Shear calc help

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

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  1. fallguy
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    fallguy Senior Member

    No problem. Thanks for posting.
     
  2. rxcomposite
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    rxcomposite Senior Member

    It is too late now to insert an anti compression bushing. Why not slip a fender washer, a flange polyurethane bushing (car suspension bushing), machined to height. I prefer the polyurethane bushing with a metal insert inner bushing. That way, when machined to correct height, you will not be able to over torque it. That will fill the gap between the fiberglass laminate and the metal flange.
     
  3. fallguy
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    fallguy Senior Member

    This goes against the general concept RW presented which is to allow for the strap gap(fun with words) to deal with any variability like a strap bent 1/32 deeper on one side or versus another strap.

    i.e. I don't think the metal inner bushing dimension can be pinned down as well as you might want...

    Thay said, I do like the idea of a larger fender washer and bolt above the plate. This would mean any force on the bolt adhesive head would also need to overcome the 9 layers of laminate.

    However, the importance of developing torque specs is even more important. And now, the lower nut is not accesible with my deep well 24mm socket, so an added dimension and need for alternative torqueing method. A washer with a neoprene head on the glass side might be enough. Torque it to deformation off the boat and it would be a fairly close estimate.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Not recommended at all:

    upload_2018-11-14_9-29-28.png

    A gap as shown is bad for so many reasons, some of which have been noted above.
     
  5. fallguy
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    fallguy Senior Member

    The strap is 316 stainless steel 3/8" thick and 2" wide, not ally(aluminum).

    I genuinely appreciate the perspective. It is difficult to understand a way to not have a gap. I don't think we can develop a poly bushing with metal that would be close enough.

    If I torque a nut, lock and fender down onto the top of the plate(fiberglass); there would be insufficient space for another nut as well.

    Scrutinizing the design is not the original intent of my question. I was only trying to determine a torque rating.

    But to the discussion, I am struggling conceptually with how the beam and strap can rotate. In order for any rotation to occur, there needs to be a failure of the strap, bolt, or vertical shear of 9 layers of glass on the other side (forward or aft) and the other straps on the other hull.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    At the very minimum you'll have simple load cases as such:

    upload_2018-11-14_12-30-40.png

    So, work out what torque is required, form the previous posts. Then you'll have to make simple shims/thick washers to fit the gap. They may likely end up being all individually tailor made, since nothing will be exact or perfect fit.

    But you must ensure there is constant contact between the bolt of the head, to the flange and in turn the flange to the shim and in turn the shim to the grp seating. So any load from the mast goes directly from the mast into the bolt and then into the grp seating. A gap prevents this and makes the bolt subjected to various loadings. Most likely the worst being the loading and unloading of the bolts (which are torqued) which will eventually slowly loosen the bolt torque and hence the bolts working themselves free.
     
  7. fallguy
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    fallguy Senior Member

    I definitely understand the fluctuating loads on the beam, thus the straps and thus the bolts. One wave can be cresting on a hull and another trough at a 13' distance. Cresting wave socket is loaded while the other unloads. I can double nut everything for safety.

    I will need to review this issue with RW which is the opposite of intent.

    I will need to take rx numbers amd convert them from ksi to psi.

    I am a little nervous about getting a torque spec based on laminate shear that is irrational, like 150 ftlbs and then denting the beam instead or even bending the strap which would be avoidable by a gusset as G recommended, although I think the beam would dent or deform firstly.

    I will work the calc and see.
     
  8. rxcomposite
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    rxcomposite Senior Member

    Multiply it by 1,000 you get psi.
     
    Last edited: Nov 13, 2018
  9. fallguy
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    fallguy Senior Member

    Ya I know that bit.
     
  10. rxcomposite
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    rxcomposite Senior Member

    Opps sorry. My bad. I corrected my post so as not to confuse others.
     
  11. fallguy
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    fallguy Senior Member

    What about using preload washers between the bottom of the strap and the top of the lower nut? It would act like a mini shock absorber, no? Might need two or three for each one.

    Isn't this sort of the hope that there is less play and some preloading?
     
  12. fallguy
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    fallguy Senior Member

    I did a calc using the above and found some other online stuff and came up with a max torque rating of the bolt of 130ftlbs, or 1567inlbs. My calc basically agreed with the online ratings for the same 316SS M16 2 pitch bolt within 2 pounds. But this is the max right? It doesn't really tell me the amount to torque the bolt. I don't feel too comfortable torqueing the bolt to say 70% of max because that is a helluvalotta force on the laminate. And 70% of the 130 number is 91ftlbs. Still a lot of torque, more than seems rational.

    This ends up -> compression stress = 15,000#/A And the area of the washer is 4 square inches less the diameter of the bolt (.629") or 3.37 square inches.

    Compression stress is 4,451 psi. Gotta admit, not sure what this means, but this number is quite a bit lower than the plate which rx cites at 18,800psi.

    Is this simply saying the bolt will fail before the laminate (as estimated)? Not what I expected at all. Or am I just confused at 1am?

    What is also bugging me is if I used a 12mm bolt, would I be torqueing it less and imparting less stress on the laminate?

    Obviously a larger bolt will be stronger and less likely to fail due to extreme waves.

    Conversely, let's say I apply 91 ftlbs to the bolt to keep the bolt closer to its maximums. I can see the potential for galling and the potential for delamination. This is probably a duh and that is why you ought to have had a bushing, but what is the problem with torqueing the operation to say 25ftlbs? Is the nut going to loose on the top? Can't I just double it?
     
  13. groper
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    groper Senior Member

    sorry- late comer to the discussion...
    Fallguy - Please listen to Adhoc and RX... despite RW doing the initial design - sometimes engineers and designers make oversights...

    Also - the bolt torque is not the elephant in the room here, your attention to this is largely irrelevant.
    Provided the bolts, straps, mast and laminate all fit snug with good contact and fitment - the bolt torque only has to be quite light as this is not where the work is done. The loads simply need to be transmitted from the beam to the hull and it does that best with maximum surface area of the mating surfaces - not 100% thru the tiny bolted area. Over torquing is detrimental in that it will likely deform the mast section and create a local buckling problem- not to mension destroying the local laminate area around the bolt heads.
    To demonstrate in practice - have a look at Wharram Catamarans - which use beams that are LASHED to the hulls with ROPE... yes, thats all that holds them together! And theyve been circumnavigating the world for decades... you can imagine the effective torque from a lashing a beam down is nowhere near what you could acheive with a bolt and lots of torque.
    Also - i realize you may not have finished the configuration yet - but make sure you "bed" the mast section to the hull with thickened resin/milled fibers so that the mast fits into the socket "like a glove". You can do the same sort of the thing to close the GAP Adhoc is referring to. Make it out of milled fibers and thickened resin and build it up onto your existing laminate. once you have a nice molded fit and just a tiny gap, torque it down lightly the last little bit onto a high density plastic compression washer or similar for the final take up to the SS strap...
    [​IMG]

    You can look at a million pictures on the web of this type of aluminium hull to beam connection that your using on your boat. No matter how many pics you look at they all share the same common elements;
    1 - very good fitment hull to aluminium
    2. No gap under the straps - they are snug to the hull forming a continuous structure that cannot move...
    [​IMG]
     
    Last edited: Nov 14, 2018
    Ad Hoc and rxcomposite like this.
  14. fallguy
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    fallguy Senior Member

    The mast section does not fit like a glove because it must be removable. The fit is good, but not ultra perfect. Since the bottom of the mast section is the widest; that width is a constraint on the width of the socket.

    This is also why preloading to a high amount would not be wise; the removing factor. Again to close the gap; what about a preload washer or two?

    I will take another picture of the fitment from a cutaway of the socket later today.
     

  15. groper
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    groper Senior Member

    I dont understand - having a very good fit does not preclude it to being removed.
    To bed the fitment - wrap the mast section in plastic film as a mold release media - then press it down into your milled fibers and thickened resin "peanut butter" and leave it cure - make sure the alignment is perfect when your doing this btw.
    After it cures - remove it and remove the plastic film from the mast. reassemble at will into a perfect socket fit...

    PS- see above beach cat pic my edited previous post. All cats with this type of beam connection are generally used when the ability to dissassemble is desired...
     
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