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Re: BOLT TORQUE/TENSION RELATIONSHIP

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>There is NO direct correlation between the torque applied to tighten a 
>fastener and the tension generated in the fastener (or the clamp load 
>generated in the connection.) 
This is a little bit of an over-simplification. Saying that there's no 
understanding of why a torque wrench produces a pre-tension in a bolt 
denies a lot of very effective and useful service experience and would be 
flat out not true. OTOH, saying that  the determination of tension from 
torque requires field experience and some measurements is right on. The 
bugger is friction. The basis for the torque tension relationship 
includes a simplified approach to friction and how it changes over time. 
I claim that there _is_ a relationship and a useful one between torque 
and tension, but that the physics is complicated enough so an engineer 
needs to know just what the hell he's about. The same thing is true in 
such commonly used techniques as  concrete prestressing and response 
spectrum analysis. If it weren't true, David's lab would have no reason 
to calibrate super-accurate torque wrenches.

The 30% figure for scatter in bolt pre-loading by torque has been around 
for years and isn't a big surprise to people who have been in the biz for 
while. That's just the way friction and fit-up work out. This fact alone 
doesn't invalidate the use of bolt torque specifications. What it does 
invalidate is the quickie-bugger-factor-in a simplified-formula approach 
to engineering design. That's what made me really nervous about the 
original question.

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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