Fun wikipedia article on preload of bolts

[304065]

I thought this was interesting. Engineers care to comment?

http://en.wikipedia.org/wiki/Preload_%28engineering%29

[304065]

Or better expressed by the board's own Jim Sims, in another thread. This is the best technical explanation of what goes on with a torque-to-yield bolt that I have read.

When the stretch bolts (also known as torque to yield fasteners) yield they do not lose their clamping force but rather due to the shape of the metal's stress/strain curve the clamping force stays rather constant after yielding even as the bolt is turned a further half turn or so. This means that the required clamping force will always be achieved despite the variability of thread friction. The variability of thread friction causes the amount of bolt tension for a given torque to be a variable; the greater the friction the more torque is used to overcome it and therefore there is less to stretch (elastically and plastically) and tension the bolt. If the bolt is taken to yield then the clamping force needed to preload the bolt and rod end cap joint to prevent fatigue failure is guaranteed without regard to friction. Torque to yield fasteners should not be reused as the torque to yield procedure causes the bolt or screw to "neck" or reduce it's cross-section (at the cross-section of a thread root usually). Therefore the second time the bolt will not achieve the same clamping load as it necks even further during the second tightening. Torque to yield fasteners are typically made from less expensive metal (compare price with Raceware and ARP), are not as thread friction sensitive and may be correctly tightened with a simple torque wrench. This may be compared to a more elaborate bolt stretch measuring rig or the repetive tightening (five cycles of tightening and loosening to reach a constant and stable thread friction) sometimes required to achieve the correct tension in the more exotic superalloy rod bolts. Cheers, Jim.

[patkeefe]

Originally Posted by 304065

Engineers care to comment?

Well, I wish Wikipedia was around when I was in school, I could have saved a lot of money on those textbooks! That is a good all purpose article for our needs.
Pat

[hcoles]

I'll attempt to explain in as few words as possible:
It is about fatique life and fatique failure.
If a bolt torqued and is subjected to cyclic stress there is a high and low stress amount. It is this cyclic stress that contributes strongly to the fatigue life failure.
If the bolt is torqued more (e.g. into the yeild range) the high value of stress goes higher and the low value also goes higher but the variation from top to bottom of the cycle of stress is a lower percentage of the total high stress. This translates to a better fatique life. This allows using a smaller bolt and still having the required fatigue life. If the Porsche engineers could have fit a bigger bolt there they would have, but no such luck for them... so they use a smaller bolt and make it work a bit harder.

I might be all wet... this is what I remember from stress analysis class.

[304065]

Originally Posted by patkeefe

Well, I wish Wikipedia was around when I was in school, I could have saved a lot of money on those textbooks! That is a good all purpose article for our needs.
Pat

Pat,

True, but what would we have sold back at the end of the year to allow us to reinvest the proceeds in beer?