Measuring Inertia ()  

The Measure Inertia command lets you measure:

  • 3D inertia properties of surfaces and volumes (explained below)

  • 2D inertia properties of plane surfaces.

This section deals with the following topics:
 

Measuring 3D Inertia

This task explains how to measure the 3D inertia properties of an object. 

Details about Inertia Measurements:
 

You can measure the 3D inertia properties of both surfaces and volumes, as well as retrieve the density or surface density if valuated from V4 model type documents. You can also retrieve inertia equivalents set in Knowledgeware formulas.

  • The area, density, mass and volume (volumes only) of the object are calculated.

  • Deactivating features does not affect the mass of the part.

  • The center of gravity G is expressed in the global axis system, it is not possible to express this value in the local axis system.

  • The center of gravity of a surface is meaningful only as long as the center of gravity of the equivalent volume
    corresponding to the surface with a unitary thickness is considered.

  • The center of gravity of a closed surface (thin skin) is different from the equivalent volume included in the surface.

  • When there is no material applied to the selected items, the center of gravity corresponds to a geometrical center of gravity. In this case, it is recommended to use the Measure item tool where it is possible to select a secondary axis system. Thus the center of gravity coordinates will be expressed in this secondary axis system.
Insert the Valve.cgr document from the samples folder. It is to be found in the online documentation file tree in the common functionalities sample folder cfysm/samples.

  1. Click Measure Inertia . In DMU, you can also select Analyze > Measure Inertia from the menu bar. The Measure Inertia dialog box appears.

    By default, 3D inertia properties are measured.

    The Measure 2D Inertia icon lets you measure 2D inertia properties of plane surfaces.

    Dialog box options
     

    • Keep Measure option in the dialog box lets you keep current and subsequent measures as features in the specification tree. Some measures kept as features are associative and can be used as parameters.

    • Only main bodies: Select this check box to measure inertia only for main bodies.

    • Measure only shown elements check box: when selected, lets you measure only the elements from the show space.
        Swapping the elements between show and hide spaces does not modify the existing measurements.
      An update (automatic or manual) of the existing measurements is necessary to modify them after the swap.

    • A Create Geometry option lets you create the center of gravity and the axis system for principal axes in a part from inertia results.

    • An Export option lets you write results to a text file.

    • A Customize... option lets you define what will be computed and displayed in the dialog box.

    In the Drafting workbench, the Keep Measure option is not available. Measures are done on-the-fly. They are not persistent. This means that they are not associative and cannot be used as parameters.
    Note: When you move the cursor over the geometry or specification tree, its appearance changes to reflect the measure command you are in

  2. Click to select the desired item in the specification tree, for example Valve. 

    Selecting Items
    In the geometry area, you can select individual faces and edges on cgr files and in Visualization mode.
     
    To...
    Then

    make a multiple selection

    Shift-click in the specification tree

    add other items to the initial selection

    Ctrl-click in the geometry area or the specification tree

    select items using the bounding outline

    Drag (using the left mouse button)

    make your multiple selection.(P2 only)

    Use the Group command
    Notes:
    • Only items of the same type can be included in a multiple selection or a bounding outline; you cannot mix volumes and surfaces.
    • Inertia measures made on a multiple selection of items are not associative.

    Dialog Box

    The Dialog Box expands to display the results for the selected item. 

    The measure is made on the selection, geometry, assembly or part. To measure the inertia of individual sub-products making up an assembly and see the results in the document window, you must select the desired sub-product.

    The dialog box identifies the selected item and indicates whether the calculation is exact or approximate:
     

    • In Design mode, measures (bounding box excluded) access exact data and wherever possible true values are given. Note that it is possible to obtain an exact measure for most items in design mode.

    • In Visualization mode, measures are made on tessellated items and approximate values are given.

    In addition to the center of gravity G, the principal moments of inertia M and the matrix of inertia calculated with respect to the center of gravity, the dialog box also gives the area, volume (volumes only), density and mass of the selected item.

    You can also compute and display the principal axes A. To do so, you must first activate the appropriate option in the Measure Inertia Customization dialog box.

    The density is that of the material, if any, applied to a product, part or part body:

    • If no density is found, a default value (1000 kg/m3 for volumes and 10 kg/m2 for surfaces) is displayed.
      You can, if desired, edit this value to re-calculate all the other inertia values and display them in the dialog box. Note: re-calculated inertia values are not stored in the measure feature.

    • If sub-products or part bodies have different densities, the wording Not uniform is displayed in the Inertia Dialog Box and the Density parameter will have the value as -1 in the Specification tree.

    To make sure, you retrieve the density on any possible selection (surface and / or volume), you must select the item (part or part body) in the specification tree and not in the geometry area.

    Notes:
     

    • The bounding box calculation accuracy depends on the one used for tessellation (SAG) on objects. This is set in the Performances tab page in Tools > Options > General > Display.
      By default, this value is set to 0.2 mm.

    • You can access the density of parts saved as CGR files and opened in visualization mode. This functionality is available in both a part and a product context.

    • To do so:

      • Select the Save density in cgr option in the Cgr Management tab (Tools > Options > Infrastructure > Product Structure).

      • Open a part to which material has been applied and save as CGR type.
        The density is stored in the CGR file.


        • Important: The material must be applied to the part node. If materials are applied to part bodies, no density is saved.

      • Close the Part document.

      • Open the CGR file or switch to DMU Space Analysis and insert the part saved as CGR, then measure the inertia.

    • You must be in design mode to access the density of part bodies to which materials have been applied.

    • Unless specified otherwise, material inheritance is taken into account.

    • Density is a measure of an item's mass per unit volume expressed in kg/m3; surface density is a measure of an item's mass per unit area expressed in kg/m2.


     

    The number of decimal places, the display of trailing zeros and limits for exponential notation is controlled by the Units tab in the Options dialog box (Tools > Options, General  > Parameters and Measure).

    Elements placed in No Show are taken into account in measure operation.

    Geometry area

    In the Geometry Area, axes of inertia are highlighted and a bounding box parallel to the axes and bounding the selected item also appears.

    Color coding of axes:

    • Red: axis corresponding to the first moment M1

    • Green: axis corresponding to the second moment M2

    • Blue: axis corresponding to third moment M3.

  3. Click Customize...  to customize the inertia computation and define what will be exported to the text file.

  4. Click OK when done.

  5. If you checked the Keep Measure option in the Measure Inertia dialog box, your measures are kept as features and your specification tree will look something like this.

    Some measures kept as features are associative and can be used as parameters.

Notes:

  • You can write a macro script to automate your task. See Space Analysis on the Automation Documentation Home Page.

  • In interactive and automation modes (using CATScript), the inertia computation is always performed on the whole body. The ‘Only Main Body’ option from mechanical properties is not taken into account.

  • In interactive mode, the inertia computation can be performed on whole body or only main body depending upon the state of only main bodies option in the Measure Inertia dialog box. In automation mode, the inertia computation can be performed on whole body or only main body depending on the granularity. For example, if GranularityMode is 0, then the only main bodies option is selected and inertia is computed only on main bodies.

Customizing Your Measure

  1. Click Customize... in the Measure Inertia dialog box. The Measure Inertia Customization dialog box opens.

    Notes:

    • The inertia properties check boxes selected here are also the properties exported to a text file.

    • You can, at any time, define what will be computed and displayed in the Measure Inertia dialog box.

  2. Click the appropriate options to compute and display in appropriate tabs of the Measure Inertia dialog box the:

  3. Click Apply or OK in the Measure Inertia Customization dialog box when done.

 

Restrictions
 

  • In the Drafting workbench, the Keep Measure option is not available. Measures are done on-the-fly. They are not persistent. This means that they are not associative and cannot be used as parameters.

  • You cannot measure inertia properties of either wire frame or infinite elements.
    For examples showing 3D inertia properties measured on surfaces.
    To find out more about notations used.

  • Measures performed on sheet metal features provide wrong results. In unfolded view, volume elements are not taken into account when measuring Part Bodies.

  • Measures are not associative when switching between folded view and unfolded view (using Fold/Unfold in the Sheet Metal toolbar).

  • When an assembly contains objects with different dimensions (for example a solid -3D and a surface- 2D) only the object with the highest dimension (i.e. solid) is taken into account for the inertia measure calculation.

  • When measuring inertia on a feature (PartBody, Product, CATPart) on which a material is applied:

    • if you modify the density value on the material or modify the material itself (adding a new one, modifying it or removing it), inertia measures previously created on this particular feature are not updated automatically and the Update icon on the measure is not displayed. Use the Force Measure Update command on the Measure or Inertia Volume object to update the measure result.

  • You cannot perform Measure Inertia within a product located in a CATAnalysis document in Product Editor workbench.

  • For elements such as Volume and Volume Extrude of Geometrical Set, material applied is not considered for density value during inertia calculation.