What is MBL (minimum breaking load)

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

  1. Stumble
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    Stumble Senior Member

    This may seem a little silly, but is there a specific definition of MBL other than "of the parts we tested this is the lowest point at which they broke?"

    We are having an issue comparing our shackles to some stainless ones, and I have a suspicion that we are being overly cautious in how we are rating our parts. Basically we tested a bunch of 5/16 shackles and they all broke between 9,000 and 9,500bs. We then down rated them to a MBL of 4,000lbs.

    Given that Harken publishes a MBL of 6080 and a swl of 3040lbs for their high test stainless shackle, I am just curious if we are fairly comparing apple to apples here, or if their parts are actually breaking at 6080? If they are taking the same safety factors we are then we have to have a design issue somewhere in the part and need to try and identify where it is.
     
  2. MikeJohns
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    MikeJohns Senior Member

    Greg
    Min Breaking load is just a statistical sample minimum ultimate strength when tested to failure, so if you test some number of random parts coming off the assembly line to breaking point the weakest specimen gives the MBL. Your local standards will determine the test procedure and sample size but it will usually be several batches from successive production runs.

    Remember working load should be related to expected life and fatigue limits which tends to set a sensible factor of safety on Ultimate Strength, where US is a sampled MBL (not calculated).

    The stainless FOS above is just 2 on breaking. Safe WL isn't usually used much anymore since it's confusing. For example SS isn't going to last all that well at 50% ultimate stress.
    Also Ultimate strength is very much influenced by the test rig, if you presume the shackle eye has a partly distributed load representing a rope and the pin is nicely loaded and restrained you'll get a different result to a shackle to shackle with a point load. (Materials are much weaker in shear relative to tension.) So look closely at what test was performed and whether they tell you the procedure used.

    Hope that helps
     
  3. Petros
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    Petros Senior Member

    You have to find an organization that rates your industry products, and find out how they define it. For example, Underwriter's Labs (UL) rates consumer products, and had definition of various tests. There are various trade groups that have standards for testing other products like automobile related products (society of automotive engineers, SAE), building materials (International conference of building officials, ICBO), and so forth.

    On purely matreials testing it is ASTM (American socitiy for testing and Materials) and they use very different names. Yeild strength is the load where the material takes on permanant deformation, Ultimate strengths is where it fails completely, and fatigue limit and the load that long term repleted loading will not causes a fatigue failure. Working load is usually the fatigue limit. These are the terms I am familar with, and using an industry terminlogy must come with a precise definition, if not it is could be anybodys guess.

    For examle, for most manufactured goods, the point at which permanent deformation occurs is usually considered their max acceptable load (plus an appropriate safety factor depending on how predictable the max load will be). but on certain safety related items, deforming an anchor is not considered relevant, but ultimate load is. It is assumed the item will be replaced after a hard impact.

    So each industry developes practical applications based on long experiance with that particular usage. ANd there are many suppliers in unpoliced industries that simply make up their own definition, or worse, knock off another manufacutrues item, even giving it the same name or part number, makes it cheaper and never makes representations of strengh. Be wary of such "off brand" products, do your own tests to be certain.

    As one of my engineering professors used to say, one simple test is worth 1000 expert opinions.
     
  4. Submarine Tom

    Submarine Tom Previous Member

    OR, you could run a series of tests including off the shelf purchased competitors product, publish the results and let the educated public decide.

    Be honest and let the product speak for itself.

    Wouldn't that be a refreshing approach?
     
  5. Stumble
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    Stumble Senior Member

    This is pretty much what I thought. I have been looking for the testing standards from lloyds, or ABS, but haven't been able to even find a reference number, let alone the standards.

    If anyone happens to know what the testing standards are for a classification society I would greatly appreciate a reference number.
     
  6. MikeJohns
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    MikeJohns Senior Member

    Greg

    You won't find classification societies approving shackles.

    Here ( Australia) there is an Australian standard and the flag requirements are simply AS compliant gear.

    If you look for local lifting equipment standards you should find what you seek.
     
  7. fcfc
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    fcfc Senior Member

    Harken has an Industrial division : http://www.harkenindustrial.com/

    Check what Harken publish as Safe Working Load for the industrial part, and leisure marine part. For parts apparently being very similar.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Here are two examples of shackles and their SWL and definitions. It is not wholly consistent, but same ball park values. I use these values when i select which shackles I wish to use.

    shackles-1.jpg shackles-2.jpg

    Where as another company I also use, they simply quote the SWL, these SWR shackles are DNV Class approved too.

    shackles-3.jpg
     
  9. powerabout
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    powerabout Senior Member

    But they will issue certificates for them
    when I worked on anchor handlers every single shackle, wire, fitting, had a classification certificate with it
    checkout vanbeest.com , yes the standards are not class ones
     
  10. MikeJohns
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    MikeJohns Senior Member

    Ad Hoc
    I think the class societies look for ISO 9000 or similar and gear compliant with an acceptable standard. In the case of steel shackles for lifting ductility at low temperatures is essential as well as strength.
    [ edit added] I see looking at DNV that they simply accept international standards such as EN13889, although that is only for steel shackles. So if it meets EN13889 and you pay DNV the registration fee you can advertise DNV approved and get certificates!

    DNV are very useful to me since they endorse a huge range of equipment paints, nuts and bolts ,concrete additives you name it. I think it's quite a clever form of marketing. But it's great trying to sell some solution to a client.

    I doubt they have anything specific on shackles except that they should be selected appropriately for the task ! And if they are endorsed you know they have good product quality control.

    Actually Greg's firm might even be able to get DNV approval for these titanium shackles. But it might not mean much in the US.
     
  11. MikeJohns
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    MikeJohns Senior Member

    All those European shackles would have been manufactured to meet the EN standards and they will set out the process of quality control and testing. But not much use to Greg since they will all be for steel.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    That is a slight generalisation, but correct.

    It is all about quality in the true sense, quality control. Thus providing the paper work from drawings office to finished product, every bit of paper work that has been checked and signed approved and the material bought from a similar approved source with traceability and then the finished product tested, (with QA cert for its own calibration of equipment used to test it etc) witnessed and approved etc. etc.

    Thus when buying a nut, bold, shackle, if DNV approved, it simply signifies that it isn't from Joe Bloggs garden shed. If it says SWL 1.00tonne, then that's what it means.....it provides confidence that the source product does what it says on the tin...or least should do!
     
  13. Stumble
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    Stumble Senior Member

    After some more research I found the ABS "Guide for the certification of mooring chain" that details their testing procedures and requirements. Of course it isn't directly comparable since it only is written for steel, but at least it gives a rough idea for what to look for.

    Basically minimum breaking load is exacally that, pull to break a representative sample of the shackle in question. They then require proof testing that is 1/2 the MBL, which requires the load be applied, then that part inspected for any dimensional change. There is no ABS definition of SWL, but as AdHoc pointed out the industry standard is roughly 1/6 the proof test load.

    So we have been massively under reporting our acceptable loads compared to industry standards. Thanks for the pointers guys.
     
  14. jehardiman
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    jehardiman Senior Member

    You should also check out RR-C-271F and ASME B30.26. FWIW, the Navy uses different factors of saftey on "minimum breaking strength" depending on the selected service (static, running, or man-safe) for determining the "test load" and the "rated load". So based upon the requirements of P-307, P-9290 or other requirement, the exact same vendor shackle could be rated for different working loads, but each shackle would be individualy identified and tracked once it was designated to a service.
     

  15. Stumble
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    Stumble Senior Member

    As I understand it every particular service a shackle could be placed in could have a different SWL, which could also be user specific. But the MBL is static and based on actual destruction testing, not calculations. The issue I was running into is that our MBL has been listed at 50% of the actual measured MBL of the tested samples.

    Basically we broke them at 9500lbs, and were reporting the MBL as 4,000.
     
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