General Mechanical

General Mechanical

not enough constraint to prevent rigid body motion- mechanism with revolute joints

    • busraekinci
      Subscriber

      Hello! I need an urgent answer for my loader arm mechanism. I modeled an basic arm structure with its bucket. I am trying to make a simple static analysis to obtain the joint forces on the structure. Normally, I have 2 pistons in this system but I modeled those as beams for simplicity. The only input is the bucket force.

      I connected the arm mechanism by three revolute joint ( body to ground). All the other joints are revolute joints ( body-to-body) in the structure. When I do a static analysis on my model, I am getting the warnings:

      1) Not enough constraints appear to applied to prevent rigid body motion. 2) The warning related to large deflection

      Also,

      Joints forces do not make any sense because there are huge forces on the rotation axis of revolute joints.

      I could not find the mistakes in my analysis. Could you please help me? Is there anyone who has an idea?

      Thanks very much!


      please see the figures related to my model and analysis.


    • Ashish Khemka
      Ansys Employee
      nnI cannot look into the model but can you turn on the large deflection on? On Rigidi Body motion - do you see excessive deformation in any of the part than expected? If yes then try to apply a joint load like rotation.nnRegards,nAshish Khemkann
    • busraekinci
      Subscriber
      nHow much rotation and on which joint do you think? nBy the way, when I use bushing joints instead of revolute joints, results make sense and match my hand calculations.ps: In the model, every components are rigid, not flexible. nThanks for your help! n
    • peteroznewman
      Subscriber
      Bushing joints are good because they provide the needed flexibility in the model.nWhen all the components are rigid and all the connections are joints, that is a mechanism and the Rigid Dynamics solver is better at solving that model than the Static Structural solver, even though there is no motion.nIf you analyze this model like a rigid body mechanism, each part has 6 DOF, and you want the number of constraints to equal six times the number of parts. It looks like there are six parts, or 36 DOF. You have way more constraints than that. Each revolute joint is 5 constraints, and I count 10 revolutes. That is 50 constraints, so there are 14 redundant constraints. The Rigid Dynamics solver has logic to analyze the constraints and eliminate the redundant constraints. For example, you have three Revolutes to ground. There should only be one, the main link in the center. The other two links to ground should only provide 1 DOF, a constraint in the axial direction of the link. nThe Static Structural solver doesn't have logic to eliminate redundant constraints because it doesn't assume the parts are rigid since that is the exception, it assumes all bodies are flexible. If you change the bodies to be flexible, you might get a better result from the Static Structural solver, but first, try the Rigid Dynamics solver.n
    • busraekinci
      Subscriber
      nFirstly, thanks for your detail answer. I got it but I have no access to Rigid Dynamics Solver. I have only static and modal analysis packages. nTo understand your constraining method, I want to make a simple analysis only on bucket. According to the method, I need 6 constraints. What kind of a constraint do you suggest for this bucket? Still using the same input force at the far edge. nI will define a single joint instead of two bottom joints for the simplicity. I have 2 joints totally. Please see the pictures. nThanks so much for your help! n
    • peteroznewman
      Subscriber
      The first General joint is exactly like a Revolute. All Fixed except for Rotation about local Z. The second General joint should have All Free except for Translation along local Y. That is exactly 6 constraints, but it is not quite right.nInstead, make the second General joint a Revolute, All Fixed except for Rotation about local Z.nThe first General Joint represents the force going through the tan-colored link that is suppressed.nEdit the Joint coordinate system to make the local Y axis point to the hole at the other end of the tan-colored link. nMake the first General joint have All Free except for Translation along local Y. That force represents the compression in the tan-colored link.nAs the line of action of that tan-colored link comes closer to the Z axis of the other joint, the compression force will increase.n
    • busraekinci
      Subscriber
      Dear thanks very much for your great and helpful answers. I applied it and see the logic. It will be so helpful for my future tasks as well. nThanks a lot! Have a good day! n
    • Eashwar Raaj
      Subscriber

      I'm Getting This error In Structural Analysis after Composite Modelling in ACP , Where I have no connections related analysis its just like Cantilever type Analysis so Why this Error Come to me . Anybody Help In solving this error ASAP please 

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