Defining point mass and rotary inertia

From the main menu bar in the Property module or the Interaction module, select SpecialInertiaCreate.

In the Create Inertia dialog box that appears, name the inertia, select Point mass/inertia, and click Continue.

Select the points for which you want to define mass and rotary inertia using one of the following methods:

Select the points in the viewport. When you have finished selecting, click mouse button 2.

If the model contains a combination of orphan mesh elements and geometry, select one of the following from the prompt area:
- Select Geometry to define the point mass and rotary inertia for the geometric portions of a part or assembly or for a reference point.
- Select Mesh to define the point mass and rotary inertia for orphan mesh components.
  
  You can use the angle method to select a group of nodes from an orphan mesh. For more information, see Using the angle and feature edge method to select multiple objects.
To select from a list of existing sets, do the following:
1. Click Sets on the right side of the prompt area to display the Region Selection dialog box containing a list of available sets.
2. Select the set of interest, and click Continue.
  Note: The default selection method is based on the selection method you most recently employed. To revert to the other method, click Select in Viewport or Sets on the right side of the prompt area.

The Edit Inertia dialog box appears. The region to which you are applying the point mass and rotary inertia is highlighted in the viewport.

From the Mass portion of the Magnitude tabbed page, do the following:

For isotropic mass, toggle on Isotropic and specify the magnitude of the mass.
For anisotropic mass, toggle on Anisotropic and specify the mass components $M_{11}$ , $M_{22}$ , and $M_{33}$ .

In the Rotary Inertia portion of the page, specify the moments of inertia (units ML²).

Toggle on Specify off-diagonal terms if you want to specify products of inertia.
Enter values for the rotary inertia.

I11

Rotary inertia about the local 1-axis, $I_{11}$ .

I22

Rotary inertia about the local 2-axis, $I_{22}$ .

I33

Rotary inertia about the local 3-axis, $I_{33}$ .
If applicable, enter values for the products of inertia.

I12

Product of inertia, $I_{12}$ .

I13

Product of inertia, $I_{13}$ .

I23

Product of inertia, $I_{23}$ .

If you want to change the coordinate system (CSYS) for the rotary inertia, click

and use one of the following methods:

Select an existing datum coordinate system in the viewport.
Select an existing datum coordinate system by name.
1. From the prompt area, click Datum CSYS List to display a list of datum coordinate systems.
2. Select a name from the list, and click OK.
Click Use Global CSYS from the prompt area to revert to the global coordinate system.

By default, the global coordinate system is used to define the rotary inertia.

Use the Damping tabbed page to define proportional damping for point masses or rotary inertia. For an Abaqus/Standard analysis, you can also define composite damping. You can specify values for both proportional damping for point masses and composite damping or for both proportional damping for rotary inertia and composite damping; however, Abaqus uses only the damping that is relevant to the particular dynamic analysis procedure being performed. If different damping values are needed for point masses and rotary inertia, you must create separate inertia definitions.

In the Alpha field, enter the $α_{R}$ factor to create proportional damping for point masses or rotary inertia in a direct-integration dynamic analysis, in a mode-based analysis that supports nondiagonal damping in Abaqus/Standard, or in an explicit dynamic analysis. This value is ignored in a modal dynamic analysis that does not support nondiagonal damping. The default value is 0.0.
In the Composite field, enter the fraction of critical damping, $ξ_{α}$ , to be used when calculating composite damping factors for the modes in a modal dynamic analysis. This value is ignored in a direct-integration dynamic analysis and in mode-based analyses that support nondiagonal damping in Abaqus/Standard. The default value is 0.0.

Click OK to save your data and to close the dialog box.

Symbols appear in the viewport that represent the point mass and rotary inertia that you just created.