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You may often encounter redundant constraints while modeling in RecurDyn.
What shall we do to remove the redundant constraints?
An example of redundant constraint is the situation where two fixed joints are attached to a single rigid body.
As one fixed joint already constrains all 6 degrees of freedom of the rigid body, the model's degree of freedom becomes 0.
Therefore, having another fixed joint does not change the model's degree of freedom and results in 6 redundant constraints.
The RecurDyn solver automatically removes redundant constraints when they occur in order to perform analysis. So the simulation is carried out without problem.
But, there are cases where you obtain a result different from what you intended.
Therefore, if possible, remove redundant constraints before performing analysis.
You may need to define constraints (joints) carefully so as not to generate redundant constraints. Despite this, however, redundant constraints may still occur.
The easiest way to remove redundant constraints is to use bushing forces, instead of joints.
For example, you may use the method described in the following link to use bushing forces instead of joints.
How can I measure the reaction force in multiple fixed joints connected to a rigid body? ( <- Click)
1. Using a bushing force instead of a fixed joint
- Enter very large values for the translational and rotational rigidities of the bushing.
2.Using a bushing force instead of a revolutional joint
- Enter 0 for the rigidity of the rotational axis of the bushing and very large values for the remaining rigidities.
3. Using a bushing force instead of a translational joint
- Enter 0 for the rigidity of the translational axis of the bushing and very large values for the remaining rigidities.
As shown above, you can enter very large rigidity values for the degrees of freedom that you want to constrain and enter 0 rigidity (0 damping force as well) for the degree of freedom that you want to keep free. This way, you can use bushings instead of joints.
Generally, 100,000 can be regarded as a very large value. After all, the rigidity of a bushing can be determined using the formula F=K*delta, just like a spring. To define the rigidity of a bushing, you may want to consider how much displacement you will allow for the degree of freedom that you want to fix.
That is, if you set K to 100,000 [N/mm] in a system where a force of 10,000 N is applied, you are allowing a displacement of 0.1 mm for the bushing.
