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Thread: Complicated Moment of Inertia Calc

  1. #1
    Associate Engineer
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    Complicated Moment of Inertia Calc

    Hoping somebody can help me solve this.

    I'm trying to figure out how to calculate the moment of inertia of a particular section of a rotational body that is made up of several parts of varying size/density.

    I have all of the variables, and I know how to calculate the I of an individual piece, but not the total effect on the system. I have attached an image that lays out an example of the problem.

    In the image, I have the actual answer as calculated by solidworks, but I need to know how it's getting the answer for Ixx for each part.

    The total system Ixx = 27750.56 which is the sum of the Ixx for each section, trying to figure out how to calculate that given the other known variables.

    Any help would be greatly appreciated: Image







    Thanks,
    Kyle
    Attached Images Attached Images
    Last edited by Kelly Bramble; 10-31-2014 at 04:51 PM. Reason: attache picture

  2. #2
    Administrator Kelly Bramble's Avatar
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    I'm assuming that this is not an academic exercise (homework).

    At the moment I’m not awake or ambitious enough to do the hand calculations. If this system has been designed with any of the modern 3D CAD systems.. there should be a function available that will determine the “moment of inertia of a particular section” for you. Very accurately I might add.

    What 3D CAD does your organization use?

  3. #3
    Associate Engineer
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    Nope, lot of years since college.. That's the problem actually

    I can do it in solidworks (the numbers in the drawing are from there) but I'm trying to set up a formula so I can swap out various bodies in a database into the positions in the assembly and get the new total without having to rebuild an assembly in solidworks.

  4. #4
    Technical Fellow jboggs's Avatar
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    kyo, If I understand what you are wanting to do correctly, I would strongly suggest you learn more about configurations. You can assign different materials to a single part with different configurations, then you establish various configurations in the assembly. You can tie each assembly configuration to selected configurations of each part using part properties and achieve what you are after. I do that kind of thing a lot. It works well.

  5. #5
    Associate Engineer
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    I do that as well, but this has to be done outside of solidworks.. it's for a website user interface where other people will be swapping out parts as they want.

    In any case I solved it.. it was the parallel axis theorem I was missing.. I just needed to add I = Ix +md^2 and I'm good.

    Thanks to everybody for the help.

    Kyle

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