Shear calc help

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Nov 9, 2018.

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  1. rxcomposite
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    rxcomposite Senior Member

    Yes AH, you are correct as Dan posted in #8. I read only the part "One of them is obviously going to be the limiting one."
     
  2. fallguy
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    fallguy Senior Member

    Adding an anti-compression bushing would be of no use because the strap in the picture is purposely positioned about 24mm away from the laminate. Or, simply there is a gap.

    I will add a close up picture of the section and the modified bolt in an hour.

    I still need to digest the number you provided rx, thanks.

    Maybe once you see the picture; you will tell me it is shear.
     
  3. fallguy
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    fallguy Senior Member

    Here is a picture. You can see the three layers of glass on edge. There are six more layers of glass under the three bonding the socket to the hull for a total of 9 layers. Three primary and the other six were done together later. This allowed us to bond the socket to the hull as a unit.

    Thanks for any comments on the torque. ... Dan

    2EAB03EC-C451-49D3-A9CB-D8F2067C3CE7.jpeg

    Here is a pic of the adhesive head.

    316ss 1/4" thick

    61C6CE49-24D0-4AFE-8DEF-1F6399707E25.jpeg
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    That is a large gap...i assume you're going to put a shim/thick washer of sorts under the flange of the ally?
     
  5. fallguy
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    fallguy Senior Member

    The strap/flange? is 316ss 3/8" thick.

    The beam held in place is mast ally.

    Here is the metalworks sans the glass.
    8522BEE4-35EB-457A-AFEE-871CCE934259.jpeg
     
  6. fallguy
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    fallguy Senior Member

    I may remove this picture later after replies to keep things right: fyi
     
  7. rxcomposite
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    rxcomposite Senior Member

    Your approach is correct but inadequate. The laminated bolt is unstable as the point of secondary attachment is too far from the base. This is shear, both vertical and horizontal.

    Initially, if you torque the nut, the welded flange will push thru the laminate. This is vertical shear, similar to a punch die. This is strong.

    But from a larger perspective, this a mast. Like a giant claw hammer pulling a nail, it will rotate from the base. As it rotates, there will be an angular displacement. The inset diagrams what will happen.

    The bolt will rotate, pressure on one side will be concentrated and delamination will start on the opposite end. The bond line, the resin, is the weakest part.

    To stabilize the bolt against rotation, a better approach would be to securely fix it as outlined. Add a bushing, fender washer, and nut. calculate according to the data given.
     

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  8. gonzo
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    gonzo Senior Member

    I think that strap needs a gusset. Otherwise, it will bend back and forth, fatigue and fail.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Im getting a tad confused. What I mean is this:

    upload_2018-11-13_22-1-28.png

    Is this gap, shown by the arrows, is this gap going to be filled with a shim or similar? So the flange of the ally is in direct contact, via a shim, to the grp girder?
    Or
    Is it left as shown in your picture, totally unsupported and with a gap as shown?
     
  10. fallguy
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    fallguy Senior Member

    AH:

    Gap is being left.

    I don't understand what you mean by flange I guess.

    Gonzo:

    The plan had a weld gusset on 1/4" strapping, but I went to 3/8". The better way to make the strap stronger might be to weld a side gusset from the end to the radius. Is this what you mean?

    RX: This is a sail mast, but configured as a catamaran beam in case that was not understood. I understand how using another bolt and washer would help avoid any rotational movement and I like the idea. I would still need a torque rating for the lower and the upper nut. I don't think I can install a bushing at this point because the bolts are already welded in place. I would need to design a hole cutter of sorts and do all the work from above. It would be easier to push a rubber lining on the bottom of the washer and just torque to a reasonable amount I'd say.
     
  11. fallguy
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    fallguy Senior Member

    My friend is here and mentioning there are actually four beam straps and 8 bolts holding the beam in place, so the amount of rotational force would be divided by all those four beam straps. The tighter I torque down the top nuts, the less rotational force as well.

    I am only showing one hull of the catamaran for simplicity.
     
    Last edited: Nov 14, 2018
  12. TANSL
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    TANSL Senior Member

    The rotational force (I would not call it that) that each beam contributes to support is not the same for all of them, but depends on where the axis of rotation is and the distance of each beam to it. One of them will support more rotational force than any other and it is convenient to calculate each beam as if it were the one that supports the maximum effort.
    I refer to the torsion efforts that act on the beams between hulls of a catamaran.
     
    Last edited: Nov 13, 2018
  13. fallguy
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    fallguy Senior Member

    Here is another pic of the star hull. I added a lower nut. The washer will need to be customized for even loading with no bushing.

    I still need a torque spec; now for two nuts!
    4DA38649-3837-491A-97BC-4621FCA19EAC.jpeg
     
  14. fallguy
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    fallguy Senior Member

    There is only one beam in this discussion.

    The boat uses this configuration in two places.
     

  15. TANSL
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    TANSL Senior Member

    Sorry, when I read "so the amount of rotational force would be divided by those four beams" I thought you were talking about the beams (plural) of the boat. Excuse my intrusion and my mistake.
     
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