4 Replies Latest reply on Aug 24, 2017 2:12 PM by Hari Padmanabhan

# Polar moment of ineria ERROR

When I use 'Section Properties' to calculate the polar moment of inertia (i.e. the object's ability to resist torsion) of a shape that is NOT circular the sold gives me an answer the doesn't mach the textbook calculation.

For example when I calculate the MOI of a square (10x10mm) the solid says it's 1666.6mm^4 and according to the textbooks it's 10^4 X 0.141 = 1410mm^4

Why does it happens and how can see the object's ability to resist torsion?

• ###### Re: Polar moment of ineria ERROR

Hi Eyal,

Are you sure you are looking at the correct formula from the textbook? Is the formula you are looking at for the polar moment of inertia about the centroid or is it about some other point on the square?

If I do the following Google search for "square polar moment of inertia", all of the formulas I find for polar moment of inertia about the centroid give the results 1666.67 mm^4

Thanks,

Jim

• ###### Re: Polar moment of ineria ERROR

Hey Jim,

Thanks for the answer.

Perhaps I didn't explained myself properly- I'm looking to calculate an object's (that is not circular) ability to resist torsion. When I calculate for a non-circular shape I cannot simply use the polar moment of inertia, I need to use the torsion constant.

I think my problem was that thought that the Polar MOI was enough. Now I'm changing my question - Can the SW tell me what is the object's  ability to resist torsion without using the SW Simulation?

• ###### Re: Polar moment of ineria ERROR

Eyal,

Jim asked me to help you out here. In Simulation we do compute the torsion constant when we do weldment analysis (treated as beams) for any cross-section. See attached picture showing you the values for these calculations for a square tube. So if you have SOLIDWORKS Premium you should be able to find the section properties of any cross section. A nice enhancement would be to leverage this piece of code inside section properties dialog box so that you do not have to go through simulation to get these values. Can you please go through your VAR and file this enhancement. The more customers request for this helps SOLIDWORKS gets a sense of priorities for features to be implemented in each release. • ###### Re: Polar moment of ineria ERROR

And one point to note - I don't even have to run any analysis  or even mesh the model to get these values. These values are provided for any structural members with any cross section when you create a simulation static study.