Message text written by INTERNET:europa-list@matronics.com
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Ted,
Would really appreciate an explanation of the reasoning behind this
please.
An intellectually handicapped mind would like to know !
With the half nut on first, I can see that the load on its threads are
likely to be less but I can't even hazzard a guess as to why it matters
which nut is held firm while the other is locked up.
Regards
Kingsley
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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.
The newsreels of the Potters Bar rail crash in England a few years ago - -
which was caused by the bolts becoming loose on the points - - - showed a
close up of the offending points and I noticed that the remaining bolts
were locked with half nuts on the wrong way round. I pointed this out to
the investigating team and they stated that they would consider this during
their investigation.
It is difficult to explain in words and if it is still not clear and you,
or anyone else, would like further details I can send you the detailed
drawings that I sent to the Potters Bar Investigation Team.
Regards
Ted
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