About Rigid Bodies
A rigid body is a group of components that are rigidly controlled with no degrees of freedom within the group. The constraints used to place a component determine which parts belong to a rigid body. The software automatically defines rigid bodies based on these constraints.
You create an assembly by combining components, including parts and subassemblies. You use the Component Placement tab in Assembly to define the relationship of the component you are adding to your assembly.
There are two types of constraint sets on the Component Placement tab. You can use user-defined constraint sets, such as Coincident and Parallel, or you can use predefined constraint sets to define connections. If you assemble two components as partially constrained, a connection is assumed.
The way that you define the rigid bodies in your assembly impacts the way you create predefined constraint sets in the following ways:
• You can create predefined constraint sets only between distinct rigid bodies.
• When specifying predefined constraint sets for a Mechanism Design connection on the Component Placement tab, you can only reference a single rigid body in the assembly and a single rigid body in the component being placed.
• When you select the first assembly entity for a predefined constraint set, you can select entities only from the same rigid body for the remaining constraints of that connection. This is also true when selecting the component references.
Ground components (parts and subassemblies) in a mechanism do not move with respect to the assembly. You can include several parts or subassemblies in the ground body. To define a ground body, fully constrain a component with constraints that reference the default assembly datums or a part or assembly already in ground. If you underconstrain the component, it is not placed in the ground body and is considered a new rigid body.
The best way to review rigid bodies is by opening the RIGID BODIES folder in the Mechanism Model Tree. Click 
> in the Model Tree to open the Mechanism Model Tree in Assembly.
> in the Model Tree to open the Mechanism Model Tree in Assembly.For more information, search the Help Center.