5 Replies Latest reply on Dec 30, 2013 3:54 PM by Shaun Densberger

    Unstable because of trusses

    Anthony Wente

      I am trying to show displacement for a frame in Solidworks, and I am putting forces on two points where front wheels would be located. I do not want the arms holding the wheels to take any of the force, so I want to treat them as trusses. Whenever I do this, the test fails because the model is unstable. It works just fine when they are treated as beams, but they hold a majority of the force. What do I need to do to make the test work with those forces as trusses?

        • Re: Unstable because of trusses
          Rukshan Wijekoon

          I think you have not applied any(or enough) constrains to your simiulation.What kind of fixtures you are using there ? can you upload your model ? (Use Attach Files here to attach your geometry)

          • Re: Unstable because of trusses
            Jared Conway

            why do you want to make them trusses?

             

            if you take a look at the cause of this error (many documented cases on the forum) it is almost always that you have not recalculated the joints and that you need to fix them. if you are still having trouble, use a process of elimination to find the bad joints

            • Re: Unstable because of trusses
              Anthony Wente

              I want them to be trusses because I do not want those arms bearing any of the load. I am alos using fixed geometry on the rear of the frame to constrain it. I recalculated the joints, but I got the same error.

              • Re: Unstable because of trusses
                Shaun Densberger

                The difference between truss elements and beam elements is that the former doesn't carry moment loads. I can't say for certain what is wrong without having the specific details on your design, but my guess is that you have either: a mechanism in your design or possibly a geometric non-linearity. I suggest you:

                 

                1. Make sure that your joints are such that rigid body motion is not allowed (like a 4-bar mechanism). I'm not sure if it can happen in SW, but make sure that on-axis rotations of truss elements are prevented.
                2. If everything looks good with respect to (1), then you might need to capture geometric non-linearities. This is typically handled with a large-displacement analysis, but I'm not sure if SW can support that for 1-D elements.