Seat Mechanism

Create and articulate a kinematic mechanism based on FE mesh using the Mechanism Browser.

From the menu bar, click Tools > Mechanism.

Restriction: Available in the LS-DYNA, Radioss, and OptiStruct user profiles.

You can undo and redo actions made in the Mechanism Browser using the Undo and Redo commands on the Restore toolbar.

The Mechanism Browser consists of two panes. The first pane displays the mechanism structure which consists of joints, bodies, and constraints. The second pane displays the Entity Editor.


Figure 1.
The following data is displayed in the Mechanism Browser:
Column Description
Entity Lists the mechanism, joints, bodies, and constraints in your model.
ID Displays the mechanism, joints, bodies, and constraints IDs.
Type Displays the joint or constraints type.
Color Displays the joint and body entity colors.

Body color is different from the component color set-up in the Model Browser. The body color is activated when a body is in Review mode.
Lock level Displays the lock level of a joint (green = free joint, yellow = locked joint).

Entity Editor

The Entity Editor is used to assign, modify and quickly view the attributes defined inside Mechanism Browser entities.

For example, once you create a new body or select a body in the browser, the Entity Editor opens and displays the bodies corresponding attributes, which you can view and modify.

From the Entity Editor, you can also create or edit an entity assigned to the selected body by right-clicking on the entity assignment field and selecting Create or Edit, respectively.


Figure 2.

Context Menu

Option Available for Description
Actuate Mechanism Mechanism Gives the ability to articulate a mechanism by defining one of the following actions:
  • Translate a node
  • Rotate a body
  • Move a node to a target location
Check Mechanism Mechanism Enables you to check the validity of a mechanism, looking for common nodes, empty bodies, and redundant joints.
Collapse All entities Closes all of the folders for the selected mechanism, so that only the mechanism item displays.
Collapse All In Mechanism Browser Closes all of the folders in the browser, so that only the mechanism items display.
Column Visibility Mechanism and in Mechanism Browser Enables you to hide or show the ID, Type, Color, Lock, Common nodes, Empty Body and Redundant Joint columns in the browser.
Create Mechanism and in Mechanism Browser Create a new mechanism, body, or constraint.

You can only create a body or constraint by right-clicking on a mechanism in the browser.
Delete All entities Deletes the selected entity.

Shortcut: To delete a selected entity in the Mechanism Browser, press Delete.
De-Link Dummy Mechanism When a dummy has been linked to a mechanism, this option is activated in order to remove the link between the dummy and the mechanism.
Expand All entities Opens the selected folder, exposing every entity nested at every level.
Expand All In Mechanism Browser Opens all of the folders in the browser, exposing every entity nested at every level.
Export Position Mechanism Opens the Export Position dialog, from which you can export a saved position of the selected mechanism into a solver file containing the corresponding solver transformation cards.
Hide All entities Turns off the entity in the graphics area. This selection affects each item’s local display control, that is, it will make the icon become ghosted indicating the display state is off.
Isolate All entities Displays only the selected entities, and turns off all other entities of the same type.
Link to Dummy Mechanism In case a dummy is present in the session, this option is activated in order to link the dummy, for example, to a seat mechanism.
Lock/Unlock Joint Joint Block (Lock) all degrees of freedom of the selected joint or make them free again (Unlock).
Move Joint Opens the Entity Editor to define the positioning values of the selected joint and activate the joint manipulator in the display that can be used to interactively move the joint.
Move Mechanism To Mechanism Moves the selected mechanism to its initial position or other positions, which have been previously saved.
Review All entities Invokes Review mode, which displays selected entities irrespective of their display state, masked, active state (Entity State Browser), but not outside of the spherical clipping (if enabled).

By selecting a body, it will be displayed with its body color while other entities are grayed out and displayed with transparency.

By selecting a joint or a mechanism, the joint marker with the bodies links between will be displayed.
Reset Review All entities Resets the review of the previously selected entity.
Rename All entities Rename an entity in the Name field. The new name must be unique. All instances of the renamed entity will update automatically. You can cancel the rename operation by pressing Esc.
Save Position as Mechanism Save the displayed mechanism position as a new position or replace an already existing position.
Show All entities Displays the selected entity in the graphics area. The entities icon changes to bold indicating that the display state is on. Using the Show option on a mechanism or joint will display all nested entities inside.
XRef Entities All entities Opens the References Browser and displays the relationship of the selected entities to other entities in the model in a hierarchical tree structure.

Any single entity or multiple entities can be selected.

Supported Entities

A Mechanism () is the root of the hierarchy in the Mechanism Browser. To create a Mechanism, right click in the Mechanism Browser and select Create > Mechanism from the context menu. A mechanism is defined by the following entities:
  • Bodies () define a kinematic assembly made of FE parts or nodes, which can be selected in the Entity Editor. To create a body, right-click on the Mechanism and select Create > Body from the context menu.
  • Joints () define the kinematic relationship between two bodies (for the following joint types: Ball, Cylinder, Revolute, Slider) or three bodies (for the DoubleSlider joint). To create a joint, select two bodies to connect in the browser, then right-click and select Connect from the context menu. For the DoubleSlider joint, the third body has to be defined in the Entity Editor.
  • Constraints () define kinematical constraints on a body at a specified node or point location.

Supported Keywords Exported in Solver Deck

The mechanism information is embedded in the input deck using keywords following the /END (for Radioss), *END (for LS-DYNA), ENDDADTA (for OptiStruct/Nastran) and *END STEP (for Abaqus).

For Radioss, a mechanism is defined between the /MECHANISM_START and /MECHANISM_END keywords, and is composed of:
  • /ASSEMBLY defines a body with the following attributes:
    • Assembly ID
    • Assembly name
    • Number of part sets and related set IDs
    • Number of parts and related part IDs
    • Number of node sets and related set IDs
    • Locked degrees of freedom
    • Optional local coordinate system
  • /CONNECTION defines a joint between assemblies with the following attributes:
    • Connection name
    • Assembly ID #1
    • Assembly ID #2
    • Connection position (defined by Node IDs or connection position coordinates)
    • Joint limits in + and - directions
    • Current joint distance/angle value
    • Lock level
    • Assembly ID #3 (for the Double Slider only)
    • Scale factors for relative displacement between Assembly #3 and Assembly #1 and #2
  • /POSITION defines, for a stored position, the position information for the mechanism’s assemblies with the following attributes:
    • Position name
    • Assemblies position matrixes
      Note: The Reference and Initial positions should never be deleted inside the mechanism.
  • /CHILD_DUMMY defines the coupling between a mechanism and a dummy with the following attributes:
    • Name
    • Main assembly ID of the mechanism
    • Dummy ID
    • Number of child assemblies of the dummy
    • Degrees of freedom linking main to child
    • Child assembly IDs of the dummy

Create a Mechanism

This task explains how to create a mechanism.

There are two ways to create a mechanism: Manual and Auto Generate. In this task, you will create a mechanism using the Auto Generate method.

  • Using the Manual approach, you create the mechanism manually by defining the bodies and joints.
  • Using the Auto Generate approach, HyperMesh will automatically create bodies and joints based on a selection of components and elements of the FE model.
In the Mechanism Browser, right-click and select Create > Mechanism > Auto Generate.
When you select Auto Generate, you will see the Mechanism Builder dialog.


Figure 3.
To enable an extraction of the mechanism, each field in the dialog must be defined or selected. Set the options according to the Mechanism Builder Options table.

Mechanism Builder Options Table

Use this table to set options for the Mechanism Builder.

Table 1.
Mechanism Builder Options
Option Action
Mechanism Type Select the type of mechanism, either Seat or General. Your selection will affect other options in the dialog.
Mechanism Name Type the name you want to give to the mechanism after the extraction is completed.
Entities Select the FE components that define the mechanism. You can choose Components, Mechanisms, Assemblies or Sets.
Node Selection on Lower Track When the Mechanism Type is set to Seat, you can select nodes on the lower track components of the seat.
Node Selection on Upper Track When the Mechanism Type is set to Seat, you can select nodes on the upper track components of the seat.
Sliding Vector on Upper Track When the Mechanism Type is set to Seat, you can specify the translation direction of the joint defined for the seat track.
Seat Track with Roller Balls When the Mechanism Type is set to Seat, you can specify whether the seat track contains roller balls. If you check this option, HyperMesh will automatically create a double slider joint for the seat track, and an additional selector for the seat track roller will appear.
Track Roller Balls When the Mechanism Type is set to Seat, and Seat Track with Roller Balls is checked, this option allows you to select the roller ball components that will be defined as third body in the double slider joint.
Joint Elements Select the FE elements (springs or beams) that have to be created as joints in the mechanism extraction process.
Excluded Elements Select the FE elements that should be excluded from the extraction process of the mechanism.

For example, nodal rigid bodies that are used to link components in the FE model but that are not physical conditions, can be selected here and will not be considered in the extraction process.

Excluded Contacts Select to exclude tied contacts that may be defined on the FE model but do not represent physical connections. This excludes them from the extraction logic.
Detect Holes Select to activate the hole detection parameters. This allows you to detect joint positions by looking for concentric holes in components.
Relative Search Distance When Detect Holes is checked, this option defines the distance of search for concentric holes that define a joint location.
Components for Hole Search When Detect Holes is checked, this option allows you to select the components on which the hole detection must be performed.
Preferred Revolute Joint Axis Type the vector that defines the revolute joint axis. This will overwrite the direction of the joint elements.
Angular Tolerance Type the angular tolerance into this field. The joint axis within this angle will be overwritten by the Preferred Revolute Joint Axis value.

Position Mechanisms and Joints

Overview of how to modify the position of mechanism and joints.

Modify the Position of Mechanisms

  1. In the Mechanism Browser, right-click on a mechanism and select Actuate Mechanism from the context menu, then choose from one of the following:
    • Choose Translate to open the Entity Editor and select a Node/Constraint, give a Direction of motion, and define the Magnitude of the translation to apply.
      The Magnitude can also be modified, using the up and down arrow buttons. Moreover, you can control the increments of the operations by changing the Increment value.


      Figure 4.
    • Choose Rotate to open the Entity Editor, select a Body, define a Rotation axis, a rotation center (Base point), and provide the Magnitude of the rotation to apply to the body.
      The Magnitude can also be modified, using the up and down arrow buttons. Moreover, you can control the increments of the operations by changing the Increment value.


      Figure 5.
    • Choose Target Point to open the Entity Editor, select a Node and define its Target location. Click Move to calculate the position of each body to bring the selected node to the target location or the nearest location, depending on joints limit values.
      This option can be applied on a set of nodes and targets, by setting Multiple pairs to Yes.


      Figure 6.
  2. Click Save to save the achieved position.
    A name will be appended to the New position name field.

Modify the Position of Joints

  1. In the Mechanism Browser, right-click on a joint and select Move from the context menu.
  2. Modify the position of the joint.
    • In the graphics area, click-and-drag the manipulator.


      Figure 7.
    • In the Entity Editor, modify the Current angle of the joint.


      Figure 8.
  3. Change the Increment value to control the increments of the operation.

Joint Coupling

Joint coupling provides functionality that allows you to create a gear joint (coupling between two revolute joints), or a rack and pinion joint (coupling between a slider joint and a revolute joint).

With joint coupling, you can link the degrees of freedom of two joints with a given multiplication factor.


Figure 9.
  1. To create a joint coupler, right-click on the mechanism and select Create > Joint Coupler from the context menu.


    Figure 10.
  2. In the Entity Editor under Joints, select the two joints to link.


    Figure 11.
  3. Under Joint Factors, in the Factor 1 field, enter an appropriate multiplication factor.
  4. Click Create.

Joint with Morphing

When creating slider joints, you can select additional components that will be automatically morphed along with the motion of the selected bodies.

This functionality is useful when morphing a wheel suspension system, for example.


Figure 12.
  1. To activate the joint with morphing functionality, from the Entity Editor, check the Slider with Morphing checkbox.
  2. Under Support, select the components you want to morph in the Component to Morph field.


    Figure 13.
  3. Click Create.