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  • 'Tie down point' load rating

    There has been a lot of discussion over the years about rated recovery points. Shiny plates with a SWL or WLL stamp are often taken to be safe for the rated capacity.

    These plates are only as good as what they are attached to, and importantly how they are attached. You will struggle to find any information that rates the holes that are used for mounting most of these recovery plates. Just as important is the method of attachment. When designing plates to bolt together to take a load in shear, the friction of the joint is an important factor. The joint should be designed so that the bolts create a friction joint and don't just take a shear load as is the case for most recovery points. And using 'high tensile' bolts doesn't do away with the need to use them correctly. They are designed to be high tensile in tension, clamping a joint together. They are not designed specifically for shear loads as they are often used when attaching a recovery point. Anyway I could go on, but I'd love to see some science to the rating of recovery points.

    The 'tie down points' as they are often called and which come on the vehicle from the factory are actually engineered by the manufacturer to take loads. Take a close look and you will see that they are not some flimsy bits of metal, not on the Prado anyway.

    I decided to do some quick calcs on the 'tie down points'. If there are any welding experts please let me know if I got this wrong.

    Each u-shaped 'tie down' is 16mm diameter and has 4 full circumference fillet welds holding it in place. That gives 5cm for each weld x 4 is a total of 20cm = 7.9inches of weld for the u shaped tie down. The fillet welds look to be about 8mm but lets round that down to 6mm (1/4inch) for safety. Rule of thumb says that 1/4 inch weld can hold 1t / inch (a conservative number backed by some complicated welding theory). So that gives a theoretical dynamic rating of 8t for each 'tie down point'. Let the flood gates open....

    edit: And by my calcs the 16mm bar should be good for about 10t, so the welds should fail first.
    krypto
    Avid PP Poster!
    Last edited by krypto; 04-12-2018, 12:17 AM.
    [B]Steve[/B]

    2010 Silver GXL Prado 150, D4D Auto, with a few non standard bits

  • #2
    if you paint the tie down points yellow or red it will double their capacity

    Adding some branding and stamps will then double that capacity again :-)

    Comment


    • #3
      hi

      just thought i would chip in on the theory a little. I'm a structural engineer and play with much higher loads, however the theory is all the same...

      your weld numbers are in the ball park and i would hazard to say conservative, without knowing all the steel grades it is hard to check - my normal go to weld would be 1 tonne / 10mm for normal structural applications (6mm cfw, E48xx). In a general sense, welds tend to be higher strength than the parent material.

      Bolts ARE normally designed to carry shear predominantly in the bolt. Whilst the clamping force from friction exists and is real, it is very hard to rely upon as you need a defined tension in the bolt to clamp the plates. The tension in the bolt is related to torque, but this is notoriously hard to correlate - you can readily measure torque, but you can not measure the bolt tension (or bolt elongation) easily - it is bolt tension / elongation you need to clamp the connection.

      The reason is that the when you torque up a bolt you are naturally building friction between the bolt (or nut) and surface under however how much of this torque is lost in direct friction that you can't quantify between all the various surfaces (underside of bolt head/ underside of nut/ each washer/ bolt threads / various plus built) versus how much torque is leading to bolt elongation is. If you think about a nyloc nut, they generate quite a bit friction with the nylon ring, this would be read as a torque on a torque wrench - in theory you can be reading the nut is done up before a nut has actually clamped onto anything.

      If you had to rely on the friction a bolt creates, the Aus standard (in the construction world) requires you to calibrate the bolts and nuts and the torque wrench and the operator with a mock up of the exact connection for each batch of bolts! In short - it gets complicated pretty quickly and design bolts for shear only and any clamping action you get is a bonus.

      One item that i feel would be governing is the lever arm / eccentricity of the loading eye versus the bracket geometry, depending on the geometry, it is very possible for the bolts to be carrying higher loads than the load being applied! I won't try to explain this in words - but if you want to understand this, google reaction of a overhanging beam and get ready for a sore head





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