7 Replies Latest reply on Jan 9, 2010 9:28 AM by Derek Bishop

    how to put constraints in a spreader bracket?

    Helen Zhao

      I need to model a spreader bracket with a shackle attached on the top (see attached pic.). The main plate is subjected to 5000 lbf on each side and 16000 lbs on the stand-off plate (all loads are in gravity direction). Now the bracket is lifted by a crane hook through the shackle. Obviously, the bracket will reach a load equilibrium position (some degree WRT horizontal). My question is:


      1. Is there a way to find the quilibrium position for the bracket when lifted by the crane through shackle by using SW simulation?? or SW motion??( I do have SW 2009 premium). I can find the equilibrium position by hand assuming all parts are rigid. but it is tedious and everytime I change the load magnitude, I have to re-do it.


      2. How to model the constraint conditions/fixture for this bracket? my thought is that the bracket is free and under quilibrium position under applied load and the load from crane hook. But all FEA supposes to have at least a constraint to prevent rigid movement. Am I right?


      Any ideas or help are really appreciated!!!



        • Re: how to put constraints in a spreader bracket?

          You could assume a single contact point where the crane hook catches the top shackle.  If you can make a point somehow on the model, you could constrain it in space in the FEA study.  (use "immovable", not "fixed" because you want to allow rotation, I gather.)  Then you would only need to apply the loads and gravity and let the thing come to equilibrium through the FEA.  Your screenshot is very close to the solution as it is.


          If you have an idea of where it's going to end up, it would be good to set up the study with the bracket already close to the final position.  (use a reference plane for the forces/gravity.)  This way, you will not have the 'large displacement detected' warning cropping up if the part rotates a few degrees from "vertical".


          Also, if you know the geometry and loads, and you have already solved it by hand, you could use a spreadsheet program (e.g. MS Excel) to solve it again when the loads change.


          Good luck!


            • Re: how to put constraints in a spreader bracket?
              Helen Zhao

              thanks for your reply, David.


              1. I am using SWS 2009 and there is no "immovable" for solid part, I can only use fixed geometry. I still do not know how to let the bracket fall into the equilibrium position by running FEM, since this is pretty much rigid rotation plus deformation. You have any experience like this?


              2. It is a good idea to set up the initial position of bracket as equilibrium position (21.6 degree from vertical based upon moment equilibrium). I did that and the results really depends a lot on the constraints that you put on.


              3. I even tried to change the stand-off plate as sheet metal, so that the mesh for this plate is shell mesh (using thin plate theory). Again I got different solution from treating it as solid part.


              4. So the questions boils down how to find the real solution (or at lease close to real one) for this bracket. Do you have any idea of how to do the hand-calculation or the reference that I can find, that way I can at least get a ballpark of stress I should expected.





            • Re: how to put constraints in a spreader bracket?
              Derek Bishop

              1. Write the moment equation and solve using a spreadsheet. YThat would give the orientationof the bracket. You could reference the result in the SolidWorks.

              2. Try applying a vertical translation restraint and see how it goes. If necessary apply restraint in other direction.