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Re: BOLT TORQUE/TENSION RELATIONSHIP

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>Have another look.  What I said was there is no DIRECT Correlation, not that
>there is NO correlation.  The distinction is an important one
I think of a correlation as either existing not not existing. If by no 
direct correlation you mean that there's a third variable involved, 
you're absolutely right--it's friction and it's damn important. Without 
it a fastener would unthread itself. There's a lot that goes into that K 
value besides a quickie bugger factor.

I think it's fair to add that there's more to the situation than fuzzy 
torque-tension behavior. The overall requirement is to clamp two parts 
together under load. An important piece of this in many areas to insure 
that the fastener doesn't loosen and fail in fatigue. The service 
requirements are also as fuzzy and the relationship between torque and 
tension, partigularly where the loads are cyclic. The whole business is 
fairly inexact before the vagaries of fabrication come into play.

>Thus, I
>believed some strong precautionary language about adopting torque values
>from some table/formula somewhere was appropriate.
Damn right--no quibble there. 

I really am curious about the S-curve behavior you mention. Seems to 
imply some sort bistability situation like snap-through behavior on a 
belleville spring. I can't say that I've ever encountered any such thing. 

>I think I read somewhere that 90% of the
>energy put into torquing the nut onto a bolt is
>"spent" overcoming friction.
Just remember--without friction the nut will unscrewing itelf like a ball 
screw spiraling down its raceway. Doesn't matter how you tighten that 
bolt--the friction acting against rotation with be the same for the same 
surface condition. So yeah, I save a lot of rotation by using a 
pretensioner, but I don't eliminate any friction.

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw



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