Plywood question

Discussion in 'Class Societies' started by willfox, Nov 1, 2011.

  1. willfox
    Joined: Oct 2008
    Posts: 79
    Likes: 1, Points: 8, Legacy Rep: 22
    Location: southampton

    willfox Junior Member

    Hi there,

    I am looking at fixing a bolts through a vertically mounted piece of plywood. The force is acting vertically therefore the bolt is acting in shear. The grain of the ply is vertical and is in the same direction of the force. What properties would you use for the plywood? Would you use the compressive properties?

    Thanks for your help
     
  2. Landlubber
    Joined: Jun 2007
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    Location: Brisbane

    Landlubber Senior Member

    willfox, the grain as you say is vertical, but that is only the outside veneers, plywood is at 90 deg veneers, so half of em are vertical the other half are horizontal. As you say, the bolts are working in shear, make sure that the bolts are supported with large washers, that will assist the loading on the ply if you feel there is a risk of the bolts tearing the ply, what load and what size ply and bolts are you proposing anyhow.
     
  3. willfox
    Joined: Oct 2008
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    Location: southampton

    willfox Junior Member

    The 16mm ply pad is actually boned in as core between 2 skins approx 3 mm on each skin. There are 10 M9 bolts clamping the ply and skins together. i am sure the bolts are strong enough in shear, the bonding area is strong enough in shear, the shear tear out of the laminate is good. I am just unsure of the tear out or shear of the bolts through the PLY and the laminate. What properties of the ply should I use? Compressive stress or shear stress with the loadcase mentioned on my previous mail?

    Thanks for your help
     
  4. willfox
    Joined: Oct 2008
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    Location: southampton

    willfox Junior Member

    sorry. Load is 8 tonnes breakins
     
  5. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    There is quite a bit more to this than you have yet mentioned. Overall geometry and bolt pattern is important. Steady or cycling loads. Bolt material. Design life both in years and in load cycles. Load always perfectly balanced on both skins? The plywood mfg should give you info on what standard the ply is made to, and there is an extensive and well sifted collection of test data available. Try googling "bolt patterns" and "timber association bolt testing" for general engineering practices. More recently, the aircraft industry has done a lot of work testing bolted composites. Usually, for small boats, there are rules of thumb for a given application that were arrived at when masts quit falling down and the like. I don't know if BS1088 has anything on it but I think the HPMA might. Don't rely on clamping pressure, rely only on shear strength. Both laminates and ply will creep over time and you will lose the clamp. Some boats with bolted hull to deck joints have been squeezed to nothing by fastidious retorquing applied over forty years. (Mine isn't one of them :D)
     

  6. cyclops2
    Joined: Sep 2010
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    Location: New Jersy

    cyclops2 Senior Member

    Much as I dislike them.

    I would be EXTREMELY tempted to go to Stove Head Bolts. Those ugly ones with the square anti-rotating section under the domed heads. They would give you a massive horizontial set of edges to prevent downward shearing of the plywood. Not sure if they are available in the size & finish metal you can accept.

    I use the ugly things in some highly stressd areas. Usually out of sight.

    You can take some square stock & cut them into the same squares as a stove bolt would have them. Then drill a hole in the center of the square pieces so you can slide them up to the head of any bolt head & finish you like.

    I was always the " Save your Bacon guy ". :)

    Rich
     
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