Inventor Nastran Troubleshooting – Normal mode


Inventor Nastran is great for creating simulations of your assembly and then analysing the results to make better design decisions for your business – but what happens when the analysis fails. If you have worked with Inventor Nastran before you will know that a failure will give a number which you have to look up. Nastran will then give a few suggestions and you have to troubleshoot which sometimes is cryptic in itself. You have to troubleshoot the troubleshooter.

Error 5004 – Stiffness matrix singular or non-positive definite. Have you seen this before? I have and plenty of times. It is the most common error message you will see. What does this mean in the common language of the common folk? Something is unattached and can still move. This can be caused by the following.

  • Lack of Constraints
  • Improperly defined constraints
  • Improperly defined contact
  • Singular elements

The most common one of these is the lack of constraints. A trick I learnt to see exactly where it is unattached, is to change the analysis to Normal Modes. When analysing this type of simulation you do not have to be completely attached and it will quickly show where the unattached parts are.

Inventor Nastran

In the above image you can see I have forgotten to put some much needed bolts. So I quickly backtrack and insert the bolts and voila, my simulation runs and I get results.

Inventor Nastran

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