Kittiwake 23 JR Conversion

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  • 09 Aug 2019 16:47
    Reply # 7820557 on 7804871
  • 09 Aug 2019 16:15
    Reply # 7820320 on 7817221
    David Tyler wrote:
    Scott wrote:The tapering technique usually takes a uniform tube and squeezes it down by rolling it.  You don't save weight with a tapered aluminum pole, but do save windage.

    Not so, Scott. Some of the material moves inwards, thickening the wall, and some is extruded lengthways*. For example, a parallel tube 9.2m long, 17.7cm dia x 3.2mm wall: 43.4kg. My actual mast tube, after tapering: 36kg.

    * exact proportions dependant on the machinery used, I suppose.

    Huh.  Now my curiosity is piqued, and your point makes sense.  It's big difference in weight - we roll titanium sheet onto a on a mandrel and then squeeze it for a taper, and don't see that much "down-shaft" migration.  I'm going to have to take a look at the tool and see if it's controlled.  It might be that for our purposes, we want a thicker wall as the diameter gets smaller.  Or maybe a grain structure?  I'm off to talk to the engineers.
  • 09 Aug 2019 09:09
    Reply # 7819905 on 7804871
    Anonymous member (Administrator)

    Finding the strength of a tube

    Shemaya and David W, this only shows that there is more than one way of doing things.
    The shown formula ( in posting # 7817591) is the standard way over here of finding the bending strength of a tube. Other, similar formulas handles rectangular tubes, I-beams and wotnot. The lovely thing with a round tube is that it is equally strong in all directions.

    There is also a set of similar formulas which deals with the stiffness of tubes and beams, but I haven't bothered with them (except, sometimes, comparing their numbers  -  higher value of Inertia means stiffer tube)

    Frankly, although 'my' formula may look intimidating at first, it is quite straightforward in use. A spreadsheet would speed up the use of it, if one needs to do these calculations frequently.

    Shemaya, to raise a number to the power of 4 is one thing. When calculating SA/Disp, one has to raise to the power of 2/3 (or 0.6667)!

    Good luck!
    Arne

    PS:
    David W’s formula only involves multiplications. That method has probably been developed for use with slide rules. These were at their best for multiplications (..we used them on my first year of technical school, but switched to calculators the second year, in 1974..).


    Last modified: 09 Aug 2019 09:38 | Anonymous member (Administrator)
  • 09 Aug 2019 05:56
    Reply # 7819801 on 7804871

    Shemaya,

    the first D and th in the calculation are in inches which relates to the yield strength in pounds per square inch and the second D is in feet, to get an answer in foot pounds that will correlate to the stress equation for the bending moment at the deck.

    I hope that explains everything.

    David.

    Last modified: 09 Aug 2019 05:59 | Anonymous member
  • 09 Aug 2019 02:17
    Reply # 7819643 on 7804871

    Thanks David W. Which units do you use, to make this work?

    Arne, thanks also to you for the earlier clarification. I've actually succeeded in getting some sensible numbers. Miraculous. Turns out there are several online scientific calculators, and this one worked out great. This was especially helpful because working through the formula turned out numbers that were bigger than my household calculator could manage. The online one was also perfectly happy to do x^4. That was fun!

    Shemaya

    Last modified: 09 Aug 2019 02:19 | Anonymous member
  • 09 Aug 2019 00:30
    Reply # 7819547 on 7804871

    Shemaya,

    multiply 1/4 of the circumpherence of the pipe by the wall thickness, multiply this by the yield strength of the aluminium and then multiply the result by 0.88 of the outside diameter of the pipe. Or use 0.8 if you want to build in some additional safety factor.

    0.25 x 3.147 x D x Th x ys x 0.88 D

    where D is the outside diameter of the pipe

             Th is the wall thickness of the pipe

             ys  is the yield strength of the aluminium

    I hope that helps, David.

  • 09 Aug 2019 00:09
    Reply # 7819518 on 7804871

    Arne,

    the factor represents the distance between the center of area of each of the curved sections of the pipe that make up the imagined I beam, expressed as a fraction of the outside diameter of the pipe. The 0.88 factor more accurately represents the actual distance but I generally use the 0.8 factor as it builds in a little extra safety factor in the calculations.

  • 08 Aug 2019 23:58
    Reply # 7819512 on 7818552

    The table is clearly wrong, Robert. Look at the line above. 0.08 wall thickness and a weight of  3.143 lbs? That's also wrong. I've checked my working, and will stand by it.

    I concede. The lookup table is wrong on those 2 rows. Starting over and computing volume of the metal (VOD minus VID) then multiply by density of 6063-t6 (0.0975 lb/in^3) I got 73 pounds. 

    Last modified: 09 Aug 2019 00:33 | Anonymous member
  • 08 Aug 2019 22:59
    Reply # 7819363 on 7818552
    Anonymous wrote:
    Robert wrote:
    David wrote:

    I compute the weight of a parallel tube 24' x 7" x ⅛" to be 33.8 kg or 74.4lbs.

    Hmmm--This chart from a metals vendor (see attached pdf) puts the weight per foot of 7" x 1/8" at 2.099 pounds per foot, times 24 ft is 50.3 lbs.

    rself

    PS--The weights in Reply # 7816554 on 7815815 are shipping weights. These poles are cone-tapered such that the bottom half or so are straight wall (see ECXA25 pdf attached). So using the standard tube sizes.pdf you can estimate the weight of the pole alone . I got an estimate of 70 lbs weight for the ECXA25 which is 5" x 5/32" x 33 ft, 6063-T6.

    The table is clearly wrong, Robert. Look at the line above. 0.08 wall thickness and a weight of  3.143 lbs? That's also wrong. I've checked my working, and will stand by it.

    In an earlier post I wrote that a 6” x 1/8” 24 ft tube of 6061t6 would weigh about 64 lbs. That was based a Speedy Metals listing of .22 lbs / inch or 63.4 lbs. By Speedy Metals specs a 7” tube of the same thickness should weigh 74 lbs.

  • 08 Aug 2019 17:35
    Reply # 7818552 on 7818467
    Robert wrote:
    David wrote:

    I compute the weight of a parallel tube 24' x 7" x ⅛" to be 33.8 kg or 74.4lbs.

    Hmmm--This chart from a metals vendor (see attached pdf) puts the weight per foot of 7" x 1/8" at 2.099 pounds per foot, times 24 ft is 50.3 lbs.

    rself

    PS--The weights in Reply # 7816554 on 7815815 are shipping weights. These poles are cone-tapered such that the bottom half or so are straight wall (see ECXA25 pdf attached). So using the standard tube sizes.pdf you can estimate the weight of the pole alone . I got an estimate of 70 lbs weight for the ECXA25 which is 5" x 5/32" x 33 ft, 6063-T6.

    The table is clearly wrong, Robert. Look at the line above. 0.08 wall thickness and a weight of  3.143 lbs? That's also wrong. I've checked my working, and will stand by it.
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