5 Replies Latest reply on Mar 10, 2012 12:07 AM by Paul Kellner

    Very strange unstable reaction

    Adrian Tayne

      I am unable to explain why a part in my assembly is turning into Sonic the Hedgehog when I run my analysis. The bottom plate is fixed at its base. The top plate has a fixed hinge on a fillet so that when a shock load is applied to it, it tips in that direction and stresses the screws holding it down. I am trying to look at the case where the screws have no preload to determine the axial loads in each screw. This particular study is a duplicate of another study that ran successfully with no abnormal behavior. The only changes I made was the direction of the acceleration. But for some reason I get an "improperly restrained" error and the bottom plate distorts at each element forming weird spikes. I had to use soft springs and inertia relief to get it to run. There is no external load on the bottom plate. I tried supressing both external loads separately, to no avail. I tried hinging only one of the "rails" as opposed to two. I've tried no penetration contact between plates. The original study was run without any component connection at all and it ran fine because there shouldn't be penetration anyways.

       

      I've been cursing over this for the past couple of hours and cannot figure it out so I'm hoping someone has seen this before and has an explanation. I've attached a picture (top) of the successfull study and one of the funky study (bottom)

       

      good study.jpg

      hedgehog.jpg

        • Re: Very strange unstable reaction
          Rohit Mitra

          See what happens when you change the deformation scale on that plot. Also try to set it up in a way that doesn't require soft spring and inertial relief. When you have to use both, it tends to mean that you're forcing a simulation to run that should be set up differently in order to more correctly reflect the real life situation.

            • Re: Very strange unstable reaction
              Adrian Tayne

              Yes I know that using soft spring and inertial relief give bad results. I am merely using them for troubleshooting to see what part of my assembly is under restrained. The problem is that, as far as I can tell, there is nothing wrong with my setup because it is identical to how I have the original study set up, which runs fine with no inertial relief or soft spring applied to it.

                • Re: Very strange unstable reaction
                  Rohit Mitra

                  Top left of the workspace with "deformation scale" on it. Double click on that area and then modify the deformation scale. Change it a few times by orders of magnitude. This will give you a better picture of what's going wrong. With just that picture it's hard to come to a conclusion on what's going on.

              • Re: Very strange unstable reaction
                Adrian Tayne

                The strange effect ended up being some kind of anomaly that occurred because I was using fixed hinge (i.e. axially and radially restrained cylindrical face). I removed the axial restraint and it solved properly. For some reason, the axial restraint was introducing an imbalanced force on the lower plate. This doesn't really make sense so I'll just chalk it up to SW being confused about an overrestrained assembly.

                • Re: Very strange unstable reaction
                  Paul Kellner

                  Well you do have a relatively compliant base attached to an infinitely stiff restraint. That can cause numerical problems.

                   

                  How many layers of elements are in the base?