2 Replies Latest reply on May 10, 2017 10:09 AM by John Willett

    Spring Creates Unbalanced Forces (Sometimes)

    John Willett

      Novice (at springs at least) Question:  Why are the forces on the two ends of my spring balanced in this case

      Balanced Forces.png

      but not in this case?

      Unbalanced Forces.png

      Both cases are compression/extension springs between plane parallel faces with total compression preload -- 1 lb in the top case and 18 lb in the bottom -- but no displacement (so far), modeled in Solidworks Premium 2017 SP2.0 (linear static) simulation running under Window 7 SP1.  It doesn't seem to matter which solver I choose.

       

      The top case is the simplest I could think of and appears to behave as expected.  The bottom case is slightly more complex: a tube sliding over the shaft of a bushing, pushed against one end by the spring's preload acting on the other.  The bushing does not need a fixture since it is a slip fit (no-penetration contact) in the fixed slider and pulled into another no-penetration contact against its end by the spring.

       

      I must be missing something obvious... -- John Willett

        • Re: Spring Creates Unbalanced Forces (Sometimes)
          J. Mather

          What is the absolute difference that you are seeing if you don't write the results in scientific notation?

            • Re: Spring Creates Unbalanced Forces (Sometimes)
              John Willett

              Mr. Mather -- I'm not sure I understand your intent, but for the lower case I expected the faces at the ends of the spring to experience equal and opposite Y-directed forces and that those forces be commensurate with the 18 lb preload.  What I get instead is -18 (lower face) + 8.63 (upper face) = -9.37, which agrees pretty well (give probably inadequate mesh resolution) with the "Sum Y" and "Resultant" in the table on the lower left.

               

              Does this address your question?

               

              Update:  But subsequently increasing the mesh resolution appears to prove me wrong, as the force balance improves slowly with resolution.  It seems to require much more resolution than I expected to get reasonable force balance with springs.  Sorry to waste you guys' time! -- John Willett