MOI calculations clarification

[CP_10719148]

Hello, 

Currently working in Creo 7.0.2.0, and was hoping to get some clarification on some of the moment of inertia calculations Creo can perform. For the assembly at hand, there is a "true reference frame," (TRF) which is what I need to calculate moments, CG distance references, etc. from. Going in part by part to find these values, I come across two MOI tensors, and I'm unsure exactly what the difference between the two are. I have pasted them directly from the mass properties analysis done in Creo.

INERTIA with respect to TRF coordinate frame: (POUND * INCH^2)

INERTIA TENSOR:
Ixx Ixy Ixz 1.6605289e+03 1.1988931e+01 4.2694060e+02
Iyx Iyy Iyz 1.1988931e+01 1.7687049e+03 -4.6548034e+01
Izx Izy Izz 4.2694060e+02 -4.6548034e+01 1.1179549e+02

INERTIA at CENTER OF GRAVITY with respect to TRF coordinate frame: (POUND * INCH^2)

INERTIA TENSOR:
Ixx Ixy Ixz 1.5889992e+00 0.0000000e+00 0.0000000e+00
Iyx Iyy Iyz 0.0000000e+00 1.1091056e+00 0.0000000e+00
Izx Izy Izz 0.0000000e+00 0.0000000e+00 5.2537525e-01

I want to say the first tensor treats the TRF as a "simulated" CG of the part, whereas the second tensor takes the parts CG to be the CG for the calculations, but again I'm uncertain.

Any help is greatly appreciated!

[tbraxton · Accepted solution]

Use Moment of Inertia to measure the moment of inertia relative to either the current coordinate system or the principal inertial axes of the model.

You can specify the reference csys used for mass property calculations. 

Does this support information answer your question explicitly?

https://support.ptc.com/help/creo/creo_pma/r9.0/usascii/index.html#page/simulate/simulate/mom_inertia.html 

[SweetPeasHub]

 FYI some info. on the parallel axis theorem which can be used to transform MOI to a new coordinate frame.

Maybe this also helps explain the two values provided.

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Sent: Wednesday, March 17, 1999 4:26 PM

When specifying a mass element in a 3D model, the user is asked for
mass and inertia terms Ixx, Ixy, Ixz, Iyy, Iyz, and Izz. For a mass
element where the attachment point (to the rest of the model) is not
coincident with the center of mass for the element, how should the
inertia terms be entered?

The inertia terms are with respect to the attachment point. Thus, the
user can figure out the inertia terms with respect to the center of
mass, and then use the parallel-axis theorem to compute the inertia
terms with respect to the attachment point:

Mxx = Ixx = Icxx + m * (y^2 + z^2)
Myy = Iyy = Icyy + m * (z^2 + x^2)
Mzz = Izz = Iczz + m * (x^2 + Y^2)
Mxy = Ixy = Icxy - m*x*y
Myz = Iyz = Icyz - m*y*z
Mzx = Izx = Iczx - m*z*x

where:
c = represents mass center C
m = mass of the body
x,y,z = coordinates of the mass center C

The user must calculate Ixx, Iyy, Izz or in terms which MECHANICA LABELS
Mxx,
Myy, Mzz.

The user now has the correct formulas and should be able to calculate
the Mass
moments of inertia which is needed to use in Mechanica. You need all
Mxx, Myy,
Mzz, Mxy, Myz, and Mzx.

--
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