2 Replies Latest reply on May 4, 2011 6:02 AM by John Ward

    Reaction forces - Force parallel to surface even with no friction

    John Ward

      Hi,

       

      I a modelling a part with a number of contact sets.  One contact set is between two flat plates.  In the definition of the contact set I have left the friction box unticked, ie. no friction.  I have also left the global friction box unticked in the studies properties.

       

      When I run the analysis and check the Result Force between the two surfaces I get a compression force normal to the two faces, which is what I would expect.  However I also get a force parallel to the surfaces.  To me this indicates that a horizontal load is being transferred between two horizontal flat plates (with no friction) which is not possible.  The deflection of the plates is not great enough to produce any significant horizontal force from the vertical component.

       

      One reason for this that I have just thought of is that in a single contact set I have selected 4 different sets of surfaces that are in contact in different parts of the model.  The sets are all the same but should I have split this up into 4 seperate contact sets?

       

      One more question that I would be grateful for a tip on...  I am trying to verify my simulation against hand calculations that I have done on the part that I am analyzing.  One way in which I would like to do this is to take a section through part of my model (through a single plate) and get the net force transferred through that section.  Is there any way to do this?  One way would be to make two parts that join at that particular section but that is not practical because I want to check the net force in a number of different locations in the model.

       

      Thanks for you help.

       

      Regards,

      Jonathan

        • Re: Reaction forces - Force parallel to surface even with no friction
          Jerry Steiger

          John,

           

          This is no doubt too late to help with the problem you were having but may help both of us understand Simulation a bit better. (We just got a seat to go along with a seat of ANSYS). What is the magnitude of the force parallel to the surface? There will always be some (tiny) forces just due to the nature of FEA. If you have forces that are significant, say a few % of the force perpendicular to the surface, then I would suspect that it is due to deflections causing your surfaces to rotate relative to the original normal direction.

           

          Since friction really does happen in the real world, you might also want to add a low friction coefficient, say 0.1, to see how the result changes. You could then try 0.2 or 0.3 to see how significant friction will be in real life. (These values assume you have solid-solid contact, not a lubricated bearing surface.)

           

          Jerry Steiger

            • Re: Reaction forces - Force parallel to surface even with no friction
              John Ward

              Hi Jerry,

               

              Thanks for your input.  Since my original post I have worked out what was happening.  The reason why Simulation was showing a contact force parallel to the flat surface was because the nodes along one edge of the surface were also included in a different contact set which was oriented perpendicular to the original surface.  The nodes along the edge had contact forces applied from the two different contact sets.  This is correct but the way it is presented is perhaps a little misleading, at least I found it to be.  You have to remember that when you list contact forces by selecting a surface then all the edges are also included and any forces applied to these edges due to other contact set definitions will be included.  It seems you cannot list the the contact forces due to a single contact set definition independantly.

               

              Cheers,

              Jonny