In a nutshell.... Bolted connections work because tightening the bolt
generates a clamping force between the two items. The trick is to
generated as much clamping force as possible without damaging the bolt
by permanently stretching it. In a range, a bolt can be
tightened(stretched) and loosened without permanent change in length-
it will return to its original length. If you stretch it too far, it
"yields" and will not return to its original length. The best amount of
tightening would be to about 90% of the point where it wouldn't return
to its original length-- a little safety factor for the unknown. Now
the trick is to try to relate "torque" to the stretch in the bolt,
including all kinds of other factors like type of metal, strength,
thread pitch, lubrication, etc. Testing and usage has verified common
engineering formulas for this and people have set up tables for common
bolt size torque values. If you think about it, "threads per inch" tell
you that with one turn of the bolt, exactly how far it will stretch.
Say... if you have 16 threads per inch, then one turn will stretch the
bolt 1/16" of an inch.


--
CraigFL
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