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1-Newbie

## Moment of inertia

I would like to be able to find the rotational moment of inertia of an assembly (that is, to disegnate which parts move around the axis and get the moment for those parts together). There are several different materials involved in these parts. I have the density (kg/m^3) of all the materials. I tried setting mass poperties but everthing in the window is greyed out or locked. And even if I could set that, I am unsure how to find the moment. How do I go about doing this? I could do it one part at a time and sum if needed, since inertia about the same axis is additive.

1 ACCEPTED SOLUTION

Accepted Solutions
4-Participant
(To:ejenner)

Initial thoughts are the first part needs to be fixed. There are two constraints on this part but it is still 'packaged' and free to move.

Also, the first part has no material applied to it. For a Mass Properties analysys you need to assign a material or density to every single component. Which parts was the material property greyed out?

So I put together a few of the parts I think should be in the rotor sub-assembly and added this to the housing with a pin connection. The attached model should be self-explanatory. You should then be able to check the moments of intertia of the rotating parts as they will be in one sub-assembly.

12 REPLIES 12
4-Participant
(To:ejenner)

With the model open in PTC Creo, open the Analysis tab and click Mass properties. Click on the glasses button and the dialog will report moments of inertia. For this to work you must assign materials to every component in the assembly/

The entire assembly is not moving (that is, the assembly includes the stationary parts in which the item is moving). How do I get the moment for only the parts that will move? Also, the density entry boxes for some of the items in the assembly are greyed out; what do I do with these items?

4-Participant
(To:ejenner)

Can you make the moving parts into a sub-assembly?

Rather than change the density apply a material definition. File > Prepare > Model Properties > Material. In the dialog choose a material and apply it to the part.

I appplied materials to everything.

I've been trying to move them to a subassmebly but it won't allow me to finish the operation and it keeps on bringing along some parts I don't want and leaving others behind even with as far as I can get.

4-Participant
(To:ejenner)

Can you zip the folder containing all the files for your design and upload to here?

As no-one else has responded you may need to contact PTC Academic support on the form below.

http://support.ptc.com/appserver/wcms/forms/index.jsp?&im_dbkey=142574&icg_dbkey=482

Here are the files if you are interested. The green bas and it's directly connected parts are not supposed to move. The setup is an air bearing. I need the moment of inertia of all the moving parts together.

4-Participant
(To:ejenner)

Initial thoughts are the first part needs to be fixed. There are two constraints on this part but it is still 'packaged' and free to move.

Also, the first part has no material applied to it. For a Mass Properties analysys you need to assign a material or density to every single component. Which parts was the material property greyed out?

So I put together a few of the parts I think should be in the rotor sub-assembly and added this to the housing with a pin connection. The attached model should be self-explanatory. You should then be able to check the moments of intertia of the rotating parts as they will be in one sub-assembly.

That does exactly what I wanted. How were you able to get it into a subassembly?

4-Participant
(To:ejenner)

I started a new assembly (Rotor.asm) to contain all the rotating parts. The first part I added was the Air_bearing_rotor_conical.PRT. This was fixed in this sub-assembly using the 'Default' assembly constraint. This is really important and is needed in most assemblies to create a fixed reference for all the other parts which were located by adding constraints to the existing parts. The key is making sure none of the parts are left floating or 'packaged'. Packaged parts have a very small square next to the icon in the model tree.

I did this assembly rather quickly and notice today the rotor wobbles a bit! You will need to edit definition and sort out the constraints for the 'wobbling' parts. 🙂

I then created a new assembly (Air_bearing_conical_2.ASM) which will be the top-level assembly. I brought in the Air_bearing_rotor_conical.prt and same as before, fixed this part in the assembly using a 'Default' constraint. I then added the Rotor sub-assembly to this assembly and used a pin connection to allow it to rotate in the main assembly.

I used the mechanism module to add a servo motor to drive it at 2 RPM. You may not have this module in the student edition but should be able to play it on the university plus version on campus.

4-Participant
(To:ejenner)

Also, the first part has no material applied to it. For a Mass Properties analysys you need to assign a material or density to every single component. Which parts was the material property greyed out?

SInce it didn't need to move I didn't apply a meterial to it. 6061 Aluminum. I didn't apply densities to any of the parts that should be stationary. All the screws are standard screw steel and tubing pieces are plastic.

I was able to get the parts to take materials (I think).

4-Participant
(To:ejenner)

Take a look at my updated reply above, it now has a model attached and brief explanation.

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