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How can I measure the reaction force in multiple fixed joints connected to a rigid body?
In the above figure, a rigid body with a mass of 251.2 kg is secured by two fixed joints. The reaction force on each fixed joint is 1231.71 N.
However, if you simulate this model in MBD for ANSYS , the two joints exhibit different reaction forces: one joint endures 2463.43 N of force, while the other experiences 0 N of force. This difference results from a constraint problem. The program thinks that the model is over-constrained and ignores this over-constraint as a redundant constraint. Although you have connected two fixed joints, the solver used in the actual simulation uses only one of them. Therefore, the reaction force of the fixed joint that the solver uses is 2463.43 N while that of the unused joint is 0.
If the joints are intended to merely secure the rigid body, then you don’t need to worry about these simulation results. However, in some models, it is important to precisely measure the reaction force on each joint.
To obtain precise reaction forces, you must use bushing forces rather than fixed joints, as follows:
When you replace the joints with bushing forces , you must enter a very high value (between 500,000 and 50,000,000) for the Stiffness setting, which corresponds to the degree of constraint.
For a fixed joint , you must constrain six axes (three for translation and three for rotation). In the above example, large values are set in the six Stiffness fields. Note that a bushing cannot constrain a rigid body as firmly as a fixed joint, so the rigid body may experience some displacement. (Higher Stiffness settings reduce the amount of displacement that occurs.) Now, if you simulate the above settings, the reaction forces of the both bushing forces read 1231.71 N.
In the above example , Stiffness is set to 50,000,000. If slight displacement is insignificant in your model, then you can set Stiffness to a lower value, for example, 500,000 or 5,000,000.
To replace a revolute joint with a bushing force, set the K and C parameters for rotation elements to 0 and use a large value for the Stiffness of all other elements.
