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Re: BOLT TORQUE/TENSION RELATIONSHIP

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Actually, I don't believe my statement is an over-simplification.

Have another look.  What I said was there is no DIRECT Correlation, not that
there is NO correlation.  The distinction is an important one that is not
made by the commonly relied upon formula for calculating torques:  T = (K) D
P.

Almost every week I receive calls from people who are trying to deal with
fatique failures because they have used this formula to extrapolate from
tensions achieved using XX torque which provided XX tension --- from which
they all-too-often get fooled into believing means that an increase in
torque by 25% will equate to an increase in tension by 25% and so-on.

The actual torque-tension relationship for a given fastener is better
described by a curve.  Quite often these curves double back on themselves
and graphically look like "S"s as different factors which make up the "K"
factor come into play.

It wasn't clear to me what the original poster might do if he were merely
presented with (or found on his own) the Torque-Tension formula.  Thus, I
believed some strong precautionary language about adopting torque values
from some table/formula somewhere was appropriate.

David Sharp
"TQH8R"




----- Original Message -----
From: "Christopher Wright" <chrisw(--nospam--at)skypoint.com>
To: "?" <seaint(--nospam--at)seaint.org>
Sent: Tuesday, June 17, 2003 11:28 AM
Subject: Re: BOLT TORQUE/TENSION RELATIONSHIP


> >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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