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Unexpected behavior ball joint in MBD eigenfrequency study
Posted Sep 24, 2020, 10:31 a.m. EDT Structural & Acoustics Version 5.5 0 Replies
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Dear All,
I am trying to include a ball joint with spring damper subnode in a 3D mechanical model. Apart from the joint I only use 2 rigid body domains with set masses in the Multi Body Dynamics module. The study I run is an Eigenfrequency search. For the ball joint I would expect 2 modes for the inclination angle and one for the rotation angle, all with real valued eigenfrequencies determined by the mass, distance from mass to joint, joint spring stiffness and Inertia. However I don't get this result. If I search for all eigenfrequencies I get a solver error, if I restrict the number of modes I get several imaginary results. It seems as if the spring damper subnode is not working.
For a hinge joint I used the exact same model (only replaced the ball-joint with the hinge joint) and then the model does give the expected single mode (5.3 Hz for the set parameters). See attached two test models.
Is there anybody who knows what I am doing wrong and why I don't get the same results for the ball joint as for the hinge? I would expect the ball joint to give the same first eigenfrequency, but then as a degenerated mode with modes around both x and y.
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Hello Henk van Weers
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