Product of Inertia Sign Incorrect in Creo 2 and 3

[cballantoni]

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

I've found recently that Creo 2 and 3 are reporting the incorrect sign for the product of inertia and I wanted to confirm if anyone else in the PTC community 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. Please see attached PNG image file for those results. Thanks!

[cballantoni]

Is anyone seeing the same result especially with Creo 4 and 5?

From my understanding if all the mass is located in the positive X,Y,Z quadrant, 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!

[cballantoni]

Via another forum a generous individual provided me an output from Creo 4.

It appears PTC might have some even more serious issues with Creo mass properties unless I'm missing something.

In the example they matched my basic geometry and now the CG is incorrectly being reported with a negative value when it should be positive. See attach PNG file.

Anyone else seeing these troubling results with any version of Creo?

[MartinHanak]

Hi,

if you think that there are serious issues with Creo mass properties then I suggest you to contact PTC Support and ask them to provide an explanation. I wish you good luck ...

[cballantoni]

I wish I could, but the support tool won't let me since I'm not provided the information needed to fill it out. I'll have to coordinate with my IT department to do it.

I'm mainly trying to find others with the same blatant issues to support my case before I make a big deal about it especially since we are using an older version of Creo. PTC is less likely to help when you're not using the latest.

So I wanted to see if others are seeing problems even with later versions.

[jbutler-2]

The mass properties in your screenshot are reported with respect to 'PRT0002 coordinate frame'. This is NOT the same as CS0 in your picture. Display the spin centre to see the default coordinate system direction (red=x, green=y, blue-z). You can select a coordinate system as the basis when calculating mass properties, rather than leaving it set to 'default CSYS'.

[Kevin · Accepted solution]

The signs are correct. To see why lookup inertia tensor. Another good source is a dynamics book; look for the sections on moment of inertia and angular momentum.

[cballantoni]

Thank you for the correction. You are definitely right. It appears the other CAD tools I had on hand are confusing and using what they are referring to as a "Positive" Inertia Tensor convention which is kind of a misnomer since it won't transform or provide proper eigenvalues to give the principal moments of inertia without flipping the sign. What a headache. Thanks again!