Bolted joint: Tapered bolt shank w/standoff at head: pre-load and interference

[Christopher.T.Buckingham.civ]

Say there are two simple plates to be joined by a bolt and nut joint: The shank is tapered towards the threads where the nut is to be installed. There is to be an interference fit between the shank and holes in the plates such that when the bolt shank is inserted by hand a standoff/clearance exists between the top plate upper face and the bottom face of the bolt head of say 0.1". The shank and the holes in the plate are assumed to be in initial contact. The nut is installed and threaded untill it just seats against the bottom plates lower face. Then the nut is to be turned which advances the bolt head/shank untill the bolt head is flush on the top plates upper face. This final configuration is something of a combination preload/interference fit where the joint has internal "spring" energy in the axial (as in standard bolt preload) and in the radial (technically it would be normal to the interference faces, which are tapered, but it a very slight taper) directions (as a pure raidial interence fit would have) I am struggling to find a reliable and efficient way to model bringing the parts into this configuration which is required such that further arbitrary loading can be applied to the joint for analysis.

-Built in bolt pretension cannot be used because the shank is not perfectly cylindrical, and if it could, i beleive it would have a significant effect on the frictional contact conditions/forces on the shank-bolt hole contact area since it would create a bit of a step in the shank taper profile.

- I dont know before hand what the force requiremed to meet the final configuration is, so somekind of prescribed displacement seems to be the solution.

-Im thinking that in a perfect world there would be some kind of loading that would couple a displacement at the nut end of the shank (pulling it into the holes) with the displacement of the bottom face of the bolt head  e.g. displace the far face of the nut side of the shank until the known standoff clearance distance has been closed and the program would apply whatever force is needed to continue pulling the tapered shank into the plate holes until that condition was satisfied. I could then extract the force necessary to compare with other models for corroberation/ verification.

- since we would pulling the overzied shank forcefully into the plate holes, simply applying the standoff clearance would not be enough since the shank would stretch by some amount so it would need to be a fractionally larger amount than the standoff clearance e.g. ~.11" or so?

- I beleive contact birth between the nut/flange and nut/shank once the proper configuration is reached would be equivalent to "locking" the joint in that state. please recommend other methods if that seems like a poor solution.

-This model and the resultant contact / clamping forces would be extremely sensitive to contact enforcement and minimal penetration is needed. part of the further loading once the clamped and interference fit state is achieved will be to analyze friction grip when applying shear excitation to the joint.

[Ashish Khemka]

Hello,

I think you can model the interfering geometry, solve the model in multiple steps. First step - resolve the interference, then apply bolt pretension in the next step.

Regards,

Ashish Khemka

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[Christopher.T.Buckingham.civ]

 

Ashish, I had thought about that however because of the taper, wouldn’t the interference  tend to push the bolt back out of the hole resulting in just the initial position of shank and holes just touching and the bolt head with a standoff? I think you’re solution would absolutely work if a purely cylindrical shank with interferece were used since the interference and pre-load would be orthogonal loading where one is not a function of the other. 

As an example in a similar FEA software, I've seen this solved by probing the position of the bottom of the bolt face and relating the force applied to the nut end shank as being an equation where F applied satisfies the condition where the continually probed bolt face position + or - (depending on convention) the known initial bolt standoff distance = 0  

Thank you for your reply.

 

 

[Ashish Khemka]

Thanks, Christopher, for your comments. Please create a support case for more help.

Regards,

Ashish Khemka