Self-equilibrating stress

[jball1]

Can anyone explain to me what a "self-equilibrating stress" is? I heard this term used in reference to bolt preload. Something to the effect that preload is a self-equilibrating stress, and therefore is a secondary stress which can be subtracted from the total stress before the safety factor is calculated.

 

[MikeHalloran]

It appears that at least one of us (you, me, and whoever came up with that term) is not understanding bolt preload.


Mike Halloran
Pembroke Pines, FL, USA

 

[jball1]

Haha, thanks Mike for your concise answer. My understanding of the analysis of a preloaded bolt is that it is a assumed to have an initial, lower bound stress of the preload stress, and any additional stress that the bolt "sees" up to its preload stress does not add to the bolt stress. However, once the preload stress is exceeded, the analysis stress is now the stress seen by the bolt.

I know this is a simplification, and that other ways of analyzing preloaded bolts are out there. So when I heard the idea that I explained in my original post, I thought it might just be a different way of looking at things. I probably misunderstood though.

 

[desertfox]

Hi jball1

I cannot quite agree with your analogy, my understanding is that if the external load exceeds the bolt preload, then the joint will separate and the bolt takes all the external force but up to that point any external load is shared between the clamped components and the bolt proportional to the stiffness of the clamped components and that of the bolt.
Now the clamped parts are normally much stiffer than the bolt itself so what happens with an external load below the preload value is that it reduces the compressive stress between the clamped faces but however increases slightly the tensile stress in the bolt, all this is nessecary to maintain equilibrium.
Please see bolt joint diagrams on this link

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein