3 Replies Latest reply on Nov 8, 2011 6:19 PM by Jerry Steiger

    Request for help with constraints in arbor press simulation

    Julian Leland

      Hi all,

       

      I'm trying to simulate stresses developed in a simple arbor press as part of a class project. The arbor press consists of three parts: the frame, the "spindle" (basically a 1.75" dia. rod, flattened in one section with a 12-pitch, 25 deg. pressure angle rack cut into the flat section), and a pinion to drive the spindle. I have successfully modeled all three components, and have been able to simulate them with SW Motion. However, I'd like to examine the stresses that develop throughout the entire assembly: I've tried a couple of times to set up a simulation using SW Simulation, but haven't had any luck.

       

      What I'd like to do is apply a torque to the pinion with the end of the spindle fixed, as though I placed a solid block in the press and applied pressure. I'd like to have the gears mesh together, and then get the stresses in the frame as well as in both the spindle and the pinion reported. Historically, I've set up the simulation as follows:

      1. Assembly set-up: I've set up the assembly with standard mates (concentric, parallel, distance) to position the spindle and pinion within the frame. I have NOT included a rack and pinion mate between the spindle and pinion, but I have made a point of positioning the pinion such that its teeth do not interfere with any of the rack teeth.
      2. Fixtures: I define both the base of the frame as well as the end of the spindle (the part that applies the pressure in an arbor press) as fixed.
      3. Loads: I apply a 1 N-m torque to the pinion
      4. Connections (this is where I think I'm going wrong): I define a no-penetration component contact between the rack and the pinion. I've also noticed that a "bonded" global contact is defined at some point in this process: this didn't make sense to me (why would I want all the components to be defined as bonded together?), so I deleted it. Was this the wrong thing to do?
      5. Meshing: Nothing fancy here - I just let SW set up a fairly coarse mesh, and ran the simulation.

       

      Results thus far have been pretty odd - I've gotten a number of error messages during simulation (I neglected to write them down, sorry), and the final simulation plot showed no variance in either deflection or von Mises stress across the model, even though the spindle had distorted WILDLY (stretched out enormously, through the frame of the arbor press). If anyone has any suggestions for what I'm doing wrong, or how I could better approach this problem, I'd appreciate them greatly. I'm using SW 2011, with Motion and Simulation.

       

      JL

        • Re: Request for help with constraints in arbor press simulation
          Jerry Steiger

          Julian,

           

          I'm not familiar with how SW Simulation transfers mates in SW into constraints in Simulation, but I have the impression that it doesn't; that the user has to establish the boundary conditions in Simulation. If that is the case, then you will need to set it up so that the pinion is constrained to rotate inside the base. You would also need to set up the constraints so that the spindle can only move up and down in the base.

           

          It seems to me to be a mistake to fix the end of the spindle. That would mean that no matter how hard the pinion pushes the spindle down, no load gets transmitted to the base. I think you want a no penetration constraint here.

           

          Check to see the scaling of the deflection image. Simulation SW often scales the plot so that you can see the otherwise very small deflections.

           

          Jerry Steiger

            • Re: Request for help with constraints in arbor press simulation
              Julian Leland

              Hi Jerry,

               

              Thanks for your advice - I'll check out the constraints in Simulation and see what I find there.

               

              Regarding fixing the end of the spindle: I also defined the bottom of the base as fixed - my intention in doing this was to simulate me placing a solid block of steel in the arbor press and pushing down on the handle. The idea was for the pinion to push against the rack because of the torque applied -> rack produces a reaction force because of the fixed constraint, which pushes the pinion into the frame - frame deflects (as do the teeth of the rack and pinion). Now that I think about this, it seems like a needlessly complicated (if true-to-life) way of simulating this - do you think I could just define the pinion as being unable to rotate in the frame, and then apply an upwards force at the end of the rack?

               

              Thanks,

               

              Julian

               

              EDIT: I've attached the assembly and the associated part files so you can get a better idea of what I'm talking about - let me know if I've missed anything.

                • Re: Request for help with constraints in arbor press simulation
                  Jerry Steiger

                  Julian,

                   

                  Regarding the end of the spindle, if you don't care about the stresses in the base where the spindle presses against it, then I guess you could fix the end of the spindle, but it seems very unrealistic to me. The first thing I would do is make some simple sketches and free body diagrams for each part.If you don't understand that, all the computers in the world won't be able to find the right answer.

                   

                  Regarding the pinion, it depends upon whether you are interested in the stresses in the pinion and the rack. If you do care about stresses in the gear teeth, then your orginal method is more appropriate. If all you care about is stresses in the base, then you can use a simplified analysis. But every time you simplify and make assumptions, you run the risk of being mistaken in your analysis.

                   

                  Jerry Steiger