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Understanding bolt connectors

Question asked by Mikael Martinsson on Oct 22, 2012
Latest reply on Jan 30, 2013 by Neil Grant

Hi.

I need help to understand the bolt connectors in SW.

 

The formulation of SF is from NASA NSTS 08307. In this standard each of Ra, Rb and Rs (SW terminology) should fulfill the requirement on their own as well as together. SW only show the SF combined. Is it possible to plot the individual Ra, Rb and Rs values as well? It is a bit frustrating to calculate them all manually when many bolts are involved.

 

Preload is an important factor for working bolt connections. But if I preload bolts in SW at about 80% of the proof load strength, the SF will be about 1,2-1,4 and decreases if a tensile load is added to the parts. (bolt carries some of the force and the joint the rest). This means that I can never set SF criteria to 2 or above unless I minimize the preload. Where can I see how much additional load the bolts can handle to get the "real" SF? Is the only way to do this to increase external load until SF falls below 1?

 

I ran a simulation on 2 plates bolted together with 21 bolts and external force on one of the plates causing tensile stress. When using 12.9 grade M16 bolts, the results reported back OK. SF average 1,2-1,3 and no separation between parts. I then tried using 8.8 grade and 2 bolts reported SF below 1. I then tried with preload set to half  and now all bolts reported OK. The two bolts that failed in previous test where now a little bit above SF1. Now I tried decreasing the preload even more, and the SF result increased even more and all bolts reported as OK. But now the parts was separated (displacement plots).

 

I find it a bit difficult to understand how much more load the bolt connection can handle just using the results. Trial and error is one way, but with a lot of connectors and no penetration contacts means that this can take a while. Or how do you do it?

 

Perhaps I'm missing something obvious here.

Thank you in advance.

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