The Fixed Joint node is mainly intended for connecting reduced-order models in the context of Component Mode Synthesis. A fixed joint connects, for example, two attachments so that they form a common rigid region.

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

Select a Source and a Destination attachment from the lists. You can select any Attachment or Rigid Domain in the model. In case that there is a Multibody Dynamics interface in the model, also such features as Spur Gear, Helical Gear, Bevel Gear, Worm Gear, Spur Rack, and Helical Rack can be selected. In Source, an additional option, Fixed, is also available. When Fixed is selected, the source side of the joint is fixed in space, and not connected to any modeled parts. If any Base Motion nodes are present in a Multibody Dynamics interface, these also appear in the list for the Source attachment. Selecting a base motion gives a behavior similar to using Fixed, with the exception that the base now can have a nonzero prescribed displacement, velocity, or acceleration.

In large models, the list of available attachments can become long. You can then create geometric filters to narrow down the search. When the Use selection filter check box is selected, two subnodes named Source Filter and Destination Filter are added to the joint node. In these subnodes you can make graphic selections of the objects whose attachments should be shown in the Source and a Destination lists.

Physics tab with Solid Mechanics selected in the Model Builder tree: