The Rigid Connector is a boundary condition for modeling rigid regions and kinematic constraints such as prescribed rigid rotations. A rigid connector can connect an arbitrary combination of boundaries, edges, and points which all will move together as being attached to a virtual rigid object.

You can add the Rigid Connector node at the boundary, edge, and point levels.

The Rigid Connector node is only available with some COMSOL products (see https://www.comsol.com/products/specifications/).

This section is present when the Rigid Connector node has been added at the boundary level. Select one or more boundaries to be part of the rigid region.

This section is present in 3D when the Rigid Connector node has been added at the boundary or edge level, and Connection Type is Rigid.

This section is always present when Connection Type is Rigid.

If this node is selected from the Pairs menu, choose the pair to use. An identity pair has to be created first. The rigid connector applies to the common part of the boundaries, and makes the parts behave as if there were an infinitely stiff layer between them.

The Global coordinate system is selected by default. The Coordinate system list contains any applicable coordinate systems that the model includes. Prescribed displacements and rotations are specified along the axes of this coordinate system. It is also used for defining the axis directions of the moment of inertia tensor of the Mass and Moment of Inertia subnode.

Select Rigid or Flexible. When the connection type is rigid, the whole rigid connector acts as a virtual rigid object. In the flexible formulation, the selected boundaries are allowed to have internal deformations, and the kinematic constraints are fulfilled only in an average sense. The flexible formulation is useful for example when applying loads, since it will reduce local constraint effects.

Select a Center of rotation — Automatic, Centroid of selected entities, or User defined.

Select the Offset check box to add an optional offset vector to the definition of the center of rotation. Enter values for the offset vector Xoffset.

Specify the rotation at the center of rotation. Select from the By list: Free, Constrained rotation, or Prescribed rotation at center of rotation.

For Constrained rotation select one or more of the available check boxes to enforce zero rotation about the corresponding axis in the selected coordinate system:

Select Evaluate reaction forces to compute the reaction force caused by a prescribed motion. The default is to not compute the reaction force. When selected, the prescribed motion is implemented as a weak constraint.

Select Apply reaction only on rigid body variables to use a unidirectional constraint for enforcing a prescribed motion. The default is that bidirectional constraints are used. This setting is useful in a situation where a bidirectional constraint would give an unwanted coupling in the equations. This would happen if the prescribed value of the motion is a variable solved for in other equations.

On the boundaries where the rigid connector is coupled to a flexible material, all nodes on such a boundary are constrained to move as a rigid body. As a default these constraints are implemented as pointwise constraints. If you want to use a weak constraint formulation, select Use weak constraints for rigid-flexible connection.

This formulation cannot be combined with the Flexible formulation of the rigid connector, which in itself is a special form of weak constraint.

This section is present when the rigid connector is added at edge or point level. In these cases, checks for selections that would result in a singularity are performed as a default. If these checks give false positives, you can turn them off by clearing the Include consistency checks check box. This could, for example, be necessary if a rigid connector, which in itself is singular, is connected to another one in a way that forms a stable configuration.

Physics tab with Solid Mechanics selected:

Physics tab with  Layered Shell selected:

Physics tab with Multibody Dynamics selected: