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Modeling fastener stiffness

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12-21-2018 04:21 PM

I am trying to find an appropriate modeling technique for bolts. I'm new to modeling bolts with beams to solve for bolt loads. If I model a "stiff" bolt, my bolt shear margins are negative. If I model a "regular" bolt, my margins are positive.

I am modeling a 6-fastener pattern with beams for the shanks and RBE2s spidering at the heads. The threaded end is attached to RBE2s that attach to a point mass that is offset perpendicular some distance from the pattern centroid. Simplified, its a box (point mass) bolted to a wall at 6 points. The wall is part of a larger structure shaped like a larger box, which is mounted to another structure. When I apply 23g in the X direction, the larger structure deflects in X, but also rotates some about Y (due to the design).

If I model my beam with a "regular" size diameter of 0.22 inches (0.006 m) and "regular" modulus (steel), then the total shear results have a maximum of 189.7 N shown below. (I should mention that all of these have NO PRETENSION modeled. Our company approach is typically to not include pretension, but model the bolts as "stiffer". However, some here take issue with high shear stiffness resulting from the stiffer bolts. I'm not smart enough to say one way or the other.)

If I __increase __my bolt __diameter__ to 1", but keep my modulus the same, then my results have a maximum value for total shear of 298.59 N.

If I model the bolts with a __regular diameter__ bolt and then assign a __modulus__ that is __X10__ greater than steel, then I get a maximum total shear of 285.21 N on any bolt.

Summary of loads in Newtons:

~190: Not very stiff since there is no pretension

~299: Stiff due large-diameter bolts (higher Area)

~285: Stiff due to greater modulus (higher E)

I was tutored by several others that just modeling the bolt as "much stiffer" allows for one to *accurately* calculate applied loads (with pretension left out). Then applied loads and pretension loads can be combined through hand calculations. The problem I see with this though my trade study is that the much stiffer beam (whether from A or E) can increase the applied shear loads felt in the bolts. If I need to refine the analysis and see if I can decrease the load in the bolts, how much can I reduce the stiffness of the beam? I have a coworker that thinks that stiff beams are great for tensile loads, but the lateral stiffness need not be so high because it artifically drives up the shear loads. What is the right approach here?

Note, the shear vectors are pointing at different directions I believe due to 1) the rotation of the larger structure and 2) the varying stiffness at the boundary condition of my submodel plate due to the complexity of the larger structure. I've simply hidden the rest of my larger structure due to the nature of my program. Thank you!

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