13 Replies Latest reply on Sep 15, 2017 11:18 AM by Bill McEachern

    Help with bearing loads vs. force applied to an inner bore.

    Dustin Smith

      I am working on analyzing the stresses inside and around a connecting rod small end.  I am trying to verify my results and the different ways to apply a force to the small end.  Currently I am running a static analysis with fixing the big end with a split line, and am applying a total force of 17000N (tension) to the small end bearing surface.  I have attached three different scenarios with vastly different results.  The first is applying a "bearing load" directly to the surface in the Y direction, the second is applying just a force directly to the surface in the Y direction and the third is using an assembly with a wrist pin inserted into the bore and then applying the force to the pin with a no-penetration contact set(the third gives very similar results whether using a bearing load or a force). All there photos are scaled the same.  I would assume the bearing load to give higher stress concentrations due to the sine wave distribution but the results are vastly different.

       

      My concern is how vastly different the "bearing loads" results are.  The stresses are much higher than my hand calculations are showing(they are similar to the other options).  Can anyone shed any light on this?  There is very little clearance(a few microns) between the wrist pin and connecting rod bore so I would assume the force would partially sinusoidal however theses results seem very dramatic. 

        • Re: Help with bearing loads vs. force applied to an inner bore.
          Keith Frankie

          Interesting.  My guess is that, while the bearing load may be more realistic, it lacks the 'support' that the actual part gives.  With contact enabled the con rod end isn't able to deform inward.  If it can't deform inward it can't take on the 'egg shape' that is resulting in your first two studies.  The ID of the con rod hole flexes a tad, but remains circular.


          Look at the contact area for your no-pen study, contact must be occurring all the way around to the sides.

           

          Here's a screen shot of the bearing load, without a wrist pin inside to prevent deformation.

           

          • Re: Help with bearing loads vs. force applied to an inner bore.
            Bill McEachern

            post the force application dialog - something seems odd about your result. Plot the pressure distribution from the contact problem.

              • Re: Help with bearing loads vs. force applied to an inner bore.
                Dustin Smith

                Top one is the "Force" application, bottom is the "bearing load".  With the assembly and a no-contact penetration, shouldn't the rod still be able to deform around the wrist pin or does it stay concentric to the wrist pin due to the contact or mating.  I guess that is why I thought that should be the most accurate model as the part can only deform into the wrist pin and in reality the bore can only deform into an "egg" shape.

                Displacement in the Z direction(or what would be squeezing the wrist pin sides) is about 90 microns with the bearing load and 40 with the force load.  Both are unrealistic due to only about 10 microns of clearance to the wrist pin or bearing anyway. 

                  • Re: Help with bearing loads vs. force applied to an inner bore.
                    Bill McEachern

                    The bearing load only applies the load to the top half of the surface. The other load applies it to the entire surface. It makes me doubt your contact solution. Post the pressure distribution. I suspect it isn't what you were aiming for.

                      • Re: Help with bearing loads vs. force applied to an inner bore.
                        Dustin Smith

                        Sorry do you mean when running simulation as an assembly or just as a part?  I guess i am not sure how to find the pressure distribution.  Thanks for all your help so far!

                          • Re: Help with bearing loads vs. force applied to an inner bore.
                            Bill McEachern

                            In whatever study you did the non penetration contact. Under edit definition of a stress plot, pick pressure distribution or something very similar instead of Von Mises. Or make a new stress plot and pick pressure distribution instead of von Mises. It should give you a carpet plot of arrows normal to the surface. Probably a good idea to hide one of the parts first. Otherwise you get a lot of arrows pointing in opposite directions.

                              • Re: Help with bearing loads vs. force applied to an inner bore.
                                Dustin Smith

                                Got ya. 

                                Here is the contact pressure plot.  Looks reasonable to me. And again this is with an actual wrist pin inserted with a no-penetration contact set and a bearing force applied to the wrist pin only. 

                                  • Re: Help with bearing loads vs. force applied to an inner bore.
                                    Bill McEachern

                                    I agree, it does look reasonable. Maybe the bearing load is busted. Quality control is not really a priority at SWX on Simulation. Try splitting the bearing surface so the load only applies to the top half you should do this for both the bearing load distribution and the normal load distribution and see how they look relative the contact solution. They should all be close.

                                      • Re: Help with bearing loads vs. force applied to an inner bore.
                                        Keith Frankie

                                        I don't think you need to split the bearing face into a top and bottom.  I believe SW knows not to apply a load to the bottom half of the face.

                                         

                                        I split the inside face at the 12 and 6 o'clock positions, and around the center.  I applied the bearing load to all 4 resulting interior faces.

                                         

                                        Then I changed the results plot to cylindrical by selecting an axis through the center of the rod end.  I show stress in the x direction, which is the radial direction.  I plot this from bottom (parametric distance 0) to top (parametric distance 1).  Note how there is no radial load for the first 90° (the bottom half).

                                         

                                          • Re: Help with bearing loads vs. force applied to an inner bore.
                                            Bill McEachern

                                            but on the force dialog you would need them split to compare them apples to sort of apples so to speak. Well at least the bearing load distribution is working. But that does explain why it is so "wrong". How are the reaction? do they equal the applied load?

                                              • Re: Help with bearing loads vs. force applied to an inner bore.
                                                Dustin Smith

                                                I did check my reaction forces and they are equal.  Can you explain why that one is so "wrong"?

                                                 

                                                  Sorry for all of the questions but we just got simulation in the past few weeks so much of this is new to me.  I am just working on validating my simulation process, which isn't going well currently!  I am working toward fatigue analysis but using a bearing load I can't even do that because the part yields on the first cycle.

                                                 

                                                Thanks for all your help thus far!

                                                  • Re: Help with bearing loads vs. force applied to an inner bore.
                                                    Bill McEachern

                                                    I have no idea why the bearing load is producing such different results. I would try the process of elimination or if you just want to move one just use the contact solution. I would up the mesh density though, make the components size on size and use the surface to surface contact. the size on size is a numerical thing - it will likely solve faster than with a gap. though SWX sim is pretty good with loose contacts.

                                                    Process of elimination: try the parabolic distribution - see if that is any better. I have never used the sinusoidal myself but I have used the parabolic and it always worked fine. You could try a line load using the force command as it would seem to me the response to the sinusoidal option almost looks like what you might bet with a line load at the top of the bearing face.

                                                    Your reference solution is the contact solution. So you can spend time figuring what went wrong with the bearing load or you can move on. Your call.

                                                      • Re: Help with bearing loads vs. force applied to an inner bore.
                                                        Dustin Smith

                                                        Thank you so much for that.  I think I have found the main issue to be is the direction of the force.  Using a bearing load the force is only in the Positive Y direction where as when using the assembly it is normal to the face of the surface which is how this should be modeled since it is supported.  I guess that being the case - where does the bearing load really used?  I could see the usefulness in a plain bearing type situation that provides a true sinusoidal distribution but in any roller bearing the bore is supported by the bearing. 

                                                          • Re: Help with bearing loads vs. force applied to an inner bore.
                                                            Bill McEachern

                                                            Well bearing in this case does not mean a mechanical bearing. It means a round thing bearing on a round thing - bearing as in pushing on it.

                                                            The bearing force is applied relative to the coordinate system used and the load direction picked. The coordinate system must be in the assembly being analyzed.

                                                             

                                                            There is a bearing connector under fixtures to simulate mechanical bearing when you don't want the bearing in the model just its effect.