Dear Ted,
Thank you for your explanaton. I had drawn the large zigs and zags as you
suggest. I understand you are refering to the bolt thread in your discussion
below.
It seems to me that the size of the nut does not really matter, because the
same problems apply to both full and half nuts, but for the sake of clarity
in this discussion I will continue to consider the lock nut as the half nut.
You suggest the half nut is applied first, to the correct torque, then the
full nut, also to the correct torque.At this point the half nut is just
about floating, as the pressure on the zig face of the bolt thread is
approaching zero.As the half nut is backed off, some of the turn will be
just taking up the slack in the thread, and this does not change anything.
But as it is backed off further, it is putting pressure on the zag face of
the bolt. This must also reduce pressure on the article being held, as the
torque on the full nut is reduced by this amount, and the torque on the full
nut is increased by the same amount.
What is unclear to me is whether this reduction is significant, as the
article being held is not under the correct pressure.
The danger of having a floating half nut, thus negating any locking effect,
is still present, if it is not backed off far enough to create a definite
lock on the full nut.
It seems to me that as the half nut is being backed off, and just starting
to lock on the zag face, you do not want it to move from that position on
the bolt . This would require then turning both nuts towards each other ,
the half nut twice as much as the full nut, to create the lock without
changing position on the bolt, and maintain the correct pressure on the
article being held.(If using both full nuts, move them together equally)
If the thread pressure produced by the required torque to hold the article
is P1, and the thread pressure produced by the locking action is P2, it can
be seen that the outer(full) nut carries P1+ P2, and the inner(half) nut
carries only P2, which is far preferable to the reverse arrangement having
the half nut on second,carrying P1+P2.
The art is judging how firmly to lock the two nuts together!
Thank you, I now consider myself educated.
Regards Graham Higgins, in OZ.
Hi Kingsley,
If you consider a section through the axis of the nut an bolt and greatly
enlarge the zigzag line formed by cutting through the thread at the
interface between the nut and bolt, you will see that, when the nut is
tightened, the pressure will be on the sloping face of the thread towards
the bolt head - - lets call this the "zig" and there will be a gap on the
rear facing slope of the thread due to the clearance between nut & bolt - -
lets call this the "zag".
If the full size nut is put on first and tightened to the specified torque
and then the half size nut is put on and tightened, to try and lock it, it
will tend to push the full nut forward thereby reducing its pressure on the
bolts zig face and could eventually force it so far forward that it reduces
the pressure to zero and may even produce a gap on the zig face AND zag
face. The full bolt is then floating free and is acting as nothing more
than a washer !! In doing so you will invariably have had to apply
greater torque. The full tension in the bolt is then being taken by the
half nut - - which it was not designed to carry and could eventually strip
its threads. There will also be a gap on the zag face which will allow the
nut to slacken with any vibration.
If the half nut is put on first and tightened to the correct torque and
then the full nut is put on and tightened to the correct torque, both nuts
then have pressure on the zig face, If the full nut is then held
stationary (to maintain the correct torque and tension in the bolt) and the
half nut is backed off (slackened) sufficiently for its pressure to be
removed from the zig face and transferred to the zag face it will then be
jammed against the full nut. The full nut will be holding its full
designed load and be locked by the half nut - - - - just like when you put
two nuts on a threaded rod and tighten one against the other. There is
pressure on the zig face of the full nut and pressure on the zag face of
the half nut thus there is no slackness and vibration will not loosen them.
... Ted.
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