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Bolt FOS in FEA

Question asked by Tyler Murray on Jan 5, 2015
Latest reply on Dec 7, 2015 by Robin Boudreault

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I was trying to compare my hand calculations to Solidworks FEA and I was always getting a discrepancy in bolt factor of safety. I dug into it deeper and realized that Solidworks calculates bending stress for a bolt using its nominal radius for area moment inertia and stress calculations. Is there a reason for this that I cannot think of? I have been using the minor diameter to be conservative and do not know what the correct radius would be but I am pretty sure a bolt does not have same bending strength as a solid cylinder(unless analyzing portion with no thread).

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Jared Conway Jan 6, 2015 6:44 PM
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hi tyler, can you point us to the reference you found for solidworks simulation and also the reference for the equation you're using?

your methodology sounds logical and i don't think it hurts to look at it from both perspectives.
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- Tyler Murray @ Jared Conway on Jan 7, 2015 9:34 AM
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  This can be easily reproduced, all you need is to do is create a situation in which a bolt will undergo some bending moment(off centered load). A simple plate with hole and force up on one side and down on the other will work. This produces a moment with no axial or shear which makes it easy to determine stresses from bending. You can look up FOS and moment in bolt using result tools then back calculate stress and diameter using simple MY/I formula where Y=D/2 and I=pi*D^2/64 so stress = 32M/pi*D and D = 32*M/stress*pi. When I do this I get diameter equal to nominal diameter so that means solidworks is effectively treating a bolt as a solid cylinder for bending stress.
  
  Another thing that I found strange is that in my situation when I use 0 preload I get a moment that matches my calculations but if I increase it to even only 1 ft lb then my moment increases 50%. If I increase the preload to where it should be I get a large displacement error and roughly 60% more moment then no preload. I don't get why preload should affect bending moment especially when a negligible amount is used.
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  - Jared Conway @ Tyler Murray on Jan 8, 2015 7:14 PM
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    if you can post references and the test models, i can take a look.
    
    and regarding your new question, once you have a large disp error, you basically can't use those results.
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Robin Boudreault Dec 7, 2015 11:33 AM
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I've got the same result like you and was able to recalculate the FOS from force, moment and shear found in the analysis.

Most of the time, my main concern is more that induced bending from the FEA is often lowering too much the FOS compared to a practical and classical calculation. Not sure If I should design with that in mind or stick with no bending calculated.
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Bolt FOS in FEA

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