8 Replies Latest reply on Feb 13, 2015 2:48 PM by Jared Conway

    Bolt Connector preload in compression

    Pk M

      Hi,

       

      I am working on a cylindrical (externally loaded) pressure vessel,with two flat end caps connected by 4 retaining threaded rods(8-32) and nuts. We generally snug tight retaining rods with nuts.I calculated preload torque for 8-32 screw with nuts to be about 14 lb-in.

       

      1.I modeled threaded rods as "blot connectors" with 14 lb-in pre-load and ran the study by applying external pressure.The "Pin/Bolt Check" plot showing FOS for these screws as only 0.85, which is very low value than I expected. When I ran study with zero pre-load, I got FOS of 15.9. Other thing I noticed is - FOS is increased with increase of pre-load to certain value (0.15) in this case and decrease gradually. In this case thread rods are under compression. I imagine this happened because "pre-load" in solidworks simulation expected to be used only under tension. I wanted to see how it will behave under tension.

       

      2. Then I applied same pressure internally to see how bolt joint is under is going to behave tension. Even in this case, I got FOS of 15 with zero pre-load torque and got 0.8 for 14 lb-in preload torque.

       

      (3. I applied force on inside of flanges(triangular corners - notice in pic. attached), in same direction as bolt, to see how its going to behave in pure tension. In this case, with zero preload, FOS is 4.5 and with 14 lb-in preload FOS is 0.75.

       

      4. I applied same load on outside of flanges (same direction as bolt) in pure compression, in this case with zero preload, FOS is 23 and with 14 lb-in preload, FOS is 0.86.)

       

      Now I am trying to understand results. I would greatly appreciate if someone could explain me based on above studies.

       

      1. Why FOS of bolt joint decreases when correct preload (75% proof load calculated from handbook) is applied?

      2. Why, bolt joint showing same behavior in both tension and compression in case 1 & case 2.

       

      Although case 3 and case 4 are not required for my application, I am curious to know

       

      3. Why thread joint showing less FOS in pure tension compared to compression? I expected to see bolt joint stronger in tension in this case (as the cylinder is sandwiched between plates & with correct pre-load) the joint supposed to be stronger?

       

      thank you so much,

      pkpavv.

        • Re: Bolt Connector preload in compression
          Mike Pogue

          The reason you get a smaller FS with the preload is because, in normal bolted joints, the tension from preload uses up the vast majority of the bolt's strength. If you torque to 75% proof, you have a FS of 1.33 with zero external load. The reason the FS is higher in compression, is because the preload is in tension. The compressive load is subtracting from the preload. The tensile load is adding to the preload.

           

          I'm not sure why the FS increases initially with preload, but it may be that SW is calculating the FS for bolt rupture, joint separation, tear-out, etc. The FS for joint separation increases with preload, and initially (zero preload) should be very low. But eventually, once the bolt gets overloaded, joint separation won't be your biggest problem anymore, and the FS for bolt rupture, which is going down, becomes the limiter.

            • Re: Bolt Connector preload in compression
              Pk M

              thank you Mike for your reply.

              "The compressive load is subtracting from the preload. The tensile load is adding to the preload"

              when I compare with my study,

              1. So, we don have to apply pre-load in compression so that we get FOS of 0.86 without pre-load. In tension if we apply pre-load then we gain advantage & better FOS of 4.5 (instead of 0.75 without pre-load)? - which is what I read some where (I am not sure of source). is my inference correct?

               

              Coming back to the application I am doing,

               

              2. Because I am getting almost same FOS values for both internal and external loading for case 1& 2 (with pre-load 0.6 FOS and with NO pre-load 15 FOS), is it correct to run study without pre-load (or very low value to replicate snug fit,which is what we are practically do)?

                - If I do that I will get better FOS of 15 or slightly higher (as I told, FOS is slightly increasing with increase of pre-load to up to certain value).

                 If I get FOS of 16 is it trust-able result? Is there any way to verify this?

               

              thank you,

              pkpavv.

                • Re: Bolt Connector preload in compression
                  Mike Pogue

                  Just to verify, I am assuming compressive loading means "from outside the vessel".

                   

                  The bolt is never going to fail in this mode if it does not fail during installation. You can leave the bolt entirely out of the model for this load condition, since the walls of the vessel are reacting the force on the lids from this pressure.

                   

                  In the case of internal pressure, I would expect the bolt to react the entire force on the lids. This would decrease the FS, but probably not by as much as you think. If you tell me the internal area of the lids, and the pressure, I'll do some back of the envelope to compare to the results.

                    • Re: Bolt Connector preload in compression
                      Pk M

                      Id of the lids (& housing) is 2 in. bolts are 8-32 & 10.5 long.

                       

                      Compression in my question 1(in previous response) refers to case 4 (attached screenshot) and tension refers to case 3.

                      So, my question 1 can be re-phrased as

                      1. So, we don have to apply pre-load case 4 & we get FOS of 0.86. In case 3 we we gain advantage with pre-load & better FOS of 4.5 (otherwise we get 0.75 FOS without pre-load)? - which is what I read some where (I am not sure of source). is my inference correct?

                       

                      I wanted to use bolt in case 1(case 2 is not my application. I just did to compare results) to see bending / shear stress on the "all thread rod" as the housing is long compared to its diameter (With application of external pressure, whole whole housing will try to compress/shrink causing flanges to bend which induce shear / bending stress on bolt).

                       

                      Is there anyway to know weather it is correct to apply pre-load in case1?

                       

                      thank you

                • Re: Bolt Connector preload in compression
                  Jared Conway

                  can you break this down to a very simple example and post here?

                   

                  i also assume you've gone through all the documentation on bolt connectors in the help and in the kb to understand how they work and that they match how you are doing your back of the envelope calculations.

                   

                  it may be helpful to summarize your results with a table rather than with text and also to describe what you expect the results to be based on your experience and hand calcs. some of that is getting lost in the way you're trying to outline what you've done.

                   

                  I would also recommend focusing on one case at a time.

                    • Re: Bolt Connector preload in compression
                      Pk M

                      Hi Jared,

                       

                      I attached model & screenshots.      

                           

                      Load (lb)Pre-load torque (lbf-in)Axial force in bolt connector (lbf)FOS
                      Case-A100 - Tension10297.50.487
                      Case-B100 - Tension015.259.97
                      Case-C100 - Compression1057.340.66
                      Case-D100 - Compression0224.86149.5

                       

                      I went thru help documentation but I m unable to find rough equation to calculate axial force on bolt in a bolt joint to verify above values & I dont have access to kb.

                      Also, I am unable to find how SW calculated above FOS from axial force in table and bolt material I used.

                      This is what I came up with.

                      I used 304 SS material from solid works with Sy = 30KSI. & At for 8-32 hardware = .014 in^2.

                      Case A FOS = (0.2*30000)/(297.5/.014) = 0.28 (solidworks uses 0.2 X Sy for FOS calculation)

                       

                      My Observations:

                      - Although I m unable to verity calculations, I expected to see only little difference in axial force for case A and case B.

                      -For case C and Case D, bolt connector is not required as force is in opposite direction.But I just wanted to run study and see results.For this also, I expected to see little difference for bolt axial force.

                       

                      1. Are Case A & Case B axial bolt force and FOS values are correct? With pre-load torque, I expected to see better FOS in case of tension load.

                      2. Is there any equation to approximately calculate axial force & FOS of bolt, to verify above values.

                       

                      thank you,

                      pkpavv

                        • Re: Bolt Connector preload in compression
                          Jared Conway

                          what documentation have you gone through? did you check the help and the solidworks kb? i believe both have the equations that you are looking for.

                           

                          also i'm not sure i really understand what you're trying to get at here. do you believe the software is calculating things incorrectly or are you trying to verify that your hand calculations match what solidworks simulation is doing or that it should match some reference (what reference)?

                           

                          have you spoken to your reseller about this?