topic Moment of Inertia Issues in Fusion Support Forum

Moment of Inertia Issues

moriarty.chch — Mon, 27 May 2019 20:30:50 GMT

Having issues with MOI calculations - simple example ...

collar ID25mm OD35mm L7.5mm Material Steel

Calculate MOI at COG for part using Properties (RMB MOI Ring v1 part) ...

Ixx Iyy Izz about COG make sense

calculate MOI at COG for body using Properties (RMB the body within the part)

COG MOI different ....

Not sure whats going on here - shouldnt they be the same?

To make life easier for us users

Principle MOI should be presented somewhere - there appear to be API functions to do this
All possible MOI data should be copied to Clipboard - not just Origin and COG MOI

Re: Moment of Inertia Issues

James.Youmatz — Tue, 28 May 2019 16:54:09 GMT

Hi @moriarty.chch ,

Can you share the .f3d with me? I'm curious if somehow the origin of the file, differs from that of the origin of the body. It shouldn't in nature, but I'd like to investigate.

Thanks,

Re: Moment of Inertia Issues

moriarty.chch — Tue, 28 May 2019 21:24:48 GMT

model attached

Re: Moment of Inertia Issues

patilsm — Wed, 29 May 2019 10:53:06 GMT

Hi @moriarty.chch ,

Thanks for posting and providing data set.

Yes, I have also checked MOI and COM for bodies and Component which show different values.

I thinks its Bug, I have logged task for further analysis.

Thanks for reporting this.

Regards,

Sagar Patil

Moment of Inertia Issues (negative MOI!!!)

Anonymous — Thu, 14 May 2020 19:31:09 GMT

I have been experiencing this issue for several days and now I got surprised to see that it has been noticed some months ago but it is still unresolved! In my case it gets even worse because I get negative values for the moments of inertia! I detected it in an assembly with many components (several parts containing several bodies each one) when I was interested in finding the Tensor of Inertia for some of the bodies but not for the whole component.

As an example, in the case of the figure below, you can see a set of bolts that join together 2 of the parts of the assembly. The set of bolts belongs to a component which origin is the one in the lower bolt head (the first bolt that was inserted form the McMaster-Carr database, the rest was placed by means of a circular pattern), the one in the center is the global origin.

Figure 1. Set of bolts (bodies) belonging to a part

If, for any reason, one needs to find the Inertia Tensor for a single bolt of the group the problems start to arise. Have a look at the figures below.

Figure 2. Inertia Tensors for the original bolt

Figure 3. Inertia Tensor for the oposite bolt (with negative MOI's at Center of Mass)

The MOI at COM should be the same for all the bolts since they all have the same geometry and density, but this is not happening. In fact, looking at the Figure 2 it seems like the computation of the MOI at COM is done taking the COM of the component (the set of 6 bolts), or something like that, not the one of the body itself (both MOI at COM and MOI at Origin have very similar values). But things get even worse when the opposite bolt is analyzed (Figure 3). In this case 2 of the MOI at COM get negative values and nothing seems to make any sense...

At this stage, I decided to check what happens with only one bolt placed in the origin, so I opened a new document and inserted the same bolt centered in the origin. And below you can see the results.

Figure 4. MOI for the bolt (body selected)

Figure 4. MOI for the bolt (component selected)

Again, a negative MOI at COM...

If anyone can shed some light on what is happening here I would be very pleased since this makes no sense at all. It would be great if this bug could be solved ASAP because the calculation of a MOI is something basic when it comes to mechanical engineering and now, after having discovered this issue, I can't trust in any of the MOI I have been using for my calculations.

Thank you

Alex

Re: Moment of Inertia Issues

moriarty.chch — Tue, 30 Nov 2021 04:32:50 GMT

fixed V.2.0.11680 Nov2021 - choice