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  • Broken Bolts

    Long ago, in a galaxy far, far,,,,,,,, No wait, that is the wrong story.

    Well, not completely, it was long ago that I came upon a small trashcan in the local scrap yard. I had been looking for something to use near the bench, this seemed like a likely candidate.

    Not only was it a fairly good looking unit, it came half filled with some 1/2" NC hex head cap screws about 1 1/2" long. They were unmarked and very obviously low quality grade 2 or maybe even grade 0 but what the hey! Scrap price and plenty of non critical applications, I can probably use them.


    So over time I have used a fair number of them, and have subsequently found why they were in the scrap. I have never seen bolts fail the way these do, and I thought I would ask here if anyone has any insight as to why or how. It is obviously a manufacturing defect, but is it wrong material, wrong process etc.

    They don't all do this, maybe 10-20% and they look fine until I apply a fair amount of torque and the greater portion of the head just pops off.

    What does the collective wisdom say about this??

    Dave




  • #2
    The break looks pretty clean, like it was made from two pieces.
    Location: Long Island, N.Y.

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    • #3
      IIRC, heads are stamped on using the existing metal of the blank rod? Maybe something went wrong with the stamping process that resulted in hairline cracks...

      Or maybe some inventive Chinese person decided to reduce scrap, by putting reject bolts them back into the stamper with a little bit of extra metal to make up the missing bit of head. (Bolts made from the last bit of a bar that ended up short?)
      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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      • #4
        It's a fascinating failure, I haven't seen anything quite like it, it would suggest there is some defect there but to repeat on lots of them, the steel is made by rolling and the defect is 90 degrees out of place, how odd. The heads are hot raised aren't they? It would be worth saving some and sending them to a university with a metallurgy department, I'm sure they would be interested, make a good project for someone.
        Mark

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        • #5
          Looks like a heat treatment gone wrong to me-
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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          • #6
            Verrrrrry Interesting!

            You could try a cut down the axis with a hack saw, file or sand paper it flat, and then an acid etch to see if it reveals anything. Go slow with the cutting and filing to keep the temperature down so you don't alter the structure.
            Last edited by Paul Alciatore; 02-12-2015, 02:46 AM.
            Paul A.

            Make it fit.
            You can't win and there is a penalty for trying!

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            • #7
              Those are 'U' grade bolts, used under machines to level them. Gravity helps keep them together.

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              • #8
                In the electrical trade we come across lots of bolts where a portion of the head is designed to snap off once you reach a certain torque. They are very common in bussduct, switch gear etc.
                Typically they are easy to identify as they have slot around the head or shoulder where they are supposed to break off, I don't see a notch like that on your bolts however...
                cheers,
                Jon

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                • #9
                  .02
                  Looks like the stock they used to make the bolts was laminated. This sometimes happens in the rolling process. I have never had it happen with stock from here or Europe.

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                  • #10
                    Cold shuts in the rod used to make them, or a similar problem created problem in the heading process, or both.
                    1601

                    Keep eye on ball.
                    Hashim Khan

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                    • #11
                      They are poor man's torque limiting bolts. You tighten them until the head shears.

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                      • #12
                        Looks like they could be useful for certain areas that need low profile bolts.
                        Andy

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                        • #13
                          The heads of most bolts are made on cold formers or "headers". We had a department full if them and I used to grind the dies for them. I have never seen that in anything that we produced but I do think it is a problem with the cold forming process.

                          Brian
                          OPEN EYES, OPEN EARS, OPEN MIND

                          THINK HARDER

                          BETTER TO HAVE TOOLS YOU DON'T NEED THAN TO NEED TOOLS YOU DON'T HAVE

                          MY NAME IS BRIAN AND I AM A TOOLOHOLIC

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                          • #14
                            Originally posted by Black_Moons View Post
                            IIRC, heads are stamped on using the existing metal of the blank rod?
                            Originally posted by boslab View Post
                            It's a fascinating failure, I haven't seen anything quite like it, it would suggest there is some defect there but to repeat on lots of them, the steel is made by rolling and the defect is 90 degrees out of place, how odd. The heads are hot raised aren't they? Mark
                            bborr01 beat me to it.

                            Yes, most fastener heads are made through a single or multi blow cold heading process. For them to shear in the middle of the head indicates a problem in the setup of the heading machine.

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                            • #15
                              Torque limiting as stated already

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