How to Troubleshoot ‘Excessive Nodal Displacement’ and ‘Possible Rigid Body Motion’ Errors in ANSYS

During calculation and solving, sometimes situations like excessive nodal displacement and possible rigid body motion occur, as shown in the screenshot below:

For the above situation, Shuige suggests troubleshooting from the following three aspects:

1. Are the units matching?

Unit system mismatch can easily cause the above situation. For example, when users adopt the N-mm unit system, the density unit should be t/mm³, and the density value for ordinary steel should be 7850e-12 at this time. In addition to materials, all unit-related inputs need to be comprehensively reviewed.

Regarding unit system conversion, you can use the finite element unit conversion table compiled by Shuige, which is simple and quick, as shown below:

The conversion table can be accessed by clicking ‘Read More’ at the bottom left of this article to go to Shuige’s blog, or directly enter the URL https://fscae.com/3617/ in your browser.

2. Are the boundary conditions reasonable?

Simulation mechanical models are mainly simplified from actual structures. In the simplification process, mechanical boundary conditions are relatively important and directly determine the reliability of calculation results. If the above situation of excessive nodal displacement occurs, it may also be due to unreasonable boundary conditions, leading to structural instability, also known as kinematic situation.

3. Are the element connections reasonable?

Connections between same or different elements have many methods, such as welding, coupling, shared nodes, and contact. If the above situation occurs, there may also be problems with element connections. For example, Boolean operations were not done properly, resulting in failure to achieve shared nodes; or contact element normals are not opposite, or contact settings have problems, leading to kinematic situations.

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Best regards,

ANSYS Structural Institute

October 9, 2025