Performing "retorque" of bolted electrical bus connections

[meengrn]

There seems to be two camps of thought on performing "retorque" of bolted electrical bus connections. One promotes regular retorquing of bolted connections while the other calls for repairs made to connections based only on the results of a thorough thermographic survey. I have also been led to understand that regular retorque can max out the elasticity of the fasteners and cause the joint to loose its clamping force. Who's more correct and what are the details?

 

[byrdj]

i would be sure you are using correct torque. i have not done a lot of bus bars, but when i did i got some one to double check what was going to used. the torque and resulting strain should be well below the plastic yield limit. thus these bolts were no where near torque used elsewhere.

if below the yield, the work harden should take many passes to occur.

 

[dewone]

Checking the torque with a torque wrench should not yeild the tensil of the bolt any greater than the original torquewas set to by the manufacture. It should be re-torqued to the same value. You are only ensuring it hasn't vibrated loose. I can understand where someone over torqued a bolt and it will loose its strengh, deforming the threads and or stretching the bolt. The values your connections are to be torqued to ussually are found in the mfgs manuals. Over torquing can also smash the connection deforming the copper, worsening the condition as well.
Before thermal imaging annual torquing of the bus connections was a common practice. Just make sure they use a torque wrench.

 

[LionelHutz]

I think dewone has it about right. Use a torque wrench and just go until the torque wrench indicates it's tight. If the person doing it gives the nut a little turn each time even when the torque is correct that's not really good either. Personally, I'd be happier with records or the retorque which also show what connections were found loose or were actually tightened.

I read an article where they said in a paper mill it was found that if loose connections were tightened that only 1 in 5 showed an improvement during infrared scans. If the joint was taken apart and cleaned before putting back together then 92% showed an improvement.

 

[IRstuff]

That probably makes sense.

If a connection was found to be loose, then contaminants and corrosion can enter. If the connection is not properly prepped, then retorquing would not guarantee a good connection.

TTFN

 

[DanDel]

My understanding is that re-torqueing should only be performed by first loosening the bolt, then torqueing to it's specification.

 

[itsmoked]

I would not agree with that.

In some cases you may want the torque reading to occur during motion as this separates the dynamic friction from the static friction. In that case you may slightly loosen a bolt just enough to allow motion in the tightening direction but only just enough.

First you should just apply force with a torque wrench. If you reach the proper torque before any movement occurs.. just move on to another.

Also if your buss bars are copper the steel bolts are probably operating far, far, below their maximum load levels.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DaveScott]

A bit extreme: on our EAF connections we re-torque as a routine measure, because waiting for temperature in the joints would not be such a good idea (60,000 Amp nominal).

 

[waross]

I have a problem with retorquing unless the bolts are removed, cleaned, and lubricated. After a few years, I find that many bolts have much more friction in the threads than when new.
Whatever happened to bellville washers?
respectfully

 

[desertfox]

Hi meengrn

If you can get your hands on a copy, a book called "Copper For Busbars" by the Copper Development Association deals with bolted busbar joints in terms of joint effiency, joint pressure, torque values, preping the joint etc.
I would go with the manufacturers recommendation with regard to checking bolt torque.
Joint preparation is extremely important, I can recall an incident where I was asked to look at some electrical bushings that were having there threads stripped during assembly with a steel bolt, it turned out that the torque figure quoted for the joint was given dry (ie no lubricant)
however the fitters on the job were coating each conductor as they went along with a electrical conducting grease which in turn inadvertently got onto the threads of the next steel bolt they were using. The result was this reduced the friction between the steel bolt and bushing and for the given torque they were using resulted in twice the expected clamping force and this caused the brass threads in the bushings to fail.

regards

desertfox

 

[itsmoked]

Yep, "dry" or "lubricated" makes all the difference.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Electic]

Check with the manufacturer.

Eaton offers switchgear with no-retork promise.

Others recommend a periodical disassembly with new paste and burnished contacts, etc.