topic Re: Negative Moments of Inertia in 3D Part & Assembly Design

Negative Moments of Inertia

atibbitts — Thu, 03 May 2018 01:45:22 GMT

I am struggling to understand why my assembly is displaying negative values of IXY and IXZ my IXX is correct with a positive value. The negative IXY and IXY seem to be correct but negative which is impossible?

Re: Negative Moments of Inertia

Chris3 — Wed, 15 Jul 2015 13:32:29 GMT

Do you have assigned (as apposed to calculated) mass values? This can create negative numbers.

Re: Negative Moments of Inertia

atibbitts — Wed, 15 Jul 2015 16:56:27 GMT

Yes, Unfortunately I have shrinkwrap models where I don't have the originals and I have to assign the mass and moments of inertia.

Other than getting real models do you have any suggestions on how to get this to represent correctly.

Re: Negative Moments of Inertia

Chris3 — Wed, 15 Jul 2015 17:07:38 GMT

The only way I know if is to get the correct information and type it in. You don't necessarily need the original models, you just need the info. For instance if your shrinkwrap represents outside vendor or customer geometry maybe you could get it from them. Short of that I don't have any additional suggestions.

Re: Negative Moments of Inertia

atibbitts — Wed, 15 Jul 2015 21:09:19 GMT

I appreciate the response. I have been grappling with this for some time. If I discover anything I will share. I can only guess my coordinate systems somewhere is rolled in the wrong direction. But I haven't figured out which one

Re: Negative Moments of Inertia

MichalLukac — Fri, 17 Jul 2015 09:47:14 GMT

Why do You think negative value for Ixy Ixz.. are impossible? It depends on orientation of coordinate system.

Re: Negative Moments of Inertia

jbush — Thu, 16 Jan 2020 03:14:16 GMT

I agree with MichalLukac, only the moments of inertia, Ixx Iyy and Izz, must be positive.

Ixy, Ixz, and Iyz are called the products of inertia and they can be negative. Search up definition of "inertia tensor".