearing area (brg. stress and delta), thickness (length in direction of analysis) in some way and just add that to the shank length of the bolt in your analysis; or just include their small contribution directly; but that’s no simplification.

They do not have the actual spring potential of the Belleville springs/washers, so footnote #9 is a reasonable statement. The “stiffness” of the plain washer is relatively immaterial since it just transmits the bearing load from the outer edge of the Belleville washer over some annular area, and its thickness delta is small. I can imagine your problem, but have not actually done the analysis you are doing. Isn’t the real problem enough pre-load; but then the soft elastic and high thermal coef. of the copper busbar and its creep potential cause joint loosening. At some point maybe the solution is two Belleville springs face to face, with a flat washer btwn. them, at the bolt head and at the nut, to maintain some pre-load after cycling many times. Is this called an elastic and thermal matching problem?

You said “large force to generate a small strain.” I think we are saying the same thing, just in different words, coming from a slightly different angle. But, words always seem to fail us, without sketches and instantaneous, face to face, ability to interact and exchange thoughts. You sure do find interesting stuff on the internet.

 

[desertfox]

Hi dhengr

Thanks for the reply.
Yes it was the sort of thing I was looking for,I was slightly thrown by the analysis given in the link I posted, because whilst they didn't include the washer stiffness in the calculation they did include its the free expansion for the temperature rise.
I had a senior moment and included the washer stiffness in my analysis initially, however I ended up with an extremely large force which would have pushed the washer straight through the copper and I realised my error.
The statement "spring rate of a flat washer is negligible" also bothered me because, spring rate to me is stiffness and the stiffness isn't negligible its hugely stiff in relation to the washers, your statement negligible elongation sits much better.
Yes its a balancing act when it comes to the copper joint I have to look at joint electrical resistance and how it relates to the compressive stress when the joint is made, on top of that you have thermal stress generated when the joint is in service and also under possible short circuit conditions,joint relaxation etc.
Anyway thanks again for clarifying my thoughts and I believe that we are saying the same thing and maybe I should have clarified my washers as incompressible lumps as incompressible in relation to the busbar joint.

Regards

desertfox

 

[unclesyd]

The washer.

http://www.energy.siemens.com/us/po...topics-application-notes/techtopics16rev0.pdf

 

[dhengr]

desertfox:
Glad to be of help. I see you helping plenty of people here, with some very sensible sketches, suggestions, links and answers. So you deserve one back at you once in awhile. And, God knows we both deserve a senior moment now and then too, I certainly have them. If you weren’t so darn smart, and doing this the old fashioned way, you never would have appreciated your error in thinking. You would have done this with FEA, squeezed the cooper bars right out of the joint, and gone on your marry way, with a perfect computer printout.

 

[desertfox]

Hi dhengr

Thanks for your kind sentiments, I prefer the old fashioned way because as you say it helps the understanding of what your doing.

Thanks for the link unclesyd, I was after a little bit more though on the actual calculation of the joint under thermal stress as opposed to the general description in your link, however it again confirms my thinking.

desertfox

 

[unclesyd]

Appreciate the acknowledgement.

I was killing a little time looking around while I was trying to contact my old buddy who designed all the bus bars for our electrolytic process, unfortunately he has passed on and is probably working on any problems that come up with lightening. I just found out today.

You might find a reference or two in the following papers

http://www.multi-contact.sk/AcroFiles/Catalogues/SZ_Multilam-F_(en)_hi.pdf

http://cdsweb.cern.ch/record/1028176/files/lhc-project-report-1004.pdf

http://www.schneider-electric.com/d...ectrical-know-how/general-knowledge/ect83.pdf

Anecdotal:
I worked with this engineer on many problems but one concerning bus bars and washers really had us pinned downed. We had a process that leaked a lot of ammonia vapor and this got really bad when they did away with asbestos gaskets. We started loosing "cages' on "stabs" in our substations and switchgear. When we stated to loose bolts and washers on the some bus bars it was obvious that stresses played a large part. We couldn't figure why as the most failures wee on connection made up by Pee Wee the smallest electrican on site. We agonized over this for several months until coming back from lunch I happened to see Pee Wee swinging on a three foot heater on his socket wrench. He would tightened a connections this way then using click type toque wrench check the connection, always good.

 

[desertfox]

Hi unclesyd

Thanks for the latest links there very interesting.

desertfox

 

[Tmoose]

That (2001) Seimens bus bar link advocates split type lockwashers and large diameter, thick flat washers. I think more modern info would replace the nasty split lockwasers would substitute belleville type washers, in quantitiew and orientation selected to produce a specific load.

Some older MIL specs for electrical equipment would only allow star type lockwashers on bolted electrical connections

Dan T

 

[EdStainless]

We used to silver plate the joints, use bellville washers, and bronze bolts. Started using steel ones but found that when the system got hot the steel bolts were so stiff that the Cu yielded and then when cold the joint would be loose.
We were only running about 10kA, but it switched on and off very abruptly.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[desertfox]

Hi Tmoose & EdStainless

Thanks for your posts

desertfox