3 Replies Latest reply on Mar 30, 2011 11:53 AM by Ben Hallett

    Resultant Force Error

    Ben Hallett

      Hi all,

       

      I'm new to using Solidworks simulation (have previously used Mechanica).

       

      I have created a model where i want a minimal 6 point contraint system.  At one of the points I have used a bearing fixture that allows rotations in all axes but fixes translations (spherical bearing).  I have run a frequency model and it seems to be doing what I want.  However when i run a 1G gravity load and check my resultant force is the same as the model mass I get an incorrect resultant force (resultant force is equivalent to just ovver 50% mass) if i change this bearing fixture to a fixed constraint then i get the correct reation force for the mass of the assembly.

       

      Has anyone come across this, is it just a reporting error or should i not rely on the stresses from a model using these constraints.  I maybe using the constrain incorrectly though any ideas appreciated..

       

      Oh by the way for the 1G case the stresses were slightly different but not by much and that may be due to using the fixed contraint?

       

      Cheers

       

      Ben

        • Re: Resultant Force Error
          Roland Schwarz

          Likely you are getting strange results because your model is overconstrained.  Try using "soft" constraints like springs to support your model.  If you are using motion analysis, activate and assign the bushing properties in your redundant mates.

          • Re: Resultant Force Error
            Paul Kellner

            Are you summing all the reactions when you restrain your node to translations only? You will need other contraints to keep the model from rotating.

            • Re: Resultant Force Error
              Ben Hallett

              Hi All,

               

              Thanks for the replys....

               

              I got over it by using a spherical bearing connection instead of a fixture then used a fixed constraint on the additional bearing housing geometry.   The model then returned the correct resultant force on application of a gravity load for the defined structure mass.

               

              So I definitely think there is something iffy in the calculation of resultant loads when this fixture type is used.

               

              To be honest i'm still not fully conversant with the connections and fixtures used in solid works.  My intention for the model  is to simulate a spherical bearing connection or ball joint.  In mechanica/patran  i would have beamed to a point then fixed the point in translations x,y, and z thereby allowing rotations.  You can't do that it seems in Solidworks so I modeled the connection using a bearing fixture (allowing off axis rotations) applied to the bearing shaft which seemd to work fine for modal and appeared to work for static loads based on the animations however the error on resultant force casts doubts on the model stresses.   As I said above when i added a geometric representation of the bearing housing and changed the fixture to a connection i got the correct resultant force reported???

               

              Ben