Moment of inertia not matching calculations 1

[rocketgofire]

This may be more of a general mass properties question than an NX question, but NX is involved, so I'm posting it here. I'm using NX 7.5.5.4, and I'm having an issue where NX's calculation of the moments of inertia (MOI) of a body do not match general engineer formulae. Specifically, I'm talking about the MOI about the WCS after a translation of the WCS. We often get block models from vendors where the mass properties have to be entered manually into a spreadsheet, so this is a problem. Here's an example to illustrate the process that isn't working:

I created a simple 1x2x3 (inches) block (uploaded), set the density to 1 lbm/in^2 (giving me a total mass of 6.00 lbm), then measured the mass properties using analyze-measure bodies. The MOI about the WCS (not the centroid) were Ixx=26.00, Iyy=20.00, and Izz=10.00. Then I moved my WCS to the origin (0.10,0.20,0.30) and re-did the measure bodies, getting Ixx=18.98, Iyy=14.6, Izz=7.3.

However, using the parallel axis theorem (there are many resources out there, one is at [URL unfurl="true"]http://www.eng.auburn.edu/~marghitu/MECH2110/C_4.pdf[/url], equation 4.12), the translated moments should be found as such:

Ixx(new) = Ixx(old)+mass*(y^2+z^2) = 26.00+6*(.2^2+.3^2) = 26.78
Iyy(new) = Iyy(old)+mass*(x^2+z^2) = 20.00+6*(.1^2+.3^2) = 20.60
Izz(new) = Izz(old)+mass*(y^2+z^2) = 10.00+6*(.2^2+.3^2) = 10.30

Obviously, these don't match, so either I'm doing the calculations incorrectly (a likely possibility since I haven't done this in a while), NX is wrong, or I'm misinterpreting what NX is doing. So, what's my error here?

Thanks for any help!

 

[cowski]

First you must calculate the moments of inertia about the mass center; then you can use the parallel axis theorem to calculate the moments of inertia about a different (but parallel) axis. In your example, neither of the moments were calculated w.r.t. the center of mass.

For your example file Ixx_C=6.5
With the WCS at (0.1, 0.2, 0.3):
Ixx = 6.5 + 6.0 * (1.4422)² = 18.98 (as reported by NX)

www.nxjournaling.com

 

[rocketgofire]

Ah, that seems to be what I was missing. Thanks for the help!