NX area moment of inertia calc - how do I specify the neutral axis?

[JoeSaw]

In NX 7.5, how do I calculate the area moment of inertia about a specific axis?

I've tried 2 methods, and NX does not let me specity the axis the area is revolved around. For example a 2mm by 8mm rectangle has 2 neutral axes, which give 2 very different area moments of inertia.

Ix = (2 * 8^3)/12 = 85.33mm^4
Iy = (8 * 2^3)/12 = 5.33mm^4

Here's the 2 methods I've tried, but both give the same result. It chooses an axis based on how the part was modeled rather than letting me choose it for each calculation.

1. Analysis, Section Inertia..., Parellel Sections, Section Normal. (The Section Normal is perpendicular to the neutral axis or axis to revolve the area around.)
2. Analysis, Section Inertia..., Existing Section.

Thanks.
Joe Sawicki, P.E.
mechanical design engineer

 

[JohnRBaker]

Simply move the WCS to line up with your axis of Rotation and then read the results labeled 'Moments of Inertia (Work)' from the listing window using...

Analysis -> Advanced Mass Properties -> Area Using Cureves...


John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

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

 

[JoeSaw]

Thanks John.

The WCS has 3 axes: x, y, and z. Do you mean line up the x-axis with the axis of rotation? Or will y and z also work?

For NX7.5, Analysis-> Advanced Mass Properties-> Area Using Curves, I there's dialog box called Analysis with these 4 choices:
-Boundary (Permanent)
-Boundary (Temporary)
-Face (Permanent)
-Face (Temporary)

Any recommendations between these 4. I've tried all 4 without success. I don't see a "moment of inertia" option. This should be easy to calculate moment of inertia for a 2D face. Imagine a solid block with one face being a 2mm by 8mm rectangle. All I need to do is calculate that face's area moment of inertia about its neutral axis. Piece of cake, right? Have you tried it before?

Thank you.
Joe Sawicki, P.E.
mechanical design engineer

 

[JohnRBaker]

Since the calculation is based on the plane of the WCS, use either X or Y.

I use the 'Boundary (Temporary)' option, since this will allow you to either select curves (but not edges) or a face. And while there is a 'Moments of Inertia' option, I usually just just press the 'List All' button and get everything in the listing window, which can be saved as a text file for later reference and to help document your work. However, you have to actually get to the proper dialog before you can continue. After

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

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

 

[JoeSaw]

John,

I followed your advice, selected the face, and then the command prompt reads "Select Object 1." What is "Object 1," and what do I select? See attached prt and ppt files. Thanks for your help.

http://files.engineering.com/getfil...le=NX_area_moment_of_inertia_calc_082813.pptx

http://files.engineering.com/getfil...0-b609-3434f240be5b&file=momnt_inrtia_chk.prt

Joe Sawicki, P.E.
mechanical design engineer

 

[JohnRBaker]

When you see the Green OK button, just hit it and see what happens. In this case the wording of the prompt is probably not correct, or at least it's misleading. In other words, once you've selected your face, just hit OK a few times until you get to a menu that looks like:

And here is where I usually hit the 'List All' button.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

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

 

[JoeSaw]

Thanks John! It's calculating the correct area moment of inertia now. This will be a good tool for designing cross sections of deflected beams.

Joe Sawicki, P.E.
mechanical design engineer

 

[JohnRBaker]

Interesting that you mention that since this functionality was one of the first things I used Unigraphics (AKA NX) for when I started using it 36 years ago. We designed machinery used to produce food products (bread and rolls to be specific) and back in the 70's we needed to redesign many of our machines in an attempt to avoid using paint (USDA was getting picky about stuff that might chip-off a machine and end-up in a loaf of bread or a hamburger bun). Up until then, most of our machines frames were welded steel tubing and rolled shape strutures but as we started to use other metals, mostly stainless ateel and aluminum, it soon became apparent that while we could basically replace the mild steel shapes with stainless, it was very expensive. Therefore we started to look into using more aluminum profiles and while we could again do an almost one-for-one replacement, due to the difference in the yield strength of aluminum versus steel, for many situations we had to increase the size which then led to higher cost due to using more material. Well we finally realized that since you could basically buy aluminum extrusions in virtually any shape you desired, we should design custom profiles with the intention of getting maximum ridgidty with the least cross section, even going so far as to design some cross member profiles with specific sized cores which would accept large self-threading fasteners which eliminated not only extra hardware (no lock washers and nuts) but also resulted in even more sanitary designs since what threads were now completely hidden.

Anyway, I was given the job of designing these profiles, determining their section moduli as well as cross sectional area, profile perimeter and minimum included circle size (these last three numbers were used by our aluminum vendor when they quoted tooling costs and production costs). So for about a year I was doing this manually by making full-size hand-drawn profiles and then using a 'planimeter', a really cool little watch-like mechanism, to compute the area of the 2D profiles (but only if you were really careful tracing the profile of course):

Of course, even with that data I still needed to calculate the section modulus using transform coordinates.

When we bought our first Unigraphics system, which we really justified so as to help get toolpaths generated to keep our machine shop busy, we didn't even know this functionality existed. Then about a month later I was messing around and discovered this capability, which was great. It took me maybe half-a-day per profile to do it manually, but now in a single afternoon I could knock-out a half-dozen fully drawn and documented designs. Later, while attending a Users Meeting, I met some people from Alcoa and Alcan, two of the earliest users of Unigraphics, and they explained that they had both demanded that this capability be part of Unigraphics since it was so key to enhancing the productivity of their extrusion business.

If you look at the options on the menu below, you'll see what I mean when I say it included all of the types of info that would be important for anyone designing extruded profiles, their tooling and then quoting the cost of the final extrusions:

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:

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

 

[JoeSaw]

Interesting! Thanks for sharing that. It's a nice feature that will either save time or lead to a better design.