Folding system loads

Discussion in 'Multihulls' started by tamas, Nov 11, 2013.

  1. hump101
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    hump101 Senior Member

    1 1/4" is 32mm, do you mean 32 or 36mm?
     
  2. bruceb
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    bruceb Senior Member

    sorry

    Whats four mm- they cant be that important:(
    32mm ! I realized after I posted, but I was on a six hour road trip.
    Sorry H.
    I really don't care what size the pins are, as long as they work. I realize a pipe or hollow pin might be better from a flexing load, but the end of the folding arm then gets excessively large. Even a 32 mm pin, with its bushing, adds quite a lot to the arm ends.
    B
     
  3. hump101
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    hump101 Senior Member

    BruceB's buc33 rev2

    Hi Bruce,

    Here is a drawing of the arrangement you previously specified. Bad news is that there is an error in your dimensions. Good news is that it is only 1.02mm!

    The upper arm needs to be kinked as shown, based on my best guess at to the folded position of the arms. If you are in agreement I will model this arrangement with the upper arm kinked thus, and the lower arm pins fully supported (near to pure shear) with only a 1mm tolerance.
     

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  4. bruceb
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    bruceb Senior Member

    Good timing

    I just finished removing the crossbeam end plugs today, so I could get accurate measurements of the total beam. I probably won't have to fold quite as much as you have shown, but lets go with that anyway.
    Also, yes on the pin/arm in shear at one mm, that should not be a problem.
    Thanks!!!
    Bruce
     
  5. hump101
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    hump101 Senior Member

    Rev1 results

    Here are the results from the rev1 model. Main features are that the kink is causing high stresses, and the bearing area in the 1/2" plates is not enough on the 36mm holes, unsurprisingly. I'm adding bearing pads and increasing the material in way of the kink. More to follow.
     

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  6. hump101
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    hump101 Senior Member

    Rev2 model

    Here are the results from the Rev2 model. In this case the pins are supported by the beam both inside and outside the plates, to a tolerance of 1mm, so substantially entirely in shear, and the inboard end of the beam has additional restraint. This may not be achievable in practice, particularly if the beam structure is not very stiff, but shows the impact on stress of such a restraint. The stresses are reduced by two order of magnitude, not just in the pins, but in the plates as well as local bending caused by bending in the pins is eliminated.

    As noted, I'm not sure how practical a solution this will be, as it will depend on beam design, but at this load (static full weight of boat at 20 degrees heel) the stress is just below yield in the pins, so still not acceptable, but not so far away.
     

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  7. hump101
    Joined: Oct 2004
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    hump101 Senior Member

    Rev2 model

    Here are the results from the Rev2 model. In this case the pins are supported by the beam both inside and outside the plates, to a tolerance of 1mm, so substantially entirely in shear, and the inboard end of the beam has additional restraint. This may not be achievable in practice, particularly if the beam structure is not very stiff, but shows the impact on stress of such a restraint. The stresses are reduced by two order of magnitude, not just in the pins, but in the plates as well as local bending caused by bending in the pins is eliminated.

    As noted, I'm not sure how practical a solution this will be, as it will depend on beam design, but at this load (static full weight of boat at 20 degrees heel) the stress is just below yield in the pins, so still not acceptable, but not so far away.
     

    Attached Files:

  8. hump101
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    hump101 Senior Member

    Rev3

    Here are the results for rev3, with the 3/4" lower brackets. The stress in the brackets are now low (arguably the 3/4" is not required over the entire bracket, just the bearing locations), but the max remains similar as this is in the lower inboard pin. This is in single sided shear, rather than double shear like the outer pin, and hence is the most highly stressed. I'm going to update the model to include contact between the pin and the plate (currently it is simplistically assumed to be monolithic) which will make things worse. Is it possible to arrange the structure on the boat to put this pin in double shear (support the pin fwd and aft of the bracket, as well as in the centre)?
     

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  9. bruceb
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    bruceb Senior Member

    more support

    At this point, it is fairly easy to add more support to the pin/pivot system, but I am not sure how "stiff" I can make the supports. The boat structure its self may start to be the weak link. However, the total load is in the range I was expecting, and the Buc's basic structure should be strong enough as long as I can distribute the forces properly.
    I will post some more drawings soon.
    Thanks H, this is a lot of information to work with.
    B
     
  10. hump101
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    hump101 Senior Member

    Rev4

    Here is the rev4 results, again the same geometry as rev3 except that the lower bracket inboard pin is extended to put it in double shear. Now max stress is in the lower bracket outboard pin, and is only 67MPa. I would suggest we stick with this geometry and develop a valid set of dynamic load cases, inc folding, then see what we can trim.

    Edit: this is rev4, not rev3. Doh!
     

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  11. warwick
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    warwick Senior Member

    thanks Bruce and Hump101, it is interesting to see how it is progressing.
     
  12. bruceb
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    bruceb Senior Member

    much better

    I am back:)
    I am appreciating how fairly small changes are resulting in large reductions in max loads. Thanks to H's modeling, this should result in a safe and practical folding system.
    H, I will see what I can do with the lower strut/beam connection. I think a center rib can be added to the beam, which should help support the pin. A short alloy C channel bolted to the beam underside where the pin crosses would also work, and might be easier.
    I have to get some measurements off the boat, but I think I have the inner lower pin supports worked out, in alloy or maybe glass if it would be stiff enough. I will be back at the boat soon.
    B
     
  13. hump101
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    hump101 Senior Member

    Bruce, when you are at the boat can you measure the distance between the fore and aft beam centres? We need to model both to capture the eccentric load cases.
     
  14. bruceb
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    bruceb Senior Member

    beam centers

    I know that one:), It is 13' center to center.
    B
     

  15. bruceb
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    bruceb Senior Member

    Progress report

    I think the report is that it is going slowly. I am working on replacing bulkheads in the first float, and it is going to take a while to get both of them finished.
    I also have never completely resolved the design and loading of the lower inner pivot supports, but I hope to have something finalized by the time my floats are done. I think I have at least a couple of weeks work in the floats, and I am somewhat dependent on warm weather.
    B
     
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