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 that 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/). Rigid Connector is available for 3D and 2D.

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.

The Global coordinate system is selected by default. The Coordinate system list contains all applicable coordinate systems in the model. 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 about the 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:

For Prescribed rotation enter an Angle of rotation ϕ0. For 3D components also enter an Axis of rotation Ω for the x, y, and z coordinates.

In some cases it can be useful to not constrain the displacement in a certain direction. For instance, the radial displacement of a Rigid Connector acting on a circular cross-section could be allowed to be free. To do so, select a local Coordinate system for specifying the directions in which the degree of freedom will be released. The Coordinate system list contains only applicable coordinate systems in the model.

The section Released Degrees of Freedom is only shown if Connection Type is Rigid (if applicable), and if the check box Use weak constraints for rigid-flexible connection in the section Constrain Settings is not enabled.

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.

Select Use symmetric formulation for geometric nonlinearity to force all matrix contributions from the rigid connector and its subnodes to be symmetric.

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.

When the rigid connector is added at edge or point level, automatic tests are performed to check for selections that would result in a singularity. 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.

When the Rigid Connector is added at the point level, select the Add rotational stiffness for two-point selection check box to automatically suppress singularities when the Include consistency checks check box is disabled. This check box allows to enter a rotational stiffness, kθ, to stabilize the rigid connector. The rotational stiffness is only added if less than three points are selected.

Select Group dependent variables in solver — From physics interface (default), Yes, or No, to choose how to group the dependent variables added by the Rigid Connector feature in the solver sequence.

Physics tab with Solid Mechanics selected:

Physics tab with  Layered Shell selected:

Physics tab with Multibody Dynamics selected: