centroidal moment of inertia

[mco]

I am running NX 4.0.4.2.

I am having a problem with calculating the centroidal moment of inertia. The answer that I am getting from the program is off by a factor of 10.

The way that I am doing this is by creating a sheet body using simple curves. The cross section is simple enough (rectangular tube) that I can quickly do a hand calculation. I then use Analysis?Advanced Mass Properties?Mass Using Sheets and Curves , Thin Shell to evaluate the cross section. My hand calculations and the answers calculated by UG are off by a factor of ten.

My units are consistant, using mm-kg.

Am I making a wrong assumption, or is UG calculating this wrong?

 

[JohnRBaker]

Can you supply the part file and both sets of numbers (the system results and what you've calculated)?

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Cypress, CA

http://www.siemens.com/plm

http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.

 

[mco]

John-
I have uploaded the file...and have provided the output that I receive on the drafting sheet. I calculate values of 475,424 and 156,376. The program calculates values of 47,542.4 and 15,637.6
Mike

 

[JohnRBaker]

OK, I agree that this appears to be giving an incorrect result. While I'll need to investigate this further to see what the problem is, I CAN offer you an alternative which will give you the correct results. It's actually a much older approach but one which I've personally used myself for over 30 years.

Instead of using the 'thin shell' approach, go to...

Analysis -> Advanced Mass Properties -> Area using Curves...

...and select the 'Boundary (Temporary)' method, then select the 'Face' option, select your sheet body as before, then hit OK a few times until you get to a dialog listing the various results options and just select 'List All' as before. Now in this case you will only get 2D results relative to the X-Y plane, but that's all you were looking for anyway, correct?

Now there is only one new restriction which you will need to be aware of that did not apply to the Thin Shell method and that is that since this is purely a 2D solver the curves MUST lie in the X-Y plane of the WCS. Now there is no need to move your sheet body, just reorient the WCS so that the Z-axis if normal to the plane of the curves. You will need to note the direction of the X and Y axis since the results will be calculated relative to those directions.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Cypress, CA

http://www.siemens.com/plm

http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.

 

[JohnRBaker]

Based on further testing, it appears that the problem may be related to the setting of the result units. Anyway, I'm going to open a PR and will report back if and when I learn anything.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Cypress, CA

http://www.siemens.com/plm

http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.

 

[mco]

Thanks, John