What is MBL (minimum breaking load)

Discussion in 'Sailboats' started by Stumble, Dec 3, 2012.

  1. MikeJohns
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    MikeJohns Senior Member

    Absolutely
    I'm told here that if the manufacturing process is ISO 9000 or above and the product meets a DNV recognised national standard (basically from Europe USA Australia/NZ ) then it's endorsable by DNV . So basically they are giving us a shortcut to quality product selection. Of all the societies you are more likely to find DNV endorsement certificates for a much wider range of goods that the others.
     
  2. DCockey
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    DCockey Senior Member

    From: http://wiki.answers.com/Q/What_do_you_mean_by_minimum_tensile_strength
    Tensile strength is determined from testing a large number of samples. Some will fail higher or lower than others, and an average strength is determined. Minimum tensile strength is usually calculated from statistics using a Weibull probability analysis. In this case the minimum tensile strength usually is reported as the Weibull A value, which is the value at which 99% will survive with 95% confidence. Weibull B, usually based on fewer samples, is the minimum value determined to survive with 90% reliability and 95 % confidence.

    My knowledge of probablility and statistics is limited but I would expect a procedure for determining the minimum breaking strength of shackles or other fittings would include testing a "sufficently large" number of samples, and then developing a probability distribution of strengthd based on the test results. The minimum breaking strength can then be defined based on the statistics of the distribution. The magnitude of "sufficiently large" would depend on the confidence level desired.
     
  3. pdwiley
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    pdwiley Senior Member

    I think that if you have broken a representative sample to get a statistically valid number, advertising the MBL as 9500 lbs is fair. I personally would also like to see the standard deviation on the average but I'm like that.

    We used to use Rochester data cable that only had a stated breaking strain (MBL) no SWL due to its nature. As we usually had in excess of $250K equipment hung on it, we respected their number and tried to stay well under it.

    As for SWL, 1/6 to 1/4 of the MBL seems acceptable, ignoring any peculiarities specific to Ti that may reduce that number.

    There is probably a niche market for Ti shackles in the marine science area where cost really isn't a consideration but reliability and freedom from contamination is. For some things we couldn't use galvanised shackles as the salt water runoff could contaminate samples & equipment. Finding 316 shackles etc with a certified SWL was an issue as no way would we permit uncertified crap from a yacht chandlery to come aboard.

    PDW
     
  4. Stumble
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    Stumble Senior Member

    PD,

    I am still working on the numbers, but at least in larger size shackles, say over 10 ton MBL we might actually be cheaper than steel. I have yet to hear back from the engineers, but the quick numbers I ran today seems to indicate that we could replace a 1" 5lb shackle with a 1/2" .75lb shackle and keep the same strength. While our material is more expensive by volume, when compared by pounds of breaking strength we may actually be cheaper.
     
  5. pdwiley
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    pdwiley Senior Member

    That could be very useful in the right circumstances. I've retired from that game some years ago but we had a strict weight budget to work to as heave on an instrument package at the end of 5000m of wire could get closer than we liked to the breaking point of the wire. In fact there are a few instrument packages on the bottom of the ocean. They're not lost, we know exactly where they are, but they aren't coming back any time soon.

    There were quite a number of times we would have paid quite a lot more if we could have reduced the weight of the pressure housings we made. Not shackles etc but I'm sure there's a market for Ti instrument housings as the current crop are either aluminium or stainless.

    My personal use, I can't see a lot of need for Ti shackles etc as I plan on using galvanised ones (but I'm building a low tech rig). In the 10mm to 16mm sizes that are the largest I'm likely to ever need, the weight saving wouldn't be significant. However, like most things technological, if the specifications and price was right I'd do it.

    Afterthought on sizing: keep in mind that while a 1/2" Ti item might be as strong or stronger than a 1" one in steel, it still has to mate with other fittings. Nobody is going to use a 1/2" shackle to join say 22mm trawl warps because it simply won't fit. You may have to make the shackles the same physical size as what you aim to replace in terms of eye size & pin length etc.

    PDW
     
    Last edited: Dec 5, 2012
  6. jehardiman
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    jehardiman Senior Member

    Yep, That is a major consideration but having shackles of the exact same size with different strenghts is also an issue. About 20 years ago there were issues in the Navy with grade A (std) and grade B (hi-strengh) shackles. After several accidents, we went through and threw out all grade A shackles and now track all procured items to ensure that they are grade B.

    I know, that is why I sent you to the specs, they are very specific about what is a "lot", lot sampling, load test set-up and requirements, and what is "failure". It is easy to use a test set-up that does not reflect actual loading conditions (like pin on pin) which can give false indications.
     

  7. Stumble
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    Stumble Senior Member

    My thought for the high load stuff is that we would make someone that was dimensionally the same size (throat, depth, ect) as say a 2" shackle, but have the shank and pin size of a much smaller one. So there would be a clear physical difference in how they looked (weight as well of course). My thought is this would solve the fitting problem PD raised, as well as the logistic issue raised by jehardiman brought up.

    This is a long way from hitting the market, and I am most defiantly not an engineer, but the more I look into it, the more intriguing it is becoming.
     
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