splicing aluminum tube with a sleeve insert

Discussion in 'Boatbuilding' started by Charly, Oct 3, 2013.

  1. Charly
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    Charly Senior Member

    Hey guys I made another goof on my cat build.

    The aluminum tubes that connect the rudder quadrants to the steering track, are supposed to be about ten feet long (see photo). I ordered eight feet sections by accident. They are here and they are beautiful, but I need a couple of more feet on each one.

    The thing is, freight charges go way up after eight foot lengths. I could just bite the bullet, order two more, pay the freight, and continue on... OR order two short sleeve pieces , much cheaper, rivet or screw them in place, inside the existing pipes (ID 1.38"), bedded in something (5200?). Seems to me that would be pretty strong. I would save roughly $250 that way. Is it worth it??

    Oh , also, I have to bed the terminal forks that attach to the rudders and track ends in some structural bog that goes inside the tube anyway, so the design already takes that into account-- it seems sleeving would be no weaker than that.

    Any advice appreciated.

    PS online metals have been great to deal with
    http://www.onlinemetals.com/
     

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  2. PAR
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    PAR Yacht Designer/Builder

    I would think sleeves would do just fine. What alloy is it, as some grades can be welded with minimal strength loses (15%). This would be easily inside most safety margins, built into the structure.
     
  3. Charly
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    Charly Senior Member

    It is 6061 schedule 40. I was hoping to avoid welds, as that would up the PITA factor and expense.
    Would 5200 work fine with adequate prep, and aluminum rivets? I can get 5200 easy. Not so sure about Araldite or more exotic stuff. Thanks
     
  4. jonr
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    jonr Senior Member

    You are right to avoid welding. It would lose more than 1/2 of its strength.
     
  5. dinoa
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    dinoa Senior Member

    Splicing aluminum primary structures is sometimes done in light aircraft. A rivet line along the neutral axis secures the splice. Occasionally a raunchier method is seen were the same diameter tube is cut axially and a slice removed that allows a tight internal fit. A double rivet line straddling the internal seem secures it sometimes with an external doubler added. Aluminum rivets with low copper content like 6061 could be used to avoid corrosion.

    Dino
     
  6. PAR
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    PAR Yacht Designer/Builder

    Not necessarily, it depends on the alloy. A good welder also has a huge impact on it as well. I tested 5383 welds this summer with less than 15% loses over the continuous tube base lines.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    What are the internal and external diameters of the tube you bought?

    Roughly where along the length of the tube, are the bearings/supports, since this will dictate what loading you shall get on the tube.

    It's best to calculate the expected loads, assuming single tube, and once you have the loads and then work out the stress, you can decide what is the best way. Since if you weld, whilst the stress values may suggest it is fine, fro example, does it take into account fatigue and the facts that the weld is subjected to a sea water environment, which again reduces the design allowable.

    Thus best obtain some facts first, in my opinion.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Paul

    The alloy is 6061, so once welded shall loose a shed load of strength. From 260MPa down to 115MPa, depending upon the temper of the alloy of course.
     
  9. jonr
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    jonr Senior Member

    Even 5383 can drop from 220 to 145 MPa (yield strength) when welded.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    You need to be careful what you're quoting.

    Those values are UTS. One never designs to UTS values.

    Proof (yield) Stress of 6061:
    T4 = 110MPa, as welded = 115Mpa
    T6 = 240MPa (mistyped 260 above), as welded = 115MPa.

    When using extrusions, 6000 series, the objective is to use the unwelded strength in the high stress regions and ensure the as-welded strength, which is significantly lower, is far from the high stress regions to the point that the FoS is the same as the unwelded region.

    With plate, 5000 series, you can't really do this, as the range between unwelded and welded is minor.

    5083, unwelded:
    O/H111 = 125MPa, as-welded = 125MPa, no change.
    H116 = 215Mpa, as welded = 125MPa.

    Thus the as-welded strength is the important value.
     
  11. alan white
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    alan white Senior Member

    I'd order new tubes and pay the $250.00 depending on how over or under built the design is. While you could weld, rivet, thread, or otherwise attach the two tubes, especially in extreme conditions I assume the tubes will be in compression half the time so why even worry about a stress riser where the tube meets the thicker joining parts?
    Tubes like anything can bow under compression loads and any change in pressure (worst towards the middle) can break that smooth curve causing failure at the edge of the joint.
     
  12. jonr
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    jonr Senior Member

  13. Ad Hoc
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    Ad Hoc Naval Architect

    I only use Class approved values, not sales literature. You also need to understand tempers too, that one is listed as H321, somewhat different to O temper!

    5083-O DNV.jpg

    The above is an excerpt from DNV rules, from a previous posting I made. Note the UTS values of 270MPa, the proof/yield stress, as noted 125 or 115MPa depending upon the filler wire.
     
  14. jonr
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    jonr Senior Member

    Using your data, as welded 5383 has a YS of 145. I quoted 145 also. What was your point?
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Well...

    DNV ally grades.jpg

    It is not carte blanch. There are many different tempers of aluminium. Without qualifying which temper you're referring to, the 220MPa is misleading, as this is only true for higher strain hardened tempers like H321/H116. As it is not true for O/H111 tempers.
     
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