If you use SOLIDWORKS Simulation for an extended period, you’ll soon see that it has some strange error messages. This is one of my personal favorites:
The Iterative Solver stopped, so what does this mean? Thankfully the SOLIDWORKS Knowledge Base has (nearly) every Simulation error message documented so it’s an easy search to learn that it represents rigid body modes because of missing boundary conditions.
Rigid Body Modes
What are rigid body modes? It means that a body is not adequately supported and can translate or rotate as a whole without deformation. This lack of boundary conditions is typically found in the form of missing Fixtures and/or Contact Sets.
To fix this error, double check that all the appropriate fixtures are added and then move on to Contact Sets. Checking the Contact Sets is more involved, but it’s usually where this error originates. It is easy to miss a contact set, especially with mixed mesh models where Global Contact is limited.
If you’ve double checked everything and you’re still getting an error for an unstable model, there is a diagnostic option that can help diagnose the study. This setting is called Soft Springs and can be enabled in the study properties.
When this option is enabled, the model is surrounded by soft springs with negligible stiffness values relative to the stiffness of the models. They will stabilize the model and possibly allow it to solve. The setting does have limitations, and the solver may still fail if the model is not self-stabilized or the external loads are higher that the soft springs can account for.
While enabling Soft Spring may allow the solver to complete, it doesn’t necessarily mean the results are correct. It’s recommended that you look at that displacement plot and make sure the study is behaving that way you expect it to. You may see some models have displacement when they should, not or you may see the opposite. Look for something unexpected. Add the necessary Fixtures and Contact Sets, turn off Soft Springs, and solve again. Hopefully the study is now able to solve. If not, enable Soft Springs and try again.
Using Soft Spring
Let’s look at a simple example. This is a two part assembly with a Shrink Fit contact set between the rim and wheel. There are Symmetry fixtures on the cut faces. We should see nothing more than radial and circumferential stresses and displacements. For this example we’re intentionally leaving the axial direction free.
When we try to solve this we get the Singular Matrix error, so let’s enable Soft Springs. The study solves this time, and when we look at the displacement plot it’s clear that there is noticeable displacement in the axial (Y direction). There is still rigid body motion, but the solver completes because the Soft Springs stabilize the model.
Armed with the information that the model can translate in Y, add the appropriate Fixture. Turn off Soft Springs and we should now have a successful solve.