26 Replies Latest reply on Mar 30, 2011 8:11 AM by Philip Lewis

    Bolts in double shear

    Philip Lewis

      I do not use simulation very often but usually can get by based on the training manual. I can only do static study as I only have the version that comes with SW Premium. I have not been able to figure out the correct way to run a simulation on this lifting lug set up (see attached jpg). If I use the physical model of the bolts (3/4" bolts in 7/8" holes) I can get wildly different results based on how the bolt connections are defined.

       

      So far I have run this with the beam as a solid. I have the global contact set to bonded. I have made sure that the 2 gusset plates are bonded by adding a bonded contact between their faces and the corresponding beam faces. All the holes have a no penitration with the bolts. The nuts are bonded to the bolts and I used virtual walls to stop the bolts/nuts from flying off into space under load.

       

      Is there a correct way to approach this problem? I can't seem to use the SolidWorks bolts or pins functions because they don't seem work across 3 parts (double shear).

        • Re: Bolts in double shear
          Philip Lewis

          Maybe I should have included the model. Here is a pack and go of it in 2011.  I know this is probably a simple problem but I just don't know how to handle it.

          • Re: Bolts in double shear
            Dougal Hiscock

            What is the goal of this simulation run?  Are you checking the integrity of the gussets welded into the beam, the bolted connection or the lifting lug itself?

             

            I have to say that regardless of what the simulation results were, I wouldn't design or use a joint like that.  The amount of restraint in the gussets either side of bolted join is too high, it won't allow the bolts to reliably clamp the joint and prevent slip.  Weld distortion will make this worse.

              • Re: Bolts in double shear
                Philip Lewis

                The goal is to have a reasonable idea of the lifting limit of this design with an acceptable FOS.  This is a lug we have been using for years with a hand calculation done by a PE. After looking at his calcs I really didn't like how he did them so I decided to try and run it in simulation and see what it came up with.

                 

                This isn't supposed to have a clamping force on the center lug. These are basically for transporting and will be removed once the system it is used to lift is installed in its' permanent location.

                  • Re: Bolts in double shear
                    Dougal Hiscock

                    Clamp load is the normal way a bolted join resists the loads applied.  The bolts are torqued up with only axial tension in them and the friction between the joint plates carries the load, the bolts don't need to take any shear loads.

                    With a join that doesn't let the bolts clamp the plates firmly together (like this one) you're simply using the bolts as pins in shear, which is not ideal and requires larger bolts than are normally required.  Bolts make poor pins as the diameter and bearing surface are significantly reduced due to the threads, you can customise bolts to get around this (find bolts with the appropriate length shank and cut them down) but that gets costly.

                     

                    If these bolts are to act as pins then practice won't see three of them sharing the load evenly, if at all.  It's likely due to the tolerance in hole size and location that one pin will take all the lifting load, another will take all the moment reaction while the third simply rattles around in the hole.  Which pins do which this will likely change from part to part even when made to the same drawings.

                     

                    I think you are not going to get a good correlation between simulation results and test results for a part like this for those reasons.  The joint could be improved by removing the second gusset plate and bolting the lifting lug up tight with the three bolts.  This will let the clamp load do the work and simulation will more accurately reflect what is actually happening.  Other benefits would include the joint behaving predictably, being easier and cheaper to manufacture and easier to protect from corrosion, if that is a concern at all.

                      • Re: Bolts in double shear
                        RICH PAUZA

                        Did you try to run the simulation using pins instead of bolts....?

                         

                        I would be curious to see the results.

                         

                        I always try to incorporate a double shear design  into any bolt on connection I build.  Weather it is machined into a billet bulkhead or a welded fabrication using tabs - I think it is superior to a single shear design.

                         

                        The difference in the load the fastener can withstand is substantial.

                         

                        But my experiance is with small & light weight parts - not heavy fabications.

                         

                        Rich

                          • Re: Bolts in double shear
                            Dougal Hiscock

                            Rich, yes double shear is a stronger joint than single shear both both bolted and pinned joints.  But a bolted double shear joint must be flexible enough that full contact pressure on both faces is acheived.  The joint shown in this thread is too rigid to let that happen.

                             

                            There is also the question of how strong a joint needs to be.  In this case the plate thicknesses seem quite large in comparison to the beam they are welded to.

                              • Re: Bolts in double shear
                                Bill McEachern

                                I am a big fan of practicing good design. However, suboptimal designs can work with demonstratable margins of safety. And in my experience sub optimal designs are far more frequent than optimal ones.When you job is to analyze other peoples designsit is easy to be critical but that doesn't mean it won't work.

                                  • Re: Bolts in double shear
                                    Dougal Hiscock

                                    Bill

                                     

                                    The point I'm making is indeed regarding the analysis.  This joint appears to be a double shear bolted joint with a group of three bolts, yet it is unlikely to behave like one.  It will behave like a double shear pin joint with two pins taking the load and taking that load un-evenly.  It's not possible to analyse any joint properly without understanding how it will behave and why.

                                     

                                    I don't know if Phillip is a PE, but I have seen before people trusting simulation results over calculations where they should not.  If a PE has performed some work that you want to know more about, calling up that PE would be a necessary first step.

                                     

                                    For running FEA on such a joint there are two scenarios that need checked, both involve the middle bolt doing nothing:

                                    One is the gusset holes being slightly larger spacing than the lifting plate.  This results in the lower bolt taking all the lifting load with the moment caused by the offset being resolved through the top and bottom bolts.

                                    The other is the gusset holes being slightly shorter spacing than the lifting plate, this results in the upper bolt taking all the lifting load with the moment also resolved between the two bolts.

                                     

                                    But both these scenarios can be checked very easily with simple hand-calcs.  I can understand wanting to run an FEA simulation on the lifting plate itself, but for the joint as a whole I don't see the benefit until lifting loads are large enough to become a concern to the beam itself.  Like the stress in the bolts, bulk stress in the beam can be quantified quickly and easily with hand-calcs.

                                      • Re: Bolts in double shear
                                        Bill McEachern

                                        Hi Dougal,

                                         

                                        I get and agree with all that. The point I was makinig is  that the connection maybe good enough for the applied loads regardless of its short commings. How you actually analyze the joint (hand calcs or FEA analysis)should result in similar margins of safety assuming the noise of hte fea in the contacts is well managed. In my view the margin of safety should be caculated on ultimate strength of the connection and not on the onset of plastic deformation, which is likely to be very much higher.   At the end of hte day it is either tear out, shear out, tension failure or pin failure with both pins in play, moments and all. As soon as things start to deform the other pin will get loaded. Further, I think to ensure assembly of the connection the hole tolerances would have to be large so both pins will be in bearing and as you point out doing them up tight is probably not worth the effort. I also agree that understanding hte PE's hand calc's is required before one questions the predicted margins and tries to investigate them further.

                                  • Re: Bolts in double shear
                                    Philip Lewis

                                    I did run it with pins but it has problems with this too. I have not been able to have the software select all 3 faces the pin will be in contact with. It will only allow me to run it with a pin from each gusset to the center lug. Therefore I end up with 2 pins where there is only 1 bolt. 

                                  • Re: Bolts in double shear
                                    Philip Lewis

                                    What we are doing is trying to condense our part stock. We have been using this design since before I got here but with 6 different gusset and lug thicknesses. We now are trying to get this down to 2 (one for W6 and one for W8 beams). For this we have used the thickest plates from the previous designs and had them rerun by a PE. When we got them back we didn't like that the calculation showed only three calculations (force on the center lug, weld capacity and bolt capacity) none of which seemed to include a moment.

                                     

                                    This design is using bolting as pins. The bolts at 4-1/4" long will have the unthreaded portion to 2-1/2" of it's length. This will allow the unthreaded portion to be used as a pin since all but 1/8" will be 3/4"  diameter.

                                     

                                    Weither this is the design that someone feels is best or not, it is the one we have the calcs for and have been using for many years. I just can't seem to get a good result out of simulation and was trying to see if anyone knew of the best way to run this. It seems that the spaces between the bolts and holes as well as the plate to gusset clearance is causing too many problems for simulation to give me a valid result. If so, we will continue to reley on the hand calculations.

                                      • Re: Bolts in double shear
                                        Dougal Hiscock

                                        Phillip.

                                         

                                        Have you tried rotating the lifting lug in the assembly to the point where the bolts would be loaded up between the lug holes and the gusset holes?  Otherwise we're into large displacements just to get the geometry correct which does cause problems.

                                          • Re: Bolts in double shear
                                            Philip Lewis

                                            I've tried them at tangent to both the top quadrent and now at different locations on the holes.  I'm gonna keep moving the bolts to see if I can line it up close enough to run sucessfully. So far I have been spending alot of time watching the runs only to have them fail at some point. then I make adjustments and try again.

                                      • Re: Bolts in double shear
                                        Paul Kellner

                                        Guess I'm wondering what you didn't like about the PE's calculations? This is a standardized calculation that you will find in the AISC handbook. In aerospace the hand calcs are also well undertood and cover a number of failure modes. So why do to the trouble of doing FEA, especially if it is a temporary feature?

                                         

                                        If you run FEA on the bolts you will have to take into account a plethora of manufacturing and use conditions and possibly the plastic behavior of the bolts in shear. Failure of a bolt in shear requires that the bolt become plastic throughout it's thickness, not just in a localized area. So you are looking at a pretty involved analysis. Elastic analysis will be difficult to interpret as yielding may occur well before the joint is anywhere near failure.

                                         

                                        As has been pointed out, you can't develop clamping on the lug, so why use bolts if this is a temporary joint? Why not use pins? It's a heck of a lot easier to disassemble in the field when done.

                                          • Re: Bolts in double shear
                                            Philip Lewis

                                            I don't have the AISC handbook to know for sure but I don't think this is what he was using. He mentioned a Dupont standard "SG 4 C" which when I looked at it seems to be a basis for calculating welded on lifting lugs. He does one equation for the allowable load on the center lug from the standard but it doesn't seem to have anything in it which represents the fact that the lug has a distance between where it is being lifted and where it is attached. He then has one equation for the weld capacity and one for bolt capacity with the threads in shear. The whole calculation takes up a half sheet of paper.

                                             

                                            To be clear, I am not an engineer by trade. I am a designer but I do have a Mech Eng degree. In a discussion with the engineering manager he thought that this calc seemed to lack some things that he felt should have been in this type of calculation. Since I am the one that uses SolidWorks I am the one who received the simulation training and therefore the one who is trying to get a valid result for this setup.

                                             

                                            If solidworks would let either the bolting portion (or pinning since this is designed to use the unthreaded area of the bolts) of the simulation connection sets contact 3 holes then I would probably already have a result.

                                              • Re: Bolts in double shear
                                                Paul Kellner

                                                Did he calculate the moment and the shear on the group of bolts. AISC does both, it is the only way to know the load on individual bolts. I wouldn't be surprised if DuPont doesn't reference AISC somewhere for this kind of thing. What FOS did he come up with? I would reference AISC anyway instead of getting into an FEA. It might be easier to defend as FEA results will be difficult to interpret without correlation to test.

                                                 

                                                In calculating the welds, did he include the stresses induced by clamping or attempting to clamp the lug? These could be quite high if there is not a good fitup. And how did you plan to model the weld residual stresses and clamping induced stresses in your model? The weld residual stresses in the corners ought to be quite high.

                                                 

                                                Did he take into account variations in the direction of the load? This would include any twisting of the beam or at least a check for significant twisting.

                                                 

                                                Still, this is a case of answering a question, not necessarily using the tool you have been trained in.

                                                 

                                                Are you worried about the bolts, tearout, bearing failure or what?

                                                 

                                                Frankly, this is probably an Excel spreadsheet type of problem unless you are working to an FOS near 1.2. I doubt that.

                                                  • Re: Bolts in double shear
                                                    Philip Lewis

                                                    I don't see any mention of a moment in any of the 3 equations he shows. The equation for the 3 bolts in double shear is (2 * bolt qty) * bolt area * Load = (6)*(.302)*(15000) = 27180. The dupont calcs doesn't have any bolting in it, just welded lugs.He is giving a FOS of 3.5.

                                                     

                                                    His weld calc is 0.707 * weld size * weld length * SA = 0.707 * 0.25 * 11 * 14500 = 28190#. This lug is not designed to clamp all 3 pieces together. It is more of a pin arrangement for the center lug with as much of the solid portion of the bolt seeing the load as possible.

                                                     

                                                    The only other calculation on the sheet is for the center lug. P = (Stress Allowable * plate thickness)/((0.33/(Outer lug radius - Lug hole radius) + ((0.76 * (Outer lug radius + Lug hole radius)/(Outer lug radius - Lug hole radius)^2) = (SA * tb)/((0.33/(rL - rH) + ((0.76 * (rL + rH)/(rL - rH)^2) = (14500 * 1) / ((.33/(2-1)) + ((0.76 * (2+1)) / (2-1)^2)) = 5555#

                                                     

                                                    These 3 equations make up the entirty of the calculation he supplied us.

                                                      • Re: Bolts in double shear
                                                        Paul Kellner

                                                        Here is what you need to do. AISC 8th Ed. 4-58 Eccentric Loads on Fastener Groups.

                                                         

                                                        You can blow through this on an excel spreadsheet a whole lot faster than doing FEA and be confident in your results. It continues on the next page.aisc-bolted-joints-elastic.png

                                                        aisc-bolted-joints-ultimatestrength.png

                                                         

                                                        Message was edited by: Paul Kellner

                                                          • Re: Bolts in double shear
                                                            Philip Lewis

                                                            Unfortunately, I can't open the pdf. Adobe Reader X says it is either not a supported file type or has been damaged (like an incorrectly decoded email attachment).

                                                             

                                                            I had this happen with a pdf before and it was a secure pdf that could only be opened by a certain person.

                                              • Re: Bolts in double shear
                                                David Paulson

                                                Phillip,

                                                 

                                                The bolts should play an insignificant role in the performance of your lifting lug.  When lifting,  the force is along the y-axis and will rotate the lifting lug within the confines of the beam and the gussets, thus transfering the load to the webs of the beam. Properly designed, the clearances between the bolt and the hole should be greater than the clearances between the lifting lug and the beam so the bolts are never in shear.  The bolts are only useful when using the lifting lug to pull your skid along the z-axis.  When lifting with a sling, the sling will impart a force on the lifting lug that will force the lifting lug into the beam pocket.

                                                 

                                                I would recommend that you run your analysis without the bolts to determine at what load the beam becomes deformed.

                                                  • Re: Bolts in double shear
                                                    Dougal Hiscock

                                                    David.

                                                     

                                                    The geometry of the pocket the lug sits in and the clearances shown cannot work in that way.  There is clearance above, below, behind and beside the lifting lug.

                                                     

                                                    I've just run this part myself, but due to not have 2011 I imported the part as an IGES and analysed it as a multibody part.  Needless to say solidworks didn't handle the clearance between the bolts and holes correctly, the results are similar to bolts being a close fit in the holes.  But the bulk stress in the lug and beam are still representative, this is a 10kN upward pull on the lifting eye.

                                                     

                                                    Z783 Export-Study 1-Stress-Stress1.jpg

                                                      • Re: Bolts in double shear
                                                        David Paulson

                                                        Dougal,

                                                         

                                                        Your analysis of the  lifting lug shows that there is significant stress on the bolts.  I don't question that that may be the case "as designed".  My point was that with a better design the entire load should be transferred from the lifting lug to the beam with little or no load on the bolt.  If your bolt had threads in your model were they deformed as a result??  The load rating of the lifting lug would not necessarily be at the breaking point of the bolts because it would probably take much greater force to deform the beam to the point where the lug could slip out of the pocket.  What happens if you supress the bolts and then run the analysis?  The lifting force may not be purely vertical dependent upon the geometry of the spreader bar.

                                                          • Re: Bolts in double shear
                                                            Dougal Hiscock

                                                            David

                                                             

                                                            Yes the bolts are shown stressed there, but it's not accurate due to the pin fit problem I mentioned just before.

                                                             

                                                            The best design is, as already mentioned, has plates which can be pulled in tight under the bolt tension.  This, like every good bolted join, carries the load in friction on the faces and creates no extra stress in the bolts.  I don't believe a "cam action" type lifting lug is appropriate here, aside from the high contact pressures potentially damaging the beam it will never meet any of the safety requirements required for lifting lugs.

                                                             

                                                            I'm a little concerned about this comment:

                                                            David Paulson wrote:

                                                             

                                                            The load rating of the lifting lug would not necessarily be at the breaking point of the bolts because it would probably take much greater force to deform the beam to the point where the lug could slip out of the pocket.

                                                             

                                                            The standards I work to when designing or analysing and certifying lifting points calls for 4x safety factor minimum.  The load rating is never-ever anywhere near the point where anything can break or deform to the point where it sheds load.

                                                             

                                                            A bolted joint has failed when the joint slips (i.e. not enough clamp load).  Shearing bolts off is unthinkable.

                                                              • Re: Bolts in double shear
                                                                David Paulson

                                                                Dougal,

                                                                 

                                                                As designed the force that it would take to create a friction bond between the lifting lug and the two  gussets would be very substantial depending upon the tolerance of  the thickness of the lifting lug and the spacing and the thickness of the gussets.  If you desire a friction type connection you would not use two gussets, just one.  To me, the design intent inherent in the pocket design is to use the lifting lug a a camlock type device.  See US Patent 4,069,991 at www.uspto.gov.  The concept of cam type devices is not new.

                                                                 

                                                                I didn't mean to suggest that a safe load would be defined by the point at which the bolts shear.  I was only suggesting that the force requried to shear the beam between the two gussets would be much greater than the force to shear the bolts if they were truly intended to carry the load in shear.  And if that were the design you would use a hardened pin instead of a bolt.

                                                                 

                                                                But wasn't this question about evaluating an existing design that was verified by a PE??  My view of the design as presented is that it was intended to be a camlock or chock type of connection and it's strength is not dependent upon the bolts, although admittedly the bolts seem rather large to keep the lifting lug from falling out during transportation.