Listers,
A bit of engineering perspective, to try to clarify things for those who are
interested (everyone else can delete & go on with more fascinating
pursuits).
Rod bolts, like any threaded fastener, require a certain amount of PRELOAD
in order to reliably hold the joint together. This "preload" is a FORCE,
measured in pounds (or newtons, if you're anywhere but the U.S.). The
amount of required preload (force) depends upon the loads (force) that the
joint will see during operation. Too little preload, and the fastener will
fail due to fatigue as the joint undergoes cycles of load/unload (most
frequent failure mode). Too much preload, and the fastener yields
(comparatively rare).
HOWEVER, we can't easily measure the preloaded force in the fastener. That
is, we can't measure the force in pounds directly -- we can't hang some fish
scale on it & measure the pounds force in the fastener. So we have to
measure something else, to get an estimate of the preload.
The easiest, and most reliable, is to measure the length of the bolt.
Easily done with a micrometer (assuming bolt does not go into a blind hole).
We measure the length of the bolt before installation, and after
installation, and the difference is bolt "stretch". The engineering term
would be deformation.
Fortunately, since the material exhibits elastic material (up to a point
...), the deformation ("stretch") is directly proportional to the amount of
force (pounds) applied to the fastener. So, by measuring stretch, we know
the amount of preload we've applied. We know whether we have reached the
target amount of preload for the joint. "Stretch" is a VERY ACCURATE method
of estimating preload.
On the other hand ... torque is a pretty awful way of estimating preload.
There are just too many variables -- principly, the coefficient of friction
in the threads and under the head of the fastener. If you could somehow
magically measure actual preload (force), you would find that you could
torque 10 fasteners to the exact same torque and have widely differing
preloads. Not a good thing on a rod bolt or bearing cap.
As an aside (this whole thing's an aside, actually) ... in critical
large-scale fasteners, like you find in civil work and heavy industry, the
preferred tightening method (if you can't measure stretch) is the
"turn-of-the-nut" method. Tighten to snug, then continue to tighten to a
specific degree of rotation of the nut. or bolt. This is actually more
accurate than torque, in terms of reaching target preload. Even civil
engineers regard torque as the least reliable method.
So ... the right thing to do, is tighten the fastener until you've reached
the recommended "stretch", which will assure that you have reached the
target preload. Use whatever torque you need, to reach that stretch. If
you somehow inadvertently exceed the recommended stretch, take the fastener
back out, re-measure in the unloaded state, and compare to the original
unloaded length. If you have caused a permanent deformation (bolt now
longer than original measurement), throw the fastener away -- you have
loaded it beyond yiel, into the plastic deformation range, and damaged the
bolt.
If, on the other hand, you are faced with a fastener in a blind hole, you
can't measure stretch ... so you have to measure torque, and HOPE that the
indicated torque has produced the target preload.
Regards,
Mark Palmer
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