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  1. #1
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    Forces on a splice

    I was thinking about how weight is supported on a splice done with a locked brummel and bury...

    Can someone explain how the bury adds structural support??

    It seems from looking at it, that the locked brummel is the actual strength portion is, and the bury is to keep the line from unravelling allowing the brummel to fail.

    Is that correct?

    If so, why is such a long bury required?

    I can see that a longer bury will provide more friction on the tail portion (working end?), preventing the forces from unravelling, but with the amount of constricting forces involved in a bury, it seems that the suggested bury lengths are significant overkill.

    Please don't get me wrong, I'm all for safety, and I am using long burries, I'm just making sure I understand the physics involved.

    For example, f you were to make a loop with a tail that is very long (say 12" on 7/64 amsteel, would there be any danger of not doing a bury?


    Again, I'm not planning on doing this!


    The reason I started thinking is that I have a use for some relatively short dogbones, and it seems that the minimum length is dictated by the bury length.

    I hope that makes sense - It's almost 4am here - a bit on the tired side. :P

    Cheers!

  2. #2
    Senior Member GrizzlyAdams's Avatar
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    You've got the roles of bury and locked Brummel reversed.

    Think of a whoopie sling as an 'adjustable' eye, rather than a fixed eye. The important distinction is that there is no Brummel, locked or otherwise, which makes it possible to adjust the size of the eye.

    All that is holding the eye size fixed under tension is the bury. Exactly the same forces are involved in the fixed eye as in the whoopie sling.

    The locked Brummel is placed to keep the cord that is buried from being pulled out accidentally when the bury end is not under tension. The same thing can happen to a whoopie sling, but the preventative measure there is to put a nice fat stopper knot at the end of the whoopie sling's tail to keep it from being pulled entirely into the bury and then out.

    A splice of two lines will hold without a locked Brummel, try it!, the same constriction forces as make whoopie slings and fixed eyes are at work.

    Depending on the application, I've made short bury dogbones, but not ones that were going to be on the main suspension line, or cause me to drop if they failed.
    Last edited by GrizzlyAdams; 10-07-2013 at 06:30.
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    (alias ProfessorHammock on youtube)

  3. #3
    Senior Member Bubba's Avatar
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    To add to what Grizz has said, the bury lengths also seem long because the manufacturer recommends the buried end be tapered a certain length. Sudden changes in diametre are areas of weakness. Shorter buries will hold fine so long as you are aware of the manufacturer's recommendations and that the risk of failure increases if you do not follow them. I don't follow the specs but am fine with it.
    Last edited by Bubba; 10-07-2013 at 07:01.
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  4. #4
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    Grizz and Bubba are correct.

    If you need shorter dogbones, the answer is to use continuous loops instead. They still require the same length of buries, but the buries are side by side instead of end to end. You can easily get down to 7" long loops. Need it shorter? Use soft shackles.

  5. #5
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    I do understand that the bury adds friction with the constriction forces, and that a long bury will hold all on it's own.

    I'm still having trouble seeing how that really comes into play in this case.



    If you take the image above and try to pull on the end of the loop, you will not be able to pull out the loop (by hand) - The brummel is taking all the forces.

    If the tail is buried, I do see that it adds additional grip that prevents the tail from unravelling, thus increasing the maximum load capabilities, but the main force is still on the brummel lock itself, isn't it?

    I don't have anything that would allow me to do any real testing, but i'm curious what the breaking strength of a non-buried tail is, compared to buried tail of equal length.

    Anyone want to indulge my curiosity?

    Also,
    Thanks for the advice guys. I know I could have used a continuous loop, but I like the loop of the dog bones.

    I have already made a few of the soft shackles too.

    I'm getting to the point where I can't think of anything else that I need to slice for though.


    Cheers,
    S

  6. #6
    Senior Member Bubba's Avatar
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    I agree that brummel itself would hold a lot of weight. I think when the tail is buried it keeps the forces in the same direction. I suppose putting a separating force could cause weakness. Maybe if enough force was applied the strands would unbraid. Just a guess.
    Don't let life get in the way of living.

  7. #7
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    That's my line of thinking, which is why I asked the question.

    If all that is true, then how strong is a locked brummel with a short tail/bury compared with a more typical length bury?

    S


    Quote Originally Posted by Bubba View Post
    I agree that brummel itself would hold a lot of weight. I think when the tail is buried it keeps the forces in the same direction. I suppose putting a separating force could cause weakness. Maybe if enough force was applied the strands would unbraid. Just a guess.

  8. #8
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    In this test of a polyester rope, a locked brummel with no bury, broke at ~60% of a locked brummel with a proper bury. Polyester rope and the different braid have different characteristics than amsteel so take it for what it's worth.

    My conclusion from this test is that, if you don't have a sufficiently long bury to keep the forces off the locked brummel, the LB will fail at 60%.

    I can't find it now, but somewhere I believe I have seen a test of a LB with a very short bury as you are suggesting and the buried end unraveled allowing the LB to pull apart without breaking.

    HYOH.

  9. #9
    Senior Member kbajg's Avatar
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    Quote Originally Posted by sunkmail View Post
    I do understand that the bury adds friction with the constriction forces, and that a long bury will hold all on it's own.

    I'm still having trouble seeing how that really comes into play in this case.



    If you take the image above and try to pull on the end of the loop, you will not be able to pull out the loop (by hand) - The brummel is taking all the forces.

    If the tail is buried, I do see that it adds additional grip that prevents the tail from unravelling, thus increasing the maximum load capabilities, but the main force is still on the brummel lock itself, isn't it?

    I don't have anything that would allow me to do any real testing, but i'm curious what the breaking strength of a non-buried tail is, compared to buried tail of equal length.

    Anyone want to indulge my curiosity?

    Also,
    Thanks for the advice guys. I know I could have used a continuous loop, but I like the loop of the dog bones.

    I have already made a few of the soft shackles too.

    I'm getting to the point where I can't think of anything else that I need to slice for though.


    Cheers,
    S
    Don't have a scientific exact amount for you but from experience pulling heavy & I mean heavy electrical wiring thru a conduit it will hold a lot.
    In a pinch we will make a quick splice or eye loop by just passing the line thru itself like your Brummel picture 3 or 4 times. Hooking wire to the eye & pulling it in thru conduits with a tugger, winch or even a vehicle. Have never had one break or pull out & that's with upwards of 500lbs or a little more according to the gauge on the tugger. FYI we are usually using normal 3 strand rope any where from 3/8" to 3/4".
    Looking at your picture I can only imagine it would have the same holding strength or close to it as the rated strength of whatever line you are using.
    Again not scientific just years of experience or maybe just dumb luck.
    For safety issues we always treat every line/rope like it may break especially when applying that much force on a rope.

  10. #10
    Senior Member Jtupnsmoke's Avatar
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    Forces on a splice

    I've always wondered this myself. Makes sense after grizz's explanation. Unfortunately I've made quite a few locked fixed eyes with this incorrect thought process.

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