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Torque Specs in various metals

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  • Torque Specs in various metals

    We are in process of rebuilding all of our large frame air circuit breakers on the plant, and I am having a hard time coming up with torque specs for things like steel bolt in threaded copper buss bar. An example of this is:

    3/8" grade 5 plain bolt through a sleeved insulator into a copper buss plate 1/2" thick.

    The sleeve precludes breaking the insulator, so the torque would be only from the steel backing plate to the copper buss. None of our breaker manuals have torque specs for their breakers. It's obvious that I can't torque to regular nut and bolt specs, the copper threads would strip out in a heartbeat.

    So is there a place where I can find any kind of torque specs or recommendations for making a bolted joint into softer materials?

    As usual, thanks for any help,

  • #2
    Very good question, as I've spent the best part of an hour searching and have yet to find any kind of definitive specs for copper torque specs.

    I did however find two links that may be somewhat interesting for you.

    Have a look at section 7 especially in this link.

    Here is another link from which you may gain a little insight as it gives you an idea of torque values in metal such as brass and aluminum for example, but not copper.

    If it were me, (and I realize this is not very scientific), I would sacrifice one or two in order to see what the maximum torque is before thread failure. After getting a handle on maximum torque before failure I would use a torque figure of 70% of the failure torque.

    Well I gave it a shot. Hopefully someone will jump in with something more concrete for you to work with.
    Home, down in the valley behind the Red Angus
    Bad Decisions Make Good Stories​

    Location: British Columbia


    • #3
      Could you set up your own test?

      Try and break about 5-10 bolts in material that you plan to use, using taps and tap drills that you plan to use. Then set a specification some comfortable percentage lower than that.



      • #4
        Try Phelps Dodge. Copper is just about their ONLY business.
        Duffy, Gatineau, Quebec


        • #5
          This is an exercise I have recently been through. Usually fastener manufacturers do their own testing and provide specs. But that is only for common items.

          We manufacture some specialty aluminum nuts. The basic engineering practice is to sacrifice a few for sample. (Or make some samples from the same material).

          When testing them here we use a torque wrench and work up until failure. Note the failure torque. We repeat this a minimum of ten times, take an average and use 50% of the average. If you are careful in taking your measurements, you will find they do not vary much from one to the next.

          I use 50% because it gives a good factor of safety, plus these items are constantly assembled/disassembled.

          Remember there are two specs. One is dry and one is lubricated. if developing your own spec, remember to specify if it is lubricated using Teflon tape, anti seize, oil etc...Even if using Loctite. Always test in the condition you plan to use them.


          • #6
            Possibly you could take the torque value for the steel in steel (base case) and multiply it by the shear strength of the copper divided by the shear strength of the steel (base case).



            • #7
              The guys have inadvertantly stripped the threads out of the bus bar twice, both times using 40 and 42 ft lbs respectively. I don't have a lot of extra material laying around, these buss bars are factory made and threaded. I would rather not screw any more of them up.

              I know what *some* electrical connections use as a bolt torque spec, and that figure is about 60-65% of the 40 ft lb failure force I have observed today. So what you guys are saying is making some sense. Therefore, I believe that 26-32 ft lbs is going to be just about the ticket for my purposes.

              I'm still looking for some kind of bible for the soft stuff... if it exists.

              Thanks guys,


              • #8
                If you cant find a "bible," how about some logic? Since these are steel to copper buss joints, they carry no current and thus dont heat much. It seems that this is a rebuild, so %age thread, etc. is what you got and you are stuck with it. Since you have 1/2" thread, you have 8 threads engaged. Why not arbitrarily select 20 lb/ft for maximum torque, and use red Locktite on the threads? This will not shake loose. It is unlikely to heat to releasing temperature, (over 400 F,) and your gorillas wont strip anymore threads. This way you have provided a standard procedure for everyone to follow. I assumed that it was not important that you obtain the maximum possible torque, just keep the insulators from falling off!
                Duffy, Gatineau, Quebec