Message text written by INTERNET:europa-list@matronics.com
>-- - - - - >
>Next, I loosened the locknut and tightened it to snug condition, and while
holding the locknut
>fixed, backed off the lower nut to lock the pair. The bolt tensile
preload decreased to 5000 lbf. <
> - - - - >
.
Hi Chuck,
This is exactly as I would have expected - - - - the tension in the bolt
is produced by the stretching of the bolt by applying torque to the nut.
When you loosened the locknut and tightened it to a snug condition (and
thus a lesser tension) you then reduced the stretch in the bolt when you
backed off the lower nut to lock the pair.
I suspect that in you first half of your experiment the nuts were not
properly locked - - To provide the correct lock there has to be a wedge
effect between the two nuts and to do this by tightening down the second
nut you would have to push the first nut further along the bolt to transfer
the pressure from the zig to the zag face. To do this you would have to
apply a far greater torque than that applied to the first nut. this would
mean that the bolt would be stretched further & therefor I would expect the
tension in the bolt to have increased.
Try the second half of your experiment by applying the correct torque to
the second nut and hold it stationary (thus keeping the stretch the same)
while you back off the first nut. In backing off the first nut it
does not move along the bolt, it only transfers the pressure from the zig
to the zag face of the thread to produce the wedge action.
By the way - - - the method that I have described as the correct method is
not my pet idea - - - as a retired professional engineer it was drummed
into me as an apprentice and again when I studied for my Mechanical
Engineering Degree many years ago.
With the development of the modern "stiff" nuts and other locking aids the
basic stuff seems to have been forgotten.
Ted
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