Remove rigid body modes
At times, it is difficult to constrain a model without limiting or preventing the deformation that would occur naturally. If you choose to leave your model unconstrained, you can select Remove rigid body modes to eliminate rigid-body, or free-body, motion during the solution. To use Remove rigid body modes with a constrained model, you must ensure that the sum of the forces equal zero AND the sum of the moments equal zero.
Remove rigid body modes is available for the following simulation types:
| Study icon | Study Name |
|---|---|
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Static Stress |
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Thermal Stress |
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Shape Optimization |
Remove rigid body modes works by applying a global acceleration to the model. This acceleration (a) produces a force (F), based on the mass (m) of the parts (F=ma). The force opposes any imbalanced loads acting on the model.
Note: In addition to a global acceleration, the solver also adds a set of soft spring constraints to ensure static stability (no free-body translation or rotation). The springs are anchored at the center of gravity of the model. The other ends are attached to the nodes nearest the corners of the model's bounding box. The springs account for slight imbalances that remain due to imperfect numerical balancing of the loads and acceleration forces. However, the global acceleration is the primary stabilizing factor. The soft spring constraints have a negligible effect on the results. The process is fully automatic, and there are no additional settings to specify.
When you might leave your model unconstrained
- Suppose that a shaft is supported on spherical bearings. This type of bearing pivots in its housing as the shaft bends under radial loads. If you apply any type of constraint to the portion of a shaft that extends through the bearings, the shaft behaves like a built-in beam. The constraint does not allow the supports to rotate as the shaft bends, unlike the spherical bearings.
- Imagine surface contact between the parts of an assembly. Often, parts are only constrained by virtue of contact with other parts (not directly constrained). Rigid-body motion can occur during the solution in these cases. Commonly, the constraints that you might apply to prevent rigid-body motion overconstrain one or more of the parts.