11 Replies Latest reply on Aug 25, 2010 12:58 PM by Ryan Werner

    Bolt Stress Distribution

    Kevin Goosie

      How would you set up an FEA analysis to see the stress distribution about a bolt under a defined torque?  Everytime I set one up the nuts and bolt tends to look as though they have been flattened. Thanks.

        • Re: Bolt Stress Distribution
          Ryan Werner

          Hi Kevin,

           

          Are you trying to analyze an assembly of a bolt and a nut?  A picture always help show what you mean.

           

          Ryan W. 

            • Re: Bolt Stress Distribution
              Kevin Goosie

              I have attached an image of what I am trying to achieve.

                • Re: Bolt Stress Distribution
                  Ryan Werner

                  Ok, thanks for the picture.

                   

                  How are you applying the torque and how do you have your restraints set up?  Just a suggestion but I think it would be easier to to calculate the preload produced by your torque and then apply that to just your bolt with the proper restraints.

                   

                  You could set it up so that the nut is bonded to just the bolt shaft and run it that way.  This should get you results though I cannot say how valid they would be.  Since you said in your original post that everything is getting flattened I think you are probably having problems with the models moving during the analysis.  You need to make sure that all the bodies are restrained in all directions.

                   

                  I hope some of this is helpful.

                   

                  Ryan W.

                    • Re: Bolt Stress Distribution
                      Kevin Goosie

                      Ryan,

                       

                      Here is an image of the bolt after I added some more "fixtures" to it.  I added cylindrical fixtures with zero radial translation. This helped out extremely.

                        • Re: Bolt Stress Distribution
                          Ryan Werner

                          Kevin,

                           

                          If you are interested in the stress in the bolt shank given a certain torque/preload than the best way to analyze it would be to get rid of the washers and nut.  The nut is only applicable if you are checking the stress in the threads of the bolt.  Since there are no threads present the nut isn't really serving a purpose.  Fix the bearing face of the bolt and add split lines where the nut threads would be engaging with the bolt threads, then add you axial force to that area. 

                           

                          If you want to know what the stress in the threads of the bolt than the following equation will give you that. 

                           

                          Determine the stress in the threads of the bolts.

                           

                          Untitled.jpg

                          Ryan W.

                      • Re: Bolt Stress Distribution
                        Michael Feeney

                        If your curious about the stress in the bolt...I would exclude the nut from the simulation. Simpler models are better and if you arent concerned about the effect on the nut...there is no point to include it.

                        Furthermore, bolts don't really experience much in the way of torque. Granted you do apply a torque on a bolt head or nut head when driving it into whatever entity, but this is quickly transfered into an axial load through the threads. What I've seen in the past with bolt analyses have been shear (perpendicular to the cylinder of the bolt) or axial (tension in line with the cylinder). This information is standard and  thoroughly documented. If you're looking at whether the bolt will torsionally shear under your driving force...I would say that you're using the bolt in the wrong way in whatever real-life situation you have.

                      • Re: Bolt Stress Distribution
                        Michael Feeney

                        One other thing, you have washers in your picture too...this really seems like an axial loading case to me, in which case you look this information up in a table.

                          • Re: Bolt Stress Distribution
                            Kevin Goosie

                            Michael,

                             

                            I will set them up now with an axial load, but would you add the force to the face of the washer or the bearing face of the bolt/nut? Also, would you take the clamp force and divide by two and apply that load to each face or apply same force to each face, and in equal/opposite directions?  So, does SW distribute the force equally among faces or does it apply the same force to every face, i.e. 7lbs of force to two faces in equal/opposite to be 3.5lbs on each face or 7lbs of force on each face when two faces are selected for the same force?

                              • Re: Bolt Stress Distribution
                                Michael Feeney

                                If you wanted 10 lbs of tension on the bolt you would apply two equal loads of 10 lbs in equal and opposite directions. Think of SUM(forces). If you have a string hanging from a ceiling...and a weight on the other end...you have the weight pulling down and an equal force pulling up. You would say that there is a tension force equal to the weight...not 2 times the weight. This is my logic at least.

                                 

                                The specified force value is applied to each face. For example, if you select three faces and specify a 50 lb force, the program applies a total force of 150 lbs (50 lbs on each face). [SW manual]

                                 

                                Either way, can you explain a little more what you want to know from the simulation? What are you trying to prove? Most people looking at bolt properties just look it up in text source.

                                  • Re: Bolt Stress Distribution
                                    Ryan Werner

                                    Hi Michael

                                     

                                    Just a note.  In Simulation 2010 you can choose Per Item or Total when defining the Forces/Torques.  So he could pick three faces and say 50lbs Per Item and the total would be 150lbs or he could choose the same three faces and input 150lbs Total.  Both mean the same thing.

                                     

                                    I am trying to remember if this was possible is 2009 or not.

                                     

                                    The strange thing is that when using beams you cannot choose a Total and can only apply Per Item to forces on nodes which makes it a little annoying when doing a distributed load.

                                     

                                    Ryan W.

                            • Re: Bolt Stress Distribution
                              Michael Feeney

                              http://www.portlandbolt.com/technicalinformation/bolt-torque-chart.html

                               

                              Standard Torque on bolts of all sizes.

                               

                              Here is there explanation:

                               

                              Question: Can someone explain how tension and torque relates to bolted connections?

                              Answer: Sure, we’ll try our best. The relationship between tension and torque should be looked at cautiously, since it is very difficult to indicate the range of conditions expected to be experienced by a fastener. Torque is simply a measure of the twisting force required to spin the nut up along the threads of a bolt, whereas tension is the stretch or elongation of a bolt that provides the clamping force of a joint. Bolts are designed to stretch just a tiny bit, and this elongation is what clamps the joint together. Torque is a very indirect indication of tension, as many factors can affect this relationship, such as surface texture, rust, oil, debris, thread series and material type just to name a few. Virtually all the torque/tension tables that have been developed, including ours, are based on the following formula:

                              T = (K D P)/12

                              • T = Torque (ft-lbs)
                              • D = Nominal Diameter (inches)
                              • P = Desired Clamp Load Tension (lbs)
                              • K = Torque Coefficient (dimensionless)

                              The value of K is a dimensionless torque coefficient that encompasses variables such as those listed above, as well as the most significant variable, friction. The value of K can range from 0.10 for a well lubricated/waxed assembly, to over 0.30 for one that is dirty or rusty. The values we used when calculating our values are:

                              • 0.10 = Waxed/Lubricated
                              • 0.20 = Plain, as received condition, slightly oily
                              • 0.25 = Hot-Dip Galvanized

                               

                              Hope this helps a bit.