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Solving for Moments of Inertia


rberger

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I'm trying to add more "stock" tube sizes in my Content Center library. If I go to my copied "Square Tube" family and right click>FamilyTable to get spreadsheet-like list of the stock Inventor sizes (see attached image). The 2 values I now believe to be moments of inertia need to be filled out in order to keep the family from getting 'sick'. If I just make up some numbers, I can use frame generator build my frame, but I am concerned that for stress analysis I might not get accurate results. I've applied the formula for solving moments of inertia and when compared to stock size data - they are not the same. I am assuming this is because the formula doesn't take into consideration the corner radii and I do not know how to calc for that.

Has anyone added sizes this way before?

Any help is appreciated!

 

Thanks

 

-Rob

Family_Table.jpg

Family_Table2.jpg

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are you entering information for existing tube sizes that just aren't in the library? if so, just look them up, i know that in my mechanical engineering text books it has pages of moments of inertia for all shapes of tubes i beam etc. and the radii and whatnot are all taken into account and are absolutelu accurate, they are the standards they use for building construction and stuff, so they'd better be

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I have been searching thru our library....I've got an 11 vol. "Metals Handbook" series and I am not seeing anything - yet.... Also I've looked it up in the "Machinists Handbook" and it does have numerous tables and formulas for everyting you can think of, but when it comes to tubing, it leaves out the corner radii....I'm stumped, for one I don't know where it applies in Inventor and Two, if I were to create the tube shapes the other way (draw and publish) there is no value to enter for MOI (it's just easier from the Family Table) - so does it even matter?

 

Any other thoughts?

 

Thanks!

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  • 1 month later...

Ok...This is an old thread, but I didn't give up and I figured it out.....There is another problem which I will get to in a minute.

To simply add more square or rectangle tube sizes in Content Center, edit the Family Table - fill in the data - when you get to Ix and Iy the numbers Inventor already has plugged in for existing tube sizes are the AVERAGE between rounded and sharp edged tube!!Family-Table3.jpg Why they did this - I don't know - but the math works out.... Now, to get the MOI for tube with rounded corners I downloaded the super handy program called Engineering Power Tools Plus - worth every penny of the $50!! It gave me the MOI info plus a whole bunch more and I was then quickly able to add more tube sizes via the spreadsheet.

 

Now, the new problem:

Sizes larger than 2" work perfect!

Sizes smaller than 2" come out as rectangles in the wrong size. I've analyzed everything I can think of to no avail. Can someone take a look and see why I can't make a 1" x 1" x .120 wall tube (for example) using the attached variables? (incidentally, the MOI values I entered are not averages - they are exact for rounded corner tube....incase you check my work :) )

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