About Masses

A mass is an idealization that you can use to represent a concentrated mass without a specified shape. The mass of an object determines how that object resists translation and rotation. If you are interested in the way your model behaves with mass at a given location and in the larger effect of its mass and inertia on other parts of the model, but not in the geometry or other features of that mass, use mass idealization. For example, you can represent the mass of an engine on a car frame without specifying the engine geometry.

Mass idealizations are typically used in modal dynamics problems to account for lumped masses.

To create a mass, select Refine Model >

Mass and use the Mass Definition dialog box to create a mass and specify its properties. For some types of masses, you may need to assign mass properties. You can create mass properties by clicking Refine Model and then click the arrow next to

Mass. Click

Mass Properties or from the Mass Definition dialog box as you are creating the spring.

Your options for creating masses differ in native mode and FEM mode:

• Native mode—You can add a mass to a vertex or a point. You can also add a mass to multiple single points, point features, and point patterns in most cases. However, if you are working in the assembly mode and want to use a component's mass for your mass idealization definition, you can only select a single point.

• FEM mode—In addition to adding a mass to a point, you can also add a mass that is distributed over geometric entities such as curves, edges, or surfaces. Use distributed masses, for example, to represent the mass contribution from paint, or from a large number of small objects. You can select several curves, edges, or surfaces. To learn how FEM mode's FEA solvers treat masses, see ANSYS and MSC/NASTRAN.

If you want to override the precedence rules for masses in FEM mode, you can use the configuration option sim_additive_mass. When you set this option to "Yes," you can create several masses that reference the same geometric entity and the software will include all of the masses. Also, with this option turned on you can override the precedence rules that normally govern mass creation. For example, if sim_additive_mass is set to "No" and you try to create two masses, mass a on a single point and mass b on a feature of points containing the single point, mass a on the single point overrides mass b on that same single point, leaving mass b on the other points in the feature unchanged. But if sim_additive_mass is set to "Yes," you can create both masses, and include both masses in your analysis. Use this option, for example, to simulate two layers of paint as distributed masses on a surface.

If you need to create additional datum points or curves before you begin the process of adding masses to your model, you can use Refine Model >

Point or click Refine Model > Datum >

Curve. Keep in mind that if you want to model a mass on a point that is separate from your model, you must connect it to the model, for example, with rigid links or beams. Otherwise, the mass will not be included in any analyses.