17 Replies Latest reply on Mar 26, 2018 4:12 AM by Unspecified Unspecified

    HOW TO DO FEA ON THREADS?

    Evan Leftwick

      I am new to FEA and I am trying to figure out how to do a simulation on threads to see if they will hold the desired weight without shearing or pulling out. I feel like the threads need to be touching to do this but I can't figure out how to mate them together or maybe there is another way?

       

      Thanks

        • Re: HOW TO DO FEA ON THREADS?
          Christian Chu

          I'm not sure if this can be done by the expression version. If yes, I really want to know too

          • Re: HOW TO DO FEA ON THREADS?
            Dan Pihlaja

            I am NOT an FEA person....however, I would think that you need to first make sure that the threads that you modeled are exactly what will be produced by the method of manufacturing them (thread forming / thread milling / tap, etc...).

             

            This is VERY rarely the case.  Which is why most people don't put threads in their models anyway....just graphical representations.

              • Re: HOW TO DO FEA ON THREADS?
                Christian Chu

                first make sure that the threads that you modeled are exactly what will be produced by the method of manufacturing them (thread forming / thread milling / tap, etc...).

                It's not about how to model the thread correctly, it's about how the thread can stand for certain stress

                I really want to perform FEA on thread using the expression version including in the SW premium, but can't figure out a way to, unless I get a full FEA package.

                Just used my common sense such as coarse for aluminum or fine thread for steel and more number of engaged threads for more strength ... It's a shame but I have no tool for it ... 

              • Re: HOW TO DO FEA ON THREADS?
                Evan Leftwick

                Dan,

                If I can figure out how to get it to work I was going to do a stress test at LMC so I can see the worst case scenario.

                  • Re: HOW TO DO FEA ON THREADS?
                    Dan Pihlaja

                    Evan Leftwick wrote:

                     

                    Dan,

                    If I can figure out how to get it to work I was going to do a stress test at LMC so I can see the worst case scenario.

                     

                    You can do that I suppose.  That would get you a number, however, in addition to LMC tap drill size, thread strength has a heck of a lot to do with the shape and the form of the thread as well as how much actual contact surface there is between the male and female threads.

                     

                    That is what I meant by making sure that the model is the same as how it will be manufactured.  Different manufacturing methods produce slightly different forms of threads. 

                     

                    If this were a safety item (like the thread needed to hold and I-BEAM over a crowd of people) then I would (in addition to the FEA), make sure that a heck of a lot of real world tests were performed.

                  • Re: HOW TO DO FEA ON THREADS?
                    Evan Leftwick

                    Dan,

                     

                    I am a machinist, I am aware of the thread form and I will be making the parts myself. I will be testing the actual parts but I would like to catch anything ahead of time if possible so we don't waste a bunch of machine time. I guess nothing is as good as real world testing.

                     

                    Thanks

                      • Re: HOW TO DO FEA ON THREADS?
                        Dan Pihlaja

                        Evan Leftwick wrote:

                         

                        Dan,

                         

                        I am a machinist, I am aware of the thread form and I will be making the parts myself. I will be testing the actual parts but I would like to catch anything ahead of time if possible so we don't waste a bunch of machine time. I guess nothing is as good as real world testing.

                         

                        Thanks

                         

                        Awesome.  I didn't mean any offense.....I just wanted to make sure that all was covered. 

                         

                        Now, on to your question......I have no idea.  LOL  Never actually run an FEA in Solidworks.

                      • Re: HOW TO DO FEA ON THREADS?
                        Evan Leftwick

                        No offense taken Dan, thanks for the thoughts.

                          • Re: HOW TO DO FEA ON THREADS?
                            Bill McEachern

                            Most people, including me, usually use a revolve feature for the thread, the helix angle is often very slight and it is best ignored as it complicates the analysis to impracticality in many cases. The initial tightening can be modeled by applying a tensile pre load to the bolt. Threads in tension have a stress riser of between 3 & 7. You can search for a GMRI presentation of the analysis of bolts in tension. If might be useful to start with that.

                          • Re: HOW TO DO FEA ON THREADS?
                            Roland Schwarz

                            The problem with threads is that they are relatively tiny features on much bulkier parts. You would need extremely fine resolution at thread roots. Perhaps possible with auto mesh refinement.

                             

                            None of this would account for the microstructure changes that affect thread strength. You may get a solution to converge, but it's not likely to be accurate.

                              • Re: HOW TO DO FEA ON THREADS?
                                Christian Chu

                                it's what I thought ...

                                assume the full FEA package would be used to perform the analysis ...

                                • Re: HOW TO DO FEA ON THREADS?
                                  Bill McEachern

                                  With all due respect to Roland Schwarz, I believe you can get an accurate stress solution to a thread that lines up with experimental data. I know others that through considerable resources at such problems but it is pretty much restricted to just modeling the fastener system. You do need a boat load of elements typically. Roland what do you mean by changes in micro structure? At least as far as I know,  typically the micro structure of a metal does not change with stress state even post yield. It would have to go from say BCC to FCC and typically that is a temperature dependent thing, no?

                                    • Re: HOW TO DO FEA ON THREADS?
                                      Roland Schwarz

                                      Good points, Bill. True, the stress solution may be accurate. However, there will be changes to other properties, most notably hardness and strength. Localized working can embrittle the material. At that scale, imperfections due to cutting or rolling will have a significant effect. Even if parts are molded, there will be significant variations in material properties due to skinning and differences in cooling rates.

                                        • Re: HOW TO DO FEA ON THREADS?
                                          Bill McEachern

                                          Hi Roland, the material property changes (strength and stiffness) are typically accounted for in the plasticity model selected, along with the hardening if multiple load applications are specified.  Imperfections while hard to model, upon yielding would tend to smooth out with a sufficiently fine mesh. This is where some experimental work can establish the impacts of imperfections so that modeling can account for them in an average sense. On the cooling effects, they can be accounted for if known by having different material properties for different zones. Most advanced programs can even model fracture and crack propagation (though not SWX Sim as yu would know). This discussion is probably beyond what was intended by the OP. Maybe its helpful in highlighting how complicated things can get and what can be done if you dig in enough.

                                    • Re: HOW TO DO FEA ON THREADS?
                                      Ananda Ganesh Madheswaran

                                      hi...
                                      if you want to see the failure / strength of the thread, you have to model thread in the part and apply contact to the thread and also to the mating hole..

                                       

                                      model the thread by sweep option with helix and the mating area by just making the cavity on it.

                                      provide the contacts between them and apply the required force / load.

                                      • Re: HOW TO DO FEA ON THREADS?
                                        Elmar Klammer

                                        Hi Evan,

                                         

                                        Rather than solving your joint in 3D us 2D analysis. That solve much faster and can provide just as good results.

                                        The kind of loading on the bolt is important just like the joint overall design plays an integral part in the load behavior of your joint.

                                        If I was to do a 3D analysis then I would consider to combine standard mechanical thread theory and 3D simulation tools.

                                        That way I can simplify my analysis and make some assumption on the joint stiffness etc. since flexible joints cause bending in the bolt.

                                        The theoretical strip out force only is not adequate to describe the joint strength. Unless you look at a eye bolt application for instance. But even there you have side loading to consider. On top of that is torque. Head friction and so forth...too much to mention. The image below highlights how nut design itself affects the load carrying capacity and how the load spreads over the first couple threads. In a) you can see that the first 4 threads carries 35%+25%+15%+10% = 85% of the max load. while in b) the load spreads to approx. 6 x 15% = 90% of max load. The that means that the threads in a) are loaded much higher (35/15=2.33) and therefore will yield differently than in case b). That's just to give an idea on the complexity of your question.

                                         

                                        There are good books on threaded connections. Look up Introduction to the design and behavior of bolted joints by John Bickford.

                                         

                                        Sample of 2D drill case axial loading FEA

                                        Elmar

                                        • Re: HOW TO DO FEA ON THREADS?
                                          Unspecified Unspecified

                                          Good day,

                                          If you figure out the axial force on a bolt, then you can cross check from standart charts if the tension exceed the value you found. I dont like to `full scale` model the thread , stress singularities at vertices, edges etc.can mislead you.

                                          - You can add bolt connection to simulation and run it,
                                          -Check the foces acting on the bolt, you have shear and axial force

                                          a-Axial force shouldnt  exceed the tensile force of the material in the given diameter
                                          b- Base  plate and screwed plate must also withstand this stress, you can use charts you can find from web.