2 Replies Latest reply on Apr 8, 2009 7:12 PM by Richard Obrien

    Shrink Fit / Pull Force

    Tyler King
      I am really new at the COSMOSWorks thing and have never had formal training. Regardless, I am trying desperately to use COSMOSWorks to validate the design of a plastic knob insert. I am running SW SP05. The 2 key things I am trying to accomplish are:

      1. Verify that the forces from the crush ribs and flaps in the X and Y direction=0. This is critical for the knob to remain concentric to the potentiometer shaft.
      -I have tried to fix the outside of the knob and the flat surface of the pot shaft and add shrink fit contact sets. I then ran the analysis and tried to see if the stress on the shaft was equal, which would indicate equal forces form the ribs/flaps. This almost seemed to work, but the stresses on the flat side of the shaft never seemed right. And I'm not sure this is the best way to accomplish this. Is there another way to do this? Perhaps withe the correct restraints I could find the displacement and measure the distance from the axis or the knob insert and pot shaft.

      2. Determine the proper interference to acheive a pull force of ~5 kg
      -I likewise am using the shrink fit contact sets and have tried adding a force to the bottom of the knob. Although this doesn't tell me anything but that the knob does in fact slide off! I have tried listing the result force>contact/friction force and selecting the faces that I used in the shrink fit contact set, but the value for the friction force is extreemly high. Not to mention it is really annoying to use this tool because it is not a standard result that is saved, maybe I'm using it wrong.

      I am also aware that ABS may not fall under the criteria of a linear elastic material, but I also cannot find how to run a non-linear study as shown in the help file. Is it part of COSMOSWorks or another tool? For what I'm doing this may be an acceptable assumption..? As of now, no one here has gone through this trouble and the knobs end up needing to be tweaked to get the concentricity and pull force correct, often with a compromise. I hope to get this working, my paper calculations have a lot of assumptions and only get me so close. Any help would be greatly appreciated!

        • Shrink Fit / Pull Force
          Tyler King

          Buehler... Buehler...
          • Shrink Fit / Pull Force
            Richard Obrien
            Hello Tyler,

            This is a fairly difficult problem that is likely to be non-linear from both a materials and boundary conditions perspective. I find it is best to start out very simply with something that will work with the static solver. In your case that might mean reducing the interferences so that shrink fit will work properly. If you apply an axial restraint to the shaft but otherwise allow the shaft to float, then shrink fit has a decent chance of showing you the final position of the shaft...if the interference is not too high. Get it working with maybe a 0.001in of interference and then creep up to the real value. You'll know when the solver assumptions are starting to fall apart.

            Another approach is to start with the shaft outside the knob and tell Simulation (COSMOS) to slide axially. Again, if the interferences aren't too high you should see the ribs and flaps deform as the assembly proceeds. You will also have a reaction force that maybe you can assume is similar to the pull out force. You'll also have a way cool animation to show your friends!

            This is really a non-linear approach however and I wasn't sure you have that available. If not, see how far you can get with the shrink fit approach in the assembled postion.

            Hope this helps