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(OP)
Has anyone noticed a issue in either Creo 2 and 3 (or later versions) that has the product of inertia showing the wrong direction (positive or negative)?

I've found recently that Creo 2 and 3 are reporting the incorrect sign or value (depending on the placement of mass) for the product of inertia and I wanted to confirm if anyone else has seen the same thing.

I created a simple block in exactly the same position, size, orientation, and density in Creo, NX, and Solidworks and ran the mass properties. Creo is the only one reporting the Product of Inertia with negative signs.

I only have access to Creo 2 and 3, so I'm wondering if the same result is seen in Creo 4 and 5?
From my understanding if all the mass is located in the positive X,Y,Z quadrant with a right hand coordinate system, the Product of Inertia should be positive. So that means Creo, at least in 2 and 3, the coordinate system is backwards when outputting the Product of Inertia. For Moment of Inertia there wouldn't be a noticeable difference based on axis +/- direction since the value is always positive. With anyone using later versions of Creo are you also seeing this result?

Thanks!

### RE: Product of Inertia Direction/Sign Incorrect in Creo 2 and 3

From Creo 4 m060:
VOLUME = 1.0000000e+06 INCH^3
SURFACE AREA = 6.0000000e+04 INCH^2
DENSITY = 9.9000000e+01 POUND / INCH^3
MASS = 9.9000000e+07 POUND

CENTER OF GRAVITY with respect to _PRT0002 coordinate frame:
X Y Z 5.0000000e+01 -5.0000000e+01 -5.0000000e+01 INCH

INERTIA with respect to _PRT0002 coordinate frame: (POUND * INCH^2)

INERTIA TENSOR:
Ixx Ixy Ixz 6.6000000e+11 2.4750000e+11 2.4750000e+11
Iyx Iyy Iyz 2.4750000e+11 6.6000000e+11 -2.4750000e+11
Izx Izy Izz 2.4750000e+11 -2.4750000e+11 6.6000000e+11

INERTIA at CENTER OF GRAVITY with respect to _PRT0002 coordinate frame: (POUND * INCH^2)

INERTIA TENSOR:
Ixx Ixy Ixz 1.6500000e+11 0.0000000e+00 0.0000000e+00
Iyx Iyy Iyz 0.0000000e+00 1.6500000e+11 0.0000000e+00
Izx Izy Izz 0.0000000e+00 0.0000000e+00 1.6500000e+11

PRINCIPAL MOMENTS OF INERTIA: (POUND * INCH^2)
I1 I2 I3 1.6500000e+11 1.6500000e+11 1.6500000e+11

ROTATION MATRIX from _PRT0002 orientation to PRINCIPAL AXES:
1.00000 0.00000 0.00000
0.00000 1.00000 0.00000
0.00000 0.00000 1.00000

ROTATION ANGLES from _PRT0002 orientation to PRINCIPAL AXES (degrees):
angles about x y z 0.000 0.000 0.000

RADII OF GYRATION with respect to PRINCIPAL AXES:
R1 R2 R3 4.0824829e+01 4.0824829e+01 4.0824829e+01 INCH

"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

### RE: Product of Inertia Direction/Sign Incorrect in Creo 2 and 3

(OP)
Thank you for the reply. Looking at the Creo 4 results you gave is that the same example as I posted, a cube in the positive XYZ quadrant?
I ask because I see the Iyz as negative, so I'm assuming PTC fixed this issue and the geometry you have shown is different than the example I gave especially with the CG also shown negative in the Y and Z.
Could you match the example or give an image to what those results are based from?

Thanks so much!

### RE: Product of Inertia Direction/Sign Incorrect in Creo 2 and 3

The cube was 100 inches per side with the start corner at the intersection of the 3 planes. It was constructed in the first quadrant.

Here are the corrected values.
VOLUME = 1.0000000e+06 INCH^3
SURFACE AREA = 6.0000000e+04 INCH^2
DENSITY = 9.9000000e+01 POUND / INCH^3
MASS = 9.9000000e+07 POUND

CENTER OF GRAVITY with respect to _PRT0002 coordinate frame:
X Y Z 5.0000000e+01 5.0000000e+01 -5.0000000e+01 INCH

INERTIA with respect to _PRT0002 coordinate frame: (POUND * INCH^2)

INERTIA TENSOR:
Ixx Ixy Ixz 6.6000000e+11 -2.4750000e+11 2.4750000e+11
Iyx Iyy Iyz -2.4750000e+11 6.6000000e+11 2.4750000e+11
Izx Izy Izz 2.4750000e+11 2.4750000e+11 6.6000000e+11

INERTIA at CENTER OF GRAVITY with respect to _PRT0002 coordinate frame: (POUND * INCH^2)

INERTIA TENSOR:
Ixx Ixy Ixz 1.6500000e+11 0.0000000e+00 0.0000000e+00
Iyx Iyy Iyz 0.0000000e+00 1.6500000e+11 0.0000000e+00
Izx Izy Izz 0.0000000e+00 0.0000000e+00 1.6500000e+11

PRINCIPAL MOMENTS OF INERTIA: (POUND * INCH^2)
I1 I2 I3 1.6500000e+11 1.6500000e+11 1.6500000e+11

ROTATION MATRIX from _PRT0002 orientation to PRINCIPAL AXES:
1.00000 0.00000 0.00000
0.00000 1.00000 0.00000
0.00000 0.00000 1.00000

ROTATION ANGLES from _PRT0002 orientation to PRINCIPAL AXES (degrees):
angles about x y z 0.000 0.000 0.000

RADII OF GYRATION with respect to PRINCIPAL AXES:
R1 R2 R3 4.0824829e+01 4.0824829e+01 4.0824829e+01 INCH

"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

### RE: Product of Inertia Direction/Sign Incorrect in Creo 2 and 3

(OP)
I might be confused. Is the first quadrant in the negative Z direction given the CG has a negative Z value?
My first thought would be the first quadrant would be considered all positive.

Thanks!

### RE: Product of Inertia Direction/Sign Incorrect in Creo 2 and 3

My cube was all positive. Square sketched on the top plane with right and front planes as edges. Extruted in the plus Z (above) the top plane.

"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

### RE: Product of Inertia Direction/Sign Incorrect in Creo 2 and 3

(OP)
PTC might have some even more serious issues with Creo mass properties unless I'm missing something.
Given that it's now reporting an incorrect CG with the outputted result showing a negative Z.
CENTER OF GRAVITY with respect to _PRT0002 coordinate frame:
X Y Z 5.0000000e+01 5.0000000e+01 -5.0000000e+01 INCH

Thank you again!

### RE: Product of Inertia Direction/Sign Incorrect in Creo 2 and 3

(OP)
Not considering the Creo 4 result shown, which was troubling, it appears Creo 2 and 3 are reporting correctly. After chatting with PTC, Siemens, and Dassault it appears I came across a convention that not all CAD tool companies do consistently. I'm not sure why they don't just follow the math exactly. It appears Siemens and Dassault follow what they refer to as a Positive Inertia Tensor which I think is confusing since you can't calculate the eigenvalues for the principal moments of inertia without first changing the signs on the non-diagonal values to the opposite shown. Creo follows what an inertia tensor is supposed to be as is with no change required.
What a nightmare to keep up with from CAD tool to CAD tool.

### RE: Product of Inertia Direction/Sign Incorrect in Creo 2 and 3

There is a config option for show_dim_sign Yes/No but I think that is only used in dimensioning.
For instance you could enter a -20 distance and flip length or direction of offset.
If options were set to
show_dim_sign no
The - sign would be dropped from the dimension and just show the value of 20. I've encountered issues where entering an offset of 0 for a dimension and then entering a value would swap the directionality of the dimension.

UGNX has Top Plane as X-horizontal,Y-vertical with Z in Positive direction. Front plane faces in the -Ydirection.
Pro/E CREO on the other hand Has the Front Plane defined with a X horizontal and Y vertical direction with a Z normal (Facing the front)out of screen.

"It's not the size of the Forum that matters, It's the Quality of the Posts"

Michael Cole
Boston, MA
CSWP, CSWI, CSWTS
@ TrajPar - @ mcSldWrx2008
= ProE = SolidWorks

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