This question came up on our Tech Support hotline this week and I thought it would make a great quick post, mostly because the title is so fun to say.
Many of you already use the Symmetry constraint when running Simulation studies, to cut your rectangular models in halves or quarters and reduce the number of mesh elements you have to solve. It’s pretty obvious that the same constraint can be used for circular models when nothing breaks the symmetry:
If there is just a force up or down on this disc (like pushing a button) or just a radial pull (stretching the disc into a bigger circle) then the model on the right can be used instead for a faster solve. Symmetry constraints on the cut faces will keep everything nice and boring there, not allowing mesh nodes to move into or out of the cut faces, only along them, like sliding on a table. That’s all Symmetry does, really, is prevent the nodes from moving normal to the selected face.
Engineers happily used this tool for a long time until the developers at SolidWorks overturned our apple carts a few years ago by introducing a new command: Circular Symmetry. Even the icon makes you think you should use it instead of regular Symmetry for this situation:
Customers did start using it, and then this started happening:
That’s a disc with the center raised section constrained to only rotate, and ONLY a radial pulling force put on the edge. The transparent gray is the original model. If you can’t see the problem yet, try this short video:
There is only a radial force! The disc shouldn’t rotate, especially not with “symmetry” applied! That was the call we got on our hotline this week, and a little bit of research revealed what’s going on.
The Circular Symmetry constraint does allow node movement normal to those cut faces. In fact, it forces nodes on one cut face to move normal the same amount as the nodes on the other side’s face! So why would you use this? The answer is tangential loads.
If you have tangential loads (like a turning wheel accelerating or braking) you will have motion exactly like the video, even though there is already material where the leading nodes are going. They push material out of the way, that material pushes more out of the way, etc, all the way around the disc, until the slack is made up when material fills in on the other end. Symmetry does not allow this, but Circular Symmetry does.
I wanted to make a simple picture with arrows and text to leave you with, but as some of you have learned, it’s hard for SolidWorks to directly create images suitable for assembly instruction manuals. So I turned to the only program capable of simultaneously dealing with 3D CAD data AND publishing tools, 3DVIA Composer:
(I’ll be posting more about 3DVIA Composer in the future.)
And as a last note, everyone knows to NEVER use any kind of symmetry with natural frequency studies, right? Even if the input (model) is symmetric, the output (results) must also be symmetric for the constraints to hold, and natural vibration modes may not be. CAPINC engineers learned this in a great session at SolidWorks World 2011 called “D’OH! 10 ways our Simulations have fooled us!”
(FYI, you don’t have to travel all the way to SolidWorks World for those kind of fun, insightful presentations; on July 25th our bi-weekly CAPU webinar will feature Elba-Gloria discussing file management in a talk she’s calling “Where are My Relatives?” She always makes it interesting!)