Fasteners - hdgasa

Fasteners
were duly disassembled. It is those
bolted joints that failed
unexpectedly, that have caused a
large degree of associated drama and
consternation.
The traditional weapon of the bolted
joint designer against the multiple
factors that give rise to uncertainty
within the bolting application, is
over-design. With failure, comes
experience. Larger and more
numerous bolts are deployed into
the joint so that it does not fail even
if not all of the bolts are correctly
tightened or if the service loads were
under-estimated. The route of overdesign
is often the only recourse
when seemingly, the theory
evaporates.
Almost all tightening methods invoke
relative rotation between the bolt
and nut (or threaded hole). The
mechanical advantage of the screw
thread (what Archimedes identified)
enables high clamping loads with
only modest torsional moments.
The methods of controlling such
tightening efforts are many and from
the simple spanner whose length has
been estimated to limit the torque
that could be reasonably exerted by
the human arm, has developed the
torque-control method, in which a
spring-loaded pin mounted inside a
wrench, limits the tightening effort to
a predefined level.
Although the accuracy and
repeatability of this method has been
widely condemned (±27%) it
nonetheless enjoys favour with many
(including the ‘cost effective’
mechanic who has just re-assembled
your vehicle’s engine).
The more reliable method and one
that is advanced by the more
discerning members of the torquing
fraternity, is the ‘angle control
method’ (turn-of-nut). This method
rotates the nut through a prescribed
angle after it has been given a
‘snugging’ torque (that is initially
chosen to exceed any influences that
localised features – such as burrs –
might incur).
In the direct measurement method,
where both ends of the fastener are
accessible, the length of the fastener
might be measured and with
advances in ultrasonic measurement
techniques, this too may be used to
determine the extension of the bolt
and thus determine through the load
extension behaviour, what preload
level has been achieved during
tightening.
The preload-indicating method that
utilises deforming washers beneath
the bolt head, has advanced to a
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Corrosion & Technology
Consultants
Specialists in Cathodic Protection (CP) of buried
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CTC also specialise in AC Predictive and Mitigation
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Specifiers of protective coatings for the construction
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GERALD HAYNES:
Cell: 082 658 0821 • Email: gjhaynes@icon.co.za
Tel: 011 456 7960 • Fax: 011 454 6304
P.O. Box 2154, Edenvale 1610
1160 CBT (011) 868 3408
Volume 9 Issue 1 2012 Hot Dip Galvanizing Today 19