5 Replies Latest reply on Jan 30, 2013 4:43 PM by Neil Grant

    Understanding bolt connectors

    Mikael Martinsson


      I need help to understand the bolt connectors in SW.


      The formulation of SF is from NASA NSTS 08307. In this standard each of Ra, Rb and Rs (SW terminology) should fulfill the requirement on their own as well as together. SW only show the SF combined. Is it possible to plot the individual Ra, Rb and Rs values as well? It is a bit frustrating to calculate them all manually when many bolts are involved.


      Preload is an important factor for working bolt connections. But if I preload bolts in SW at about 80% of the proof load strength, the SF will be about 1,2-1,4 and decreases if a tensile load is added to the parts. (bolt carries some of the force and the joint the rest). This means that I can never set SF criteria to 2 or above unless I minimize the preload. Where can I see how much additional load the bolts can handle to get the "real" SF? Is the only way to do this to increase external load until SF falls below 1?


      I ran a simulation on 2 plates bolted together with 21 bolts and external force on one of the plates causing tensile stress. When using 12.9 grade M16 bolts, the results reported back OK. SF average 1,2-1,3 and no separation between parts. I then tried using 8.8 grade and 2 bolts reported SF below 1. I then tried with preload set to half  and now all bolts reported OK. The two bolts that failed in previous test where now a little bit above SF1. Now I tried decreasing the preload even more, and the SF result increased even more and all bolts reported as OK. But now the parts was separated (displacement plots).


      I find it a bit difficult to understand how much more load the bolt connection can handle just using the results. Trial and error is one way, but with a lot of connectors and no penetration contacts means that this can take a while. Or how do you do it?


      Perhaps I'm missing something obvious here.

      Thank you in advance.

        • Re: Understanding bolt connectors
          Kevin Corr

          The list pin/bolt/bearing force menu pick on the right mouse menu of the results item sounds likely to satisfy your Ra, Rb and Rs needs.

            • Re: Understanding bolt connectors
              Mikael Martinsson

              Absolutely. I can list all forces for the individual bolt connectors. But there are no listing of the calculated Ra,Rb and Rs on the right click or am I missing something? Of course you can calculate them manually by using the result forces, or perhaps you can because I see now in SW2013 that the forumlas are changed. Now they more represent the NSTS 08307 standard. But I still get exactly the same results for SF between 2012 and 2013 which is odd. 


              SW2013 Pin Bolt Safety Check: (click to enhance)


                • Re: Understanding bolt connectors
                  Kevin Corr

                  For a bolt in tension only...notice Rb and Rs are zero,  FOS = 1 / Ra² = 1 / [F /( At * S)]² = At² * S² / F².


                  the following may be true:

                  § increasing the bolt Strength S by a factor of 2 increases fos by a factor of 4
                  § increasing the axial load F by a factor of 2  reduces fos by a factor of 4
                  § increasing the tensile stress area At by a factor of 2  increases fos by a factor of 4


                  If the shear and bending terms are not negligible, the cube of rs and the square of ra + rb and  are felt so a sense of the trend given by changing d is not a simple exchange rate.


                  So if you see loads in the list that are negligible in bending and shear, it may be true that you can relate the rate of fos and the given load, bolt strength and tensile stress area by "power" rule.

              • Re: Understanding bolt connectors
                Neil Grant

                Just a couple of thoughts.  The new FoS has been since 2012 sp 4 per spr below

                The help file is noted as being wrong and the new FoS is about the sq root of the old one.

                One thing I have noticed is the FoS shows you need very big bolts if you do not include friction which greatly reduces bending and shear in the bolts.  In reality it is there as no one would design a mount for most anything to have it float around in its bolt clearance holes...

                When you add friction at the contact surfaces, bending and shear are mostly taken up by the resisting friction and you get something realistic. The ironly of not including friction is the tighter the bolt, the lower the shear load allowble which is exacty opposite to reality where friciton will take more of the fastner shear load away and lower the fastener applied shear. The safety factors calculated from the new way are smaller now.Capture.JPG