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Simulating Stresses on Bolt Threads inside larger Assembly

Question asked by Victoria Nedbal on Oct 21, 2019
Latest reply on Oct 22, 2019 by Bill McEachern

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HI all,

I am trying to manually put in two sets of threaded nuts and bolts into a larger assembly in order to determine if the forces will create stresses on the bolts that will potentially cause them to be stripped/fail. I am aware of bolt connectors, but I would like to use the bolts I have for this simulation. I also was having trouble with the bolt connections because I do not have a circular edge where the bolt head would be placed.

I am having trouble mating the threaded nut to the bolt so that there is no interference and that the nut is also touching the bottom face of a plate in the assembly.

I am applying two main forces to the largest part of the assembly that the bolts are trying to keep stationary and a clamping force/axial load on the bolts. My fixtures are on the sides of the bottom plate and in one simulation, I tried to fix the bottom of the screws because they were the only parts moving freely.

The assembly is symmetric, so I tried to start a 2D static simulation but I cannot figure out how to model/mate the threaded nuts and bolts on the assembly.

If anyone has any advice on how to proceed, it would be greatly appreciated. I have attached some photos of the assembly model for reference.

Attachments

forces.jpg43.1 KB
assem.png65.8 KB
nut and bolt on assem..png53.8 KB
closeup of threaded nut and botl.png332.3 KB
closeup of nut and bolt in assem..png40.7 KB
side view of assem..png17.1 KB
where bolt head would be placed.png76.4 KB
Thread Interference.png69.4 KB

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Shawn Mahaney Oct 22, 2019 8:42 AM

I think most people would advise against ever trying to model threads explicitly. A cylindrical surface at the root diameter of the softer material, or the smallest diameter, will give useful forces and relative stresses for handbook calculations.

If you insist, and have the resources to run an ultra fine mesh in full contact analysis, model the threaded parts together in context. The exact same sketch-sweep should cut both sides of the thread. Then you can be sure the helixes are clocked together with uniform clearance.

>The assembly is symmetric,

A helical thread is not symmetric in any plane or axis.

>I do not have a circular edge where the bolt head would be placed

It is common to edit base geometry to force a circular edge, e.g. making a slotted hole a cylinder in a simulation configuration. The 'bolt connector' is the only way I've found to model a fastener with accurate preload and bolt stretch.

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Bill McEachern Oct 22, 2019 8:53 AM

As shawn says the lining g up of helices can be tricky. The best approach if insisting on a helix is to use the cavity option and zero clearance would be my suggestion to minimize over closure issues at the start. An alternative approach which I found to be a good approximation in all material ways is to revolve the threads and ignore the helix angle. It produces stress in the right places with a distribution that aligns with what one would expect (the first three thread roots are the highest loaded and if do stiffness tapering the stress distributes as expected. The revolved thread approach allows for symmetry so you could use an asymmetric 2D analysis. If it were me I would do it that way but maybe that doesn't work in your case. Trickier still would be to do up the thread and then load it. I have seen that done but i would strongly advise against it. I have no idea how it was pulled off.

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Shawn Mahaney Oct 22, 2019 8:42 AM
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I think most people would advise against ever trying to model threads explicitly. A cylindrical surface at the root diameter of the softer material, or the smallest diameter, will give useful forces and relative stresses for handbook calculations.

If you insist, and have the resources to run an ultra fine mesh in full contact analysis, model the threaded parts together in context. The exact same sketch-sweep should cut both sides of the thread. Then you can be sure the helixes are clocked together with uniform clearance.

>The assembly is symmetric,
A helical thread is not symmetric in any plane or axis.

>I do not have a circular edge where the bolt head would be placed
It is common to edit base geometry to force a circular edge, e.g. making a slotted hole a cylinder in a simulation configuration. The 'bolt connector' is the only way I've found to model a fastener with accurate preload and bolt stretch.
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Bill McEachern Oct 22, 2019 8:53 AM
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As shawn says the lining g up of helices can be tricky. The best approach if insisting on a helix is to use the cavity option and zero clearance would be my suggestion to minimize over closure issues at the start. An alternative approach which I found to be a good approximation in all material ways is to revolve the threads and ignore the helix angle. It produces stress in the right places with a distribution that aligns with what one would expect (the first three thread roots are the highest loaded and if do stiffness tapering the stress distributes as expected. The revolved thread approach allows for symmetry so you could use an asymmetric 2D analysis. If it were me I would do it that way but maybe that doesn't work in your case. Trickier still would be to do up the thread and then load it. I have seen that done but i would strongly advise against it. I have no idea how it was pulled off.
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Simulating Stresses on Bolt Threads inside larger Assembly

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