Constraints are the natural anchoring points for your design model. A constraint represents the method and location of fixing the structure or part to the ground or to other parts. When you set a constraint, you effectively reduce the translational freedom of a portion of your design model to zero at selected vertices, along an edge, or over a face.
The two types of constraint symbols are shown here:
In the image, the magenta symbols represent a translational constraint and the yellow symbols represent a rotational constraint. These symbols are shown in the default colors. To change the colors, click on the Settings icon in the Creo Elements/Direct Finite Element Analysis task bar.
Rigid body motion and mechanisms
When you set loads and boundary conditions, it is important to remember to apply sufficient constraints to prevent rigid body motion.
Rigid body motion occurs when the structure or part can move freely in one or more displacement directions (displacement without strain). When you push the coffee cup on your desk with your finger so that the cup slides on the desktop, you are creating rigid body motion.
A subclass of rigid body motion occurs when part of an otherwise constrained structure is capable of rigid body motion. This is called a mechanism. In linear static analysis, the presence of a mechanism also produces a singularity failure in the solution.
Two lengths of rod connected by a free-moving ball joint illustrate this problem when only one length of rod is correctly constrained prior to the analysis run.
To set a constraint on a face, edge, or vertex,
1. Click FEA and then, in the Mechanical LBC group, click Constraints.
2. Click Vertex, Edge, or Face in the Translational or Rotational section of the Constraints dialog box.
3. Click Study and select a study in the Structure Browser.
4. Click Name and type a name for the constraint. Each constraint is displayed under the study in the Structure Browser.
5. Click Ref Vertex, Ref Edge, or Ref Face and select a vertex, an edge, or a face in the viewport.
6. You can change the degrees of freedom to Free or Fixed for the X, Y, and Z axis directions.
7. You can change the coordinate system used for the constraint to:
◦ Global: Uses the normal Cartesian coordinate system.
◦ Local: Allows you to define a local coordinate system for the constraints
◦ Cylindrical: With a translational constraint within a cylindrical coordinate system, you can effectively block the rotation of a shaft (Fix Tangential). Although you are blocking the rotation of the shaft, you are not dealing with the rotation of a node because Solid-Model-Finite-Analysis does not have a rotational degree of freedom. By such a constraint each constrained node can only move in the plane given by the original location of the node and the axis of the coordinate system.
8. If you select Local or Cylindrical in the previous step, you must define the coordinate system. Click Define CS and follow the prompts in the prompt bar, which is directly above the Structure Browser and viewport.
9. Click Next to set additional constraints of the same type.
to complete the operation.