8 Replies Latest reply on Jul 29, 2015 4:03 AM by Stephen White

    Static Analysis of a short beam

    David Flemming

      Hi guys and gals,

       

      I have a short simple question. To put it simply I'm making a seesaw. Now the pivot or the teeter bar (seesaw bar) is a short piece (12") of 1.25" solid bar. The bar goes through the center of the teeter bear and each end of it its secured in pillow block bearings.

      I've done the FEA for the beam so far and have gotten my results however, I'd like to to some sort of hand calculations to verify if the result of the FEA is as I predicted.

       

      Does anyone know what sort of stress theory I'd have to use to do the hand calculations for this?

       

      I've added some images to explain the case.

       

      Thank you,

      David

       

      TOP VIEW

      FEA SETUP

       

      THE POINTS WHERE THE TEETER BEAM IS WELDED TO THE SHORT BAR

        • Re: Static Analysis of a short beam
          James Riddell

          Yes, stress = F/A.  This looks to be a case of double shear and bending that you would need to resolve to find the principal stresses.  Then you can find von Mises stress from there.

            • Re: Static Analysis of a short beam
              David Flemming

              Thanks James,

               

              one more thing...I've done some hand calculations for this problem with respect to SF and BM diagrams but the beam diagrams I'm getting from solidworks are different. I understand mine but not what solidworks is producing.

               

              Do you see any problem in mine or the solidworks result?

               

              Thanks,

              David

               

                                                                MINE

              FB SF BM.jpg

               

                                                                     SW Shear force & Bending Moment

               

                • Re: Static Analysis of a short beam
                  David Flemming

                  The values given by solidworks are 543.7 N and -543.7N as max and min shear forces.

                  and 10.7 Nm and -10.7 Nm.

                   

                  I'm not as concerned with the values that much. I'm more concerned about the shape of the SF and BM diagrams so I'd know if I'm interpreting the behaviour of the forces properly.

                   

                  Thanks

                  David

                    • Re: Static Analysis of a short beam
                      James Riddell

                      That's more a question for DSS but my gut reaction would be that it is a function of the solid model elements vs. theory and the idealized inflection points (which is usually where the part actually fails and why engineers use these equations in the first place.)  If it is any consolation, it looks like your numbers are correct.  (Is your bending moment diagram shaped correctly?) 

                        • Re: Static Analysis of a short beam
                          David Flemming

                          I believe it is shaped correctly. To be completely honest, when I set this short beam to be treated as a beam, I got the message from SW that it was too short to be considered a beam. But when I treat it as a solid, it shows behaviour like a beam so I did it both as a solid and a beam. The stresses in the solid analysis are about twice that of the beam analysis.

                      • Re: Static Analysis of a short beam
                        Stephen White

                        Hi David,

                         

                        Further to my previous comments, the shape of the plots I attached are related to the fixture you have applied to the joint points. Because you have fixed the points, a moment is developed at the joint, so the bending moment will be non-zero at the fixture. If you change the fixture to immovable to only restrain the translational degrees of freedom the result will match your hand calcs.

                         

                        Or you can apply the following to your example to get the results to match in;

                         

                         

                        Cheers

                         

                        Steve

                    • Re: Static Analysis of a short beam
                      Stephen White

                      Hi David,

                       

                      Would be good to get your files to have a look, but from the images you have attached you seem to have an endpoint that it not correctly generated as a joint on the beam;

                       

                       

                      That might be throwing the results for the direction of the shear. I have ran a  quick test and got this for the moment;

                       

                      So I think the missing joint is causing a bit of mischief. Try editing and recalculating the joints to see if this improves the results. Hand calcs are looking good on my model.

                       

                      Cheers

                       

                      Steve