Bolt preload

[digger200]

I thought by preloading a bolt then loading the joint with a smaller load, the bolt should not see any increase in stress.
When checking the bolt loads in Cosmos it always seems to adds a bit. I've tried all sorts of setups and even very low direct pull loads to try & eliminate any bending etc. but it always adds this little bit to the preload.
Why?
Thanks

 

[Albigger]

I think, in general, the additional load gets split by the bolt and the joint. The additional load the bolt "sees" is the additional load, times the ratio of stiffness of the bolt to the sum of the stiffnesses of the joint and bolt.

For most applications (say bolting 2 metal plates together) the joint stiffness is much higher than the bolt stiffness, so the bolt only sees a very small additional load amount. Here is a link that I've found helpful:

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

 

[cbrn]

Of course !!!!!
The bolt preload is only a means to preset a compressive load on the bolted parts (at the expense, of course, of a strong traction stress in the bolt). Due to the different compliances of bolt, on one side, and flanged parts, on the other side, the effect of a high-prestress joint is that the working load is "split" on a part which acts to make the traction load on the bolt HIGHER, and a much bigger part which acts to "decompress" the flanged parts. BUT IN NO CASE THE PRELOAD CAN CANCEL OUT THE EXTERNAL WORKING LOAD !!!!! This is not physical !!!
See UNI-10011 or, better, VDI-2230.
Regards