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Help Required on Area problem

  1. Sep 27, 2012 #1
    Hi guys, hope im posting in the correct place.

    At work im creating a excel spreadsheet that will work out the cross sectional areas of a manufactured bi metal tape'.
    And i cant figure out the domed part.

    ive uploaded an image of the shape that needs calculating.

    any idea if it can be done or does it require more infomation?


    Attached Files:

  2. jcsd
  3. Sep 27, 2012 #2


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    These are quite troubling problems. Are you expecting exact answers, or approximations?
    Anyway, I believe I could flesh out the first problem because it does have enough information, but for the second, you've got the circular part labelled as 9.00, while the angle it makes at the end is 7o, but the "diameter" is 3.36.
    How is it possible that the circular part is 9.00 (or is that counting the slanted edges as well?), but even so, if we made the slanted edges and circular part longer by turning the figure into a rectangle with length 3.36 and height 0.50, then all edges excluding the bottom will add to 4.36, which is much less than 9.00, so your numbers just don't add up.
  4. Sep 28, 2012 #3
    i was hoping for exact answers.
    im not sure why you think the numbers dont add up.

    the tape is 3.36mm wide with an overall height of 0.50mm there is a taper of 7° (either side) and a radius of 9.00mm

    we produce this tape at work millions of times aday and many others relativly similar.

    if you could figure out the top problem that would be great as i could use that and subtract that area from a theoretical trapezoid (factoring out the radius of the tape)

    hope that makes sense, ive uploaded another drawing if not :)

    Attached Files:

    Last edited: Sep 28, 2012
  5. Sep 28, 2012 #4


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    So, just to make this clear:
    The circular arc is a segment from a circle with a 9.00 mm radius?
  6. Sep 28, 2012 #5
    exactly :)
  7. Sep 28, 2012 #6


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    First problem:
    (W+h tan(θ))2+(R-h)2=R2
    You can turn that into a quadratic in h and solve.
    Second problem:
    Consider one end of the curved part. Let X be its horizontal distance from the centre line and Y be its height above the base line.
    X = W/2 -Y tan(θ)
    R2 = X2 + (Y+R-H)2
    From those, get a quadratic in X or Y and solve.
    Let the angle the arc subtends at the centre of its circle be 2ψ.
    Cos(ψ) = (Y+R-H)/R
    Solve for ψ.
    Area = ψR2-R2sin(ψ)cos(ψ)+2XY+Y2 tan(θ)
  8. Sep 28, 2012 #7


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    I don't see why an engineer would bother with finding analytical formulae for arbitrary area shapes.
    A much simpler approach in general would be to work out the area ratio of two pieces of the same material by calculating their weight ratio. By controlling the area of one of the pieces, you'll readily find the other.
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