Calculating System Inertia of Rotating Mass in Dynamic Simulation

[jrohit1110]

Hi! 

    I was working on Dynamic Simulation of an assembly, which consists of a motor onto which there is a load attached.

Is there functionality to calculate this(Jo) in Inventor2021?

@JDMather 

@mcgyvr 

Warm Regards,

Rohit Kumar

[imajar · Accepted solution]

The mass moment of inertia of parts is calculated in the parts iproperties (File --> iproperties).  You can get it calculated from the principal axes, global coordinate system, or the body CG.

[jrohit1110]

Hi! @imajar 

     This may be a conceptual doubt in physics, but what is the difference between principle moments and mass moments calculated byInventor? 

     And which one and how do we use it to calculate the mass moment of inertia(J)?

(Assuming that this computational functionality is scalable to assemblies) ref: https://www.mikipulley.co.jp/EN/Services/Tech_data/tech24.html

Best Regards

[jrohit1110]

I'm essentially trying to calculate the Moment of Inertia of the system, considering that the motor will be attached to one of the entities in the below-shown subassembly, and I would ultimately need to size the correct motor for the application.

@imajar (apologies for not being able to cascade both my replies into one reply, the page didn't allow me to upload the 3rd photograph upon editing my already posted reply).

Best Regards,

Rohit Kumar

[imajar]

Hi @jrohit1110 ,

They are all "mass moment of inertia", the different tabs are just calculating it from different coordinate systems / origins.  The principal moment of inertia are calculated about the principal axes.  The Globals are calculated from the global coordinate system along the x,y, and z axes, and the center of gravity is calculated at the center of gravity along the x,y, and z axes (I believe - best check the documentation on this one).  If you want to calculate the mass moment of Inertia of the entire system by hand, then you will need to calculate the mass moment of inertia of each body (using the same orientation, this can be challenging if the indidual parts are not oriented the same, or if their principal axes are not shared) and use the parallel axis theorem to add them up into one.  This subject occupies a couple of weeks in a university dynamics course, If this does not make sense or jog your memory, then I suggest you buy a book on dynamics, or spend some time on youtube and wikipedia learning more about mass moments of inertia and dynamics.  They will do a better job than me at explaining the topic correctly.

Otherwise Inventor will calculate the mass moment of inertia for assemblies as well.  If you locate your axis of rotation on the x (or y, or z) axis, then the Global Ixx (or Iyy, or Izz depending on which axis you choose) will give you the mass moment of inertia  of the rotating system about the rotation axis.  I think this would be easiest and less prone to error.

I'm curious what you are designing, if you can share?

[jrohit1110]

Thank you @imajar for your reply. It is clear now, the quantities that need to be calculated. Although, I cannot share the design as I an under an NDA. However, I can give you the representative files. I reckon that I need to incorporate a 'rule' to calculate and export the IXX, IYY, and IZZ values:  https://forums.autodesk.com/t5/inventor-customization/ilogic-code-to-find-area-moment-of-inertia/m-p...

Warm Regards,

Rohit Kumar

[imajar]

The mass moment of inertia about the axis of rotation (Izz) in the sample part is 1.690 kg mm^2 (calculated in iproperties).  

I'm not sure why you would need a rule to export the values, as they are directly available in iproperties - unless you have moving parts on the flywheel and you need to export a bunch of different values based on different configurations?  I am also not sure why you need Ixx or Iyy (unless the whole motor assembly is in some sort of gimbal and you need to account for torque variations resulting from the gyroscopic effect?)

[jrohit1110]

HI! @imajar 

The model that  I provided is representative. The original model has multiple components on its driveshaft and the data needs to be updated in the excel sheets, as I remove, add, and vary the configurations of the elements of the drive shaft. This is a micromechanical system, and we need very precise values of Izz, for sizing the motor.
    Yes, I do not require Ixx and Iyy values. Only Izz. 
    Also, yes the system could possibly be placed in a centrifuge and its operations tested.

Warm Regards, 

Rohit Kumar