For any multibody dynamics model in COMSOL Multiphysics, you should finalize the geometry using Form Assembly. To connect the different domains, you need to define Attachment boundary conditions. In order to enable motion between the different domains, there are several different Joint types available within the software. In this example, we show how Prismatic Joints and Hinge Joints are used to enable the motion in a centrifugal governor.
For more detailed information on the differences between Form Assembly and Form Union, watch the Form Union and Form Assembly Geometry Finalization Methods video.