8 Replies Latest reply on Jul 25, 2018 12:04 PM by Matthew Zola

    Bolt connector preload and factor of safety

    Chris Fernald

      Hi,

       

      Is there a way to ignore bolt preload when SW calculates its Factor Of Safety.  For example, on an M10 bolt I have applied a ~45,000N preload, with a desired FOS of 2.  The applied load on the joint is small (~100N).  When Simulation reports the connector's results, it says its FOS is just above 1.  This is becuase it considers the tension due to preload when calculating the FOS.  In reality, a M10 bolt would have lots of FOS on such a small load.  If the preload is reduced, bending moment in the bolt will increase because the joint would separate.  Is there a way to apply the preload but remove it from the FOS calculation?  The only work around I know of is to multiply the applied force (and friction coefficient if shear is present) by the desired FOS - then make sure the axial force does not exceed the preload.  There must be a better way...

       

      Thanks

          • Re: Bolt connector preload and factor of safety
            Chris Fernald

            Thanks Kevin.  This is a good summary of how bolt connectors work, and it does mention "absolute value of the axial force on the bolt should be  about the value of the preload that was entered".  However, I'm still not sure how you can get the bolt's true factor of safety when it considers the absolute force.  The images I attached show two different load cases on an M10 bolt that is preloaded to ~45,000N.  In the real world, the bolt in the 100N load case would have a much higher safety margin than the one in the 10,000N load case - yet the FOS's are the same because it sees the preload as the dominate force.  This is what I'm trying to figure out. 

             

             

             

            100N.jpg10000N.gif

              • Re: Bolt connector preload and factor of safety
                Russ Johnston

                Chris,

                 

                I think you're missing the theory behind bolt preload.  Check out this link for more information:

                 

                http://en.wikipedia.org/wiki/Bolted_joint

                 

                "When a fastener is tightened, it is stretched and the parts being fastened are compressed; this can be modeled as a spring-like assembly that has a non-intuitive distribution of strain. External forces are designed to act on the fastened parts rather than on the fastener, and as long as the forces acting on the fastened parts do not exceed the clamp load, the fastener is not subjected to any increased load."

                 

                Russ

                  • Re: Bolt connector preload and factor of safety
                    Chris Fernald

                    Thanks Russ.  I guess my question isn't about the theory, because I understand the reason for preload.  I'm just wondering how I can use Solidworks to quantify how much more my applied load can increase before it exceeds that clamping force (in other words, safety margin).  This is easy for simple axial cases where it's just a hand calculation, but more difficult when there are axial, bending, and shear components on a pattern of bolts.  My only approach so far was to multiply the applied load by my desire factor of safety (in this case 2), and then look at the bolt loads to make sure axial load has not increased beyond the preload value.  This sort of works, but requires a separate analysis, and if shear isn't negligible I have to mulitply the coefficient of friction by the same safety factor.  Then, in order to get a definitive number for my bolts factor of safety, I would have to iterate the force until I converge on the point where the axial load is just starting to exceed the preload.

                     

                    It's kind of a clumsy process.  I just thought that since SW has nice FOS plots for bolt connectors, it would be smart enough to ignore the tension from preload.

                    • Re: Bolt connector preload and factor of safety
                      Jonathan Bechtel

                      This reply is not correct.

                       

                      When a bolted joint is subjected to loads that would try to separate the joint an additional tensile load is added to both the bolt and compressed members.  Assuming no joint seperation, typically the fastener will acquire 25% of the load, while the compressed members will acquire the remaining.  Therefore, the tensile load in the bolt increases beyond the preload value and the compressive force in the joint lessens.

                       

                      When the joint seperates, all preload is lost and the fastener takes 100% of the external loading.

                       

                      This information is from Shigley's Mechanical Engineering, 8th Edition.

                       

                      ISBN: 0-390-76487-6

                       

                      Jonathan Bechtel

                    • Re: Bolt connector preload and factor of safety
                      Jonathan Bechtel

                      If the external loading is tensile the FOS in Solidworks is correct.  Even without an external load, the preload in a bolt for a permanent joint is typically close to yield.  In fact it is not uncommon for the bolts to be intentionally yielded during installation.  This redistributes the load for evenly within the bolted group.  As such if the preload is 75% of the yield, your factor of safety is less than 1 right of the bat.

                       

                      If the external loading is compressive the FOS in Solidworks is wrong.  The load increases the compression in the joint even more, thus reducing the tension and hence preload of the fastener.

                       

                      How to compensate for this Solidworks issue is why I'm here today.  There has to be some workaround using spring elements in compression only to compensate for this issue.

                        • Re: Bolt connector preload and factor of safety
                          Matthew Zola

                          I understand that this thread is seven years old, but I wanted to see if anyone has a solution to what Mr. Bechtel said in his last post (quoted below).

                           

                          Jonathan Bechtel wrote:

                           

                          If the external loading is tensile the FOS in Solidworks is correct. Even without an external load, the preload in a bolt for a permanent joint is typically close to yield. In fact it is not uncommon for the bolts to be intentionally yielded during installation. This redistributes the load for evenly within the bolted group. As such if the preload is 75% of the yield, your factor of safety is less than 1 right of the bat.

                           

                          If the external loading is compressive the FOS in Solidworks is wrong. The load increases the compression in the joint even more, thus reducing the tension and hence preload of the fastener.

                           

                          How to compensate for this Solidworks issue is why I'm here today. There has to be some workaround using spring elements in compression only to compensate for this issue.