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Re: Loose Bolts with DTI Washers

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>
>     <<  Even if you put the lock washer in there, all it would be doing is
> taking up space, since the pretension in the bolt would quickly overcome
any
> of the spring force it could provide.  >>
>
> Perhaps I have been laboring under a misconception on this item.  My
> thinking was that the split washer was not to maintain "spring force" so
> much as it was to prevent rotation by one split lip "biting" the material
> and the other "biting" the underside of the nut.  I can see where the
> hardness of heat treated nuts would inhibit this effect and the
interposing
> of a hardened washer would leave a nonrestrained mating surface, but it
does
> look to me that the industry could develop a dimpled nut/washer
combination
> to achieve this "biting" effect. I can see a use for this product in
cyclic
> loading conditions where only a bearing type connection is required and
100
> percent testing is not warranted.
>
>

Indeed, Lock Washers of various types (ANSI B18.21.1) have been marketed as
providing both a spring action and a 'biting-in' function.  The Fastener
Industry has developed a wide variety of fasteners intended to take
advantage of these two beneficial attributes.  However, research into
various systems has provided no universal solution.

Consider, for example, that unless the 'biting-in' takes place on both sides
of a connection, the mode of loosening will simply switch to the 'unlocked'
bearing surface.   Yet --- if one provides any significant level of such
'bite' on both or all bearing surfaces -- this will dramatically impede
one's ability to tighten the fasteners.  In fact, on fasteners intended to
be tensioned into yield (like structural fasteners), torsional stresses
during tightening may limit the fastener's ability to attain required
tension before tension/torsion fracture.

As the previous poster noted, the level of 'spring' provided by lock washers
is too minimal to provide much benefit to most high-strength fastener
applications.  The most consistent system for maximizing such spring action
would likely be that of 'Belleville' washers, hence their acceptance in ASME
practice.

SPS Technologies (Jenkintown, PA) for many years tested various mechanical
and chemical locking devices for aerospace, automotive, and defense
industries.  A gentleman by the name of Gerhardt Junker developed a system
for them to quantify the beneficial attributes of various proposed fastener
locking systems.  His 'Junkers Vibration Loosening Test' became the standard
method for quantification of such tests.  A MIL standard was developed from
the same principles, although it was an 'attribute' test that didn't
quantify results.

One key finding within the work performed at SPS' Contract Research
Laboratory was the "Locking system can not compensate for undertensioned
fasteners."  That said, most fatigue failures today are misdiagnosed as a
failure of the fastener system to remain 'locked'.  Assuming one does attain
high clamping force to begin with, the better locking systems included (from
memory) micro-encapsulated epoxy, nylon patches and rings, 'Nord-Lok'
washers, and 'Flex-Lok' nuts.

Other industries such as manufacturers of Class 8 over-the-road truck frames
or railcar builders avoid concerns for loosening entirely by . . . well . .
. ditching the inclined plane.  They use Huck's lock, pin, and collar system
in critical dynamic applications.  Huck's is a sophisticated rivet-like
system that swages a collar onto a bolt.  (It does not, as most mechanics
will wrongly tell you, pull the pin to tighten.)  Huck's system is very
accurate in achieving clamp loads of about +/-5% in a production facility,
although the tools do not lend themselves to use in high steel.  Plus, it's
darn hard to 'snug' a Huck bolt without installing it completely.

Hope this helps.

David Sharp
TurnaSure LLC