9 Replies Latest reply on Mar 6, 2013 11:58 AM by Jared Conway

    Simulation for a plastic chair

    Ben Kirkby

      Hi Everyone,

       

      I was just wondering if you would be able to give me some advice about how to successfully run simulations on a chair I am currently designing.

       

      I understand the basics of Solidworks Simulation but I'm struggling to set up a test that I need.

       

      Basically, I want to be able to apply a load to the chair (somebody sitting on it) to see how much displacement the feet experience relative to the floor. Can somebody please tell me how to do this?

       

      I've tried putting a coincident mate on the feet and a "floor plane", but this didn't work. I have fixed the feet to a florr plane too, this worked but didn't allow the feet to move, as they would in real life, when the load was applied.

       

      Any help or advice you can give me would be greatly appreciated!

       

       

      Cheers,

       

      Ben

        • Re: Simulation for a plastic chair
          Mikael Martinsson

          Hi.

          Mates are not considered in simulation.

           

          You can use a plane as a "viritual wall" and add a no-penetration contact between the viritual wall and the feets of the chair. This is simlar to what you tried with coincident mate. (you find viritual wall and no-penetration under "connections").

           

          Another alternative is to place points in the center of all feets, add coordinate systems in the points, and then use "remote load/mass" and tick the "Displacement (rigid connecton)" button. Then you bind each coordinate system to it's corresponding surface and set all translations to 0 (x,y,z) but allow rotation. This will allow the feets to bend in a more natural way compared to fixed restraint.

            • Re: Simulation for a plastic chair
              Ben Kirkby

              Hi Mikael,

               

              Thank you for the response! I'll give it a try

              • Re: Simulation for a plastic chair
                Ben Kirkby

                Hi Mikael,

                 

                I've tried the "virtual wall" thing and it keeps failing. do i have to apply the no-penetration contact seperate to the virtual wall?

                 

                Cheers,

                 

                Ben

                  • Re: Simulation for a plastic chair
                    Irfan Zardadkhan

                    Can you upload the model so I may give you some direction on how to simulate your model?

                    • Re: Simulation for a plastic chair
                      Mikael Martinsson

                      Hi Ben.

                      Sorry for late answer.

                       

                      You don't need a "no penetration" contact since you have this already with the viritual wall contact. (sorry for misinformation in my last post)

                      The problem is that with viritual wall it seems like you can't add friction, wich you can with the no penetration contact.

                       

                      If you try to activate "soft spring" it should work, so I would try this first.

                      Otherwise you need to lock a node on one of the legs to prevent rigid body motions which I belive is the problem right now.

                       

                      Brg

                      Mikael

                        • Re: Simulation for a plastic chair
                          Jared Conway

                          careful with the soft spring, the results won't really be good in that mode. it is best used for troubleshooting.

                           

                          the problem is the rigid body motion developed from mesh/force imbalance. the only way to do that is to try and eliminate it. since you can't prevent the mesh from not being symmetry or the force balance, by adding symmetry, you'll help your situation (assuming that symmetry is a good assumption at the center).

                           

                          adding an actual part to interact with your chair sounds like another good idea. adding the friction might hold things from flying off into space.

                           

                          if you still can't get it to solve, fixing a point along the symmetry plane will help. or maybe a point on one of the feet. you have to be careful here. maybe try a few ways and see what you get.

                           

                          it might be a stretch, but there might be a way to use nonlinear to apply a force to hold the chair to the ground at the beginning of the analysis before you apply the person force. that might keep things from flying about.

                    • Re: Simulation for a plastic chair
                      Ben Kirkby

                      Hi Everyone,

                       

                      Thank you for all of your advice. Sadly it's all gone over my head a bit. I'm a product designer with very minimal experience of using the simulation add-in. If i put a mock up of the chair on here, would somebody mind setting up the simulation with regards to the nodes etc?

                       

                      That way I can just look through the feature tree and see how it's done?

                       

                      For confidentiality reasons with the project, I can't include the actual model, but I've included a basic representation.

                       

                      I use Solidworks 2012 so if anybody uses that or previous versions, that'd be brilliant.

                       

                      Many thanks,

                       

                      Ben Kirkby

                       

                       

                       

                      Below is a link to the parasolid file for my chair

                       

                      https://www.dropbox.com/sh/9w90i1bqsfqnq9d/7KE38e48li?m

                      • Re: Simulation for a plastic chair
                        Jared Conway

                        Hi Ben, my sample loading was 100lb. I used a square split line on seat pan for the application of the load and it goes straight down. In addition to the force, I also included gravity. See the results.

                         

                        I set this up and ran it in 2013 so I didn't attach the model. (Sorry, forgot you were working in 2012)

                         

                        The setup:

                        1. build a "ground" component

                        2. set it to rigid and fix the top face

                        3. set global contact to no penetration, friction I set to 0.5

                        4. add load

                        5. add gravity

                        6. mesh with draft quality and run, i had to use the "fine" side of the slider.

                         

                        some considerations you'll need to make:

                        a. 0.5 friction probably isn't correct for your application. make sure to update for your application.

                        b. right now i've run the problem with draft quality but would run a final analysis with high quality

                        c. you'll need to be careful about element size here. this part seems like it might be best for mixed mesh with a shell in the main section and solids/beams in the supports. if you want to stay with solids, get at least 2 elements across the thickness.

                        d. the loading condition needs some thought. i'm thinking you have to have a load both in the seat pan and on the back to match what really happens. same thing goes for a square seat pan load. you might need something fancier to match a person's behind and the loading that they generally provide as you get closer to what you want to learn.

                        e. my load may be too low for your application. in the end you may need nonlinear to handle the deformation, contact and/or the materials.

                        f. i would also consider running a buckling analysis on this chair. I have a feeling that is a potential mode of failure.

                        g. eventually the software might get upset about the gravity being applied to the rigid body. you may eventually need to eliminate it

                         

                        overall things look pretty good. going back to my original comment about symmetry, that might help hold things in place better. the assymetry of the results are because it is walking a bit on the ground and potentially near a buckling problem.