See Section 14.15.1 of the Theory Reference manual.

Keith DiRienz, PE
FEA Technologies
Dana Point, CA

At 11:19 AM 1/26/2022, you wrote:

Keith DiRienz, P.E.
FEA Technologies
Office: 949.481.4946
Cell: 949-910-7049 (New !)

Principal masses are calculated mass moments of inertia taken with respect to coordinate axes, called principle axes, which have zero cross-product (Ixy, Ixz, Iyz, etc.) values. Moments taken about principal axes of a plane section produce only motion or deformation in one direction. Likewise applied moments taken about the principle axes of a mass only produce rotation about the corresponding axis of rotation. Moments about non-principal axes produce rotation about both in-plane axes. For example if you were to apply a moment about one leg of a structural angle, you'd obtain rotation (and bending stress) about both legs of the angle.

I don't know what 'psolve,elform' means—I've never used it.If I were to guess I'd bet that it has to do with how ANSYS handles cross-products (Ixy, Ixz, Iyz)

Christopher Wright P.E. (ret'd) |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
| John Sedgwick, Spotsylvania (1864)
http://www.skypoint.com/members/chrisw/

psolve,elform
Partial solve, perform element formation (element formation only, without solving the full system of equations).

--
Mitch Voehl
CEO and Engineering Consultant

Summit Analysis, Inc.
78748 410th Ave
Lakefield, MN 56150

651-287-2360
www.summitanalysis.com
Specializing in the use of ANSYS (R) finite element analysis software

Thanks.    Now to figure what that that section is saying!  😊

-----Original Message-----
From: Keith DiRienz fea-technologies@cox.net
Sent: Wednesday, January 26, 2022 2:30 PM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Subject: EXT: [Xansys] Re: (APDL) Principal Mass solution from partial solution.

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See Section 14.15.1 of the Theory Reference manual.

Keith DiRienz, PE
FEA Technologies
Dana Point, CA

At 11:19 AM 1/26/2022, you wrote:

Keith DiRienz, P.E.
FEA Technologies
Office: 949.481.4946
Cell: 949-910-7049 (New !)

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Dan,
I was curious as well and found this online from NX Nastran documentation.

If scalar masses are used, the total mass may have different properties in each direction, and the center of gravity may not be a unique location. This effect is shown in the output by providing the direction and center of gravity for each of the three mass principal components.

When using directional mass, the axes about which the inertia matrix i[S]is calculated are referred to as the principal mass axes. The principal mass axes may not necessarily intersect. However, these axes provide uncoupled rotation and translation mass properties. If the structural model is constructed using only real masses, the three principal mass values printed out are equal, the center of gravity is unique, and the principal mass axes intersect at the center of gravity.

Copied from the following website.
http://www2.me.rochester.edu/courses/ME204/nx_help/index.html#uid:id471971

Keith DiRienz
FEA Technologies
Dana Point, CA

I've never been deeply involved in this but I think the math is roughly analogous to the stress tensor calculations. With a general stress state (3 normal stresses and 3 shearing stresses) taken with respect to some particular axes, it's possible to find an orientation where the shearing stresses become zero. I think you're looking at the same sort of thing here.If you can find an introductory  book on tensor analysis or a fancy book on advanced dynamics or stress analysis, it'll probably go into detail. I got into a little of that in school, but it was abstract enough so I never got what you'd call a 'feel' for it.

Christopher Wright P.E. (ret'd) |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
| John Sedgwick, Spotsylvania (1864)
http://www.skypoint.com/members/chrisw/

Hartog talks a bit about it in Chapter 7 (I think) in his book on Mechanics.

Peter Attar

From: Christopher Wright chrisw@skypoint.com
Sent: Thursday, January 27, 2022 9:52 PM
To: XANSYS Mailing List Home
Subject: [Xansys] Re: (APDL)  Principal Mass solution from partial  solution.

I've never been deeply involved in this but I think the math is roughly analogous to the stress tensor calculations. With a general stress state (3 normal stresses and 3 shearing stresses) taken with respect to some particular axes, it's possible to find an orientation where the shearing stresses become zero. I think you're looking at the same sort of thing here.If you can find an introductory  book on tensor analysis or a fancy book on advanced dynamics or stress analysis, it'll probably go into detail. I got into a little of that in school, but it was abstract enough so I never got what you'd call a 'feel' for it.

Christopher Wright P.E. (ret'd) |"They couldn't hit an elephant at
chrisw@skypoint.com            | this distance" (last words of Gen.
| John Sedgwick, Spotsylvania (1864)
http://www.skypoint.com/members/chrisw/

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