Folding system loads

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

  1. bruceb
    Joined: Nov 2008
    Posts: 1,211
    Likes: 40, Points: 48, Legacy Rep: 214
    Location: atlanta,ga

    bruceb Senior Member

    more questions

    Thanks H, I hope this is what you need, I really appreciate the help, this is way out of my comfort zone.
    I used CT's diagram, which should at least be good for comparisons, but I am suspect of the actual load vectors. I guess that is why there are "safety factors" ?
    The 32mm pivot pins are as specified on the S-32 and the folding arms are 12mm thick 304 stainless. The inside of the beam is about 200mm across with extra glass where the pivots are installed.
    Bruce
     

    Attached Files:

    1 person likes this.
  2. bruceb
    Joined: Nov 2008
    Posts: 1,211
    Likes: 40, Points: 48, Legacy Rep: 214
    Location: atlanta,ga

    bruceb Senior Member

    More?

    Have I missed something necessary?
    B
     
  3. hump101
    Joined: Oct 2004
    Posts: 261
    Likes: 14, Points: 18, Legacy Rep: 58
    Location: Brittany, France

    hump101 Senior Member

    No, sorry for delay but real work has had me tied up for a while, I'll get to it shortly.

    One question: your geometry creates an upward force on the inboard end of the beam structure, which holds the beam in place (unfolded). Is this force resisted by the end of the beam, or some other location?
     
  4. bruceb
    Joined: Nov 2008
    Posts: 1,211
    Likes: 40, Points: 48, Legacy Rep: 214
    Location: atlanta,ga

    bruceb Senior Member

    This is for fun

    No hurry, jobs come first! I didn't include the thrust pad in the second drawing, but the underside of inboard end of the beam contacts the upper arm near the upper inboard pivot. Two bolts through the top of the beam and threaded into a plate on the arm keep the beam secured in the extended position. When the float is extended and the beam bolted, the upper arm becomes an extension of the beam.
    I have attached a drawing of a similar system, my geometry is a little different so the beam contact pad can be more inboard.
    B
     

    Attached Files:

  5. hump101
    Joined: Oct 2004
    Posts: 261
    Likes: 14, Points: 18, Legacy Rep: 58
    Location: Brittany, France

    hump101 Senior Member

    That is clear now. I had thought this was the arrangement from your earlier illustrations, but was surprised to see a lateral load being applied to a strut that is also primarily in compression, hence my request for clarification. This is generally a bad idea as it greatly reduces the buckling load in the strut. I suspect that, when the numbers are run, buckling is not an issue, but it does lead to a less than optimum (so heavier) structure.
     
  6. bruceb
    Joined: Nov 2008
    Posts: 1,211
    Likes: 40, Points: 48, Legacy Rep: 214
    Location: atlanta,ga

    bruceb Senior Member

    compression

    I thought the same, but it is as designed on the S-32.
    My boat is a little narrower, and has more inboard mounting height so the upper arms don't need as much bend in them, and the beam can have the compression pad almost over the upper inboard pivot. I think it should eliminate most of the bending load on the upper arm. Of course, unlike a Farrier style system, my upper arm takes the whole compression load of the beam through the upper inboard pivot so the upper arms must be stiff. Since the lower arms are much shorter than an F style, the total weight of metal in the arms seems to work out about the same. It is also possible to design the inner end of the beam to notch into a compression pad more like an F style beam, but it would complicate the installation.
    I haven't decided whether to use aluminum or stainless for my arms, there seems to be good arguments for both. It appears the aluminum would be a little lighter and less expensive to fabricate but might require more inspections and service. I am in a city with access to machine shops that can work in either material.
    B
     
  7. redreuben
    Joined: Jan 2009
    Posts: 1,693
    Likes: 85, Points: 48, Legacy Rep: 349
    Location: Beaconsfield Western Australia

    redreuben redreuben

    What about carbon ? I know it's exe but you are looking at short enough pieces to be able to scrounge offcuts. My understanding is that if your fibre orientation is good the carbon piece is substitutable size wise with aluminium ?
    And no disimilar metals issues. Although I think titanium matches carbon best. Oops $$.
     
  8. Gary Baigent
    Joined: Jul 2005
    Posts: 2,985
    Likes: 113, Points: 63, Legacy Rep: 509
    Location: auckland nz

    Gary Baigent Senior Member

    Go with carbon, Bruce, so much easier and you can beef up by extra lamination suspect strength areas. The other materials are retrograde by comparison, weight/strength/stiffness/compression etc. And the carbon amounts used is minor in cost.
     
  9. bruceb
    Joined: Nov 2008
    Posts: 1,211
    Likes: 40, Points: 48, Legacy Rep: 214
    Location: atlanta,ga

    bruceb Senior Member

    materials

    Funny about that- I have priced titanium, and yes , it IS expensive. Probably 4-5K US before fabrication, at least. Stainless and aluminum are both well under 2k and "easy" to get machined.
    The folding arms on this design will have compression loading, and carbon isn't especially good in compression, not much better than fiberglass since it is the resin system that is the weak point. I have no experience at building high strength carbon parts, so I don't think I could trust anything I built:( This would be a very bad place to have a failure. I am planing on using carbon or fiberglass chain plates on the floats and fiberglass attachment points for the folding system.
    I am open to ideas, but it is my understanding that parts like the arms are best produced in a pressure mold/prepreg system.
    B
     
  10. bruceb
    Joined: Nov 2008
    Posts: 1,211
    Likes: 40, Points: 48, Legacy Rep: 214
    Location: atlanta,ga

    bruceb Senior Member

    new pics soon

    I should be able to get back to my boat next week, and I will try to photograph the mounting points for the folding system. These are all I have right now.
    The upper beam will be cut back and the pivot installed on/in it, and the lower pivot will attach to the stainless bar stub with a fiberglass extension and brace.
    B
     

    Attached Files:

  11. hump101
    Joined: Oct 2004
    Posts: 261
    Likes: 14, Points: 18, Legacy Rep: 58
    Location: Brittany, France

    hump101 Senior Member

    The double pivot system is statically indeterminate, so requires an iterative technique to solve for the forces. You could use a solver such as in a spreadsheet, but you would then need to model all the details. Simpler to generate an FE model and solve all at the same time, so that is what I'm doing.

    Regarding material choice, I would also go with carbon composites. They are excellent in compression if made consistently with appropriate layup, which is fairly trivial for such small components. The problem with using stainless or alloy is that the component geometries are driven by the stress around the pins, so the metal has to be thick enough here, which makes them excessively thick/heavy elsewhere. You can use thinner sections with bearing pads, and taper the section depths, but this is a lot more fabrication work and you still have to pay for the material you are cutting away, and pay to have it cut.
     
  12. bruceb
    Joined: Nov 2008
    Posts: 1,211
    Likes: 40, Points: 48, Legacy Rep: 214
    Location: atlanta,ga

    bruceb Senior Member

    carbon?

    Ok, I am listening. Carbon would save at least 100lbs over aluminum, and probably not cost any more, if you think I could build reliable arms. I am guessing with a safety factor, they would have to be good for at least 50,000 lbs in tension. H might say higher;)
    I have never built anything like these in carbon or glass. Guidelines?
    Bruce
     
  13. redreuben
    Joined: Jan 2009
    Posts: 1,693
    Likes: 85, Points: 48, Legacy Rep: 349
    Location: Beaconsfield Western Australia

    redreuben redreuben

    I would not think that pressure moulding would be required for such relatively beefy components, infusion should be more than adequate.
    My first thought was to build them as tubes and then wrap them but I see some have a kink in them.
    Second thought is lay up carbon around a rod as a bush, lay up a flat sheet with appropriate uni orientation from which to cut out your arms and then insert the carbon bushes in later. This may be a bit iffy ?
    Construct a female mould from melamine sheet with the bolts for the rods installed, assemble the whole laminate as one and infuse.
    It would look something like a crankshaft laying flat I imagine.
     
  14. bruceb
    Joined: Nov 2008
    Posts: 1,211
    Likes: 40, Points: 48, Legacy Rep: 214
    Location: atlanta,ga

    bruceb Senior Member

    infusion?

    The top arm could be a "single" bar but it has to have a kink. The bottom arm would still have to be a double for clearance. I do have vacuum pumps but I have never tried infusion.
    B
     

  15. hump101
    Joined: Oct 2004
    Posts: 261
    Likes: 14, Points: 18, Legacy Rep: 58
    Location: Brittany, France

    hump101 Senior Member

    Some geometry issues

    Bruce, I've been checking the dimensions given for the FE model by plotting out the arrangement in CAD, see attached. There seems to be an issue with the dimensions as given, or my interpretation of them, as at no point is a kink required in the upper arm, and the geometry will not fold the float into the main hull. To achieve this you need to either reduce the length of the lower arm, or increase the length of the upper arm, the choice being dictated by where you want the float to end up relative to the waterline. I've used your imperial dimensions, not the metric.
     

    Attached Files:

Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.