Lumped mass universal connection

Add lumped mass universal connections to your model to include parts whose mass needs to be accounted for, but where the stiffness is not important to the final result. For example, in an automotive analysis, you might create lumped mass connections to represent trim, mirrors, lights, speakers, batteries, and so on.

A lumped mass is defined by its mass value, location, and how it is connected to the rest of the model. You can assign the lumped mass directly to nodes, or you can connect it using spiders.

Lumped mass on nodes	Lumped mass on spider

Lumped mass connections work the same as other universal connections. First you create the connection, and then you mesh it to solver-specific elements. To realize (mesh) the lumped mass, you need to select the type of elements to be created for the connection. You can select the Nastran CONM2+RBE2 spider(s) or CONM2+RBE2 spider(s) elements, which consist of a concentrated mass element and spider elements.

• Concentrated mass element (CONM2)The CONM2 allows a concentrated mass about its center of gravity, and it defines the mass value, axis type, and inertia values.

• Spider connection element (RBE2 or RBE3)The connection element specifies the type of element to use to mesh the main spider. You can connect the main spider to the target via a local spider. Using the local spider mesh or elements, you can quickly connect multiple spider elements. For example, the mass can be connected to the main spider, and the main spiders legs can be connected to the target through the local spiders. For more information on using local spiders, see Local spider element targets in universal connections.

Lumped mass on main spider	Lumped mass on main spider with two local spiders

Lumped mass connection elements

The following table shows the valid combinations of elements for realizing (meshing) a lumped mass.

Elements combination	Description
CONM2 + RBE2	For the RBE2 spider, which is a rigid spider, the center node has all degrees of freedom fixed. The degrees of freedom for the leg nodes are set as follows, depending on the type of elements they connect to. If the legs are connected to solid elements, the translational degrees of freedom are fixed.If the legs are connected to shell elements, all degrees of freedom are fixed.Note: When you model a lumped mass on nodes, only the CONM2 element is created even though you select CONM2+RBE2.
CONM2 + RBE3	For the RBE3 spider, which is an interpolation spider, the center node has all degrees of freedom fixed, and the leg nodes have the translational degrees of freedom fixed.

Note:

For parabolic elements, the midnodes are not used for the spiders.

Lumped mass connection status

After you create and realize (mesh) your lumped mass connections, if the status columns shows Invalid, check the following:

• The mass may be set to zero.

• All of the legs may not be paired with the target.

• A connection may be missing a target. For example, this can occur when a mesh that was selected as a target is deleted.

Exporting and importing lumped mass connections

To use your lumped mass connections with other models, you can export the lumped mass connections using XML or spreadsheets, and then import them to another model. Or, you can add and update connections in the XML or spreadsheet and then import them to the same model.

The exported information includes the following:

• Connection type, name, and description.

• Unit system. The unit system is exported one time (in the element in the XML, and on the Assembly worksheet in the spreadsheet). You can set the unit system on the export and import dialog boxes.

• Connection folder. In the XML, the folder name is an attribute for the element, and in the spreadsheet, each folder is a separate worksheet.

• Mass, inertia, search, element type, and other values defined in the Lumped Mass dialog box.To create elements automatically when you import the lumped mass connections, specify the element type in the Connection Element column in the spreadsheet, or in the element in the XML.

For more information, see Export and import lumped mass connections.

If you plan to use your lumped mass connections in NVH Composer, see NVH Composer and Mass List view.

Where do I find it?

Creating a lumped mass connection

Application	Pre/Post
Prerequisite	A FEM or assembly FEM file as the work part and displayed part
Command Finder	Lumped Mass

Importing or exporting lumped mass connections

Application	Pre/Post
Prerequisite	A FEM or assembly FEM file as the work part and displayed part
Command Finder	Export Mass Import Mass
Simulation Navigator	Right-click the Universal Connections container→New Connection→Import Mass or Export Mass

Setting lumped mass customer defaults

Command Finder	Customer Defaults
Location in dialog box	Simulation→Pre/Post→Universal Connections node→Lumped Mass tab

How do I

Export and import lumped mass connections

Quick links

Command reference

Pre/Post video examples

Bulk Entry Descriptions

Simcenter 3D tutorials

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Lumped mass universal connection, Simcenter 3D 2021.1 Series

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