Calculated Moments of Inertia

[billtati]

I have CAD models and the software I used (Mechanical Desktop) is capable of calculating moments of inertia. However, three different versions are returned; (1)Mass Moments of Inertia (2)Mass Products of Inertia & (3)Principal Mass Moments. The coordinates for (1) are given in Lx, Ly & Lz, for (2) in XY, XZ, YZ, and for (3) I, J, K. I am used to seeing MOI expressed in Ix, Iy, Iz.

Does anyone know which if these would be the "correct" one for me?

 

[JStephen]

It sounds like you are wanting the moment of inertia of a surface (like a beam cross section), and the software is giving you moments of inertia of a three-dimensional solid. Is this right? In that case, none of the moments given correspond to the values you're looking for.

 

[billtati]

I have a 3D design of a housing and covers and require the MOI for each part. I hope this helps.

 

[CESSNA1]

BILLTATI: It depends on what you wnat to do. I beleive you need to go back to the fndamentals to determine what you want to do.

Regards
Dave

 

[billtati]

I want the moments of inertia for each part rotated about the X, Y & Z axis.

 

[CESSNA1]

BILLTATI: You then want the principle moment of inertia. So that part is solved. The next question is do you want them about an axis other than the principle axis. If so, then you will need to use the parallel axis theorem.

Regards
Dave

 

[tlee123]

Billtati, make sure you use Mechanical Desktop to run a test case for a rigid body that has principle moments that you can easily verify from a textbook.

My question is why does someone want this information. The moment of inertia calculations are relative to the three axes, x, y, and z, which are different for each part. I would think that maybe a moment of inertia calculation of the whole machine as an assembly might be helpful or maybe the moment of inertia of the cover about the hinge axis might be interesting.

Please ask whoever is demanding this info for an explanation and post again.

Tom

 

[tygerdawg]

I have used MechDesk to calc mass moments of inertia for robot end effectors. It has been a few years, but I vaguely remember that it was necessary to assign the user coordinate system to the robot wrist joint (my axis of reference). AND to make the UCS aligned in the proper orientation. In my case, the wrist joint axis was always offset from the actual part. It was also necessary to provide the correct mass density value in some obscure text file buried somewhere in the AutoCAD directory (see Help for this). Only then did MechDesk generate the correct mass moments of inertia of the object about the axes in question. I tested the procedure on known sample parts to be sure because those numbers always looked weird.

TygerDawg