About the Computation of Inertia

This section describes the Inertia concepts.

This page discusses:

Introduction
User Interface
Inertia in the Context of a Product

Introduction

The Inertia command allows you to measure inertia data in the context of the active product. Inertia data cannot be saved in product context.

The inertia measures are based on the density associated with the selection, and the density is defined as follow:

either by the material applied to the selected element, see Material Definition User's Guide: Applying Materials .
or by the default density defined in preferences.

or from the declared weight of the selected element, see Weight Analysis User's Guide: Declaring Weight.

Important:

A declared weight of a null value is not taken into consideration; in this case weight and inertia are computed.
If the declared weight is not computed in Weight Analysis, the center of gravity and the inertia matrix are not defined. This will generate an error or warning message during the computation of the assembly inertia.
A covering material is accounted for if all of the body is covered.
Surface where a thickness is applied are accounted for when no inertia covering thickness is specified, otherwise the inertia covering thickness overrides all existing surface thicknesses.

You can measure volume geometry or surface geometry.

Important: You can mix volume and surface geometries in the selection but only volume geometries are accounted for.

To do this you can:

Select the root product, a sub-product, a part, a 3D Shape in Volume or Surface mode.
All representations in Show space in the product structure are accounted for.
Select exact or tessellated bodies in Volume mode, planes or surface features for Surface mode.
Select exact or tessellated features.

User Interface

The Measure Inertia dialog box offers several sections to manage inertia data.

Inputs

Lets you specify what you want to measure. According to the selected object, the other sections of the dialog box reflect the selection.

The Customization button allows you to define which information will be displayed in the Result section.

Export Geometry

Allows the creation of the corresponding geometry of the inertia measure.

Creates an axis system and a point corresponding to the current inertia measure.

Important: The created point and the axis system are not associative, therefore they are not updated when the product is modified.

The axis system and the point require a representation for their creation, and you can either create a representation under an existing product or select an existing representation.

The point is created in a new Geometrical Set under the representation.

Material Properties

Allows you to manage the material associated with the selected element.

By default the Density box is filled in with the default density (no material applied) or the applied material density.

Note: You can modify the existing density applied on a 3D Shape. You can specify the density value to zero.

For a volume, you can either use the default density specified in the preferences or specify a density for the Core Material.

For a surface, you can either use the default density and thickness specified in the preferences or specify a density and thickness for the Surface Properties.

You can also use a Covering Material by specifying its density and thickness.

Important: Specify a material excessively thick or a with a density higher than the body material will generate erroneous results.

Each time you customize or reset to default a density you need to click Recompute to update the inertia measure.

Export to file

Allows you to export inertia information of the assembly in a CSV file.

The CSV file provides information about:

The selected component,
Each product containing a 3DShape representation that belong to the selected product, including their instance paths.

By default, the area, volume, density and mass information is exported.

Additionally, you can export the following information:

Center of Gravity according.
Principal Moments.
Principal Axis
Inertia / G matrix.
Inertia / O matrix.
Bounding box.

Coordinates are defined according to the axis system of the root product.

Important: For the bounding box, the BBLx, BBLy, and BBLz values do not correspond to the x,y, and z directions of the root product's axis system. See Bounding Box Definition.

To view an example of a CVS file, see Assembly Inertia Export CSV.

Results

Displays results in dedicated tabs as specified in the Customization dialog of the Inputs section.

The Main results tab displays Area, Volume, Thickness, Mass and the coordinates of the center of gravity of the selected element.

Inertia tabs display Inertia Matrix, Principal Moments, and Principals axis according to the center of gravity.

Inertia in the Context of a Product

In the context of the active product, you can measure inertia. Inertia measures are not persistent and they are not saved with the active product.

The following rules are applied:

The instance location and orientation for each representation are taken into account.
For representations that are not loaded in session, inertia measures are approximated.
For applicative products (Electrical, Composite, or Piping) inertia data are provided by the dedicated apps.
Thread or tap features in a 3D shape representation are taken into account to compute the mass and the volume of a body but other inertia parameters like area, center of gravity, inertia moment or inertia box are ignored.
It is strongly recommended that you generate technological results for thread or tap features to ensure they are taken into account. Otherwise, only thread and tap features in the body are taken into account, other features in functional bodies or operated bodies are ignored.
Generating technological results for thread or tap features to retrieve their specification in imported features is recommended.